Question about weird gdt output waveform

Thread Starter

Ferrawinner

Joined Apr 27, 2021
8
Im trying to generate a signal to power a full bridge of mosfets through a GDT. Im using the tl494 into some gate drivers and then into a GDT. The output of the gate drivers (UCC37322) looks relatively okay except for the weird pulses after the turn off on the yellow signal. When input into the gdt the signal becomes very messed up. I was curious as to what is causing this and how to mitigate it. Im assuming a bunch of this is due to poor gdt design on my part but I wanted to know if there are any underlying things at play. I've also heard that the miller effect could be destroying my signal. I'm not sure how to reduce this so any solutions would be welcome.20210723_235649.jpg(pre-GDT)20210723_234732.jpg(GDT out - 250khz)
 

LowQCab

Joined Nov 6, 2012
1,096
What is a GDT ?
It doesn't sound like you're talking about a Gas-Discharge-Tube.
We need a Schematic and Spec-Sheets in order to make any kind of meaningful suggestion.
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Thread Starter

Ferrawinner

Joined Apr 27, 2021
8
What is a GDT ?
It doesn't sound like you're talking about a Gas-Discharge-Tube.
We need a Schematic and Spec-Sheets in order to make any kind of meaningful suggestion.
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A gdt in this case is my gate drive transformer. The gdt was homemade therefore I don't have a datasheet for it. The two other main components in this circuit are the gate drive ICs and my PWM generator. Respectively they are the UCC37322 and a TL494. All measurements shown were done under no loading of the gdt or gate drive IC's outputs. Part of the main schematic is attached below. The TL494 can practically be ignored since I do not have many problems pre-gate IC.

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gatedrivingcircuit.png
 

Veracohr

Joined Jan 3, 2011
759
Notice how the transients happen when the other signal changes? I think it’s current pulses through the capacitor on the transformer primary, transferring to the secondary.
 

LowQCab

Joined Nov 6, 2012
1,096
1)
Pins 9 and 10 need pull-down Resistors, 150-Ohms would be fine.
2)
The Enable-Pins on the Gate Drivers need to be tied to VCC, NOT the Input.
3)
The Gate Drivers MUST have a 100nf Bypass-Capacitor between VDD and Ground.
( and a 2.2uf Tantalum-Capacitor will have a lower Impedance than your 4.7uf Electrolytic )

A Gate-Drive-Transformer is a bucket of worms that I just don't have enough appreciation for.
I would certainly never try to wind one myself,
because there are quite a few other, better alternatives, that I'll try to talk You into.

Please provide your Bridge-Schematic, Part Numbers, and Load-Characterization.
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gatedrivingcircuit 1 Flat .png
 
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Thread Starter

Ferrawinner

Joined Apr 27, 2021
8
Thanks for your response. The whole GDT plan was also something I was pretty hesitant on. I stuck with it mainly because that's what I've seen replicated in circuit akin to mine. I don't have access to the schematic currently but this GDT is driving an intermediate stage of MOSFETS which will then drive the gates of IGBT bricks. The reason I stuck with a gdt is also because I didn't want to spend too much on high current ICs. My brick's input capacitance extremely high (39nf). This bridge will eventually drive a ferrite transformer. The hope is to softswitch it but for now it'll be hardswitching.
 

Veracohr

Joined Jan 3, 2011
759
So you have a PWM IC driving a gate drive IC driving a transformer driving a MOSFET driving an IGBT driving another transformer? Sounds like an awful lot of intermediate stages.
 

LowQCab

Joined Nov 6, 2012
1,096
It sounds like what You need is 2 Isolated-Switching-Power-Supplies,
one for the High-Side, and one for the Low-Side, with Optical-Input-Isolation.

Check the edited picture above for some ideas.
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