External Disable on profdc9 SSTC driver

[KrisPringle]

Hi all, I'm working on making my first solid state tesla coil and plan on using profdc9's PCB pack for the full bridge, driver, and interrupter. I think I have a good understanding of the driver and GDT, but I am unsure on a few things for the driver.

• Can the external disable pins on the driver be used at high frequency, such as for music? If so, I would like to use the interrupter provided in the PCB pack. The schematic for the SSTC driver calls the external pins "External Disable Optocoupler" but it's only 2 pins, and it seems to me all fiber receivers require at least 3 pins. I see three paths forward here, either I have made an incorrect assumption and these pins can't used for music, I can use some sort of shielded wire as the interrupter cable (what would that be? From what I've seen almost everyone uses fiber), or I can modify the schematic for the driver to accommodate a fiber receiver.
• What is the reactive power meter and current transformer? Is the current transformer an alternative to the antenna, a la lonoceans SSTC 2?

Forgive me for the inexperience, I'm an engineer but not an electrical engineer

PCBs: https://github.com/profdc9/DRSSTC-PCB-Pack
(full-bridge-transistor, half-bridge-sstc, interrupter)

[KrisPringle]

I done some research and spent some time looking at profDC9s schematics more, and I've answered my previous questions. I have some new questions though, mainly around preliminary testing of the solid state tesla coil:

How do I test the output of my driver before running at full mains power to the bridge? I've seen some people test their driver without even the GDT connected but I've seen other sources say that will blow up your MOSFET drivers. If I connect the secondary of my GDT to my full-bridge without mains power to the bridge is this an accurate load? What if I do not have a primary coil connected?

The driver I'm using has both a feedback and manual frequency option. I'll use feedback for the final coil, but for testing I will just be using the frequency generator.

I would appreciate any help!

[davekni]

I've seen other sources say that will blow up your MOSFET drivers.

I certainly can't think of any reason for that.  Perhaps someone had issues due to an unintentional stray-capacitance coupling back to feedback CT input or scope probe connected to feedback path causing high frequency oscillation.

If I connect the secondary of my GDT to my full-bridge without mains power to the bridge is this an accurate load? What if I do not have a primary coil connected?

Without H-bridge power, gate load will be a bit low due to lack of Miller capacitance (Ccg).  Close enough though.  If you want to be slightly more accurate, use a bench supply to power H-bridge.  Even 30V will add most of the Miller charge, as Ccg drops with more voltage.  If you add both some H-bridge power and connect primary, allows checking CT operation.

Best for any testing to be done with interrupter connected and operating at low duty cycle (low repetition rate).  That way any issue will cause less total heating.

[KrisPringle]

Thanks Dave,

In Mads article for Kaizer SSTC1 he states "When I first tried to run the driver circuit separately to test the driver before connecting it to the MOSFETs, it only resulted in the MOSFET driver chips (UCC37321/UCC37322) catching fire and burning up like a small volcano. This did of course upset me when it happened once more when I had changed the chips. This led me to seek help and I learned that running the driver chips unloaded, without a MOSFET or GDT connected to the outputs, the chips will oscillate into oblivion and burn them self down." Maybe this is something specific with his driver? I didn't look into the specific, maybe he can elaborate.

I'm sure I'll have more questions as I move along, but for now:

First, the same Kaizer SSTC1 article mentions primary phasing being wrong, how do you determine the proper phasing of the primary coil? Is it basically "guess-and-check" the way secondary feedback transformers are?

Second, (this may be an obvious question, but) if I use an overcurrent protection transformer, it is a coupled to the primary, not the secondary, correct?

[davekni]

In Mads article for Kaizer SSTC1 he states "When I first tried to run the driver circuit separately to test the driver before connecting it to the MOSFETs, it only resulted in the MOSFET driver chips (UCC37321/UCC37322) catching fire and burning up like a small volcano. This did of course upset me when it happened once more when I had changed the chips. This led me to seek help and I learned that running the driver chips unloaded, without a MOSFET or GDT connected to the outputs, the chips will oscillate into oblivion and burn them self down." Maybe this is something specific with his driver? I didn't look into the specific, maybe he can elaborate.

Kaizer SSTC1 is an antenna-feedback design.  Easy for driver output to couple back to antenna.  It is also constructed on a proto-board rather than ECB.  Supply wire inductance could be an alternate path for feedback.  Either way, a well designed ECB with a signal generator driving feedback should have no issue with such oscillation.   I don't think there is any issue with those driver chips.

First, the same Kaizer SSTC1 article mentions primary phasing being wrong, how do you determine the proper phasing of the primary coil? Is it basically "guess-and-check" the way secondary feedback transformers are?

For that SSTC, there is no feedback CT, so phasing is done by swapping primary leads.  With a feedback CT, swapping either those leads or primary leads will reverse phase.  No need to do both.  UD2.7 and similar drivers include jumpers to swap phase, so neither primary nor feedback CT leads need to be swapped.  (Of course, make sure GDT is wired correctly so upper FET/IGBT is off when lower one is on and visa-versa, for each half-bridge if using a full H-bridge.  That is separate from proper feedback phasing.)

Second, (this may be an obvious question, but) if I use an overcurrent protection transformer, it is a coupled to the primary, not the secondary, correct?

Yes, correct.  OCD is to protect H-bridge FETs/IGBTs, which conduct primary current, not secondary current.