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UCC28070: Current Loop Instability

Tony Bussan
Prodigy 70 points
Part Number: UCC28070

Hi,

I am approaching 80% load in my testing and modifications.  I am now seeing what initially looked like an intermittent current spike on the AC input current. 

I was able to trigger on this while monitoring the CSA and GDA, just before the FETs blew, again. 

The current is actually going into destructive resonance.  The current signal is quite strange, almost like it has inverted. 

My duty cycle limit is approximately 95%.

Can anyone interpret what has happened here?  I'm not sure where to start.

Thanks,

Tony

Current spike, Channel 3- Mains Current:

Smaller time scale. Channel 1- GDA, Channel 2- CSA, Channel 3- Mains Current:

  • 0
    TI__Mastermind 34465 points
    Hi Tony,
    It may be that the current transformer is saturating.
    Around the zero crossings the duty cycle is maximum and the current transformer has a short time to reset.
    You should look at the output of the CT's and check what happens when the current spike occurs.
    If the CT is saturating you should re evaluate your reset and/or lower your Dmax value.
    Did you use the Excel calculator for your design ?

    Regards
    John
  • 0 in reply to John Griffin1
    Prodigy 70 points

    Hi John,

    The blue trace above is the CT output.  I think it is already past the point of no return.  Can you point me to any examples of good/bad CT reset behavior before total loss of current sensing? 

    Yes, we used the excel calculator.  I am examining the reset circuit, one of my colleagues put it together, it uses the diode/zener combination from PM4259.  I may change it to the basic R C for simplicity.

    Thanks,

    Tony

  • 0 in reply to Tony Bussan
    TI__Mastermind 34465 points
    Hi Tony,
    I dont have any scope pictures for what you ask.
    Your CT needs to be a high frequency type with adequate Volts.Secs rating forr your application and you need to ensure that the CT can adequately reset during the max on time of the FET. Those are the main requirements for this part.

    Regards
    John
  • 0 in reply to John Griffin1
    TI__Mastermind 34465 points
    Hi Tony,
    Since there has been no response to these suggestions in two weeks I am going to close this post.
    Please open a new post if you need further help with this device.
    Regards
    John