Hall Effect Current Sensors in Telecom Rectifiers and Server Power Supplies

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Hall Effect Current Sensors in Telecom Rectifiers and Server Power Supplies [Copy link]

The power factor correction (PFC) circuit and inverter circuit in telecom rectifiers and server power supply units (PSUs) need to detect the current signal on the high voltage side to the controller on the low voltage side, so isolated current sensors are used. There are many ways to implement isolated current detection, such as current transformers (CTs), isolation amplifiers, and Hall effect current sensors. Among them, Hall effect current sensors are an ideal choice because of their ease of use, accuracy, small size, and DC detection capability.

The current transformer samples the current based on the principle of transformer. The CT can 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, the CT based on the transformer coupling principle cannot sense DC or very low frequency current, which results in the inability to directly detect the power frequency AC current, or the loss of measurement accuracy due to the indirect method of only detecting the turn-on current (no turn-off current). In addition, since CT needs to use ferrite cores, it is difficult to make it small, and a larger CT will increase the power switch loop, generating higher voltage spikes and noise interference.

The Hall effect current sensor is a more accurate and smaller option. It can work under DC conditions and can measure the total AC current including the opening and closing with good linearity and accuracy. At the same time, the volume of the Hall effect current sensor can be packaged in SOIC-8, 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.

	Hall Effect Current Sensors	Current transformer
principle	Hall effect (magnetic field measurement)	Transformer (Flux Coupling)
Measurement capabilities	DC and AC	Communication only
size	Minimum: SOIC-8 (5 mm x 6 mm x 1.8 mm)	Smallest: EE5 (7.7 mm x 6.9 mm x 5.4 mm)
Accuracy	High – up to 1%	Low – Depends on many factors
application	DC/AC current detection, PFC current control, inverter current control	Peak current control, overcurrent protection

Table 1: Comparison of Hall Effect Current Sensors and Current Transformers

When applying Hall effect current sensors to telecom 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 host computer. At this time, high-precision Hall current sensors (such as TI's TMCS1100) can help the system achieve a current detection accuracy of ≥1%.

Figure 1 shows a typical application circuit of a Hall effect current sensor when powered by 3.3 V and 5 V, respectively. Compared with 3.3 V, 5 V can extend the current detection range of the Hall sensor. Taking TMCS1100A1 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 a 5.0V power supply is used, the current detection range can be extended to -50 A to +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.

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(a)

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(b)

Figure 1: Common applications of Hall-effect current sensors: Hall-effect current sensor with 3.3 V supply (a); Hall-effect current sensor with 5 V supply (b)

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

• Heat dissipation: Increasing the copper area of the primary current conductor as much as possible 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 PCB with thicker copper foil, or place some heat dissipation vias on the primary traces, or place the Hall current sensor and PCB traces in the air duct, which can improve the average current tolerance of the Hall current sensor.
• Primary side current magnetic field: When laying out, try to avoid routing large currents close to the Hall current sensor.
• Isolation requirements: Consider the creepage distance and electrical clearance from the overall system perspective. When the Hall current sensor cannot meet the required PCB creepage distance, grooves can be dug on the circuit board to meet the 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 large and not very accurate. Hall effect current sensors are small, accurate, simple and convenient to use, and more suitable for detecting AC line current. I hope that some of the uses of Hall current sensors introduced in this article will be helpful to you.