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UCC2818: The mosfets blow up at the start of UCC2818

Rushi Jariwala
Expert 1225 points
Part Number: UCC2818

Hi,

I am testing the PFC stage with UCC2818. 

Input: Slowly increasing AC voltage using a Variac. At 85V rms, the control starts up and the mosfets short circuit.

Power Level: 3kW

Output Voltage: 380V

Test Load: 150W bulb load

I am trying to figure out what could be going wrong.

The gate drive is working fine.

I am suspecting that the voltage overshoot at startup is causing the mosfet failure (its just a possibility which I think).

This same setup has worked once nicely with the output voltage being well regulated to 380V. So my current sense, power circuit, etc seem to be fine. But on the next attempt, it failed. And it keeps failing thereafter even though the gate driver has been verified to be fine.

Is there any open loop configuration in which I can test UCC2818?

What else can I do?

 

 

  • 0
    TI__Mastermind 31580 points
    Hi Rushi

    As a test setup you open loop the Voltage loop to produce max duty cycle and then drive the SS pin with a DC voltage to control the duty cycle. Note that bringing SS to 0V may not command 0% duty cycle so please add some load to the output of the boost and increase Vin slowly.

    I the circuit has been damaged once by a failure and is not repeatedly failing after start-up I would double check that some other part of the circuit is not damaged.

    I recommend using a DC source for the test with the current limit reduced in case there is an issue.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Expert 1225 points

    Hi Peter,

    Yes, there was a problem due to dry solder which I have fixed and the circuit is working now.

    Just for knowledge, how does soft start work? I understand that it will increase the Voltage Error Amplifier slowly which will increase the duty cycle slowly. But what is the internal circuit that makes the voltage error amplifier to increase slowly? Is there any document on this? 

    Also, what voltage on soft start pin will yield max duty cycle? How is it decided?

    Thanks 

  • 0 in reply to Rushi Jariwala
    TI__Mastermind 31580 points

    Hi Rushi

    The SS voltage has to rise to 7.5V for the SS period to end, see the the formulae used in section 10.2.2.2. it shows that the final voltage level is 7.5V for the SS period.

    The SS input to the voltage error amp overrides the 7.5V reference so the error amp follows the SS pin voltage until the SS pin charges above the 7.5V level.

    TI cannot share any internal circuitry for the controller for IP reasons.

    Regards

    Peter