• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 64 subscribers
  • Views 352 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC28180: max Vsense resistor limitation?

Martin Chien
Expert 3590 points
Part Number: UCC28180

Hello team,

As customer concerning about the Vsense resistor power losses, they want to increase them total resistance, what is the max limitation you can suggest? the EVM has totally ~1Mohm which is too low, can they up to 10Mohm? can you help calculate the impact of PFC Vout regulation impact?

Thanks.

Martin

  • 0
    TI__Expert 6080 points
    Hello Martin,

    If the customer is concerned about the power loss across the Vsense resistor divider, they can increase the resistor as long as the ratio between RFB1 and RFB2 remains the same because it is basically a potential divider.
    I have attached the excel calculator tool that does a very good job in calculating the RF2 and the total power loss as well. The datasheet does not mention any abs max rating on it, but the OVP and UVD trigger voltage levels will change according to your choice of RFB1/2 resistors.

    Hopefully this answers your question.

    Regards,
    Sonal

    www.ti.com/.../getliterature.tsp
  • 0 in reply to Sonal Singh
    TI__Mastermind 34465 points
    Martin,
    This is a question that comes up a lot and there are two issues
    that influence the upper limit of the sense resistor:
    (1) The input bias current to the voltage sense amplifier of the UCC28180 has an upper limit of 250nA
    If for example you use a 10MOhm part with a 400V output then the sense current in the divider chain is 40uA
    So the error could be 0.625%.
    (2) With high sense impedances there should be concern about keeping the pcb surfaces clean or protected from deposition of moisture and dirt which act as a leakage path around the sense resistors. High voltage across a moisture path will effectively reduce the upper sense resistor value.
    This could cause an undetectable drift in VOUT and VOVP. Leakage around the upper resistor will cause VOUT to drift down and across the lower resistor to drift up. This effect can be mitigated by using a conformal coating on the pcb.
    A large value of upper resistor is sometimes achieved by putting 2 or more large value resistors in series and this will tend to increase the leakage effects.

    It is possible to use a sense resistor larger than 1M Ohms but the disadvantage is a decrease in output voltage accuracy.
    Regards
    John