Power Supply Design

Question: How much current is needed to drive a power MOSFET?

Original Question: The email described a full-bridge topology with MOSFET switches and their drive, then commented: The drive is not working because the MOSFET is not fully on and off. In our view the driving current for turning on the MOSFET seems to be OK So can you tell us how much drive current is required to turn on and off the MOSFET or suggest another drive circuit?

Answer: MOSFET drive circuits are complicated and beyond what I can answer in an email. At the 2001 Series TI/Unitrode World Power Supply Seminar, Laszlo Balogh gave a paper, "Design and Application Guide for High-Speed MOSFET Gate Drive Circuits". I highly recommend you get this paper from TI (it is available at no cost as a pdf download on their website). This paper should be in the personal library of every power supply designer. The main thing is that since the gate drive must provide the charge needed by the power MOSFET, you must know its requirements and monitor the current into the gate versus time to determine if the gate charge requirements are being met. This is the only way you can tell what is actually happening in your circuit. In trouble shooting a circuit you must use both voltage and current probes. In my opinion not using current probes and synchronizing voltage and current waveforms is the most common mistake made by power supply designers in troubleshooting circuits.