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UCC28780EVM-002: low power efficiency

yang h
Prodigy 30 points
Part Number: UCC28780EVM-002
Other Parts Discussed in Thread: UCC28780

Hi experts,

I am testing UCC28780EVM-002 with 90Vac(60Hz) input and 45w@20V 2.25A output  ,but the power efficiency is much lower than that in EVM user‘s guide. I wonder what cause the efficiency loss, and here is my waveform.

the green one is  SWS,  the bule one and yellow one is PWML and PWMH

It seems like ZVS has been achieved, but the power efficiency is really low.

  • +1
    TI__Mastermind 40580 points

    Hello Yang, 

    Thank you for your interest in the UCC28780 ACF controller and its EVM.

    The efficiency value listed in the EVM's User Guide at 90Vrms is a typical value and each board may vary up or down by a bit. 
    Certainly, 90.24% (43.3W/47.988W) is significantly lower than expected.  
    However, the numbers used for the calculation are readings from the AC source and the DC load modules. It is not clear how well calibrated these devices are.
    Also, the losses in the input cable and output cable are not accounted for.   

    I recommend to measure the output voltage and current using independent, known calibrated DMMs.
    For input power, I recommend to use a known calibrated power meter (such as Yokogawa WT310, or equivalent) and measure the voltages at the EVM input connectors. 
    Also, allow about 30 minutes or more warm-up time for the transformer temperature rise to settle to a steady state.  The core loss goes down as it heats up.  
    You may not measure exactly the same number as in the User Guide, but it should be much closer to 94% than to 90%. 

    For no-load input power, make sure to measure the current at the input connector first and measure the input voltage BEFORE the current measurement so that the V-meter current is not included in the current measurement.  Make sure there are no other probes connected to the no-load EVM so that probe impedance does not add to the measured input current. 

    Regards,
    Ulrich 

  • 0 in reply to Ulrich Goerke
    Prodigy 30 points

    Hello Ulrich，

    Thanks for your suggestions，after checking the resistance of input cable and output cable,I found their input resistance cannot be ignored(0.06 ohm and 0.15 ohm)，and later I will retest my EVM with more professional  power meter, I think this time I will get the precise power efficiency.

    Best regards.