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Gate Drive Waveform gets distorted when SMPS DC power supply is turned ON

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renga92

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Hi everyone. I am trying to design a Full bridge DC-DC converter. Vin = 24v(Battery) and gate drive pulse of 50Khz is passed through a Gate drive Transformer. One strange thing I observed was that my Gate drive pulse gets distorted(Blue Waveform) when I turn on my 24v (Battery).
IMG_20160124_135124875[1].jpg
IMG_20160124_135115344[1].jpg
I am probing my Gate drive pulse from the primary of the Gate drive Transformer and the Yellow Waveform is probed across the secondary of the Isolation Transformer. Can you please tell me why it is so and how to avoid it.

My initial guess is the drain source stray capacitance when is causing the coupling. But I am not sure if it is the reason behind this.
 

The effective gate charge that must be delivered by the gate transformer will considerably increase when the supply voltage is turned on. In addition, transformer inter-winding capacitance can have an effect.

The first (no supply) waveform already suggests that the driver circuit is much too weak. It takes several microseconds to charge the gate capacitance completely.
 

Your top waveform shows barely any gate drive waveform in the sec. I think you may need to post GDT schem.
 

Hi FVM. Thanks for shredding some light on it. Can you give me some reference from where I can learn about it ? I think the gate transformer I designed is poorly done. Like you said I tried with some other gate transformer that I have build some time back and it is work properly in that gate drive transformer.
 

I believe there are some reference circuits with gate transformers in switch mode controller applications notes, e.g. in TI power seminars. Personally I didn't use it in the last 20 years or so.

As you said that the blue waveforms are from the transformer primary, it seems to me that the problem is caused by a weak driver rather than a poor transformer design. How are you driving the transformer?
 

I have resource on GDT's for you...simulations are in ltspice
 

Attachments

  • Gate drive.zip
    3 MB · Views: 91
  • Gate drive transformer.zip
    3.1 MB · Views: 120
Hi Treez. Thanks a lot. I will now head to running this simulations. I have some progress in debugging this problem. I said the other gate drive transformer which was working normally. Now when I connected to the H bridge, the gate drive pulse also drooped for the second drive transformer. This problem started happening once I powered the H bridge converter from 320Vdc. It is Mosfet that is causing the voltage to droop. I think I have damaged the Mosfet. Correct me if I am wrong.
 

Does it mean Diff probe problem exist when the DC bus voltage for H bridge is turned on or even when it is turned off. Because the voltage droop was not occurring previously when the diff probe was probed to view the waveform using scope.
 

There may be problems with the differential probe. Actually, the "yellow" oscilloscope traces are mysterious in both screenshots, and the description how they have been captured isn't particularly clearly.

It's however rather clear that the primary waveforms are already distorted which is unlikely a transformer neither a probe problem. Without fixing the driver circuit, you will hardly achieve good results.
 
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