Q0465R 查看數據表（PDF） - Fairchild Semiconductor

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Q0465R

Green-Mode Fairchild Power Switch (FPS™) for Quasi-Resonant Operation - Low EMI and High Efficiency

Fairchild Semiconductor 

4.1 Overload Protection (OLP): Overload is defined as

the load current exceeding its normal level due to an

unexpected abnormal event. In this situation, the

protection circuit should trigger to protect the SMPS.

However, even when the SMPS is in the normal

operation, the overload protection circuit can be

triggered during the load transition. To avoid this

undesired operation, the overload protection circuit is

designed to trigger only after a specified time to

determine whether it is a transient situation or a true

overload situation. Because of the pulse-by-pulse

current limit capability, the maximum peak current

through the SenseFET is limited, and therefore the

maximum input power is restricted with a given input

voltage. If the output consumes more than this maximum

power, the output voltage (VO) decreases below the set

voltage. This reduces the current through the opto-

coupler LED, which also reduces the opto-coupler

transistor current, thus increasing the feedback voltage

(VFB). If VFB exceeds 2.5V, D1 is blocked and the 5µA

current source starts to charge CB slowly up to VCC. In

this condition, VFB continues increasing until it reaches

6V, when the switching operation is terminated, as

shown in Figure 29. The delay time for shutdown is the

time required to charge CFB from 2.5V to 6V with 5µA. A

20 ~ 50ms delay time is typical for most applications.

VFB

FSQ 0465 R ev.00

O verload protection

6.0V

3R

OSC

PWM

LEB

200ns

SQ

RQ

R

Gate

driver

AOCP

FSQ0465 Rev.00

Rsense

VOCP

2

GND

Figure 30. Abnormal Over-Current Protection

4.3 Output-Short Protection (OSP): If the output is

shorted, steep current with extremely high di/dt can flow

through the SenseFET during the LEB time. Such a

steep current brings high voltage stress on the drain of

SenseFET when turned off. To protect the device from

such an abnormal condition, OSP is included in the FSQ-

series. It is comprised of detecting VFB and SenseFET

turn-on time. When the VFB is higher than 2V and the

SenseFET turn-on time is lower than 1.2µs, the FPS

recognizes this condition as an abnormal error and shuts

down PWM switching until VCC reaches Vstart again. An

abnormal condition output short is shown in Figure 31.

MOSFET

Drain

Current

Rectifier

Diode

Current

Turn-off delay

ILIM

VFB

2.5V

t12= C fb*(6 .0 -2 .5 )/Idelay

T1

T2 t

Figure 29. Overload Protection

4.2 Abnormal Over-Current Protection (AOCP): When

the secondary rectifier diodes or the transformer pins are

shorted, a steep current with extremely high di/dt can

flow through the SenseFET during the LEB time. Even

though the FSQ-series has overload protection, it is not

enough to protect the FSQ-series in that abnormal case,

since severe current stress is imposed on the SenseFET

until OLP triggers. The FSQ-series has an internal

AOCP circuit shown in Figure 30. When the gate turn-on

signal is applied to the power SenseFET, the AOCP

block is enabled and monitors the current through the

sensing resistor. The voltage across the resistor is

compared with a preset AOCP level. If the sensing

resistor voltage is greater than the AOCP level, the set

signal is applied to the latch, resulting in the shutdown of

the SMPS.

0

D

Vo

0

Io

Minimum turn-on time

1.2µs

output short occurs

FSQ0465 Rev. 00

0

Figure 31. Output Short Waveforms

4.4 Over-Voltage Protection (OVP): If the secondary

side feedback circuit malfunctions or a solder defect

causes an opening in the feedback path, the current

through the opto-coupler transistor becomes almost

zero. Then, Vfb climbs up in a similar manner to the

overload situation, forcing the preset maximum current

to be supplied to the SMPS until overload protection is

activated. Because more energy than required is

provided to the output, the output voltage may exceed

the rated voltage before overload protection is activated,

resulting in the breakdown of the devices in the

secondary side. To prevent this situation, an over-voltage

protection (OVP) circuit is employed. In general, VCC is

proportional to the output voltage and the FSQ-series

uses VCC instead of directly monitoring the output

voltage. If VCC exceeds 19V, an OVP circuit is activated,

© 2008 Fairchild Semiconductor Corporation

FSQ0465RS/RB Rev. 1.0.1

14

www.fairchildsemi.com

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