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uzzors2k

Thu Aug 03 2006, 06:33PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I'm building a smps for my HV projects, and I need someone to just make sure the components I'm using will survive, it costs 2 good semi's a failure after all.

Its off-line, half-bridge topology and runs off 240v mains. The main switching mosfets are IRFP450s with PR1503s to bypass the internal diodes. All I'm really wondering is if the PR1503s are good enough, like voltage and current-wise. I'm not sure what kind of spikes I can expect, and if the diodes short the bridge is ruined.

So will these

work for bypassing? I can post the whole schematic if my description is unclear.

Marko

Thu Aug 03 2006, 07:34PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

You have a lot more to tell.

Are you using high or low side driver, wich IC, regulation, and etc.

A schematic would also be nice...

uzzors2k

Thu Aug 03 2006, 08:56PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Currently I'm just trying to get an output from a smps transformer. I have two taken from a computer PSU and one from a monitor. I plan on rewinding one of them once I get the main switching circuitry in place. Its driven by a TL494 -> mini-h-bridge -> GDT -> mosfets -> xfrmr. Pretty crude, but I'm just experimenting.
Here is the GDT output as well. When switching a mosfet it slopes a little more of course, but by just 0.1Âµs or so.

I just want to know if its safe to plug into the wall yet.

1154638599 95 FT14056 Uzzors Waves Smps1

Steve Ward

Thu Aug 03 2006, 09:19PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Those diodes are only rated 200V at 1.5A. The diodes will see 350V plus any voltage transients (so maybe 400V). Also, the diode should usually be rated about the same as the FET, so a 15A diode would be better than 1.5A. The MUR1560 comes to mind as a suitable diode.

So, dont plug it in until you get new diodes!

Marko

Thu Aug 03 2006, 09:52PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

For such frequencies and power level internal diodes inside the mosfets are good for beggining.
You can bypass them altough if you want 'for insurance'.

Maybe you could lower the deadtime in order not to push everything to maximum instantly.

Take care around transformer, im. use one that was previously used with halfbridge (ATX transformer for example).

Steve Conner

Thu Aug 03 2006, 11:01PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Just think how much you'll learn for every two MOSFETs you trash. :P

The extra diodes shouldn't be needed. I've repaired and modded several commercial SMPSs that use MOSFET half-bridges, and they all seem to get by with the MOSFETs' own body-drain diodes.

The diodes only carry the transformer's magnetizing current, which is small with a well designed transformer. Even if you have a transformer with lots of magnetizing current, it's always an inductive current. What really stresses the diodes is capacitive current.

The SMPS circuit you chose is a fairly old and well-tried one. It should be pretty hard to blow up unless you make a gross mistake wiring it up, or get the transformer design so badly wrong that it saturates.

...

Thu Aug 03 2006, 11:13PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

It would have been nice if you got the formatting right for the schematic so we could actually read it (quote me to see how it was done) when we click it

But other than that you shoud be fine without diodes

I should also point out that you might need to put a schotty diode in series with the fet if you use an external diode as if the diode you are using has a higher Vdrop than the fet's it won't do anything (the schotty adds an extra .5v or so to the Vdrop of the internal diode, so the external one takes all of the current)

Steve Ward

Thu Aug 03 2006, 11:59PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Thanks for fixing the schematic ... .

the schotty adds an extra .5v or so to the Vdrop of the internal diode, so the external one takes all of the current

I suggest you look again at the direction of the schottkey diode. It doesnt add a .5V drop, rather it adds whatever the reverse stand off voltage of the schottkey diode is (so 40V or whatever). The schottkey is forward biased when the MOSFET conducts, but is *reverse* biased when the current reverses and tries to go through the body diode.

I agree with the rest of the people here, your application is low enough in frequency that you can get by just fine on the internal body diode (though it is particularly slow at over 1uS recovery time!).

uzzors2k

Fri Aug 04 2006, 11:56AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Thanks for the advice. I took out the diodes and plugged it in, only to have the breaker trip.

I was about to despair, but I found the bridge recticfier had shorted. I forgot to replace it after the first failure. (my avatar) Now would it be ok to just feed the smps with half-wave recticfied mains?

Steve Ward

Fri Aug 04 2006, 06:02PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Now would it be ok to just feed the smps with half-wave recticfied mains?

I would not personally do that, but if you really cant wait...

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