Simplify the isolated CAN and power interface of HEV 48-V system

“The demand for isolation in 48V automotive applications continues to grow. This is a compact, efficient, robust, and low-noise method that can isolate the 48 V system through the CAN interface.

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The demand for isolation in 48V automotive applications continues to grow. This is a compact, efficient, robust, and low-noise method that can isolate the 48 V system through the CAN interface.

Designing for today’s cars is a balancing act. Between meeting increasingly stringent emission standards and powering more and more on-board systems and gadgets, today’s vehicles need to be provided with high power to achieve high efficiency.

In order to achieve the integration of efficiency and power, engineers rely more on systems that combine 48V power operation with traditional gas engines, such as hybrid electric vehicles (HEV). This method ensures that the vehicle meets strict carbon dioxide (CO2) emission standards, while also improving performance and drive quality.

Although there have been many statements about the dual-battery car system itself, I am concerned with a key and sometimes overlooked component of these combined 12 and 48V systems: galvanic isolation. Galvanic isolation is used to resist ground noise and protect the 12 V system when the ground is disconnected or faulted in the 48 V system connected to it.

In this article, I will discuss the need for isolation in 48-V automotive applications and describe a compact, efficient, robust, and low-noise method to isolate the 48-V system through a control area network (CAN) interface.

The necessity of galvanic isolation for vehicles using 48V batteries

Even in vehicles that use 48V batteries (usually lithium-ion batteries), traditional 12V lead-acid batteries can still power control electronics and low-power devices. The systems running on these two consumables need to communicate with each other. For example, the 48 V starter generator is controlled by the engine controller and uses a 12 V battery. The grounds of the two systems are connected to the car chassis. Although in theory, two systems can be directly connected to each other (Figure 1a), galvanic isolation (Figure 1b) is almost always necessary for the following reasons:

Transient ground potential difference: The ground of the 12 V system is directly connected to the car chassis using bolts. The ground of the 48 V module is connected to the chassis of the car using a few feet of cable. The large amount of switching currents in the 48 V system (such as starting generators or AC compressors), combined with the inductance characteristics of the grounding cable, may cause instantaneous grounding noise, which can easily damage low-voltage 3.3 V or 5 VV communication signals. Galvanic isolation is necessary to ensure reliable data transmission.

Ground disconnection on the 48 V side: Sometimes under fault conditions or during maintenance, GND_48V in Figure 1a may be disconnected from the chassis. The module’s 48 V power supply, instead connected to a 48 V battery, may still be intact. In this case, all internal nodes of the 48 V system (including the interface of the 12 V system) can float to 48 V. This poses a danger to a 12 V system because its input/output ports may not be designed to handle 48 V. In Figure 1b, the same fault conditions will not stress the 12 V system. 48 V appears on the current barrier, and the rated voltage is usually much higher (such as 2.5 kV).

Short circuit situation: In Figure 1a, any short circuit inside the 48 V system may result in a 48 V voltage at the interface with the 12 V system. This potential hazard may endanger multiple circuits that operate on 12 V power supplies, including circuits that are critical to the safe operation of the vehicle. Galvanic isolation helps to ensure that any short circuits on the 48 V system will not propagate to the 12 V side of the vehicle.
　

Figure 1. Direct and electrically isolated connection between 12 V and 48 V systems.

Isolate 48 V system using CAN interface

Galvanic isolation can be achieved in a variety of ways, and isolation boundaries can be drawn at different locations in the system. Figure 2 shows a general method for isolation on the CAN interface. Isolating the CAN interface from other places in the system has the advantage of using a minimum number of isolation channels-only two isolation channels are required. This reduces cost and board space.

2. Shown is an example of galvanic isolation between the 12V and 48V sides in a mild hybrid electric vehicle.

The isolated DC-DC converter can provide an isolated power supply VISO to power certain parts of the 48 V system. Even if the 48 V battery is fully discharged, VISO can ensure that the digital isolator and key components of the 48 V system have a power source available for operation. If GND_48V is disconnected, VISO can also be used to put the 48V side in a safe state.

New integrated isolated CAN transceivers and isolated DC-DC power supply controllers are now available to help simplify isolated CAN interfaces in 48 V systems. Figure 3 shows an example 48-V starter generator. You can use similar isolation architectures for other 48 V systems, such as DC-DC converters, battery management systems, heaters, and air compressors.

Simplify the isolated CAN and power interface of HEV 48-V system

3. This 48V starter generator uses isolated CAN transceiver and push-pull isolated power supply.

Monolithic integrated isolated CAN transceivers, such as Texas Instruments (TI) ISO1042-Q1 (Figure 3), integrate high-voltage galvanic isolation with high-performance CAN transceivers, helping to reduce board area while improving timing parameters. From a CAN perspective, low loop delay and skew use CAN flexible data rates to achieve high-speed data communication. Isolation provides immunity to conduction and radiation interference. Redundant or reinforced isolation will provide additional protection margin under fault conditions.

When used with an external transformer, a push-pull transformer driver such as Texas Instruments’ SN6505-Q1SN6505-Q1 (also shown in Figure 3) can generate an isolated power supply VISO_HV (in the range of 10 to 15 V) to supply power to the metal oxide Semiconductor field-effect transistor (MOSFET) gate driver, and can generate low VISO (in the range of 3.3 to 5V) to supply power to the digital side of the microcontroller and isolated CAN devices.

The push-pull topology uses two low-side switches. These switches are turned on in alternate clock phases in order to continuously transmit power on the central tap isolation transformer. The topology adopts feedforward regulation, and the output voltage is controlled purely by the transformer ratio. Compared with other topologies, continuous power transmission can produce lower peak currents, thereby reducing emissions and improving efficiency. The symmetrical drive also prevents the transformer from saturating, resulting in a compact transformer.

On the 12 V side, a non-isolated DC-DC converter or step-down converter can generate 5 V power to supply power to the CAN transceiver, and it can also be used as the input voltage of a push-pull isolated DC-DC converter. The use of front-end step-down makes the system insensitive to changes in the 12 V battery power supply, which may be caused by changes in the load. In addition, operating at a lower input voltage (5 V vs. 12 V) will cause the transformer to become smaller.

in conclusion

Galvanic isolation is an extremely important consideration in cars powered by 48V batteries. Isolation is used to resist ground noise and protect the 12 V system when the ground is disconnected or faulted in the 48 V system connected to it. Examples of systems that use 48V power in HEV include starters-generators, electric turbochargers, electric pumps, air conditioners, heaters, electric suspensions, and driver assistance. The integrated isolated CAN transceiver combined with a push-pull-based isolated DC-DC power supply can provide a compact, efficient, robust and low-noise technology for isolating 48 V systems.