L6591

Application information

17/32

Figure 10.

How the L6591 can switch off a PFC controller at light load

5.3

PWM control block

The device is specific for secondary feedback. Typically, there is a TL431 at the secondary

side and an optocoupler that transfers output voltage information to the PWM control at the

primary side, crossing the isolation barrier. The PWM control input (pin #7, COMP) is driven

directly by the phototransistor’s collector (the emitter is grounded) to modulate the duty

cycle. It is recommended to place a small filter capacitor between the pin and GND (#11), as

close to the IC as possible to reduce switching noise pick up, to set a pole in the output-to-

control transfer function.

5.4

PWM comparator, PWM latch and hiccup-mode OCP

The PWM comparator senses the voltage across the current sense resistor (Rs) and, by

comparing it with the programming signal derived by the voltage on pin COMP (#7),

determines the exact time when the high-side MOSFET is to be switched off. The PWM

latch avoids spurious switching, which might be caused by the noise generated (“double-

pulse suppression”).

A second comparator senses the voltage on the current sense input and shuts the IC down

if the voltage at the pin exceeds 1.5 V. Such an anomalous condition is typically generated

by either a short circuit of one of the secondary rectifiers or a shorted secondary winding or

a saturated transformer. This condition is latched as long as the IC is supplied; hence if the

IC is supplied by an external source it is necessary to disconnect the source to restart the

IC.

To distinguish an actual malfunction from a disturbance (e.g. induced during ESD tests), the

first time the comparator is tripped the protection circuit enters a “warning state”. If in the

next switching cycle the comparator is not tripped, a temporary disturbance is assumed and

the protection logic will be reset in its idle state; if the comparator will be tripped again a real

malfunction is assumed and the L6591 will be stopped.

If the device is self-supplied no energy is coming from the self-supply circuit, then the

voltage on the Vcc capacitor will decay and cross the UVLO threshold after some time,

which clears the latch. The internal start-up generator is still off, then the Vcc voltage still

needs to go below its restart voltage before the Vcc capacitor is charged again and the IC

restarted. Ultimately, either of the above mentioned failures will result in a low-frequency

intermittent operation (Hiccup-mode operation), with very low stress on the power circuit.

The timing diagram of Figure 11 illustrates this operation.

L6591

PFC_STOP

8

L6562

6

Vref

ZCD

5.6 k

47 k

BC547

BC547

L6591

PFC_STOP

8

L6563

PFC_OK

(AC_OK)

L6591

PFC_STOP

8

L6562

6

Vref

ZCD

5.6 kΩ

47 kΩ

BC547

BC547

L6591

PFC_STOP

8

L6563

PFC_OK

(AC_OK)