NCP1255 查看數據表（PDF） - ON Semiconductor

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NCP1255

Current-Mode PWM Controller for Off-line Power Supplies featuring Peak Power Excursion

ON Semiconductor 

NCP1255

40.0

20.0

N1 (Vout + Vf)

off−time

0

−20.0

−40.0

−N2Vbulk

on−time

464u

472u

480u

time in seconds

488u

496u

Figure 5. The signal obtained on the auxiliary winding swings negative during the on−time.

Let’s assume we need to reduce the peak current from 2.5 A at low line, to 2 A at high line. This corresponds to a 20%

reduction or a set point voltage of 640 mV. To reach this level, then the negative voltage developed on the OPP pin must reach:

VOPP + 640 m * 800 m + −160 mV

(eq. 6)

RoppU

This point will

be adjusted to

reduce the ref

at hi line to the

desired level.

Vcc

aux

Vdd

swings to:

Vout during toff

−NVin during ton

from FB

reset

OPP

Iopp

RoppL

K1 SUM2

ref

CS

K2

0.8 V

±5%

ref = 0.8 V + VOPP

(VOPP is negative)

Figure 6. The OPP circuitry affects the maximum peak current set point by

summing a negative voltage to the internal voltage reference.

Let us assume that we have the following converter

characteristics:

Vout = 19 V

Vin = 85 to 265 V rms

N1 = Np:Ns = 1:0.25

N2 = Np:Naux = 1:0.18

Given the turns ratio between the primary and the

auxiliary windings, the on−time voltage at high line

(265 Vac) on the auxiliary winding swings down to:

Vaux + −N2Vin,max + −0.18 375 + −67.5 V (eq. 7)

To obtain a level as imposed by Equation 6, we need to

install a divider featuring the following ratio:

Div

+

0.16

67.5

[

2.4

m

(eq. 8)

If we arbitrarily fix the pull−down resistor ROPPL to 1 kW,

then the upper resistor can be obtained by:

ROPPU

+

67.5 * 0.16

0.16ń1 k

[

421

kW

(eq. 9)

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