原创 Simple sense resistor: Is it really that simple?

As with so many other engineering topics, the reality is that what looks simple at first actually involves tradeoffs and conflicts. That's true for the sense resistor, as well, with issues such as the resistor's value, location, and physical installation. Two application notes (found in the References section) made this quite clear.

Using a low-value sense resistor is a common technique for measuring current through a load, but applying even this simple component for such a basic, straightforward application has its subtleties and tradeoffs.

 

Consider the most obvious parameter: the resistor value. A higher-value resistor develops a larger voltage drop for a given current, and that larger voltage is easier to use as a feedback signal. Whether you digitize it or use it in analog form, the higher voltage provides greater noise immunity and better resolution.

However, that larger value also means there is increased voltage drop between the rail and common (often referred to as "ground," even if it is not a true Earth ground) and less voltage for the load, reducing system performance and efficiency. Further, the resistor itself dissipates power. This wastes available power and means there is more heat to be removed from the system. Dissipation in tens of watts is fairly common. Finally, the sense resistor is within the load-control loop, and this will affect the loop dynamics, stability, and performance, since it is in that loop but not part of the "real load" that the system is driving.

Balancing these factors, most sense resistors are chosen with sub-ohm, milliohm, and even sub-milliohm values to minimize IR drop, I²R self-heating dissipation, and load disruptions. The corresponding voltage across the resistor is usually about 1 V full scale, meaning that the sensing circuit needs to be designed for good analog response and resolution at relatively low levels.

The low resistance value also has a ripple effect on design, layout, and physical configuration in a way that many engineers may not be used to considering, especially if most of their experience is with the resistance in the more familiar kilohm range. At the milliohm and lower values for the sense resistor itself, the associated resistances of the PC board, solder connections (if any), and the placement of the voltage sense-lead pickoffs become significant.

Even a few centimeters of PC-board track between the sense resistor and the sense-circuit input may be a significant fraction of the sense-resistor value. There's also the temperature coefficient of resistance α of the PC board's copper to consider: ΔR/R₀ = αΔT; for copper, α = .00386.

I also wonder: If you are interviewing a candidate for a power-related or analog-circuit role, would a good place to start be to ask about an apparently simple subject such as current-sensing options, especially the use of current-sensing resistors? Then you could dive deeper into topics such as the pros and cons of high-side versus low-side sensing or techniques for isolated or differential sensing (often needed, especially with high-side sensing). Perhaps we'll look at those issues in future columns.

What's been your experience with current sensing and sense resistors? Are there other topics which you assumed at first would be simple, only to surprise you when you dug deeper, did the math, and looked at topologies?

References

1. Choosing the right sense resistor layout, Texas Instruments
2. Optimize High-Current Sensing Accuracy by Improving Pad Layout of Low-Value Shunt Resistors, Analog Devices