AN3065 datasheet(22/33 Pages) STMICROELECTRONICS

22 / 33 page

Test results and significant waveforms

AN3065

22/33

Doc ID 16279 Rev 2

The event of an open loop is captured in

Figure 30. We can notice the protection

intervention, latching the operation of the L6563S. As mentioned previously, to restart the

system the input power must be recycled.

4.6

TBO (tracking boost option)

To use the TBO function on L6563S, a dedicated input of the multiplier is available on pin #6

(TBO). The function can be implemented by simply connecting a resistor (RT) between the

TBO pin and ground.

Usually, in traditional PFC stages, the DC output voltage is regulated at a fixed value

(typically 400 V) but in some applications, it may be advantageous to regulate the PFC

output voltage with the "tracking boost" or "follower boost" approach. In this way the circuit

with the TBO function provides better efficiency and thanks to the lower differential voltage

across the boost inductor, the value of L2 can be reduced as compared to the same circuit

without the TBO function.

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

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

V(TBO)/RT, that is internally 1:1 mirrored and sunk from pin INV (pin 1) input of the error

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

increases also as well as the current flowing through the resistor connected between TBO

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

PFC pre-regulator will be forced to go higher. Obviously, the output voltage will move in the

opposite direction if the input voltage decreases.

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

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

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

Figure 29.

EVL6563S-100W TM PFC load

transient at 115 Vac, 60 Hz, full load

to no load

Figure 30.

EVL6563S-100W TM PFC open loop

at 115 Vac, 60 Hz, full load

CH1: PFC output voltage

CH2: PFC_OK (pin #7)

CH3: gate drive (pin #13)

CH4: Iout

CH1: PFC output voltage

CH2: Vcc (pin #14)

CH3: gate drive (pin #13)

CH4: PWM_LATCH (pin #8)

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