What type of diode is best suited for the RCD Clamp network in Flyback converters? How does the recovery speed of the diode factor in the overall efficiency of the converter?
In an RCD clamp Flyback, as soon as the MOSFET turns off, the energy in the primary winding first charges the MOSFET’s drain-to-source capacitance. When this voltage rises above the input DC voltage + the reflected voltage, the diode in the RCD clamp starts to conduct. The leakage inductance of the transformer discharges into the C of the RCD clamp and the voltage across the cap rises in a resonant fashion. As the voltage reaches its peak, the current in the diode reaches zero and changes polarity. If a standard recovery diode is used, the reverse recovery time of the diode is between 0.5 us to 1.5 us. During this reverse recovery time, reverse/negative current flows in the diode. This transfers some of the energy stored in the C to the transformer and then ultimately to the load. This improves the efficiency and also allows for a higher resistance value of the R in the RCD clamp. Care must be taken to account for the negative current spike in the diode during this resonant transfer of charge and energy from the C to the transformer.