Application information
L6562A
Example: Vo = 400V, ∆Vo = 40V. Then: R1 = 40V/27µA ≈ 1.5MΩ ;
R2 = 1.5 MΩ ·2.5/(400-2.5) = 9.43kΩ. The tolerance on the OVP level due to the L6562A will
be 40·0.13 = 5.3V, that is 1.2%.
When the load of a PFC pre-regulator is very low, the output voltage tends to stay steadily
above the nominal value, which cannot be handled by the Dynamic OVP. If this occurs,
however, the error amplifier output will saturate low; hence, when this is detected the
external power transistor is switched off and the IC put in an idle state (Static OVP). Normal
operation is resumed as the error amplifier goes back into its linear region. As a result, the
device will work in burst-mode, with a repetition rate that can be very low.
When either OVP is activated the quiescent consumption of the IC is reduced to minimize
the discharge of the Vcc capacitor and increase the hold-up capability of the IC supply
system.
6.2
6.3
Disable function
The INV pin doubles its function as a not-latched IC disable: a voltage below 0.2V shuts
down the IC and reduces its consumption at a lower value. To restart the IC, the voltage on
the pin must exceed 0.45 V. The main usage of this function is a remote ON/OFF control
input that can be driven by a PWM controller for power management purposes. However it
also offers a certain degree of additional safety since it will cause the IC to shutdown in case
the lower resistor of the output divider is shorted to ground or if the upper resistor is missing
or fails open.
THD optimizer circuit
The device is equipped with a special circuit that reduces the conduction dead-angle
occurring to the AC input current near the zero-crossings of the line voltage (crossover
distortion). In this way the THD (Total Harmonic Distortion) of the current is considerably
reduced.
A major cause of this distortion is the inability of the system to transfer energy effectively
when the instantaneous line voltage is very low. This effect is magnified by the high-
frequency filter capacitor placed after the bridge rectifier, which retains some residual
voltage that causes the diodes of the bridge rectifier to be reverse-biased and the input
current flow to temporarily stop.
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STMICROELECTRONICS [ ST ]
