The power factor correction (PFC) circuit and inverter circuit in telecom rectifiers and server power supply units (PSU) both require the detection of the current signal on the high voltage side to the controller located on the low voltage side, thus requiring the use of isolated current sensors. There are various implementation methods for isolated current detection, such as current transformers (CT), isolation amplifiers, and Hall effect current sensors. Among them, Hall effect current sensors have become an ideal choice due to their simplicity, accuracy, small size, and DC detection capability.

Current transformer is based on the principle of transformer to sample current, and CT can be used to detect the turn-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 the principle of transformer coupling cannot sense DC or very low frequency currents, resulting in its inability to directly detect power frequency AC currents, or loss of measurement accuracy due to indirect methods that only detect turn-on currents (without turn off currents). In addition, due to the use of ferrite cores in CT, it is difficult to make the volume small, and 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, which can operate under DC conditions and measure the total AC current including on and off with good linearity and accuracy. At the same time, the volume of Hall effect current sensors can be packaged in SOIC-8, with the same size as an integrated IC, making PCB layout easier and helping to achieve higher power density.

Table 1 compares Hall effect current sensors with current transformers.

Application of Hall current sensors in telecommunications rectifiers and server power supplies

When applying Hall effect current sensors to telecommunications power supplies or server PSUs, it is necessary to evaluate the current detection range, continuous current tolerance, response speed (/bandwidth), and voltage isolation level. In some cases, telecom power supplies or server power supplies may also need to report the current operating power to the upper computer. In this case, high-precision Hall current sensors (such as TI's TMCS1 100) can help the system achieve a current detection accuracy of ≥ 1%.

Figure 1 shows the typical application circuit of a Hall effect current sensor when powered by 3.3 V and 5 V respectively. Compared to using a 3.3V power supply, using a 5V power supply can broaden the current detection range of the Hall sensor. Taking TMCS 110A1 as an example, the sensitivity of the Hall sensor is 50 mV/A. If a 3.3V power supply is used, the current detection range is -33 A to+33 A (bidirectional); When using a 5.0V power supply, the current detection range can be extended to -50 A to+50 A. In addition, in the design, it should be noted 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.

(a)

(b)

Figure 1: Common applications of Hall effect current sensors:

Hall effect current sensor using 3.3 V power supply (a);

Hall effect current sensor using 5V power supply (b)

In the layout of circuit boards using Hall effect current sensors, the following factors should be noted:

Heat dissipation: Increasing the copper coating area of the primary side current wire as much as possible can improve the heat dissipation capability of the Hall current sensor, thereby increasing the maximum average current tolerance of the sensor. In addition, thicker copper foil PCBs 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 to improve the average current tolerance of the Hall current sensor. Primary side current magnetic field: During layout, it is advisable to avoid high current wiring near the Hall current sensor as much as possible. Isolation requirements: Considering the overall system creepage distance and electrical clearance, when the Hall current sensor cannot meet the required PCB creepage distance, grooves can be dug on the circuit board to achieve system level isolation requirements.

In summary, CT is more suitable for peak current control and overcurrent protection in telecommunications rectifiers and server PSUs, but it has a larger volume and lower accuracy. Hall effect current sensors have small size, high accuracy, simple and convenient use, and are more suitable for detecting AC line currents. I hope the usage of Hall current sensors introduced in this article is helpful to everyone.