mosfet failure reasons?

What can cause a nchannel mosfet(irf510) to fail? I've gone thru 2, the first one acted as a short but always did from the start the second one worked for a little while then went dead(not nearly as loud and strange waveform even driven direct dc) sometime while I was moving the speaker it was driving I have a 555 timer driving it directly at 5khz I have it in a push pull configuration with a pchannel mosfet, tho that's not hooked up to v - yet

Usually, too much current or too-much power-dissipation.

I have it in a push pull configuration with a pchannel mosfet, tho that's not hooked up to v - yet

What is connected? Do you have a schematic, or a link to your schamatic? Something might be wired wrong...

You can more power dissipation if you use it in an "analog" mode (or accicently) to turn it partially-on so that it has current through-it, and voltage across it, at the same time. When used as a switch, which you should be doing with a (digital) 555, it's either off (with voltage across it but almost no current) or on (with current through-it, but with most of the voltage across the load), it shouldn't get hot (or dissipate power)

Sounds like you have shoot-through show your circuit. But its hard to say not seeing how you have the speaker hooked up. But you should be using a cap to feed the speaker.

Yeah they got quite warm, which confused me as well I thought I was spending too much time in linear mode but my oscope confirmed it wasn't I can't draw a diaragram on my phone but I had the gate directly to the 555 timer output(~4500hz 0-12v supply), the source to my load and the drain to +12v its the way basically every push pull mosfet diagram I've seen, which isn't how I've seen it as a switch using the drain side but I guess you can't do that in pushpull type method

For an n-channel MOSFET you must have the load between positive supply and the drain, connect the source to ground. You have a source-follower which is ALWAYS in linear mode - it won't be able to pull the output much higher than +9V either.

You are confusing a MOSFET class AB push-pull linear amplifier with a MOSFET switch circuit. The linear amplifier configuration generates a lot of heat and heatsinks are mandatory because you are in the linear region - this is how you might design an audio amplifier, not a switching application.

For switching first get a decent switching MOSFET (Rds(on) should be low, something more like 0.01 ohm, so a heatsink isn't needed - but calculate this according to your load). Always use common-source configuration, not source-follower. If you have to switch on the high-side of the load you need a p-channel device.

Ah ok thatbmakes sense, That would cause the mosfet to fail u think? It was warm when I touched perhaps it had already burnt out?

Could I connect my load to both drains, then have the sources go to v+ and v and it will work without being in linear mode and burning up?

No.

How would I connect it then to get either v+ or v - full on I guess like hald an hbridge?

I guess like hald an hbridge?

If that means half a h-bridge then yes.

Yeah typo, srry so for that how would I configure my fets?

Configure? You don't configure components. I assume you mean wire them up? What sort of FETs, what sort of voltage do you need to switch, what sort of drive do you have?

Generally you need a p-channel and n-channel, along with a few resistors to ensure that the two are not on at the same time. Do a google image search for some schematics.

How's this link work for a shematic, seem correct to you? http://www.cadvision.com/blanchas/hexfet/npch-sw-sch.htm im gonna use an irf9520 I think i and an irf 540 My plan to drive it is from a 555 timer, im gonna set up a dual supply opamp to take the 555 output from 0/12v to -12/+12,

If what you're trying to do is drive a speaker or similar load with a 5KHz square wave, you might be better off using bipolar transistors instead of mosfets. Use a cmos 555 (which drives the output right to the supply rails, unlike the bipolar version), and replace your N-channel source follower with an NPN small signal transistor driving a PNP power transistor (see http://en.wikipedia.org/wiki/Sziklai_pair but also add a resistor between the base and emitter of the PNP transistor). Then add an opposite pair (PNP small signal transistor driving NPN power transistor) to pull the output down to ground. This isn't quite as efficient as using a mosfet half h-bridge (because you will have maybe 1.5v drop across whichever power transistor is turned on), but it avoids the problems of shoot-through that you tend to get with home-made h-bridges.

If the power required in the load is very large, then the mosfet h-bridge is the most efficient solution; but in this case I recommend you buy a mosfet h-bridge driver chip. You won't need any p-channel mosfets because these driver chips are always designed so that both the upper and lower switches in each half h-bridge are n-channel.

I don't like that because it is possible to turn both FETs on at the same time. You should look for a circuit that can't do that even by accident.

Yeah looking at that id rework it so it is both driven from the same wire, so +12 activats the nchannel, -12v will activate the pchannel Anyone know any good examples? And one of the things I plan on doing with this circuit is playing around with homeade higher frequency transformers, so ill probably want like 1-2 amps available

So basically this is how id wire it, should I add 10k resistors to ground for the gates? http://www.electronics-tutorials.ws/transistor/tran_7.html

winner10920: How's this link work for a shematic, seem correct to you? http://www.cadvision.com/blanchas/hexfet/npch-sw-sch.htm im gonna use an irf9520 I think i and an irf 540 My plan to drive it is from a 555 timer, im gonna set up a dual supply opamp to take the 555 output from 0/12v to -12/+12,

That schematic is for slow-speed switching rather than PWM - you need to reduce those 10k resistors between gate/source so the MOSFETs turn off fast - with 10k they are likely to spend 20 to 200us in turn-off, all the time in the linear region dissipating lots of power. For a 5kHz signal this clearly isn't sensible. Remember power MOSFET input capacitances can be around 10nF or so. To switch off in 1us you might need the gate-source resistors as low as 100 ohms (which would mean using 2W resistors!)

Proper MOSFET driver chips use active drive to the gate rather than one-sided in order to get good switching speed both on and off.

wouldn't the switch from +12 to -12 actively drive it? just realized tho if I do it the way in that last picture it'll put too much voltage on the gate since the gate will be the opposite rail which is atleast 24v difference, being over the 20 vgs spec, or is my thinking missing something somewhere

you might need the gate-source resistors as low as 100 ohms (which would mean using 2W resistors!)

A 2W resistor to switch a 10nF capacitor, that sounds a bit excessive. It is not a continuous current rating we are working with.

just realized tho if I do it the way in that last picture it'll put too much voltage on the gate

Yes it will, your thinking is correct.