UCC28C56L: Issue with startup circuit

Dimple sai krishna Tadikamalla

Part Number: UCC28C56L

Tool/software:

I have designed a flyback circuit with UCC28C56L by taking the reference from the datasheet, Webench designer.
When I connected the board to the power supply I noticed that the constant input voltage which is supposed to be converted to 18V inorder to power up VDD of the IC through resister divider is fluctuating from 14V to 18V. The Vref is also 0V. The IC is not turning on.

I dont think the voltage divider as suggested in the Webench works. That may be the reason why in the eval board they have implemented it using an N-channel depletion type MOSFET. I tried to recreate the same using N channel Depletion MOSFET. This time I got a stable 16V at VDD and 4.2V at Vref. However the IC was still not turning on and no PWM switching was observed.

Please help me on how I can resolve this issue. Can I make the circuit work with resister divider as shown in webench as well?

6 months ago

0 Hong Huang 6 months ago

TI__Guru 73825 points

Hi,

Please provide VDD, VREF, COMP, CS and OUT PIN waveforms during power on to understand the issue and solutions.

0 Dimple sai krishna Tadikamalla 6 months ago in reply to Hong Huang

Prodigy 120 points

I have attached two photos, one while the startup and the second while it is continuously on.

Here the yellow line indicated VDD and read as 16.8V on the multimeter
Pink line indicates Vref and read as 4.18V
Blue Line indicates COMP pin and as 1.1V
Green line indicates CS Pin and as 0.202V

0 Dimple sai krishna Tadikamalla 6 months ago in reply to Dimple sai krishna Tadikamalla

Prodigy 120 points

And there was no voltage waveform observed in the OUTPIN of the IC

0 Dimple sai krishna Tadikamalla 6 months ago in reply to Dimple sai krishna Tadikamalla

Prodigy 120 points

The IC finally turned on when Soft start, current filtering, slope compensation circuits were enabled. I previously didn't use it as it was not recommended in the WEBENCH. Now the PWM is being generated. However the circuit is deigned for 12V output and we are getting only 2.5V output.
The VDD was 16.86V
VREF was 4.22V only. Why is VREF dipping to 4.2V instead of 5V?

In this picture-1; Yellow-VDD Pink-VREF Blue-COMP GREEN-CS

In this picture-2 is of startup; Yellow-VDD Pink-VREF Blue-COMP GREEN-CS

In this Picture-3; Green-Voltage waveform across VGS of the MOSFET