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UCC28060: Circuit from SLUA625 not functioning

Matthew Bloom5
Intellectual 330 points
Part Number: UCC28060
Other Parts Discussed in Thread: TL431

Hi,

I built up the circuit from SLUA625, Fig. 15, but it does not work correctly. I noticed that the auxiliary coil for T1 was not labeled correctly (should be ZCDA, not ZCDB), so I corrected that before I built the circuit. 

The chip seems to be working, as VREF is putting out 6V when I inject 18V from a separate source into the VCC pin. 

VSENSE is reading: 2.84V

COMP is reading: 0.675V

HVSENSE is reading: 2.84V

VINAC is reading: 4.95V. For some reason, the rectified 120VAC is coming out as 310VDC. Would this cause issues? I can change the VINAC resistor network as a bandaid. 

I expect to see ~60V on my output, since I changed the FB resistor network to regulate the TL431 around 2.5V @ 60V DCBUS voltage. 

Let me know what I can do.

Thanks!

  • 0
    TI__Prodigy 750 points
    Hi,
    DO you see any switching pulses from the gate when you power up the VCC?

    Thanks
    VA
  • 0 in reply to VA Viswanathan
    Intellectual 330 points
    Hi,
    Just a few more details:
    I was powering the IC from an external supply. This supply somehow depressed the primary side ground below 0, making the HV-PriGND voltage look like 310V; which explains why VINAC was reading so high. I've since removed the external supply, and now VINAC looks normal.
    However, now the IC does not power from the bootstrap circuit.

    The issue now, is that VCC1 builds up as the 47uF capacitor charges, and once the voltage reaches ~13V (the IC's turn on voltage), the voltage collapses. This is most likely because the capacitor cannot support the IC current draw. I believe to be because the IC draws 5-8mA from VCC, and the 150kOhm resistor between HV and VCC1 will not permit that much current to flow (5mA*150kOhm = 750V!). I'm going to try decreasing the resistor to ~25kOhm, which should permit enough current to flow to the IC and jump start the flyback to the point where the ZCDA aux coil will power the IC.

    Will keep posting with updates.
  • 0 in reply to Matthew Bloom5
    Intellectual 330 points
    Hi VA,

    I am now using an external supply for the VCC voltage.
    I have the circuit working (regulating the output to the appropriate voltage) with <25W on it, however once I exceed 25W the voltage collapses.

    Thoughts on why this is happening?

    PS - in order to get the circuit working, I found that I have to lightly touch (w/ either a probe, wire, etc) the ZCDx pin. I think there is some issue where this causes a falling edge, which is jumpstarting the gate drive circuitry, as described in the datasheet: "...the ZCD input must drop below the falling threshold, approximately 1 V, to cause the gate drive output to rise."

    Any thoughts on this as well?