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NCP1607

Cost Effective Power Factor Controller

ON Semiconductor 

NCP1607

input current to stay in phase with the input voltage. These

circuits operate at a higher frequency and so they are

smaller, lighter in weight, and more efficient than a passive

circuit. With proper control of an active PFC stage, almost

any complex load can be made to appear in phase with the

ac line, thus significantly reducing the harmonic current

content. Because of these advantages, active PFC circuits

have become the most popular way to meet harmonic

content requirements. Generally, they consist of inserting

a PFC pre−regulator between the rectifier bridge and the

bulk capacitor (Figure 22).

Rectifiers

PFC Preconverter

Converter

AC Line

High

+ Frequency

Bypass

Capacitor

NCP1607

+ Bulk

Storage

Capacitor

Load

Figure 22. Active PFC Pre−Converter with the NCP1607

The boost (or step up) converter is the most popular

topology for active power factor correction. With the

proper control, it produces a constant voltage while

drawing a sinusoidal current from the line. For medium

power (<300 W) applications, critical conduction mode

(also called borderline conduction mode) is the preferred

control method. Critical conduction mode (CRM) occurs at

the boundary between discontinuous conduction mode

(DCM) and continuous conduction mode (CCM). In CRM,

the next driver on time is initiated when the boost inductor

current reaches zero. CRM operation is an ideal choice for

medium power PFC boost stages because it combines the

lower peak currents of CCM operation with the zero current

switching of DCM operation. The operation and

waveforms in a PFC boost converter are illustrated in

Figure 23.

Diode Bridge

+ VIN

+

IN

−

IL

L

Vdrain

Diode Bridge

VIN

+

+

IN

−

IL

Vdrain

L

+

VOUT

The power switch is ON

With the power switch voltage being about zero, the

input voltage is applied across the coil. The coil current

linearly increases with a (VIN/L) slope.

Coil

Current

VIN/L

IL(pk)

The power switch is OFF

The coil current flows through the diode. The coil voltage is (VOUT −

VIN) and the coil current linearly decays with a (VOUT − VIN)/L slope.

(VOUT − VIN)/L

Critical Conduction Mode:

Next current cycle starts as

soon as the core is reset.

Vdrain

VOUT

VIN

If next cycle does not start

then Vdrain rings towards VIN

Figure 23. Schematic and Waveforms of an Ideal CRM Boost Converter

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