part 1 astrodyne sourcebook'11

1Part I

2IFC

810.75

Letter from the CEOThe entire Astrodyne team wants to thank you for your continued vote of confidence over the past year. It is an exciting time at Astrodyne as we accelerate our new product development ef-forts for traditional product lines as well as in the ultra high reliability RO portfolio and our new line of low leakage medical adapter products from our recent acquisition Jerome Industries. Additionally, we have recently added significant additional capacity to our manufacturing facili-ties and we have expanded our design centers in California and Taiwan. You can be assured that these efforts are dedicated to our basic principle of satisfying our customers with innovative technology, superior quality, and best in class service.

We are also energized by the opportunities we will pursue going forward. We will continue to increase our product offering with an expanded range of power options, packaging alternatives and environmentally responsible designs. We will qualify our production operation to comply with new medical certifications and we will aggressively add engineering resources to facilities in the United States and Asia.

Our workforce remains passionate about what they do, striving for technical excellence and un-compromising integrity. As our partners you will help direct us on a path that adds value to your products and your businesses. Our commitment is to be flexible, professional and responsive when you come to us seeking a power solution.

Sincerely,

Peter J. Murphy President & CEOAstrodyne Corporation [email protected]

1Table of Contents

3 Principles of Power 28 Alternative Energy TS200 TS400 TS700 TS1000/1500

41 Lighting Power LPC20 LPV20 ELN30 LPC35 LPV35 PLN30/60 OLP48 ELN60 LPC60 LPV60 CLG100 PLN100 CLG150 HLG240/H

80 Open Frame EFM ASL10/15 OFM20 PS/PD25 LPS PS/PD/PT45 ASL40/60 PS/PD/PT65 ASL150 ASL365 BSL40 BSL60 PQ100/PPQ1003 LPP PD110 PPS125 PPT125 PMK130 PMK150 PMK225U PMK450U PWC PWC12

167 Encapsulated ECA MCA

180 Medically Approved EMA MMA EFM/M ASM10/15 MPS30 MP45 ASM40/60 ASM150 ASM365 BSM40 BSM60 MMK PMMK130 PMMK150 PMMK225 PMMK320 PMMK450 AMP1201 AMP1501 AMP300 ASA30M AMP6301 AMP500 AMP101 MP200

278 DIN Rail ( Please see Part II of PDF ) MDR DR DR75/120 SDR240 DRP480

297 External SPU10 SPU12A SPU16 SPU20 SPU25A ASA30 SPU31 SPU46 SPU63

Continued

2Table of Contents

External (Continued) SPU68 SPU80 SPU131 VPU10 VPU11 VPU15 VPU26 VPU40 VPU60

334 Enclosed PV15 RS25 RS/RD35 R50 R65 RS75 R85 RS100 R125 RS150 MK PMK150E/D PMK225E/D PMK320E/D PMK450E/D SP/TP75 SP/TP/QP100 SP/TP/QP150 SP/QP200 SP240 SP PSP RSP RSP1500 SCP

433 High Density DU1P0 ASD105 ASD03 ASD35 VKC03 ASD06 ASD ASDH ASD05 ASD08 ASD15/15H LED/LCD15 FED20 ASD20 ASD30 FEC40 ASD50 ASD75EB ASD150QV

481 High Reliabilty/COTS Application Notes SMV-28-500 PFC PV300 PicoVerter ASD75Q ASD100W NV300 NanoVerter ASD150QB SV28 SuperVerter SV48 SuperVerter UV28 MicroVerter UV48 MicroVerter UV24-164 MicroVerter UV48-164 MicroVerter UV300-164 MicroVerter UV300 MicroVerter MV380 MegaVerter MV24-28-600S

548 Jerome 556 Other Products Available 557 Custom Order Capabilities 560 Glossary548 Price List568 Product Index

*Liability Waiver:All specifications in this catalog are typical at nominal input, full load and 25C unless otherwise noted. Items noted with an asterisk (*) are stress ratings. Exposure of the devices to any of these conditions may adversely affect long term reliability. Proper operation under conditions other than the standard operating conditions is neither warrantied nor implied.

3Principles of Power Conversion

AC/DC Power Conversion 4 Linear Vs. Switch Mode Power Conversion Linear Power Supply Linear Vs. Switching Power Supply Comparison 6 AC/DC Power Conversion AC/DC Power Conversion Topologies Basic AC/DC Switching Topology Universal Input Power Factor Correction Passic PFC Active PFC

8 DC/DC Power Conversion Topologies Non-Isolated DC/DC Converters Buck Converter Boost Regulator Isolated DC/DC Converter Topologies Forward converter Two-Switch Forward Converter Push-Pull Converter Half Bridge Converter Full Bridge Converter DC/DC Application Unregulated DC/DC Converters

12 Other Important Conversion Principles Multiple Output Switchers SynchronousRectification

13 Key Switching Power Supply Considerations Input Voltage and Frequency Inrush Current Leakage Current DC Input Voltages Input.Output Isolation Output Voltage Tolerance and Adjustment Output Voltage Trim TmperatureCoefficient Maximum Output Current/Power Ripple and Noise Line and Load Regulation Minimum Load Balanced Loading Setup Time, Rise Time and Hold Up Time Switching Frequency Efficiency EMC, EMI and RFI Over Current/Over Load Protection Over Voltage Protection Output Power Derating Factors Output Power Rating Vs. Temperature Output Power Rating Vs. Input Voltage

Remote On/Off Control Operation Idle Current Series Operation Parallel Operation for Increased Power Output Parallel Operation for Redundancy Input Surge Suppression Lightning Environments

20 Power Supply Package Styles Open Frame U-Channel and Enclosed Encapsulated Modules DIN Rail Mount External Power Adapters Specialty LED Lighting 21 DC/DC Converter Package Types Single In-Line Package (SIP) Dual In-Line package (DIP) Modular and Open Frame Packages Large Format Open Frame with Case High Density Brick Packages Input Fusing

22 Reliability Considerations Operating Temperature and Derating Factors Reliability and Lifetime MTBF Reliability Prediction Thermal Budget Considerations Thermal Management Safety and Regulatory Standards26 CommonPower-RelatedSpecifications26 EssentialSafetySpecificationDefinitions Isolation/Electrical Strength Creepage and Clearance Spacing Requirements Insulation Strength Leakage Current

Service and Support Experienced Engineers ready to talk From Stock to Distribution - JIT Shipping Schedules Over 4,000 Models and Over 150,000 Stock Items

Technical Capabilities Standard and Custom Product Engineering High Volume Production and Testing Full Ratings and Compliance including RoHS 2002/95/AC

4Principles of Power Conversion

On any given day we come in contact with a wide variety of electronic devices. Addition-ally, there is a plethora of instrumentation, control systems, communication devices, and any number of miscellaneous electronic devices or systems in place in the back-ground of society as a whole upon which we depend on a daily basis.

No matter what type of electronic device, be it a consumer-based product such as computers, cell phones, game systems and the like or more industrially-based OEM monitoring and control systems, they all have one common requirement for their operation-- a reliable and stable source of DC power.

There are three types of power conversion devices in use today: the AC/DC power supply, the DC/DC converter, and the DC/AC inverter. Of the three, AC/DC power supplies and DC/DC converters are the most commonly used.

Whether an AC/DC off line power supply or an embedded DC/DC converter in a distributed power system, no other single component has as much direct effect on

overall system reliability and performance as the omnipresent power supply.

The power supply must not only provide reliable power to the device, but also is called upon to meet safety and emissions requirements, often while operating in harsh thermal or other less than ideal system environments.

Prior to the introduction of switching power supplies to the commercial marketplace, linear converter solutions which were bulky and inefficient were the only AC/DC power supplies available. The more efficient and compact switching type supplies became commercially available in the 60s. Prior to that having been considered, specialized solutions were limited to aerospace and military applications.

Luckily for todays system designer, advanc-es in virtually all component technologies including power semiconductors, integrated circuits, magnetic materials and capacitors in the more than 40 years since has led to the development of a vast array of efficient, reliable, off-the-shelf power conversion products from literally hundreds of world-

wide manufacturers at economical prices. Prior to this, the designer had few packaged power converters to select from, and often had to resort to designing their own power product for use in their system.

This off-the-shelf availability has freed the system designer from having to homebrew his or her own power supply solution and from having to contend with the myriad of safety agency requirements relatedto power supplies and power systems. Power supply design and manufacturing has matured into a $25 billion industry and is now considered a specialty unto itself. Power converter designers and those firms in the business of power conversion design and manufacturing are uniquely qualified to provide the most reliable and economical solution for virtually any application.

This designers guide seeks to acquaint the system designer with the specification and implementation guidelines for power converters. Being a key design element of any system or product, selecting the cor-rect power converter product and required features will assure overall system perfor-manceand reliability.

The transition from linear power supplies to switch mode technology and a push to higher switching frequencies has allowed the compact devices of today to be smaller, lighter, and more efficient than their linear counterparts. Linear power supplies utilize conventional 50/60 Hz power transformers followed by rectifier, bulk capacitor filter and a linear regulator. These supplies, still widely used in low noise requirement applications, are large in size related to output power achieved and are about 40-50% efficient. Conversely, switching power supplies are generally off-line devices. They directly convert the AC line voltage

to high voltage DC without the use of a low frequency (50/60 Hz) transformer. This DC voltage is chopped by a primary power switch that is connected to a high frequency transformer. The secondary or output of the transformer is then rectified and filtered again. Because the transformer operates at high frequency which can be from 20 kHz to MHz, it can be much smaller and lighter than its 50/60 Hz counterpart.

In addition, the power transistor is oper-ated predominately in the low loss on or off modes instead of the highly dissipative linear mode reducing the size of heatsinks

and other cooling devices. These properties of switchers increase efficiency and reduce overall size and boost output power density. Todays AC to low voltage DC switching power supplies reach efficiencies between 75 and 85% while AC to DC PFC units and DC to DC units can operate from 80% and up to 97% efficiency.

Introduction

Linear Vs. Switch Mode Power Conversion

*Liability Waiver:All specifications in this catalog are typical at nominal input, full load and 25C unless oth-erwise noted. Items noted with an asterisk (*) are stress ratings. Exposure of the devices to any of these conditions may adversely affect long term reliability. Proper operation under conditions other than the standard operating conditions is neither warrantied nor implied.

5Linear Vs. Switch Mode Power Conversion

The linear power supply converts AC line voltage to DC output power with just a few components:

Line frequency transformer: Converts the high voltage AC line down to a more suitable low-voltage as required by the system circuitry - usually from 3.3V to 24V. The line frequen-cy transformer also provides electrical safety isolation.

Rectifiers: Converts the transformed AC voltage to a unipolar alternating voltage (rectified AC)

Capacitor: The bulk capacitor, smoothes the ripple of the rectified AC voltage at an average value close to the peak value of the rectified AC voltage

Series pass or Linear mode device: Through the use of a series device and regulator components, the voltage drop across the series device is electronically adjusted to provide constant output voltage while compensating for line, load, and temperature changes.

The diagram illustrates the common linear power supply circuit nor-mally used, with the basic transformer, rectification, bulk capacitor filter, and regulation elements shown.

Linear power supplies have many desirable characteristics such as design simplicity, low leakage current, low output ripple and noise, and fast recovery time. They are not however, noted for high-efficiency, or small size and weight, especially when compared to todays modern switching power supplies. The following table illustrates the primary specification differences between linear and switching power supplies.

The overwhelming difference in size and weight (compare power densities in the table) between the two power conver-sion techniques have allowed switching power supplies to gain in popularity and in fact completely dominate the worldwide power supply market. Another key perfor-mance difference of note is output ripple. Switching power supply output filters are usually designed to filter the switching frequency ripple down to a level of 100 mV or something on the order of 1% of the DC output voltage. While higher than the ripple produced by linears, this is acceptable for most applications; however it can be prob-lematic in low noise requirements such as instrumentation. Of course the ripple can be filtered further to arbitrarily small limits if required and the relatively high switch-ing frequency allows the additional filter components to be small and inexpensive. Switching supplies may have slower tran-sient recovery times than linears, but much longer output holdup times. These char-acteristics may be important in computer, instrumentation and other applications and should be considered. A more detailed

explanation of both transient recovery and hold up time specifications can be found in the glossary at the end of this sourcebook.

Linears are also sometimes chosen for low EMI noise and leakage currents. These differences are similar to the output ripple consideration and likewise, although more complex, a switcher designed with additional EMI filtering and to low leakage current requirements will still be smaller, lighter and less expensive than a compa-rable linear solution.

The switching power supply has the advan-tage of a wider input voltage range than its linear counterpart. The linear power supply input range is usually limited to +/-10% and to make this range wider would reduce the efficiency of the linear supply from its al-ready low levels. AC/DC switching supplies commonly operate over a wider input range, approaching 2:1 while DC/DC converters readily achieve a 2:1 input voltage range and AC/DC supplies incorporating PFC usu-ally achieve a 3:1 range.

Essentially the circuit converts one DC voltage into another by regulating the output voltage by means of a pulse width modula-tion (PWM) control circuit. The inherent wid-er input voltage range of a switcher makes the power supply useful under abnormal input power conditions such as brownouts or surges. Also, by using auto switch or uni-versal input PFC circuitry, switching power supplies no longer require jumpers or wiring changes to adapt to worldwide input line voltage differences, greatly simplifying their application. These topics are all discussed in some detail in this guidebook.

Lastly, the overall efficiency gains of switching supplies and the reduction in heat dissipation requirements have given them a distinct power density advantage over linear types. The increased efficiency factor alone has contributed more to the replacement of linear power supplies in most applications to that of switching power supplies.

Linear Power Supply

Linear Vs. Switch Mode Power Supply Comparison


AC/DC converter topologies can be subdivided into two categories, those with Power Factor Correction (PFC) and those without PFC. The simplest AC/DC converters are those without and will be discussed first.

This diagram shows a simple flyback type switching power supply. This type of switcher is normally referred to as an off-line type because the primary circuit is directly connected to the line without first passing through a line frequency transformer, as is the case with linear type designs. This line voltage is rectified and smoothed by capacitor C1. As C1 operates as a line frequency energy storage device, it is commonly referred to as the input bulk capacitor. The voltage across C1 is a DC voltage essentially equal to the peak of the line voltage, about 300 VDC for a 220 VAC line. A double circuit is often used for 110/120 VAC inputs so that worldwide voltages can be accommodated with 300 VDC storage in the bulk cap.

The flyback circuit that follows is essentially a DC/DC converter, converting the unregulated high voltage DC of the bulk cap into a highly regulated low voltage DC output useable by a system load.

Regulation of the output voltage is achieved by controlling the duty cycle of the primary switch by means of a pulse width modulation (PWM) control circuit. PWM is the method of controlling the ratio of on-time to off-time of a switching element. In the flyback type shown, the longer the on-time is compared to that of the off-time, the more energy is stored in the transformer and then transferred to the output and load.

The switching transistor is controlled by the PWM circuit, and serves as a chopper inverter of the input DC voltage. When the transistor is on, current flows in the primary of the transformer storing substantial energy in its magnetic core. When the transis-tor turns off, the current decreases, reversing the polarity of the transformer windings which causes current to flow in the second-ary of the transformer. The secondary current charges the output

capacitor and also flows into the load. If the output load increases, it is only necessary to increase the on-time of the transistor during which additional current builds up to a higher value to which a higher current flows in the secondary during the off-time. The op-posite occurs for a lighter load. By comparing the output voltage dif-ference with a reference voltage in the feedback loop of the PWM, the output is tightly controlled and the PWM automatically keeps the output voltage at a constant value despite changes in load demand or input voltage. It is important to note that while directly connected to the output and fed back to the PWM and high power switching elements, the feedback loop must contain safety isolation in order to provide electrical isolation between the DC output and the AC line and to comply with safety agency requirements. This is normally accomplished by using an opto isolator, or in some cases a transformer.

Ideally, the flyback regulator circuit is lossless since at any time since the switching element is either at zero voltage or zero current. In practical application however, there are always some switching and conduction losses in the circuit elements due to the high cur-rents involved, particularly in the switching transistor, transformer, diode, and capacitors. To minimize these losses, specialty high power MOSFETs, rectifiers and low ESR (equivalent series resis-tance) capacitors are used. The combination of the switching power design and specialized components make the overall efficiency much higher than that of a conventional linear regulator power supply and allows the flyback topology to be operated efficiently for power levels up to around 150W.

As noted earlier the basic AC/DC flyback converter has both an AC to DC conversion, followed by a DC to DC conversion. Many topologies are in common use for the DC/DC converter portion of the AC/DC converter and are discussed in the DC/DC Power Conversion section of this guidebook.

Although the DC/DC converter portion has many common forms, the AC/DC conversion is generally limited to a few common topolo-gies including those that provide Power Factor Correction (PFC).

AC/DC Power Conversion Topologies

Basic AC/DC Switching Topology


Worldwide input voltages range from 100 VAC to 240 VAC nominal, in reality a 90 VAC to 265 VAC range. Older power supply designs normally required a mechanical jumper or different terminal strip wiring to accommodate this roughly 3:1 input voltage range. Modern switchers utilize universal input, which al-lows these devices to operate from any worldwide line voltage without the need for physical jumper wires or wiring changes. This universal input function can take two forms, auto-ranging types and active PFC types. AC/DC units with auto-ranging circuitry use input voltage detection circuitry and a solid state device to perform the mechanical jumper function in automati-cally. This allows operation typically over either the 100 to 130 VAC range or 200 to 264 VAC range, the auto-switch allowing doubling to take place when operating in the lower range so that a 300 VDC bus is established across the bulk cap either way.

Power Factor Correction (PFC) is an important consideration in selecting the appropriate AC/DC converter for any given application and so a guide to understanding the concept of Power Factor and methods commonly used to improve it is presented here.

Power Factor, as applied to AC/DC power conversion, is a measure of the harmonic distortion of the input current waveform to the power supply. It is defined as the ratio of the True Power to the Apparent Power and is therefore a number between 0 and 1 and alternatively can be expressed as a percentage. The Apparent Power is the product of the RMS voltage and the RMS current whereas the True Power is the integration of the instantaneous power and is the actual power drawn by the converter.

In an electric power system, a load with low power factor draws more current than one

with high power factor for the same amount of useful power that is transferred. These higher currents increase the energy loss in power distribution systems and require larger wires and equipment to minimize their effects. Because of the costs required for additional power equipment to supplant the wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers where greater power is used due to low power factor ratings.

The rectifier and bulk capacitor found at the input of an AC/DC power supply draws cur-rent from the AC line in short pulses when the mains instantaneous voltage exceeds the voltage across the bulk capacitor which results in high RMS current and low Power Factor. A typical current waveform for an AC/DC converter without PFC circuitry shown in the figure here is highly non-sinu-soidal and rich in harmonic distortion. The Power Factor is about 0.65 which is about

as high as can be expected for an AC/DC power supply without PFC also referred to as Line Harmonic Reduction.

In 2001 the European Union put standards in place per the IEC/EN61000-3-2 to set limits on harmonics of the AC input current for power supplies above the 75 W power level. To comply with these requirements which have been adopted worldwide, high values of Power Factor are needed. Switch-ing power supplies designed to meet these specification normally include an additional PFC stage in their design. Power factor cor-rection circuitry can take two forms, either passive, or active.

Universal Input

Power Factor Correction

Country Volt VAC Freq. Hz. Country Volt VAC Freq. Hz.Argentina 220 50 Mexico 127 60Australia 240 50 Netherlands 220 50Brazil 127 60 China 220 50Canada 120 60 South Africa 220 50Denmark 220 50 Span 127/220 50Finland 220 50 Taiwan 110 60France 127/220 50 UK 240 50Germany 220 50 USA 120 60Ireland 220 50 Russian 127 50Japan 100 50 Venezuela 120 60

AC/DC units with active PFC are generally designed to operate continuously over the entire 90 VAC to 265 VAC universal input range. Ac-tive PFC circuits usually use a boost topology and require the output be higher than the peak of the highest line voltage and therefore the bulk cap storage is generally at a slightly higher voltage of 380 VDC.

To be truly universal input devices, some PFC units are able to cope with both 50/60Hz ground-based power as well as military and com-mercial aircraft input frequencies which can be fixed at 400Hz or can vary between 360 and 800Hz.

8Although DC/DC converters often differ significantly in packaging and application from AC/DC converters, the topologies commonly used for the DC/DC portion of AC/DC converters are the same as those used in DC/DC converters.

DC/DC converter topologies can be subdivided into two significantly different categories: Isolated and non-isolated.

The most common non-isolated topologies are the buck and boost topologies.

Used for power levels from a few Watts to KWatts, the buck regulator shown below operates by alternately applying the input voltage to the LC output filter section. During the off time of the switch, energy stored in the inductor continues to flow to the load through the diode producing a rectangular unipolar pulsed waveform at the input of the filter. The filter acts as an integrator

and low pass filter. A regulated DC output is produced by pulse width modulation of the switch. Today the diode in the circuit is usually replaced with a synchronous switch, especially for low voltage applications. Syn-chronous bucks find use in all sorts of step down applications ranging from high current multi-phase arrays to auxiliary outputs in multiple output converters.

Linear Vs. Switch Mode Power Conversion

Passive PFC uses a low pass filter as a simple way to control the harmonic current by using a filter design that passes current only at line frequency (50-60Hz). This type of filter requires large value and high current inductors which are large, heavy and expensive due to the relatively low corner frequency of the filter. The passive PFC filter approach is very simple and reliable and can provide power factors in the range of 0.7 to 0.75 for single phase applications and even higher for 3 phase circuits.

Most often, active PFC utilizes a boost converter circuit. The boost switch is controlled by a special PWM controller that is programmed to force the peak or average current in the boost switch to follow the input voltage waveform. The inductor in the boost circuit must provide sufficient smoothing of the current at the input to produce a clean sinusoidal current with low harmonic distortion. Since the boost switch can be operated at higher frequency, typi-cally between 75KHz and 150KHz, the boost inductor can be considerably smaller than the inductor used in the passive PFC approach and high Power Factor, above 0.9 and even approaching 1 can be achieved.

Whether of an active or passive type, the additional components required make PFC enabled power supplies larger and more expensive than their non-PFC counterparts. Therefore, careful consideration should be given to the level of PFC that may be required.

DC/DC Power Conversion Topologies

Non-Isolated DC/DC Converters

Buck Converter

Passive PFC

Active PFC

9DC/DC Power Conversion Topologies

Another non-isolated DC/DC converter is the boost regulator shown below. The boost converter operates in a similar manner to the buck by storing energy in the inductor during the on-time of the switch like a buck regulator except that the output voltage in this case is higher than the input voltage. During the off-time of the switch, the inductor voltage polarity is reversed and a voltage higher than the input is produced al-lowing energy to flow through the diode to the output capacitor and load. Again, PWM techniques are used to modulate the on and off intervals of the switching device and regulate the output voltage. Boost regulators are used over a wide range of power levels and find application when a voltage higher than the input is needed and isolation is not required, and as discussed above are very well suited to active PFC circuits.

The most common isolated DC/DC converter topologies are shown below with the exception of the flyback converter which is described in the Basic AC/DC Converter section.

Another popular switching configuration as shown below is known as the forward converter. Although the forward converter bears some similarity to the flyback type, there are some key differences. The forward converter does not store significant energy in the transformer, but rather in the output inductor. When the transistor switches on, and output voltages gener-ated in the secondary current flows through the diode into the inductor. The longer the on-time of the switch relative to the off time; the higher the average secondary volt-age and the higher the open load current. When the switch is off, the current in the

inductor cannot change instantaneously. Because of this, current flows from the magnetic energy storage element during both halves of the switching cycle, unlike that of the flyback type circuit. Because of this, the forward converter exhibits lower output ripple voltage than a flyback circuit for the same output power level. This type of configuration is used for power levels up to around 400W. The forward converter is most suitable to low voltage DC inputs and is most often used in DC/DC converters but also finds use in the DC/DC section of AC/DC switching power supplies.

The two-switch forward is commonly used for the DC/DC converter portion of offline switchers because it reduces the voltage rating required for the main switching MOS-FETs. Another benefit of this topology is that the diodes shown on the primary side provide voltage clamping for the switches.

Boost Regulator

Isolated DC/DC Converter Topologies

Forward Converter

Two-Switch Forward Converter

10

The push pull converter is another two-switch topology and can be thought of as another variant of the forward converter. In this topology the transformer can be driven in both polarities and power transfer from primary to secondary can take place over nearly 100% of the switching period which results in reduced filtering requirements for the output filter and low output ripple.

Some of the first commercial switching power supply products available in the 60s used the half bridge topology. This is another two-switch topology that uses a capacitive voltage divider so that the primary can be driven in either direction with half the input voltage.

The full bridge topology is a four-switch configuration that, like the half bridge topology, can drive the power transformer in both polarities but with the full input voltage applied. This more complex topology is generally reserved for high power applications ranging from 500W and up into the Kilowatts. It is used for both DC/DC converters and as the DC/DC portion of off line AC/DC converters.

DC/DC Power Conversion Topologies

Push-Pull Converter

Half Bridge Converter

Full Bridge Converter

11

DC/DC Application

Aside from being an essential component of AC/DC power converters, DC/DC converters find application in both distributed and central-ized power architectures. Both of these power architectures involve one or more intermediate DC bus voltages from which DC/DC conver-sion can take place to arrive at the required system voltages. The intermediate bus voltage can be 370VDC from an AC/DC PFC unit for example, or could be as low as 12 VDC from which low voltages can be derived with high conversion efficiency.

A typical on-board or single-point application is shown below. A primary power supply main voltage is presented at the circuit board, and an in-board DC/DC converter is used to derive the necessary differing voltage for the local circuit. In some cases, a non-isolated IC type DC/DC may be used. Many handheld or other battery operated portable consumer type devices utilize this approach.

In higher power applications, one or more discrete modular DC/DC converters may be required to not only provide the required voltages, but also to provide galvanic isolation and tightly regulate the output voltages.

In distributed power applications, the single point DC/DC converter configuration is replicated across numerous system boards, utilizing the same or different output DC/DC converters as required by each sub-circuit in the system. The distributed power concept has several advantages over routing a main supplys multiple outputs throughout the system:

1) Reduction of wiring. In systems requiring high current, multi-voltage power, routing multiple power outputs (such as +5V, +/-12V, +3.3V) throughout a system requires complex power harnessing, and may be physically impractical due to the large AWG sizes required. Voltage drops and power-to-signal noise crosstalk can develop, especially in high frequency logic or communications systems.

2) End point adaptability. As newer technology develops, updating legacy systems may require the introduction of new voltage levels on portions of the system circuitry, as seen by the recent migration of logic and instrumentation circuitry from the +5 V to the +3.3 V power standard. Another requirement may be to provide a differential dual output (such as +/-12V or +/-15V as often required for analog control or instrumentation amplifiers) from a single-output +5V main supply. In power hardwired systems such a change would necessitate additional power wiring to be added, or in some cases, replacement of the main system power supply itself. DC/DC converters are an efficient and convenient way to upgrade sub-circuit sections without complete power architecture revisions.

3) Circuit isolation. In mixed-signal systems containing both high-speed logic and sensitive analog instrumentation circuitry, the gal-vanic isolation and common-mode power and ground isolation provided by DC/DC converters becomes mandatory in preserving system signal separation and integrity.

4) Voltage regulation. In distributed power systems where a large bulk power supply is used for main branch power, tight regulation of the output may be affected by AC line variances, or in the case of generator or battery-backed systems, by inherent instability of these sources. Due to the lower branch circuit power demands, DC/DC converters can provide significantly improved output power regulation at the point of power.

5) Fault tolerance. By separating power demands into distinct isolated branches powered at the local level with DC/DC converters, failure of one circuit within a system will not result in failure of the entire system. Repair of encounter problems can be addressed quickly by hot swapping out the affected sub circuit with a replacement board.

DC/DC Application

In applications where the input voltage is applied by a regulated main power supply and output load is fairly constant, an unregulated DC/DC may suffice. The main advantages of this approach are low cost and smaller size, often in IC form. With constant loading, typical regu-lation is within +/-2%, acceptable in many applications.

Unregulated DC/DC Converters

12

While the examples of switching power supplies to this point have shown single output versions, it is not uncommon for applications to require multiple and varied outputs. As an example, an applica-tion may require a switcher with a main +5V output for digital logic, +/-15V outputs for analog circuits or an additional 24V output for relays, sensors or other devices. Multiple output requirements vary widely and selecting a standard multiple output power supply with just the right outputs for a given application can be challenging but in almost all cases where a multiple output supply fits, the solution is far more cost effective than several single output units.

Not all topologies are well suited to producing multiple outputs. This figure shows a multiple output flyback switcher. The main V1 output is regulated by a feedback loop including a pulse width modula-tor. The additional outputs are not directly regulated by the PWM control loop but with careful design the main loop can be used to produce fair regulation of the voltages across C2 and C3. Where

tight regulation of the auxiliary output voltages is not required, these voltages can serve as the auxiliary outputs. For applications where these outputs must be tightly regulated, post regulation, as shown in the figure is provided. Post regulators can take the form of either linear regulators or switching regulators, such as the buck circuit described above.

Conversion efficiency can be a deciding factor when specifying a power supply. Although efficiencies above 75% for AC/DC supplies and above 85% for DC/DC converters are common nowadays, the losses in the conversion process are concentrated in a relatively small number of power components that experience a temperature rise as a result.

In low DC output voltage converters, the highest losses are in a single component -- the output rectifier. A commonality between all switch-ing supply topologies is that all the output power must flow through an output rectifier. In a 5V supply utilizing a low drop Schottky diode with a drop of 0.5V, the maximum efficiency that can be obtained is 5/5.5 (about 91%) even if all the other components in the supply were ideal and their losses zero. In a real 5V DC/DC converter with a low-drop Schottky diode and 84% efficiency, the losses in the output recti-fier make up about half the total losses in the supply. Notice two other key points to this rule: First, since the drop in the diode is nearly constant, this efficiency rule is independent of output current. Secondly as output voltage requirements decrease (to 3.3V, 2.5V and lower), the rectifier drop cannot be decreased proportionally and therefore the problem is compounded.

Synchronous rectification is a technique that replaces the output rectifier or rectifiers with switching devices, low Rds (on) power MOSFETs in todays designs. These power MOSFETs, called synchronous rectifiers in the application, must be driven in synchronization with the primary switching which adds some cost and complexity. The synchronous switches can be chosen such that their drop at rated current is significantly less than that of a power Schottky rectifier diode and efficiencies for low voltage outputs can be increased by 5 to 10% or more, reaching into the 90s. Such efficiency improvements are generally well worth the added cost and complexity since they result in smaller more compact power conversion units due to reduced cooling requirements for example heat sink size or airflow requirements.

When selecting any power supply, it is important to consider the power supply efficiency, power loss and output power de-rating at the maximum expected load and ambient temperature conditions. This becomes even more important with synchronous rectifier units which normally experience efficiencies that decrease as operating temperature increases. The voltage drop across power Schottky diodes used in low voltage outputs decreases at higher temperature, like any diode. This helps to offset other losses in supplies with conventional rectification that increase at high temperature, such as MOSFET conduction losses and copper losses. In synchronous rectifier units, there is no such offset; the losses in the synchronous switches also increase with increasing temperature.

Other Important Conversion Principles


Multiple Output Switchers

SynchronousRectification

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When specifying a switching power supply, there are a number of factors to be considered. Some of the key specifications and features are described in this section. Please be aware that these descriptions are for general information, and the manufacturers product data sheet should be referenced for detailed parametric performance and product information.

Because power supplies utilize a large capacitor across the input, a sizeable input peak current is required to charge this empty bulk ca-pacitor when the AC is initially turned on. This momentary peak is called the inrush current and the magnitude varies according to the type of current limiting circuit that is being used within the supply. Switching supplies commonly use a negative temperature coefficient (NTC) thermistor in the input line to limit the inrush current. The high initial resistance of the thermistor limits inrush current at start up. While this high inrush current is only momentary, it may be enough to blow fast acting input line fuses. As such, an appropriately sized slow blow fuse should be used. Additionally, to avoid stresses associated with the inrush current, the power supply should not be cycled on and off rapidly. Generally, after turning off the supply, there should be a delay of several seconds before re-powering in order to allow the NTC device to cool and return to its higher resistance state and effectively limit the inrush current.

Key Switching Power Supply Considerations

Inrush Current

Most switching power supplies are rated for nominal 100VAC, 110/120VAC or 220/240VAC operation, with the input range specified over 85-264VAC to account for variances in the input AC supply line. The frequency of the AC mains is generally 50Hz or 60Hz. Switching power supplies are designed for 50/60Hz operation are usually rated for input frequency within the range of 47-63Hz. Those products may also be acceptable for 400Hz applications but increased leakage current should be expected.

he leakage current is defined as the small currents flowing from the AC lines to earth ground (such as the case). EMI filters in switching power supplies utilize capacitors connected between the AC line and earth, and between the neutral line and earth, called Y caps. This configuration causes a small current to flow through the Y caps to the frame ground. In practice, leakage current is regulated to complywith applicable safety standards. ITE equipment is governed by EN 60950-1, where leakage current should be less than 3.5 mA for por-table Class I equipment, 0.75 mA for handheld Class I equipment, and 0.25 mA for Class II equipment. In the case of medically approved power supplies, the IEC 60601-1 specification applies and is more stringent, the leakage current requirement being as low as 50A in some cases.

Isolation of the DC output section from the AC mains input is an essential safety feature of power supplies. In general, this breakdown volt-age rating is 3KV AC for ITE products and 4KV AC for medically rated products. Per safety agency requirements, this specification is 100% tested at the factory to assure compliance. Such testing, called hipot testing should only be performed according to the factory procedures by trained personnel as it involves dangerous voltages.

Isolated modular DC/DC converters with 300V or 380V input intended for use as part of an AC/DC system generally carry the same isola-tions ratings and approvals as for AC/DC supplies, since the DC/DC portion of an AC/DC converter is where the input to output isolation is normally provided.

For products intended for 48V, 24/28V and 12V systems, isolation ratings are typically lower from 500V to 2500V.

Isolation in DC/DC converters is not only important for providing safety protection, but often is used to reduce overall common mode noise and ground loop coupling. In applications where user safety is mandated, check the manufacturers specifications to assure that the con-verter is approved for compliance to the appropriate safety standards.

Input Voltage and Frequency

Leakage Current

Input/Output Isolation

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The nominal output voltage of the switching supply is specified as being measured directly at the output terminals with minimum load as specified, usually 5% of the total rated power output. In general the tolerance specification takes into account line regulation, load regula-tion, cross regulation, and initial setup tolerance. Typically, an output adjustment potentiometer is provided to compensate for initialand wire losses for remote located loads, and usually provides -5/+10% adjustment range. When adjusting the voltage, care should be taken not to exceed the overall voltage rating as referenced to the output connections of the power supply. In addition, increasing the out-put folder each above nominal will reduce the maximum output current that can be supplied with out triggering the over-current protectioncircuitry.

Stated as a percentage of output change under normal operating conditions and directly related to ambient temperature, is expressed in terms of %/ C. Typical specification for temp coefficient is +/-0.03%/ C over the rated operating temperature range. Once the power sup-ply is turned on and stabilized, temperature coefficient effects, though additive to regulation, are usually minimal.

Due to the line frequency AC/DC conversion, and the high frequency switching and fast rise time pulses encountered in switching power supplies, the DC output contains an AC component known as ripple and noise and also referred to as periodic and random deviations (PARD). AC/DC converters have a low frequency component from the sine wave rectification which is 2 times the input line frequency. The second com-ponent is that of the higher switching frequency, normally 25 kHz or more. Additional higher frequency components due to steep edges on the pulse waveforms presented to the output filter, referred to as the spike component may be present as well. Atypi-cal waveform is shown. It is generally good practice when using switching power supplies to add small capacitors such as 0.1uF ceramics and small electrolytic, for example 47uF across the output at the point of load, especially for applications where low noise may be required, such as instrumentation. Additionally, care should be taken with input and output wiring and PCB layout in order to minimize noise. Generally, input wiring should be kept away from output wiring and runs and routes should be kept short and avoid loop area.


Output Voltage Tolerance and Adjustment

TemperatureCoefficient

Ripple and Noise

Many encapsulated DC/DC converters provide a trim connection, where an external adjustment of the output can be made. Usually, the simple connection of a single resistor to this pin is all that is needed to program the output voltage to a non-standard value or to increase the output slightly to make up for load line drops, or a potentiometer can be connected to provide output voltage adjustment.

Trim up range is normally limited to +10% to stay below overvoltage trip points, but the trim down capability of various DC/DC products can vary widely, sometimes being limited to -10% and sometimes being as much as -50% or more. Always check the specification and observe maximum power and current limits when using the trim function.

Simply put, maximum output of a power supply is defined as Iout x Vout. If the nominal output voltage is increased through adjustment as described above, the output current must be reduced proportionately as to not exceed the overall power rating of the power supply. As an example, a power supply with a 50W rating can provide 5V at 10A. A 10% adjustment of the output to 5.5V reduces the output current to 9A. Exceeding this rating places stress on the power semiconductor components and reduces overall operational life (MTBF).

Output Voltage Trim

Maximum Output Current/Power

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Line regulation is usually specified from low line to high line with full load applied.

Load regulation may be specified from no load to full load or from a minimum load such as 20% of full load to full load. Though small per-centages, these, as well initial accuracy and temperature coefficient are additive when considering total regulation performance.

Some multiple output units require the loads to be maintained within a specified range for the outputs to be within regulation limits. For con-verters with such requirements, highly imbalanced loading between outputs can also result in premature overvoltage protection triggering and can contribute to an overall reduction in converter efficiency.

Line and Load Regulation

Balanced Loading

Due to their inherent design, switching power supplies often require a minimum output load to maintain regulation of the output voltage to stated specification tolerances. This minimum is typically 5% of the total output power, though may be as low a requirement as 1%. In cases where the minimum load cannot be maintained during steady-state operation, a pre-load resistor may be required on the output. This resistor must be rated to continuously sustain the minimum load power dissipation. Alternatively, electronic preload circuits can be used so that energy is not wasted in the preload resistor when the load is sufficiently large, increasing overall efficiency.

Setup Time is the time required for the power supply to reach 90% of the rated output voltage per regulation specifications. This specifica-tion is directly related to output load, and may be as long as1 to2 seconds in duration.

Rise Time is the time it takes for the output voltage to rise from 10% to 90% of the rated output and assumes a minimum starting load of 10%. The specification is usually within the range of 50ms.

Hold Up Time is the time that the DC output sustains itself down to 90% of rated output once the AC power is shut off, typically 16 to 32mS. This specification is important when the system is used in conjunction with a UPS or other backup power system, in that it gives the backup power time to switch in before the output of the power supply collapses. This hold up time also serves to isolate the power supply output from the effects of momentary line sags or dropouts.

Typical AC/DC PFC circuits operate in the 50 to 200KHZ range while DC/DC operating frequencies are in the range of 100Khz - 450Khz. Depending on the design, this frequency may be fixed or may vary with load and line changes. Because of the potential EMI and con-ducted noise issues that may result, care should be taken in ground loop prevention and shielding to minimize propagation of the potential frequency effects throughout the system, especially where sensitive instrumentation circuits are used. Modular products may utilize internal shielding or metal case shields to minimize the broadcast effects, though good PCB layout guidelines should be observed to locate on-board sensitive circuitry away from the DC/DC section of the board.

Minimum Load

Setup Time, Rise Time and Hold Up Time

Switching Frequency

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Efficiency is the ratio of output power to input power. Todays AC/DC power supplies incorporating PFC may be 75 to 85% efficient while DC/DC converter efficiencies start at 85% and can exceed 90%. It is important to evaluate the efficiency at the actual operating point and translate this number into the power loss, which is the amount of heat to be dissipated.

Although a power converter that is operating at 85% efficiency needs only dissipate 15% of the input power, it generally resides in the same enclosure with the load, where the other 85% of the input power is turned to heat and probably occupies a lot less space. In most systems, compared to the power supply, the load is spread out over a larger space and therefore the power loss density and temperature rise of the power supply is a key reliability consideration and must be managed.

It is also important to understand the relationship between efficiency and power loss.

For example, the table below shows that the power loss in a con-verter operating at 92% efficiency is less than half that of a converter operating at 85% efficiency.


Efficiency

Switching power supplies, unlike linear supplies, can be a source of electromagnetic and radio frequency interference. There are two basic types of interference: conducted and radiated. The source of this interference is the short burst of high frequency energy caused by the rapid switching voltage and current transients in the power supply and are repeated at the switching frequency of the power supply.

.Conducted EMI is the noise fed back from the power supply onto the AC power line, which can then present itself at other AC line connected components. To minimize these effects, an input Pi filter for differential mode noise or balun and capacitor filter for common-mode noise can be used. Examples of each are shown here. Both types of filters are commonly used in the internal EMI filters found in many AC/DC switching power supplies and are often specially designed, tested and certified to meet applicable EMI compliance standards for the intended product application.

For many DC/DC converter applications, strict adherence to an EMI standard may not be required and so a high attenuation level

filter may not be included in the converter. In such cases, the manufacturer will generally provide an Input Reflected Ripple Cur-rent specification. If further filtering is required, ripple current and conducted emissions can be suppressed to a low percentage by using a capacitor or an input Pi type filter. Most higher performance DC/DC converters include such a filter internally, though if required, a simple Pi filter can be implemented externally. For modular prod-ucts, a standard filter may be available as a separate module.

Radiated emissions are tested with an antenna. Generally radi-ated emissions are addressed by use of filters, shielding and good layout practice. Filters used to reduce conducted emissions will also reduce radiation when placed close to the converter. Other-wise, the load side of the filter wiring can be a significant radiation source. Metal enclosures used on many power supply products are very helpful in containing radiated noise. When an open frame supply is selected, it is good practice to route input and output lines away from the supply and each other.

Some DC/DC products designed for modular applications may not provide a metal shielding case as standard but may offer this as an option. DC/DC converters, being board mounted devices, often find themselves on the same board as load circuitry that may be sensi-tive to radiated noise. In such applications, special care should be taken with regard to routing and shielding if necessary to prevent such troubles.

EMC, EMI and RFI

efficiency,% power loss, Pout = 100W100% 0.099% 1.098% 2.097 3.196 4.295 5.394 6.493 7.592 8.791 9.990 11.189 12.488 13.687 14.986 16.385 17.6

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To safeguard the system circuitry and prevent power supply damage in cases of temporary catastrophic overload, most switching power supplies include over current and over load protection on the output. In general, this range is between 105% and 150% of the rated output. This protection may take several forms as illustrated in the diagram below:

1. Fold Back Current Limiting. The output current decreases to perhaps 20% or less of the rated load current as the load imped-ance is reduced. (Curve A)

2. Constant Current Limiting. The output current remains constant and within the specified range while the output voltage drops in proportion to the load impedance (Curve B)

3. Overpower Limiting. The output power remains constant. As the load impedance decreases, output voltage decreases but the current continues to increase until a maximum current value is reached, above which constant current limiting is usually used. This is necessary because as the output voltage approaches small values, very high values of current which could damage the supply would otherwise result. (Curve C)

4. Hiccup or Pulsed Current Limiting. Under overload conditions, the output will keep pulsing on and off repeatedly when the protection is active. The power supply will recover when the fault condition is cleared on the next pulse.

5. Shut Off. The output voltage and current are cut off and locked when the output power reaches of protection range. Recovery of the power supply from the over current protection mode usually occurs in one of the following ways:

- Auto Recovery. The power supply recovers automatically when over load condition is removed. - Repower. The power supply requires a manual restart by cycling the input AC power off and on.

In the case of an output regulation malfunction, the output voltage of the power supply may rise above the rated value. Under this condi-tion, the OVP circuit will trigger to prevent catastrophic system component damage. This OVP protection takes two forms:

1. Shut Off. The output shuts down completely and can be restarted by cycling the AC input power off and on.

2. Hiccup Voltage Limiting. Similar to over current protection, the output voltage keeps pulsing on and off repeatedly when the protection is activated. The power supply automatically recovers when the fault condition is cleared on the next pulse.

In general, consideration should be given to the type of loads that are being driven by switching power supplies to prevent erroneous or premature triggering of either the over current protection or over voltageprotection functions. Static resistive loads have virtually no effect in false triggering of these functions. Highly capacitive or inductive loads, such as those from motors or electrically driven mechanical actuators,can present high levels of out rush currents or back/reverse EMF voltages that can trigger the current overload protection or OVP circuitry. Care should be taken to minimize these effects through the use of blocking diodes or similar means of protection. In extreme cases, output fusing may be required to prevent repeated events that may dam-age the power supply protection features or the power supply itself. It may be necessary in some instances to utilize a larger power output supply to accommodate these loads.

Additionally, proper grounding and ground loop elimination practices should be used. Modifying or disabling the OVP or current overload protection circuitry should never be attempted, as this can lead to circuit damage and possible personal injury.

Over Current/Over Load Protection

Over Voltage Protection

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The output power and overall performance of the switching power supply is affected by two primary conditions: ambient temperature and input voltage.

The curve to the right shows the relationship of output power to input voltage for a switching power supply with a rated input specification of 110VAC to 264VAC. If the input AC power drops below the 110VAC lower limit, the efficiencies caused by dynamic component operation in the input section of the power supply circuitry limits the capabil-ity of full power operation, and must be derated accordingly. Should the input voltage drop below the lower limit, as in the case of severe brownout conditions, the input fuse may blow or the power supply may shut down completely.


Output Power Derating Factors

Output Power Rating Vs. Input Voltage

The graph shown to the right illustrates the relationship between the ambient tem-perature and output power rating of the switching power supply. Ambient temperature affects all electronic systems, none more so than the power supply, due to the self heating caused by power conversion process itself.

As shown for a typical switching supply with an ambient operating range of 0-50C, the power output capability rolls off dramatically above 50, and shuts down completely upon reaching 60C. Conversely, temperatures lower than 0 affect the efficient startup and operation of the power supply, due to the higher component resistance encoun-tered at low temperatures. To minimize the derating vs. temperature effects, proper ventilation and mounting of the power supply for maximum convection airflow should be considered. If necessary, fan cooling should be provided.

For high density conduction cooled brick type converters, the converter may be rated in terms of the base plate temperature and may not require derating as long as the base plate temperature can be maintained within its rated operating temperature range by conduction cooling.

A number of switching power supplies and DC/DC converters offer a remote On/Off control function of the output. Most types utilize posi-tive logic where Logic 1 = Output On, and Logic 0 = Output Off. In other cases, negative logic is considered standard. Generally, these are designed to be operated by pulling the ON/OFF (Control) Pin low (Minus input or Output Return in some cases) to enable or inhibit converter operation, depending on logic type. In many cases an external pull up is not required. If this function is not utilized, the Control pin may be left floating, as it is pulled high internally. Refer to the manufacturers detailed specification sheets concerning this feature.

When placed in shutdown mode by use of the on/off logic control, the converter will operate in an Idle or quiescent state. Output power is effectively shut off, and input current drops to a low standby level to assure instantaneous restart when the on/off control is activated back to the on state. This input idle current is of a minimal level, typically 1-10mA.

Output Power Rating Vs. Temperature

Remote On/Off Control Operation

Idle Current

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In application situations requiring output voltages outside of the normally available standard outputs, power converters can be operated with the outputs connected in series as shown in the diagram to the right. However, it is important to note some limitations:

1. The total combined output voltage should not exceed the rated output-to-case isolation voltage of any one of the converters.

2. Reverse bias protection diodes should be utilized across the output of each supply to protect each output from the possibility of reverse voltage being applied by another. Such a condition exists when tow supplies share a common load and one supply is producing voltage and the other is not.)

Connecting standard power converters in parallel for power redundancy is an acceptable prac-tice. The converters must have their outputs connected in parallel through two blocking diodes, as shown to the right.

Each converter must be capable of driving the full load. The diodes permit one output to fail without affecting the other. The blocking diodes should be sized and heat sunk, if necessary, to accommodate the individual power supplys maximum power output.

Unless the power supply is specifically designed for parallel operation, parallel operation is not recommended. Two output voltages from two discrete fixed output converters cannot be held exactly equal. The converter with the higher output will try to provide the full load, causing it to go into premature current limit. The second supply will then do the same. Even if an adjust function is used to set the outputs, any drifts with time and temperature will cause the same reaction. An example of parallel operation with power supplies equipped with this function is shown to the right. General guidelines for parallel operation:

1. Output voltage tolerance should be as small as possible (< +/-2%). 2. Parallel connections between the power supply outputs should be kept as short as possible

and use large wire gauges. 3. Maximum output power should be limited to 90% rated power per unit to account for current

sharing tolerance and assure continuous operation. 4. Follow connection diagrams with regard to terminating sense lines for supplies with

remote sense.

In situations where power converters may share the main AC trunk line with heavy industrial equipment, the power supply input may be subjected to surge voltages in excess of the rated line specification due to the effects of the shared heavy equipment cycling off and on. In addi-tion, the local power grid in some countries may be subject to frequent sag and surge condi-tions. In either of these situations, it is prudent to provide some form of protection on the power supply AC input.

Accepted methods for protection include basic Metal Oxide Varistors (MOVs) and solid state transient suppression TVS devices. These devices normally work better in combination with each other and X caps which are across-the-line caps used in EMI filters.

Series Operation

Parallel Operation for Redundancy

Parallel Operation for Increased Power Output

Input Surge Suppression

For harsh DC input environments such as vehicle power systems, a power conditioning module designed to handle sags and surges used as a front end to a standard 2:1 range DC/DC converter may provide the best solution.

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In extreme applications, power supplies and their associated electronic systems may be subjected to physical phenomena, such as lightning strikes, that far exceed the power supply ratings as well as those of the protection devices that may be used. Classic methods employed may include fast acting circuit breakers and/or gas discharge tube protection devices. While these may provide complete protec-tion under remote strike and effect conditions; in reality, most electronic devices will not survive a direct or near strike of this magnitude. External grounded shielding or enclosures rated for industrial or outside use are the best protection under these circumstances.

Perhaps the most widely used of all switching power supply formats -- the open frame style, offers the designer both moderate to high power density and low-cost. Available in sizes ranging from 1 x 2 miniature module types up to 7 x 4.25 high power versions with single and multiple outputs. The open frame switching supply is available in both commercial and medical versions. The most popular format for this product type is the industry standard 3 x 5 package which is available in power levels up to 125W.

Interconnections are typically made utilizing standard type locking pin connectors though screw connections and ultra-miniature connec-tors are also offered. Features may include universal input, PFC, OVP, and EMI filtering.


Lightning Environments

Open Frame

Switching power products are available in a number of package styles that are suited to particular application requirements. Some of-fer broad range application use, while others are geared to conform to a particular set of mechanical, environmental, or industry-specific defined performance and certification requirements.

Incorporating an open frame type design with a rigid three-sided metal enclosure, the U- channel style enclosure provides not only me-chanical stability but a greater degree of user protection over open frame configurations. The additional metal shell also provides needed heat sinking surface that allows conduction cooling and extends the ambient operating range and overall MTBF of these higher power products. Interconnections are typically made via either locking style pin connectors or screw terminal blocks. Integral threaded holes in the U-channel simplify mounting.

The fully enclosed variant of the U-channel incorporates a top cover to fully shield all components of the power supply. This provides not only full user protection, but may also incorporate a ventilation fan into the top cover to provide additional power supply cooling in extreme ambient conditions. Since it is enclosed, this type of packaging can be used internally to a system or adapted for open air use as in DIN channel type mounting configurations.

Due to their fully encapsulated construction, these types of modular products are ideally suited for applications that may be subjected to shock and vibration or humid and dirty ambient environments.

Available in either pin styles for onboard use or screw terminal versions for wall or bulkhead mounting, encapsulated modules were originally offered as linear power types though switching power types are now the standard. Besides providing hermetic protection for the internal power supply components, the encapsulation material is thermally conductive. This eliminates both internal hotspots within the device as well as extends the ambient operating range of the power supply through efficient dissipation.The encapsulation material also provides a high degree of internal isolation, making medical versions of this type of product commonplace. Though more costly than open frame styles, the small size and ruggedness of the encapsulated power supply make it the ideal choice for many environmentally harsh applications.

Power Supply Package Styles

U-Channel and Enclosed

Encapsulated Modules

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DIN rail mount power supplies are characterized by the unique package and mounting style as defined by the former German DIN 46277 standards, now European standard EN50022 (35mm width), 50045(15mm width), or 50023 (7.5mm width) mounting rail. Originally specified for use in Europe, it has become a worldwide standard for mount-ing power supplies, circuit breakers and industrial control equipment inside standardized equipment racks and system enclosures.

Besides the mechanical standardization, DIN power supplies are designed to meet the UL508 electrical performance standards for use in industrial equipment. The unique package shape and mounting allows multiple DIN power supplies to be stacked side by side on the rail for secure mounting and connection of electrical wiring via screw terminal connections.

A new class of power supply and packaging has been developed specifically for the LED lighting markets, where NEMA 3 enclosures and UL1310 class 2 compliant outside wet location operation is required.Similar in appearance to conventional fluorescent light fixture ballast packages, this class of power supplies offer power factor correction, IP64 level isolation, and constant current control operation to meet the requirements of LED lighting systems. The specific packaging style of these power supplies allows retrofit of existing standard enclosures for LED lighting, while assuring compliance to all safety specifica-tions.

Moreover, input and open connections are made through standard stripped wire leads and can be installed and serviced by qualified elec-tricians per applicable construction and wiring codes.

With the development of portable electronic products it became impractical to include a power supply within the design, not only because of the additional weight but also the small size of the device itself often precluded a built-in power supply. As such, the external power adapter has come into wide use today. Common configurations range from the wall plug type most often seen with cell phones or portable entertainment devices, to the higher power external boxes commonly associated with laptop computers and associated peripherals. Taking the power out of the box of the system itself presents several advantages to the designer. First and foremost, by having the high power line connected components remote to the system eliminates the need for safety agency approval of the entire system, making product updates and revisions easier to do without the need of safety recertification. Also, since the power supply is usually a prime source of heat, overall system thermal considerations are significantly reduced.

The external power adapter is most often a high impact plastic package surrounding an open frame switching power supply. An IEC 320 type power socket and universal input allows one basic design to be used in a variety of countries with differing inputs voltages and plug configurations by simply utilizing an input line cord with connectors specific to that country. In the case of wall-mounted external adapters, snap-in-plug adapters allow easy configuration for multi-country use. Because of these design advantages, external adapters are now used in most consumer products as well as many test instrumentation and portable medical devices.

DC/DC converters are available in a number of package and pin-out styles to fit virtually any output power and size requirement.

The SIP package offers the ultimate in a small (0.24x0.77x0.4) package size for power needs of under 1W. Available in single and dual outputs on standard footprints, these solid encapsulated devices offer isolation up to 500V and internal filtering. Input voltage is fixed (within +/-5% being typical).

DIN Rail Mount

Specialty LED Lighting

External Power Adapters

DC/DC Converter Package Types

Single In-Line Package (SIP)

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Formatted on the standard (0.8x1.25x0.4) 24 pin DIP socket pin out, these DC/DCs are available in single and dual outputs at power lev-els up to 6W. Fully encapsulated and isolated to 500V, the DIP style is commonly specified for on-board use due to a standardized pin out.

Designed for use in applications requiring 50W or greater, this type of DC/DC converter uses an open frame board and metal case/heat-sink configuration similar to U or enclosed switching power supplies.Though a pin module in connection form, the additional case/heatsink surface greatly improves power dissipation in the higher power operating ranges of these products. Available in single and multi-outputs with isolation of 1500V and efficiencies to 85%, these products are commonplace in large distributed power telecom systems.


Dual In-Line Package (DIP)

Large Format Open Frame with Case

Most commonly offered in the standard 1x2 and larger power 1.6x2 encapsulated pin module styles, this group of products has become available in recent years in the open-frame non-encapsulated format to meet the unique needs of telecom applications. With power output up to 15W and wide input voltage ranges of up to 4:1, this class of DC/DC converters has seen widespread use across many applications from instrumentation to telecomm. In addition the larger format over those of SIP/DIP packages allows isolation up to 1500V and greater internal filter capability, thus simplifying overall system design. Metal cases on the encapsulated versions, and integral finned heat sinks on the open frame types extend their overall operating temperature range and MTBF rating.

There is some standardization within the industry for brick sizes in the 1/8 brick (2.3 in. x 0.9 in.) to full brick range (2.4 in x 4.6 in.) however packaging styles range from open style to encapsulated types and pin outs may not be standard from manufacturer to manufacturer. The open style bricks usually incorporate a heavy copper multilayer board that captures all circuitry including the windings for the transformer and choke and are designed for applications using forced convection cooling. The most advanced high density encapsulated types use thermally conductive potting compound and insulated metal substrate technology and provide a metal mounting surface for conduction cooling. This type of brick package usually finds applications where fans are not permitted and thermal management via conduction cooling is required.

Should safety requirements or best practice design rules require input fusing of the DC/DC converter, the manufacturer may specify an appropriate fuse value for the converter based on input voltage and total output power demands. The primary specification to note is that of input surge current, usually specified with nominal input voltage and maximum rated output current conditions for a resistive load. If using the DC/DC converter to drive highly capacitive or inductive loads, output filtering and suppression circuitry should be used to minimize the instantaneous current demands or back EMF presented by these types of devices, which can greatly increase the surge current beyond that which is specified.

The biggest single factor determining a power supplys reliability in an application is operating temperature. Secondary factors, usually having an influence on operating temperature are the output loading and, the input voltage. For some converters attention to input voltage derating requirements may be necessary for reliable operation.

Modular and Open Frame Packages

High Density Brick Packages

Input Fusing

Reliability Considerations

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In many cases operational temperature performance can be ex-tended by forced air cooling across the converter, usually provided by a fan and rated at a particular level of cubic feet/minute (CFM) airflow. Due to their often small overall size and high power output/square inch ratios, converter performance declines dramatically once critical maximum temperature is reached. Exceeding the maximum rating for an extended period may result in protection shutdown of the converter, or, worse, irreparable damage to the device.

For any electronic product or system, the prime factor affecting useful life is that of heat. Over time, heat will take its toll in the form of either reduced or intermittent performance, leading to eventual failure. All products have an anticipated lifespan, but reaching that point or even extending a product or systems useable life beyond it requires a good design, quality components, and effective thermal management.

The operating temperature of a given product is normally a strong function of loading and therefore derating of the output power will

normally result in lower internal losses, operating temperatures and increased lifetime. It is important to understand the efficiency vs. load and more specifically, the power losses vs. load in order to quantify the effects of output power derating. Since the efficiency will normally drop off a very light loading, excessive derating of the output loading is not recommended.

Many converters behave as constant efficiency devices when oper-ated under load and within a specified input voltage range. This means that as input voltage is reduced, the input current must rise in inverse proportion to maintain constant power flow to the con-verter. The higher currents at low input voltage may cause power losses and temperatures within the converter to increase to the point that the converter is unable to operate reliably at full output power and very low input voltage. Always check the input voltage derating curves to make sure that the product is being operated within its capabilities in order to realize reliable operation.

The graph to the right shows a bathtub curve typical failure rate vs. time for power converters. The failure rate, expressed as is shown over the lifetime of the power converter. The A portion of the curve defines the early life operation and failures during this period gener-ally occur prior to shipment, such as during testing and burn-in and are termed infant mortality. Manufacturing defects or component failures are usually responsible for failure in this period. Manufac-turers test and monitor failures at this point and will burn in product under load to eliminate infant mortalities from reaching the field. Should failure rates rise to unacceptable levels, detailed failure analysis is done to correct any design, manufacturing process, or component flaws.

Region B illustrates the useful life of the converter and depends on use within specified parameters, particularly those of line, load, and ambient temperature and derating thereof. To better determine this theoretical life, the manufacturer may also subject test lots to HALT (Highly Accelerated Life Testing) and HASS (Highly Ac-celerated Stress Screening) procedures to prematurely age the power converter while monitoring degradation in key performance specifications. Though requiring more costly environmental simula-tion and test equipment, HALT and HASS tests provide insight into product lifespan and wear out mechanisms, and may be mandatory for military and other high reliability applications.

Region C shows the wear out period when one or more converter performance parameters becomes out of spec or when the

converter fails completely, depending on the wear out mechanism.

High reliability products normally come from a complete design for reliability program that includes strict adherence to conservative design rules regarding electrical design and component derating, parts count, component selection, mechanical design, thermal management, manufacturability and may also include qualifica-tion testing such as power and temperature cycling combined with shock and vibration and other environmental factors such as humid-ity, altitude, salt fog, explosive atmosphere and the like.

Operating Temperature and Derating Factors

Reliability and Lifetime

24

Mean Time Before Failure can be calculated or demonstrated. There are two popular standards for MTBF calculations, MIL-HD-BK-217, developed by the US Department of Defense and accepted for most military work and Belcore TR-332 which is an often sited standard for MTBF reliability predictions for commercial equipment. Both approaches use a similar statistical prediction method that assigns a failure rate to each and every component based on the component type and construction, quality level, application of the component (derating and stress), application environment for the system, and temperature.

Mathematically, failure rates are expressed in units of failures per million hours and are summed up to find the total failure rate for

the device. The MTBF is then taken as 1/(Total Failure Rate) and is then expressed in hours. Typical power converters have MTBF ratings of 100,000 to 500,000 hours at 25C ambient operating temperature when operated within specified ranges in a benign en-vironment. High reliability products may have MTBFs that exceed 2 million hours.

MTBF can also be demonstrated using field data or controlled environment data such as accelerated life test data from a sample lot. Again, statistical analysis and generally accepted reliability prediction formulas may be called upon to predict the mean time before failure.

In reality, most power converters will be operated in ambient condi-tions far above the idealized room temperature of 25C. The power supply itself, through losses in the power conversion process such as switching losses, conduction losses, magnetic core and copper losses and capacitive ESR effects, will be a prime contributor to its own ambient temperature. By nature, component self heating and ambient temperature are additive. Without proper consideration, the operational range and resultant derating in output power can quickly reach maximum in relation to operational ambient temperature rise. In a typical 150W AC/DC switching power supply with 75% efficien-cy, a 20C rise in internal temperature can be expected. Coupled with a 25C ambient, this raises the internal ambient temperature of the power supply to 45C. Once maximum ambient is reached, power converters typically derate the output at 3%/C, resulting in rapid output power roll off above the manufacturers rated maximum ambient. At 70C ambient, power output drops by 30%, resulting in an effective constant output capability of 105W. MTBF is also severely downgraded by elevated temperature operation, as shown in the following curve.


MTBF Reliability Prediction

Thermal Budget Considerations

Several temperatures must be known: actual ambient immediately around the converter under normal operation (including self-heating contribution), the manufacturers temp rating for the converter (when properly mounted and installed), and the maximum system ambient operating specifications of the overall system itself. These combined factors will identify to the designer the temperature envelope under which reliable power converter operation is assured. If these cumu-lative temperatures are expected to exceed the operating specifica-tions of the converter, heat dissipation and control methods must be employed. These can include:

- Additional heat sinking, either through mounting the unit to an outer case wall as a dissipating surface, or by use of finned heat sinks on the device itself.

- Forced air cooling using fans, though the introduction of mov-ing part components can greatly reduce overall system MTBF.

- Operating with a derated output power level, or oversizing the converter to conform to the manufacturers power derating curve specification.

Thermal Management

25

- Alternative mounting and location of the converter away from other heat-generating parts of the system. The manufacturers recommendations for proper mount orientation and clearance spacing (if recommended and as shown below) should always be followed for maximum cooling performance. Failure to do so may result in operational ambient temperature reduction by 8-10% or more.

For baseplate rated conduction cooled products, the heatsink or cold plate thermal resistance equation should be known. This value will normally be given in C/W. From this, and the converter power loss, the temperature rise of the converter can be easily calculated. Although easily managed, the interface and temperature drop be-tween the converter and the heatsink should be considered as well. Many excellent materials are available to maximize the heat transfer across this interface, the selection of which depends on such factors as surface roughness and flatness, and cost.

Any one or a combination of these methods may be necessary to assure the full performance of the power converter product over the anticipated system operating range. It should be kept in mind that good thermal practice would be to keep the power converter operat-ing at a temperature below that of the rated maximum specification. This will contribute to the reliable and extended life operation of the power converter specifically, and the entire system as a whole. A general rule of thumb used by many reliability engineers is that a 10C reduction in temperature results in roughly double the lifetime. Although a rough rule of thumb only, it gives insight into the order of magnitude and importance of thermal management with regard to the power conversion system.

Understanding thermal specifications and management is critical, and the designer should become familiar with all aspects of proper power converter operation related to temperature effects. Advances in power supply design and the specialized components used have resulted in highly efficient and reliable power products. Misapplica-tion and/or poor thermal management accounts for most failures in the final application. If the designer is in doubt concerning proper thermal management, Astrodynes technical support staff can pro-vide guidance in this important area.


26

USAUL 60950-1: 2001, 2nd edition 2007-03-27(Information Technology Equipment- Safety-Part 1:General Requirements)UL 60601-1, 1st Edition 2006-04-26(Medical Electrical Equipment Part 1: General Requirements for Safety)UL 1310 and CSA C22.2 No. 223-M91(except for 48 volts)(Class 2 Equipment: Power supply with Extra Low Voltage and limited output power, North America only)UL 8750 Safety Standard for Light Emitting Diode (LED) Equipment for Use in Lighting Products

CANADACSA C22.2 No. 60950-1-03 1st Edition, 2006-07(Information Technology Equipment- Safety-Part 1: General Requirements)CSA-C22.2 No.601.1 M90, 2005(Medical Electrical Equipment- Part 1: General Requirements for Safety)

GERMANYIEC 60950-1:2001, First Edition inclusive of CENLEC Common Mod

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