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Posted on 01 September 2019

New Applications for Hall-Effect Current Sensors

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Sectors where sensors provide optimal solutions

Hall-effect current sensors offer an inherent benefit over other solutions in that there is isolation between the current path and the sensing and interface electronics.

By Andreas Friedrich, Allegro MicroSystems, Europe

 

The innovative packaged current-sensing solutions featured in this article are based on a low-resistance primary current path and a monolithic linear Hall IC which integrates the Hall element and stateof- the-art BiCMOS interface circuitry (Figure 1). The sensors cover a measurement range of up to ±200 A, and can also be designed into higher current applications by using a current divider configuration. They also offer the benefits of low cost, high accuracy, and small size. This article describes two application sectors where these current sensors provide optimal solutions.

Hall-effect current sensors. Low profile surface-mount device for currents up to 20 A

Hall-effect current sensors

Battery monitoring

Smart battery systems require circuitry to monitor cell voltages, temperatures, and currents. For capacity monitoring applications, all these measurements are critical. The most difficult to design in properly is the current measurement. The reasons for this are accuracy, power dissipation, and solution size.

Current measurement accuracy is essential to ensure that the capacity monitoring algorithms are working well. The traditional method of measuring this current is with a shunt in the ground path or on the low side. The key problem with this is that to minimise I2R losses, the value of the shunt needs to remain very small. With this approach, lowcurrent measurement accuracy becomes compromised. What it means for notebook applications is that, during suspend, hibernate, or other low power states, it is difficult for the battery to accurately monitor the current flowing into the system.

If the battery is using a 10 mOhm sense resistor to minimise power dissipation at nominal loads in a low power state with only 50 mA of power draw, the voltage across the shunt would be 500 mV. This voltage is very difficult to resolve, and so complicated algorithms for estimating the residual capacity must be developed for the battery to compensate for this effect. These routines are conservative in nature, meaning that they tend to assume that they lose a bit more capacity than is calculated. This can result in the battery appearing to lose capacity over time.

Depending on the battery and the application, 1-2 W sense resistors would be required to monitor the currents. Typically, in portable solutions, there is not enough space for 2 W resistors, and so the solution is usually limited to 1 W resistors. For higher current solutions, multiple resistors are used in parallel to keep the power ratings within the devices limitations. Both solutions have a large impact on the available board space required to fit these components.

By using a Hall-effect device as a shunt solution in the battery pack, the power dissipation in the pack can be reduced. The advantage of using Hall-effect devices is readily apparent with the low insertion loss of the device. For the latest SOIC-8 packaged devices, the lead-frame insertion loss can be as low as 1.5 mW. The difference in power consumption over a range of load current is shown in Figure 2.

Power loss in a shunt compared with a Hall-effect current sensor in battery monitoring applications

The use of a Hall-effect device can also increase the accuracy of the current measurements, as shown in Figure 3. This block diagram shows a high current path and a low current path – the latter being enabled for better accuracy when monitoring small currents.

Improved accuracy and efficiency in battery monitoring with Hall effect devices

Not only does the solution shown in Figure 3 provide higher accuracy for lower charge and discharge currents, it also provides more signal than the shunt solution over the measurement range. Assuming that the Hall effect device has a gain of 100 mV/A, this signal is much larger than the resulting signal across a shunt resistor (Figure 4).

Output voltage of a Hall-effect current sensing solution compared with a 20 mOhm shunt

The step increase in gain with the Hall-effect solution assumes that the application allows the high-current path shown in Figure 3. The actual threshold for the transition and level of hysteresis desired will be a function of the application as well as the value of the shunt employed.

The use of Hall-effect devices in battery systems can help to reduce the PCB area required for a shunt sensing solution and allows high-side sensing which does not interrupt the ground path. The two major benefits in using a Hall-effect device will be in improving current measurement accuracy over a wider current range, and reducing power consumption by significantly reducing the I2R loss of the shunt.

UPS and invertor applications

Both Hall-effect devices and current transformers are used for current sensing in UPS systems. While current transformers are seen as low-cost solutions, they actually require more support components than a Hall-effect solution, and are strictly limited to AC applications. Another secondary cost involved when using current transformers to monitor the AC line voltage is the additional circuitry to manage the effects of inrush and possible core saturation during an inrush event.

UPS systems use the line voltage to charge a battery that is used to supply line voltage for a system in the event of a power failure. The goal of the UPS is to supply as much energy as possible with the maximum efficiency. For example, a 2200 VA UPS requires a typical 3-hour charge time. This same UPS can only supply around 24 minutes of power at half load (990 W) and 6.7 minutes at full load (1980 W). The input and output currents are monitored both for protection and to be able to show the battery state of charge with a level of confidence.

A high-performance Hall-effect current sensor is ideal for monitoring the input power or battery charge current for several reasons. The obvious benefits for a small form factor Hall-effect solution are discussed next.

The volume required is a fraction of the equivalent current-transformer solution the elimination of gain and additional protection components, resulting from the fact that the Hall sensor cannot overshoot the voltage on the isolated side of the device.

When powering the invertor stage at high loads, the optimal place to have the Hall-effect sensor is at the line voltage itself to monitor the load currents directly. The reason is that the line voltage current may be as high as 15-20 A RMS, whereas the battery sourcing current may be in excess of 50-60 A, depending on the voltage of the battery stack and the efficiency of the convertor.

Figure 5 shows how a Hall-effect device can be used in an UPS power train.

Using Hall-effect current sensors in a UPS power train

This latest generation of Hall-effect devices is helping to resolve known issues with current transformers and to improve the reliability of the system. By using Hall-effect devices in the battery charging system and invertor power train, the efficiency of the convertors can be optimised. This can help to reduce the overall size of the system as well as saving costs.

 

 

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