Meet the Challenge of Accurate Voltage Sensing in Electric Vehicles with Isolation Amplifiers

Designers of electric vehicles (EVs) and hybrid electric vehicles (HEVs) need to meet the demand for higher performance, faster charging, and greater efficiency. One of the many electronic functions that can help satisfy these demands is accurate voltage sensing for optimal power control.

However, automotive applications are particularly challenging. Power electronics must function reliably for decades despite temperature extremes and the presence of high voltages that demand suitable isolation. Voltage-sensing circuits for these applications must offer high bandwidth, low error and drift, and high common-mode transient immunity (CMTI) while meeting automotive standards like AEC-Q100. These requirements are especially relevant for critical components in EVs and HEVs, including inverters, DC/DC converters, and onboard chargers.

Transformer-based isolation amplifiers are well suited to these applications. These devices use advanced technology to achieve excellent performance over decades of exposure to harsh conditions.

This article examines the operating principles of isolation amplifiers. It then introduces a transformer-based example that uses iCoupler technology from Analog Devices, reviews its potential applications in EV/HEV development, and presents an evaluation board to help begin the design process.

Operating principles of transformer-based isolation amplifiers

Isolation amplifiers are specialized differential amplifiers that provide electrical isolation between input and output circuits. This isolation can be achieved through several means, but transformer-based isolation amplifiers like the ADuM3195 (Figure 1) offer unique advantages for EV/HEV applications.

Figure 1: The ADuM3195 isolation amplifier uses transformer-based isolation. (Image source: Analog Devices, Inc.)

In transformer-based designs, isolation is achieved through transformer coupling. The basic principle of operation involves the following steps:

1. The input signal is converted into a high-frequency carrier signal.
2. This carrier signal is then transmitted across the isolation barrier via a transformer.
3. On the secondary side of the transformer, the original signal is reconstructed from the carrier.

The transformer serves two crucial functions. It provides galvanic isolation between input and output circuits, allowing safe measurement of high voltages and protecting sensitive circuitry. It also enables signal transfer without a direct electrical connection across the isolation barrier.

Transformer-based isolation offers significant advantages for voltage-sensing applications. These amplifiers effectively reject common-mode voltages, crucial in noisy electrical environments. In addition, modern designs achieve wide bandwidths suitable for many power electronic applications.

Performance advantages of planar micro-transformers for isolation amplifiers

iCoupler technology, developed by Analog Devices, represents an advancement in isolation amplifier design. iCoupler devices feature planar micro-transformers with a typical diameter of approximately 0.5 millimeters (mm), enabling remarkably compact solutions. The small size also provides inherent resistance to external magnetic fields, enhancing reliability.

Central to iCoupler performance is a polyimide insulation layer (Figure 2). This insulation provides high thermal and mechanical stability, making the device exceptionally durable. It can withstand surge voltages exceeding 10 kilovolts (kV) and offers long-term reliability when operating continuously at 400 V root mean square (V_RMS).

Figure 2: Central to iCoupler performance is a polyimide insulation layer that provides high thermal and mechanical stability. (Image source: Analog Devices, Inc.)

An essential feature of iCoupler technology is its ability to operate at high frequencies, supporting data transfers up to 150 megabits per second (Mbits/s). This is achieved in part through a highly efficient signal encoding methodology. Data is encoded into 1 nanosecond (ns) pulses that enable fast data transfer and low power consumption, typically less than 1 milliampere (mA) per channel (Figure 3).

Figure 3: A highly efficient encoding method allows iCoupler devices to transfer data at 150 Mbits/s and draw typically less than 1 mA per channel. (Image source: Analog Devices, Inc.)

Additionally, iCoupler devices incorporate input glitch filters to reduce noise and ensure clean signal transmission, enhancing performance in electromagnetically noisy automotive environments.

Key features of automotive-qualified isolation amplifiers

iCoupler technology has been implemented in several devices, including the ADuM3195WBRQZ isolation amplifier. This AEC-Q100-compliant version of the ADuM3195 is specifically designed for automotive environments. It has an isolation voltage of 3,000 V_RMS, an output offset voltage of ±6 millivolts (mV) (max) at 25°C, a gain error of ±0.5% (max), a bandwidth of 210 kilohertz (kHz), a gain drift of ±27 parts per million per °C (ppm/°C) (max), and an offset drift of -22 microvolts per °C (μV/°C) (typical). The device has a CMTI of 150 kV per microsecond (kV/µs) (typical), an operating temperature range of -40°C to 125°C, configurable gain settings, and comes in a 16-lead QSOP.

These features make the ADuM3195WBRQZ suitable for accurate, isolated voltage measurements in challenging automotive applications, including:

• Voltage monitoring in battery management systems (BMSs)
• Feedback loops in power supplies
• Inverter and motor drive systems

The high accuracy, wide bandwidth, low power consumption, and robust isolation capabilities make the ADuM3195WBRQZ a particularly effective solution for voltage sensing in EV/HEV systems.

Isolation amplifier requirements for inverters, DC/DC converters, and onboard chargers

The ADuM3195WBRQZ isolation amplifier addresses critical challenges in EV/HEV power systems, including inverters, DC/DC converters, and onboard chargers.

Its 210 kHz bandwidth enables sub-5 μs response times, crucial for efficient charging, precise inverter control, and minimized voltage ripple in DC/DC converters. This high bandwidth also allows for smaller passive components and supports wide-bandgap device integration, enhancing overall system efficiency and power density.

The high-impedance input of the ADuM3195WBRQZ minimizes measurement-related power loss and stabilizes converter and inverter operations. Reducing current draw also decreases stress on auxiliary circuits, improving system reliability.

The ADuM3195WBRQZ’s high temperature tolerance allows it to be placed near heat-generating components like electric motors, onboard chargers, and regenerative braking systems to help prevent thermal runaway, manage thermal cycling, and avoid hotspots in power electronics.

For DC/DC converters handling various output voltages, the ADuM3195WBRQZ’s low offset error and offset drift ensure accurate voltage feedback across temperature variations. This accuracy contributes to precise control, reduced ripple, and improved drivetrain performance.

The 3,000 V_RMS isolation voltage of the ADuM3195WBRQZ protects low-voltage electronics and occupants from high-voltage systems (up to 400 V). It provides effective noise rejection between power stages and control circuits in EV battery systems while interfacing with low-voltage systems (12/48 V).

By meeting these critical requirements, the ADuM3195WBRQZ enhances the performance, efficiency, and safety of EV/HEV power systems.

It is worth noting that the ADuM4195 is available for higher voltage system requirements, providing an isolation voltage up to 5,000 V_RMS and low-voltage electronics protection up to 800 V.

Jumpstart ADuM3195 development

The EVAL-ADuM3195EBZ (Figure 4) is a compact evaluation board designed for testing and evaluating the performance characteristics of the ADuM3195 isolation amplifier. It is an isolated voltage monitoring board that can be configured for both DC and AC measurements. This board is pre-configured to handle input voltages up to 1,000 V_DC (continuous).

Figure 4: The EVAL-ADuM3195EBZ evaluation board is designed for setup and testing of the ADuM3195. (Image source: Analog Devices, Inc.)

The features of the EVAL-ADuM3195EBZ evaluation board can help jumpstart the development of EV/HEV applications in several ways:

• High-voltage isolation and measurement: The board is designed to safely measure high DC voltages up to 1,000 V, which is especially relevant for EV/HEV battery systems. This allows developers to monitor battery pack voltages, measure individual cell voltages in a BMS, and interface with high-voltage DC bus lines.
• Configurable input range: The input voltage divider can be adjusted to accommodate different voltage ranges common in EV/HEV systems. For example, a 400 V_DC bus is typical in many EVs, 800 V systems in newer EV architectures, and lower voltage ranges for 48 V mild hybrid systems.
• AC measurement capability: With minor modifications, the board can measure AC voltages, which can be helpful for motor drive inverter output monitoring, AC charging system measurements, and electromagnetic interference (EMI)/noise analysis on high-voltage lines.
• Low-power option: For lower power consumption, the power disable (PDIS) input can disable the internal power supply when energy needs to be used judiciously.

Conclusion

Designers of EVs and HEVs require precision sensing in various subsystems to achieve performance and efficiency goals. A micro-transformer-based isolation amplifier, such as the AEC-Q100-qualified ADuM3195WBRQZ, is well suited to this application, offering a mix of performance, miniaturization, and longevity that meets the critical design requirements. The associated evaluation board for this isolation amplifier helps designers quickly get up and running.