Output protection

[R.G.]

I don't know if this has been around before, so I thought I'd try it here.

I don't like the idea of protection with relays. Too prone to relay failure. So I comtemplated my navel for a while and decided to use MOSFETs in series with both the power supplies to the amp.

Big digital switching MOSFETs are cheap, and if you have them switched full on or off you're not worried about the effect on audio quality. I set up a pretty standard DC voltage sense and used that to turn the MOSFETs in the power supply lines off when a fault was found.

It worked pretty well on the somewhat modest amp (100W) I tried it on.

[mikeks]

Originally posted by R.G.
I don't like the idea of protection with relays. Too prone to relay failure.

True...

Originally posted by R.G.

So I comtemplated my navel for a while and decided to use MOSFETs in series with both the power supplies to the amp.
.......................

......It worked pretty well on the somewhat modest amp (100W) I tried it on.

Schematic please?

[R.G.]

It's not much of a schematic, and I'd have to go draw it.

Lemme see if I can describe it. For the negative rail:
one or more n-channel MOSFETs, source to the DC supply, drain to the amp. Resistor and gate-protection zener from gate to source; drive resistor to pull-up-to-ground driver. I used a PNP, but obviously any arrangement to switch the drive resistor up to ground would work. The PNP pulls the gate up, to turn on the MOSFET, gate voltage is limited by the zener, and turn off time is determined by the gate capacitance and the gate-source resistor. I used two MOSFETS in parallel, but as many as are needed can be parelleled there to keep DC drop low.

I used P-channels on the top side because I had a tube of IRF9640's on ebay cheap, but a high-side driver chip would let you use n-channels on the positive side. In my amp, the positive side was the mirror image of the negative side.

The MOSFETs are used purely as high speed, high current switches. You put enough current handling there by picking big MOSFETs and paralleling to handle the max current excursion on the power rail and most of the time they act like smallish resistances in series with the power rails. When you detect a fault, you drop the drive to the MOSFETs and they turn off before you smoke your speakers.

The MOSFETs can switch slowly because they only switch once per power cycle, and you only have to heatsink them to get rid of the saturated-on power losses.

By turning off both power rails on a fault, you keep the speakers from being toast. Selected properly, the MOSFETs cannot have their contacts welded...

Of course, anything that you want to detect can turn them off.

[AndrewT]

Hi,
sounds interesting.
A schematic would really gild the lily.
How fast do you turn off? just to save the speakers (hundreds of mS) or to save the output stage (a few mS)?

[R.G.]

The original idea is that the outputs are already dead, at least one of them. If you get a sudden DC offset, that's the presumption anyway, and the reason for protection circuits that open relays.

With that rationale, I just let them have maybe a millisecond to turn off.

I suppose you might want to try something like monitoring other junk to turn the amp off, but I was just after eliminating the output relay.

Hate relays. ptuui...

[Kevinbd]

I was wondering if VI protection was better than protection in a output stage voltage regulator. I think FETs can hack more current for a very short duration than BJTs.

Maybe for BJTs VI protection is better than having regulator protection?

Whereas with FETs DC protection in reg's maybe adequate ?

Maybe it's a response related question(s)



------

Live on the Power Mosfet......

[R.G.]

I was wondering if VI protection was better than protection in a output stage voltage regulator.

I had no idea that the MOSFETs were part of any regulator. They're switches, pure and simple. When they're on, current flows to the outputs. When they're off, no current goes to the outputs. Any regulator would be separate.

It is possible to use the MOSFETs as part of a regulator, I suppose, but that would be a sophisication I didn't envision, and might compromise the regulator.

Maybe for BJTs VI protection is better than having regulator protection?

This is not intended to replace VI protection. The amplifier remains exactly as it was, internal protections, and everything as it was. All I added was a fast, high current switch in both V+ and V- lines. No regulator or protection for the outputs is intended - this is to keep my very expensive speaker drivers from going up in smoke.

[mikeks]

schematic would be good...

[R.G.]

Yes, schematics are good.


I'll draw one when I get a chance.

[cunningham]

If your protection circuit is set off by an 'over current' sensing circuit, you could have only the drivers and biasing current sources be switched off and use a smaller device to do the switching, in the case of a big amp. As long as the output devices don't fail, the result would be the same in that no current would flow. The only problem though with setting a max current is when driving an inductive load, where the phase margin between V & I, might create a condition that will exceed the SOA. Of course in that case... (pooooofffff!)