NCP1351 データシートの表示（PDF） - ON Semiconductor

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NCP1351

Variable Off Time PWM Controller

ON Semiconductor 

NCP1351

+

CBulk

CS

C4

22p

Roffset

DRV

VCC

Rsense

LP

DRV

Daux

+

CVCC

Laux

ILp

R1

150k

D2

1N4148

C3

270p

ROPP

100k

Figure 12. The OPP is Relatively Easy to Implement and It Does not Waste Power

Suppose you would need to reduce the peak current by

15% in high−line conditions. The turn−ratio between the

auxiliary winding and the primary winding is Naux. Assume

its value is 0.15. Thus, the voltage on Daux cathode swings

negative during the on time to a level of:

Vaux_peak + −Vin_max Naux + −375 0.15 + −56 V

(eq. 7)

If we selected a 3.7 kW resistor for Roffset, then the

maximum sense voltage being developed is:

Vsense + 3.7 k 270 m + 1 V

(eq. 8)

The small RC network made of R1 and C3, purposely limits

the voltage excursion on D2 anode. Assume the primary

inductance value gives an on time of 3 μs at high−line. The

voltage across C3 thus swings down to:

VC3

+

tonVaux_peak

R1C3

+

−

3

150

m

k

56

+ −4.2 V

270 p

(eq. 9)

Typically, we measured around –4 V on our 50 W prototype.

By calculation, we want to decrease the peak current by

15%. Compared to the internal 270 mA source, we need to

derive:

Ioffset + −0.15 270 m + −40.5 mA

(eq. 10)

Thus, from the –4 V excursion, the ROPP resistor is

derived by:

ROPP

+

4

40.5

m

+

98

kW

(eq. 11)

After experimental measurements, the resistor was

normalized down to 100 kW.

Feedback

Unlike other controllers, the feedback in the NCP1351

works in current rather than voltage. Figure 13 details the

internal circuitry of this particular section. The optocoupler

injects a current into the FB pin in relationship with the

input/output conditions.

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