FSES0765RG

12

Figure 6. Auto restart operation

3.1 Over Load Protection (OLP) : Overload occurs when

the load current exceeds a pre-set level due to an unexpected

event. In this situation, the protection circuit should be

activated in order to protect the SMPS. However, even when

the SMPS is in the normal operation, the over load protection

circuit can be activated during the load transition. In order to

avoid this undesired operation, the over load protection

circuit is designed to be activated after a specified time to

determine whether it is a transient or overload situation.

Because of the pulse-by-pulse current limit capability, the

maximum peak current through the Sense FET is limited,

and therefore the maximum input power is restricted with a

given input voltage. If the output consumes beyond 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 2uA current source

condition, Vfb continues increasing until it reaches 7.5V,

then the switching operation is terminated as shown in figure

7. The delay time for shutdown is the time required to charge

delay time is typical for most applications.

Figure 7. Over Load Protection

3.2 Abnormal Over Current Protection (AOCP) : Even

though the FPS has OLP (Over Load Protection) and current

mode PWM feedback, these protections are not enough to

protect the FPS when a secondary side diode short or a

transformer pin short occurs. The FPS has an internal AOCP

(Abnormal Over Current Protection) circuit, as shown in

figure 8. When the gate turn-on signal is applied to the power

Sense FET, the AOCP block is enabled and monitors the

current through the sensing resistor. The voltage across the

resistor is then compared with a preset AOCP level. If the

sensing resistor voltage is greater than the AOCP level, the

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

of SMPS.

Figure 8. AOCP Block

3.3 Over Voltage Protection (OVP) : If the secondary side

feedback circuit malfunctions or a solder defect causes an

open in the feedback path, the current through the opto-

coupler transistor becomes almost zero. Then, Vfb climbs up

in a manner similar to the over load situation, forcing the

preset maximum current to be supplied to the SMPS until the

over load protection is activated. Because more energy than

required is provided to the output, the output voltage may

Fault

situation

9V

12V

Vcc

Vds

t

Fault

occurs

Fault

removed

Normal

operation

Normal

operation

Power

on

V

FB

t

2.5V

7.5V

Over load protection

T

T

1

T

2

2

S

Q

Q

R

OSC

R

2.5R

GND

Gate

driver

LEB

PWM

Vaocp

AOCP

R

sense