MP44011 – BOUNDARY-MODE PFC CONTROLLER WITH HARMONIC INJECTION

MP44011 Rev. 1.01

www.MonolithicPower.com

11

1/12/2012

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Frequency of traditional

BCM flyback PFC

Frequency of

MP44011

t

Figure 7: Switching Frequency

Even with the same transformer core as used for

a traditional BCM PFC, the efficiency improves

with harmonic injection.

With harmonic injection, the minimum frequency

occurs not at the top of the line but at the line’s

choppy regions. Compared with traditional BCM

PFC, the frequency at the top of the line

increases.

Power Factor Correction

The MP44011 senses the inductor current

through the current sense pin and compares it to

the choppy sinusoid signal which is generated

from the multiplier output. When the external

power MOSFET turns on, the primary inductor

current rises linearly. When it reaches the choppy

sinusoid signal, the external power MOSFET

turns off and the secondary diode turns on. Then

the secondary inductor current begins to fall.

When the secondary inductor current reaches

zero, the power MOSFET turns on again, which

causes the primary inductor current to start rise .

The power circuit works in boundary conduction

mode and inductor current’s signal envelope has

a choppy sinusoidal shape. The average current

also has a choppy sinusoidal shape. This control

method can achieve a high power factor.

Layout Guide

For boundary-mode PFC operation, the output

feeds back to the FB pin for comparison with the

reference

voltage.

Therefore,

a

constant

reference voltage is very important for an

accurate output. Use short traces to connect the

FB pin to the feedback resistors.

A rectified AC voltage and COMP voltage

generates the multiplier output, which generates

the inductor current envelope. Place a small

ceramic capacitor from the MULT pin to S-GND

and place compensation components close to

the COMP pin and S-GND to improve noise

immunity.

For zero-current sensing, place R5 close to the

ZCS pin to prevent noise caused by long wire.

For inductor current sensing, keep the trace from

the current-sensing resistor to the CS pin pin as

short as possible—even though there is an on-

chip filter on the CS pin—to prevent falsely

turning off MOSFET. If the design bars the use of

a short trace, add an external filter from the

sense resistor to the CS pin. To prevent noise

from P-GND, limit the connection between P-

GND and S-GND to one point.

To keep the chip operational with a stable VIN

voltage, keep the VIN capacitor as close to the

VIN pin to limit voltage fluctuations.