Main characteristics and circuit description

AN3014

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Doc ID 16064 Rev 2

order to avoid capacitive mode operation, but to allow the resonant stage to operate even in

case of mains sag and consequent PFC output dip.

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 the peak-holding

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

loop gain, which 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, thus causing distortion of the

current reference (resulting in high THD and poor 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 preregulator'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 to a set threshold (40 mV typically), a

mains dip is assumed and an internal switch rapidly discharges the VFF capacitor via a 10-

k

Ω resistor. Thanks to this feature, it is possible to set an RC circuit with a long time

constant, assuring a low THD and maintaining a fast response to mains dip.

Resonant power stage

The downstream converter employs ST’s L6599A, which incorporates all the functions

necessary to properly control the resonant converter with a 50% fixed duty cycle and works

with a variable frequency.

The transformer uses the integrated magnetic approach, incorporating a resonant series

inductor. Thus, no additional external coil is needed for the resonance. The transformer

configuration chosen for the secondary winding is center tap.

On the secondary side, the output rectification is controlled by the SRK2000, an SR driver

dedicated to LLC resonant topology.

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

which quickly discharges the output capacitors when the converter is turned off. It has been

implemented to quickly decrease the residual output voltage after the converter is turned off

at no load.

Output voltage feedback loop

The feedback loop is implemented by means of a typical circuit using a TL431 to modulate

the current in the optocoupler diode.

On the primary side, R34 - connecting the RFMIN pin (pin 4) to the optocoupler

phototransistor - closes the feedback loop and its value sets the maximum switching

frequency at about 130 kHz. This value has been chosen to limit the switching losses at light

load operation. R31, which connects the same pin to ground, sets the minimum switching

frequency. The R-C series (R44 and C18) sets both the soft-start maximum frequency and

duration.