current limiting using top243 or top245

Lower side ZD has been implemented because till the ZD voltage it will limit the current and after that it will behave normally. Upper side ZD could be replaced by a resistor.

Sorry , don't understand your explanation at all.
How is the lower zener supposed to limit current?
The output of your optocoupler (the transistor) appears to be incorrectly connected , if it is going to the topswitch( C) pin.
According to their examples anyway.
Neddie

Upper side ZD will provide the reference voltage upto which current limiting through the transistor will be there and once ZD voltage limit is reached no current limiting is there.

Do you know how zener diodes work?

Plz tell me where is the mistake. Coz here i have one of such a circuit there it has been implemented like this. I got the same answer as i gave you. Moreover SMPS was tripping, so after adding this ZD tripping was stopped. That's it. Now what should be the role of lower side ZD according to you. Because there is ZD in circuit you have posted too.

There must only be 1 zener diode. Can be on the to or bottom , does not matter. In your case , because you are adding the extra current limiting circuitry , the zener must be on the bottom. A zerer diode should always be used in conjunction with a resistor to limit
the current else it could burn out.
The output voltage of your supply is determined by the zener diode voltage. Assume you have a 12V zener , and assume the attached circuit. Ignore the forward voltage of the opto diode for now.
When the power supply starts up , its output is zero and no current flows in D1 , U1 and R1. Now the power supply output starts to climb. Until the power supply output EXCEEDS the zener voltage of 12V , NO current is flowing in D1, U1 ,R1. At just over 12V (assuming perfect components here :0) ) D1 starts to conduct. Once this happens , there is current flowing in U1's diode and the U1's transistor turns on. The topswitch regulator "sees" this and stops increasing the pwm drive to hold the outout voltage steady at just over 12V. If R1 is not present , as soon as the zener starts to conduct there is nothing to limit the current and it could burn out.
In practice zeners are not perfect and it's internal resistance will limit the current somewhat. If the regulator is fast enough it could
regulate the output before the zener is damaged , but it would still be a mistake to leave out R1.
Your circuit with 2 zeners has the same flaw. When the output voltage exceeds the SUM of the zener voltages current will start to flow in the circuit , with nothing to limit the current something could be damaged. Again the internal resistance of the zeners will limit the current and MAY prevent damage , but the circuit is still flawed.
Getting tired of this now....

I may be wrong somewhere however language of reply is certainly not pleasant. Anyways, transistor is also there for current limiting. I think it also must be helping for current limiting. I don't know what's right but here I have a card which is working fine on this arrangement. I wish I could post you the schematic.