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UC3843A: UC384x Family: variable frequency

marco forno
Prodigy 40 points
Part Number: UC3843A
Other Parts Discussed in Thread: TL431

Tool/software:

Good morning
Is there an Application Note or a White Paper that addresses the topic "how to reduce the oscillation frequency of the UC384x family PWM controllers as the duty cycle decreases"?
Reason: I'm sizing a flyback without PFC preregulator due to space & cost (total requirements: Vin = 85-269Vac 47-63Hz; Vout = 30-95Vdc under uC control by DAC output, Pout = 10-180W variable under load needs, Poutmax,avg = 150W for thermal purposes; Ioutmax = 1.9A, Ioutmax,avg = 1.6A for thermal purposes), it would be interesting to be able to vary the switching frequency in a 1:2 range from minimum duty cycle to maximum duty cycle in order to have sense currents and ON times that are not very small even in very bad conditions (example: Vin = 269Vac, Pout = 10W).
Thank you for the information and references you want to give me.
Greetings
Marco

  • 0
    TI__Guru 73495 points

    Hi,

    No paper on your specific requirement as we never heard such requirement.

    It looks my understanding to your need is the duty cycle is varied by the load, right?

    Then as duty cycle varies from small to big, ten you want the frequency increase and the increase range is 1 : 2, for example from 50kHz to 100kHz (what are your real numbers for frequency change you want to have?)

  • 0 in reply to Hong Huang
    Prodigy 40 points

    I try to clarify

    We're talking about an isolated DCM flyback, supplied by rectified 85-269Vac line voltage (no PFC boost pre-regulator), with isolated output, with "standard" voltage feedback by optoinsulator + TL431 shunt regulator

    Primary MOSFET duty cycle changes from a few% to 50% due to load power requirements and line input voltage:

    working point A - a few % in case of 269Vac and min output power

    working point B - about 50% in case of 85Vac and max output power

    My idea is: to have a switching frequency in working point A significantly lower than in working point B (i.e.: 50..60kHz vs. 100kHz, it will be ok as a first try)

    Thanks in advance for your help

    Bye

    Marco    

  • 0 in reply to marco forno
    TI__Guru 73495 points

    Hi,

    To change the frequency between point A and B, one way is to change the resistor value on RT/CT pin.