Application information

L6563 - L6563A

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Essentially, the circuit artificially increases the ON-time of the power switch with a positive

offset added to the output of the multiplier in the proximity of the line voltage zero-crossings.

This offset is reduced as the instantaneous line voltage increases, so that it becomes

negligible as the line voltage moves toward the top of the sinusoid. Furthermore the offset is

little offset at low line, where energy transfer at zero crossings is typically quite good, and a

larger offset at high line where the energy transfer gets worse.

The effect of the circuit is shown in Figure 36, where the key waveforms of a standard TM

PFC controller are compared to those of this chip.

To take maximum benefit from the THD optimizer circuit, the high-frequency filter capacitor

after the bridge rectifier should be minimized, compatibly with EMI filtering needs. A large

capacitance, in fact, introduces a conduction dead-angle of the AC input current in itself -

even with an ideal energy transfer by the PFC pre-regulator - thus reducing the effectiveness

of the optimizer circuit.

6.5

Tracking Boost function

In some applications it may be advantageous to regulate the output voltage of the PFC pre-

regulator so that it tracks the RMS input voltage rather than at a fixed value like in

conventional boost pre-regulators. This is commonly referred to as "tracking boost" or

"follower boost" approach.

and ground. The TBO pin presents a DC level equal to the peak of the MULT pin voltage and

is then representative of the mains RMS voltage. The resistor defines a current, equal to

amplifier. In this way, when the mains voltage increases the voltage at TBO pin will increase

as well and so will do the current flowing through the resistor connected between TBO and

GND. Then a larger current will be sunk by INV pin and the output voltage of the PFC pre-

regulator will be forced to get higher. Obviously, the output voltage will move in the opposite

direction if the input voltage decreases.

To avoid undesired output voltage rise should the mains voltage exceed the maximum

specified value, the voltage at the TBO pin is clamped at 3V. By properly selecting the

multiplier bias it is possible to set the maximum input voltage above which input-to-output

tracking ends and the output voltage becomes constant. If this function is not used, leave

the pin open: the device will regulate a fixed output voltage.

Starting from the following data:

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Vox = absolute maximum limit for the regulated output voltage;

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∆Vo = OVP threshold,