What happens if common mode choke is not used in LNK417

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Dear Sir
Please clear my following doubts

1) What is the use of common mode choke in LED driver ?

2) If common mode is not used what happens to LNK switch?

3) how to design the common mode choke and how to decide the inductance ?

Common mode choke is used as an EMI filter especially for common-mode noise generated by the LED driver. You may find some Design Example Reports (DERs) that use a common-mode choke, but in most design examples, the emi section consists of drum core because using common mode choke will not fit on some LED bulb enclosures.

I advise that you download several DERs and compare how the EMI sections are implemented. And if you working on a new project, it will be easier if you find a similar design based on your specs and implement the same emi filter section. In choosing DERs, you may find the LED Design Selector to be of great use. Here's the link:

http://www.powerint.com/en/applications/led-driver-ics/led-design-selector