Measure Currents Accurately with High-Precision Monitors

By Steve Hsiao, WW Strategic Technical Marketing

Measuring current may seem like a simple task, but it can be a challenge to obtain the accuracy required without excessive power losses. This is particularly true for high currents; for example, in fault monitoring by overcurrent detection across industrial and telecoms applications.

A popular approach is to use resistive current sensing, where a shunt resistor is deployed in series with the load. By measuring the voltage drop across the resistor, we can derive the current.

For high currents, we need to use a very small shunt resistor to minimize power dissipation, often just a few milliohms. This means that the voltage drop across the shunt resistor will also be very small, with the maximum drop possibly as low as 10mV—hence the need for precision measurement.

If we look at key trends in current monitoring today, this precision is increasingly important and requires a current monitor component (including the resistive shunt) with a low offset voltage such as 100µV. To also achieve rapid response times to current changes, the current monitor requires a high bandwidth, typically between 300kHz and 1MHz.

Another important performance metric is the common-mode voltage supported by a current-sense monitor, which may need to be up to 100V. This depends on the application; for example, 48V or 54V for an AI server 3kW+ power supply, or 48V for the brushless direct-current motor in a hybrid electric vehicle.

A significant market that requires slightly lower voltage is industrial power supply, which typically operates at 12V or 24V. Another big market in the same kind of voltage range is USB Power Delivery (PD 3.0 Standard) for applications such as laptop computers, docking stations, and portable chargers. This can require a range of voltages, including 5V, 9V, 15V, and 20V.

For industrial power and USB PD, an example of a suitable component that meets all these demands is the ZXCT21x current monitor series from Diodes Incorporated. These are single-stage instrumentation amplifiers, designed to accurately measure very small sense voltages across a wide range of common-mode voltages up to 26V.

As well as power supply applications, the ZXCT21x series is suitable for current sensing of load/rail currents in BLDC motor control, telecom power, solar inverters, and battery-charged equipment.

The ZXCT21x series provides multiple gain options to measure low voltage drops of 10mV across a small shunt resistor with minimal error. This enables high-precision measurement of large currents and reduces power loss caused by the measurement. The low quiescent-current consumption of 100μA (maximum) also reduces power consumption.

The series uses a zero-drift architecture and is manufactured by post-trim technology to achieve low offset voltage (±30μV maximum), low gain drift, and low gain error (±0.5% or ±0.8%) for precise measurement. The devices have both a wide common-mode range and wide operating-voltage range, giving flexibility in the applications for which they can be used.

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