NCP1200A 데이터 시트보기 (PDF) - ON Semiconductor

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NCP1200A

PWM Current-Mode Controller for Universal Off-Line Supplies Featuring Low Standby Power

ON Semiconductor 

VCC

12 V

10 V

5.4 V

NCP1200A

REGULATION

OCCURS

HERE

LATCHOFF

PHASE

TIM

Drv

E

INTERNAL

FAULT FLAG

DRIVER

PULSES

DRIVER

PULSES

TIM

E

STARTUP PHASE

FAULT IS

RELAXED

TIM

FAULT OCCURS HERE

E

Figure 21. If the fault is relaxed during the VCC natural fall down sequence, the IC automatically resumes.

If the fault still persists when VCC reached UVLOL, then the controller cuts everything off until recovery.

When this level crosses 5.4 V typical, the controller enters

a new startup phase by turning the current source on: VCC

rises toward 12 V and again delivers output pulses at the

UVLOH crossing point. If the fault condition has been

removed before UVLOL approaches, then the IC continues

its normal operation. Otherwise, a new fault cycle takes

place. Figure 21 shows the evolution of the signals in

presence of a fault.

Calculating the VCC Capacitor

As the above section describes, the fall down sequence

depends upon the VCC level: how long does it take for the

VCC line to go from 12 V to 10 V? The required time

depends on the startup sequence of your system, i.e. when

you first apply the power to the IC. The corresponding

transient fault duration due to the output capacitor charging

must be less than the time needed to discharge from 12 V to

10 V, otherwise the supply will not properly start. The test

consists in either simulating or measuring in the lab how

much time the system takes to reach the regulation at full

load. Let’s suppose that this time corresponds to 6 ms.

Therefore a VCC fall time of 10 ms could be well

appropriated in order to not trigger the overload detection

circuitry. If the corresponding IC consumption, including

the MOSFET drive, establishes at 1.8 mA for instance, we

can calculate the required capacitor using the following

formula:

Dt

+

DV @

i

C

, with DV = 2 V. Then for a wanted

Dt of 10 ms, C equals 9 mF or 22 mF for a standard value.

When an overload condition occurs, the IC blocks its

internal circuitry and its consumption drops to 350 mA

typical. This happens at VCC = 10 V and it remains stuck

until VCC reaches 5.4 V: we are in latchoff phase. Again,

using the calculated 22 mF and 350 mA current

consumption, this latchoff phase lasts: 296 ms.

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