STMicroelectronics 19 V - 90 W adapter with PFC for laptop computers using the L6563H and L6599A EVL6599A-90WADP EVL6599A-90WADP Fiche De Données

Doc ID 17230 Rev 1

7/31

Voltage on the L6563H VFF pin (#5) has the same value as the peak value of the voltage on 
the MULT pin (#3) and it is generated by the RC network (R15+R26, C12) connected to VFF, 
completing an internal peak-holding circuit. This signal is necessary to derive information on 
the RMS input voltage to compensate the loop gain which is mains voltage dependent.

In general, if the VFF time constant is too small, the voltage generated is affected by a 
considerable amount of ripple at twice the mains frequency. Because the VFF signal is fed 
into the multiplier the excessive ripple causes distortion of the current reference resulting in 
high THD and poor PF. On the other hand, if the time constant is set 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. 

To overcome this issue, the L6563H implements the new Fast Voltage Feed Forward 
function. As soon as the voltage on the VFF pin decreases from a set threshold (40mV 
typically), a mains dip is assumed and an internal switch rapidly discharges the VFF 
capacitor via a 10 kΩ resistor. Thanks to this feature it is possible to set an RC circuit with a 
long time constant, assuring a low THD, but keeping a fast response to mains voltage 
variations.

The downstream converter implements the ST L6599A, incorporating all the functions 
necessary to properly control the resonant converter with a 50 percent fixed duty cycle and 
working with variable frequency.

The transformer uses the integrated magnetic approach, incorporating the resonant series 
inductance. Therefore, no additional external coil is needed for the resonance. 

The transformer configuration chosen for the secondary winding is centre tap and makes 
use of a couple of power schottky rectifiers p/n STPS30H60CFP. A small LC filter has been 
added on the output, filtering the high frequency ripple. 

D15, R56, R62, R65, R66, Q5, and Q6 implement an output voltage “fast discharge” circuit, 
quickly discharging the output capacitors when the converter is turned off. It has been 
implemented to quickly decrease the residual output voltage once the converter is turned off 
at no-load.