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MRacerxdl

Wed Feb 06 2008, 01:34AM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Hi all, I built my first sstc here, I get a good sparks (5cm with 48V).

ok, but I have a really bad problem here. I blow up 8 IRFP250 putting it at more than 90VDC.

Here is my setup:

Ok, that things I know on my circuit:
The C5 cap its on the wrong place, i needs to be after the Primary Coil, I know. But if I put it after the Primary coil (as convetional sstc's circuit) It decrease HIGHLY the spark output (5cm to a few mm), I dont know why but its true.

I think I blow up the mosfets because C5, but I dont know why its need to be there.
Someone can help me?

Thanks

Here is a video of last sparks at 80VDC:

and here a pic of my first spark at 48VDC:

PS: My primary coil doesnt have 1mH :P it has 4 turns arround the botton of secondary coil, and its insulated by PVC and air.

Coronafix

Wed Feb 06 2008, 01:44AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Have you tried more turns on the primary?
Shouldn't the zeners be between the gate and the gate resistor?
Can you take a picture of the setup, it might help.

MRacerxdl

Wed Feb 06 2008, 02:29AM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Here is a pic of my old setup:

My new setup is with less wire (not to long as that in the picture).

I didnt try more turns on the primary, but it blow the mosfets after some time (but mosfets wasnt really hot)

Kizmo

Wed Feb 06 2008, 05:44AM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

One thing i can see here is that IRFP250 is rated only for 200V D-S and you dont have any protection for them. SSTC produces very nasty voltage kickbacs so i suggest you could try it with IRFP460 (500V, 20A) and add 400V TVS diode (1.5KE400) across D-S of each mosfet.

MRacerxdl

Wed Feb 06 2008, 04:18PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

If I put reverse MUR diodes it will solve the problem?
Because Steve Ward uses IRFP260 (Like mine, but 13A more), and the IRFP250 its cheaper :P
Other thing, My secondary botton is grounded at DC Power Supply ground. That is correct? (Same as Second FET Source)

Dr. Dark Current

Wed Feb 06 2008, 04:23PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I see two problems in your schematic: 1) capacitor C5 should be "after" the primary, so C5 and C6 form a voltage divider
2) increase the 100n cap on the primary of the GDT, I'd use something like 1uF (film or foil, not lytic) cap.

MRacerxdl

Wed Feb 06 2008, 04:25PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

I have some problems with heat on the 2 TC's, if I put more capacitance on the GDT Cap, I think it will heat more, that is a problem?

Of the C5 problem, as I say, if I put it after the primary, it decreases the output of tesla coil, but I dont know why.

Dr. Dark Current

Wed Feb 06 2008, 05:17PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Mestre Racerxdl wrote ...

I have some problems with heat on the 2 TC's, if I put more capacitance on the GDT Cap, I think it will heat more, that is a problem?

Of the C5 problem, as I say, if I put it after the primary, it decreases the output of tesla coil, but I dont know why.

You absolutely must connect it like this:

With the schematic as you have it now, the upper FET shorts out the capacitor every time it turns on, this is NOT good.
The reason why are you getting bigger sparks might be because you may be getting something close to class E operation, but if you want proper class E, it's a different topology that uses one FET and a resonant capacitor across it. Look at Steve Ward website there's something on the subject (Micro SSTC).

But if you want half bridge then you must connect it as in the diagram I posted.

Zum Beispiel

Wed Feb 06 2008, 07:06PM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

Mestre Racerxdl wrote ...

Other thing, My secondary botton is grounded at DC Power Supply ground. That is correct? (Same as Second FET Source)

Nonononono no! The secondary must be grounded to earth! (either a grounding rod pounded in the ground, or mains ground)

MRacerxdl

Wed Feb 06 2008, 07:20PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Ok, I returned now of the store, I buyed 2 IRFP250 and 2 IRFP460 (The last IRFP's of store, I think I buyed all :P) I will try to put on the original schematic and put the voltage at variac until reaches 110VAC

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