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Galvanic isolation in electric vehicles

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By Nick Flaherty

For people familiar with conventional engines, looking under the hood of a fully electric vehicle (EV) is a mystifying experience. The main difference will, of course, be the lack of an internal combustion engine (ICE). Instead, they are likely to find the electric traction inverter, which is often the same size and mounted in a way that resembles a conventional engine. Other systems will look even less familiar, but there is a good chance they will be able to identify one component that has changed little – the 12 V battery.

In a non-EV vehicle, the 12 V system is required to power the starter motor, which provides the initial rotation of the ICE to start the four-stroke combustion cycle. Given that an EV has no need for a starter motor, it may be surprising to discover the presence of a 12 V battery. However, the majority of an EV’s electrical systems still operate from 12 V. With no ICE or alternator present, the 12 V system must be completely powered from the high voltage, traction battery.

This presents an interesting design requirement. The traction inverter system is likely to operate at a DC voltage in the region of 800 V. This high DC voltage is converted to AC in order to drive the traction motors. However, the traction battery in an EV is not simply multiple 12 V batteries connected in series to create 800 V; it is a sealed unit. The addition of this high voltage system, and its role in the vehicle, means the 12 V system is now commonly referred to as the auxiliary system. It powers everything that is auxiliary to the traction system (including the traction control system).

The main high voltage battery is now responsible for providing power to the 12 V auxiliary system in order to keep the battery charged. For safety reasons, it needs to do this in a way that maintains electrical isolation between the two voltage domains.

 

 

Figure 1: The key components of an electric vehicle (Source: Energy.Gov)

Isolation is critical

Figure 1 is a typical EV diagram and depicts a number of functions, including the traction inverter, climate control and heating, and the on-board charger. These systems operate at radically different voltage levels and must be galvanically isolated. Galvanic isolation prevents the flow of current between the different voltage domains while still supporting the flow of data and power.

Historically, galvanic isolation for data transmission has been implemented using optical technology, with an LED source and photodiode receiver. However, the demands of the automotive market in general and electric vehicles, in particular, have spurred the development and adoption of digital isolation technologies.

Auxiliary power

The auxiliary power system is typically controlled by a dedicated module, called the Auxiliary Power Module (APM). This is effectively a dc-dc converter that takes the high voltage (HV) from the traction battery and converters to a low voltage (LV). This LV bus supplies the auxiliary systems and charges the 12 V battery. Initially, this may seem like a relatively simple function, however the need for galvanic isolation introduces additional complexity.

Many DC-DC converter topologies use a transformer to provide both voltage step-down and galvanic isolation in a single step. While this is an effective way of isolating the HV and LV circuits, it does require additional conversion steps in order to make use of the transformer. Specifically, the HV voltage needs to be converted from DC to AC and then the LV needs to be converted from AC back to DC. The circuit diagram in Figure 2 shows a common full bridge implementation.

Figure 2: Circuit diagram of an APM (Source: Silicon Labs)

The full bridge converts the DC voltage into an AC voltage, so it can excite the primary side of the insulating transformer and induce a current in the secondary side. The secondary side AC voltage then needs to be converted back to DC. In order to use smaller magnetic components, and reduce the size and weight of the end solution, many systems use a switching frequency of 100 kHz or higher.

The example shown in Figure 2 uses a full bridge on the primary (HV) side of the transformer and a full-bridge synchronous rectifier on the secondary (LV) side. The choice of switches on the HV side will be based on cost versus efficiency; typically, IGBTs would be used, but newer APMs are likely to use Silicon Carbide (SiC) MOSFETs to achieve maximum efficiency.

Regardless of the switch technology, isolated gate drivers play a critical role. Digitally isolated gate drivers leverage CMOS technology to create both the device itself and the isolation barrier. Figure 3 shows a block diagram of a single channel in the Si8239x isolated gate driver, which uses an RF carrier to pass information across the isolation barrier. This digital isolation technology provides a robust, isolated data path which is easy to integrate with other CMOS technologies, like gate drivers.

 

Figure 3: A single state of the automotive qualified Si8239x isolated gate driver family from Silicon Labs (Source: Silicon Labs)

Extending digital isolation

The circuit shown in Figure 3 is managed by an APM controller, which generates the PWM signals to control the power switches’ gate drivers. To achieve maximum efficiency, the controller needs to monitor the voltages being generated, a process that also requires an isolated solution such as a galvanically isolated analog amplifier. Isolation is also required for the system bus that connects the APM to the wider automotive control system. Many designs use a CAN bus and the APM includes digital isolators for the CAN bus signals. A multichannel digital isolator with 5 kVrms isolation, such as the Si86xx from Silicon Labs, is optimized for this application. Just like with isolated gate drivers, the CMOS isolation barrier allows the integration of high performance analog and digital I/O functions.

Conclusion

The move to EVs involves significant design challenges for OEMs and Tier 1s. Maintaining, at least for now, 12 V power rail for auxiliary power simplifies the task by supporting legacy systems. However, the removal of the main source 12 V battery energy source – the alternator driven by the engine – increases the complexity of the auxiliary power module. The advances in integration, brought about by CMOS isolation technology, simplify the APM’s design while providing safe, reliable operation for the lifetime of the vehicle.


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