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Driving a beanpole OP stage with squarewaves

Hi, I'm planning to use a couple of complementary power fets in a beanpole config for a power output stage. Power probably 5 watts or less, frequency around 500 KHz, the thing has to tolerate an unpredictable load which will be part capacitive, part resistive. I'm not too fussed about how efficient the power transfer is, and there'll be a series resistor on the output for protection, so that part's probably OK. Here's the potential issue: for simplicity I'm thinking of driving the two gates with 180 deg out of phase squarewaves (with the appropriate level shifts), so that one device is on, the other off, and the centre point of the beanpole toggles between the rails. Might phase shifts that result in both devices being on during the transitions cause problems? Is there a good way to handle this situation? TIA

Reply to
Bruce Varley
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"Bruce Varley"

** I think you mean " Totem Pole " output stage.

Heap powerful magic ........

Jack and the beanstalk stuff is bit outre nowadays.

** Way to go.
** Load phase angle will have no effect on your square wave drive circuit arrangement - provided there are diodes able to conduct the back emf voltage spikes generated. Luckily, all power mosfets have the needed diodes inbuilt.

Fussy folk may like to parallel these with special high speed ones - ie Schottky types, if the voltage requirements are as low as you indicate.

..... Phil

Reply to
Phil Allison

Normally folk worry a bit about shoot-through, depending on how fast you can drive the gates up and down - total gate charge is the issue here. If you drive the two gates with synchronized waveforms you *will* get some shoot-through, even if your drivers can deliver several amps. The shoot-through will cause dramatic RFI emission, and collapse the PS rails enough to affect other parts of your circuit, not to mention the neighbour's TV.

Google for chips and circuits that provide dead-time and find how it's done - you don't need to do much to provide a few 10s of ns of dead-time and remove the shoot-through. Simple RC circuits and NAND gates are one easy way.

Make sure your gate drivers can deliver enough current to switch fast, and make sure the switching doesn't overlap in nasty ways.

Clifford Heath.

Reply to
Clifford Heath

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