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FAN7554

Versatile PWM Controller

Fairchild Semiconductor 

FAN7554

Protection Circuits

The FAN7554 has many built-in protection circuits that do not need additional components, providing reliability without cost

increase. These protection circuits have the auto-restart configuration. In this configuration, the protection circuits reset when

Vcc is below UVLO stop threshold (9V) and restarts when Vcc is above UVLO start threshold voltage (15V)

Over Voltage Protection

Abnormalities may occur in the SMPS secondary side feedback circuit. First, when the feedback pin is short to the ground, the

feedback voltage is zero and the FAN7554 is unable to start switching. Second, when the feedback circuit is open, the

secondary voltage generally becomes much greater than the rated voltage as the primary side continues to switch at the

maximum current level. This may cause the blowing off the fuse or, in serious cases, fires. It is possible that the devices

directly connected to the secondary output without a regulator could be destroyed. Even in these cases, the over voltage

protection circuit operates. Since Vcc is proportional to the output , in an over voltage situation, it also will increase. In the

FAN7554, the protection circuit operates when Vcc exceeds 34V. Therefore ,in normal operation, Vcc must be set below 34V.

Over Load Protection

An overload is the state in which the load is operating normally but in excess of the preset load. The overload protection circuit

can force the FAN7554 to stop its operation . The protection can also operate in transient states such as initial SMPS operation.

Because the transient state returns to the normal state after a fixed time, the protection circuit need not to operate during this

time. That is, the FAN7554 needs the time to detect and decide whether it is an overload condition or not. The protection

circuit can be prevented from operating during transient states by ensuring that a certain amount of time passes before the

protection circuit operates. The above operations are executed as follows: Since the FAN7554 adopts a current mode, it is

impossible for current to flow above a maximum level. For a fixed input voltage, this limits power. Therefore, if the power at

the output exceeds this maximum, Vo, shown in figure21, becomes less than the set voltage, and the KA431pulls in only the

given minimum current. As a result, the photo-coupler’s secondary side current becomes zero. The same goes for the

photo-coupler’s primary side current. Consequently, when the full current 1mA flows through the internal resistor

(2R + R = 3R), Vfb becomes approximately 3V and from that time, the 5uA current source begins to charge Cfb, the

photo-coupler’s secondary current is almost zero. The FAN7554 shuts down when Vfb reaches 6V.

6V

S Q Shutdown

R

Vo

Vfb

UVLO out

1mA

2R

Cfb

5uA 5V

R

Vcc

OSC

SQ

R

1

KA431

FB

FAN7554

V

6V

Shutdown start point

3V

t1

Time Constant = 3R * Cfb

t2

t

5uA = (Cfb *3V)/t2

Figure 21. Delayed Shutdown

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