12-6-15 Ridley Engineering - Current Limiting and DCM Flyback Design1/3 www.ridleyengineering.com/index.php/dc1/56-13-current-limiting-and-dcm-flyback-design.html?shWelcome back xie haibingLogoutSearchSearchHome About us Design Center Books Workshops Analyzer Software Consulting Contact Shopping CartDesign Center LibraryMore ArticlesRegisterEdit My DetailsDESIGN CENTERCurrent Limiting and DCM FlybackDesignDCM Design Constraint at Low LineChoosing a ControllerOverload TestingSummary and Additional ReadingAll PagesCurrent Limiting and DCM Flyback DesignPower Supply Design Tips For very low power design, around 1 W or less, the DCM flyback converter is avery popular topology. When pushing up to higher power, however, thisconverter can create difficulties if you do not properly take care of the controlparametersespecially current limiting.Current Limiting and DCM Flyback DesignThe discontinuous-mode (DCM) flyback converter is viewed by many designers as being the simplest converter of all. For lowpower design of several watts, it is often the most prudent choice. The DCM flyback converter works by charging up theprimary inductance of a transformer when the power switch is on, and fully discharging the current in the secondary of thetransformer when the switch is off. The inductor value, a crucial circuit element, is accurately controlled by gapping the core ofthe transformer.The DCM flyback has the following features that lead designers to its selection:Natural commutation of the output diode, minimizing switching loss and secondary noise;Low-noise turn-on of the primary switch;Simple control characteristics; andPossibility of a quasi-resonant mode of operation to lower noise even further.The disadvantage of the DCM operation is that the currents in the circuit are higher than for CCM operation.Figure 1: DCM Flyback converter. In the field of power electronics, there is always a tendency towards higher power. System designers see the features theycould add to new products, and inevitably increase the power requirements. Similarly, in applications like appliances, therole of power electronics is on the increase, and older mechanical parts are being replaced by electronics parts that needmore power.The same designers who worked on the low power applications are pushing to 5 W, 10W, and beyond. They are taking withthem their design experience at low power, and are encouraged by silicon chip makers who are pushing the capabilities oftheir integrated switchers to higher and higher power levels.This is getting a lot of designers into trouble. Design practices that they could get away with at 1 W are not so forgiving ofrigorous testing at higher power levels.DCM Design Constraint at Low LineAlong with increasing power requirements, there are very wide input ranges available for design. It is common for a powersupply to need to operate from 85 to 265 VAC continuously, and even wider than that for high-reliability power supplies. Withinput ripple considerations, and surge requirements, the input line can range from about 85 to 425 VDC. DCM operationmust be maintained under all conditions according to many design engineers, even at low line condition.Home >>> Design Center Library >>> Current Limiting and DCM Flyback Design12-6-15 Ridley Engineering - Current Limiting and DCM Flyback Design2/3 www.ridleyengineering.com/index.php/dc1/56-13-current-limiting-and-dcm-flyback-design.html?shFigure 2: Power switch drain current at low line, full load. Lets suppose we have a 60 kHz converter, with a 10 W output. Figure 2 shows the current in the power switch at low line.Typically, when using controllers with integrated power switches, most designers will restrict operation to 50% duty cycle inorder to limit the peak voltage on the power switch.For this converter, the choice of inductor to make the converter operate in DCM is 1.1 mH. The peak current, shown in Figure2, is 0.61 A.Choosing a ControllerNow lets suppose you want to choose an integrated converter controller for this application. ST Microelectronics [1] providesa popular series of parts with their VIper chips.Possible choices from ST Microelectronics are:VIper 22This chip has a current limit of 0.7 A, and has a recommended power of 12 W for wide range input. However, this current limithas a tolerance of +/-20%, so the converter cannot work with this. It also has an on-resistance of 31 ohms at hightemperature, which is too high for low-line operation.The VIper 32has a current limit of 1.45 A, again with a 20% tolerance. This seems to be the most logical choice with an on-resistance of11 ohms at high temperature. For the operating conditions of figure 2, this causes a conduction loss of 0.62 W.The VIper 53 isdesigned for considerably higher power with a current limit of 2 A.Overload TestingThere are many conditions which must be tested on your power supply design. High-line, low-line, light load and full load arejust the beginning. Another test which is recommended is short-circuit testing, as described in [2].The short circuit test is severe, and thoroughly tests the capability of the controller to shut down quickly without the currentrunning out of control.Another test that must be done is overload testing. The load on each output of the power supply should be increasedgradually until the converter starts entering current limit. With the DCM flyback converter, overload testing can produce somesurprises.For the converter described so far, the normal peak current is set at 0.61 A. The DCM flyback operates by sending pulses ofenergy from the primary to the secondary and the resulting total energy transfer is given by LIp2f.For the control chip chosen, the worst-case peak current is 1.8 A. This is three times the normal peak current, which leads tonine times the power capability for the converter if it stays in DCM all the time! Figure 3: Power switch drain current with seven times the rated load at nominal operating frequency. Figure 3 shows the overload current in the power switch with 7 times the rated load. A power supply which was designed for10 W operation must now be able to operate around 100 W (taking into account variations in frequency and inductortolerance) without any safety issues. Clearly, this presents a significant challenge to the designer in making a safe converter.Magnetics must be grossly over-designed for safe thermal parameters and non-saturation of the transformer.Notice that the waveform of Figure 3 shows that converter is now in continuous conduction mode, shown by the initial step inthe current when the switch turns on. This causes current limit to activate at just 7 times rated current instead of 9 times. Ingeneral, operating in CCM reduces sensitivity to inaccurate current limiting. However, it does result in a different transformerdesign.The large overhead in current limiting also produces considerable stress on the converter during start up and other transient12-6-15 Ridley Engineering - Current Limiting and DCM Flyback Design3/3 www.ridleyengineering.com/index.php/dc1/56-13-current-limiting-and-dcm-flyback-design.html?shconditions. The tighter the current limit can be kept, the more reliable the converter will be.SummaryAs power levels rise above 2-3 W, the choice of the DCM flyback converter for the power topology must be given carefulthought. If it is to be used with integrated controllers, the most difficult aspect of the design procedure is incorporating propercurrent limiting. If you are running into problems with high power testing, design options include:Select a topology that can run in CCM and DCMUse a controller properly matched with the right current limit for your designChange to a discrete controller and FET for higher power applications so you can set precise current limits by choosing yourown current sense resistor.Do not overlook the fact that you should be doing overcurrent testing, in addition to short-circuit testing. Short-circuit testing isactually quite benign in terms of thermal stress placed upon your design.Power supply designers that work in the kW power range often add the additional expense of very precise current limiting,within 1 or 2 percent of the rated load. Lessons should be taken from the high power designs and applied to the low powerfield.Additional Reading[1] ST Microelectronics website, www.stmicro.com[2] Power Supply Stress Testing