“Hybrid” Hall Current Sensors Have Higher Bandwidth

A new family of “hybrid” coreless magnetic current sensors could help bring more efficient and reliable power conversion to applications ranging from EV charging and renewable-energy systems to data centers.

In power electronics, current sensing is crucial for everything from control (such as peak- and average-current-mode control loops) to circuit protection (against overcurrent, short circuits, and other fault conditions). But as silicon-carbide (SiC) and gallium-nitride (GaN) devices switch at increasingly high frequencies, isolated current sensors that deliver both high bandwidth and accuracy are in demand. Improving one of these metrics usually comes at the expense of the other — or adds circuit complexity.

Infineon is trying to tackle the challenge with its TLE4978, which combines Hall-effect and coil-based current sensors in a unique hybrid architecture. The chip integrates differential Hall-based sensing with monolithic differential air-core coils similar to Rogowski coils on a single die. The current sensors are all arranged on the chip differentially, delivering excellent common-mode transient immunity (CMTI) and strong immunity to electromagnetic interference (EMI).

The ultra-low-noise architecture delivers 9 MHz of bandwidth with 38-mA RMS noise and high accuracy characterized by ±1.2% sensitivity error and ±200 mA of offset error over its temperature range of –40 to 150°C (and over its lifetime).

The isolated current sensor comes with less than 1 mΩ of current rail resistance, enabling nominal current measurements up to 60 A.

The Pros and Cons of Current-Sensing Technologies

Current sensors act as the eyes of power electronics, enabling advanced power-management strategies that maximize energy efficiency while ensuring system protection. Each method of measuring current comes with its pros and cons, though

The most common approach is placing a shunt resistor in series between the power supply and the load, measuring the voltage droop that occurs as current races through it. Using the relationship of voltage = current × resistance (V = I × R), current can be determined. These shunt-based current sensors can measure AC or DC currents, and they handle high bandwidths of more than 1 MHz, which are key for fast-switching power electronics.

In general, the sensors are very accurate across the entire current range. The issue is that the resistors add a very small amount of resistance to the circuit, which means that the output voltage is equally small and it can be difficult to detect. By using high-performance current-sense amplifiers (CSAs) and high-precision analog-to-digital converters (ADCs) to condition and convert the resistor’s output, engineers can achieve high accuracy with less than 1% drift over the entire current measurement range, temperature, and lifetime.

However, shunt resistors aren’t inherently isolated. In high-voltage battery packs in EVs and increasingly high-voltage DC power supplies in AI data centers, galvanic isolation must be integrated into op amps or other components in the signal chain.

One of the other problems with shunt-based current sensors is that they must be placed within the power path. By adding resistance to the circuit, they contribute to power losses as current races into the load. These losses can pile up, becoming a limiting factor in power-switching circuits that handle very high currents. The shunt resistor also adds parasitic source inductance to the power path and gate loop of low-side switches, where it’s most unwanted, causing delayed turn-off (propagation delay) and voltage spikes.

One way to measure massive amounts of current is to use a current transformer (CT). Similar to shunt resistors, current transformers are often placed directly within the power rail to monitor average and/or peak currents as required for power control and circuit protection. However, these passive components are galvanically isolated, which helps prevent ground loops and reduce EMI and other noise between the power circuits and the controller.

When AC current races through the primary winding of the device, it creates a proportional magnetic field, which produces a smaller current in the secondary winding wrapped around the magnetic core. The step down in current is based on the transformer's turns ratio. The magnetic iron core at the heart of it can handle higher currents than shunt-based current sensors. Current runs through the burden resistor placed at the output of the transformer, which produces a voltage proportional to the primary-side current.

One of the tradeoffs with current transformers is that they tend to be large, heavy components, in many cases constructed with donut-shaped coils. As a result, it can be challenging to fit them into high-density power systems.

The Highs and Lows of Hall-Based Current Sensors

Hall-based current sensors are also widely used because they’re compact, isolated, and enable non-contact measurements of several amps to thousands of amps very efficiently.

Instead of detecting AC or DC currents directly, these devices measure the magnetic field created by current racing through copper wires on the PCB or other power rails, such as busbars. They convert the magnetic field into a proportional voltage or other output signal. By measuring it, the current that produced it can be determined indirectly.

These current-sensor ICs can be coreless, directly measuring the magnetic field generated by current racing through the package. Without a magnetic core, the solution can be smaller. However, it adds resistance and inductance to the power’s path, causing power losses (and heat).

Hall current sensors can also be paired with a magnetic core wrapped around the conductor, measuring magnetic fields through proximity alone. This capability can be important when working with very high currents because it reduces the impedance.

Additionally, there’s no need for high- and low-side power supplies when using Hall-based current sensors. You can use just one low-side power supply to power the device, easing design complexity and simplifying system integration.

One other hallmark of Hall sensors is that they have inherent galvanic isolation. But since they rely on measuring magnetic fields, they’re also vulnerable to external magnetic fields generated by motors and other nearby power components.

Moreover, Hall-based current sensors can struggle to stay accurate in increasingly hot and crowded power supplies. They could be impacted by temperature-dependent offset and sensitivity drift, particularly in high-voltage power converters where operating temperatures may vary significantly. They will also drift due to degradation of the device over its operating life. While many Hall current sensors use temperature compensation to these issues, thermals remain one of the main sources of measurement error.

Hall-based sensors are bandwidth-limited, too, when compared to shunt-based current sensors. Their bandwidths typically range from tens to hundreds of kilohertz and, in some cases, can reach 1 MHz depending on the device architecture.

Hybrid Hall Current Sensing Hits Higher Bandwidths

Infineon said it addresses these limitations with the hybrid architecture of the TLE4978. It integrates Hall-based current sensing with on-chip, air-core coils akin to miniature Rogowski coils. Traditionally, Rogowski coils are donut-shaped devices used to measure a wide range of currents without the bulky magnetic cores used in current transformers. However, they operate on the same principle: measuring the magnetic field caused by current flowing through wires, busbars, and the like.

The coil is wrapped around the conductor without touching it, and unlike Hall-based and other magnetic current-sensing technologies, it outputs a voltage proportional to the rate of change of current rather than the current itself. As a result, Rogowski coils feature accurate phase response and fast transient performance, which are key advantages when measuring the complex, rapidly changing waveforms commonly found in today's power-conversion systems.

These coils can accurately measure currents ranging from a few amps to several thousand amps while operating at high frequencies. Without a ferromagnetic core, they eliminate the risk of core saturation that comes with current transformers operating at very high currents. Their coreless, or air-core, construction also delivers inherent galvanic isolation, keeping current-sensing circuitry electrically separated from high voltages and helping protect against voltage spikes and other transients.

It’s possible to manufacture the coils in various sizes and geometries to fit around conductors of different shapes and dimensions. They can even be integrated directly into a circuit board to create compact, lightweight current-sensing solutions.

Infineon said it has taken things a step further by fabricating the coils directly on the silicon die. The TLE4978 uses Hall-based current sensing to measure magnetic fields created by electric current at lower frequencies (DC to several kHz), while the coils corkscrewed on the chip can respond rapidly to changing currents, delivering high bandwidth (up to 9 MHz).

For higher accuracy, proprietary magnetic shielding is used to deliver robust CMTI, ensuring reliable operation in electrically noisy environments.

According to the company, the TLE4978 is the first coreless magnetic current sensor to offer integrated zero crossing detection (ZCD), simplifying power-control loops. Its protection features include fast overcurrent detection (OCD) with 100-ns response time. The OCD thresholds can be configured with the digital configuration and diagnostic interface (DCDI) or using an external resistive divider. The chips come in industry-standard DSO-16 300-mil packages with both basic and reinforced isolation, 8-mm clearance, and creepage.

The compatibility with high currents and the ability to handle high frequencies are a strong fit for high-density power electronics in AI data centers, where power demands are growing exponentially. Infineon said the TLE4978 is optimized for use with its broad server power-delivery portfolio, spanning grid to core. With ISO 26262 capability up to ASIL B and AEC-Q100 Grade 0 qualification, the current sensor is also targeted at onboard chargers (OBCs) and high-voltage DC-DC converters in EVs.