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UCC256303: OUTPUT OVERSHOOT.

KIM Andy
Expert 2530 points
Part Number: UCC256303
Other Parts Discussed in Thread: UCC28063,

HI

My client is currently working on the 800W design with UCC28063 + UCC256303.

During the test, the output voltage of UCC256303 is overshoot.

In Practical Design Guidelines 5.2 (Circuits to Avoid Optocoupler Deep Saturation), we added a 4.7V zener diode.

This has improved the problem.

Do you have any criteria for setting this Zener value?

  • 0
    TI__Mastermind 27145 points
    Hi KIM Andy,

    Thanks for your interest in UC256303. You want to choose the zener breakdown voltage to be approximately equal to RVCC - FB typical voltage. For example, on the UCC25630-1EVM the typical FB voltage is ~6V so a 6.1V zener was chosen.

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Expert 2530 points
    Good morning
    thank you for your quick reply.
    I would like to know the zener diode setting of the optocoupler stage in item 2.
  • 0 in reply to KIM Andy
    TI__Mastermind 27145 points
    Hi KIM Andy,

    The premise is by clamping the voltage of the resistor feeding the opto, you can clamp the current through the secondary side of the opto to some maximum value. Based on the CTR of the opto coupler you are using, you can calculate a maximum secondary side current so that the optocoupler does not pull more than IFB from the feedback pin. You can then choose the resistor and zener breakdown voltage accordingly.

    Best Regards,
    Ben Lough