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Original question:

ISO1211: ISO1211 AC detection

ISO1211: ISO1211 AC detection (Rthr thermal issue)

Rick Liu
Prodigy 30 points
Part Number: ISO1211

Tool/software:

Hi Koteshwar Rao ,

Sorry ,We have thermal issue in ISO1211 AC detection design ,need your suggestion. 

Please refer below picture .

1. The AC inpurt source is 220V ,

We check the Rthr resistor's temperature is too hight (5min rise up to 90‘C)

2. We change the Rthr resistor to SMD(2512) or MELF resistor (MMB02070C3002FB200)x2pcs ,

but the Rthr temperature could not uder 50‘C (this sepc from customer requirement)

3.  Could I modifly Rthr or Rshunt value to reduce AC innput power dissipation ? 

4. For this thermal issue ,could you advise Rthr resistor type or PCB layout design 

(maybe can remove the Rthr's PCB area (just only PAD or through hole) ,to reduce heat compoment's conduction )

Thanks a lot .

  • 0
    TI__Mastermind 29665 points

    Hello Rick,

    Thank you for reaching out and the detailed post. 

    Rick Liu said:
    Could I modifly Rthr or Rshunt value to reduce AC innput power dissipation ?

    Yes, you can modify the resistor values. Please use the ISO121x threshold calculator which should help simplify the calculation.  http://www.ti.com/lit/zip/SLLC457. Let me know if you have any questions here. 

    Rick Liu said:
    For this thermal issue ,could you advise Rthr resistor type or PCB layout design 

    The larger MELF resistors are a good selection and help the thermals. In general, increasing the coper thickness and trace width helps provide a low-impedance path to reduce board heating. Pad thickness and size also play a large role.

    Additionally thermal vias can be placed underneath the component and connect to a large copper plane to act as a heat sink. 

    Best,
    Andew