Odd MOSFET Driver Behavior

[KrisPringle]

I've been testing my driver for my first SSTC and am having some very weird problems with my MOSFET drivers. I've looked through the forum and never seen anything like it. It seems like each driver has a little extra blip of on-time after the actual on-time.

I attached some images of relevant scope traces and the schematics I'm using (Profdc9's). For the unloaded case, I'm scoping the low and high side of the GDT driver outputs with no transformer on the outputs. For the loaded case, I'm scoping the outputs with the gate drive connected to the full bridge.

I have no idea what would be causing this. Any clues? The MOSFET drivers get pretty warm after a few minutes of running the driver (CW mode, 15V).

[davekni]

I have no idea what would be causing this. Any clues?

I might have time to make a better answer in a day or two.  Most likely your issue is with physical circuit construction.  Too much wire inductance in driver ground and supply connections, bypass caps not close enough, etc.

The MOSFET drivers get pretty warm after a few minutes of running the driver (CW mode, 15V).

CW mode often stresses drivers, so warm isn't unusual for CW operation.

[KrisPringle]

I might have time to make a better answer in a day or two.  Most likely your issue is with physical circuit construction.  Too much wire inductance in driver ground and supply connections, bypass caps not close enough, etc.

Not saying this definitely couldn't be true, but others seem to have had success using the same PCB layout. https://highvoltageforum.net/index.php?topic=1898.0

Is there anything I could scope that would confirm or reject your hypothesis?

I've attached a picture of the board as well as the board layout.

[davekni]

I've attached a picture of the board as well as the board layout.

Layout looks good.  I'd been guessing you were using a hand wired breadboard for my previous comment.
However, this circuit design looks quite sensitive to noise.  TC4420 input traces are slow, especially falling edge, where voltage must fall to ~1.5V for output to switch.  With only 0.3V hysteresis, it doesn't take much noise (such as ground bounce inside TC4420) to make the extra output edges you see.  Notice that these noise pulses are all on falling edges.
40106 is slow and 4.7k series resistors will slow output further by the time it reaches TC4420.
One option is to add ~1k to 1.5k resistors from TC4420 inputs to ground.  That will attenuate signals to be roughly symmetric around TC4420 input threshold.
Sockets contribute to ground bounce inside chips.  Convenient for chip replacement, but have a significant down side.

Good luck with debug!  Thank you for posting details.

[KrisPringle]

Thanks for the help!

I will try removing the sockets. Also, the BOM called for 10k on those resistors but the schematic says 4.7k. I think I’ll also try replacing those 10k’s with 4.7k’s.

I will report back with results.

[davekni]

I will try removing the sockets.

It's the driver (TC4420) sockets that are of particular concern.  The other low-power chips are much less likely to have any issues due to sockets.

Also, the BOM called for 10k on those resistors but the schematic says 4.7k. I think I’ll also try replacing those 10k’s with 4.7k’s.

You could try even lower than 4.7k.  Also you may want to try pull-down resistors after 4.7k.  Driver chip input signal could be even better with a combination of pull-down resistors after 4.7k and small caps across 4.7k resistors to peak high frequencies, something in the range of 33pF to 100pF.  See what makes driver chip inputs look best (sharp rise and fall transitions, especially in region around 1.5V which is driver input threshold voltage).

[KrisPringle]

In an effort to not have to "jankify" the PCB, I tried just removing the MOSFET driver sockets and lowering the resistance to 4.7k. This didn't help, and the issue was still occurring. I wonder what is different about my board compared to others who have had success.

I tried using a 4.7k pull down resistor, and the GDT output looked good unloaded, but loaded, something was still wrong. (I think still the same issue as my original post, just to a lesser degree? I'm not sure through what mechanism the solution identified in the next paragraph works if it isn't the same issue.)

Unfortunately, I am not into much for hobby electronics outside this project, so I don't have a bunch of random capacitors lying around. I found a single 4700nf capacitor to throw across one of the 4.7ks, and voila! The rising edge of the waveform looks pretty good! Maybe a little over damped? The high frequency ringing before the output goes high: is that a problem? I don't have another capacitor to check if that's a symptom of the other driver not having the capacitor across it's resistor yet.

[davekni]

I tried using a 4.7k pull down resistor, and the GDT output looked good unloaded, but loaded, something was still wrong. (I think still the same issue as my original post, just to a lesser degree? I'm not sure through what mechanism the solution identified in the next paragraph works if it isn't the same issue.)

I'd guess that the 4.7k pull down resistors are working fine.  Loaded waveform is likely due to IGBT miller capacitance loading driver chip outputs.  However, it could be that 4.7k isn't enough pull-down.  Any ~2k resistors around?  Or enough 4.7k resistors to parallel 2 or 3 for each pull-down?

I found a single 4700nf capacitor to throw across one of the 4.7ks, and voila!

Did you mean 4700pF?  Even if so, that is still large.  Probably OK when used without a pull-down resistor.  I was suggesting a much smaller capacitor combined with a pull-down resistor.  However, combining the two does require getting capacitance correct to avoid driver chip inputs glitching too far below ground.

The rising edge of the waveform looks pretty good! Maybe a little over damped?

Once the input is clean enough to not glitch due to driver chip ground bounce, any remaining characteristics of driver outputs are due to driver output impedance and load impedance (GDT and IGBT gates).

The high frequency ringing before the output goes high: is that a problem?

Probably not an issue.  For such relatively small ringing, it is difficult to know if it is real or magnetic pickup in loop from scope probe ground lead to probe tip.

I don't have another capacitor to check if that's a symptom of the other driver not having the capacitor across it's resistor yet.

Probably best to have both channels modified to be the same.  There can be some advantage to a slight delay in one driver relative to the other.  However, any of these fixes (caps or pull-down resistors) will affect rising and falling delays differently.  If you have any lower-value resistors (ie. 2k), I'd guess that to be a more reliable fix than capacitors alone.  (If you have extra 4.7k resistors, two in parallel for each pull-down would be better than just one each.)  The pull-down resistor solution should move duty cycle closer to 50%.

[KrisPringle]

I'd guess that the 4.7k pull down resistors are working fine.  Loaded waveform is likely due to IGBT miller capacitance loading driver chip outputs.  However, it could be that 4.7k isn't enough pull-down.  Any ~2k resistors around?  Or enough 4.7k resistors to parallel 2 or 3 for each pull-down?

Gotcha, I have a resistor pack. The pulldown resistors are at 2k now, and unloaded output is good. No capacitors on it.

Loaded output still has a weird jump on one MOSFET driver. If this indeed from Miller Capacitance, that peak should reduce dramatically when the bridge is at mains voltage? Would you expect the traces to be asymmetric? The spikes are only on of the outputs.

[davekni]

Loaded output still has a weird jump on one MOSFET driver. If this indeed from Miller Capacitance, that peak should reduce dramatically when the bridge is at mains voltage? Would you expect the traces to be asymmetric? The spikes are only on of the outputs.

This capture does look less like Miller capacitance, more like driver chip ground bounce.  Loading may be adding just enough additional ground current spike to cause trouble.  Perhaps one chip has slightly higher inductance in ground path or has slightly faster output transition or more shoot-through current.  Previous loaded captures may have been ground bounce too.  They looked more like flat spots than spikes, which is typical of Miller capacitance.