AN4339 datasheet(5/27 Pages) STMICROELECTRONICS

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AN4339

Description

At startup, the L6564 device is powered by the VCC capacitor (C14), which is charged via

resistors R3 and R9. As the capacitor C14 charges to turn on threshold of the L6564 device

(typically at 12 V), the transformer T1 auxiliary winding (pins 5 - 6) generates the VCC

voltage, rectified by the D7 and R12, that powers the L6564 during normal operation. The

R11 is also connected to the auxiliary winding to provide the transformer demagnetization

signal to the L6564 ZCD pin, turning on the MOSFET at any switching cycle. The MOSFET

used is the STP4N80K5, a standard, low cost 800 V device housed in a TO-220 package,

and needing only a small heatsink. The transformer is layer type, using a standard ferrite

size EF25. The flyback reflected voltage is close to 170 V, providing enough room for the

leakage inductance voltage spike still within the reliability margin of the MOSFET even at

300 VAC input.

The RCD snubber circuit using the R4, C5 and D6 clamps the peak of the leakage

inductance voltage spike at the MOSFET turn-off. The resistor R2 is usually inserted in

series with a subbing capacitor C5 to kill the spike and reduces further the EMI generated

due to leakage spikes. The resistors R31 and R32 sense the current flowing into the

transformer primary side. Once the signal at the current sense pin has reached the level

programmed by the internal multiplier of the L6564, the MOSFET turns off. The divider R7,

R10, R13 and R21 provides to the L6564 multiplier pin with instantaneous voltage information

which is used to modulate the current flowing into the transformer primary side. The C16 is

a small noise suppression capacitor, of course the purpose of this capacitor is not to disturb

the actual sinusoidal mains information.

The output regulation is done by means of an isolated voltage loop by the optocoupler U2,

and using an inexpensive TL431 device to drive the optocoupler. The opto-transistor

modulates the input voltage of the L6564 internal amplifier, thus closing the voltage loop.

The output rectifier is a Schottky rectifier, selected according to its maximum reverse

voltage, forward voltage drop and power dissipation. A small LC filter is added on the output,

filtering the high frequency ripple.

1.3.1

PFC_OK

PFC_OK function in PFC operation

PFC pre-regulator output voltage monitoring/disable function

This pin senses the output voltage of the PFC pre-regulator through a resistor divider and is

used for protection purposes. If the voltage on the pin exceeds 2.5 V the IC stops switching

and restarts as the voltage on the pin falls below 2.4 V. However, if the voltage of the INV pin

falls 40 mV below that of the pin PFC_OK, a feedback failure is assumed. In this case the

device is latched off. Normal operation can be resumed only by cycling VCC, bringing its

value lower than 6 V before to move up to turn on threshold. If the voltage on this pin is

brought below 0.23 V the IC is shut down. To restart the IC the voltage on the pin must go

above 0.27 V. This can also be used as a remote on/off control input. Refer to Figure 3:

PFC_OK function in PFC operation, the R3 and R4 composes the network to activate PFC

output overvoltage shutdown.

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