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Signal Chain Basics #161: Isolating signals and power in HEV/EV isolated subsystems

The need to reduce carbon footprint in automobiles has driven the use of multiple power supplies in hybrid electric vehicle/electric vehicle (HEV/EV) subsystems. As a result, robust and reliable signal and power isolation have become a necessity in many automotive applications.

Isolation is an integral part of powertrain systems such as battery-management systems, traction inverters, onboard chargers, and DC/DC converters and body systems including air-conditioning (AC) compressors, interior heaters, and power-distribution boxes. In these systems, isolation ensures that the circuits on the low-voltage side (12V) can communicate reliably with those on the high-voltage side (48 to 400V or higher), while protecting against noise transients on the low-voltage side and faults on the high-voltage side. This article will highlight the various isolated solutions, such as digital isolators, isolated interfaces, isolated amplifiers, isolated gate drivers, and isolated power that are available to you when designing discrete or integrated isolated subsystems.

Two key terms that we will be using are robustness and reliability. For the purposes of this article, “robustness” will refer to the dielectric strength of the isolation barrier to withstand the voltage difference between the multiple power domains, while “reliability” will refer to the signal integrity of the data being transferred across the isolation barrier. To learn more about common terminology used with isolation devices, please refer to the TI isolation glossary.

Signal isolation

Designers typically begin by selecting signal isolators that meet their interface communication needs. Finding the right isolation device for a system will depend on your interface standard design requirements. For example, if you’re isolating universal asynchronous receiver transmitter (UART) or serial peripheral interface (SPI) communications in a 400-V battery-pack system (Figure 1), a digital isolator such as the ISO7721-Q1 or ISO7741-Q1 is the recommended option. In this application, the isolators are used in a SPI/UART interface to enable communication between the microcontroller on the high-voltage cell-supervision circuitry side and the microcontroller on the 12-V side.

A simple two-channel digital isolator like the ISO7721-Q1 would be sufficient when designing for one transmit and one receive signal in UART communication between the two microcontrollers, while a four-channel isolator such as the ISO7741-Q1 with three channels in one direction and one in the reverse direction facilitates SPI communication. In some cases, you may need additional isolation channels for any fault or diagnostic signals.

block diagram of a battery management system using a digital isolatorFigure 1 A digital isolator is recommended for isolating UART or SPI communications in a 400-V battery-pack system.

A digital isolator is also the recommended solution for isolating the TR/RX side of controller area network (CAN) and local interconnect network (LIN) communications given its ease of implementation. For example, a 400-V HVAC compressor will require a digital isolator between the CAN or LIN transceiver on the 12-V side and a microcontroller on the high-voltage side (Figure 2). In some subsystems, the inclusion of the isolator does add additional propagation delay to the CAN communication path, which may be unacceptable. In these cases, using integrated isolated CAN devices, such as the ISO1042-Q1, in place of discrete solutions will save space and meet system timing requirements as well as system emissions requirements.

block diagram for an HVAC compressor highlighting the digital isolatorFigure 2 A high-voltage HVAC compressor subsystem will require a digital isolator between the CAN or LIN transceiver on the 12-V side and a microcontroller on the high-voltage side.

Considering the high-voltage traction inverter shown in Figure 3, two other isolated device types will allow data transfer across the isolation barrier: isolated gate drivers and isolated amplifiers. Isolated gate drivers enable the transfer of pulse-width modulation (PWM) signals to insulated gate bipolar transistors (IGBT) and metal oxide semiconductor field-effect transistors (MOSFET), while isolated amplifiers facilitate the transfer of analog current or voltage measurements to the PWM controller. It is possible to achieve these functionalities with discrete digital isolators, but the integrated solutions enable smaller designs and thus lighter systems. AMC1311-Q1 is a 2-V input, high-precision reinforced isolated amplifier for voltage sensing while AMC1302-Q1 is a ±50-mV input, reinforced isolated amplifier for current sensing. Examples of a single channel isolated gate driver for SiC/IGBT are UCC21710-Q1, which has an overcurrent protection, and UCC5870-Q1, which is an isolated gate driver with advanced diagnostics and protection features.

block diagram of a high-voltage traction inverter subsystemFigure 3 In a high-voltage traction inverter subsystem, isolated devices allow data transfer across the isolation barrier.

Power isolation

The signal isolators discussed in this article can derive their power from isolated power supplies. For example, one side of the isolators can be powered from the 12-V battery, which is stepped down to the supply range of the isolator, while the other side is powered from the high-voltage battery. If the isolator is supporting critical signals, however, redundancy is important in case battery power is lost. In that scenario, isolated power from the 12-V side to the high-voltage side, or vice versa, is recommended.

Another example is when an independent bias supply is needed for a traction inverter, similar to what is shown in Figure 3. To avoid a single point of failure, each of the phases of the isolated gate driver must use a separate bias supply. In any of these cases, much like signal isolation, there are discrete and integrated solutions for isolating power.

Discrete solution options include flyback, fly-buck, and transformer drivers. Transformer drivers provide the most simple isolated power-supply solution. As shown in Figure 4, combining the transformer driver with an external transformer, rectifying diodes, and an optional low-dropout regulator provides up to 1 A of output current that is sufficient for isolated signals. The discrete solution works best when the size of the transformer is within an acceptable range and high-efficiency, very-low emissions are required.

isolated power supply schematic wiht a transformer driverFigure 4 An isolated power supply with a transformer driver provides up to 1 A of output current, sufficient for isolated signals.

There are instances where further integration results in board space savings that are beneficial for automotive systems; integrated isolated signal and power solutions do exactly that. They combine the isolated signal path and isolated power path in a single device, eliminating the need for you to design a power supply. Figure 5 shows a device with four channels of isolation along with an isolated power supply like the ISOW7841A-Q1, which is capable of providing 130 mA of output current. This output current can drive external CAN transceivers, LIN transceivers, and analog-to-digital converters (ADCs), helping you maintain a low device count.

block diagram of a device with four channels of isolation and an isolated power supplyFigure 5 A device with four channels of isolation along with an isolated power supply is capable of providing 130 mA of output current.

With small planar transformers integrated in the device, the efficiency of the integrated solution is lower (~50%) compared to discrete solutions (~ 90%). In most cases, the isolated data and power device is powering up CAN transceivers, amplifiers, and ADCs that are not “power hungry,” therefore efficiency may not be a critical design consideration. With higher switching frequencies employed to keep the inductors small, radiated emissions with integrated transformers are higher than discrete solutions, so additional components may be needed to mitigate EMI. The reason for the adoption of the integrated signal and power device is the simplicity of the solution versus a discrete solution with bulky transformers that could be an issue in compact designs.

Also, integrated power and data isolation devices can simplify the system certification process. Integrated solutions are certified by agencies such as Underwriters Laboratories, Verband der Elektrotechnik, or the Canadian Standards Association, so it reduces the burden on designers to find an automotive-grade transformer that meets the necessary certifications for their application.

Figure 6 shows an integrated signal and power isolation device and a non-isolated CAN-flexible data rate (CAN-FD) transceiver that can be used to communicate between the MCU on the high-voltage side to the CAN transceiver on the low-voltage side. When the output voltage of the integrated signal and power isolation device drives the supply of the CAN transceiver, the ISOW7841A-Q1 and TCAN1042-Q1 provide the entire signal and power solution for an isolated CAN system.

block diagram for an isolated CAN subsystemFigure 6 An integrated signal and power isolation device and a non-isolated CAN-FD transceiver can be used to communicate between the MCU and the CAN transceiver.

Figure 7 shows a battery-management system where an ADC and isolated data and power device measure the high-voltage battery insulation to the chassis. Similar to the previous example in Figure 6, the VISO signal of the integrated signal and power isolation device drives the ADC supply current, along with any other devices that need powering.

block diagram of a battery management system that measures insulationFigure 7 A battery-management system measures high-voltage battery insulation.

Figure 8 illustrates another integrated solution typically used for shunt-based current and voltage measurements in high-voltage applications such as on-board chargers (OBC). In this case, the high-voltage domain must be isolated from the low-voltage domain. Precision isolated amplifiers such as AMC3301-Q1 and AMC3330-Q1 with a fully integrated isolated DC/DC converter enable single-supply operation from the low side of the device for current sensing and voltage sensing respectively. Typical applications are DC link voltage and phase current measurements in traction inverters, the power-factor correction stage in onboard chargers, and primary and secondary current sensing in DC/DC converters.

schematic for a shunt-based current voltage sensing solutionFigure 8 This shunt-based current and voltage sensing solution uses isolated amplifiers with an integrated DC/DC converter.
Click to enlarge.

A growing number of HEV/EVs require isolation to keep the low-voltage side protected while reliably driving communication to and from the high-voltage side. There will be some trade-offs on the compactness of the solution versus its flexibility and performance. With the subsystems being different depending on voltage, e.g. 48 or 800V, you need to clearly identify the isolation ratings for each system and then partition the systems (discrete or integrated) to keep the solutions small enough without compromising performance.

Neel Seshan is the marketing manager of isolation products at Texas Instruments.

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