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NCP1606

Cost Effective Power Factor Controller

ON Semiconductor 

NCP1606

When the switch is closed, the inductor current increases

linearly to its peak value. When the switch opens, the

inductor current linearly decreases to zero. At this point,

the drain voltage of the switch (Vd) is essentially floating

and begins to drop. If the next switching cycle does not

start, then the voltage will ring with a dampened frequency

around Vin. A simple derivation of equations (such as found

in AND8123), leads to the result that good power factor

correction in CRM operation is achieved when the on time

is constant across an ac cycle and is equal to:

Ton

+

2 @ POUT @ L

h @ VacRMS 2

(eq. 1)

A simple plot of this switching over an ac line cycle is

illustrated in Figure 24. The off time varies based on the

instantaneous line voltage, but the on time is kept constant.

This naturally causes the peak inductor current (ILPK) to

follow the ac line voltage.

The NCP1606 represents an ideal method to implement

this constant on time CRM control in a cost effective and

robust solution. The device incorporates an accurate

regulation circuit, a low power startup circuit, and

advanced protection features.

VOUT

Vinpk

ILpk

Iinpk

Vin(t)

IL(t)

Iin(t)

ON

MOSFET

OFF

Figure 24. Inductor Waveform During CRM Operation

ERROR AMPLIFIER REGULATION

The NCP1606 is configured to regulate the boost output

voltage based on its built in error amplifier (EA). The error

amplifier ’s negative terminal is pinned out to FB, the

positive terminal is tied to a 2.5 V ± 1.5% reference, and the

output is pinned out to Control (Figure 25).

ROUT1

FB

ROUT2

CCOMP

Control

EA

−

+

2.5 V

VCONTROL

PWM BLOCK

TON(max)

TON

Slope

+

Ct

ICHARGE

TPWM

VEAL

VCONTROL

VEAH

Figure 25. Error Amplifier and On Time Regulation Circuits

A resistor divider from the boost output to the input of the

EA sets the FB level. If the output voltage is too low, then

the FB level will drop and the EA will cause the control

voltage to increase. This increases the on time of the driver,

which increases the power delivered and brings the output

back into regulation. Alternatively, if the output voltage

(and hence FB voltage) is too high, then the control level

decreases and the driver on times are shortened. In this way,

the circuit regulates the output voltage (VOUT) so that the

VOUT portion that is applied to FB through the resistor

divider ROUT1 and ROUT2 is equal to the internal reference

(2.5 V). The output voltage can then be easily set according

to the following equation:

VOUT

+

2.5

V

@

ROUT1 ) ROUT2

ROUT2

(eq. 2)

A compensation network is placed between the FB and

Control pins to reduce the speed at which the EA responds

to changes in the boost output. This is necessary due to the

nature of an active PFC circuit. The PFC stage absorbs a

sinusoidal current from a sinusoidal line voltage. Hence,

the converter provides the load with a power that matches

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