Slowing down MOSFET turn ON/OFF

[jophuds]

Hi,

I have a problem whereby I have a set of circuits for use with Helmholtz Coils that were initially designed to allow for rapid switching ON and OFF of the coils. The circuits are a High Side Power MOSFET and a H-Bridge configuration using Power MOSFETs. All the power MOSFETs are currently being turned ON by a logic signal from a microcontroller. They are all logic MOSFETs.

The problem I'm facing is that we wish to alter the setup so that the turn ON and turn OFF switching times of the circuits are much much longer than they currently are. The reason for this is because of an experimental setup involving a sensor that will become unstable if magnetic fields change at too high a slew rate.

I would like to avoid having to redesign the setup, and I'm wondering if I can just place inductors in series with the gates of the MOSFETs to slow down the charge being delivered to them and therefore slow down their turn ON and OFF times?

If this introduces more problems then it is worth is there any other way I can retain the same circuitry with minimal changes?

Any assistance would be greatly appreciated,
Regards,

Joe

 

[VEBill]

You wouldn't need inductors. Maybe series R and probably some parallel C to stablize the time constant (so as to not depend only on uncontrolled gate C).

Heat dissipation will skyrocket as you spend more time in the region between OFF and ON.

Standby for others...

 

[Skogsgurra]

Not much to add to VE1BLL's answer.

How rapid is "rapid"? If several thousand times/second, you may get a lot of heat in the transistors. If around ten times/second or slower, the extra dissipation may not be too bad.

What currents are you handling? And what voltages?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[IRstuff]

Isn't your controller able to change the effective field through altering the power control to the drivers? You would then program the actual field value to ramp up to the desired final value.

TTFN

FAQ731-376

 

[jophuds]

Thanks for the replies.

To answer Skogsgurra's question:

The voltages are up to 12V and the Current is up to 5A. Heat dissapation shouldn't be a problem as they intended usage would be for the MOSFET to turn ON over maybe 10-15ms, stay on for 5 seconds and then turn OFF over 10-15ms. As I said before the issue is that the current can't be supplied or removed from the helmholtz coils at a rate that would cause the generation of fields above the slew rate of the sensor.

To answer IRStuff's question:

I misrepresented what the setup actually is when I said microcontroller. It isn't a H-Bridge Driver or anything like that rather a pretty cheap Digital I/O Board that is providing logic pulses to the MOSFETs. The power to the MOSFETS is supplied from a Bench-Top Power Supply.

 

[IRstuff]

Sure, but the concept still applies. It would be easier, and more repeatable, to find a power supply that can do the ramping under software control.

TTFN

FAQ731-376

 

[benta]

I know the OP didn't want to redesign, but to me this sounds like a job for a function generator and an audio power amp.
Something like this one:

http://www.national.com/mpf/LM/LM3886.html

Then you have complete control over the waveshpes sent to the coil.

Benta.