UD 2.7c "sawtooth" output

[Felix M.]

Hello everyone,

For around 2 months now I've been building my first drsstc. Lats week my bridge showed its first life sign with Vbus=60V and OCD=200A (it never tripped).
However I can't conduct more power tests due to the strange looking output of my UD 2.7C, since it might cause IGBT damage a higher power.
I have hooked up a signal generator at my desired frequency to simulate feedback. I did some scoping but couldn't find the problem.
This shows the UD's output with a 10ohm load resistor:


The 24V regualtor does its job and the ripples are minimal so my guess would be the logic. Here is an other shot of the MOSFET driver IC.


The signal on the gate looks like this:


After going to 24-25V, it ramps down to around 18V. (There is a 5.1ohm gate resistor, a diode for faster discharge and some 33V zener diodes for protection)


I appreciate every peace of information

Cheers

[Weston]

What are the specifications on your gate drive transformer / what does it look like / what core did you use?

The waveforms look like the magnetizing inductance of your GDT is too low (too few turns / core with too low an Al value). A low magnetizing inductance causes a high magnetizing current. The IR drop due to the magnetizing current causes the waveforms to look sawtoothed.

[Felix M.]

I'm using a large N30 ferrite core with 15 windings for my fullbridge setup. It's driving two SKM200GB128D bricks.

I think the problem isn't actually the GDT because like the first picture shows, the output without the GDT is allready not a square wave.
The GDT without any gates connected spits out the same sawtooth wave it gets as an input.

[Late]

Try probing along the signal path and see if waveform in all places looks okay. Mainly check the input of the high speed comparator, both outputs of it, inputs to the AND gate and inputs and outputs of the gate drive ic. Send all those scope shots here.

I have hooked up a signal generator at my desired frequency to simulate feedback.

What signal are you feeding in the input? sine or squarewave or smth else?

[Felix M.]

I'll do the scoping tomorrow, it's already late.

The feedback is a 80kHz sine wave at around 6-8V

[Late]

I'll do the scoping tomorrow, it's already late.

Okay, sounds good. No rush

The feedback is a 80kHz sine wave at around 6-8V

That sounds correct

[Felix M.]

One thing I forgot to mention: I don‘t have the SMD but the THT version of the UD 2.7C. It probably doesn‘t matter anyways but I saw you have experience with this driver in particular so if there‘s anything you had problems with, please let me know.

[Late]

It probably doesn‘t matter anyways but I saw you have experience with this driver in particular

Yeah it doesn't matter, all the parts and circuit is the same just tht. And I've made the driver 2 times and in neither of them did I have issues that the output is sawtooth. Usually when the output is sawtooth it means that the GDT is saturating.

I don't know of you mentioned it before but how many turns did you have on the GDT and what wire did you use to wind it? Also what's the core size and what igbts are you using?
Maybe send a pic of the GDT

[davekni]

Here is an other shot of the MOSFET driver IC.

Looks like roughly 4V amplitude.  If I'm understanding correctly that this is output of UCC27423 chip, it should be 9V amplitude.  Suggests one of three possible issues:
     9V supply is low (4 to 5V).
     UCC27423 chips are bad.
     Something is loading down UCC27423 chip output(s) excessively.

Good luck with debug!

[Anders Mikkelsen]

The UD output stage is not designed to be loaded by a resistor, but by the gates which are mainly capacitive. A 10 ohm resistive load in series with the 2.2 µF GDT coupling capacitor gives a 22 µs time constant, which would explain the sloping of the gate driver output waveform. If you use a more representative load, maybe some ten nanofarads in series with a few ohms, the waveforms should be closer to reality.

[Felix M.]

Thanks for your advice. I'm gonna do some test and post some scope shots soon.

[AeraCura_]

I believe what you're seeing is expected. See https://www.loneoceans.com/labs/ud27/,

Specifically the section labeled "Above shows the secondary of the GDT across a small resistor at 50kHz and 500khz." which looks exactly like your image with the 5.1 Ohm resistor.

Further, if you look at this image. https://www.loneoceans.com/labs/ud27/ud27atopview2.jpg you can see that Lone Oceans is using the exact same 5.1 Ohm resistor on the GDT.

[Felix M.]

Further, if you look at this image. https://www.loneoceans.com/labs/ud27/ud27atopview2.jpg you can see that Lone Oceans is using the exact same 5.1 Ohm resistor on the GDT.

The unloadet output of the driver should be a square wave:


In my case there is no GDT connected and it's already a sawtooth:

[Late]

In my case there is no GDT connected and it's already a sawtooth:

What does the input and output of the driver IC looks like?

[Felix M.]

Driver IC input:


Driver IC output:

[Late]

This looks very wrong. At least looks like the driver IC and the output stage isn't the issue. Could you probe along the signal path? from the input up until the driver ic.

Also what are you using as your interrupter signal?

[Late]

Here i attached the UD2.7C THT schematic. Probe in all the places I've circled red. for the driver IC inputs and outputs probe both at the same time using both of your scopes channels.

[Felix M.]

Thanky Late, I'm off to the lab 

[Felix M.]

Feedback input:


Ultra fast comparator input:


Ultra fast comparator output:


74hc08 output:


UCC27243 input:


UCC27243 output:


It's getting more and more strange since the 74hc08 output is directly connected to the UCC27243 input so the signal should be the same but it's not. For some reason my trigger and othe functiones of the scope didn't do what they are supposed to do while testing. So lastly I measured the output of the driver with channel 2 an I got this:


The scope was likely the problem all the time because it measures those ramps in square waves. It's good because it means the driver works but it's also really bad because now I have to troubleshoot on my scope. I will runn some further tests to verify this but it makes sence to me.

[Late]

Comparator output and 74hc08 output looks very weird and not like it should be. Looks like there's a problem with your ultra fast comparator.
Looks like one of the inputs and outputs for the driver ic looks okay

Have you connected the phase setting jumpers correctly? and are both of them installed? Also could you send a pic of the driver board?