Power Supply Design

Question: How does diode recovery time affect power losses?

Original Question: In a boost PFC (Power Factor Correction) application it is said that the boost diode recovery characteristics can contribute some loss in the switching MOSFET. I do not really understand this. Please explain it to me.

Answer: Since the boost diode conducts in the reverse direction until it recovers, the turn-on of the power MOSFET is controlled by the diode characteristics, not the power MOSFET or its drive. In fact, best performance is usually had by matching the turn-on of the MOSFET to the diode by slowing down the MOSFET to what the diode reverse recovery is. If you switch the MOSFET into a short, you lose power. For more information see the paper: A Design and Application Guide for High Speed Power MOSFET Gate Drives by Laszlo Balogh described in a previous blog. This paper is available at no cost as a pdf download on the TI website.

Question: Also brief me about the major parameters to be considered while choosing the boost diode.

Answer: Until proven I need something different, I usually start with a fast diode with soft recovery (not snap-back recovery). Since the diode is critical to maximum switching speed, you want it fast. Since it is a major source of EMI, you want a soft recovery to minimize this.