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UCC2894: Secondary Feedback circuit using TL431

Umamaheswararao Vuyyuru
Prodigy 175 points
Part Number: UCC2894
Other Parts Discussed in Thread: TL431

Hi Sir,

I am designing the feedback network using TL431. Can you please explain the need of circled part in the given below 

Thank you in advance for your inputs. 

Reference: www.ti.com/.../PMP7391

Regards

Umamaheswararao

  • 0
    TI__Guru 58665 points

    Hi,

    This circuit is to help VOUT turn smoothly during initial power on. C19 force a high current into the opto and then pull down FB  low and FB can only increase following C19 charge up so the duty cycle increase is smoothed.

  • 0 in reply to Hong Huang
    Prodigy 175 points

    Hi, 

    Thank you for the reply. Is this circuit create any effect on control stability. How to select the capacitor and resistor in this circuit. Please also explain the need of two diodes. 

    Thank you

    Umamaheswararao

  • 0 in reply to Umamaheswararao Vuyyuru
    TI__Guru 58665 points

    Hi,

    The capacitor value needs try for a design, but 1uF is a good start.

    The diode on the left is to help block the 1 uF capacitor after its charge up so the 1 uF capacitor has no effect after start up then no effect for stability for operation. The diode on the right is to discharge the 1 uF when power off so to help next power on the 1uF still ready for start up