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PMP30720: UCC28780 behavior at negative temperature

Daria Kiseleva
Intellectual 1620 points
Part Number: PMP30720
Other Parts Discussed in Thread: UCC28780

Hi,

My customer is working on power supply unit and uses PMP30720 as a reference, they faced the same problem with their design and with our reference design.

The problem is with the operation of the power supply based on UCC28780 at a negative temperature of -40 degrees.

The power supply does not start up. Pulses do not generate at pin 4. The reference voltage is 0V at pin 16.

18.5V is setting on pin 1 and maintaining until the board warms up to -5 (C).

In normal operation mode the voltage changes from 18.5V to 15V with a slight dip during the start.

They see the same behavior with our board - PMP30720.

If they firstly apply power to the power supply at +25 (C) and then cool it down, then the power supply works normally without shutdowns.

The question is what is the reason of such behavior of the UCC28780 at the start up at negative temperatures and how to fix this problem?

 

Thank you,

Daria

  • 0
    TI__Expert 7080 points

    Hi Daria,

    The first step of UCC28780 start up is VDD voltage ramp up to VDD(ON) voltage (typical value is 17.5V) , look at the PMP30720 schematic , you will find that there are some zener connected to VDD and SWS pin , such as D4 and Dz2 , The zener voltage of Dz2 is from 17.1~18.9V . the problem in here is maybe that the zener voltage is decreasing while UCC28780 internal VDD(on) voltage is increasing at -40c temperature . the VDD voltage been clamped lower than VDD(ON) voltage. as you know , Generally , for >6V zener . the zener voltage has a positive temperature coefficient . as temp. down the zener voltage will drop. 

    My recommendation is change all of zener which connected to VDD and SWS pin to a higher voltage level , such as from 18V to 20V , Then we can let the VDD voltage ramp up to reach to VDD(ON) voltage .

    Thanks.