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uzzors2k

Thu Nov 29 2007, 05:12PM

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

BD1XXes are just no good for more than 150 kHz, but other common signal transistors will work at high frequencies. I've ordered some better signal transistors so I'll see how they work. My next SSTC will be gatedriver-chip-free, guaranteed. (I hope I don't have to eat my words later.

)

I like PLL too, it has so many benifets, I don't know if I'll every use anything else.

Reaching

Thu Nov 29 2007, 06:12PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

i used a bdxx and irf9530/530 stage in 2 of my sstcs and it works just fine even up to 2,1mhz in case of my hf driver. so i dont know how you wired them but for me those bd types worked well.. at 2mhz the bd stage put out a nice squarewave with load resistor, so i dont know.
i already own some (without driver chip) sstcs. its not that difficult at all

uzzors2k

Thu Nov 29 2007, 07:16PM

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

The BD1XXs have no problem running gates directly, but through a GDT they really slow down. I actually used something similar to your discrete driver to begin with, but I would get massive amounts of ringing on the output with an IRF9530/630 stage. The switching speed was good, but the ringing ruined everything. Even with coax cable the waveform was poor. I was under the impression that the output capacitance of the fets ruined the waveform on me. And using just BD1XXs alone with 4nF of gate capacitance total doesn't work too well. Is this the circuit you use?

And you've tested it with a half-bridge of IRFP450 equivalents at 2 MHz and gotten good waveforms? There must have been something wrong with my core then, even with UCC's the waveform isn't as good as I had expected.

Firnagzen

Fri Nov 30 2007, 02:43AM

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

Nice sparks!

Is there any reason there's a lower limit on the frequency of this? I mean, an upper limit seems perfectly fine... But a lower limit?

And another thing. No audio modulation? Steve's PLL has that option too, you know. Audio modulation seems all the rage these days.

EDIT:
Was about to close this topic when I realized something. You haven't reported any MOSFET deaths yet. Is that due to not mentioning them, or your first set of MOSFETs are still alive? If they're still alive, I am amazed! And then I'm going to scrap my current circuit board and replicate yours.

Steve Ward

Fri Nov 30 2007, 07:12AM

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

That scope shot looks like you might be switching a little too early... have you tweaked the phase control much? Getting the switching down better should help the mosfet heating a good deal. I usually like to scope both primary current and gate driver output at the same time so i can verify the phase relationship between the switching and the primary current.

uzzors2k

Fri Nov 30 2007, 12:00PM

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

I play with the phase control all the time, but it seems to alter the switching frequency. Turning the pot from one direction will give no output, then suddenly noisy sparks, then perfect streamers, and then steadly decreasing streamer size. This is when turning counter-clockwise. I recently noticed that the coil seems to detune when objects are brought near, a sign of running fixed frequency. Or that I've set the frequency range to tight, one 15k and 16k don't give much roaming range... Will the fres rise or lower when objects are brought near? I haven't scoped the GDT output and primary current at the same time yet. I've just turned the pot until the streamer length peaks. I'll scope everything and check whether it's running properly or just fixed frequency.

Firnagzen, there's no lower limit, but the BD1XX's are too slow to switch heavy loads properly at high frequencies. Audio modulation would be a piece of cake to implement, if I could run the SSTC from filtered mains. I've killed about 3 pairs of mosfets so far, IIRC.

Marko

Fri Nov 30 2007, 02:30PM

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

One thing I missed out, absolutely do use a small value (500nf..1uF) DC blocking cap in series with your halfbridge output. That should save lots of mosfet lives if anything goes cardinally wrong (but not from overcurrent in operation).

uzzors2k

Fri Nov 30 2007, 03:01PM

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

I put one in a few days ago, but its good you noticed Marko. Will it be needed once the coil is working properly?

I scoped the outputs without becoming any wiser. At 50Hz half-wave it's hard to make anything out, and at 50V DC the waveforms were still fairly fuzzy. But I did see something. Turning the pot does changes the phasing difference between the primary current and GDT output as expected, but it also seems to alter the frequency. Is this normal? I couldn't hit resonance exactly at 50V because it would suddenly drop out of resonance, and I would have to turn the pot back until a pop came, and then back to get close to fres again. Bringing anything near the topload would bring the phasing closer to perfect switching, before the popping thing occurs and I have to reset the system. Very strange. I haven't had that problem from half-wave rectified, but that might be because the power is applied in bursts, allowing the system to reset.

Is the gate drive good enough? I really expected better from UCC's. I'm using a 470nF blocking cap and 5 ohm resistor on the primary side.

Bauerb2

Fri Nov 30 2007, 09:12PM

Registered Member #973 Joined: Tue Aug 28 2007, 07:32PM
Location: Pittsburgh, PA
Posts: 66

5 ohm on the pirmary side?? i thought they are supposed to go on the secondary side in series with the mosfet gate....

anyway, i believe the resonant frequency of the coil increases as you bring things closer to the topload. the close objects decrease the secondary side capacitive value and increase Fres.

Try removing the 5ohm resistor from the primary side of your gdt, and keep those 1.2ohm resistors on the mosfet gates. i think the extra resistance in your circuit causes some rise/fall delays, especially at +500kHz. i had fet gate resitors of 10ohm and dropped them down to 2ohm and it helped out a great deal.

-Andrew

...

Sat Dec 01 2007, 12:43AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I had that popping effect on my pll coil as well, the problem was due to the r1 and r2 (ie, pins 11/12 iirc) not being correct. One of them sets the center frequency, the other the amount of tuning range possible with the pll. Then just twiddle them until it starts working

That scope shot looks great for the gate drive (especially at the frequency you are operating at). The fets heating could be from the coil not running perfectly in tune, but it more likley that you just need more primary turns.

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