Application of Hall current sensor in telecom rectifier and server power supply - Acry Lu Linyu

Both power factor correction (PFC) circuits and inverter circuits in telecom rectifiers and server power supply units (PSUs) require that the current signal on the high side be detected to the controller on the low side, so isolated current sensors are used.

Both power factor correction (PFC) circuits and inverter circuits in telecom rectifiers and server power supply units (PSUs) require that the current signal on the high side be detected to the controller on the low side, so isolated current sensors are used. Isolated current sensing can be implemented in a variety of ways, such as current transformers (CTs), isolated amplifiers, and Hall-effect current sensors. Among them, Hall-effect current sensors are ideal because of their ease of use, accuracy, small size and DC detection capabilities.

Current transformers sample current based on the principle of the transformer, and CT can be used to detect the turn-on current of MOSFETs or IGBTs. The CT's fast response speed makes it ideal for peak current control and overcurrent protection control. However, CT based on the transformer coupling principle cannot sense DC or very low frequency currents, resulting in it not being able to directly detect the power frequency AC current, or losing measurement accuracy (no shutdown current) because of the indirect method of only detecting the turn-on current. In addition, because CT requires the use of ferrite cores, it is difficult to make a small volume, and a larger CT will increase the power switch loop, resulting in higher voltage spikes and noise interference.

Hall-effect current sensors, on the other hand, are a higher-precision, smaller option that can operate in DC conditions and measure the total AC current with good linearity and accuracy, including on and off. At the same time, the volume of the Hall-effect current sensor can be achieved in the SOIC-8 package, the same size as the same integrated IC, making the layout of the PCB easier and helping to achieve higher power density.

When applying Hall current sensors to telecom power supplies or server PSUs, the sense range of the current, the ability to withstand continuous current, the response speed (/bandwidth), and the voltage isolation level need to be evaluated. In some cases, telecom or server power may also need to report the current operating power to the host computer, where high-precision Hall current sensors such as TI's TMCS1100 can help the system achieve ≥ 1% current sense accuracy.

Figure 1 shows a typical application circuit for Hall-effect current sensors using 3.3 V and 5 V respectively. Using a 5 V supply broadens the current sensing range of the Hall sensor compared to operating from a 3.3 V supply. Taking the TMCS1100A1 as an example, the Hall sensor has a sensitivity of 50 mV/A: if a 3.3V supply is used, the current detection range is -33 A to + 33 A (bidirectional), while when using a 5.0V supply, the current detection range can be extended to -50 A to + 50A. In addition, in the design, it should be noted that in addition to the current detection range, it is also necessary to consider the continuous current tolerance of the sensor, and when the current tolerance is insufficient, it can be optimized by improving the heat dissipation of the sensor.

Figure 1: Common Applications for Hall Current Sensors: Hall Current Sensors with 3.3 V Supplies (a); Hall Current Sensors with 5 V Supplies (b)

In board layouts using Hall-effect current sensors, the following factors should be noted:

Heat dissipation: Maximizing the copper coverage area of the primary side current wire can improve the heat dissipation capacity of the Hall current sensor, thereby increasing the maximum average current tolerance of the sensor. In addition, pcBs with thicker copper foil can be used, or some thermal vias can be placed on the primary trace, or Hall current sensors and PCB traces can be placed in the air duct, which can improve the average current tolerance of Hall current sensors. Primary side current magnetic field: When laying out, large current traces should be avoided as close to the Hall current sensor as possible. Isolation requirements: Considering the creepage distance and clearance from the system as a whole, when the Hall current sensor cannot meet the required PCB creepage distance, it can be grooved on the circuit board to achieve system-level isolation requirements.

In summary, in telecom rectifiers and server PSUs, CT is more suitable for peak current control and overcurrent protection, but it is larger and less accurate. Hall-effect current sensors are small, highly accurate, easy to use, and better suited to sense AC line current. I hope that some of the usages of Hall current sensors introduced in this article are helpful to you.