Buffered Flyback Converter

Previously, we looked at how to buffer the voltage across the output rectifier of a forward converter during its on-time. Now, let’s look at how to buffer the FET turn-off voltage of a flyback converter.

Figure 1 shows the flyback converter power stage and primary MOSFET voltage waveforms. The converter stores energy in a transformer primary winding inductance and releases it to the secondary winding when the MOSFET is turned off. Because the transformer drain inductance causes the drain voltage to rise above the reflected output voltage (Vreset), a snubber is usually required when the MOSFET is turned off.

The energy stored in the drain inductance can cause the MOSFET to avalanche, so a clamping circuit consisting of D1, R24 and C6 is added. The clamping voltage of this circuit depends on the amount of leakage energy and the power dissipation of the resistor. Smaller resistors can reduce the clamping voltage but increase power loss.

Figure 1: Overvoltage across the drain inductance when the FET is turned off

Figure 2 shows the transformer primary and secondary current waveforms. On the left is a simplified power stage when the MOSFET is turned on. The input current ramps up through the series combination of the drain inductance and the mutual inductance. On the right is a simplified circuit during turn-off. Here, the voltage has reversed to the point where the output diode and clamping diode are forward biased. We show the output capacitor and diode reflected to the primary side of the transformer.

The two inductors are in series and initially carry the same current when Q1 turns off. This means that there is no current in the output diode D2 immediately after turn-off, while the total transformer current flows in D1. The voltage across the drain inductor is the difference between the clamping voltage and the reset voltage, and tends to release the drain current quickly. As shown in the figure, a simple calculation shows how much energy is shunted to the snubber. Therefore, you can reduce the shunted energy by reducing the time it takes to release the energy in the drain inductor. This can be achieved by increasing the clamping voltage.

Figure 2: Leakage inductance steals output energy

Interestingly, you can calculate a tradeoff between the clamp voltage and the snubber power dissipation. As shown in Figure 2, the power into the clamp is equal to the average clamp diode current multiplied by the clamp voltage (assuming a constant clamp voltage). After rearranging some terms, we get 1/2* F *L * I2, which is related to the discontinuous flyback converter output power. In this case, the inductor is the drain inductance. This expression is a little surprising because the power loss here is not just the energy stored in the drain. It is always large, but depends on the clamp voltage.

Figure 3 shows this relationship. The graph plots the normalized losses of the drain inductor energy loss versus the ratio of the clamp voltage to the reset voltage. At high values ​​of the clamp voltage, the snubber losses approach the energy in the drain inductor. As the reduced resistance reduces the clamp voltage, energy is diverted from the main output and the snubber losses increase dramatically. At a 1.5 Vclamp/Vreset ratio, they are almost three times the losses associated with the drain inductor stored energy.

Figure 3: Increasing the clamping voltage reduces snubber losses

As it happens, the leakage inductance is typically about 1% of the magnetizing inductance. This makes Figure 3 more interesting, as it shows us the effect of reducing the clamping voltage on efficiency, so we just need to change the vertical axis to efficiency losses. So, reducing the clamping ratio from 2 to 1.5 will have a 1% effect on efficiency.

In summary, the drain inductance of a flyback converter can produce unacceptable voltage stress on the power switch. An RCD snubber can control this stress. However, there is a trade-off between clamping voltage and circuit losses.