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UCC28070 incorrect input current waveforms in low temperatures, IMO signal changes shape

Lukasz Zyskowski
Prodigy 20 points
Other Parts Discussed in Thread: UCC28070, UCC3817

1106.UCC28070 incorrect input current waveforms in low temperatures.docx

Hello,

PFC is controlled by UCC28070. It was observed during low temperature tests (+10C and lower temp.) that input current waveforms changed their shape from rectangular to ones resembling diode rectified mains voltage (see Pic. 1 in the attached file). It is worth mentioning that IMO voltage changes its shape with temperature too.

This is a non destructive phenomenon. When UCC28070 warms up, input currents look OK.
I attached a piece of schematic containing UCC28070 (Pic. 2, Pic.3).

I appreciate any suggestions what is the root cause of such phenomenon and how (if possible at all) can it be prevented from occurring.

Other info about circuit:
PFC boosts voltage to 600V
Pout=16kW
Mains=3x175V

Thank you,

Lucas

  • 0
    TI__Guru* 86286 points

    This device was really developed for two phases and to work correctly needs VINAC to be a rectified sine wave.  Or a square wave at 200 Hz with 300 us of time being 0V.  For QVFF to work correctly.  However, with three phases if you have your design working already with the highest QVFF level you probably don't need the 200 Hz signal.

    The IMO pin's voltage shape will track whatever is on the VINAC pin.  I am not sure what is at this pin at 25 degree C.  Coming off a three phase rectifier this shape of  IMO signal should be in the inverse of what you are showing.  It should have a DC level and have ripple pointed in the opposite direction.

    I don’t believe this issue is related to the UCC28070 but what you are feeding the interleaved boost converter.  Also note that the same THD and PF levels that can be achieved off a single phase using this device, cannot be achieved in three phase.  However, that does not mean that you cannot use this device to meet your harmonic requirements.

    Good Luck,

  • 0 in reply to Mike O'
    Prodigy 20 points

    Thank you for your post.

    1)

    << The IMO pin's voltage shape will track whatever is on the VINAC pin.  I am not sure what is at this pin at 25 degree C. 
    << Coming off a three phase rectifier this shape of  IMO signal should be in the inverse of what you are showing. 
    << It should have a DC level and have ripple pointed in the opposite direction.

    IMO’s behaviour I showed can be explained. Due to very strict mechanical constraints the installed output PFC capacitance is smaller than resulting from equations given in IC’s datasheet. To provide sufficient output voltage stability at decreased output capacitance value, the output circuit connected to VAO is also different from the one given in the datasheet (see attachment in my previous post). The circuit regulates Uout at programmed level what results that VAO and IMO voltage waveforms also differ from the ones given in standard application of UCC28070.

     2)

    Regulation loop seems to be working correctly as IMO and VAO signals change their values following load change (see attached file).

    However, I noticed that as temperature decreases, PWM signal measured directly on PFC transistor (Ugate-source), stops to change its duty cycle which means regulation is not working any more (see scopes: “PWM at 25C.” – regulation OK, “PWM when UCC28070 cooled” – regulation NOK). “Persistence” function was used to show how PWM signal changes over time.

    I would like to mention that this phenomenon can be observed at 3x170V mains (high duty cycle) but NOT visible at 3x200V and higher (smaller duty cycle) .

    Additionally, I performed a test with a different Dmax resistor (Dmax=0.83 => Dmax=0.93) to verify if this functionality limits maximal duty cycle. Result: no influence.

    During the test the maximal duty cycle was:

    • 0.7 at 25°C
    • 0.6 – during cooling of UCC28070 (independent of Dmax resistor)

    I am looking forward to your comments on the described behaviour.

    Lucas.

    PWM at different temperatures.docx
  • 0 in reply to Lukasz Zyskowski
    TI__Guru* 86286 points

    I am not exactly sure why the two waveforms are different.  However, I had noticed that you are using the UCC38070 in a three phase application.  This device was designed for single phase applications.  If the VINAC pin does not see a rectified sine wave the multiplier will not work correctly. Customers have had better luck using the UCC3817/8A devices for these applications.

    Regards,