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Original question:

UCC28C56H-Q1: Questions about UCC28C56EVM-066, (doc bumber sluucn1c)

UCC28C56H-Q1: Purpose of R8 and D10 in UCC28C56EVM-066

bseck
Prodigy 40 points
Part Number: UCC28C56H-Q1
Other Parts Discussed in Thread: UCC28C56EVM-066

Tool/software:

Hello,

what is the exact purpose of R8 and D10 in UCC28C56EVM-066?

I know that D10 is there for overvoltage protection. If Vout is above 18V it will be conducting, limiting the output voltage. However theres no resistor limiting the current, so I guess D10 will be destroyed in an overvoltage event? 

Also, the datasheet specifies the maximum output voltage as 19.3V at no load, isnt D10 always conducting when theres no load at the output then? 

Regarding R8, maybe its purpose is to discharge the capacitors? 

See attached image.

  • 0
    TI__Mastermind 26215 points

    Hi,

    R8 is to introduce at least some minimum load needed to help regulation during the no-load condition. Normally the feedback divider resistors are present as part of the secondary side regulation and can serve the purpose of R8 but since this design is configured for primary side regulation, the feedback divider resistors are not present. Without R8 in place, no load is truly NO load and no flyback regulates well in this condition.

    Even with R8 in place, VOUT is likely still drifting during no load. D10 crudely clamps the output voltage to 18V. D10 is PDZVTFTR18B which is rated for 1W (~55mA). The further breakdown of D8 will present the load needed to lower the output voltage below 18V and cause D8 to not conduct. There are other ways to introduce no load regulation but for low component count, evaluation purpose, this works as intended.

    Regards,

    Steve

  • 0 in reply to Steven Mappus1
    Prodigy 40 points

    Thanks a lot for your detailed answer, that makes sense!