Why signal isolation is important in 48V HEV/EV systems

One of the main differences between a traditional internal combustion engine vehicle and a hybrid electric vehicle ( HEV ) or electric vehicle ( EV ) is the presence of multiple batteries and voltage levels. Internal combustion engines are run by a single 12V or 24V battery, typically a lead-acid battery. However, HEVs and EVs use secondary high-voltage batteries that range from 48V (HEV) to higher voltages of 400V to 800V (EV).

The presence of multiple voltage levels requires isolation to protect low voltage circuits from high voltage. Obviously, for batteries at 400V and above, you need isolation, but is isolation required in a 48V mild hybrid system? Let’s analyze it.

Isolation in 48V HEVs

Even though the voltages are not as high as 400V or 800V, isolation is important for 48V hybrid vehicles for a variety of reasons, including enhanced noise immunity and fault protection.

Figure 1 shows a starter generator system where the power stage including the H-bridge and field effect transistors (FETs) is on the 48V side. The switching of these FETs causes voltage transients (dv/dt), which can generate some common-mode noise on the 48V ground side. Without any isolation, this noise will couple to the 12V side and affect the signal integrity of the low-voltage side circuits. By adding isolation between the two sides, as shown in Figure 1, you can improve common-mode transient immunity and signal integrity.

Figure 1: Starter/generator subsystem in a 48 V HEV

In Figure 2, the 48 V battery pack and microcontroller ( MCU ) in the battery management system ( BMS ) are on the high-voltage side, and the MCU communicates with the electronic control unit using the controller area network ( CAN ) protocol . If a fault occurs on the 48 V side, the voltage may appear on the 12 V side. The circuit components on the low-voltage side (the CAN transceiver in this case) may not be able to withstand the high voltage and may be damaged. Using an isolator between the CAN transceiver on the low-voltage side and the microcontroller on the high-voltage side will ensure the safety of the low-voltage circuit even if a fault occurs on the high-voltage side.

The German Association of the Automotive Industry 320 (VDA320) standard specifies a fault current test (E48-20) for automotive electrical and electronic components, where the test voltage is applied to the 48V/12V isolation barrier and the prospective current between the 12V and 48V systems must be less than 1 microampere. Equipped with an isolator to ensure that the current meets this standard.

Figure 2: 48V BMS block diagram

If you are designing a 48V HEV system and are looking for an isolation device to interface with the 48V side. There are a few options available for communication between the 48V side and the 12V side based on the interface standard.

For designs that require serial peripheral interface (SPI), universal asynchronous receiver transmitter ( UART ) , or general-purpose input/output (GPIO) communication between the 12V and 48V sides, you can use digital isolators such as the ISO7741-Q1  or ISO7721-Q1 , depending on the number of isolation channels required.

When you are using I2C communication to save signal trace count, isolated I2C devices such as ISO1540-Q1 (bidirectional data, bidirectional clock) ISO1541-Q1  (bidirectional data, bidirectional clock) can meet this purpose.

If there is CAN communication between the two sides and isolation is required, you can add a digital isolator (such as the ISO7721-Q1 ) in series with the CAN transceiver or use an integrated isolated CAN device (such as the ISO1042-Q1 ) to save space.

Data communication is only part of the solution. You must also isolate the power between the two sides, which you can achieve using a flyback, flyback buck, or push-pull topology. For local power supplies (for example, power for an isolated CAN transceiver), consider a transformer driver such as the SN6501-Q1 , SN6505A-Q1 , or SN6505B-Q1 that can be used with an external transformer, rectifier, and low-dropout regulator to generate a simple isolated power supply, as shown in Figure 3.

Figure 3: Simple circuit for isolated power supply with regulated output

The main differences between the SN6501-Q1 , SN6505A-Q1 , or SN6505B-Q1 are the output current of each driver, the presence of spread spectrum to reduce emissions, and different switching frequencies. These options enable you to select the right device to meet the emissions standards and power requirements of your system.

Although I have discussed these solutions in the context of 48 V HEVs, the isolation specifications and wider packaging options of these device families make them suitable for EVs with higher battery voltages as well. The isolation portion of the HEV subsystem can be reused with minor modifications to the EV design, saving design and layout time.