High voltage mosfet driving

I'm a bit confused about how to drive a mosfet as a switch when dealing with high V_DS voltages. I have several nch mosfets with 100V+ up to about 1000V for max source to drain voltages.

I want to create a few buck converter circuits and others to test the quality of the output but I cannot for the life of me figure out how to drive the gates without requiring other high voltage components and without the circuit becoming overly complicated.

If you take a simple voltage diving circuit with the nch mosfet replacing the high-side resistor and try to drive the gate then the source terminal is "floating" in the sense that it depends on the load. If I tie the gate to the source through a resistor then this solves the problem by turning off the mosfet. Unfortunately it then causes major problems when trying to drive it. Not only that the gate then has to discharge through that resistor plus the low-side resistor, which here is the load, which reduces the fall time significantly for low loads.

One can't simply use another nch HV switch to connect the first mosfet's gate to ground because it brings the source of the first mosfet way below spec.

The issue is driving the gate so that it is only within +-20V of the source which can be up to several hundred voltages above ground almost up to Vcc.

Is there something I'm missing here? I imagine using a p-ch mosfet would be much easier yet I can't find ones with spec for my application yet N-ch's are abundant.

Reply to
James Rollins
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You need the actual driver to fly up and down with the source of the MOSFET. It needs its own little power supply.

You could also use a gate driving transformer.

Smaller MOSFET

--- ------- -------- Gate of big MOSFET )!!( s! ! !d )!!( ----- )!!( !

--- ---------+---------- Source of big MOSFET

Reply to

Yes but I'm not sure how to do this without running into similar problems and making it too complex. I believe the problem of the the gate attached to the load is still an issue. The load is 100kohms which will effect the fall times.

Are there transformers created just for this sort of thing? I suppose it would work but I'm a little confused how I would drive the smaller mosfet. I assume the transformer simply floats from the source and one applies the primary side voltage to the gate. So if 10V input and it's a 1:1 transformer then it would lift the gate 10V? The small mosfet is to remove the transformer from the gate? I still have the problem then of bringing the gate low quickly. I'm thinking that some voltage follower might work where it would follow the source voltage and sink the gate charge but this introduces similar problems with the powering of the opamp and keeping it within spec.

Is this correct? Or do I not need to worry about the gate discharge when floating since it is at a higher voltage and hence will be quicker and might act as a virtual ground?

Reply to
James Rollins

You have some options where, use a Gate driver with a self generated inverter powered from the high potential or, do what I did once and that was to use a 555 timer into a toroid xformer where on the other side I used a high speed switching full way rectifier config (2 diodes CT), no caps were needed due to the gate capacitance but you may want to have that your self. I had fets that already had high value cap on the gate. A shunt R is needed of course.

By connecting the (-) output to the source and (+) to the gate, I was able to increase the gate voltage above the source voltage.

something to think about I guess.

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Notice how I hooked it directly to the transformer. Try spicing it up with a squarewave driving the transformer primary through a resistor. You get blips in the up and down direction on the transformer instead of a squarewave. The small MOSFET turns this back into a squarewave.

When you yank the transformer the other way, the small MOSFET is turned on and snaps the ate down.

Try spice and see what happens.

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I once tried that (or rather, a somewhat more complicated approach, but same idea).

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Had erratic behavior though. Nasty business when you're humming along at 20kHz like a good half bridge, then suddenly a transient (I never decided what it was, but I'm guessing related to dV/dt coupled in the input transformer there) ticks it "ON" and gulps three hundred amperes short circuit through your bridge. Oops. So the desat kicks it off, but it takes a cycle before either gate drive's state is determinate.

That's what I don't like about these sorts of "I trust you'll stay put" methods. To mix quotations: trust but verify, while carrying a big stick. ;-)


Reply to
Tim Williams

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