ESD problem (LNK501)
Hello, I am now using LNK501. it makes 9V Flyback converter.
If I introduce/apply ESD pulse(8kV Air) into GND net (of 9V(VDD1)), it seems to lose control, so BD21 goes to fail with A1-A2 short and consequently R21 and R22 go to fail(opened)(See attached file)
I would like to know how I improve ESD immunity.
Could you examine my design and advise me?
Comments
First of all, Thank you for your kind answer.
Unfortuhately, It is not easy to implement spark gap in my layout design.
(No space available)
Is there any other suggestion to reduce ESD effect?
and If I implelement spark gap. How many gap between A1 or A2 and Spark Gap can be effective? 1mm? 0.5mm?
and Is there any potential risk when some surges (or insluation test like impulse) are applied A1-A2?
A spark gap is one of the only ways to reduce ESD damage on your board. You need to provide a path for ESD to return to the AC Neutral input without damaging any sensitive components in your design. A spark gap is the most straightforward approach because you provide a direct path (through air) for the discharge to travel.
An alternative to a spark gap would be to place a y-cap between DC ground and AC Neutral or Earth Ground. This will provide a return path to the AC input which bypasses all primary side components.
The gap between the floating spark gap and AC Neutral can be as small as 0.5mm. There is no danger of line surge sparks because the copper closest to the AC input is completely floating.
Standard cell-phone charger requirements for ESD immunity are 15kV and we usually meet this in our charger reference designs. The way to achieve good ESD immunity is entirely through layout. You will need to insert a spark gap in your design which will allow the ESD to arc across the isolation barrier. This spark gap should provide a path for the ESD current to return directly to the AC input while completely bypassing all primary side components. A good example of this can be found in the RDR-157 design: http://www.powerint.com/sites/default/files/PDFFiles/rdr157.pdf
On page 10 of RDR-157, you'll find highlights of the special layout considerations taken for this design. Note the that the spark gap included is a floating piece of copper run parallel with a corresponding strip on the opposite side of the isolation barrier. This copper is returned to the AC input terminal, but electrically isolated from the AC terminal by a small gap to prevent EMI currents from coupling back into the AC line. Because the copper runs right underneath the transformer, it has the potential to pick up EMI noise and directly inject it back into the AC line (bypassing all primary filter components) if it's not electrically isolated.