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SN6501: Design improvements to pass EMC CISPR25 testing?

Part Number: SN6501
Other Parts Discussed in Thread: SN6505A


Could you please provide some suggestions for further EMC optimization?
(I will provide the schematics and further design details off-line)

we are currently in EMC tests with the SN6501 and are having problems achieving the desired EMC class:
- CISPR25 CP - Class 3: Deviations at approx. 97MHz
- CISPR25 E-field Class 3: deviation at 1.1MHz

We have already carried out various investigations and we always come back to the combination of transformer and push pull driver as the cause.
So far we have mainly tried to optimize the snubber.
We are now considering what further investigations we can do and what can still be optimized. Do you have any suggestions for optimization from your side?

Thanks in advance,


  • Hi A.,

    Thank you for posting to E2E!

    For SN650x systems, transformer selection impacts emissions significantly since emissions are generated during switching by resonance of the parasitic inductance of the transformer and capacitance of the trace.

    We have a few options to help mitigate emissions based on your flexibility, mentioned below:

    • Reduce the load current if possible
    • Use a transformer with a lower leakage inductance spec. Transformer vendors can often modify existing part numbers for customers if needed
    • Modify the snubber circuit. Snubber circuits can be calculated as explained in How to Reduce Emissions in Push-Pull Isolated Power Supplies (slla566a), and the values can be adjusted to reduce emissions further, for example by increasing snubber capacitance. 
    • We strongly recommend using SN6505A in emissions-sensitive applications since it offers spread spectrum modulation, a slower default switching frequency, and a slower switching slew rate, all which reduce conducted and radiated emissions. Its size and pinout is very similar to SN6501, and the transformer V-t product needs to to be sufficient for use with SN6505A.

    The emissions are likely not coming from the SN6501 subcircuit alone, and the DC/DC converter supplying the power supply for Vcc or other switching elements around the SN6501 may be contributing at the failing frequencies. Please test the system without the SN6501 power supply to measure how much the circuitry around SN6501 is contributing to emissions as well and address those accordingly.

    I will mark this thread closed since we will continue this conversation offline.

    Thank you,
    Manuel Chavez