• Resolved

ISOW7821: output freqency

Part Number: ISOW7821

Hi teams,

Please let me know, how much frequency does it work as power delivery.

Because the my customer's board is affected to big noise in test of EMI.
Their application is communication of RS232C.
The noise band is 30-70 MHz,200 MHz-300 MHz, 500 MHz especially.

First of all, I want to know the oscillation frequency of the power supply part of this device.
Would you be able to provide guessed items if possible?

Best regards

Hayashi

  • Hi Hayashi,

    The switching frequency of the power converter in ISOW7821 is about 60MHz, the frequency components you are observing are probably the fundamental and harmonics.
    Radiated emissions test results get largely impacted by the overall PCB size and the cables connected to the PCB. The bigger the PCB and the longer the cables you have in your application, higher would be the emissions. Could you please tell us what is the emissions standard that you are planning to meet and how much is the violation?

    To reduce radiated emissions, I recommend you to use a stitching capacitor that is implemented using PCB layer as described in Section 4 of the below application note. The value of the stitching capacitor depends on the emission standard that you are planning to meet and the amount by which it is currently failing. I would recommend implementing a min of 50pF of capacitance to see improvements in radiated emission results. Let me know if you have any questions, thanks.

    www.ti.com/.../litabsmultiplefilelist.tsp


    Regards,
    Koteshwar Rao
  • In reply to Koteshwar Rao:

    Hi Koteshwar Rao-san

    Thank you for your reply.
    I understand necessity of stitching cap.

    So, I have some question when I read the application note you recommended.
    Please let me know your advice.

    1.Is The EVM conform the spec of Fig.9?
    I seem to distance L direction is not possible to ensure.

    2.If my customer will only Keep the 30mm in L direction, do it need isolation of 0.66mm certainly from primary to secondary in Vcc and GND?

    I cannot calculate for keep 50pF. I doubt the number are typo.

    Best regards.
    Hayashi
  • In reply to user5031980:

    Hi Haysahi-san,

    Sure, please find below my inputs.

    1. ISOW7841EVM doesn't incorporate any stitching capacitor in the PCB.

    2. I understand your concern on length requirement. Please note that L=77mm is required only if you need d=0.6mm. With d=0.6mm, you PCB will not limit in meeting all of ISOW7841 isolation requirements. In the actually application, the isolation requirement can be very low especially when you are using the device only for function or basic isolation. In such cases, d can be reduced to lower values to achieve higher capacitance with smaller L.
    Alternatively, you can also reduce the L requirement if you have multiple VCC/GND layers overlapped. This will be equal to keeping two capacitors in parallel resulting in capacitance add-up.

    I recalculate the values given in the table again and I see the calculation yields 30pF as results correctly. While doing the calculations, I recommend converting all the units to SI units. (like mm -> 10^-3 m) Thanks.


    Regards,
    Koteshwar Rao
  • In reply to Koteshwar Rao:

    Koteshwar Rao san

    Thank you for confirm with reply.
    My customer wants to design by the follow conditions.
    1.They can not extend in the L direction.they can extended only 30mm.
    2.They want to extend W direction,

    How long the W direction if they can enough the 50pF?

    I think they must design by more narrow d (the Vcc to GND plane )

    Best regards.
    Hayashi
  • In reply to user5031980:

    Hi Hayashi-san,

    Extending W beyond what is recommended in the app note is not advised as this is going to reduce the isolation voltage that the PCB can support. If the isolation voltage requirement is not very high then you can extend in W direction but you will not be able to have any via in this region otherwise the via will cross the PCB layer from the isolated side.

    If the isolation requirement is not very high then you can reduce the value of 'd' and achieve 50pF. Let me know if customer faces any difficulty in achieving this, thanks.


    Regards,
    Koteshwar Rao