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Original question:

UCC28740: UCC28740

UCC28740: UCC28740

Robert Bachus
Prodigy 30 points
Part Number: UCC28740

Tool/software:

Thanks for the reply. You were right, of course, about the three DRV pulses. I have attached the resulting waveforms at testpoint Vsw, after recommended changes per sluaac5.pdf Figure 1.1 were made on the primary side of the transformer. Still no continuous DRV pulses, just the same three random pulses. 

There can't be any over current fault based upon voltages at the CS pin, There can't be an over temperature since there is no continuous DRV pulses. The Rs1 and Rs2 within proper operating range for the UVLO. Any further suggestions?

  • 0
    TI__Guru* 76365 points

    Hello,

    Your design is giving three sample pulses and stops switching.  It is either shutting down for OVP, OCP, temperature or input under voltage. 

    It could also not be starting up due to input under voltage.

    When evaluating this you should look at CS, Vout, Vaux and VDD of the UCC28740.

    Since you do not have any switching your converter may be triggering input under voltage lockout .  Below information describes what this looks like from application note sluaac5.

    The below information describes how the under voltage lock out works.  Please note that IVSL(run) and IVSL(stop) ,might be different values from what is listed below.  Please double check the UCC28740 for the correct values.

    Your input voltage just might not be high enough to startup or the high side resistor in aux divider is not set high enough.

    Please note filtering the VS pin with a capacitor will prevent startup as well.  If a filter capacitor is needed on this pin it should be less than 3.3 pf.

    Regards,