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Original question:

PMP40328: Unable to start up when connected to power

UCC28780EVM-002: Unable to start up when connected to power

Nisarg Mehta
Prodigy 160 points
Part Number: UCC28780EVM-002
Other Parts Discussed in Thread: UCC28780, , UCC28780EVM-021

Hello,

I have EVM of UCC28780 with Silicon MOSFETs. Initially it was working fine. But suddenly there is no output Voltage and output current.

I am also facing exactly the same problem as Mr. Chiao Fu. As suggested by Mr. David, I have also checked RTZ, RDM and HVG pin but there is no short circuit. Waveforms are same as Mr. Chiao has posted in his thread. Vdd is triangular from 10V - 17.5V and Vref drops to 0V. No RUN and no PWML signal.

Please help me with your suggestions.

Thank you.

  • 0
    TI__Genius 9710 points
    Dear Nisarg,

    Thanks for interested in TI's ACF solution.
    In your title you mentioned UCC28780EVM-002, this EVM is with GaN device, but you mentioned you have the EVM with silicon FET.
    I want to make sure you are testing UCC28780EVM-021(which is not online yet) with silicon FET or UCC28780-002 with GaN.

    As you mentioned, you had gone through the protections checklist in datasheet 7.4.11 and find out no short circuit. How about the open circuit?

    The board was working fine, do you know what is any reason damage the board?
    Did you change any components on it?
    Did the board break after you finish the test for a while?
    When you test the circuit you connect probe's ground to PWRGND or CS pin or anywhere?

    Best Regards
    Kevin
  • 0 in reply to Kuan Wei Chen (Kevin Chen)
    Prodigy 160 points
    Hello Mr. Kevin,

    Thanks for your quick reply. Yes, I am using UCC28780EVM-021. There is no short circuit as well as no open circuit. Rrdm and Rrtz are properly connected from the respected pins to ground.

    I have not changed any components so far.

    I wanted to see the waveform of feedback current Ifb, so I cut the track between optocoupler collector terminal and resistor R9. I soldered a wire and then used current probe CP030 to measure current. But as that probe's sensitivity was 10mA per division, I wound 50 turns of the wire (to increase the sensitivity as described in its user manual, sensitivity increases with the factor of turns ratio) and then tried to measure but that time i was not getting a good waveform of current. Input Voltage was 100Vac, At the output I was getting 20V and the load was 1A. With this connection, when I tried to increase the Vin above 100V, the output voltage was falling to 0V and no current was flowing through the load.

    When I did that connection, at 100Vac input, I directly turned ON the electronic load with 1A. So initially, it was 0A and then direct 1A. Could this cause some problem?

    The next time when I disconnected the load and the current probe just to check the output voltage, at any input voltage, there was no output.

    I have measured only one signal at a time when the board was working good. And the oscilloscope is powered through an isolation transformer as well.

    I don't know what is wrong. Please help.!
  • 0 in reply to Nisarg Mehta
    TI__Genius 9710 points
    Dear Nisarg,

    I am not sure what is the reason you want to capture feedback current Ifb. Because this pin is quite sensitive to the control loop. Did you try to recover the board to the original to check is working or not?
    I think directly turn on the load will not be an issue, because the EVM should work well on full load turn on. If you want to capture Ifb current, use voltage probe with tip and barrel to measure R9 voltage will be better.
    Can you also check CS pin is short or open circuit? (Base on the information in datasheet 7.4.11)

    Best Regards
    Kevin
  • 0 in reply to Kuan Wei Chen (Kevin Chen)
    Prodigy 160 points

    Dear Mr. Kevin,

    I wanted to see the current ripples and the value of Ifb, in the case when output is equal to 20V. (i.e. required output)

    I have some doubts regarding the Ifb, it is not clear to me from the datasheet that Ifb should be between 0-75uA. At what value of Vout, what should be the value of Ifb, that I am not understanding. For eg., If output is 20V, it should be 50uA or 75uA or 0uA or what? at which exact value of Ifb, the control unit will consider the output is ok? To design the component values for the feedback section, I think it is very important. Please help me with this query.

    And I have recovered the board in the original form and still the problem is same.  CS pin is also not open circuited or short circuited.

    Could you provide me in detail the range of the Rdm and Rtz? As in the following description from the datasheet, it says about too small and too large current, could you specify the limits ? And also what is the voltage level to check the pin open or short condition?