NCP1251 데이터 시트보기 (PDF) - ON Semiconductor

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NCP1251

Current-Mode PWM Controller for Off-line Power Supplies

ON Semiconductor 

RoppU

This p oin t will

be adjusted to

reduce the ref

at hi line to the

desired level.

Io p p

OPP

R oppL

NCP1251

VCC

aux

swings to:

Vout during toff

−N V in during ton

VDD

from FB

K1

K2

0.8 V

$5%

SU M2

re f

ref = 0.8 V + VOPP

(V O P P is negativ e)

reset

CS

Figure 42. 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 Vrms

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:

Peak current

setpoint

Div + 0.16 [ 2.4m

67.5

(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

0.16ń1k

[

421

kW

(eq. 9)

If we now plot the peak current set point obtained by

implementing the recommended resistor values, we obtain

the following curve (Figure 43):

100%

80%

375

Vbulk

Figure 43. The Peak Current Regularly Reduces Down to 20% at 375 Vdc

The OPP pin is surrounded by Zener diodes stacked to

protect the pin against ESD pulses. These diodes accept

some peak current in the avalanche mode and are designed

to sustain a certain amount of energy. On the other side,

negative injection into these diodes (or forward bias) can

cause substrate injection which can lead to an erratic circuit

behavior. To avoid this problem, the pin is internally

clamped slightly below –300 mV which means that if more

current is injected before reaching the ESD forward drop,

then the maximum peak reduction is kept to 40%. If the

voltage finally forward biases the internal zener diode, then

care must be taken to avoid injecting a current beyond

–2 mA. Given the value of ROPPU, there is no risk in the

present example.

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