Principle and Application of Hall-effect current sensor

August 06 , 2020
The power factor correction (PFC) circuit and inverter circuit in the telecom rectifier and server power supply unit (PSU) all need to detect the current signal on the high-voltage side to the controller on the low-voltage side, so an isolated current sensor is used. There are many ways to implement isolated current detection, such as current transformers (CT), isolation amplifiers, and Hall effect current sensors. Among them, the Hall-effect current sensor has become an ideal choice because of its simplicity, accuracy, small size, and DC detection capability.

The current transformer is based on the principle of the transformer to sample the current, and the CT can be used to detect the turn-on current of the MOSFET or IGBT. The fast response speed of CT makes it very suitable for peak current control and overcurrent protection control. However, the CT based on the principle of transformer coupling cannot sense DC or very low frequency currents, so that it cannot directly detect power frequency AC currents, or lose measurement accuracy due to the indirect method of detecting only the on-current (no off-current). In addition, because the CT needs to use a ferrite core, it is difficult to make it small, and a larger CT will increase the power switch loop, resulting in higher voltage spikes and noise interference.

The Hall-effect current sensor is a more accurate and smaller choice. It can work under DC conditions and can measure the total AC current including turn-on and turn-off with good linearity and accuracy. At the same time, the volume of the Hall-effect current sensor can be packaged as SOIC-8, and the same integrated IC is the same size, making the layout of the PCB easier and helping to achieve higher power density.

Table 1 compares the Hall-effect current sensor and current transformer.
When applying a Hall-effect current sensor to a telecom power supply or server PSU, it is necessary to evaluate the current detection range, continuous current withstand capability, response speed (/bandwidth) and voltage isolation level. In some cases, the telecommunications power supply or server power supply may also need to report the current operating power to the host computer. At this time, a high-precision Hall current sensor (such as TI's TMCS1100) can help the system achieve a current detection accuracy of ≥1%.

Figure 1 shows the typical application circuit of the Hall-effect current sensor when using 3.3 V and 5 V power supply respectively. Compared with using a 3.3 V power supply, using a 5 V power supply can broaden the current detection range of the Hall sensor. Take TMCS1100A1 as an example, the sensitivity of the Hall sensor is 50 mV/A: If you use a 3.3V power supply, the current detection range is -33 A~+33 A (bidirectional); when using a 5.0V power supply, the current detection range can be extended To -50 A ~ + 50A. In addition, it should be noted in the design that in addition to the current detection range, the continuous current tolerance of the sensor also needs to be considered. When the current tolerance is insufficient, it can be optimized by improving the heat dissipation of the sensor.
Figure 1: Common applications of Hall-effect current sensors: Hall-effect current sensors with 3.3 V power supply (a); Hall-effect current sensors with 5 V power supply (b)

In the layout of the circuit board using the Hall effect current sensor, the following factors should be paid attention to:

l Heat dissipation: Try to increase the copper area of the primary current wire, which can improve the heat dissipation capacity of the Hall current sensor, thereby increasing the maximum average current tolerance of the sensor. In addition, you can also use a thicker copper foil PCB, or place some heat dissipation vias on the primary trace, or place the Hall current sensor and PCB trace in the air duct, which can improve the average current resistance of the Hall current sensor. Ability.

l Primary-side current magnetic field: During layout, try to avoid high-current traces close to the Hall current sensor.

l Isolation requirements: Consider the creepage distance and electrical clearance from the overall system. When the Hall current sensor cannot meet the required PCB creepage distance, grooves can be cut on the circuit board to achieve system-level isolation requirements.

In summary, CT is more suitable for peak current control and over current protection in telecom rectifiers and server PSUs, but it is larger and less accurate. The Hall-effect current sensor is small in size, high in accuracy, simple and convenient to use, and is more suitable for detecting AC line current. I hope that the usage of Hall current sensor introduced in this article will be helpful to everyone.
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