NCP1027_15 datasheet(13/30 Pages) ONSEMI

Part #	NCP1027

Description	High-Voltage Switcher for Medium Power Offline SMPS Featuring Low Standby Power

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Manufacturer	ONSEMI [ON Semiconductor]

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NCP1027

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13

Fault Condition – Output Short−Circuit

As soon as VCC reaches VCCON, drive pulses are

internally enabled. If everything is correct, the auxiliary

winding increases the voltage on the VCC pin as the output

voltage rises. During the start−sequence, the controller

smoothly ramps up the peak current to Imax setting, e.g.

Ipeak_HI, which is reached after a typical period of 1.0 ms.

As soon as the peak current setpoint reaches its maximum

(during the startup period but also anytime an overload

occurs), an internal error flag is asserted, Ipflag, indicating

that the system has reached its maximum current limit set

point (Ip = Ip max). The assertion of this flag triggers a

55 ms counter. If at counter completion Ipflag remains

asserted, all driving pulses are stopped and the part stays off

during eight periods of 55 ms (440 ms). A new attempt to

restart occurs and will last 55 ms providing the fault is still

present. If the fault still affects the output, a safe burst mode

is entered, affected by a low duty−cycle operation (11%).

When the fault disappears, the power supply quickly

resumes operation. Figure 27 depicts this particular mode.

Figure 27. In case of short−circuit or overload, the NCP1027 protects itself and the power supply via a low

frequency burst mode. The VCC is maintained by the current source and self−supplies the controller.

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