Author Topic: Question with GDT  (Read 302 times)

Offline omega45820

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Question with GDT
« on: January 02, 2023, 11:47:26 PM »
Hello all im new here and this is my fist post. My scope is a Siglent SDS1202x-e.My Coil is a EV Micro Brute with Secondary around 200Khz. I was trying to test my setup other night without secondary and top load and blew my 60N60's. My interrupter is set at between 30Hz and 160Hz. Ive been testing my GDT with my Signal gen and at 150HZ the output is a no go it looks like but at 200Khz not to bad Square wave on output. Is this normal and when the driver pickups the feedback will it be right? When i hookup the GDT to driver even on output at Gate its still the same kinda. I wanted to try to test my setup with the primary hooked up and power but i don't know how to setup the probes to safely look at it. I'm still kinda new with scopes
Thanks for any and all help
Forgot to add My Fer-rite is a TDK T38 21.3Uh
I also have a T38 12Uh and T37 6.97







« Last Edit: January 02, 2023, 11:51:38 PM by omega45820 »

Online davekni

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Re: Question with GDT
« Reply #1 on: January 03, 2023, 05:41:20 AM »
Quote
My interrupter is set at between 30Hz and 160Hz.
Use 30Hz or even lower for testing.  2Hz or so is ideal.  Allows real-time scoping (2 captures per second) with reduced chance of mistakes damaging parts.  Initially test with short enable pulse width, on the order of 20-50us long.  Further reduces chance of part damage during testing.

Quote
Ive been testing my GDT with my Signal gen and at 150HZ the output is a no go it looks like but at 200Khz not to bad Square wave on output. Is this normal and when the driver pickups the feedback will it be right? When i hookup the GDT to driver even on output at Gate its still the same kinda.
GDT is designed for 200kHz.  150Hz is way below GDT capability.  Also, GDT is designed for square waves.  Looks like 10% duty cycle pulses sent to GDT.  Needs to be 50%, which is the definition of a square wave.

For testing, I suggest setting signal generator for 200kHz square wave, +-5V amplitude.  Connect signal generator output to end of C1 adjacent current transformers.  Run interrupter at low duty cycle as above (short enable pulses repeated at low rate).  Then scope GDT inputs and outputs and IGBT Vge for both IGBTs.  All without power to the half-bridge yet.  Only after that all looks good (including that the two IGBT Vge signals are opposite phase) should power be applied to half-bridge.  Start with low voltage such as output of a small AC transformer.  That leaves half-bridge floating for easy scoping.  (Still all with signal generator to C1.)  Ideal to test with primary winding disconnected before adding it.

Edit to add one more general thought about possible cause for fried IGBTs:
Quote
I was trying to test my setup other night without secondary and top load and blew my 60N60's.
I can't find the post now, but I think an issue came up with similar UD1.3 driver when feedback polarity is reversed.  There is enough delay through feedback path (CT load diode capacitance, HC14, gate driver) that reverse feedback could oscillate with this delay providing the missing 180 degree phase shift.  Oscillation is much higher frequency than normal TC operation, so damages gate driver chips and/or IGBTs.
« Last Edit: January 04, 2023, 05:45:38 AM by davekni »
David Knierim

Offline omega45820

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Re: Question with GDT
« Reply #2 on: January 04, 2023, 03:31:29 PM »
Ok thanks for the reply I will try these things this evening when I get off work and post screen shots. I'ma going to use the signal generator to run signal into the feedback and check and see what the gates look like without the igbt installed and with them installed and go from there.

Offline omega45820

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Re: Question with GDT
« Reply #3 on: January 04, 2023, 11:33:30 PM »
I had some time to test with the new IGBT on the board and using my signal gen through the feedback circuit. Some overshoot on the wave but this is without power on the IGBT side




Online davekni

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Re: Question with GDT
« Reply #4 on: January 05, 2023, 06:38:23 AM »
Quote
I had some time to test with the new IGBT on the board and using my signal gen through the feedback circuit. Some overshoot on the wave but this is without power on the IGBT side.
Looks reasonable.  Undershoot (negative overshoot) is normal since diodes bypass gate resistors.  Positive overshoot isn't too bad.  Could be reduced with larger value gate resistors (R41 and R42).  Larger gate resistors would also increase dead time (time from IGBT turn-off to turn-on) which looks a bit low.  May be fine given plenty of these coils have been built before.  Overshoot/undershoot can also be reduced by improving GDT winding.  Slightly more complex than MicroBrute instructions, but not too hard:  Wind with two parallel primary wires, one twisted together with each secondary wire before winding.  Wind the two twisted pairs, then wire as shown in instructions, with two primary wires connected in parallel to primary ECB pads.

+-13.5V is also a bit low compared to normal +-15V for IGBTs.  Partly due to design of MicroBrute, as UCC3732x chips don't drive full current all the way to 15V.  Also could be due to scope probe compensation adjustment not being accurate.
David Knierim

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Re: Question with GDT
« Reply #4 on: January 05, 2023, 06:38:23 AM »

 


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