Figure 1 shows the forward converter power stage. This converter operates by transformer-coupling the input voltage into the secondary circuit where it's rectified and filtered. A snubber is often needed when D2 is forced to commutate off through a low impedance circuit formed by the reflected primary voltage and the transformer's leakage inductance. D2 may be a silicon p-n diode with a reverse recovery charge that must be depleted before it turns off. This loads up excess current in the leakage inductance, which results in high-frequency ringing and excess diode voltage. A similar situation exists for Schottky diodes due to their large junction capacitance, and even for synchronous rectifiers due to their turn-off delay times.

Figure 2 shows some of the circuit waveforms, the top trace is the drain voltage of Q1, the middle is the voltage at the junction of D1 and D2, and the bottom is the current through D1. In the top trace, you can see as Q1 turns on, its drain voltage is reduced below the input voltage, which forces the diode D1 current to increase. If D2 has no reverse recovery charge, the junction voltage rises when the D1 current equals the output current. Since it has a reverse recovery charge, the D1 current increases further, which begins to deplete the charge. Once the charge is depleted, the diode turns off, causing the increased junction voltage to increase. Note that the current continues to increase until the junction voltage equals the reflected input voltage because there's a positive voltage across the leakage inductance. While it is increasing, this current is charging parasitic capacitances and leads to further ringing and losses in the circuit.

These ringing waveforms may be deemed unacceptable as they may cause an EMI issue, or they may put unacceptable voltage stresses on the diode. An RC snubber across D2 can reduce the ringing substantially with little impact on efficiency. You can find the ring frequency with the following expression:

But how do you know what the values of L and C are in your circuit? The trick is to lower the ring frequency by adding a known capacitance across D2. You then have two equations and two unknowns. You can make it even easier on yourself if you add just enough capacitance to halve the ring frequency. For half the frequency, you need a total capacitance that's four times the parasitic capacitance that you started with. Then, simply divide the added capacitance by three to get the parasitic capacitance. Figure 3 shows the waveforms again with 470 pF across D2, and with half of the original ring frequency. Therefore, the circuit has about 150 pF of parasitic capacitance. Note that just adding capacitance does little for the amplitude of the ring. The circuit requires some resistance to dampen the ringing. This is another reason that the factor of three on the capacitor is a good place to start. With the proper resistor choice, it will provide good damping with minimal impact on efficiency. The optimal value for the damping resistor is nearly equal to the characteristic impedance of the parasitic elements:

Using Equation 1 with a ring frequency of 35 MHz and a parasitic capacitance of 150 pF, the leakage inductance is calculated to be 150 nH. Substituting 150 nH into Equation 2 yields a snubber resistor of around 30 Ω. Figure 4 shows the impact of adding the snubber resistor. The ringing is virtually eliminated and the voltage stress is reduced from 60 to 40 V. This may allow a lower voltage rating diode to be selected, resulting in an efficiency improvement. The last step to this process is to calculate the losses in the snubber resistor. This is done with the following equation, where f is the operating frequency:

Once calculated, you'll need to decide if the circuit can afford the losses in the snubber. If not, you'll need to compromise between ringing and snubber losses. To learn how to choose the optimum damping resistor, refer to Figure 3 in an earlier Power Tip.

To summarize, snubbing the forward converter is a simple process: 1) add capacitance to halve the ring frequency; 2) calculate the parasitic capacitance and inductance; 3) calculate damping resistor; and 4) determine if the circuit losses are acceptable.

References
Middlebrook, R. D., and Slobodan Cuk, "Advances in Switched-Mode Power Conversion," Volumes I and II, 2nd Edition, TESLAco, 1983. P. 533. Robert Kollman, Power Tip #4: Damping an Input Filter " Part 2 of 2, Power Management DesignLine, September 30, 2009.

Previously published Power Tips Articles by Robert Kollman are available on-line.

About the author
Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.