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[fullbridge] mosfets dying, why???

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Marko

Tue Apr 25 2006, 02:12PM

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

You are talking about the gate driver, but whats wrong with it? The scope screenshots looks good, isn't it?
I'm using darlingtons becouse they were laying around in my bench. As an oscilator i'm using a function generator.

What about ringing on a not loaded bridge? Is it normal? And maybe I should try a half-bridge first?

In the last pic transistents seem large for only 2nF, you should not be able to see them actually. (compare the first and the last picture)

Try hooking the gate driver to the bridge without applying power to it and scope the gates.

Ringing is quite normal as long as it is not swinging over gate voltage rating or down to linear region of mosfet (it becomes a large resistor and blows up)

It seems to be at 20V but zeners and resistor should reduce it.

Hazmatt_(The Underdog)

Tue Apr 25 2006, 07:14PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

The only reason I brought up equalizing resistors is because every commercial circuit I have looked at, with multiple devices in parallel, always had this feature.

ped

Thu Apr 27 2006, 07:27PM

Registered Member #305 Joined: Sat Mar 11 2006, 04:27PM
Location: Liverpool, UK
Posts: 29

not to mention these resistors cost next to nothing, and will help eleminate one possible cause of your mosfets dying.

c4r0

Sat Apr 29 2006, 12:28PM

Registered Member #151 Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153

How big these resistors should be? (How many Ohms)

And i just noticed one thing. On many SSTC schematics there are a MUR diodes in parallel with transistors. I thought that it is becouse there is no diode in the MOSFET structure. There is a diode inside of my IRFP451 MOSFETs so i didn't connected additional ones. But recently i noticed that here

these diodes are added. I also noticed that e. g. IRFP260 includes a diode in structure, although it's not marked on Steve Ward's SSTC schematics. So maybe lack of these diodes in my bridge is the problem?

HV Enthusiast

Sat Apr 29 2006, 01:04PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

These resistors are usually on the order of milliohms. What happens is that when they heat up, their resistance increases which reduces the current through them and this action somewhat equalizes the current through the parallel MOSFETs.

The size resistor will depend on heat dissipation, current, etc...

Marko

Sat Apr 29 2006, 01:50PM

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

These resistors are usually on the order of milliohms. What happens is that when they heat up, their resistance increases which reduces the current through them and this action somewhat equalizes the current through the parallel MOSFETs.

Just note that carbon resistors have very weak, negative temperature coefficient and they won't work as PTC current equalisers. Wirewound or metalfilm resistors can be used but it is questionable will normal resistors help there at all.

I don't know if this is really important in low-current system with mosfets that already have high-ish ON resistance.

There are some low-resistance PTC's in various switching supplies (ATX, saver bulbs, cell phone chargers, etc..)
They can have couple of ohms and may need to be paralleled.

You can also improvise them using pieces of heater wire (best from hair - dryer), thinnest possible so it heats up faster and changes resistance more quickly.

Except from PTC behaviour resistors close to paralleled devices just themslves make stray inductance/resistance less relevant, as resistors themselves tend to equalize the current since resistances of both switching devices get more similar.

This is specially important for high-current systems and low ON-resistance devices.

c4r0, did you try to scope mosfet gates, while thing was running (with un-powered bridge)?

Does the coil output small streamers, just higher voltage, anything on the secondary (at least light a neon bulb/neon probe touching it) when running with few volts bridge input?

Steve Ward

Sat Apr 29 2006, 04:40PM

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

Extra parallel diodes were often used because it was thought that the internal diodes were too slow, so they were bypassed with better diodes. I have often just used the mosfets internal diode and it worked just fine. It might be worth trying the UF diodes, but i dont have a strong feeling that it would be the problem.

I do notice that you dont have any local decoupling on the bridge. Also, since you made the bridge so large, some of the inductances are a bit higher than they would be optimally. Those 2 things could lead to excessive voltage spikes during the switching transitions.

Also, about my # of turns vs. heating quirk, im really not sure what the cause was. The reduction in heating was not a whole lot, but it did seem that the fets ran a bit cooler. Perhaps a better impedance match was found, giving less reactive power flowing in the primary and mosfets.

c4r0

Wed May 31 2006, 06:05PM

Registered Member #151 Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153

Ok, now the problem is back

I made a new beautiful halfbridge.

And a secondary

Resonant frequency is about 200kHz. It works quite well:

But the mosfets explodes when I reach 200V (It must run directly from mains voltage 230V~). I tried irf820, irf840 and irfp450, all for 500V. I can run the bridge with 100W bulb on load from even 250V~ and it works well, the mosfets are only warm WITHOUT heatsinks after 10min of work with the bulb. It can work for a long time too when I connect the tesla resonator and set about 150V on variac (i'm using 555 interupter with about 10% duty cycle), and the mosfets doesnt get warm (now with a little heatsink). But when i go up to 200V the mosfets blows immediately. I'm using the same gate driver as in the first post, but the gate driver can't be bad if the bridge works so well with bulb, right? Aaagh, what should i do now?

HV Enthusiast

Wed May 31 2006, 06:29PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Try adding some TVS across your MOSFETs. Also, do your MOSFETs have internal diodes in them?

vasil

Wed May 31 2006, 07:25PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

More primary turns?

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