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UCC28710: Low Power Mode, help for an explanation

Filippo Monti
Prodigy 20 points
Part Number: UCC28710

Dear TI,

we are actually designing a flyback converter with input 400VAC and output 15VDC @ 0.5A.

We have done the design, the custom transformer, the PCB and now we are trying it.

We have designed all the device for a switching frequency of 100kHz. Unfortunately, it seems that the switching frequency is unstable and it cannot go higher than 33kHz.

In the datasheet I have found that there is this "low power mode" that sets the switching frequency to 33kHz. Maybe we are in this mode, but I cannot understand when the IC will enter this mode of operation and how to get out of it in order to obtain higher switching frequencies.

Can someone help me?

Thank you very much.

Monti Filippo

  • 0
    TI__Expert 6080 points
    Hello Monti,

    Thank you for your interest in UCC28710.
    1 The UCC28710 use a frequency (FM) and peak current mode modulation to control the duty cycle.
    a. UCC28710 fmax = 100 kHz
    b. UCC28710 f1 = 33 kHz
    c. UCC28710 fmin = 3.3kHz
    This is described in further details in datasheet page14, fig16. The converters switching frequency depends on the power demand and may I suggest trying to increasing the load. Please share your design spec and schematic for further understanding.

    Regards,
    Sonal
  • 0 in reply to Sonal Singh
    Prodigy 20 points

    CHENCHEN_FLYBACK.pdfDear Mr. Sonal,

    thank you for your fast reply.

    Taking the load constant, how can we increase the switching frequency? As I understand, do we have to manage the voltage sense resistors (control voltage) and the current sense resistor? If yes, in which way (increasing/decreasing R)?

    Attached you can find our schematics.

    Thank you for the support.

    Best regards,

    Monti Filippo

  • 0 in reply to Filippo Monti
    TI__Expert 8210 points
    Monti

    The switching frequency should increase as the load is increased or the input voltage is decreased. Something is incorrect if this does not happen.

    How did you create this design? If you have not already, I recommend using our excel design calculator to review what the recommended component values are for this design www.ti.com/.../sluc590. Please verify that the transformer turns ratio and inductance are correct. Also what is the purpose of R9 and R10? Typically there is no resistor in series between the flyback transformer and the FET.

    I recommend reading this TI PSR Flyback debug app note, it includes information on common issues seen with our devices like UCC28710 and gives recommendations on how to solve them www.ti.com/.../slua783.pdf

    Best Regards,
    Eric