FAN7930CM Ver la hoja de datos (PDF) - Fairchild Semiconductor

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FAN7930CM

Critical Conduction Mode PFC Controller

Fairchild Semiconductor 

Figure 37. Startup Control without Overshoot

8. 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 determined 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 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.

Figure 39. Input and Output Current Near Input

Voltage Peak Zero Cross

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

cross region is a well-known technique, though the

method may vary and may be proprietary. FAN7930C

optimizes 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 determine 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 38. Input and Output Current Near Input

Voltage Peak

© 2010 Fairchild Semiconductor Corporation

FAN7930C • Rev. 1.0.0

15

Figure 40. Circuit of THD Optimizer

www.fairchildsemi.com

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