DRSSTC Fails on ZCS with Driver 1.3b

[Alex_1996]

Hello to all!

After the knowledge I have been acquiring in tesla coil construction, I have decided to build a new model. This time, a small, portable, robust and powerful DRSSTC model.

The specifications are (attached photos of the set):

Primary frequency: 195 kHz
MMC Capacitor: 66nF / 12kV
Secondary: 27 cm high and 10 cm diameter.
Secondary copper wire: 0.2mm
Number of secondary turns: 1350
Toroid: 28 cm major diameter and 10 cm minor diameter

Full bridge of 8 fga60n65smd IGBTs (previously used 4 fga60n65smd IGBTs but they failed)
OCD: 300A previously (I think it was too much for the initial 4 IGBTs). Now set to 315A for the 8 IGBTs.
DC BUS: 450VDC / 2000uF
Buffer capacitors: 2 capacitors 1200VDC / 1uF
Feedback transformers: 1:25:25

As I say in the title, when I look at the waveform of the full bridge and primary current, the expected ZCS with a 1.3 driver is not apparent. The inverter bridge turns on and off when the current is maximum and I don't understand why this happens.

I am using a 30 turn transformer connected to a 10 ohm resistor to measure the primary current. Also. I have used a 100 ohms resistor the same thing happens: no perfect ZCS is performed.

I know that the 1.3 driver does not perform a perfect ZCS, but these waveforms I have never seen before. I don't know where the problem comes from.

I don't know if it's a problem in the measurement or in the construction, but those waveforms are very strange. The circuit behaves completely the opposite of how it should behave.

I have never had anything like this happen to me before. I hope you can help me.

Best regards. Thank you very much for your help.

I attach photos of the waveforms.

[Mads Barnkob]

I think we can quickly determine this to be a measurement error, those IGBTs would have been flying off in bits or you should see some SERIOUS heat dissipation on that bridge?

Have you tried measuring with a identical CT as to what your driver uses? 1:25:25 and same burden resistor.

You could also read up upon Tektronix 30-page guide to do measurements on SMPS/Inverters : https://highvoltageforum.net/index.php?topic=111.0 maybe there was something on your oscilloscpe you forgot to reset or adjust? I recently spend 15 minutes troubleshooting on phasing of a bridge, only to find out I had one of the channels on the oscilloscope inverted in settings

[Alex_1996]

I think we can quickly determine this to be a measurement error, those IGBTs would have been flying off in bits or you should see some SERIOUS heat dissipation on that bridge?

Have you tried measuring with a identical CT as to what your driver uses? 1:25:25 and same burden resistor.

You could also read up upon Tektronix 30-page guide to do measurements on SMPS/Inverters : https://highvoltageforum.net/index.php?topic=111.0 maybe there was something on your oscilloscpe you forgot to reset or adjust? I recently spend 15 minutes troubleshooting on phasing of a bridge, only to find out I had one of the channels on the oscilloscope inverted in settings

Hello, thank you very much for your reply. The problem has finally been solved 

In the end it was a measurement error. I used CT 1:25:25 with a burden resistor of 10 ohms. The driver behaved as it should, with almost perfect zero switching.

I didn't think that the old 1:30 CT with a 10 ohms burden resistor could cause so much phase shift in the waveform during the measurement. 

I am now trying to solve a new problem. During a test the other day the system went down for a moment and after that the discharges started to lose power and there was like an interference in the signal. I replaced everything: driver and full bridge (that's why I now use 8 IGBTs) thinking that this might be where the problem is coming from, but it still persists.

I think it might be due to the battery of my interrupter, which may need to be recharged. I am currently recharging it. I will try and let you know here