Author Topic: Why is mosfet turning off slowly?  (Read 10621 times)

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Offline TheoB

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Re: Why is mosfet turning off slowly?
« Reply #50 on: December 22, 2016, 07:30:18 pm »
Quote
Can anything be done for these, I mean to prevent those spikes and the negative dips?
Yes, use a mos with smaller Cdg. But ask yourself why you want to limit it? It's harmless. The mos is chosen for a specific load. Often you want a low Rdson as that results in low loss and cheap cooling.
If your load drives some piece of wire in excess of 5cm, you have an inductive load. Then you should take care. The back emv (the current cannot stop immediately) can lead to high voltage peaks on the drain. T his can destroy the gate of the mos. Lowering the switching frequency does not help. Lowering the switch time is a poor man's solution as it increases the switching loss. Just add a diode and perhaps a snubber network (small filter).A scope is a prefect tool to measure this.
And finally don't forget to use good power supply decoupling! The wire feeding your circuit (if you use this) also has self inductance. It cannot deliver the pulsed current to feed your load. Result can be a large ripple on your supply.
 
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Offline tatus1969

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Re: Why is mosfet turning off slowly?
« Reply #51 on: December 22, 2016, 07:38:05 pm »
ok. if your load is (even slightly) inductive, then you may run into problems with ringing. i have posted a tested circuit that switches a wirewound power resistor that is connected through a cable. i deliberately had to slow down switching speed to compensate for excessive ringing from lead and part inductance. pwm freq is 2khz here, it cannot go higher without excessive efficiency loss. go for push-pull config otherwise. heres the link, first picture on the right: https://www.eevblog.com/forum/projects/variable-frequency-drive-design/msg1087549/#msg1087549

Thanks for the tip tatus... are you referring to the totem-pole output that goes to 'U_INT" by any chance?
sort of both. the BRAKE circuit on the right is comparable to your initial circuit but allows reducing switching speed and ringing in a controlled way. the additional gate-drain capacitance and the gate resistor control this. the U_INT circuit is a the totem-pole / push-pull / h-bridge like i meant. but that circuit is probably way overdone here, it is designed for higher voltages and output power in the kW range.
We Are The Watt - Resistance Is Futile!
 
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