GDT Experiments

[Uspring]

@profdc9:

I don't think it's a problem to have zero voltage in the gate drive transformer.

I should have checked IGBT specs. They indeed turn off at 0 gate voltage. But I still have a little doubt: Think of a hard switching turn off situation. And suppose, there is a little internal inductance between emitter of die and package outside. The dropping collector-emitter current will cause a negative voltage jump at the emitter, which fakes a positive gate voltage. Therefore it might be an advantage to drive the gate to negative voltages as fast as possible. Just a guess.

[profdc9]

Well you have to realize that there's a lot there in the PCB Pack and it could always use some improvement.  But I wanted to try to create something so that all the circuits one needs could be in one place like Mads with his series of guides.  One of the hardest parts about building a Tesla Coil is trying to figure out where the accurate information is, and this was a struggle for me getting started, so feedback like yours is a big help to try to make the situation better.

As for the dead time issue, here is my understanding of it.  When driving an inductive load like a tesla coil primary, one achieves ZVS (zero voltage switching) for a half/full bridge by having a dead time between the turning off of the upper/lower transistor and turning on the lower/upper transistor.  When both transistors are shut off, there is still current flowing from the inductor.  The current is diverted from the transistor that is now shut off to the the body diode or reverse pack diode of the other transistor.  Since the other transistor is now in forward conduction, its voltage drop is near zero.  So you do get some energy loss from excessive dead time which is current X diode drop voltage X dead time duration.  Now the other transistor is turned on at nearly zero voltage which results in a lower switching loss.   The disadvantage of excessive dead time is you get a little more loss in the reverse diodes but the benefit of ZVS switching for the on condition and a low probability of shoot-through.

Dan 

Well ! This certainly took a different turn, I wanted to make a test circuit for GDT’s, and it resulted in a circuit change !

Dan, thank you very much for your help with this. More than likely I will use this driver in a TC build in the future so thanks for troubleshooting and making changes.

One thing that I am not clear with is jumper settings, so far I figured out AC/DC input jumper, but others are a bit harder to figure out from the schematic ( for me at least ).

I’d like to do a test with FG connected to the antenna input next.

[Thunderstruck]

Well you have to realize that there's a lot there in the PCB Pack and it could always use some improvement.  But I wanted to try to create something so that all the circuits one needs could be in one place like Mads with his series of guides.  One of the hardest parts about building a Tesla Coil is trying to figure out where the accurate information is, and this was a struggle for me getting started, so feedback like yours is a big help to try to make the situation better.

As for the dead time issue, here is my understanding of it.  When driving an inductive load like a tesla coil primary, one achieves ZVS (zero voltage switching) for a half/full bridge by having a dead time between the turning off of the upper/lower transistor and turning on the lower/upper transistor.  When both transistors are shut off, there is still current flowing from the inductor.  The current is diverted from the transistor that is now shut off to the the body diode or reverse pack diode of the other transistor.  Since the other transistor is now in forward conduction, its voltage drop is near zero.  So you do get some energy loss from excessive dead time which is current X diode drop voltage X dead time duration.  Now the other transistor is turned on at nearly zero voltage which results in a lower switching loss.   The disadvantage of excessive dead time is you get a little more loss in the reverse diodes but the benefit of ZVS switching for the on condition and a low probability of shoot-through.

Dan 

[/quote]

To be honest, I stumbled on that issue, I was not even aware that is an issue, but I am glad that it lead to an improvement.
So far I built 2 circuits out of your github pack, interrupter and driver, both working well ( yet to program Attiny85 and try Midi function on the interrupter )
I also had Pcbway manufacture your half bridge and full bridge PCB's, so I might try to put one of them together as well.

Anyway, I did more tests on my driver board.
Image 1 shows waveform with dead time of 1.16us
I swapped UCC37322's with TC4420's to see what is going to happen, and as Dexter pointed out, TC's are slower therefore the problem  got worse, as you can see in Image 2.
Not only did the dead time increase to 2.2us, the waveform itself has a weird spike in it.

Then I put the UCC's back and replaced D3 and D4 with 1N4148's, dead time reduced to 230ns - Image 3
I also added 270 Ohm resistor to get rid of the voltage spikes.

Image 4 shows waveform at 200kHz with FG connected to the antenna input
Image 5 shows all 4 GDT outputs running, only CH 1 has 270 Ohm resistor connected, rest show voltage spikes.

[profdc9]

I have to confess I had an ulterior motive for designing the SSTC circuit this way.  I hope to try to use to make an induction heater using the SSTC board.

Basically, use a feedback coil rather than antenna near the workpiece to drive it into resonance.  So I designed it with some stuff that may not have been necessary.  I did not really worry about the dead time but I thought I should look into it after you brought it up.

If you need even less dead time, you can swap R5 and R11 for 4.7k resistors.  The limit is that the overcurrent circuit can only sink 6 mA and so if the resistors are too low resistance, the overcurrent protection does not work.

Dan

[Thunderstruck]

I have to confess I had an ulterior motive for designing the SSTC circuit this way.  I hope to try to use to make an induction heater using the SSTC board.

Basically, use a feedback coil rather than antenna near the workpiece to drive it into resonance.  So I designed it with some stuff that may not have been necessary.  I did not really worry about the dead time but I thought I should look into it after you brought it up.

If you need even less dead time, you can swap R5 and R11 for 4.7k resistors.  The limit is that the overcurrent circuit can only sink 6 mA and so if the resistors are too low resistance, the overcurrent protection does not work.

Dan

I have no idea how much dead time is enough for the correct operation of a SSTC, I suppose I’ll just have to build one and see what happens, then adjust accordingly.

Since you are looking for feedback, there is one more thing that I noticed when assembling the driver.
Two pots are very close to each other, which is not a problem if you are using 16mm pots or panel mounting them, but for some reason in Australia I can only get 17mm pots which are just a little bit too big if you mount them directly onto the PCB.
I had to remove some material from each of them to make them fit.
I am not sure if anyone else had a similar problem with slightly larger pots, but moving each pot only by 1mm in opposite directions would help.

Also, one of the pots will come very close to touching the opto-isolator if opto-isolator is mounted into the ic socket. Again, this is not a problem if pots are mounted onto a panel.

[profdc9]

I moved the OC LED backwards so the potentiometers may be better separated, there should be plenty of space now.

Dan

[Uspring]

profdc9 wrote:

As for the dead time issue, here is my understanding of it.  When driving an inductive load like a tesla coil primary, one achieves ZVS (zero voltage switching) for a half/full bridge by having a dead time between the turning off of the upper/lower transistor and turning on the lower/upper transistor.  When both transistors are shut off, there is still current flowing from the inductor.  The current is diverted from the transistor that is now shut off to the the body diode or reverse pack diode of the other transistor.  Since the other transistor is now in forward conduction, its voltage drop is near zero.  So you do get some energy loss from excessive dead time which is current X diode drop voltage X dead time duration.  Now the other transistor is turned on at nearly zero voltage which results in a lower switching loss.   The disadvantage of excessive dead time is you get a little more loss in the reverse diodes but the benefit of ZVS switching for the on condition and a low probability of shoot-through.

Certainly a nice way to avoid shoot through. And, as you write, turn on is at 0 voltage for inductive loads. In a DRSSTC, though, the current might change polarity during dead time, so you won't switch on at 0 voltage. That will happen at comparatively low currents, so it probably doesn't matter much.

[Teravolt]

what is the input vs output waveform look like on the UCC's