Current Sense Resistors Improve Safety and Efficiency

Shunts as Current Sense Resistors

Current sensing relies on the fundamental equation of network analysis: Ohm’s Law. Any resistive element with current flowing through its terminals experiences a proportional voltage drop equal to the product of the current and resistance. Current sense resistors – most commonly employed as shunt resistors – package a conductive metal in various forms for a broader range of suitable applications. The key to current sensing is a low resistance to minimize power loss and near modeling of a short circuit. A resistive element dissipates power as Joule heating, equal to the square of the current times the resistance. Since the current sense resistor cannot directly control the current, the device contributes to power savings with low resistance. The Ohmic relationship raises two further notes:

1. Shunt resistors may be less useful in high-current circuits where the current squared term more readily dominates the power loss regardless of resistance.

2. The low resistance of the shunt resistor equates to a moderately low (and sometimes lower, depending on the current value) voltage drop. Differential or other amplification methods will be necessary to obtain a viable signal reading for most sensors.

For the layout, current sense resistors lay in series with the voltage source to not divide the current between different circuit branches. It’s worth a brief digression to mention the effect temperature change has on current sensing: as temperatures stray towards extremes, there is a reduced accuracy in current readings. Material selection ensures shunt resistors possess excellent thermal stability over a wide operating range; the resistor’s temperature coefficient of resistance (TCR) value is mildly positive, which means it only weakly exhibits a positive correlation between resistance and temperature. However, it is not just the material of the shunt resistor that comes into play during current sensing: the copper traces of the PCB have a significant TCR. The TCR value of the traces amounts to a cumulative series resistance much greater than that of the shunt, invalidating much of its intended design.

The Kelvin or 4-wire method can bypass the issue of series trace resistance. The Kelvin method operates on the principle of Kirchhoff’s Junction Rule: current is the same throughout a series circuit, therefore if voltage drop is measured only across the shunt resistor (i.e., excluding the leads of the copper traces), an accurate current reading is attainable. A second, perpendicular pair of traces can overlap the leads and the shunt resistor, effectively isolating the contact points for voltage measurement from the current. The 4-wire mode is possible with a 2-terminal shunt resistor (typically a through-hole device with current and voltage traces on opposite sides of the board). However, 4 terminal variants are available that enhance the reading accuracy with the downside of a more expansive land pattern.

How to Choose and Use Shunts for Circuit Applications

Current sensing avails itself to many circuit applications, with power management, metering, and control circuits among the foremost uses:

• Battery management - Portable systems must maximize efficiency between charges. Control systems must monitor the load with a high degree of precision to detect spikes in usage and avoid energy reserve depletion. At the same time, a shunt needs to present little resistance and temperature drift to avoid excessive power loss while withstanding appreciably large currents. 
• Hot swapping - Certain industries like telecommunications rely on the ability to switch components from a system on-the-fly without interrupting the power interruption. Microcontrollers in these circuits must protect against accidental shorts while gently ramping the supply rail on a newly inserted component. A current sense resistor is ideal for preventing incoming or outgoing damage to components from dangerously high or low power conditions.
• Smart meters - The addition of series current sense resistors to mains’ hot and neutral bus bars with a microcontroller for self-reporting metering.
• DC-DC converters - These power converters see use in lightweight, portable devices where maximizing the efficiency of switched-mode supplies is a crucial design goal. 
• Motor control - Current sensing can improve the responsiveness of digital motor control, which smooths output voltage and improves direct torque control. Current sense resistors allow the modulator to measure the current without additional components directly, simplifying layout and dense assemblies. However, shunts must be rugged enough to withstand temporary shorts from the motor.

The selection of a shunt resistor depends on a few factors. Choosing the lowest possible resistance (the lowest value of detectable peak voltage that complies with the signal-to-noise ratio) is vital to minimizing power losses. Given the possibility of a sizeable current sink, shunts must also possess a power rating that exceeds surge conditions. However, the thermal reliability of shunts is heavily reliant on good layout practices. Shunt traces must be wide with many thermal vias for current capacity and heatsinking, but an immediate symmetry of copper features on either side of the shunt is necessary. Without this symmetry, voltage imbalances between the pair of lead-trace junctions can occur and disturb measurements.

Cadence Brings Sense to ECAD Tools

Current sense resistors have vast applicability for safety and efficiency in electronic devices. By monitoring the state of current draw, control circuitry can quickly react to spikes in usage that can hamper battery life or represent a potential hazard. Whatever the use, shunts require modeling to ascertain their effectiveness based on in-circuit parameters and the component value. Cadence’s PCB Design and Analysis Software suite offers an extensive DFM simulation environment for ECAD teams. Transitioning to layout, OrCAD PCB Designer can leverage simulation results with constraint-driven rulesets that minimize revision count to accelerate production schedules.

Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. To learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.