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UCC28950: UCC28950 No output PWM

vikas salvi
Prodigy 160 points
Part Number: UCC28950

Tool/software:

Hello ,

I am working on UCC28950 controller for PSFB design of DCDC,

I have assembled controller board as per schematics attached and trying to check the PWM waveform (OUT A~ D) without connecting Power MosFET .

UCC28950 not producing any PWM output. Trying to capture the pin voltage of UCC28950 as below,

 VCC =12V, Vref = 5V, EA+ = 2.5V, EA- =0V 

At SS /EN pin as per datasheet should be 0.55v while getting the ramp type voltage (Waveform attached) 

At COMP also getting pulse waveform (Waveform attached) 

No output seen at Sync pin ..

Can you please help to identify the issue and provide your feedback ,

Thank you  ,ISOLATED_DC-DC Control_Circuit.pdf

Regards ,

Vikas 

  

  • 0
    TI__Guru* 86266 points

    Hello,

    I am reviewing your inquiry and will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86266 points

    Hello,

    I think your issue that you are testing open loop with feedback included.

    To test the device open loop, I would remove all of the feedback components.  You would then tie EA - to the voltage amplifier output to form a voltage follower.

    You would then build a voltage divider from Vref to EA + of the voltage amplifier with a 5 k ohm resistor and a 10 k ohm, 100 turn, potentiometer.  You would put the 5 k from EA + to VREF and the 10 k ohm potentiometer from EA + to ground.  This will give you the ability to adjust the output voltage of the voltage amplifier which is FET to PWM comparator.  By adjusting this voltage you will be able to adjust the duty cycle of the UCC28950.

    Thank you for interest in Texas Instruments (TI) products.  If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

    Regards,