• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 64 subscribers
  • Views 348 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

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.

UCC28730: How to adjust frequency

Koyo Asaishi
Intellectual 1960 points
Part Number: UCC28730

Hello team,

As the title, I want to know how to adjust switching frequency.

Vs pin looks to be able to change frequency but the relation between Vs voltage and frequency is not specified, specified with just VcL that we can't measure.

Customer want to use 170Hz frequency but they can't reduce the frequency down to 170Hz.

Could you advice how 170Hz fsw is realized?

Best regards,
Koyo

  • 0
    TI__Genius 9790 points

    Hi, Koyo

    I've assign your post to expert, please give some time to get response.

    Yunsheng

  • 0 in reply to Yunsheng Qu
    TI__Guru* 85896 points

    Hello,

    The UCC28730 uses an FM/AM/FM modulation scheme.  Selecting the primary inductance sets the maximum frequency.

    Equation in 14 in the data sheet shows the mathematical relationship between frequency and transformer primary magnetizing inductance.

    Figure 16 shows the control law of the device and how the frequency and current will vary.  To keep the size of the flyback transformer to a minimum, I would recommend setting the maximum frequency to 70 kHz rather than 170.

    If you do this in your design at maximum load the switching frequency will be 70 kHz at no load the switching frequency will be adjust down to 32 Hz to maintain minimum duty cycle.  The controller monitors the aux winding voltage to sampled the output voltage and will adjust the frequency and peak current to maintain the duty cycle.

    Regards,