Driving a beanpole OP stage with squarewaves

[Bruce Varley]

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

 

[Phil Allison]

"Bruce Varley"

Hi, I'm planning to use a couple of complementary power fets in a beanpole
config for a power output stage.

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

Heap powerful magic ........

Jack and the beanstalk stuff is bit outre nowadays.

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.

** Way to go.

Might phase shifts that result in both devices being on during the
transitions cause problems?

** 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

 

[Clifford Heath]

Bruce Varley wrote:

Might phase shifts that result in both devices being on during the
transitions cause problems?

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.