A question about Mosfets....

Correction -- IRF840

Seems like a pretty sorry circuit IMO...

Yes, I should have taken more time to figure out how it worked (and whether it worked!) before diving in and building the thing... Sometimes one puts too much faith in things found randomly on the Internet.

Well, how does the TL494 turn those fets off?

E1 and E2 don't sink current.

Cheers

That circuit doesn't work. Actually, you shouldn't be getting spikes OR = squares out of it *anywhere*. The emitter outputs (pins 9 and 10) have = no pulldown.

Use a gate driver like this,

Better still, use a half bridge gate driver, e.g., IR2101 series. Nix = the full bridge, it's only good for increased losses. Replace C7 with =

Incidentially, T1 will happily saturate if the voltage is too high for = the frequency and primary turns. Saturation means dead FETs. This is = very easy to calculate from core geometry and material properties (all = flybacks use mix 77 ferrite, as far as I know... Bmax =3D 0.4T). = Further reading:

Tim

--=20 Deep Friar: a very philosophical monk. Website:

Looks POW-ful, to me.

Please review the 494 spec.

...and stop calling this rubbish a full-bridge flyback.

RL

R squares out of it *anywhere*. =A0The emitter outputs (pins 9 and 10) have= no pulldown.

Ok, everything you've said makes sense, I like the looks of this IR2101 driver, and might order a few and give it a shot. I've also been looking at other drivers (TC4420, TC4429, MIC442x equivalents, etc).

However, I'm confused about why the circuit works at all. I understand the lack of a pull-down and how that should be causing the gate to be stuck high and the mosfet to be stuck on. It does actually mostly work, as does this schematic:

Albeit this last one works rather poorly, due to overheating mosfets (most of the comments speculate that adding a mosfet driver would solve that problem). The problem with these instructables websites is that the original design never seems to get updated; somewhere down in the ~ 1100 comments someone who knows what they're doing has probably submitted a corrected design. At least I have learned a bit about the TL494, which was part of my goal with trying to build the project.

Thanks again for all the help!

No surprise there. And it needs a snubber. That primary is not well = coupled, most of the energy delivered will go into the transistor as = avalanche. It's a wonder it survives at all.

Tim

--=20 Deep Friar: a very philosophical monk. Website:

The schematic shows them as JFET's not mosfets. if that makes any difference.. Could be a miss use of symbols. there is a difference..

Agreed, full bridge flyback is a oxymoron self-contradiction.

Not quite. There is one load condition where it actually counts as = flyback. Of course, it's not very useful.

The load is none, so the flyback pulse dumps back into the supply.

Tim

--=20 Deep Friar: a very philosophical monk. Website:

While on the topic of mosfets...

I'd like to build a crude dc-ac inverter. I salvaged 10 12volt batteries from a large office UPS.

I have a 7.5 KVA transformer with 120-120 centre tapped primary and

I'd like to feed the centre tap on the transformer primary from the battery bank then switch the outer legs of the same windings in an alternate fashion, back to ground.

Getting a square wave output from this is pretty trivial.

I would like to get something resembling sine wave on the output.

Now, my problem. How can I get the mosfets to conduct as much sine wave shaped current as physically possible without burning them up? I understand that mosfets don't like being 'half on'.

I've been playing with an Arduino microcontroller and learned enough to be able to set whatever frequency I need on a couple of pins. These could switch the mosfets. Pulse Width Modulation is available.

If I could get 10 or 20 amperes throughput I'd be happy. Efficiency isn't that critical, but should be reasonable. The closer to a true sine wave, the better.

Any insights welcome.

mike

Check the Microchip site, I believe there's a appnote on a Sine UPS.

Cheers

First, you need 160-170V supply, not 120, or an 85V winding.

Set up PWM values from a sine table, or use magic sinewaves (Don = Lancaster's favorite).

Tim

--=20 Deep Friar: a very philosophical monk. Website:

favorite).

Thanks Martin and Tim. So, if I get a 30 or 40 amp rated mosfet and drive it with a sine wave input I will have no problems with the smoke getting out at 10 to 20 amps output?

I'm good at cooling stuff down. I solved my rectifier bridge overheating by suspending it in a glass of Canola oil. 6 amps and 120 volts makes it warm. It works, but sometimes drowns unsuspecting insects.

I'm surprised, actually, as the PIV rating on the bridge is only 50V but has been working for a few months. The unit is one of those aluminium cased Radio Shack / Source ones, with a hole in the centre.

mike

========================================= Don Lancaster? If only he'd worked a bit harder, he could have BEEN somebody.

Archie =========================================

No. I clearly stated "PWM sine". Driving a raw sine wave makes a large = linear amplifier with low efficiency and big magic smoke potential.

insects.

This gives me less confidence.

...

Hint: if this circuit

Tim

--=20 Deep Friar: a very philosophical monk. Website:

Oh ye of little faith. I've already been taught by the best.

mike

ts.

TMI again

Merely giving credit where it's due. It's the very least I can do.

mike