AN2994 datasheet(21/38 Pages) STMICROELECTRONICS

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AN2994

Test results and significant waveforms

Doc ID 15796 Rev 2

21/38

2.6

Start-up at light loads

The board, as it is, is able to properly handle an output load as low as 12 W. With lower load

levels, the system will not start up correctly at high lines because burst pulses last so long

that the Vcc voltage drops below the UVLO of the L6563S. In such conditions, if the load

increases suddenly, the PFC output voltage will drop until the IC resumes normal operation.

When the L6563S is supplied from an external source (typically, it is the auxiliary voltage

that powers the controller of the downstream converter or an auxiliary power supply), the

minimum load that can be handled properly goes to virtually zero. The start-up behavior at

the minimum load (12 W) is reported in Figure 32. It can be noted that the self-supply circuit

connected to the auxiliary winding of the inductor is working properly.

Figure 33 shows the circuit start-up at no load, with the L6563S powered by an external Vcc

(15 V). The PFC regulates the output voltage perfectly.

Figure 32.

EVL6563S-400W start-up

at 265 V, 50 Hz, 30 mA load

Figure 33.

EVL6563S-400W start-up at 265 V,

50 Hz, no load and with external

Vcc

CH1: Q1/Q2 drain voltage

CH2: Output voltage

CH3: Pin #13 - gate drive

CH4: Pin #14 - Vcc voltage

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