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Low output and mosfets burn

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LAN_403

Unleashed

Wed Jul 18 2012, 01:38AM

Registered Member #5171 Joined: Tue Jun 05 2012, 11:32AM
Location:
Posts: 67

Hello everyone.
I've finally finished making my SSTC following the mads barnkob kaizer sstcII nice guide.
The first time i run it, (of course with variac soft start) All output i got was some corona on the breakout point.
Then, when i started raising up the voltage, i had a *puff* *boom*
that's what i mean :

what could have caused this?
Thanks.

brandon3055

Wed Jul 18 2012, 08:53AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

If I had to guess I would say it was the gdt do you have a scope? I had a similar problem with mine it took a lot of playing with different cores to get it working
Another thing it could be maby one or two of your mosfet legs tuoched the heatsink (but I'm far from being an expert so wait for some more replys)

Dr. Dark Current

Wed Jul 18 2012, 09:28AM

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

-bad phasing if the TC uses feedback
-bad GDT core material
-bridge design not low inductance
-insufficient shielding of control circuits

Sigurthr

Wed Jul 18 2012, 10:41AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

I'd go with these possible problems:

Cross Conduction (both fets on at same time)
No over-voltage protection zeners on the G/S of each fet.
No Freewheeling diodes to eat the Back EMF / inductive kick
High Inductive bridge design.

1) Verify your phasing scheme.

2)Use adequate diode protections.

3)Look at other's successfull bridge design/layouts and try to emulate it.

zzz_julian_zzz

Wed Jul 18 2012, 11:44AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

My first question is are you using snubber caps??? =) (on kaizer i see nothing about snubb caps)
What is the voltage inputted(in AC and DC) when it explode?
well, what is the corona length?? if it is coronizing big, then the gdt might be working good but not efficient and need to check the material of the core(please attach photo of your GDT), then try to add some zeners on its ouput on the gate (specially when using 1:2 gdt ratio, what is the driver voltage output that is being inputted on the GDT primary?),
Phasing?? do you need to have phasing? when we drive the signal from the antenna, phasing coulb be negligible in some cases, because this is an SSTC, we just need to make sure that the antenna gets enough signal to make the gain higher than 1, but as you are experience with corona with unknown length (phasing can be change on primary coil to make it work properly, and the primary of the GDT, try it one by one)
do you feel your variac humming at the same time when running the coil?

hmmm...
can you tell us which fet is connected to what (pos and neg supply), base on your picture, your 1st and 3rd fet did not explode violently like the other 2. if possible, draw your phasing output of your GDT. THanks. Don't worry, i believe you can make it =)..

Unleashed

Wed Jul 18 2012, 12:15PM

Registered Member #5171 Joined: Tue Jun 05 2012, 11:32AM
Location:
Posts: 67

Yeah sure, the variac was humming and vibrating a lot while raising the voltage.
The biggest mistake i make, i guess it's about the feedback antenna, that wasn't placed properly.
Anyway, for the gdt, i used the material suggested directely by Mads.

this one.
here is a photo of the driver circuit

it stands in a alluminium box grounded together with the secondary (the line transformer you see, i removed it and replaced with a external dc adapter.

I know to the photo it looks like the mosfets are differentely broken, but they're all fucked up (two of them are gate-source short).
Anyhow, here is a photo of the bridge layout :

(two on the right are connected to the positive and two on the left to negative)

The GDT is 10:10:10:10:10, as mads said.

Thanks.

EDIT.
The corona was very very tiny, just some little blue light on the breakout point.

Thanks.

Unleashed

Wed Jul 18 2012, 07:17PM

Registered Member #5171 Joined: Tue Jun 05 2012, 11:32AM
Location:
Posts: 67

Hello everyone.
20minutes ago i had another test.
This time i tried with 9 windings on the primary, but nothing changed.
I also used a clamp meter to see how much power the coil was draining, and the results are pretty scarying
around 11.5 amps with a very low voltage (40-50v) i didn't go more to not burn my variac.

And the same results, just a tiny corona.

What could be?

NOTE
for resonant capacitor on the primary, i used 8 of 0.64uF in parallel, to handle more current.

Thanks.

EDIT
i forgot to say that another mosfet blew for sure (puf) and i will test the other ones later.

Mads Barnkob

Wed Jul 18 2012, 09:36PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I do not have time to go throgh the whole thread right now, but are you sure all your MOSFETs and recifier bridge is isolated from each other?

I see pads, but no plastic throughhole spacers to avoid the screw shortcircuting to the heat sink!

Will check back for a proper lookthrough tomorrow :)

Unleashed

Wed Jul 18 2012, 09:37PM

Registered Member #5171 Joined: Tue Jun 05 2012, 11:32AM
Location:
Posts: 67

Yes, i checked with the multimeter and they're good isulated from the heatsink and each other.

I will for you to help me more tomorrow.

thanks.

brandon3055

Wed Jul 18 2012, 10:42PM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

The cap on the primery is for dc blocking only unless you wer trying to build a drsstc try removing some and increasing your prymery terns to about 20 also the gdt looks like it has a lot if leakage try removing the outer cover and winding it tighter and as far as I kn

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