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Mini SSTC Bugs

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Mads Barnkob

Mon Feb 23 2009, 07:00AM

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

Do you use a variac to control the input voltage?

Did you check the gate resistors?

Are any of the fets short circuited?

semiconductor

Mon Feb 23 2009, 07:14AM

Registered Member #1989 Joined: Fri Feb 20 2009, 05:28AM
Location: Edmonton Alberta, Canada
Posts: 17

No, I don't have a variac. I make sure the driver circuit is on before I apply power to the bridge.the gate resistors are 10ohm 1/2 watt and the are good. I had replaced the feedback antenna with a secondary base CT. This is how I got my first results. The CT has 60 turns of #26mag wire and 3 turns off the bottom of the secondary. I did blow some FET's but I replaced them.

semiconductor

Thu Feb 26 2009, 01:39AM

Registered Member #1989 Joined: Fri Feb 20 2009, 05:28AM
Location: Edmonton Alberta, Canada
Posts: 17

Update: I just blew the last of my FET's and I have a feeling that most my driver board IC's are dead! Now I have to try and convince the wife to let me drop $100 bucks on components!!! Its not sounding too good though so this project will be put on hold for a while.... TO BE CONTINUED...

Tom540

Thu Feb 26 2009, 06:54PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

BlowinFets wrote ...

Update: I just blew the last of my FET's and I have a feeling that most my driver board IC's are dead! Now I have to try and convince the wife to let me drop $100 bucks on components!!! Its not sounding too good though so this project will be put on hold for a while.... TO BE CONTINUED...

Maybe change your screen name to Blowing on FETS(so they cool and don't blow). Then tell your wife you bring home the bacon and therefore you can buy the parts.

When all that is said and done and a bunch of primary turns to decrease current draw. It looks like you only have only a few. That might be the problem and would be a good place to start. Probably the simplest thing you can do to reduce stress on them. Your setup looks very nice by the way.

good luck,

-Tom

semiconductor

Sat Feb 28 2009, 05:12AM

Registered Member #1989 Joined: Fri Feb 20 2009, 05:28AM
Location: Edmonton Alberta, Canada
Posts: 17

Last night I took my multimeter to my FET's and found a few that still work. Also took Toms advice and added a few more turns to the primary coil to cut down on current draw.
I can hear the GDT emitting a high frequency buzzing, should this mean its working? With power to the halfbridge, as soon as I turn power on the the driver board I get a 1" snap from the breakout poin and thats it. Its like something is not starting the oscillation?

I made a new driver board today to see if I could get some kinks out. Now it won't work at all and I lost a few UCC's in a puff of smoke!! This is very frustrating!!

Marko

Sat Feb 28 2009, 11:43AM

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

Your low voltage power transformer looks way too small for a CW SSTC of that size. I would use something like 20VA at least, even more for fans.

Still that doesn't explain why are UCC's dying. With DC blocking capacitors, there's just no way of that happening (since coil can't oscillate with a shorted mosfet), and even if it was externally driven the gate resistors would burn first.
So I would stand back at this point and check what's wrong with the design first.

Marko

Arcstarter

Sat Feb 28 2009, 05:36PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

That reminds me, perhaps you should check your DC blocking on the gdt. Change to film when you can. Find some TV boards. Those likely red or blue caps are film. Look here to see what they look like:

They are however not always those colors. I have seen orange too. Then look at the code on it. It will be 3 numbers, with a letter after it. Those three numbers are the capacitance. then take that code here:

Those film caps would be good for GDT DC blocking, CT/antenna capacitor, and if they are big enough, even bridge DC blocking.

semiconductor

Sat Feb 28 2009, 09:25PM

Registered Member #1989 Joined: Fri Feb 20 2009, 05:28AM
Location: Edmonton Alberta, Canada
Posts: 17

Cause everyone likes pictures...

Arcstarter

Sat Feb 28 2009, 09:53PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Not sure if this will help, but change to film, those ceramics look too small to pass the current needed for the GDT. Also, the wires to the GDT are much to long. Those GDT dc blocking must be too small, just seems they would heat and short. Also, use a new film on the CT. Those caps could see high voltage, depending on how high the voltage rises. When the zeners clamp, the capacitor is still getting the full voltage of the CT output.

StevenCaton

Mon Mar 02 2009, 07:27PM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

I'm trying to resurrect my mini SSTC but I've run into a problem.
I had it working in the past but now I seem to have one problem after another.
When I applied power to the bridge, the secondaries on the GDT got extremely hot, and one nearly melted. That happened at like 15VAC input from the variac.

I know the bridge is connected correctly and everything within the bridge, (caps, fets, bridge rectifier) is new.

Does anyone have an idea what would have caused the secondaries to heat up like that.

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