Author Topic: Measuring DRSSTC gate - how to?  (Read 260 times)

Offline patracy

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Measuring DRSSTC gate - how to?
« on: November 02, 2022, 10:16:50 PM »
I have searched, but I'm not finding anything that clearly spells out the how to when it comes to measuring the inverter of a DRSSTC.  I do understand that I need to measure the bus to check for ringing and adjust for phase lead possibly.  I understand that you need to measure the current of the primary on one lead of the oscilloscope with a PT. And the other simply needs to measure the inverter bridge output.  I understand that I need to adjust phase lead to take care of any ringing on the IGBTs.  And measuring the primary current with the PT will let me see the current peaks and adjust my OCP from there.  (Although I believe I can simply limit that by adjusting the voltage on the driver at the probe points of the comparitor circuit)

So I started out with a Aliexpress coil.  Blew a number of IGBTs simply pushing it to hard.  I have built a new inverter (This one to be exact https://github.com/WaskaLabs/Fullbridge-Bus-V2 with FGA60N65SMD IGBTs).  I also swapped GDTs as the cheap Chinese made one that came from the first coil showed some ringing when I scoped it with a signal generator.   The new GDT does not show all the ringing of the old one.  I also used cat6 wire to wire from the GDT to the IGBTs.  And made sure the lengths of wire were all the same. 

Now for equipment.  I have a hantek DSO5072P scope.  Currently only have 10x probes for it, but I ordered a 100x.  I also have a variac as well to power up/limit the voltage of the bus.  I also have a isolation transformer I can use with the scope as well.

So is there a "how to measure your DRSSTC for dummies" guide or video out there? 

And for my specific case, I am correct that one lead simply is connected to the output of the inverter bridge to the primary correct?  Do I probe before or after the MMC?  Can I probe with the 10x if I keep bus voltage very low or should I just wait for the 100x probe?  Oh yeah, the primary current PT.  I was going to try to get by with simply looping the ground loop of the probe around it and try to measure from that.  Or can I use the coils PT?  I probably have a PT laying around somewhere I could alternatively use.  (I think I know where it's at from a broadcast transmitter I scrapped out)

Offline davekni

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Re: Measuring DRSSTC gate - how to?
« Reply #1 on: November 03, 2022, 04:44:48 AM »
Quote
I understand that you need to measure the current of the primary on one lead of the oscilloscope with a PT. And the other simply needs to measure the inverter bridge output.
What does "PT" refer to?  If a current transformer, I think those are usually abbreviated "CT".  Current transformer with appropriate burden resistor across secondary and scope across burden resistor.

Quote
I understand that I need to adjust phase lead to take care of any ringing on the IGBTs.
Yes.  However, ringing is also determined by parasitic inductance of IGBT bridge.  Large parasitic inductance can make spikes even with proper phase adjustment, and make much worse spikes when mis-adjusted.  Quick look at the linked ECB appears reasonable (presuming IGBT leads are short - IGBT body close to ECB edge).

Quote
I also have a isolation transformer I can use with the scope as well.
Won't help with scope.  If floating the scope, scope safety ground needs disconnecting, not line power isolated.  Better to use isolation transformer on variac input or output to isolate H-bridge bus supply.  Then scope can remain grounded, which is safer.

Quote
And for my specific case, I am correct that one lead simply is connected to the output of the inverter bridge to the primary correct?  Do I probe before or after the MMC?  Can I probe with the 10x if I keep bus voltage very low or should I just wait for the 100x probe?
With isolated H-bridge bus supply, scope probe ground clip to VBus- and scope probe tip to one output.  Check the other output too just to be sure there isn't any issues.  (One output goes to MMC, other to primary.  Don't scope connection between MMC and primary, as voltage is much higher there.)  10x is OK as long as voltage is low enough to be within probe rating.
« Last Edit: November 03, 2022, 04:46:56 AM by davekni »
David Knierim

Offline patracy

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Re: Measuring DRSSTC gate - how to?
« Reply #2 on: November 03, 2022, 12:54:15 PM »
Sorry I meant CT. Not sure why I was thinking potential transformer.  Not all the case here. 

The leads from the IGBTs do have a little length to them.  The ECB is raised up off the heatsink with spacers of course.  They're mounted and the leads laid up on top of the board and soldered there.  The past cheap inverter I used had through holes.  They were a pain to replace.  So about half the leads are on the PCB, the other half make up the span to the IGBT on the heat sink.  A trade off.  But hopefully not too terrible.

I'll use the isolation transformer on the variac side of things.

Vbus- for ground, then probe one of the outputs.  Got it. 

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Re: Measuring DRSSTC gate - how to?
« Reply #2 on: November 03, 2022, 12:54:15 PM »

 


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