Hi Viktor,

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

The recommended size of spark gaps across common mode choke is based on your normal operating voltage on the input.  This will ensure that the spark gap will not interfere during the normal operation but will activate during overvoltage event such as ESD, Lightning Strikes, or Transient Events.

The thumb rule for the distance of spark gaps is based on Paschen's Law, which describes the breakdown voltage of spark gaps, and for air it is 30kV/cm. So, let's say your normal operating condition is around 250V. The spark gap should have a breakdown voltage of around 300V to ensure no interference, which means around 0.1mm for the spark gaps.

In regard to the second questions, in the context of quality factor during surge events, it indeed affects how the LC tank circuit behaves during such condition. Under normal operation, the spark gap doesn't conduct so the LC Tank still retains its original Q-Factor indicating sharper resonance and narrower bandwidth. However, during the surge events, spark gap will now conduct which effectively lowers the circuit resistance and thus changing the Q-Factor of the LC tank circuit, making it much lower which indicates wider bandwidth. In terms of primary-secondary voltage, having a wider bandwidth allows more frequency range to dissipate which helps prevents the voltage stress between primary-secondary. This broader bandwidth and reduced Q factor are only temporary, lasting as long as the spark gap is conductive. Once the ESD pulse subsides and the voltage drops below the breakdown threshold of the spark gap, the spark gap becomes non-conductive, and the circuit returns to its normal high-Q state.

Regards,
PI-Book of PI

Thanks!

One more question. In this structure, the rectifier bridge is not covered by spark gaps in the continuous path ( grid - CM Choke - rectifier - InnoSwsitch - Out ).

-is it necesessory  for the diodes bridge, to be protected such way?
-or in case of insufficient protection, anyway need to made a spark gaps from grid to out? 

Hi  Viktor,

If ever you live in a place where ESD or surges are a common occurencre. Then you need a protection on the input side of the circuit. This can be realized using TVS (Transient Voltage Suppression) Diodes. You can reach out to the PowerPros using our Online Technical Support and they can give you full details on the implementation or different transient overvoltage protections such as TVS.

If ever your area are mostly connected to Earth Ground and your application includes such conenction to Earth, there is no need for an additional circuit to protect the bridge rectifiers. Just make sure to have atleast 1000V rating for the Bridge Rectifier to ensure that the worries for ESD damages can be mitigated

Regards,

PI-Book of PI