AN2941 datasheet(17/37 Pages) STMICROELECTRONICS

17 / 37 page

AN2941

Functional check

Doc ID 15447 Rev 4

17/37

Standby and no-load operation

In Figure 13 and Figure 14, some no-load waveforms are given. As shown, the L6566A

works in burst mode. When the feedback voltage at pin COMP falls below 2.65 V (typical),

the IC is disabled and its consumption is reduced. The chip is re-enabled as the voltage on

pin COMP rises again over this threshold. Additionally, in order to get the best efficiency,

during light-load operation the PFC stage is turned off. In fact when the voltage on pin

COMP falls below the burst mode threshold, the L6566A pin #6 (Vcc_PFC) supplying the

PFC controller, is opened. Thus the residual consumption of the PFC control circuitry is

minimized to a negligible level. Whenever the IC is shut down, either latched or not, the

Vcc_PFC pin is open as well.

In Figure 15 and Figure 16 the transitions from full load to no load and vice versa at

maximum input voltage have been checked. The maximum input voltage has been chosen

because it is the most critical input voltage for transition. In fact at no load the burst pulses

have lower repetition frequency and Vcc could drop, causing restart cycles of the controller.

As visible in the graphs, both transitions are clean and there isn't any output voltage or Vcc

dip.

Figure 13.

No-load operation at 90 V-60 Hz

Figure 14.

No-load operation at 265 V-50 Hz

CH1: Flyback drain

CH2: COMP pin

CH3: Output voltage

CH4: PFC drain

CH1: Flyback Drain

CH2: COMP pin

CH3: Output voltage

CH4: PFC drain

AN2941 Datasheet(PDF) 17 Page - STMicroelectronics