MOSFET gate circuit for PWM drive

Oct 05, 2008 4 Replies

Can anyone please explain (or suggest online reading that explain) what factors have to be taken into consideration when driving a power MOSFET with a PWM signal for power control such as motor speed or power supplies, especially at the gate?



I understand that a MOSFET gate is essentially an open circuit at low frequencies, but at higher frequencies, capacitive effects modify the necessary drive power and switching times (and possibly other things). Simple hobby-type circuits often drive the gate with resistive coupling from an opamp or something with a similar output, but more sophisticated circuits usually include capacitors, diodes and inductors between driver and MOSFET. Please help me understand the principles.


Read through these.

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AN-944: Use Gate Charge to Design the Gate Drive Circuit for Power MOSFETs and IGBTs PDF

AN-947: Understanding HEXFET Switching Performance

Go through there archives tons of info!

Thanks. I don't have time to go through everything right now, but it appears to be a very good source. I've bokmarked it. Thanks again.

The gate has capacitance. You must charge the capacitance. The faster you charge it the more power you are using. The mofset also has it's own internal resistance that dissipates power(R_DS(on)) along with power wasted from cross conduction in an hbridge.

There are some good docs about this out there such as AN786 from Microchip which gives equations for calculating the total power dissipation.

Thanks. I've downloaded AN786 and it also appears to be very helpful. I'll study it in detail.

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