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UCC28070

ron f
Prodigy 130 points
Other Parts Discussed in Thread: UCC28070

I am using 2 controllers for a 4 phase interleaved pfc. Both controllers are working separately but when I try to sync them by tying the CDR pin to Vref (on both) and driving the RDM pin with a 3.3V pulse that is approx 500nS duration and 180 degrees out of phase with the other channel (as recommended in the data sheet), the 2 controller fail to sync. It appears as though the parts will just run at their separate freqs determined by the RT resistor (75K). I am trying to sync the parts at 145khz so my sync signal is 2x this or 290Khz

Please advise.

Neither channel with sync separately or together

I looked at the signals and they are pretty clean as seen below. The purple is the drain voltage of a single phase (vo=~370V) so the parts regulate etc but donot sync

not

  • 0
    TI__Guru* 85616 points

    I had looked a similar issue for a customer the other day.  We recommend that the sync pulse should be between 200 and 250ns.  The controller frequency should be set at a switching frequency where the master clock frequency is 90% of the sync frequency.  For 100 kHz sync frequency the master frequency should be 90kH each phase would be 45 kHz.

    I gave this advice to a customer the other day and he was able to sync his two controllers to 290kHz.  When this is done correctly each phase will be 90 degrees out of phase.

  • 0 in reply to Mike O'
    Prodigy 110 points
    I have exactly the same problem as "ron f".  The UCC28070 seems to just ignore the PWM clock.  However, I wasn't able to figure out what "ron f" was doing wrong from Mike1414007's solution.
    I tried to follow Mike1414007's solution exactly.  I have CDR pulled up to 6V, and I am sending a 100kHz 3.3V PWM with a 222ns on-time into the RDM pin.  I set the RT resistors to 165k (each phase switches at 45.5kHz).  In case I misunderstood your instructions, I also tried 82.5k (each phase switches at 90.9kHz) and 332k (each phase switches at 22.6kHz).  In all cases my 100kHz PWM was completely ignored and the phase never ran at 50kHz like I expected.