EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: rentner on May 15, 2013, 11:30:32 am
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Hello folks. I will keep that very simple. I want to switch a mosfet at 200kHz with PWM as exact, as possible. I would like to use a mosfet with a recovery time of 570ns. Is this acceptable for 200kHz with PWM? (It should not be distorted more than 2% under ideal gate driving conditions.)
PW will reach from 1% to 95%.
Is a recovery time for that of 570ns enought, or what would you recommend?
Thanks.
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Recovery time? MOSFET? Err.. that does not compute...
FETs don't have a recovery time as such, instead they have gate charge and apparent gate capacitance that you need to charge/discharge. The gate drive circuit determines the timing to a large extent, subject to the properties of the FET naturally.
In itself 200 kHz is an entirely reasonable switching requency but you need to pay attention to the specs of the FET you plan to use. Properties like gate charge should be below average already. If your application operates by hard switching there will be inevitable power losses during switching events. The alternative would be some kind of resonant or semi-resonant zero voltage or current switching but that is advanced stuff at these frequencies.
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I built a gate driver, that would charge a Gate with 1uF capacity so fast, that at 100kHz it still is a very clean square wave.
Well, I need to charge or discharge the Gate very fast. But anything else? Is that enough?
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If your gate timing is OK then you are basically done. You sould be aware though, that overdriving the gate heavily is prone to cause oscillations in the 10s to 100s MHz range. These have a nasty tendency radiate as EMI into the environment and cause other kinds of unpleasantness as well. So you might want to watch out for that.
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You probably will need a ferrite bead on the gate or drain lead to suppress oscillations with very fast drive. Will not damp totally but can stop it killing the device instantly.