L6566ATR datasheet(27/51 Pages) STMICROELECTRONICS

27 / 51 page

L6566A

Application information

27/51

Figure 15.

Externally controlled burst-mode operation by driving pin COMP: timing

diagram

5.6

PWM comparator, PWM latch and voltage feedforward blocks

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

comparing it to the programming signal delivered by the feedforward block, determines the

exact time when the external MOSFET is to be switched off. Its output resets the PWM

latch, previously set by the oscillator or the ZCD triggering block, which will assert the gate

driver output low. The use of PWM latch avoids spurious switching of the MOSFET that

might result from the noise generated ("double-pulse suppression").

Cycle-by-cycle current limitation is realized with a second comparator (OCP comparator)

that senses the voltage across the current sense resistor Rs as well and compares this

voltage to a reference value VCSX. Its output is or-ed with that of the PWM comparator (see

the circuit schematic in Figure 17 on page 29). In this way, if the programming signal

delivered by the feedforward block and sent to the PWM comparator exceeds VCSX, it will be

the OCP comparator to reset first the PWM latch instead of the PWM comparator. The value

of Vcsx, thereby, determines the overcurrent setpoint along with the sense resistor Rs.

The power that QR flyback converters with a fixed overcurrent setpoint (like fixed-frequency

systems) are able to deliver changes with the input voltage considerably. Obviously, this is

not a problem if the flyback converter runs off a fixed voltage bus generated by the PFC pre-

regulator; however, with a tracking boost PFC (a "boost follower" PFC), the regulated output

voltage at maximum mains voltage can be even twice the value at minimum mains voltage.

In this case the issue is still there, although not as big as without PFC and wide-range

mains. With a 1:2 voltage change, the maximum transferable power at maximum line can be

50 % higher than at minimum line, as shown by the upper curve in the diagram of Figure 16.

The L6566A has the line feedforward function available to solve this issue.

Vcc

(pin 5)

(pin 4)

Vcc_OK

VccON

VccOFF

Vccrestart

Icharge

0.85 mA

Standby is commanded here

Vcc_PFC

(pin 6)

COMP

(pin 9)

Vout

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