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Crazed half bridge!

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Ken M.

Sun Jun 17 2007, 09:16PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

So after the 1st attempt with the new coil, I ended up blowing up the positive rail fet, after replacing it the carnage was mad when I powered it up again, heres some pics along with some the half bridge schematic with parts and what not "X'ed" out.

I'm thinking it was the gate driver, I also have some pics of the waveforms from the 555 interrupter and then pics of the wave forms after they come out of the GDT. Heres the 555's waveform with the 2.2uf and the 10k pot

,heres the wave form wit ha .1uf and the 10k pot

. Now heres a pic of the 2.2uf and pot after it comes out the GDT

, and then with the .1uf and pot after the it comes out the GDT

. So needless to say I'm confused as hell as to how to get the GDT's waveforms looking good, and as to how I managed to blow up so many parts.

Andrew L.

Sun Jun 17 2007, 09:57PM

Registered Member #515 Joined: Sun Feb 11 2007, 02:06PM
Location: Slovenia/Ljubljana
Posts: 94

I had the same problem so I rewound my GDT (adding a few more turns) and now it works perfectly.

uzzors2k

Sun Jun 17 2007, 09:59PM

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

Your bridge failure is characteristic of switching device failure. Most likely bad a waveform killed one or both, which then took the other components along for the ride.

I didn't quite understand your waveforms, the first two were directly from the 555 output, right? They look alright for interrupting, you are feeding them into the enable of the gate drive circuitry? The GDT waveforms look like crap. The first pic leads me to believe your GDT is saturating, and the second just looks wrong. Are you running the GDT from the 555 output, without antenna feedback/ oscillator?

First of all, which frequency is the GDT being driven at, and how many primary turns are on it? You need to test the GDT at the operating frequency of the SSTC, so 200 kHz - several MHz depending on your coil's size. Also use enough turns to avoid saturation of the core. From the pictures it looked like 256 Hz, the same as the 555? Way too low.

Get the gate drive sorted out, and the bridge failures will stop, -as long as you don't short it.

Ken M.

Sun Jun 17 2007, 10:15PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Yes the waveform pics where simply off the 555 because I was still trying to get it to work over range and I have no (easy) way of conneting the the GDT to the fet drivers. Yes when I'm loading them to the Ucc's the 555 hooks to the enable and the antenna to the input. The 1st GDT out pic was the best results I was getting with it from either ucc's or 555, also when I took that pic, I had the gnd clamp of teh scope probe connect to circuit gnd, and the test clamp connected to one of the output wires of the GDT. The 2nd pic had the gnd clamp on 1 output wire, and the probe clamp on the other end. Also all these were with no load and not connected to any fets or anything. The GDT has 3 wires the input and the two outputs, I originally had the output wires wrapped around the core, and then the input over top them, then when i took these pics I actually had the three wires spiraled\wrapped around each other, and then all 3 wrapped around the core. When I blew up the electronics I had 16 primary turns, and 16 secondary turns, with the 1st test (the pics) I think I had maybe 13-17, I rewrapped it a little bit ago and still am getting the crappy waveforms but have no idea how many turns I have I'd say close to 20 turns primary and secondary. BTW If your curious about the core, I ganked it from a PC PSU, it was the largest toroid I could find (and yes, the PSU was dead).

Steve Ward

Sun Jun 17 2007, 11:36PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

You dont seem to have any RF going into the gate drivers, so the waveforms will be junk. This circuit relies specifically on having RF feedback into the antenna. Without it you cant expect anything to look right. To check the gate waveforms feed 200khz square wave into the antenna. Just looking at what the interrupter produces on the output of the GDT is 100% useless.

Ken M.

Sun Jun 17 2007, 11:43PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

After uzzors said to try 200kHz, I setup it up like you originally had the schematic (only because I don't have a function generator), except I'm having two issues, 1) the 555 circuit is only producing about 20kHz, and 2) stage one of the 74hc is working but when it hits the 2nd inverter it will not change states, and this is the second 7414 I've tried and they both refuse to change states from high to low.

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