NCP1603D100R2G【PDF 24/30 页】IC CTLR PFC/PWM COMBO 16-SOIC

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Sheet目录45

20页 21页 22页 23页 [24页]25页 26页 27页 28页

NCP1603

http://onsemi.com

Feedback in PFC Section

The output voltage of the PFC circuit (i.e., bulk voltage

bulk

) is sensed as a feedback current I

FB1

flowing into the

FB1 pin (Pin 9) of NCP1603. The FB1 pin voltage V

FB1

typically smaller than 5.0 V referring to Figure 31. It is

much lower than V

bulk

that is typically 400 V. Therefore,

FB1

is generally neglected.

FB1

bulk

* V

FB1

[

bulk

FB1

(eq. 19)

where R

FB1

is the feedback resistor connected the FB1 pin

(Pin 9) and the output voltage referring to Figure 45.

Then, the feedback current I

FB1

represents the bulk

voltage V

bulk

and will be used in the PFC section voltage

regulation, undervoltage protection (UVP), and

overvoltage protection (OVP).

Bulk Voltage Regulation in PFC Section

PFCstage feedback current I

FB1

, that presents bulk

voltage V

bulk

or the PFCstage output voltage, is regulated

with a reference current (I

ref

= 203 mA typical) as shown in

Figure 63. When I

FB1

is lower than 96% of I

ref

, the V

reg

that

is the output of the regulation block is as high as

control(max)

(1.05 V typical) that it gives the maximum

value on V

ton

and the maximum MOSFET on time and

bulk

increases. When I

FB1

is higher than I

ref

, the V

reg

becomes 0 V that gives no MOSFET on time and V

bulk

decreases. As a result, the bulk voltage V

bulk

is regulated

around the range between 96% and 100% of the nominal

value of R

FB1

?/SPAN> I

ref

Figure 63. Regulation Block

reg

ref

96%

FB1

1 V

Based on Equations 13 and 14 for a particular power

level, the V

control

is inversely proportional to V

. Hence,

in high V

condition V

control

is lower. It means that I

FB1

output voltage is higher based on the regulation block

characteristic in Figure 63. In other words, the V

control

the low V

condition is much higher than the high V

condition. In order to not overdesign the circuit in the

application, the V

control

in the low V

condition is usually

very closed to V

control(max)

. It makes the output voltage be

almost 96% of the nominal value of R

FB1

ref

in high V

condition.

The feedback resistor R

FB1

consists of two or three high

precision resistors in order to set the nominal V

bulk

precisely and for safety purpose.

The regulation block output V

reg

is connected to control

voltage V

control

through an internal resistor R

control

(300 kW typical) for the lowpass filter in Figure 62. The

control

and the time information of zero current are

collected in the V

control

processing circuit to generate V

ton

that is then compared to a ramp signal to generate the

MOSFET on time t

for power factor correction.

Current Sense in PFC Section

The PFC section senses the inductor current I

by the

current sense scheme in Figure 64. This scheme has the

advantages of: (1) the inrush current limitation by the

resistor. R

CS1

. and (2) the overcurrent protection and zero

current detection implemented in the same pin.

Figure 64. Current Sense in PFC Section

CS1

NCP1603

Gnd1

CS1

Inductor current I

passes through R

CS1

and creates a

negative voltage. This voltage is measured by a current I

flowing out of the CS1 pin (Pin 11). CS1 pin has an offset

voltage V

. This offset voltage is studied in the setting of

zero inductor current I

L(ZCD)

and the maximum inductor

current I

L(OCP)

(i.e., overcurrent protection threshold). A

typical variation of offset voltage V

versus sense current

is shown in Figure 35. Based on Figure 64, Equation 20

is derived.

* R

+ R

CS1

(eq. 20)

Zero Current Detection (ZCD) in PFC Section

The device recognizes zero inductor current when CS1

pin (Pin 11) sense current I

is smaller than I

S(ZCD)

(14 mA

typical). The offset voltage of the CS1 pin in this condition

is V

S(ZCD)

(7.5 mV typical). The inductor current I

L(ZCD)

at the ZCD condition is derived in Equation 21.

L(ZCD)

S(ZCD)

* V

S(ZCD)

CS1

(eq. 21)

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