Power Supply Design: Buffered Forward Converter

Figure 1 shows the power stage of a forward converter. The converter is operated by a transformer that couples the input voltage to a secondary circuit that rectifies and filters the input voltage. A snubber is usually required when D2 is forced to commutate off through a low impedance circuit formed by the reflected primary voltage and the transformer leakage inductance. D2 can be a silicon pn diode that has a reverse recovery charge that must be depleted before it turns off. This loads up excess current in the leakage inductance, resulting in high frequency ringing and excessive diode voltage. A similar situation exists with Schottky diodes, the former because of their large junction capacitance, and the latter because of their turn-off delay time.

Figure 1: Leakage inductance delays the turn-off of D2.

Figure 2 shows some circuit waveforms, with the top trace being the Q1 drain voltage, the middle trace being the voltage at the junction of D1 and D2, and the bottom trace being the current through D1. In the top trace, you can see that when Q1 turns on, its drain voltage is pulled below the input voltage, which causes the diode D1 current to increase. If D2 did not have a reverse recovery charge, the junction voltage would rise when the D1 current equaled the output current. Since D2 has a reverse recovery charge, the D1 current increases further, which begins to consume the charge. Once the charge is depleted, the diode turns off, causing the increased junction voltage to increase further. Note that the current continues to increase until the junction voltage equals the reflected input voltage because there is a positive voltage across the leakage inductance. As the current increases, it charges the parasitic capacitance and causes more ringing and losses in the circuit.

Figure 2: D2 causes excessive ringing when it is off.

These ringing waveforms may be unacceptable because they may cause EMI problems or unacceptable voltage stress on the diode. An RC snubber across D2 can significantly reduce the ringing with little loss in efficiency. You can calculate the ringing frequency using the following equation (see Equation 1):

Equation 1:

But how do you know the values ​​of L and C in the circuit? The trick is to lower the ringing frequency by adding a known value of capacitor across D2, which gives you two equations and two unknowns. Solving for these values ​​is much easier if you add just enough capacitance to cut the ringing frequency in half. To cut the frequency in half, you need a total capacitance that is four times the parasitic capacitance you started with. Then, just divide the added capacitance by three to find the parasitic capacitance. Figure 3 shows the waveform for 470 pF of capacitance across D2 at half the original ringing frequency. So the circuit has about 150 pF of parasitic capacitance. Note that just adding capacitance does very little to the amplitude of the ringing; the circuit also needs some resistance to damp the ringing. This is another reason why a capacitance factor of 3 is a good place to start. If the resistor is chosen appropriately, it will provide excellent damping with minimal impact on efficiency. The best value for the damping resistor is almost the typical resistance of the parasitic element (see Equation 2).

Equation 2:

Figure 3: Parasitic calculations are performed by increasing the ringing frequency by a factor of two.

Using Equation 1 with a 35 MHz ringing frequency and a parasitic capacitance of 150 pF gives a leakage inductance of 150 nH. Substituting 150 nH into Equation 2 gives a snubber resistor value of approximately 30 Ohms. Figure 4 shows the effect of adding the snubber resistor. The ringing is completely eliminated and the voltage stress is reduced from 60V to 40V. This allows us to select a lower voltage rated diode, thus achieving an improvement in efficiency. The last step in the process is to calculate the snubber resistor losses. This is accomplished using Equation 3, where f is the operating frequency:

Equation 3:

Once you have done the calculations, you need to determine if the circuit can tolerate the losses in the snubber. If not, you need to make a trade-off between ringing and snubber losses. See Figure 3 on page 3 for more details on how to choose the best damping resistor.

Figure 4: Proper selection of snubber resistors can completely eliminate ringing.

In summary, buffering a forward converter is a simple process: 1) add capacitance to halve the ringing frequency; 2) calculate parasitic capacitance and inductance; 3) calculate the damping resistor and inductor; 4) determine if the circuit losses are within acceptable limits.