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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Christopher Miles wrote ...
I am concerned that you are poping FETs, uggggggggggggg. Check the gate signal and ensure you are not over shooting spec value. Also check B+ for spiking. You can stress silicon (Si) by over stressing for a while and create Si called walking wounded. ESD will also do this. Then one day the wounded Si fails. I write test at work to stress Si till it fails. There is a lot of “Walking wounded†IGBT’s/FETs on ebay indeed!
Here's the gate waveform with ~400khz testsignal, the coil's resonant frequency is probably slightly below that:
I have no idea how it looks like with feedback signal since I haven't checked that, but I believe my scope would act up by picking up rf from the coil and display a distorted signal even if it would be clean.
The gates are clamped by 18V zeners so I'm sure I'm not overstressing them.
Christopher Miles wrote ...
P.S., can you post a pic of the entire setup?
for sure
The streak on the secondary is just left over glue from a sellotape.
I guess I'm pushing the breadboard a bit beyond its component capacity limits, but it seems to work well.
The transformer on the back is just 230/115V for powering the irfp250 halfbridge. It's rectified and filtered by 220u cap, and two 1.5u PP decoupling caps are soldered right to the FETs.
The CT is a subject to change, since I'm getting stable feedback just with very low pri/sec ratios (now 2:25). A 4k7 resistor is connected across the CT output, then it goes through a 0.22u cap to the 74hc14, where it is clamped by 1n4148 diodes.
One thing I don't get is why always the low side FET fails and it is never shorted completely...
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Christopher Miles wrote ...
Would not trust another FET on breadboards.
There's no FET on the breadboard, the GDT and half bridge are outside it.
Here's one observation: when the coil fails to oscillate (ie. no feedback signal to gate drivers), what happens is as the interrupter turns on, one gate driver chip is high while the other is low. This results in that the decoupling cap that is in series with the GDT primary starts to oscillate with it, which results in a nasty burst of low frequency sine wave on the output of the GDT. I'm wondering, couldn't this be what is killing my FET?
Update: I've completely rebuilt the circuit on the breadboard and added a 555 interrupter (to eliminate the old one was the source of the problems). I've also went for 320V and IRF740's since they are cheap to blow. So far it works (low power tests), but I've spotted something unusual. This is a scope shot taken by holding the probe near the secondary: (sorry for the poor quality, the wave on the scope seems to wander around)
notice the weird "dent" in the waveform, what could it be?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
So far the coil works, sparks 4" or so (will be removing turns from primary, currently 14).
I tried running halfwave CW (connecting the gate drivers' enable input to +5V) but it did not work! It just made single spark as I applied power to the bridge and then nothing. What do I have to do to get it working halfwave CW?
Registered Member #514
Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295
jmartis wrote ...
I tried running halfwave CW (connecting the gate drivers' enable input to +5V) but it did not work! It just made single spark as I applied power to the bridge and then nothing. What do I have to do to get it working halfwave CW?
Do you have a start-up osc? Since with halfwave input there is no feedback for half the time it may fail to oscillate... A 555 running at ½ the resonant frequency should work.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Zb wrote ...
Do you have a start-up osc? Since with halfwave input there is no feedback for half the time it may fail to oscillate... A 555 running at ½ the resonant frequency should work.
ah, the start up osc, isn't it better if it runs above fres? This way I think there is less risk blowing the FETs with poor/no feedback
--- It works, the sparks are shorter but much more juicier, and also more VTTC-like, and I like that. Only if it didn't draw so much power
heh, do you think this thing can stand drawing arcs to ground? and do I draw them do the secondary base or to ground (before/after CT)?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Problems, weird ones!!!
The coil runs fine with a 500W bulb in series, but when I bypass the bulb, the spark starts emitting really bad sputtering sounds! Here's a video of the problem, first with the bulb, then I bypass it.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
It works!
I found that all the stability problems lie in the CT and how is it connected up to the control electronics. Here's a connection that I currently use, any tips on improving it? edit- I forgot a .1u cap in series with the CT
And of course some sparks The little IRF740's get just a bit warm so far.
How tightly can I couple the primary to the secondary without a risk of arc-over?
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It's time for an SSTC
