virgile valente - application notes

Buck Converter Applications

Author: Virgile ValenteArizona State UniversityApril 22nd, 2015

Content

1. Summary

2. Terminology

3. Topologies

4. Switch Selection

5. Control and Load

6. Filtering

7. Efficiency

8. Glossary

9. Test Set-Up

10. References

SummaryMany systems require that the primary source of voltage be regulated and converted to other voltages for different components. Buck converters can be used when efficiency, size, or weight requirements are mandated. Buck converters are switch-mode power supplies used to step down a high voltage to a lower voltage efficiently. The DC-DC switch mode buck converter steps down a DC input voltage to a desired DC output voltage using an active device, a switch, that toggles on and off to maintain an average value of output voltage, hence the term ‘switch mode’. The input voltage is regulated by a controller that implements and adjusts pulse-width modulation to the switch. The ratio of on-to-off time of the switch is varied by the controller to regulate the output voltage. The output voltage of an ideal buck converter is equal to the product of the switching duty cycle of the PWM signal and the supply voltage. This is the basic premise of how a buck converter works.

The following notes will introduce common terminology, then the two common buck converter topologies; asynchronous and synchronous, and give details as to how they function and which parameters have the most effect on efficiency. Various applicable components and control methods as


well as basic filtering are also presented in relation to buck converters. The efficiency of the buck converters is broken down and detailed, as it is paramount to this form of power supply.

There are many factors and parameters to consider when planning a buck converter, and with these in mind, the notes will guide you through the process of basic buck converter design. Finally the notes present the ON Semiconductor Capstone Team’s design and selection process used to design and build asynchronous and synchronous buck converters.

TerminologyInput Range

The range of input voltage the device can handle to function effectively at full load.

Load Regulation

Load regulation is the change in output voltage over the specified change in output load, expressed in percentage. As the output load changes, the output voltage should remain consistent (typically millivolt scale).

To the right, the Load Regulation of the TI TPS65251 Buck Converter is shown to have a maximum change of approximately 5mV in output voltage, a 0.05% change over the specified range of output load [1].

Line Regulation

Line regulation is the change in output voltage for a given change in input voltage, also expressed as percentage (typically millivolt scale).

To the right, the Line Regulation of TI TPS65251 Buck Converter is shown to have less than 10mV increase in output voltage over a large range of input voltage, approximately 0.05% [1].

Input and Output Ripple and Noise

Input and output ripple pertains to the amount of voltage or current drop at the input or output between switching cycles. A waveform appears from the switching of the device which gives a slightly inconsistent voltage or current value at the input or output.

At the input, capacitors are used to filter the input current so the current from the host source is approximately an average current. These input capacitors however can cause an input ripple due to

Figure 1 - Load Regulation [1]

Figure 2 - Line Regulation [1]


parasitic equivalent series resistance (ESR) and parasitic equivalent series inductance (ESL) of the capacitor. The ESR and ESL are defined in the glossary.

The input ripple current is typically estimated by dividing load current by 2:

IRipple=I Load2

The input capacitor must be selected based on the calculated input ripple current; however the input voltage ripple requirement is not as stringent as the output voltage ripple requirement. The input voltage ripple can be defined and chosen based on specific need and function. A helpful guide from the EEtimes on calculating appropriate capacitor values can be found in the References section [2].

The worst case ripple current occurs when the duty cycle is 50% as demonstrated by the graph on the right;

For a DC signal, the smaller the ripple, the better the voltage regulation. The output voltage typically rises during the on state and falls during the off state of the device. The ripple observed on the input and output is typically at the converter’s switching frequency and may look similar to the graph below:

The input and output ripple waveforms with Vin = 12V and Iout = 30A for Maxim Integrated MAX5060 buck board. The input waveform is purple, output waveform is teal [3]. A nominal mV change in ripple is effective for steady line regulation.

The effects of ESR and Capacitance on output ripple as illustrated by Texas Instruments [4]:

Figure 3 - Inductor Ripple Current vs Duty Cycle

Figure 4 - Input and Output Ripple Waveforms


Difference on output voltage ripple based on capacitor types [5], as the diagram below illustrates, ceramic capacitors offer the lowest ESR and ESL:

Figure 7 - Ripple Based Capacitor Comparison

Efficiency at Full Load

Operating at 100% load conditions at 25°C, the ratio of power delivered to power supplied for the device.

Temperature Drift

As ambient temperature changes, this is the associated change in voltage, expressed as percentage of the nominal.

Switching Frequency

Switching frequency is the nominal frequency of operation of the switching circuit inside the Buck converter. The frequency represents how many times per second the switch device switches on/off.

TopologiesThere are two types of Buck Converters; Synchronous and Asynchronous

Asynchronous Buck Converter

The main components of an asynchronous buck converter consist of:


Switch (S) Diode (D) Filter:

o Inductor (L)o Capacitor (C)

Controller

A typical asynchronous buck converter is shown to the right. The device typically uses a transistor and a Diode as switches. These are the two main switches that control power to the load. The high side switch is controlled using PWM. The diagram to the right describes its states.

As the switch is turned ON Vin charges the inductor, capacitor and supplies the load current. Once it reaches its set output voltage, the control circuitry turns the switch off. This disrupts the current flowing through the inductor, and without a path for the current, the inductor will resist this change creating a large voltage spike. To avoid this troublesome spike, a path is provided by the bottom side diode for the inductor current to continue flowing in the same direction. As the top side switch is turned off, the inductor voltage reverses its polarity forward biasing the diode on and allowing the current flow. When the output voltage drops below a set point, the control circuitry will turn the top side switch back ON and the cycle repeats to regulate the output voltage to a set value.

The diode’s forward voltage drop and characteristic losses in an asynchronous buck can account for almost 50% of the total losses. This buck converter design is asynchronous because the low side (diode) switching is independent of the high side switching.

Synchronous Buck Converter

A synchronous buck converter consists of the same main components as an asynchronous buck except the diode is substituted for another switch device. The synchronous term refers to the concurrent and complementary nature of the switches.

The high-side switch (S1) and low-side switch (S2) are controlled using PWM by the control circuitry. The low side switch is considered as the synchronous switch and the high side is referred to as the

Figure 8 - Asynchronous Buck

Figure 9 - ON/OFF Switch State Diagram

Figure 10 - Synchronous Buck


switching or control switch. The low side switch does not turn on automatically however and is driven such that it is the complement of the high side switch. This means that whenever one of these switches is on, the other is off and vice-versa. The high side switch remains responsible for the inductor current however, but using a switch on the low side decreases the amount of loss compared to a diode and therefor increases efficiency. This is because using a transistor negates the forward voltage drop of the diode, only a small impedance is present, but the significance is more closesy detailed in the Efficiency section.

The current during the charge (closed high side switch/open low side switch) and discharge (open high side switch/closed low side switch) cycles follow these outlined paths in a DC-DC buck converter:

Figure 12 - Current paths during charge (a) and discharge (b) [13]

To ensure both switches are not turned on simulatenously, dead time, or a fixed delay can be introduced before a switch is turned on. If both switches are simultaneously on, shoot through occurs. Shoot through can occur when the both switches are either fully or partially turned on, providing a path for current to “shoot through” from Vin to GND. Figure 7 shows typical PWM cycles of both switches, notice that the PWM cycles are not exact recipricals and illustrate some dead time to avoid shoot through.

Although asynchronous buck topology is less complicated and requires smaller, relatively inexpensive ICs for control, the efficiency losses due to the diode can be substantial, which is detailed in the Efficiency section. Synchronous buck topology optimizes the overall conversion efficiency however; more complicated drive circuitry is required to control the switches, increasing complexity and costs.

Switch SelectionThere are three main choices of switches to consider for implementation in a buck converter; BJT, MOSFET, or IGBT. Each switch provides many different advantages and disadvantages and are more favorable for certain applications. To determine the best applicable switch for the buck converter a variety of factors are investigated. Here is a brief breakdown of each switch, and a final comparison table to highlight which features are most desirable.

BJTs – Bipolar junction Transistors are characterized by linear current transfer function between the collector current and the base current. BJTs are current controlled devices that can readily be used as


switches. BJTs therefor require a constant current to remain in the on-state, and therefor typically exhibit moderate switching losses compared to MOSFETs. BJTs offer fast switching speeds, and can switch faster than MOSFETs due to less capacitance at the base control pin; however the losses associated with the current needs makes them less efficient.

IGBTs – The Insulated Gate Bipolar Transistor is a minority-carrier device with high input impedance and large bipolar current-carrying and low-saturation-voltage capability. It is meant to combine the best attributes of both MOSFETs and BJTs. It has a large current-voltage operating boundary before it fails or experience breakdown. IGBTs have low on-state voltage drops due to conductivity modulation and have superior on-state current density. IGBTs are also better suited for soft switching due to reduced tail current. IGBTs exhibit conduction losses that are dictated by their voltage from the Collector to Emitter, typically a value (V CE(on)) of 1V to 4V.

Conduction Loss of an IGBT: Pcond=I V CE(on)

Switching losses for IGBTs are comparable to that of MOSFETs of similar performance, although IGBTs can have higher delay time, rise time and fall time, which can amount to higher losses.

IGBTs are generally more favorable for more high voltage, high current and low switching frequency applications.

MOSFETs – Metal oxide semiconductor field-effect transistors are ideal for power switching circuits as opposed to BJTs as they do not require a continuous flow of current to remain in the on-state. MOSFETs can also offer higher switching speeds, lower power losses, lower on-resistances, and reduced susceptibility to thermal runaway. As MOSFETs can switch at higher speeds, they also exhibit lower switching losses than BJTs because as MOSFETs switch from ON/OFF states, they pass through its linear region. During this time in the linear region, it consumes much higher power than when it is fully ON or OFF. Therefore, the faster it switches between the ON/OFF states the less the loss because it spends less time in its linear region. Faster switching also enables the use of smaller inductors, which also reduces losses.


Figure 13 - Turn On Switching Loss (Left) and Turn Off Switching Loss (Right)

Figure x. Turn on Switching Loss (Left) and Turn off Switching Loss (Right).

Conduction losses in MOSFETs are directly dependent on RDS(on) values, which for low current applications can be in the low milliohm range, amounting to smaller losses than that of IGBTs. The conduction loss of a MOSFET can be determines by:

Conduction Loss of a MOSFET: Pcond=I2 RDS(on)

MOSFETs are generally more favorable for low voltage, low current and high switching frequency applications – ideal for a buck converter.

Comparison of switches:

Table 1- Comparison of Switches

BJT MOSFET IGBTControl Method Current controlled. Output

is controlled by controlling base current

Voltage controlled. Output is controlled by controlling gate voltage

Voltage controlled. Output is controlled by controlling gate voltage

Temperature Coefficient

Negative Positive Positive

Paralleling and Drive Circuitry

Difficult Easy Easy

Switching Losses Medium Low Low to Medium

Conduction Losses Low Medium Low to Medium

Applications High Power Low Power Medium to High PowerCurrent Rating High Low Very high


Voltage Rating High Low Very highSwitching Frequency

Low High Medium to High

Colors based on desirability, red is least desirable, green is most desirable. This table was designed to help the Team decide on an appropriate switch based on the load requirements.

Switch Selection ConclusionIn summary there are advantages to all three devices, and each device may be more appropriate based on the application of the converter or relative to the required load. Although IGBTs exhibit low to medium switching and conduction losses, they are more ideal for high power applications with high current and voltage. IGBTs exhibit slower switching frequencies than MOSFETs however, and at lower voltages, MOSFETs prove to be more efficient as they do not exhibit a diode like voltage drop similar to IGBTs. BJTs are also ideal for high power applications, however as they are current controlled, a constant current is required to keep them in on state, due to this they can exhibit moderate switching losses at high frequencies but are great at low frequencies. For synchronous buck applications, drive circuitry is essential, for this reason MOSFETs are typically used.

An important parameter to consider for a MOSFET is the Gate Capacitance (QG). This parameter is of primary interest along with on-resistance (Rdson). The MOSFET must have a QG within the range of the DC-DC converter.

Based on these criteria, the Team chose the ON Semiconductor NTMFS4927NT1G MOSFET for the buck converter. The NTMFS4927N has a low Rdson value of 7.3 mOhms to minimize conduction loses. It also has low capacitance to minimize driver losses and optimized gate charge to minimize switching losses. It has a high break down voltage of 30V, can handle high switching speeds and comes in a convenient package that will be easy to apply to our PCB design. These parameters were ideal for the team buck converter design. Specific characteristics for the MOSFET can be found from ON Semiconductor data sheet in the Reference section [6].

Control and LoadThe most common technique to control switch mode power supplies is Pulse-width-Modulation (PWM). There are two methods of control for DC-DC Buck Converters, Voltage Mode of Control (VMC) and Current Mode of Control (CMC). A controller unit typically compares and assesses the signals of the buck converter at various stages, and controls the switch signal based on those signals. The switch is directly operated by a gate-driver, part of the controller which turns the switch on/off.

Voltage Mode of Control - VMCThe voltage mode of control uses voltage feedback from the output of the buck converter as the input. It contains only a single feedback loop making it easier to design and implement.


Figure 14 - Voltage Mode Control Loop

In this method, the control voltage (V Con) is generated and compared with the ramp voltage (V Ramp) and the switching signal (q) is sent based on the following conditions [7]:

If V Ramp < V Con; q = 1 (switch closed)

If V Ramp > V Con; q = 0 (switch open)

Current Mode of Control - CMCIn the current mode of control there are typically two feedback loops: a current feedback loop, and a voltage feedback loop, with the current on the inductor is typically used as a feedback state.

Figure 15 - Current Mode Control Loop

At the start of the switching cycle, an SR flip-flop is used to set q=1, effectively closing and turning on the MOSFET switch. During this interval, the switch current and inductor current increase linearly and the inductor current (I L) is compared to the control signal (IRef ) from the controller. When ILbecomes greater than I Ref , the output of the comparator goes high and resets the flip-flop (q=0) effectively


opening and turning off the switch. The process is then repeated at each clock cycle as the switch is turned back on [7]. This is the basic control method that controls the gate driver which opens and closes the switch or gate. The PWM signal produced is a by this switching is thereby controlled from the controller.

There are three things to consider for current mode control:

1. Current mode operation – Ideally the converter is only dependent on the dc or average inductor current.

2. Modular gain – This is dependent on the effective slope of the ramp presented to the modulating comparator input. A unique characteristic equation for modulator gain applies to each operating mode.

3. Slope compensation – This is dependent on the relationship of the average current to the value of current at the time when the sample is taken.

Featured ControllerThe appropriate controller for a buck converter may differ based on the selected load. The controller the Team selected for the buck converter is the NCL30105D PWM Current Mode Controller for LED Applications from ON Semiconductor. The NCL30105D has an effective dimming feature for LEDs, which the buck converter will power as a representative load. Although CMC is more involved than VMC, for LED applications, current control is more practical. Datasheet for the NCL30105D can be found in the Reference section [8].

LoadThe load for a buck converter can come with various power requirements, but typically demands high efficiency. Buck converters are designed be efficient power supplies, and are used to reduce losses, where significant boosts in efficiency can save quantifiable amounts of power over other power supplies.

To provide an appropriate load to illustrate efficiency, the Team selected LED’s as they are efficient and have specific power requirements. The LEDs selected were CREE XPLs, they are powerful 3W LEDs with a typical forward voltage of 2.95V and forward current of 1000mA. The Team decided to use three XPLs in series as a load. Datasheet for CREE XPL can be found in Reference section [9].

CalculationsThe NCL30105D has an adjustable off time for stability. The off time duration for the controller is set by a resistor to ground on Pin 1 of the controller. Correct off time is important to avoid shoot through, where both switches are simultaneously in the ON state. First, the off time (t off ) required for our application was determined:

t off=(1−V LED

V ¿)∗T S=(1− 9

12 )∗10us=2.5us


Based on provided equations and our LED load voltage requirements, the resistor value (Rtoff ) was then calculated:

Rtoff [k Ω ]=t off [us ]−0.1214

0.1864=2.5−0.1214

0.1864=12.76k Ω

Additional LED and Controller Parameters:

LED Controller Parameters Derivation*Indicates Internal Constants of the Controller

Parameters Symbol Unit Value

Switching Frequency fs kHz 100

Switching Period Ts us 10

PWM Off Time toff us 2.5

Off Time Resistor Rtoff kΩ 12.76

Current Sensing Resistor Rsense mΩ 50

Minimum Peak LED Current IpkLED(min) mA 500

Maximum Peak LED Current IpkLED(max) mA 1500

Minimum Current Sensing Voltage Vsense(min) mV 10

Current Sensing Threshold Voltage* VILIM V 1

Vsense Amplifier Gain Avsense 14SSTART Voltage to Current Set Point Ratio* Iratio 3

Minimum SSTART Voltage VSSTART(min) V 1.5

Maximum SSTART Voltage VSSTART(max) V 4.5

Soft Start Duration tSSTART ms 15

Soft Start Current ISSTART uA 30

Soft Start Capacitor CSSTART nF 150

VCC Decoupling Capacitor CVCC nF 100

Efficiency There are two major sources of loss that can affect efficiency in buck converters:

Conduction losses – Power dissipated as heat

Conduction losses occur anywhere there is resistance, and particularly in the inductor

Switching losses – Power dissipated from switching component


Refer to the Glossary for terms.

Asynchronous Buck Converter LossesThe total losses associated with a typical asynchronous buck converter are due to:

Input Capacitor Losses:o ESR o ESL o Power Dissipatedo PDissipated=I Ripple

2 ∗ESR MOSFET Losses:

o Conduction - Rdson (mOhms)o Switching – Gate Charge (nC)o Power dissipated in the MOSFET amounts to

both conduction losses (Rdson) and switching losses (Gate Charge)

o PDissipated=PConduction+PSwitching

o PConduction=I L2∗Rdson∗D

I L = Load Current

D = Duty Cycle

o PSwitch=(V∗ID2 )∗(T on∗T off )∗F sw+(Coss∗V

2∗F sw ) T on = Time on

T off = Time off

F sw = Switching Frequency

Coss = Capacitance

Diode Losses: o Switching – Negligibleo Forward Voltage Drop (V D) o PDissipated=V D∗IDo ID=I L∗(1−D)

Inductor Losses: o ESRo ESLo Hysteresiso Eddy Currento Skin Effect

o PDissipation= I L2∗ESR

Output Capacitor Loss:o ESR

11%

29%

45%

14%

1%

Asynchronous Buck Converter Losses

Input Capacitor

MOSFET

Diode

Inductor

Output Capaci-tor

Figure 16 - Example Asynchronous Buck Converter Losses [10]

Figure 17 - Example Asynchronous Buck Converter Loss Percentages [10]


o ESLo PDissipated=I Ripple

2 ∗ESR

Based on the total losses, the efficiency can be calculated by dividing the output power of the buck by the output power plus the loss power.

Synchronous Buck Converter LossesThe total losses associated with a typical synchronous buck converter are due to:

Input Capacitor Losses:o ESR o ESL o PDissipated=I Ripple

2 ∗ESR MOSFET Losses (High Side):

o Conduction – Rdson o Switching – Gate Chargeo PDissipated=PConduction+PSwitching

o PConduction=I L2∗Rdson∗D

I L = Load Current

D = Duty Cycle

o PSwitch=(V∗ID2 )∗(T on∗T off )∗F sw+(Coss∗V

2∗F sw ) T on = Time on

T off = Time off

F sw = Switching Frequency

Coss = Capacitance

MOSFET Rectifier Loss (Low Side):o Conduction – Rdson – Very low almost

negligibleo PConduction=I L

2∗Rdson∗(1−D)o Switching – Very Low almost negligible

Inductor Losses: o ESRo ESLo Hysteresiso Eddy Currento Skin Effecto PDissipated=I L

2∗ESR

Output Capacitor Loss:

20%

51%

2%

25%

2%

Synchronous Buck Converter Losses

Input CapacitorMOSFETMOSFET RectifierInductorOutput Capaci-tor

Figure 18 - Example Synchronous Buck Converter Losses [10]


o ESRo ESLo PDissipated=I Ripple

2 ∗ESR

Efficiency ConclusionsBoth asynchronous and synchronous buck converters present high (>90%) efficiency ratings as power supplies. Close inspection shows that the synchronous topology generates nearly half the over losses compared to the asynchronous topology [10].

It is clear that the losses from the diode in an asynchronous buck converter amount to a significant portion of the total losses, accounting for almost half of the total losses as shown in Figure 17.

In the synchronous buck converter, a low side switch or MOSFET rectifier losses substantially less power than a diode. It is apparent that synchronous rectification can increase a power converter’s efficiency significantly, in the examples of Figure 8 and Figure 10, an almost 4% increase is expected. This increase can become substantial when considering the relative losses associated with large power applications and the period of the use of the converter.

Overall efficiency of buck converters can easily be measured and calculated using the following equation relating Power out to Power in:

n=Pout

P¿=V out I outV ¿ I¿

Filtering Filtering is necessary to smooth and clean up the output and input voltages of the buck converter. Ripple current Filtering helps remove noise at the input and output of the buck converter, such as the harmonics created by PWM at the output which need to be filtered out. Different arrangements of capacitors, inductors and resistors are used to create different types of filters. Depending on the chosen values and arrangement of these components, a low-pass or high-pass filter can be created, which can allow low or high frequency signals to pass. The use of resistors in these filters should be avoided due to their large impedance and relatively large losses. For buck converter applications, only light filtering is needed at the input,

Figure 19 - Synchronous Buck Converter Loss Percentages [10]

Asynchronous Synchronous0.00

0.20

0.40

0.60

0.80

1.00

1.20

Comparison of Total Losses (W)

Figure 20 - Comparison of Total Losses


assuming a stable power source, and low pass filtering is typically used at the output, to stabilize and remove the PWM harmonics.

Capacitor SelectionAt the input a capacitor is sufficient to smooth the input signal from the power supply. Input filtering can reduce the reflected input ripple current and reduces both peak current drawn from the input supply and radiated noise to other elements of the system. The input capacitor should have a ripple-current rating near the selected switching frequency. The input capacitor ripple current can be approximated as [13]:

Inductor Selection The low pass filter used on the output effectively allows a low frequency signal to pass and attenuates high frequency signals. The following diagrams illustrate both the ideal and real models of an LC filter.

Figure 22 - Output Filter Circuit

Again the ESR and ESL values directly affect the output signal, as described in the ripple frequency section [1].

The following equation calculates the frequency of the filtering components based on the cutoff frequency (f c):

f c=1

2π √L∗C

This frequency should be significantly lower than the switching frequency of the converter to adequately attenuate high frequency noise and ripple.

An appropriate inductor should have low DC resistance (DCR), and should meet the peak current requirement. It must also be designed to operate at the desired switching frequency.

Figure 21 - Input Capacitor


The losses caused by the inductor in the low pass filter, due to ESR, hysteresis and Eddy currents as described in the efficiency section can amount to measurable losses. It is difficult to improve upon inductor losses as they are such inexpensive parts that improvements in efficiency are often negated by the cost. One way to lower inductor losses would be to reduce ESR and Eddy current losses by using better core material and improving the structure of the inductor. Special winding and braiding using litz wire, an insulated thin gauge wire could help reduce those losses.

Testing ProceduresFor both the Asynchronous and the Synchronous board designs testing parameters include:

Efficiency Line Regulation Load Regulation Inductor Parasitic Current sensor power loss Low side switch loss

Efficiency

The efficiency of the system is determined by the ratio of power out vs power in. To test these values, a signal generator is connected to the input of the board to power it. Voltage and current meters are connected to both the input and output sides of the board. The input side measures the power coming from the signal generator, while the output will measure voltage across the load and current from the inductor. After measurements are taken, efficiency will be determined using the formula:

Efficiency%= InputVoltage∗InputCurrentOutput Voltage∗Output Current

Current sensor power loss

The voltage across the current sensor will be measured with a voltmeter using an allotted test point. Current is calculated using ohms law. The product of voltage and current across current sensor will determine power loss of the current sensor.

Low side switch loss


For the low side switch, an oscilloscope can be used to measure the signal across the switch. These measurements give the voltage curve of the low side switch. Dividing these measurements by the known current sensor resistance gives the current curve of the switch. The area where the two signals are both on as well as the switching frequency will determine the switching loss.

Glossary

MOSFET:

Rdson – The inherent on resistance between the drain and the source of the MOSFET during the on state. The Rdson of NTMFS4927N [6]:

Figure 24 - NTMFS4927N Rdson Characteristics

Gate Charge – The total Gate Charge (Qg) is a function of the gate to source voltage. As the gate-

source voltage increases, the total Qg increases. It is a capacitance that builds up between the

Figure 23 - NTMFS4927N MOSFET Rdson


gate as current flows into it. Qgs is the gate-source charge, Qgd is the gate-drain or “Miller charge” [11].

Inductor/Capacitor [1]:

ESR - Equivalent series resistance – An inherent resistance value that is parasitic. Inductors have resistance inherent in the metal conductor, the DC parameter in SMPS design is an important parameter. Can be modeled as a resistor in series with an inductor.

ESL – Equivalent series inductance – The inductance built up inherently from the component. A parasitic inductance value.

DCR – DC Resistance, inherent resistance of the metal in component. Hysteresis – The history of the input and its current

state create a dependence of the output that affects the value of its future state. Values change and predicted future values are different based on the increase or decrease of the input and its history.

Hysteresis of a ferroelectric material. D is the electric displacement field, E is electric field. This curve forms a hysteresis loop based on the direction of the values.

Eddy Currents – Faraday’s law of induction states that circular electric currents are induced within conductors by a changing magnetic field. Eddy currents generate resistive losses.

Diode:

Figure 25 - Gate-to-Source Voltage vs Total Gate Charge Figure 26 - Gate-Source Voltage vs Charge

Figure 27 - Hysteresis of Ferroelectric Material


Forward Voltage – The ‘threshold’ or forward voltage potential required for the diode to be turned on and conduct during forward bias. This voltage is different for different types of diode materials.

References[1] Texas Instruments. (2015). Understanding, Measuring, and Reducing Output Voltage Ripple

[Online]. http://e2e.ti.com/support/power_management/simple_switcher/w/simple_switcher_wiki/2243.understanding-measuring-and-reducing-output-voltage-ripple

[2] Cooper, Chris. (2007). Choosing the right input capacitor for your buck converter. EETimes. http://www.eetimes.com/document.asp?doc_id=1273212

[3] Prakash, Surya. (2009). Reference Design for a High-Current Power Supply with Lossless Current Sensing Using the MAX5060. Maxim Integrated Products, Inc. http://www.maximintegrated.com/en/app-notes/index.mvp/id/4375

[4] Singh, Surinder P. (2014). Output Ripple Voltage for Buck Switching Regulator. Application Report. Texas Instruments. http://www.ti.com.cn/cn/lit/an/slva630a/slva630a.pdf

[5] Texas Instruments. TPS65251 4.5V to 18-V Input, High-Current, Synchronous Step-Down Three Buck Switcher With Integrated FET. Specifications. http://www.ti.com/product/TPS65251/datasheet/specifications

[6] ON Semiconductor, NTMFS4927N Power MOSFET Datasheet. http://www.onsemi.com/pub_link/Collateral/NTMFS4927N-D.PDF

[7] Shu, Hi Man; Khanna, R. (2012). Various control methods for DC-DC buck converter. Power India Conference, IEEE Fifth. http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6479548

[8] On Semiconductor, NCL30105D PWM Current Mode Controller for LED Applications Datasheet. www.onsemi.com/pub/Collateral/ NCL30105 -D.PDF .

[9] CREE, XLamp XP-L LEDs Datasheet. http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/Data%20and%20Binning/ds%20XPL.pdf

[10] Microchip Technology Inc. (2006). Buck Converter Design Example. Microchip web seminars. http://simonthenerd.com/files/smps/SMPSBuckDesign_031809.pdf

[11] McArthur, Ralph. Advanced Power Technology. Making Use of Gate Charge Information in MOSFET and IGBT Data Sheets. October 31, 2001. http://www.microsemi.com/document-portal/doc_view/14697-making-use-of-gate-charge-information-in-mosfet-and-igbt-data-sheets

http://www.microsemi.com/document-portal/doc_view/14697-making-use-of-gate-charge-information-in-mosfet-and-igbt-data-sheets

http://www.microsemi.com/document-portal/doc_view/14697-making-use-of-gate-charge-information-in-mosfet-and-igbt-data-sheets

http://simonthenerd.com/files/smps/SMPSBuckDesign_031809.pdf

http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/Data%20and%20Binning/ds%20XPL.pdf

http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/Data%20and%20Binning/ds%20XPL.pdf

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