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Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
Sounds like you're making progress. I also got all the parts from mouser, but it's very possible I killed the components :( I had to replace IC 9 as it was not producing any output. I cannot recall which diode, but it was toast - essentially a short circuit. I probably cooked it when soldering. By following the signal from IC to IC I was able to find the general area of where things stopped working. Then by probing all components in between I located the offender.
Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
Graham Armitage wrote ...
Sounds like you're making progress. I also got all the parts from mouser, but it's very possible I killed the components :( I had to replace IC 9 as it was not producing any output. I cannot recall which diode, but it was toast - essentially a short circuit. I probably cooked it when soldering. By following the signal from IC to IC I was able to find the general area of where things stopped working. Then by probing all components in between I located the offender.
It seems to be the path from the optical receiver that is the problem i wonder.
All four MOSFET outputs puts out a square wave of about 24V on the scope, but this is something ive just noticed and i will try and post some more screenshots soon. Ive noticed that the second output on each pair is above 0V.
The first output on each pair will put out a square wave with 0V in the centre (+/-12VAC on each side of 0V) but the second pin on each output will give me a square wave above the 0V mark (between 0V and +24V)
If that makes sense to you, 0V is where the trace line will sit with no signal.
Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
What do the outputs look like on IC 9 and 10 (Pin 5 and 7)? Is there any DC offset or does that look good? How do those outputs compare to the U1-4 outputs? Check C30 as I believe this should eliminate DC bias and maybe what you are seeing if it's not working. I am no expert on this driver - just trying to share some similar debugging experiences I have gone through in the past.
Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
Graham Armitage wrote ...
What do the outputs look like on IC 9 and 10 (Pin 5 and 7)? Is there any DC offset or does that look good? How do those outputs compare to the U1-4 outputs? Check C30 as I believe this should eliminate DC bias and maybe what you are seeing if it's not working. I am no expert on this driver - just trying to share some similar debugging experiences I have gone through in the past.
I think ive found the issue and my friend who is an electrical engineer pointed this out to me, but i was not overly concerned until he told me this, but...
Basically all four MOSFET outputs should put out the same identical square wave, except that the second MOSFET on each pair inverts it.
What im getting on the output of U2 and U4 seems to be a shift up about 12V higher than the outputs from U1 and U3. So if you take the trace line at 0V, the square wave is perfectly balanced on U1 and U3 with +/-12V either side of the 0V mark on the grid.
With the output on U2 and U4, im getting the same amplitude square wave, only this time its 12V higher sitting between 0V and +24V peak to peak, and he tells me this is wrong and they should all be the same as U1 and U3.
Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
Got my UD all working properly now it appears. However ive got alot of ringing on the GDT. Loneoceans told me it should be much less than what ive currently got. This should drop when connected to the gates of my IGBT's, but i believe i can get it down if i shorten the leads.
How long is too long? Both ends for the primary and secondary are 300 long. If i have to shorten them, I will have to find somewhere else to mount the GDT other than the heatsink.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
That looks perfectly fine, running a transformer unloaded will ofc create overshoot and oscillations in a much higher frequency as it oscillates with the parasitic capacitance of your probes.
You could experiment with different gate resistor values to get a faster rising edge or dampen the overshoot even more.
Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
Mads Barnkob wrote ...
That looks perfectly fine, running a transformer unloaded will ofc create overshoot and oscillations in a much higher frequency as it oscillates with the parasitic capacitance of your probes.
You could experiment with different gate resistor values to get a faster rising edge or dampen the overshoot even more.
Yes loneoceans also confirmed to me that its normal, so i hope i can test everything out tomorrow with a UPS transformer and adjust the phase lead initially at low voltage, and hope to perhaps see some short sparks from the secondary.
Im using 5.1 ohm currently for the gate resistors. How high can you typically go? I take it that overshoot is undesired, but i expect its difficult to remove completley?
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Building my first DRSSTC - Updated thread...
