When selecting switching components for a flyback converter, it’s important to consider voltage and current stress. Let’s take a look at how these two parameters can affect the design.

Figure 1 shows a simplified flyback converter, built using an input capacitor CIN, a power transformer T1, and an output rectifying diode D1. The circuit in the blue box is a simplified equivalent circuit for the output capacitor, which employs ESR. RL is the output load resistor, M1 is the primary side switching FET, and RCS is the current sense resistor.

AEP3094 Fairchild Intro to Flyback Converter Tools Part 5 Figure 1

Figure 1. Simplified flyback converter

Figure 2 gives the voltage and current waveforms for the flyback converter while it’s running in continuous conduction mode (CCM).

 AEP3094 Fairchild Intro to Flyback Converter Tools Part 5 Figure 2

Figure 2. Voltage and current waveforms (CCM)

The voltage stress on M1 is VIN+VO x(Np/Ns). The peak reverse voltage on the secondary side rectifying diode can be as high as VO+VIN x(Ns/Np). This means we have to consider the turns ratio of the transformer before selecting the switching components.

In a real-world application, the transformer T1 of the flyback converter has a leakage inductance and a magnetizing inductance. The energy stored by T1 leakage inductance resonates with the parasitic capacitance of the primary-side switching MOSFET M1. The generated peak resonant voltage across the drain and the source of M1 can be much bigger than the derived

Similarly, the leakage inductance on the secondary side of the transformer resonates with the parasitic capacitance of the secondary-side rectifying diode, and this creates a sizable resonant voltage spike on D1.

 AEP3094 Fairchild Intro to Flyback Converter Tools Part 5 Figure 3

Figure 3.  Behavior during steady operation

Figure 3 shows what happens during steady operation. The blue waveform is the VDS of the primary-side M1. When the secondary-side rectifying diode begins to conduct, the output VO reflects back to the primary side of the transformer. The reflected voltage is VOx(Np/Ns). The VINMAX is the maximum input voltage. VOSC is the resonant voltage generated by the leakage inductance plus the output capacitance of M1. BVDSS is the maximum rating for the VDS of M1. To ensure M1 is safe, whether the converter is in steady state or under dynamic load conditions, BVDSS must be higher than the sum of the three voltages with a margin of 15 to 20 percent.

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