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Should Current sense resistors heat

Ajay P
Prodigy 230 points

Hi,

I am facing an issue when I am testing motor controller. During testing, I noticed that the temperature of the current sense resistor of the driver IC is getting heated at a very fast rate. Is this normal?

Also can anyone please help me with the doubt that should current sense resistors get heated at a fast rate. The temperature rise was observed from 30 deg celcius to 50 deg celcius within 15 minutes.

Kindly resolve my doubt. 

  • 0
    TI__Mastermind 19255 points

    Ajay,

    Yes, current sense resistors will heat up, and it is one of the fundamental challenges of their design. Shunt resistors are like any resistor, and generate power by I^2*R. If the shunt is handling a lot of current, they can heat up rather quickly, and often well beyond 50C. A key aspect of shunt resistor design is optimizing the shunt where the power generated is acceptable for your system, but so is the accuracy, as the two are directly proportional to one another. Selection of a current sense amplifier with low offset will help to achieve this. 

    Please check out this TIPL on shunt resistor selection for more information.

  • 0 in reply to Carolus Andrews
    Prodigy 230 points

    Can we use low wattage resistors with the same value as that of current sense resistors as current sense resistors?

  • 0 in reply to Ajay P
    TI__Mastermind 19255 points

    Ajay, 

    Can you expound on what you mean by low wattage resistor? Are you just meaning like an SMT resistor here? Regardless, any resistor can technically be used as a shunt, its just that shunt resistors often possess qualities that are advantageous for their given application (low drift, higher power for load survivability, high precision resistance values, etc.)

    If you are trying to use a typical SMT resistor here, like an 0805 or similar, the biggest challenge you will run into is power. You need to ensure that the resistor you select is capable of withstanding the full load you intend to send across it, and if that load is large enough, you often need to thermally derate it to ensure survivability of the resistor. 

  • 0 in reply to Carolus Andrews
    Prodigy 230 points

    Carolus Andrews, I meant to use 0805 resistors or less package which have lower power rating. Also, If we connect two resistors in parallel, say that if we connect resistors of wattage 1/2W in parallel, does the wattage of the equivalent resistors add up?

  • 0 in reply to Ajay P
    Prodigy 230 points

    Also, I have another doubt that if we increase the resistance of current sense resistor, does that increase the power loss?

    In other words, while adding current sense resistors, does it causes the power loss of a circuit?

  • 0 in reply to Ajay P
    TI__Mastermind 19255 points

    Ajay, 

    The calculation for loss in the sense resistor is straightforward, its just the square of the current multiplied by the resistance (P_loss = I^2*R). If you increase the resistance value of the individual sense resistor, then yes, the power loss will go up as per the formula. If you place multiple resistors in parallel, yes, you will increase the allowable power, by dividing the current over multiple resistors. Keep in mind that by placing the resistors in parallel you are also changing the resistance value (i.e., 2 100mΩ resistors in parallel is 50mΩ effective resistance).