In the past, the simple 12 V lead-acid battery was a ubiquitous feature of almost all automobiles, which meant protecting vehicle electrics against electrostatic discharge (ESD) was a relatively straightforward task. However, automobile architectures are changing radically and quickly and this means designers now need a broader range of ESD protection device options to keep vehicles operating safely on the road. This blog article looks at some of the factors driving the change in board net voltages used to power electric systems in automobiles and discusses features of a new AEC-Q101 qualified portfolio of ESD protection devices from Nexperia which includes flexible options to protect boards operating from 12 V, 24 V, and 48 V nets.
Why board net voltages are increasing
The number of electrical and electronic components in automobiles continues to increase and the amount of power they require is now pushing 12 V battery systems to their limit. Increasing the battery voltage is the logical solution to this problem and to accommodate this change, automotive companies have developed electrical systems that can operate from 24 V (now commonly used in heavy commercial vehicles) up to 48 V, increasingly a feature of newer hybrid electric vehicles (HEV). This trend towards higher battery voltages means that traditional lead-acid batteries may eventually be replaced with lithium-ion batteries. Compared to 12 V batteries, 48 V systems offer a significant increase in power output and efficiency. Another advantage of higher voltages is that they allow for reduced current levels for the same amount of power, meaning thinner, lighter cables can be used, an important requirement for electric vehicles. For this reason, higher voltages are better for powering devices like pumps, compressors, and suspension components which draw larger amounts of current. Vehicles with a 48 V battery still feature 12 V systems to power electronic control units (ECUs) and legacy automotive networking interfaces like CAN and LIN. However, the 12 V supply for these systems is generated by stepping down the 48 V battery voltage using a buck converter. The conditions used to describe various board net voltages are shown in Figure 1.
Protecting 12 V CAN systems
Controller Area Network (CAN) is a trusted legacy automotive communications protocol which typically uses a two wire, twisted pair cable to transmit and receive serial data (Figure 2). A CAN transceiver provides the physical link between the protocol controller and the physical bus wires in a network. High-speed CAN specifies transmission rates up to 1 Mbit/s while lower-speed, fault-tolerant CAN, specifies data rates up to 500 kilobits per second. CANL is the LOW-level CAN bus line and CANH is the HIGH-level bus line. In normal operating mode, the value of dominant state is about 1.4 V and the value of recessive state is 5 V. In low-power modes, the voltage of CANL is the same as the battery voltage (typically 12 V) but in normal operation, the dominant state voltage is about 3.6 V and the recessive state is 0 V. To protect against ESD and ensure communication can still take place, external clamping circuits can be applied to the CANH and CANL lines. Nexperia offers ESD protection devices specifically designed to protect two CAN bus lines from damage caused by ESD and other transients. Since CAN networks could accidentally be shorted to voltage sources like the car battery, ESD protection devices at the CANL and CANH lines must be able to withstand higher voltage levels. To safely withstand jump-start conditions or where two 12 V batteries are connected in series in the case of a commercial vehicle, Nexperia’s ESD protection devices meet the minimum required standoff voltage (VRWM) of 24 V and adhere to the ISO7637-2 and ISO 16750-2 ESD protection standards.
Protecting 24 V and 48 V systems
While 12 V battery voltages are commonly found in cars and smaller vehicles, 24 V board nets are typically used in trucks and commercial vehicles. According to the ISO7637-2 and ISO 16750-2 ESD protection standards, ESD protection devices with operating voltages typically above 32 V are required to safeguard sensitive signal lines in 24 V board nets. Nexperia has designed devices in its new portfolio to have a maximum reverse standoff voltage of 36 V and to provide up to 22 kV of ESD protection. This performance is combined with a low clamping voltage of VCL= 48 V at IPP = 1 A to provide intra-vehicle networks with excellent system-level robustness. Nexperia also offers an ESD protection device for 48 V networks which meets the additional requirements defined in ISO 21780.
Nexperia’s ESD protection devices maximize design flexibility
For maximum flexibility, all devices (12/24/48 V) in this portfolio are available in SOT23 and SOT323 packages with three different capacitance classes of 4 pF, 6 pF and 10 pF, which help ensure smooth communication between interfaces without impacting signal integrity. This combination maximizes flexibility in PCB design and offers design engineers several performance options. Specifications for Nexperia’s full portfolio of automotive ESD protection devices are shown in Table 1 below.
