Did I kill my MOSFETS? (ZVS) / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

Did I kill my MOSFETS? (ZVS)

previous 1 2 3 4 next

Move Thread

LAN_403

Myke

Sun Jul 15 2007, 07:09PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

The surface mounted zeners might have been overheated during soldering. HV things die with a bang at high powers not at the low powers you are running it at. But I Think the fets are fine. I would check to see if the zeners are still working or if they were overheated during the soldering proses.

Shaun

Sun Jul 15 2007, 07:11PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

The TO-220s are the "400+V fast diodes" from the ZVS schematic. I didn't know you could use the ones in the MOSFETs, if so then why are they included in the original schematic?

Edit

I tested the zeners with my DMM, they don't respond like normal diodes. I get about .62 V in on direction, and about 1.5 V in the other. Does this mean they're dead, or is this because they're Zeners? (I get the exact same readings for both)

Electroholic

Sun Jul 15 2007, 08:52PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

no you have to use external fast diodes to pull the gate low, thats what the fast diodes in the sch are for.

yes the zeners are toasted, it should not conduct in both direction, at least not at 1.5V. they should hold up to 12V in reverse bias.
Get bigger zeners >500mW.

Marko

Sun Jul 15 2007, 09:04PM

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

yes the zeners are toasted, it should not conduct in both direction, at least not at 1.5V. they should hold up to 12V in reverse bias.
Get bigger zeners >500mW.

Zeners are fine if they show 0.6V forward.

Reverse leakage can usually be enough to confuse some DMM's. Most cheap ones I have show 1-2V in reverse for working zeners. When they fail they go either fully open or short.

In this case they really couldn't have died with 8V input, for voltages that low they aren't even needed.

...

Mon Jul 16 2007, 01:31AM

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

The diodes going to the gate don't need to be huge ones, a little 1a diode is more then enough. But as long as the diode is fast and doesn't have much capacitance you should be fine...

In any case, what is the power draw of the circuit when you connect it to the batteries? And what is the battery voltage when it is connected?

Shaun

Mon Jul 16 2007, 04:23AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

I did not test the power draw, but I suspect it was at least a few amps because at 16.8 V my inductor heated up quickly. I am going to desolder all the components, test them, and if they show okay I'm gong to get some bigger Zeners and rebuild. Hopefully the $7 FETs are OK...

I'll also wind a new inductor on that powdered iron core. I'm thinking 30 turns of 12 awg?

I though I might have killed the zener diodes, because I used a 25 watt iron which seems high for this application.
Even though I'm not going to reuse them, I want to test them while not in circuit because I think other components are skewing the reading on the DMM.

Myke

Mon Jul 16 2007, 05:06AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I would wind as many as I could fit. I had a bigger blue core and it fit 50 turns of 20awg wire onto it and it's inductance was just over 200uH. 12awg is king of big for the core. I was putting ~100W through the driver at 12V and the only thing that got warm was the wires going to the supply and the flyback (annoying thin alligator clips)

Shaun

Mon Jul 16 2007, 06:37PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Well I thought I would use some thick wire because I am actually planning on putting a lot more than 100 watts thru it. I was hoping to push it to around a kilowatt.

I might need a bigger core for this, but as I don't even have it working yet I think I'll start with the small one and upgrade later.

Anyway, I desoldered everything, and all the components test okay except the zeners and the MOSFETs. The zeners have gone totally open-circuit, and the mosfets have also gone fully open except one which still responds to tests of the internal diode. I am not sure what voltage my meter provides; it is powered by a 9-volt battery.

I'll have to try what ... suggested, using a 12v limited power supply with a voltmeter on it.

Marko

Mon Jul 16 2007, 07:24PM

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

I though I might have killed the zener diodes, because I used a 25 watt iron which seems high for this application.

No, it's nearly impossible to happen these days. I never managed to fry a component that way in my life. 25W iron is actually pretty small.

I don't understand you people's philosophy about testing mosfets.
17 replies and still nobody has got around the point.

Simply use a DMM in low ohms setting across mosfet's drain and source.
Then connect a 9V battery from source to gate (positive pole at gate). Mosfet should go on and show it's ON resistance which is very low, 0.085 ohms in this case.

When battery is removed, mosfet should stay on. When gate and source are shorted, it goes off.

You are not going to run a flyback out of 9V batteries. YOu need a power suplly capable of at least few tens of watts and >10V.

Shaun

Mon Jul 16 2007, 08:34PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Thanks very much Firkragg, I now know my MOSFETS are in good condition. ON resistance is about .1 ohm, when OFF its about 10.5 MOhm.

I guess it must have been my wiring, possibly my power supply, or maybe the Zeners really were dead. In any case, I have ordered some new higher-power zeners and will be re-assembling it when they come in.

previous 1 2 3 4 next

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