NCP1217P65 데이터 시트보기 (PDF) - ON Semiconductor

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NCP1217P65

Enhanced PWM Current−Mode Controller for High−Power Universal Off−Line Supplies

ON Semiconductor 

NCP1217, NCP1217A

Startup Sequence

When the power supply is first powered from the mains

outlet, the internal current source (typically 7.0 mA) is

biased and charges up the VCC capacitor. When the voltage

on this VCC capacitor reaches the VCCON level (typically

12.8 V), the current source turns off and no longer wastes

any power. At this time, the VCC capacitor only supplies the

controller and the auxiliary supply is supposed to take over

before VCC collapses below VCCmin. Figure 16 shows the

internal arrangement of this structure.

12.8 V/5.6 V

+

−

8

6 mA or 0

6

4

HV

CVCC

Aux

Figure 16. The Current Source Brings VCC Above 12.8 V and then Turns Off

Once the power supply has started, the VCC shall be

constrained below 16 V, which is the maximum rating on

Pin 6. Figure 17 portrays a typical startup sequence with a

VCC regulated at 12.5 V.

13.5

12.8 V

12.5

REGULATION

11.5

10.5

9.5

3.00 M 8.00 M 13.0 M 18.0 M 23.0 M

t, TIME (sec)

Figure 17. A Typical Startup Sequence for

the NCP1217

Overload Operation

In applications where the output current is purposely not

controlled (e.g. wall adapters delivering raw DC level), it is

interesting to implement a true short−circuit protection. A

short−circuit actually forces the output voltage to be at a low

level, preventing a bias current to circulate in the

optocoupler LED. As a result, the auxiliary voltage also

decreases because it also operates in Flyback and thus

duplicates the output voltage, providing the leakage

inductance between windings is kept low. To account for this

situation and properly protect the power supply, NCP1217

hosts a dedicated overload detection circuitry. Once

activated, this circuitry imposes to deliver pulses in a burst

manner with a low duty−cycle. The system auto−recovers

when the fault condition disappears.

During the startup phase, the peak current is pushed to the

maximum until the output voltage reaches its target and the

feedback loop takes over. The auxiliary voltage takes place

after a few switching cycles and self−supplies the IC. In

presence of a short circuit on the output, the auxiliary

voltage will go down until it crosses the undervoltage

lockout level of typically 7.6 V. When this happens,

NCP1217 immediately stops the switching pulses and

unbiases all unnecessary logical blocks. The overall

consumption drops, while keeping the gate grounded, and

the VCC slowly falls down. As soon as VCC reaches typically

5.6 V, the startup source turns−on again and a new startup

sequence occurs, bringing VCC toward 12.8 V as an attempt

to restart. If the default has gone, then the power supply

normally restarts. If not, a new protective burst is initiated,

shielding the SMPS from any runaway. Figure 18 portrays

the typical operating signals in short circuit.

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