NCP1653_15 datasheet(13/21 Pages) ONSEMI

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NCP1653, NCP1653A

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As a result, the on time t1 is reduced. Reduction of on time

makes the loss of the inductor and power MOSFET smaller.

Hence, it allows cheaper cost in the inductor and power

MOSFET or allows the circuit components to operate at a

lower stress condition in most of the time.

Output Feedback

The output voltage Vout of the PFC circuit is sensed as a

feedback current IFB flowing into the FB pin (Pin 1) of the

device. Since the FB pin voltage VFB1 is much smaller than

Vout, it is usually neglected.

(eq.14)

IFB +

Vout * VFB1

RFB

[

Vout

RFB

where RFB is the feedback resistor across the FB pin

(Pin 1) and the output voltage referring to Figure 2.

Then, the feedback current IFB represents the output

voltage Vout and will be used in the output voltage

regulation,

undervoltage

protection

(UVP),

and

overvoltage protection (OVP).

Output Voltage Regulation

Feedback current IFB which represents the output voltage

Vout is processed in a function with a reference current

(Iref = 200 mA typical) as shown in regulation block

function in Figure 32. The output of the voltage regulation

block, low−pass filter on Vcontrol pin and the Icontrol =

Vcontrol / R1 block is in Figure 30 is control current Icontrol.

And the input is feedback current IFB. It means that Icontrol

is the output of IFB and it can be described as in Figure 32.

There are three linear regions including: (1) IFB < 96% ×

Iref, (2) 96% × Iref <IFB < Iref, and (3) IFB > Iref. They are

discussed separately as follows:

Figure 32. Regulation Block

Icontrol

Iref

96%

IFB

Icontrol(max)

Region (1): IFB < 96% × Iref

When IFB is less than 96% of Iref (i.e., Vout < 96% RFB

× Iref), the NCP1653 operates in follower boost mode. The

regulation block output Vreg is at its maximum value.

Icontrol becomes its maximum value (i.e., Icontrol =

Icontrol(max) = Iref/2 = 100 mA) which is a constant. (eq.13)

becomes (eq.15).

Vout + h

2RS R vac Icontrol(max) Vref Vac

RM RCS Pout

(eq.15)

Vac

Pout

The output voltage Vout is regulated at a particular level

with a particular value of RMS input voltage Vac and output

power Pout. However, this output level is not constant and

depending on different values of Vac and Pout. The follower

boost operating area is illustrated in Figure 33.

Figure 33. Follower Boost Region

Vout

ac(max)

ac(min)

Pout(min)

Pout(max)

Vin

96% Iref RFB

1. Pout increases, Vout decreases

2. Vac decreases, Vout decreases

Region (2): 96% × Iref < IFB < Iref

When IFB is between 96% and 100% of Iref (i.e., 96% RFB

× Iref < Vout < RFB × Iref), the NCP1653 operates in constant

output voltage mode which is similar to the follower boost

mode characteristic but with narrow output voltage range.

The regulation block output Vreg decreases linearly with

IFB in the range from 96% of Iref to Iref. It gives a linear

function of Icontrol in (eq.16).

(eq.16)

Icontrol +

Icontrol(max)

0.04

1 *

Vout

RFB Iref

Resolving (eq.16) and (eq.13),

Vout +

Vac

RM RCS

2RS R vac Vref

0.04

Icontrol(max)

Pout

h )

Vac

RFB Iref

(eq.17)

According to (eq.17), output voltage Vout becomes RFB

× Iref when power is low (Pout ≈ 0). It is the maximum value

of Vout in this operating region. Hence, it can be concluded

that output voltage increases when power decreases. It is

similar to the follower boost characteristic in (eq.15). On

the other hand in (eq.17), output voltage Vout becomes RFB

× Iref when RMS input voltage Vac is very high. It is the

maximum value of Vout in this operating region. Hence, it

can also be concluded that output voltage increases when

RMS input voltage increases. It is similar to another

follower boost characteristic in (eq.15). This characteristic

is illustrated in Figure 34.

Figure 34. Constant Output Voltage Region

Vout

ac(max)

ac(min)

Pout(min)

Pout(max)

96% Iref RFB

1. Pout increases, Vout decreases

2. Vac decreases, Vout decreases

Iref RFB

Region (3): IFB > Iref

When IFB is greater than Iref (i.e., Vout > RFB × Iref), the

NCP1653 provides no output or zero duty ratio. The

regulation block output Vreg becomes 0 V. Icontrol also

becomes zero. The multiplier voltage VM in (eq.8)

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