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UCC28740: Intermittent operation of Flyback Converter

Gerasimos Maroulis
Prodigy 60 points
Part Number: UCC28740

Hello,

I designed a flyback converter using the UCC28740. The DC input voltage ranges from 150V-1000V. And the output voltage consists of 2 outputs of 18V each. Unfortunately, the circuit does not respond as expected. In particular, while the circuit operates good for the first few seconds, suddendly it switches off. When Vdd reaches the low threshold (7.15V), the circuit switches on again, working properly, but then after a few seconds it switches off again.

In the following pdf you can find attached the waveforms of important parameters.

---------------

PDF 1:

Blue=Vdd

Red=DRV

Green=FB

Yellow=Output Voltage.

4571.1.pdf

Thanks in advance

  • 0
    TI__Expert 8210 points

    Gerasimos

    From the waveforms you shared it appears that a fault condition is being triggered in UCC28740.  When a fault condition is triggered in UCC28740, PWM operation is disabled until VDD discharges below the UVLO turn off threshold, where it then attempts to restart.  Section 7.3.4 Fault Protection (page 12) in the UCC28740 datasheet has details.

    What we recommend is that you measure the CS, VDD, DRV and aux voltage during this operation to see which fault condition is triggering this shutdown.  We do not recommend directly measuring the VS pin since parasitic capacitance from the probe can interfere with operation.  When you determine what fault is triggering this shutdown, you can then adjust the corresponding external component to prevent the fault from triggering.

    In addition to above, many of the debug recommendations in this app note are applicable to UCC28740 http://www.ti.com/lit/an/slua783/slua783.pdf.  UCC28740 uses the same base control as the PSR devices described in this app note.

    Best Regards,

    Eric