The demand for isolation in 48V automotive applications continues to grow. This is a compact, efficient, robust, and low-noise method for isolating 48V systems via a CAN interface.
Designing for today's cars is an act of balancing. It requires providing today's vehicles with high power for high efficiency, balancing increasingly stringent emissions standards with powering a growing number of onboard systems and gadgets.
To achieve a balance between efficiency and power, engineers are increasingly relying on systems that combine 48V electric operation with traditional gas engines, such as hybrid electric vehicles (HEVs). This approach ensures that vehicles meet stringent carbon dioxide (CO2) emission standards while also improving performance and driving quality.
While there's been much to say about dual-battery car systems themselves, I'm focusing on a key, and sometimes overlooked, component in these combined 12V and 48V systems: current isolation. Current isolation is used to resist ground noise and protect the 12V system in the event of a ground fault or disconnection in the 48V system to which it is connected.
In this paper, I will discuss the isolation requirements in 48-V automotive applications and describe a compact, efficient, robust, and low-noise approach to isolating 48-V systems via a Control Area Network (CAN) interface.
The necessity of current isolation for vehicles using 48V batteries
Even in vehicles using 48V batteries (typically lithium-ion), conventional 12V lead-acid batteries can still power control electronics and low-power devices. Systems operating on these two consumables need to communicate with each other. For example, the 48V starter alternator, controlled by the engine controller, is powered by a 12V battery. Both systems are grounded to the vehicle chassis. Although theoretically the two systems could be directly interconnected (Figure 1a), current isolation (Figure 1b) is almost always necessary for the following reasons:
Transient ground potential difference: The 12V system grounding is directly bolted to the vehicle chassis. The 48V module grounding is connected to the vehicle chassis using a cable several feet long. The large switching current present in the 48V system (such as starting an alternator or AC compressor), combined with the inductive characteristics of the grounding cable, can cause transient ground noise, which can easily damage low - voltage 3.3V or 5V communication signals. Current isolation is necessary to ensure reliable data transmission.
48V Side Ground Disconnection: Sometimes, under fault conditions or during maintenance, GND_48V in Figure 1a may become disconnected from the chassis. The module's 48V power supply, instead connected to a 48V battery, may remain intact. In this case, all internal nodes of the 48V system (including interfaces to the 12V system) could float to 48V. This poses a danger to the 12V system, as its input/output ports may not be designed to handle 48V. In Figure 1b, the same fault conditions do not stress the 12V system. 48V occurs at the current barrier and is typically a much higher rated voltage (e.g., 2.5kV ).
Short circuit scenario: As shown in Figure 1a, any short circuit within the 48V system could result in a 48V voltage at the interface with the 12V system. This potential hazard could compromise multiple circuits operating on the 12V supply, including those critical to the safe operation of the vehicle. Current isolation helps ensure that any short circuit on the 48V system does not propagate to the vehicle's 12V side.
Figure 1. Direct and electrically isolated connection between 12V and 48V systems.
Use CAN interface to isolate 48V system
Current isolation can be achieved in various ways, with isolation boundaries drawn at different locations within the system. Figure 2 illustrates a general method for implementing isolation at the CAN interface. Isolating the CAN interface from other parts of the system has the advantage of using a minimal number of isolation channels—only two are required. This reduces cost and board space.
2. This is an example of current isolation between the 12V and 48V sides in a mild hybrid electric vehicle.
Isolated DC-DC converters provide isolated power supply (VISO) to power certain parts of a 48V system. Even when the 48V battery is fully discharged, VISO ensures that the digital isolator and critical components of the 48V system have a usable power source. VISO can also be used to bring the 48V side to a safe position if GND_48V is disconnected.
New integrated isolated CAN transceivers and isolated DC-DC power controllers are now available, simplifying isolated CAN interfaces in 48V systems. Figure 3 shows an example 48-V starter generator. Similar isolation architectures can be used for other 48V systems, such as DC-DC converters, battery management systems, heaters, and air compressors.
3. This 48V starter generator uses an isolated CAN transceiver and a push-pull isolated power supply.
Monolithically integrated isolated CAN transceivers, such as the Texas Instruments (TI) ISO1042-Q1 (Figure 3), integrate high-voltage current isolation with a high-performance CAN transceiver, helping to reduce board area while improving timing parameters. From a CAN perspective, low loop delay and skew enable high-speed data communication using CAN's flexible data rate. Isolation provides immunity to conducted and radiated interference. Redundant or reinforced isolation provides additional protection margin under fault conditions.
When used with an external transformer, Texas Instruments' SN6505-Q1 push-pull transformer drivers (also shown in Figure 3) can generate an isolated power supply VISO_HV (in the range of 10 to 15V) for metal-oxide-semiconductor field-effect transistor (MOSFET) gate drivers, and can generate a lower VISO (in the range of 3.3 to 5V) for powering the digital side of microcontrollers and isolated CAN devices.
The push-pull topology uses two low-side switches. These switches conduct in alternating clock phases to continuously transfer power across the center-tapered isolation transformer. The topology employs feedforward regulation, with the output voltage controlled purely by the transformer ratio. Compared to other topologies, continuous power transfer results in lower peak current, thus reducing emissions and improving efficiency. The symmetrical driver also prevents transformer saturation, leading to a compact transformer configuration.
On the 12V side, a non-isolated DC-DC converter or buck converter can generate 5V to power the CAN transceiver, and can also serve as the input voltage for a push-pull isolated DC-DC converter. Using a pre-buck makes the system insensitive to changes in the 12V battery power supply, which may be caused by load variations. Furthermore, operating at a lower input voltage (5V vs. 12V) results in a smaller transformer.
in conclusion
Current isolation is an extremely important consideration in vehicles powered by 48V batteries. Isolation is used to resist ground noise and protect the 12V system in the event of a ground fault or disconnection in the connected 48V system. Examples of systems using 48V power in HEVs include starter-generators, electric turbochargers, electric pumps, air conditioning, heaters, electric suspension, and driver assistance systems. The combination of an integrated isolated CAN transceiver and a push-pull-based isolated DC-DC power supply provides a compact, efficient, robust, and low-noise technology for isolating 48V systems.
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