The power factor correction (PFC) circuit and inverter circuit in Telecom rectifier and server power supply unit (PSU) need to detect the current signal on the high-voltage side to the controller on the low-voltage side, so isolated current sensor is used. There are many ways to realize isolated current detection, such as current transformer (CT), isolation amplifier and Hall effect current sensor. Among them, Hall effect current sensor has become an ideal choice because of its simplicity, accuracy, small volume and DC detection ability.

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

The Hall effect current sensor is a choice with higher accuracy and smaller volume. It can work under DC conditions, and can measure the total AC current including on and off with good linearity and accuracy. At the same time, the volume of Hall effect current sensor can be the same as that of soic-8, which makes the layout of PCB easier and helps to achieve higher power density.

Table 1 compares Hall effect current sensor with current transformer.

Table 1: comparison between Hall effect current sensor and current transformer

When applying Hall effect current sensor to telecom power supply or server PSU, it is necessary to evaluate the current detection range, continuous current tolerance, response speed (/ bandwidth) and voltage isolation level. In some cases, the telecom power supply or server power supply may also need to report the current operating power to the host computer. At this time, high-precision Hall current sensor (such as TI’s tmcs1100) can help the system achieve a current detection accuracy of ≥ 1%.

Figure 1 shows a typical application circuit of Hall effect current sensor with 3.3 V and 5 V power supply respectively. Compared with 3.3 V power supply, 5 V power supply can broaden the current detection range of Hall sensor. Taking tmcs1100a1 as an example, the sensitivity of Hall sensor is 50 mV / A: if 3.3V power supply is used, the current detection range is – 33 A? 9? 1 + 33 A (bidirectional); When using 5.0V power supply, the current detection range can be extended to – 50 a? 9? 1+ 50A。 In addition, in the design, it should be noted that in addition to the current detection range, the continuous current tolerance of the sensor needs to be considered. When the current tolerance is insufficient, it can be optimized by improving the heat dissipation of the sensor.

(a)

Figure 1: common applications of Hall effect current sensor: Hall effect current sensor with 3.3 V power supply (a); Hall effect current sensor with 5 V power supply (b)

In the circuit board layout using Hall effect current sensor, pay attention to the following factors:

L heat dissipation: try to increase the copper clad area of the primary side current conductor to improve the heat dissipation capacity of the Hall current sensor, so as to increase the maximum average current withstand capacity of the sensor. In addition, a thicker copper foil PCB can be used, or some heat dissipation vias can be placed on the primary wiring, or the Hall current sensor and PCB wiring can be placed in the air duct, which can improve the average current tolerance of the Hall current sensor.

  • Primary side current magnetic field: during layout, try to avoid large current routing close to Hall current sensor.
  • Isolation requirements: considering the creepage distance and electrical clearance from the overall system, when the Hall current sensor cannot meet the required PCB creepage distance, a slot can be dug on the circuit board to meet the system level isolation requirements.

In conclusion, CT is more suitable for peak current control and overcurrent protection in Telecom rectifier and server PSU, but it has large volume and low precision. Hall effect current sensor has the advantages of small volume, high precision, simple and convenient use, and is more suitable for detecting AC line current. I hope some usage of Hall current sensor introduced in this paper will be helpful to you.

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