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Stupid ZVS Flyback Question

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Matt Edwards

Sat May 01 2010, 12:53AM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

Hello everyone! I am new to the forum (and a circuit rookie) but am hoping that someone can lend me a hand here... Please hang in there here is some back info...

I setup the standard zvs flyback driver with the IRFP250 mosfets that is posted everywhere online and it worked great. I then setup a small tesla coil that I made and things were going pretty good as well. The only difference between the standard ZVS and mine was a rectifier diode on the HV out and a 500pf cap for the coil... For this setup I was varying between a 25.5v 2A transformer with a rectifier and several lantern batteries connected at 48v. I was trying to figure out how much this flyback could handle. And at the same time fine tune the coil. Unfortunately the circuit seemed to want more amperes out of the transformer and it was getting way too hot so I started using a re-wound MOT at 48v with a 400v 8A rectifier. The MOT produced vary good results for a few minutes and then one of the mosfets shorted and then the rectifier quickly followed. I replaced the mosfet and the rectifier and after a few minutes the mosfet shorted again. This happened three times!

After re-checking the circuit for errors, I purchased the IRFP460 mosfets and a 50v 25A rectifier thinking that maybe i needed a more durable setup. After installing the new transistors the results were great! Much better then the IRFp250's. Unfortunately after a few minutes one of the new transistors shorted as well. It seems to be the same transistor in the circuit (the lower one in the schematic) that is shorting. Perhaps this one has more load?

Does anyone have any suggestions? I would really like to get this circuit running. I can post a schematic and parts list if that helps. Any constructive feedback would be greatly appreciated!!!

Just to add a bit of extra info:
Here is the zvs driver schematic that I was following.

Here is the parts list from mouser that i purchased.
1. Mosfet's------ IRFP250PBF-------------- 2
2. Zener Diodes------ 1N5349BG--------------- 2
3. FRED Diodes------ HFA15TB60PBF ---------- 2
4. 470 Ohm 2W Resistors------ 282-470-RC----------- 2
5. 10k 2W Resistors------ 282-10K-RC----------- 2
6. 1uf 100v Bipolar Capacitor------ 140-NPAL100V1.0-RC------------ 1
7. 47UH 10.7A Inductor------- 2309-V-RC----------- 1

Here are the additional parts that I added after the HV out.
Rectifier Diode
30kv 500pf ceramic capacitor
Tesla Coil (primary 20 turns center tapped every 2 turns 14awg wrapped around 2" pvc/secondary-approx 900-1000 turns 30awg wrapped around 1" pvc)

Kipmans

Sat May 01 2010, 06:28PM

Registered Member #91 Joined: Thu Feb 09 2006, 03:03PM
Location: The Netherlands
Posts: 45

Have you got any means of observing the waveforms at, say, the gates of your transistors?

Matt Edwards

Sat May 01 2010, 10:11PM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

No. It would be great if I had better equipment. The best that I have is a multimeter that measures Hz. Not sure if that will do the trick.
I am beginning to suspect a few things as the cause.
1. The circuit calls for a inductor ranging between 47-200 uH. I used 47. I have no way to determine this besides switching to a larger inductor.

2. The circuit calls for 10-40v. I am using 48v. I thought that with the IRFp460's this would be ok... I am now second guessing this. Also, My circuit does not produce any results when supplied below 20-22v. I assume that this is due to the flyback that I am using. It is pretty large.

If it is due to the 48v input, does anyone have any recommendation how to modify the circuit with the IRFP460 fets so that they will be ok with a 48v input. The reason that I am asking is because the flyback produced significantly better results with 48v verses 12 or 36.

Again, here is the circuit that I am using. I exchanged the IRFP250's for IRFP460's.

cjk2

Sat May 01 2010, 11:58PM

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

Pick fets rated for at least 4 times your supply voltage and as low Rds(on) as possible.

Kipmans

Sun May 02 2010, 07:49AM

Registered Member #91 Joined: Thu Feb 09 2006, 03:03PM
Location: The Netherlands
Posts: 45

Well, the IRPF460's are rated for 500 V so technically they should work. Also, I've had a ZVS driver run from 45+ V without the IRFP250's without any problems.

You can of course try another inductor. Inductor design is not critical for this circuit. If you happen to have one lying around, just take one of those yellow toroidal cores from an old pc power supply and wrap it with maybe 10-20 turns of wire. I've had excellent results with something like that.

If constructed properly, the ZVS driver is a very reliable circuit. I think it would be helpful if you posted some pictures of your setup so that we can spot any obvious construction issues.

condor

Sun May 02 2010, 11:37AM

Registered Member #2780 Joined: Mon Apr 05 2010, 12:39AM
Location:
Posts: 7

I dont know if you caught it on the data sheet for the irfp460 but the drain leg and the back plate are connected. Connecting it directly to the heatsink will cause a short. The irfp 450 isnt that way but has a lower ampe rating 16 if I remeber correctly.

I built this circuit off an incorrect copy.

I used 300 ohm for the first two resistors, it showed 3.0 uf for the capacitor, so I took (3) 1.0 250v caps and hooked them in parallel.

The windings on the ferrite core were only 8 turns, center tap for the feedback , then 8 turns.

I notice the sources were tied together in a n and p transistor fashion . I have see versions of this circuit that both mosfets were wired the same direction in a n and n connection. I didnt know if it was a mistake on the drawing. I followed the drawing.

only thing I added was 2 sets of ultra fast diodes on each mosfet.because thats what my drawing called for .

The circuit I found didnt have the 5 plus 5 turns on the ferrite or the 10 ohm resitors with the 3 windings each tied into the feed back.

It worked great on 12v throwing 2 to 3 inch fine blue sparks from the flyback. crt.

Interesting results I placed the circuit on a 12v battery it made a perfect sine wave that was 22 v ac.without the Pwm.
I didnt try hooking it to the flyback.

I placed a pwm between the battery and mazzilli circuit before I tried the flyback.

I plan on revising the circuit to the way you built it to see what the difference is. My guess is that it will preform even better.

Matt Edwards

Sun May 02 2010, 05:40PM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

I was tinkering with it last night and fried my breadboard. Perhaps it was the board that was shorting out the fets? Anyway, I am waiting for some new ones to arrive. I will be able to figure it out then. Thanks to everyone who has posted on my problem!

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