Power Tip: Snubbing the forward converter

Do you struggle with picking snubber components? Figuring out how much capacitance and resistance to add can seem challenging. Here's a quick way to get through the problem.

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.

1. Leakage inductance slows D2's turn off.

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.

2. D2 causes excess ringing as it turns off.

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.