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How to adjust output PWM of UC3825

Siyu He
Expert 1040 points
Other Parts Discussed in Thread: UC3825, TL5001

Hi,

I am about to built a 12V to 48V boost circuit using UC3825, the circuit below is my design

The input of Feed-Back varies from 0-48V(lab DC power supply). The outputs PWM did not change and stay at  46% while increase Feed-Back from 0 to 47V. After the Feed-Back went across 48V, the output PWM changed to 0.  Can someone help me?

  • 0
    TI__Mastermind 38230 points

    Move to non-isolated group

  • 0
    TI__Mastermind 25130 points

    Siyu,

    Change r16 to 10 K, short C17, run Fb between 2.5 V and 9.5 volts and watch E/A out and the pulse width.

    John

  • 0 in reply to John Bottrill
    Expert 1040 points

    Hi, John

    I tried your suggestion, the duty cycle of output PWM vared from 0%  to 46%, but I need a 75% PWM , the E/A out vared from 4.82V to 0V.

    I need to design a type 3 compensator, why I can't change PWM while having C17?

    Thanks for your reply.

    Siyu

  • 0 in reply to Siyu He
    TI__Mastermind 25130 points

    Siyu,

    I think the toggle flip-flop in the block diagram on page one of the spec should have let you know that the max you will get with this device is 50% duty.

    Go to page 43 and 44 of the attached and select a more suitable part. 

    Power management guide 2011.pdf
  • 0 in reply to John Bottrill
    Expert 1040 points

    The datasheet of UC3825, it shows that the maximum duty cycle is 85%, the image below is part of datasheet

    .

    Is it ok to use type 3 compensator on UC3825, why I can't get different duty cycle while using type 3?

    I think I got same problem on IC TL5001, I set maximum duty cycle 85%, if the feed-back voltage is below 48V, the output is 85%, once the feed-back goes beyond 48V, the output PWM goes to 0.

  • 0 in reply to Siyu He
    TI__Mastermind 25130 points

    Siyu,

    That applies to the comparator not the outputs. Cut that in half for each of the outputs. If you are driving a push-pull design then the total duty cycle will be up to 85% put each half will only see half of that.

    John

  • 0 in reply to John Bottrill
    Expert 1040 points

    John,

    Is it ok to use type 3 compensator on UC3825, why I can't get different duty cycle while using type 3?

    I think I got same problem on IC TL5001, I set maximum duty cycle 85%, if the feed-back voltage is below 48V, the output is 85%, once the feed-back goes beyond 48V, the output PWM goes to 0.

    These questions bothered me for a long time, if you can give some help about these, I would appreciate a lot.

    Siyu