Hi Mayur@111

Thank you for reaching out to Power Integrations and for your trust in our devices. 

To answer both of your questions, may I ask for the source of information regarding your question number 1, that the TopSwitch265KG-TL have a negative current limit of -200mA. Our TopSwitch devices doesn't have a feature of negative start-up current limit, but these controllers do have features to reduce the stresses during start-up such as the leading edge blanking and the soft-start feature.

Kindly allow me to clarify first what causes this negative current during start-up in terms of TopSwitch family. 

When rectified DC high-voltage is applied to the DRAIN pin during start-up, the MOSFET is initially off, and the CONTROL pin capacitor is charged through a switched high-voltage current source connected internally between the DRAIN and CONTROL pins. When the CONTROL pin voltage VC reaches approximately 5.8 V, the control circuitry is activated and the soft start begins. Generally, a power supply experiences maximum stress at start-up before the feedback loop achieves regulation. For a period of 17 ms, the on-chip soft-start linearly increases the drain peak current and switching frequency from their low starting values to their respective maximum values. This causes the output voltage to rise in an orderly manner, allowing time for the feedback loop to take control of the duty cycle. This reduces the stress on the TOP264-271 MOSFET, clamp circuit and output diode(s), and helps prevent transformer saturation during start-up. Note that as soon as the loop closes the soft-start function ceases even if this is prior to the end of the 17 ms soft-start period.

TopSwitch controllers have specific startup sequences that include soft start features to gradually ramp up the switching frequency and current limit. However, during the very first switching cycles, there can be irregularities or transients that cause negative inrush currents as the system stabilizes. The ringing caused by the transformer's leakage inductance and the MOSFET's parasitic capacitance may also show as a negative inrush due to its ringing nature.

I will be waiting for your response.

Regards,

Are you using a bypass capacitor across the input supply ahead of the TOPSwitch stage?  A value of around 1uF, 450 VDC would be appropriate, with the capacitor placed to minimize the primary loop area. (plus side of capacitor close to B+ side of transformer, minus side of capacitor close to the TOPSwitch source). Capacitor should be polypropylene film type. 

This capacitor may be a suitable type:

https://www.digikey.com/en/products/detail/panasonic-electronic-components/ECW-FD2W105J/3844563

You can experiment with the value of bypass capacitor to find a value that is suitable. Something in the range of  0.1-1 uF is likely appropriate. This film bypass capacitor can also be combined with a 10uF, 450V electrolytic capacitor for extra damping.

So, you are suggesting keep film capacitor at input side after bulk capacitor. We will check with this one.  Can you explain B+ side of transformer in attached diagram? I have confusion regarding B+ side of transformer.

We have requirement that starting or continuous current of top switch must be less than -200mA. Customer have observed that device is failing because of negative current. 

To reduce this negative current:

1. Leakage inductance of transformer required to decrease.
2. Use RC snubber across output diode. 
3.  Use RC snubber across the TOP switch. 

Please conform above three options are correct or not.

Please provide suggestion from your end to decrease the negative starting current.

If you are running the TOPSwitch without any capacitance across the input supply, you will get negative current excursions.  Adding a film cap or a combination of film + electrolytic across the input supply should fix this. I suggest a combination of film plus electrolytic, because the ESR of the electrolytic cap can provide some damping. This is a  problem we have seen in the past when a previous customer actually removed the input bulk capacitor to "test" his circuit.

I have drawn in the caps on the circuit you provided for purposes of clarification. You can see them in the attached file.