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LM3478: LM3478-Isolated Flyback design

GOWRISANKAR THANNEERU
Prodigy 60 points
Part Number: LM3478
Other Parts Discussed in Thread: LM3481,

HI,

I am using LM-3478 IC for isolated flyback design. In this design i have connected the FB pin to GNd and the Optocoupler  colletor pin to Compensation pin. i have the following quieres.

1. Do i need to give fixed voltage to compensation pin so that opto coupler will get powersupply.

2. if we need to give fixed voltage to compensation pin, can i connect to SS pin through a resistor.

your inputs are much appreciated.

Regards,

Sankar.T

  • 0
    TI__Guru* 78170 points
    It will be easier to discuss on a schematic. I am afraid I might have misunderstood you. Anyway, when the optocoupler controls the COMP pin, the feedback signal feeds into the COMP to modulate the PWM . If you give a fixed voltage to the COMP pin, the feedback info will be overridden by this fixed voltage and you do not have a close loop operation for output regulation. If you mean something else, please clarify by showing your schematic.
  • 0 in reply to Youhao Xi
    Prodigy 60 points

    HI Youhao,

    i have attached the image of schematic. In case of LM3481 design the VCC pin is connected to compensation pin via a pullup resistor.(Refer document SNVA761).

    But incase of LM3478 IC there is no internal reference voltage or VCC pin  available for biasing the to optocoupler transistor.

    Awaiting for your inputs.

    Regards,

    Sankar

  • 0 in reply to GOWRISANKAR THANNEERU
    TI__Guru* 78170 points

    I see. Since the LM3478 and LM3481's COMP pin has internal high and low voltage level clamps, directly pulling it down by the opto may cause conflict and damage the part. Yes, you should add an external pull up.

    Not sure what SS pin do you talked about. There is no SS pin for LM3478. You should create a dc voltage rail by a simple resistor and Zener and shunt capacitor to stabilize the Zener voltage, then feed this Zener voltage rail to pull up the COMP pin, like what you see in the LM3481 implementation where the COMP is pulled by VCC.