AN2521 datasheet(9/33 Pages) STMICROELECTRONICS

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AN2521

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To achieve optimal efficiency the PFC stage implements the tracking boost function. It

consists of a PFC output voltage that follows the input voltage. Typically, in traditional PFC

stages, the dc output voltage is regulated at a fixed value (typically 400 volts) but in some

applications, such as this one using a flyback as the downstream converter, it could 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 improved 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. In the present case a

400 µH inductor has been used, while with a fixed output voltage PFC working at a similar

operating frequency, a 700 µH inductor is required.

To achieve the TBO function on the L6563, a dedicated input of the multiplier is available on

TBO pin #6. This function can be implemented by simply connecting a resistor (RT) between

the TBO pin and ground.

Figure 4.

L6563 tracking boost and voltage feed-forward blocks

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 the current, equal to

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

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

increase as well, and so will the current flowing through the resistor connected between

TBO and GND. A larger current will then be sunk by the INV pin and the output voltage of

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

opposite direction if the input voltage decreases.

To avoid an unwanted rise in output voltage 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

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

should be left open; the device will regulate at a fixed output voltage.

L6563

L6563A

INV

Vout

TBO

1:1

CURRENT

2.5V

E/A

COM

ITBO

MUL

Rectified mains

"ideal"

diode

MULTIPLIER

1/V

current

reference

ITBO

9.5V

VFF

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