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UCC28070: input sense line on UCC28070

Natthapol Vanasrivilai
Prodigy 240 points
Part Number: UCC28070

Hello TI,

on the datasheet the input line voltage is sense directly from the main full-bridge rectifed diodes, but many of the TI reference designs add another set of rectified diodes only just for AC sense line (slua479b page 18, slua517 page 7) .

  • What are the advantages of this additional diodes for input sense line?
  • What will be the disadvantage if i omitted it in my design?

from UCC28070 datasheet

from SLUA479B

Best Regards

Natthapol Vanasrivilai

  • 0
    TI__Mastermind 25570 points

    Hello Natthapol

    At normal loads the voltage Vin - after the bridge rectifier, as in SLUA479 - tracks the line voltage accurately. However at very light loads it may not do so because of peak detecting in the input capacitor - not shown in your first schematic but it's the capacitor shown as C1 in the second schematic. This capacitor holds enough charge to supply the load and is replenished when Vac increases towards its peak. So Vin is accurate for most of the load range and you get good PF as a result but becomes less accurate at light loads. This is not normally a problem.

    The second schematic samples Vac directly and follows Vac accurately over the entire load range. The price you pay is the additional rectifier diodes.

    Regards

    Colin