The proliferation of high-voltage and AC-connected electronics in systems such as electric vehicles and industrial automation is increasing the need to control, monitor, and protect the equipment we interact with daily. Isolated current sensing fulfills a key need in these areas by providing critical operational measurements. There are multiple implementations and solutions for isolated current sensing, with significant trade-offs between isolation levels, performance, printed circuit board (PCB) space, and cost.

This white paper explains how to meet the growing need for isolated current sensing while overcoming the technical and cost barriers typically associated with it. You will learn how zero-drift Hall-effect current sensors offer many advantages over implementations such as isolated shunt-based, closed-loop magnetic and isolated in-package Hall-effect sensors – including higher energy efficiency and operating voltages, and increased performance.

As high-voltage systems increase in numbers and power levels, there is a commensurate increase in energy consumption of equipment connected to or charged by the power grid. Regulation, energy costs, and technical limitations such as power density force these systems to become more efficient and use energy more intelligently. Industrial automation equipment, electric battery packs in automobiles and even home appliances waste significant energy during power conversion from the AC grid input to DC loads or energy storage elements.

Even minor improvements in power-delivery systems can have a significant effect considering the scale of modern power systems. For example, server and data centers consume as much as 23% of all U.S. electricity consumption, yet studies show an overall efficiency of approximately 65%. Improving the efficiency of these systems even 0.5% with more efficient power conversion and smarter load monitoring could save up to 850 million kWh, equivalent to the energy usage of almost 80,000 households annually, as Figure 1-1 shows.

Figure 1-1 Efficiency Improvements Could Save Significant Energy

Power systems such as uninterruptable power supplies (UPS) or industrial AC/DC converters use switching converter topologies to optimize efficiency while converting energy from the AC grid. For these systems, isolated current measurements are required for diagnostic information and converter control loops such as power factor correction (PFC) circuits. Figure 1-2 highlights a PFC topology that requires an isolated current measurement directly on the AC input current. Because the current measurement controls the switching converter, the accuracy and lifetime stability of these current measurements can significantly impact the overall efficiency and power factor of the converter.

Figure 1-2 PFC Converter Topology With Isolated Current Sensing

Historically, isolated current-sensing measurements have a high cost-performance trade-off, requiring significantly more expense for equipment manufacturers to make high-precision measurements. Hall-effect current sensors provide the simplest implementation with no external components and an improved cost structure, but are typically not capable of providing low drift measurements over temperature.

Isolated shunt-based and closed-loop magnetic devices can provide a higher level of accuracy, but require more PCB space and external components, and are more expensive. The TMCS1100 and TMCS1101 from Texas Instruments (TI) couple the ease of use and low solution cost of a Hall-effect current sensor with a zero-drift precision signal-chain architecture to enable < 1% total error measurements and improved isolation quality. These innovations improve upon the typical negatives of Hall-effect current sensors, while retaining their key positive attributes. Table 1-1 highlights relative strengths and weaknesses of multiple isolated current sensing technologies.

The 1-kW, 80 Plus Titanium, GaN CCM totem pole bridgeless PFC and half-bridge LLC reference design highlights the impact of such a high-precision sensor, where the TMCS1100 enables a small form factor design that achieves a power factor of 0.98 with > 99% efficiency. The thermal stability of the TMCS1100 in both sensitivity and offset drift contributes to a reduction in total harmonic distortion of as much as 5%.

The galvanic isolation of the TMCS1100 decouples the system from the AC grid and simplifies measurements on AC lines with no additional components while providing < 0.5% of measurement drift. This provides improved efficiency over a variety of load and environmental conditions in compact, high-power systems such as server or network power supplies. Because the grid infrastructure chain relies on multiple UPSs, conditioners and power-conversion steps, improvements in performance, and cost structure are compounded by each stage in the chain.

Electrification enables improvements in performance, reliability, and total lifetime cost, but these systems require isolation from their high-voltage AC or DC domains in increasingly dense form factors. Many existing isolated current-sensing solutions take up significant space on high-voltage PCB designs due to additional external components, housings or elements. Hall-effect current sensors provide a monolithic, single-chip solution incorporating both the isolation and sensing elements in surface-mount form factors, such as 8-pin small-outline integrated circuit (SOIC) packages with 5 mm by 6 mm of PCB space.

However, most sensors in 8-pin SOIC packages only provide around 400 V of a working lifetime isolation voltage, which limits the usability of these devices to systems operating below this level. For example, many high-power battery systems have 400-V nominal DC buses, and they can significantly exceed this level with load and switching transients. Industrial systems that operate from 240 V_AC and convert power to a 340-V_DC rectified voltage level will often include a boost stage, as Figure 2-1 shows, which can reach voltage levels of 400 V_DC–600 V_DC. Many solar string inverters operate with photovoltaic cell and maximum power-point tracker (MPPT) buses in the 480-V to 600-V range, which are also outside the capabilities of most available 8-pin SOIC Hall-effect current sensors.

Figure 2-1 AC-Rectified System Including Boost Circuit and String Inverter Circuit With MPPT Boost.

To provide a sufficient lifetime isolation margin, the TMCS1100 and TMCS1101 devices provide 600 V of working voltage in an 8-pin SOIC form factor, with a higher lifetime margin than what is required by industry standards such as Verband der Elektrotechnik 0884-11 and Underwriters Laboratories 1577. This working voltage provides a sufficient isolation margin for equipment manufacturers without the need to transition to a larger solution size.

It is possible to extrapolate the expected lifetimes of a device from a qualification test known as time-dependent dielectric breakdown (TDDB), which measures the expected lifetime of a device versus the device voltage stress. Figure 2-2 shows the TMCS1100 family TDDB curve. The operating lifetime zone greatly exceeds industry standards for a 26-year lifetime with a 20% voltage safety margin.

Figure 2-2 TDDB Curve of the TMCS1100 Family.