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UCC28740: Startup Testing

Nick Glascock
Prodigy 10 points
Part Number: UCC28740

I am currently attempting to view the waveform coming from the DRV pin on a UCC28740 on startup. I am applying 25 V directly to the VDD pin using a power supply, and 4 V directly to the VS pin using another power supply. I am using an oscilloscope to measure the waveform at the DRV pin. I am not observing any voltage, switching or DC, at the DRV pin using this setup. The FB and CS pins are connected to ground via a 50kOhm and 750mOhm (sense resistor),respectively. HV has been left open.

I chose these voltages at VDD and VS based on the test configurations used for measuring the Electrical Characteristics in the datasheet. I would expect to see the startup switching waveform at fmax,followed by the controller trying to command a HIGH output at the DRV pin since CS is tied to ground and there is no current being fed into CS.

Am I correct in expecting these results given the test configuration I am using?

Thank you.

  • 0
    TI__Guru* 85406 points
    Hello Nick,

    For the UCC28740 to work correctly requires the VS pin to see a switcihng voltage through ground, It will not work with a DC voltage on the pin. For the device to work correctly it needs to have the VS pin connnected to a resistor divider off an aux signal. This device can not be tested open loop.

    Please review pages 14 and 15 of the data sheet that talks about how the device determines sampling and switching freqeuncy and peak current. The VS signal needs to cross zero after the FET turns off and cross zero after the transformer deengerises for the contoler to work correctly. If you have any questions after that we can discuss it. Also there is an application note that discuss how to trouble shoot PSR flyback conveters at teh following link that you might find helpful.
    www.ti.com/.../getliterature.tsp

    Even though the UCC28740 is not a PSR controller it uses similar control law and protection schemes; such as, aux winding sensing for input under voltage and output over voltage through transformer turns ratio sensing.

    Lastly you might want to order the UCC28740 EVM so you can evaluate peformance waveforms. You could even modify the EVM to support your individual design requirements. The following link will get you to an application note.
    www.ti.com/.../getliterature.tsp

    Regards,

    Mike