+

OUT

IN

FB/NC

NC

GND

UCC28810

UCC28811

Secondary Bias Regulator

Normally, only D11 is needed for a secondary bias supply to charge the bias capacitor, C9, during the switch-on

time, based on the turns ratio between the output and bias winding. U2 provides a stable 5-V bias for the

secondary-side circuitry. For this application, D12, D13, D14 are added to provide a copy of the input voltage on

the secondary side during the switch on time. This input waveform is divided down and filtered to remove the

switching frequency waveform by R22, R25, and C17. The signal is then offset by R23 and fed into a

comparator, U5, shown in Figure 23, to detect triac dimming and adjust the feedback loop.

Figure 22. Secondary Bias Regulator Schematic

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Product Folder Link(s): UCC28810 UCC28811