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

over 1 year ago

0 Hong Huang over 1 year ago

TI__Guru 53385 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 Umamaheswararao Vuyyuru over 1 year ago 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 Hong Huang over 1 year ago in reply to Umamaheswararao Vuyyuru

TI__Guru 53385 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

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UCC2894: Secondary Feedback circuit using TL431