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UC3842: How to design a startup circuit driven by auxiliary circuit for UC3842?

Xueshen Zhang
Prodigy 30 points
Part Number: UC3842

Dear TI team,

Like this circuit, how do we design the startup circuit? like aux cap, vcc cap, startup resistance. Though I have read the datasheet, I am still confused about it.

  • 0
    TI__Mastermind 31580 points

    Hi Xueshen,

    The AUX cap must be sized such that it can maintain the operation of the UC3842P device until the auxiliary bias rail comes into regulation. Once the voltage on Vcc exceeds the UVLO start-up threshold of 16V the AUX cap must be able to provide enough hold up to power the UC3842P while the output and auxiliary bias come into regulation. Since the difference between UVLO on and off is 6V, we can calculate the hold up time based on the aux capacitor value once we know the bias current consumed by the IC, we could assume 20mA of bias current, so a 22uF aux cap will provide 6.6ms of hold up until Vcc hits 10V and the IC turns off. Some adjustment of the AUX cap value may be necessary during the prototype stage.

    The start-up resistance is a balance between start-up time, since it forms an RC circuit with the AUX cap, and power dissipation in the resistors. Faster start-up times require smaller resistor values and hence higher power dissipation since the resistors will be continually dissipating power as they will have a constant voltage dropped across them.

    Raux limits the peak current into the AUX cap and is typically in the region of 4.7R to 100R, it also helps with the cross regulation between the output and the bias winding by limiting the value of the peak voltage at turn-off due to the leakage inductance in the transformer. Again the value of this resistor may have to be varied during the prototype development.

    Regards

    Peter