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Shaun

Sat Nov 17 2007, 08:43PM

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

Just a quick update...IT WORKS! on open loop, at any rate.

I wound a new GDT using insulated instead on enameled wire...waveforms are PERFECT, breakout, fluorescent bulb, etc... all fine. After work some fine tuning, antenna feedback, higher power, CW...lets see where this goes.

Firnagzen

Sun Nov 18 2007, 01:47AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

So there was a break in the insulation? Ummm... Are ferrite cores conductive? (runs off to test) Would seem not.

Anyway, congratulations on getting this to work! (Gonna try one myself... As soon as I get better from this sore throat...)

Marko

Sun Nov 18 2007, 01:57AM

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

*don't* use enameled copper wires for your GDT's. Ferrite is conductive and wires *will* get damaged during winding sooner or later, and when 170V finally arcs to core it will shatter in pieces, kill your mosfets, and possibly low voltage section.

Use wires from CAT5 cable or something similar, never even think about enamelled wire unless you have a plastic form and some kind of insulation between windings.

Shaun

Sun Nov 18 2007, 04:17AM

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

Yeah, I think I figured out not to use mag wire on my own. The thing is, my DMM tested infinite resistance between all windings of my old GDT...oh well, that was then and this is now, anyway I have since gotten off open loop and am now running antenna feedback.

Once I build a topload, test it at high power, and find an interrupter combo I like (+ a switch for CW), I will put it in a nice box. (did I mention I'm using an ATX PSU case?)

Looking back I think it was pretty obvious, I mean, GDT primary waveform was good, and GDT secondary waveforms were bad...can't believe I didn't notice before. (before I blew 4 FETs!)

Shaun

Sun Nov 18 2007, 05:42AM

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

Two more FETs dead...
I was up to about 90V on the variac, running a very low duty cycle (about 1bps), and getting nice snapping 2 inch arcs, and then....blown fuse, two more IRFP260s dead.

How are they still dying? I mean, its a half bridge, so each one only had about 63V D-S! What else is there to do to make these things last?

Dr. Dark Current

Sun Nov 18 2007, 07:44AM

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

Shaun wrote ...

How are they still dying? I mean, its a half bridge, so each one only had about 63V D-S! What else is there to do to make these things last?

uhh, half bridge does not cut the voltage on each switch to half 0_o Were you running open or closed loop? If open, then your transistors could have died from hard switching.

On a first version of my SSTC, the fets were too dying at low powers, then I figured out this is due to two things 1) primary had too low coupling and 2) my CT was badly implemented.

Shaun

Sun Nov 18 2007, 05:26PM

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

I had to draw a diagram of a half bridge, but now I realize the full rail voltage is switched through the primary. If that's true, then why do people use full bridges at all?

Anyway, it was running antenna feedback, and the heatsink was cold. Before I run it again I am getting some TVS diodes for the FETs. I'll see if that fixes the problem.

uzzors2k

Sun Nov 18 2007, 05:28PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Shaun wrote ...

...why do people use full bridges at all?

More power. Twice the voltage over the same impedance, you do the math.

Shaun

Sun Nov 18 2007, 06:13PM

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

I just remembered I have some 220V TVS diodes from my DRSSTC. Would it be worth it to stick them on my 200V IRFP260s?

And about the half bridge vs full bridge, if a full bridge gives twice as much voltage across the load as a half-bridge, how do the switches in a half bridge still see the same voltage D-S as a full bridge?

uzzors2k

Sun Nov 18 2007, 07:21PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

In both cases one switch will have full supply voltage across it when the other switch turns on. This holds true for both high and low side switches. Just analyze the circuit and it becomes pretty clear. As for 220V TVS on a 200V mosfet, there's no point. The mosfet will avalanche/ die before the voltage can reach 220V.

I read through earlier posts here, and you said full rail voltage is switched through the primary, which isn't correct for half-bridges. The capacitor divider halves the voltage, so the primary sees +/- half supply voltage. In a full-bridge the primary sees +/- full supply voltage.

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