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NCP1251

Current-Mode PWM Controller for Off-line Power Supplies

ON Semiconductor 

NCP1251

Finally, please note that another comparator internally

fixes the maximum peak current set point to 0.8 V even if the

OPP pin is inadvertently biased above 0 V.

Frequency Foldback

The reduction of no−load standby power associated with

the need for improving the efficiency, requires a change to

the traditional fixed−frequency type of operation. This

controller implements a switching frequency foldback when

the feedback voltage passes below a certain level, Vfold, set

around 1.5 V. At this point, the oscillator enters frequency

Frequency

foldback and reduces its switching frequency. The peak

current setpoint follows the feedback pin until its level

reaches 1.05 V. Below this value, the peak current freezes to

Vfold/4.2 (250 mV or 31% of the maximum 0.8 V setpoint)

and the only way to further reduce the transmitted power is

to reduce the operating frequency down to 26 kHz. This

value is reached at a voltage feedback level of 350 mV

typically. Below this point, if the output power continues to

decrease, the part enters skip cycle for the best noise−free

performance in no−load conditions. Figure 44 depicts the

adopted scheme for the part.

Peak current setpoint

Fsw

FB

65 kHz

max

VCS

0.8 V

max

26 kHz min

350 mV 1.5 V

Vfold,end Vfold

VFB

3.4 V

[ 0.36 V

[ 0.25 V min

Vfreeze Vfold

1.05 V 1.5 V

VFB

3.4 V

Figure 44. By Observing the Voltage on the Feedback Pin, the Controller Reduces its Switching Frequency for an

Improved Performance at Light Load

Auto−Recovery Short−Circuit Protection

In case of output short−circuit or if the power supply

experiences a severe overloading situation, an internal error

flag is raised and starts a countdown timer. If the flag is

asserted longer than 100 ms, the driving pulses are stopped

and the VCC pin slowly goes down to around 7 V. At this

point, the controller wakes−up and the VCC builds up again

due to the resistive starting network. When VCC reaches

VCCON, the controller attempts to re−start, checking for the

absence of the fault. If the fault is still there, the supply enters

another cycle of so−called hiccup mode. If the fault has

cleared, the power supply resumes normal operation. Please

note that the soft−start is activated during each of the re−start

sequence.

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