AN2994 Datasheet, PDF (21/38 Pages) STMicroelectronics – 400 W FOT-controlled PFC pre-regulator with the L6563S

AN2994
2.6
Start-up at light loads
Figure 32. EVL6563S-400W start-up
at 265 V, 50 Hz, 30 mA load
Test results and significant waveforms
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
CH1: Q1/Q2 drain voltage
CH2: Output voltage
CH3: Pin #13 - gate drive
CH4: Pin #14 - Vcc voltage
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.
Doc ID 15796 Rev 2
21/38