AN2941 datasheet(10/37 Pages) STMICROELECTRONICS

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Main characteristics and circuit description

AN2941

10/37

Doc ID 15447 Rev 4

Output voltage feedback loop

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

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 modulating the voltage on 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 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 also limited.

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 L6566A 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.

L6566A short-circuit protection

In case of short-circuit, an internal comparator senses the COMP pin after the soft-start at

which time the COMP pin goes high, activating an internal 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 L6566A stops the operation and enters in the “hiccup mode”. The L6566A restarts

with a startup sequence when the Vcc voltage drops below the VCCrestart level (5 V).

Because of the long time needed by the VCC capacitor to drop to 5 V, the duration of the no-

load operation increases, thus decreasing the power dissipation and the stress of the power

components. This sequence is repeated until the short is removed following which normal

operation of the converter is automatically resumed.

Another comparator, whose threshold is 1.5 V and dedicated to protecting the circuit in case

of transformer saturation or secondary diodes short, is also provided. 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 resumed as soon as the VCC voltage drops below 5 V. In this way a hiccup

mode operation is still obtained, avoiding consequent failures due to overheating of the

power components.

Overvoltage protection

Pin #11 (ZCD) 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 the IC is

shut down. This protection can be set as latched or auto-restart by the user with no

additional components. On the board it is set as latched, therefore operations can resume

after a mains recycling.

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