PWM drive of DC contactor

Rs is probably a small value, so it makes little difference.

I think this was the classical case of an unintentional Hartley oscillator, with the tank circuit composed of the load + gate drive inductances in parallel with Cgd.

Jeroen Belleman

Den torsdag den 24. oktober 2013 09.21.18 UTC+2 skrev P E Schoen:

Several manufacturers of protected mosfet in that voltage range, they are used in cars so I don't think obsoletion will be a big issue

the protection is mostly for temperature and current, many standard fets are avalanche rated for switching inductive loads

with pwm the voltage should be clamped to the supply via a flyback diode

-Lasse

On a sunny day (Thu, 24 Oct 2013 11:22:24 +0200) it happened Jeroen Belleman wrote in :

Yes, if you want to limit to say 10A, for .7 V for the NPN to start turning on, you get .07 Ohm for Rs.

When it oscillates with a bit of [bad]luck the drain voltage swings to equal or more than 2x Vsupply...

This sort of current limit circuit is quite common in audio amp output stages... short circuit protection, in my case the 1/10 of normal resistance of a cold bulb needed current limiting.

On a sunny day (Thu, 24 Oct 2013 03:50:58 -0500) it happened John Fields wrote in :

Sort of reduce gate voltage for high Id peaks. Normally that would be short (few mS) and should have no effect on the average dissipation.

Can I take a hacksaw and soldering iron to the shunts ('free' feedback winding)? ;-)

Tim

Strictly speaking, you are right, but it doesn't make much difference here. It's purpose is to swamp the drain load inductance, so that the thing won't burst into oscillation.

Jeroen Belleman

diode

I looked closely at the PWM signal at 10% and there is ringing at turn-on with a maximum swing of -13V and a TC of about 0.6 uSec. The relay coil is

The FQD13N06L I was using is avalanche tested and rated, and I have a flyback diode across the coil, although it is a 1N4004 and it is located

with the capacitor some distance from the coil (about 12"). I will have to deal with that because that is how the application is wired, and this must be a simple PCB replacement without wiring changes.

I think there will be a lot more energy and higher voltage spikes and oscillations when the contactor first pulls in with 6 amps, and when the

current is first switched off prior to applying the PWM signal. The energy stored is 144 mJ which can produce 144W for 1 mSec. The single pulse avalanche energy rating of the MOSFET is 90 mJ so perhaps that is being exceeded, although much of it should be dissipated in the diode. But perhaps it is not fast enough?

The protected MOSFET does have an overvoltage clamp which drives the gate when the drain voltage exceeds the limit. Perhaps that would be an effective way to protect the standard MOSFET. But the TVS diode should do the job as well.

Thanks,

Paul

On a sunny day (Thu, 24 Oct 2013 23:14:15 +0200) it happened Jeroen wrote in :

Even more so when Rs is a wire wound, few turns resister wire, perfect inductor...

On a sunny day (Fri, 25 Oct 2013 05:54:09 -0500) it happened John Fields wrote in :

I bet you I can make a good 100 MHz oscillor with a coil made of resistor wire.

That 'Q' thing is IMNSHO very often mis-interpreted or mis-understood.

Okay. Make it with an inductor having a Q of .0001.

Den torsdag den 24. oktober 2013 23.26.12 UTC+2 skrev P E Schoen:

only without a diode will the energy will be dumped in the FET

There shouldn't be much dissipated in FET with a fly back diode, the current will be circulating through the diode

I might be wrong but I think that the active clamp it to make the clamping voltage more predictable, not that the FET can't handle the same energy when just avalanching

-Lasse

On a sunny day (Fri, 25 Oct 2013 11:51:55 -0500) it happened John Fields wrote in :

The circuit diagram rules.

Then do not say it!

hey, I thought you only picked on J.L. ;-)

I dunno -- if Joerg is the judge, you could ask him!

Perhaps you'd get extra credit for making it work on an AC relay, even with the shunt.