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UCC27714 used for 3Ph AC to DC , upper switch gate signal does not work properly.

ashkan rashedi
Prodigy 110 points
Other Parts Discussed in Thread: UCC27714

Hi there,

I am trying to design a fully controlled bidirectional 3ph AC to DC unit connected to power grid in US. I am using 3 of UCC27714 as the driver to drive 6 IGBTs (IXGN200N60B3). I have an issue with the higher switch Gate signal. When I send the control signal from the uC to driver to turn on the IGBT, the driver should raise the Gate voltage about 15 volts higher than Emitter so the IGBT turn on (boot strap). But it wont. As an experiment I connected the Emitter of the higher IGBT to DC GND and the driver started to work. To me, it looks like that Emitter in the middle needs a reference voltage or something.
I do not have this issue when coming from DC to AC. Means if I connect the DC source and try to turn on the IGBT that is fine, but it does not fire reversed.

I appreciate your ideas.

BTW in the schematic you see a Mosfet with diode and snubber circuit but on the actual board I have only IGBTS with no diodes no snubbers. 6 switch bridge and a driver circuit is attached.

Best

Ashkan

  • 0
    TI__Mastermind 31580 points
    Hi Ashkan,

    I have asked one of the engineers from our drivers team to respond to your post.

    Regards

    Peter
  • 0
    TI__Intellectual 2061 points

    Hello Ashkan,

    Thanks very much for your detailed illustration.

    Would you please help checking the following waveforms to understand the rootcause?

    1. What is the switching frequency?

    2. are the two IGBTs in the leg complementary switching? I mean the duty cycle is D and 1-D?

    3. Does LO works good when AC-DC? or only HO won't work?

    4. Can you help capture the waveform of HI-COM, HO-HS (Differential probe) and HB-HS (Differential Probe) and LO?

    I would like to check the HI and HB-HS to make sure the conditions are all good for normal operation.

    I look forward to your reply.

    Wei

  • 0 in reply to Wei Zhang
    Prodigy 110 points

    Thanks Wei for your response.

    Questions 2 and 3: I was testing the higher IGBTS separately and I guess that it is wrong, because it was not complementary and the lower switch was not turning on and I guess boot cap had no way to get charged. Right?  Anyway, I do it correctly now and higher switches are also firing but still have an issue. I am testing one phase for now so just concentrate on IGBT1&2&3&4. The issue is when both IGBTs 1&2 are off, the HS between them (POINT A) has a voltage of 5 volt, this is when NO AC voltage is applied yet. And at HS between IGBT 3&4 (Point B) when both IGBTs are off I get 15 volts. As a result when I try to apply AC voltage between point A and B, the DC and AC voltage suppliers start to fight and one of them gets distorted.

    1. AC frequency is 60Hz so one cycle period is about 16 ms. I would like to get a little of the first half cycle and a little of the second half cycle. I attached a picture below. That makes the switching frequency 60Hz for each IGBT.

     

    4. Here are some wave forms with explanations

    Thanks a lot for your help,

    Ashkan

  • 0 in reply to Peter Meaney
    Prodigy 110 points

    Thanks Peter. Sorry somehow missed your massage.

    Wei, thanks again for your help. I am looking forward to know if there is any update on my last massage. I tried my best to make everything clear but I know it is a complex issue. So please let me know if I could explain about anything which makes it easier for you to answer.

    Best

    Ashkan 

  • 0 in reply to ashkan rashedi
    TI__Intellectual 2061 points

    Hello Ashkan,

    Sorry for the late response. Yes, to make sure the HO turns-on properly, we shall refresh the boostrap capacitor for normal operation.

    I am reviewing all your results now and I shall give my feedback asap. Btw, would you mind send me a Email to w-zhang@ti.com. I think it might be more efficient for us to have a call to discuss more details.

    Thanks again,

    Wei

  • 0 in reply to Wei Zhang
    Prodigy 110 points

    Wei,

    Thank you very much for your help. I got busy and forgot to follow up here. Our  conversation was very helpful. This was my first post and I am not sure how it should end but I will check and try to close the conversation or flag it as answered.

    Best

    Ashkan