What is the difference between high-side and low-side resistive current sensing

    What is the difference between high-side and low-side resistive current sensing? This article explains the basics and when each is a more appropriate design choice.

    Many applications, such as power management, battery charging, motor control, and overcurrent protection, can benefit from resistive current sensing. There are two options for placing a current sense resistor in series with the load: low-side and high-side current sensing.

    In this article, we will examine both arrangements and discuss their basic pros and cons.

    Resistive Current Sensing

    Resistive current sensing is widely used in printed circuit board assemblies when dealing with low to medium current levels. Using this technique, a known resistor, Rshunt, is placed in series with the load and the voltage developed across the resistor is measured to determine the load current. This is shown in Figure 1.

    Figure 1

    Current sensing resistors, also called shunt resistors or simply shunts, typically have values ​​in the milliohm range. For applications with very high currents, the shunt resistor value may even be only a fraction of a milliohm to reduce the power dissipated in the resistor.

    Note that even with small resistor values, shunt power dissipation can be a problem, especially for high current applications. For example, when R = 1 mΩ and I = 100 A, the power dissipated in the shunt resistor is

    A small resistor value will also result in a small voltage drop across the resistor. This is why an amplifier is needed to convert the small voltage developed across the shunt resistor to a large enough voltage for the upstream circuit.

    We will discuss that in high-side current sensing, the amplifier may have stringent requirements for common-mode rejection ratio (CMRR) specifications.

    Low-side and high-side sensing

    There are two options for placing a shunt resistor in series with the load. These two arrangements are known as low-side and high-side current sensing methods, as shown in Figure 2.

    Figure 2. (a) Low-side current sensing and (b) high-side current sensing techniques.

    In the low-side configuration, the current sense resistor (R shunt ) is placed between the ground terminal of the power supply and the ground terminal of the load. With the high-side method, the shunt resistor is placed between the positive terminal of the power supply and the power input of the load.

    Let’s see what the pros and cons of each method are.

    High-side and low-side sensing: common mode value

    Assume R shunt = 1 mΩ and I = 100 A. Even with such a large current, the voltage drop across the shunt resistor is only 100 mV. Therefore, the common-mode value of the voltage across the low-side shunt resistor is only slightly above ground potential. Moreover, for the high-side configuration, the common-mode level of the voltage across the shunt resistor is very close to the load supply voltage.

    Since the amplifier used in low-side current sensing handles a small common-mode voltage, it does not require a high common-mode rejection ratio (CMRR). Common-mode rejection ratio specifies how much attenuation the amplifier exhibits to signals that are common to both inputs of the amplifier. Since the common-mode value of the low-side current sensing configuration is almost zero, the amplifier CMRR requirement is significantly relaxed, so a simple amplifier configuration can be used.

    Figure 3 shows a basic amplifier that can be used for low-side current sensing.

    Figure 3

    In this case, the amplifier consists of an op amp and two gain setting resistors, R1 and R2. Note that this is actually a non-inverting configuration of the op amp. The more familiar schematic for this amplifier is shown below:

    Figure 4

    On the other hand, amplifiers used for high-side current sensing need to handle large common-mode voltages. The amplifier should have high CMRR to prevent large common-mode inputs from appearing at the output. This is why specialized amplifier configurations are required for high-side current sensing. These amplifiers should have high CMRR and support an input common-mode range up to the load supply voltage.

    It is worth mentioning that there are many high-side current sensing applications, such as three-phase motor control applications, where the load supply voltage is much larger than the supply voltage used by the amplifier. Therefore, in a high-side sensing configuration, the input common mode of the amplifier usually needs to be much larger than its supply voltage - a requirement that makes the amplifier design very challenging.

    Low-side approach may cause ground loop problems

    Although the low-side sensing method simplifies amplifier design, it also has some disadvantages. Low-side current measurement places an additional resistor in the ground path. Therefore, the ground potential of the monitored circuit is slightly higher than the system ground potential. This can become a problem for some analog circuits.

    Since the ground of the monitored circuit is at a different potential than the other loads in the system, there can be ground loop issues that can cause audible noise, such as hum, or even interfere with nearby equipment. Due to this limitation, low-side current sensing is often used in applications where we are dealing with an isolated load or where the load is not sensitive to ground noise. Cost-sensitive motor control in applications such as drones, drills, and reciprocating saws often employs low-side sensing in order to be able to compete in the consumer market space.

    Low-end methods cannot detect fault detection

    There are several fault conditions that low-side current sensing cannot detect. Figure 5 shows an example where a short circuit occurs between the power supply to the monitoring circuit and the system ground.

    Figure 5

    The fault current, I short , flows directly from the bus voltage to the system ground without passing through the shunt resistor. Therefore, the current monitoring circuit will not detect this fault condition. Low-side current sensing also cannot detect a short between the monitored circuit ground and the system ground (Figure 6).

    Figure 6

    However, high-side current sensing can detect fault conditions occurring downstream of the shunt resistor, as shown in Figure 7.

    Figure 7

    In this case, the fault current flows through the shunt resistor. Therefore, the current measurement circuit can detect the short circuit condition and trigger appropriate corrective actions.

    High-side current sensing simplifies wiring

    Another disadvantage of low-side current sensing is that even if the system ground is available, two wires are required to power the monitored circuit. For example, in automotive applications, the chassis of the car is used as a common ground. Since the chassis is on the system ground plane, we only need one wire to power the load. However, if the current through the load is monitored by low-side measurement techniques, the system ground cannot be used and two wires are required for the load. Since high-side sensing technology uses the system ground to monitor the load, it is not subject to this limitation. This is why high-side sensing is more suitable for automotive applications.

    in conclusion

    The main advantage of low-side sensing is that a relatively simple configuration can be used to amplify the voltage across the shunt resistor. However, low-side current sensing is susceptible to ground interference and cannot detect fault conditions. Low-side current sensing is often used in cost-sensitive motor control applications that need to be able to compete in the consumer market space.