Passing Radiated Immunity with no input common mode choke?

[cupoftea]

Hi,
Supposing you have a boost converter, 5V to 24V, 100W, 250kHz.
Under what circumstances might it ever be possible to pass EN radiated immunity compliance testing when you have no common mode choke or Y caps at all?

 

[Easy peasy]

soft switched / zvs type boost topology, also with fairly generous snubbing across all semis

 

[FvM]

Do you really mean emc immunity? Why should a dc/dc converter be susceptible to radiated rf? The problem mainly applies to sensitive analog circuits.

 

[cupoftea]

Do you really mean emc immunity? Why should a dc/dc converter be susceptible to radiated rf? The problem mainly applies to sensitive analog circuits.

Thanks, yes, though wouldnt the radiated noise couple in through the power cable, (which would act like a receiving antenna) which (the RF noise) would likely then go through the input SMPS converter, and on to the sensitive circuitry.....so if the input SMPS had no common mode choke, then there would be an RF immunity fail?

 

[FvM]

Thanks for clarifying the question. Difficult to answer it generally without knowing the system details. All connected cables can act as antenna, not only power cables. But their effect on susceptibility depends on the design of sensitive circuit, e.g. ground layout. With good design, you don't necessarily need common mode chokes. On the other hand, we prefer metal cases and filtered power connectors for sensitive equipment.

 

[cupoftea]

Thanks, also, may i ask what you connect the Y caps to when there is no earth connection to the PCB, the case is plastic, and there is no chassis ground either?

 

[mtwieg]

If your circuit doesn't have any isolation, and no earth connection, then there's no place for a Y capacitor, or anything with a similar function. A common mode choke may be used, but its purpose and impact are completely undefined without some sort of return path to earth.

 

[cupoftea]

A common mode choke may be used, but its purpose and impact are completely undefined without some sort of return path to earth.

Thanks, i would take it that the stray capacitances will form the "Y capacitors".

 

[Calm down]

Hello, we can try multi-layer parallel transformer, but I'm not sure whether it can meet the requirements.