As power electronic switching technology advances, there is an increasing need to accurately measure current for feedback control and system monitoring. There are several ways to achieve this, each with advantages and limitations. This article focuses on current measurement methods that require relatively high accuracy and bandwidth, such as those used to measure computer/telecom board input currents, inverter phase currents, and other circuits with currents ranging from a few amps to 100A.
In particular, this article will review specific details on how to measure current using Hall-effect current sensors, as well as sensors using current mirrors integrated into load switches or electronic fuse (e-fuse) devices. These methods will be compared to current sensing architectures using shunt resistors.
Historically, current shunts and current transformers have been considered the best way to detect current in electrical and electronic circuits. However, these approaches have significant drawbacks, as the current shunt requires a trade-off between signal-to-noise ratio (SNR) and power loss in the sensing element. This tradeoff makes accurate measurements over a wide current range difficult. Furthermore, current transformers are usually large, expensive solutions and are only suitable for measuring AC current.
1.Current measurement methods
Advances in semiconductor technology have introduced Hall effect sensors and current mirrors, which are essentially nondestructive current detection devices with outputs that are easily scaled to obtain optimal SNR.
Current mirror induction mode
Current mirror sensing is commonly used in devices with internal power MOSFETs such as smart power stages, load switches, and electronic fuses. This method uses several cells of the power FET as a current mirror, producing a current output proportional to the current flowing through the main switch.
When this current flows through an external resistor, a voltage proportional to the current through the main FET is easily generated. Half-bridge power stages are ideal for this measurement method because they offer wide current capability (10 A to 90 A) and current mirror outputs with 5 µA/A to 10 µA/A gain. These power stages are particularly useful for applications using synchronous buck regulators in single-phase or multi-phase configurations.
For other applications, there is a load switch or electronic fuse circuit to protect downstream electronics from inrush current or overload conditions. Such circuits typically deploy power MOSFETs as switching elements, so current mirrors are a cost-effective way to monitor the current through these devices. An example is the MP5921, a hot-swappable electronic fuse device that provides multiple levels of protection as well as current and temperature monitoring
The electronic fuse device supports continuous current up to 50 A in standalone mode and higher current when configured in parallel with multiple devices
The MP5921 device can work with the MP5920 device, allowing the current signal to be converted into a digital format for monitoring via I 2 C or PMBus. These devices support current ratings from 2.5 A to 50 A and offer different features such as output discharge, configurable current limit and slew rate.
These solutions are ideal for applications related to computing and DC/DC converters on server motherboards. Other applications, such as those with motor drivers and AC/DC circuits, benefit from current detection equipment that generates a bidirectional signal consistent with the AC current of the motor driver or power inverter.
2.Hall effect current detection method
High voltages in applications such as motor drivers and AC/DC circuits can be hazardous to surrounding logic level circuits. Hall based current sensors can provide current isolation between high voltage, high current, and logic level circuits used to control equipment. It does this by sensing the magnetic field generated by each current. These sensors then output a voltage proportional to the current.
The current sensor provides high precision Hall effect current sensing with isolation voltage up to 2,200 V and operating voltage up to 280 V. These current sensors can be used for applications with currents up to 50 A. A common application of the Hall effect current sensor is the inverter.
This article will examine three-phase inverters, which are ubiquitous in power supply applications such as UPS, motor drivers and solar panels. It can be easily generalized to offline applications by removing one of the stages. Typically, sensors are used to feed phase current back to the controller. In Figure 4, the high voltage side is marked red and the logical voltage is green. Isolation is achieved by electrically separating the primary side (current-carrying lead) from the secondary side (output lead).
The MCS180x current sensor lead frame and tube core are shown below. In Figure 5, the primary side is copper colored and the current is indicated by a red arrow. Note that in practice, current can flow in either direction. The flow of current creates a magnetic field (shown in blue) according to Ampere's law, which states that the size of the magnetic field is proportional to the density of the current.
It is also important to note that the primary does not touch silicon (shown in black); Instead, it is separated by an air gap and an insulating layer of silica. The direction and amplitude of the magnetic field are detected by a Hall sensor in the silicon chip, which is then amplified and output to one of the auxiliary pins (shown in silver). Other secondary pins are used for V CC, GND, and a filter. Filter pins make a tradeoff between bandwidth and signal noise.
The MCS180x series current sensor has a proportional output, which means it is centered on V CC /2. When the forward current flows, the output voltage rises from V CC /2 to V CC in proportion to the current. When the negative current flows, the output drops from V CC /2 to 0 V. Here, I PMAX is indicated by the part number suffix; For example, the I PMAX of McS1802-10 is 10 A. It allows designers to select the right current range for their applications. Available options are 5 A, 10 A, 20 A, 30 A, 40 A and 50 AI PMAX.
