FAN7930BMX Ver la hoja de datos (PDF) - Fairchild Semiconductor

Número de pieza

componentes Descripción

Fabricante

FAN7930BMX

Critical Conduction Mode PFC Controller

Fairchild Semiconductor 

9. THD Optimization: Total harmonic distortion (THD)

is the factor that dictates how closely input current

shape matches sinusoidal form. The turn-on time of the

PFC controller is almost constant over one AC line

period due to the extremely low feedback control

response. The turn-off time is decided by the current

decrease slope of the boost inductor made by the input

voltage and output voltage. Once inductor current

becomes zero, resonance between COSS and the boost

inductor makes oscillating waveforms at the drain pin

and auxiliary winding. By checking the auxiliary winding

voltage through the ZCD pin, the controller can check

the zero current of boost inductor. At the same time, a

minor delay time is inserted to determine the valley

position of drain voltage. The input and output voltage

difference is at its maximum at the zero cross point of

AC input voltage. The current decrease slope is steep

near the zero cross region and more negative inductor

current flows during a drain voltage valley detection

time. Such a negative inductor current cancels down the

positive current flows and input current becomes zero,

called “zero-cross distortion” in PFC.

To improve this, lengthened turn-on time near the zero

cross region is a well-known technique, though the

method may be different from company to company and

may be proprietary. FAN7930B embodies this by

sourcing current through the ZCD pin. Auxiliary winding

voltage becomes negative when the MOSFET turns on

and is proportional to input voltage. The negative

clamping circuit of ZCD outputs the current to maintain

the ZCD voltage at a fixed value. The sourcing current

from the ZCD is directly proportional to the input

voltage. Some portion of this current is applied to the

internal sawtooth generator together with a fixed-current

source. Theoretically, the fixed-current source and the

capacitor at sawtooth generator decide the maximum

turn-on time when no current is sourcing at ZCD clamp

circuit and available turn-on time gets shorter

proportional to the ZCD sourcing current.

Figure 37. Input and Output Current

Near Input Voltage Peak

Figure 39. Circuit of THD Optimizer

Figure 38. Input and Output Current Near

Input Voltage Peak Zero Cross

© 2010 Fairchild Semiconductor Corporation

FAN7930B • Rev. 1.0.2

Figure 40. Effect of THD Optimizer

By THD optimizer, turn-on time over one AC line period

is proportionally changed, depending on input voltage.

Near zero cross, lengthened turn-on time improves THD

performance.

www.fairchildsemi.com

15

Share Link: