[fullbridge] mosfets dying, why??? / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

[fullbridge] mosfets dying, why???

1 2 3 next

Move Thread

LAN_403

c4r0

Mon Apr 24 2006, 09:06PM

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

Hi,
I'm trying to build a fullbridge SSTC. I have a setup of 12 IRFP451s (well, now it's only 10

) on a hudge heatsink (but i'm using only 4 of them):

This is my gate driver:

GDT: 5 x 20turns

Schematic:

Here a scope screenshots (5V/div , f=300kHz):
1. GDT secondary, no load

2. GDT secondary, tiny 12V 1W lightbulb on load (about 120 Ohm when shining)

3. GDT secondary, 2nF cap on load

I connected this primary coil (8 T):

and started increasing voltage with a variac. When i reached about 100V two of the mosfets died (they didn't even get warm

). Secondary resonance freq is about 400kHz. So why the mosfets died?

Before it happend i connected scope to not loaded bridge and noticed that there is some ringing (i dont have a scope screenshot unfortunately), what is a bit strange to me (becouse of no load). There was also little current pulses on power supply.

Please help me! Unfortunately, I cant afford killing mosfets one by one before it start working.

BTW.
Steve Ward wrote on his site (mini SSTC):
"I removed 2 primary turns for a total of just 4 turns. This surprisingly helped with heating problems of the MOSFETs!"
How it's possible that removeing turns can cause less mosfets heating? And is it bad that i'm using as many as 8 turns?

vasil

Mon Apr 24 2006, 10:19PM

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

Lower your res frequency. Discrete drivers add some signal delay. Why using darlingtons?
Steve Ward SSTC was a pulsed one.This helps too.
I dont know an explanation. Maibe if the inductance decrease, then the dI/dT in the circuit will be higher and the FETs will stay lesser time in the liniar region? I dont know....

HV Enthusiast

Mon Apr 24 2006, 11:00PM

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

One thing to check is to see if you are getting voltage break-down between MOSFETs on the heatsink. I believe the MOSFETs you are using have tabs which are "hot" and although you are using insulators, you may be getting a break-down. Just one thing to check.

cjk2

Tue Apr 25 2006, 02:21AM

Registered Member #51 Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263

I'm sure you would have already checked, but are the transistors in the bridge in correct phase?

Hazmatt_(The Underdog)

Tue Apr 25 2006, 02:41AM

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

Question: Your H-bridge confituration, does it have any Source or Drain resistor to equalize the current differences between device to rail?

If the case is no, one device is almost certian to pull more current, and will die.

williamn

Tue Apr 25 2006, 03:03AM

Registered Member #55 Joined: Thu Feb 09 2006, 04:56AM
Location:
Posts: 149

Your H-bridge confituration, does it have any Source or Drain resistor to equalize the current differences between device to rail?

If the case is no, one device is almost certian to pull more current, and will die.

I have never used any such load sharing resistors in my H bridges and they do not die, I don't think I have heard of anyone using them either. Sounds to me like your gate driver might be suspect. These type of drivers are a real pain to get working. Try getting your secondary Fres as low as possible. You can try adding some deadtime by sticking in a fast diode in parallel with the gate resistor (K facing away from the gate).

teravolt

Tue Apr 25 2006, 04:48AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

If the mosfets have the same batch number for mosfets and or if they are mached you may not have to use resistors. in genral a low resistance in the emiters or collectors of .1 to.3 help sharing. unfortunatly this may not fix your problem.other things to do is make shure there are tvs's between the collector and emitter and a good zener circuiut on the gates. Good luck

Marko

Tue Apr 25 2006, 08:24AM

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

I got almost the same waveform when I used H-bridge of bipolar transistors as driver.
I used ICL7667's to drive them directly, and got somewhat better waveform (at 2nf load there was really no such large transistent, but it did apper on 10, 20nF loads and it was too high to run IGBT's fast (another story).
It seems you can't really drive, evnen darlingtons, from 4049IC.
You need something that can provide higher peak current, it doesn't need to be much, if you are already driving it from oscillator you can use TL494. It can deliver significant peak current, it should be even able to drive the bridge alone.

And it will definitely provide high enough peak base current for bridge to swing even far bigger loads.

Also you should wind your GDT for less leakage inductance. Try using few 0.2mm litz wires together, for each winding, solder about 3 or 4 together for each winding and then use motor to wrap then tightly together (I used a lego motor with a wheel) to make tough rope-form of litz wires. Then just wind it like a single wire around a GDT core.

I greatly reduced leakage inductance with it, makes almost no difference between primary and secondary GDT signals.

good luck!

HV Enthusiast

Tue Apr 25 2006, 11:34AM

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

wrote ...

If the mosfets have the same batch number for mosfets and or if they are mached you may not have to use resistors. in genral a low resistance in the emiters or collectors of .1 to.3 help sharing. unfortunatly this may not fix your problem.other things to do is make shure there are tvs's between the collector and emitter and a good zener circuiut on the gates. Good luck

Just a note that MOSFETs do not have emitters or collectors. The correct names are Drain and Source.

c4r0

Tue Apr 25 2006, 01:01PM

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

There is no break-down to heatsink. These MOSFETs are in insulated casings. They are also in correct phase.

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? What do you think? I can't make a new secondary, probably until ... June

1 2 3 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob