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UCC28070: interleaved channels working unsymetrical

Ramin Fardi
Intellectual 280 points
Part Number: UCC28070

Tool/software:

Hi,

We have designed a PFC stage with UCC28070 with the following specs:

100-240VAC in

385VDC 1.9A out

Fsw 150KHz

We have used Webench for components selection and the design worked seemingly fine, we get 98% efficiency at 230VAC and 95% at 120VAC with better thatn 99% PF at full load. But one of the MOSFETS gets a lot hotter than the other. And upon closer inspections it was seen that the CSA/CSB and GDA/GDB are widely dissimilar. My question is what aspects of the design if not done right can result in such an asymmetry and how we can proceed to find the fault.

 

Thanks

Ramin

  • 0
    TI__Mastermind 40580 points

    Hello Ramin,

    Ideally, the CSA and CSB signals should be identical, or nearly identical.  GDA and GDB duty cycles should then also be the same. 

    I recommend to examine the reset voltage of the current-sense transformers (CTs), to ensure that sufficient volt-seconds are applied during the reset intervals (MOSFET off-times) to completely match the volt-seconds applied to the CT during the MOSFT on-times. 
    Please see Section 8.2.2.7 in the datasheet (page 32) on how to account for the worst-case V-s product during the on-times.    

    Personally, I prefer to use a Zener-diode reset network because I know exactly what reset voltage will result, although the resistor-diode network can also be used with confidence.  I don't know how to calculate the values for the capacitor-resistor reset network, so I personally don't like to use it. 

    Regards,

    Ulrich