AN4027 datasheet(9/41 Pages) STMICROELECTRONICS

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AN4027

Main characteristics and circuit description

Equation 3

The value of the mounted resistor is 68 k

.

RH sets a small debouncing hysteresis and is in the range of mega ohms. Rlim is in the

range of tens of k

 and limits the current flowing through the optocoupler's diode. Both

L6699 and L6563H implement their own burst mode function but, in order to improve the

power supply overall efficiency, at light load the L6699 drives the L6563H via the

PFC_STOP pin and enables the PFC burst mode: as soon as the L6699 stops switching

due to load drops, its PFC_STOP pin pulls down the L6563H PFC_OK pin, disabling PFC

switching. Thanks to this simple circuit, the PFC is forced into idle state when the resonant

stage is not switching and rapidly wakes up when the downstream converter restarts

switching.

1.2

Startup sequence

The PFC acts as master and the resonant stage can operate only if the PFC output is

delivering the rated output voltage. Therefore, the PFC starts first and then the LLC

converter turns on. At the beginning, the L6563H is supplied by the integrated high-voltage

startup circuit; as soon as the PFC starts switching, a charge pump circuit connected to the

PFC inductor supplies both PFC and resonant controllers, therefore, the HV internal current

source is disabled. Once both stages have been activated, the controllers are supplied also

by the auxiliary winding of the resonant transformer, assuring correct supply voltage even

during standby operation. As the L6563H integrated HV startup circuit is turned off, it greatly

contributes to power consumption reduction when the power supply operates at light load.

1.3

L6563H brownout protection

Brownout protection prevents the circuit from working with abnormal mains levels. It is easily

achieved using the RUN pin (pin 12) of the L6563H: this pin is connected through a resistor

divider to the VFF pin (pin 5), which provides the information of the mains voltage peak

value. An internal comparator enables the IC operations if the mains level is correct, within

the nominal limits. At startup, if the input voltage is below 90 Vac (typ.), circuit operations are

inhibited.

1.4

L6563H fast voltage feed-forward

The voltage on the L6563H VFF pin (pin 5) is the peak value of the voltage on the MULT pin

(pin 3). The RC network (R15 + R26, C12) connected to VFF completes a peak-holding

circuit. This signal is necessary to derive information from the RMS input voltage to

compensate the loop gain that is mains voltage dependent.

Generally speaking, if the time constant is too small, the voltage generated is affected by a

considerable amount of ripple at twice the mains frequency, therefore causing distortion of

the current reference (resulting in higher THD and lower PF). If the time constant is too

large, there is a considerable delay in setting the right amount of feed-forward, resulting in

excessive overshoot or undershoot of the pre-regulator's output voltage in response to large

line voltage changes.

RL 1.25V VBM

–



VBM

----------------------------------------------

69.67k



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