Main characteristics and circuit description

AN2690

6/31

The L6563 has a similar protection on the RUN pin (#10) but in this schematic it is not used

because in this architecture it acts as slave, therefore the main controls are managed by the

flyback stage.

Output voltage feedback loop

The output regulation is done by means of two control loops, a voltage and a current one

working alternatively. A dedicated control IC, the TSM1014 (U5), has been used. It

integrates two operational amplifiers (used as error amplifiers) and a precise voltage

reference. The output signal of the error amplifiers drives an optocoupler SFH617A-4 (U3)

to get the required insulation of the secondary side and modulates the voltage on the COMP

pin (#9) of the L6566A.

L6566A current mode control and voltage feed-forward function

R52 and R53 sense the Q5 MOSFET current of the flyback and the signal is fed into pin #7

(CS), connected to the PWM comparator. This signal is thus compared with the COMP (pin

#9) signal, which is coming from the optocoupler.

The maximum power that the converter can deliver is set by a comparator limiting the peak

of the primary current, comparing the CS and an internal threshold (VCSX). If the current

signal exceeds the threshold, the comparator limits the MOSFET duty cycle, hence the

output power is limited too.

Because the maximum transferable power depends on both the primary peak current and

the input voltage, in order to keep almost constant the overload set point that would change

according to flyback input voltage, the controller implements a voltage feed-forward function

via a dedicated pin. Hence, VCSX is modulated by the voltage on pin #15 (VFF) sensing the

bulk voltage by a resistor divider. A higher voltage causes a smaller VCS,MAX so that the

maximum power can be kept almost constant at any input voltage.

The resistor R60 connected between pin #7 (CS) and pin #12 (MODE/SC) provides the

correct slope compensation to the current signal, required for the correct loop stability.

L6566A short-circuit protection

In case of a short, an internal comparator senses the COMP pin after the soft-start time. In

that condition the COMP pin goes high, activating an internal current source that restarts to

charge the soft-start capacitor from the initial 2 V level. If the voltage on this pin reaches 5 V,

the L6566A stops the operation and enters in "Hiccup mode". The device restarts with a

duration of the no-load operation, thus decreasing the power dissipation and the stress of

power components. This sequence is repeated until the short is removed, subsequently the

normal operation of the converter is automatically resumed.

Another comparator having its threshold at 1.5 V dedicated to protecting the circuit in case

of transformer saturation or secondary diodes short is provided too. If the voltage on the CS

pin (#7) exceeds this threshold two consecutive times, the IC immediately shuts down and

latches off. This is intended to prevent spurious activation of the protection in case of

temporary disturbances, for example during the immunity tests. Even in this case the IC

operation is still obtained, avoiding consequent failures due to the power components

overheating.