DocID16493 Rev 2

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AN3089

Main characteristics and circuit description

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1.5

L6566B current mode control and voltage feed forward

function

R16 senses the flyback MOSFET current and the signal is fed into the CS pin (#7)

connected to the PWM comparator. This signal is compared with the COMP pin (#9) signal,

which comes from the optocoupler.

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

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

output power is limited too.

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

input voltage, in order to keep the overload set point almost constant, which changes

according to the flyback input voltage, the L6566B implements a voltage feed forward

(#15) sensing the mains voltage through a resistor divider. A higher voltage causes

voltage.

On this board, VFF is implemented via the same circuit of brownout, saving components

and reducing consumption at light-load.

1.6

L6566B short-circuit protection

An internal comparator senses the COMP pin after the soft-start time: in case of a short, the

COMP pin goes high, and the said comparator activates a current source that restarts

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

5 V, the L6566B stops the operation and enters into the so-called “hiccup mode”. The

L6566B restarts with a startup sequence when the Vcc voltage drops below the Vcc restart

level (5 V). Because of the long time needed by the Vcc capacitor to drop to 5 V, it results in

an increase of the duration of the no load operation, therefore decreasing the power

dissipation and the stress of the power components. This sequence is repeated until the

short is removed, after that normal operation of the converter is automatically resumed.

A second protection, dedicated to protecting the circuit in the case of MOSFETs or output

diode short or transformer saturation, is implemented by another comparator on the CS pin

(#7). If the voltage on this pin exceeds the 1.5 V threshold, the IC immediately shuts down.

In this way a hiccup mode operation is still obtained, avoiding consequent failures due to the

power components overheating. To prevent spurious activation of the protection in the case

of temporary disturbances, for example during immunity tests, the comparator must be

triggered two consecutive times.

1.7

Overvoltage protection

The ZCD pin (#11) is connected to the auxiliary winding by a resistor divider. It implements

the OVP against feedback network failures. When the ZCD pin voltage exceeds 5 V four

consecutive times, the IC is shut down. This protection can be set as a latch or an

autorestart by the user with no additional components. On the board it is set as latched.

Therefore the operations can be resumed after a mains recycling.