NCP1603D100R2G【PDF 19/30 页】IC CTLR PFC/PWM COMBO 16-SOIC

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Sheet目录45

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NCP1603

http://onsemi.com

Frequency Jittering

93.6 kHz

100 kHz

106.4 kHz

5 ms

time

PWM Section Oscillator Frequency

Figure 51. Frequency Jittering of PWM Oscillator

Frequency jittering is a method used to soften the EMI

signature by spreading the energy in the vicinity of the main

switching component. The PWM Section offers a typical

?.4% deviation on the nominal switching frequency

(100 kHz typical). A sweep sawtooth modulates the

100 kHz clock up and down with a 5.0 ms period.

Figure 51 illustrates the ?.4% variation of the jittering

oscillator frequency versus time.

Fault Condition

Figure 52 illustrates the fault detection circuitry and its

timing diagram. When fault (or output short circuit)

happens, the output voltage collapses and the optocoupler

is opened. V

FB2

is internally pulled to be higher than 3.0 V

(3.8 V typical). Then, the controller activates an error flag

when (V

FB2

/3) is greater than the softstart voltage V

that is 1.0 V after the 2.5 ms from startup.

When the circuit is powering up in the beginning, the

output voltage is not yet established and FB2 pin (Pin 2) is

opened. Therefore, there is a 125 ms timer to allow the

circuit to establish an initial output voltage. Then, a fault

(or short circuit) condition is recognized when an error flag

FB2

q 3.0 V) can last for 125 ms. When a fault is

detected, Out2 (Pin 13) goes low. The power supply stops

delivering power to the output. On the other hand, the V

aux

(= V

CC1

) also goes low. The V

aux

will restore immediately

when the error flag disappears.

This fault detection method offers advantage of getting

rid of the auxiliary winding information that cannot truely

represent the output voltage when the flyback transformer

is badly coupled.

FB2

Vdd

20 k

55 k

25 k

FB2

SoftStart

1 V Max

Start Vaux

Enable Vaux/PFC

125 ms

Delay

Fault

Disable

Vaux/PFC

and Out2

SoftStart Period 2.5 ms

Figure 52. Block Diagram and Timing Diagram of Fault Detection

1 V

FB2

aux

125ms

aux

starts when V

FB2

within regulation window

FB2

< 3 V).

(i.e., normal operation)

aux

stops when V

FB2

is out of

regulation window (V

FB2

> 3 V)

for more than 125 ms.

(i.e., fault condition)

1 V

time

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