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Archive of posts published in the category: Designing
May
9

Designing a Wide Voltage Range Automotive Circuit Protector

Ignition cranking during startup and load dumps during shutdown are common sources of voltage transients on an automotive supply line. These undervoltage (UV) and overvoltage (OV) transients can have significant magnitudes and will damage circuits that are not designed to operate during these extremes.

Specialized UV and OV protection devices have been developed to disconnect sensitive electronics from power supply transients. For example, protection devices can monitor the input supply using a window comparator and then validate that it is within range. Similarly, the supply voltage can be monitored by a resistive divider network connected to the UV and OV monitor pins. The window comparator output can then drive the gates of two N-channel MOSFETs that make or break the connection between the supply and the load. The window comparator can be designed with hysteresis on its monitor pins to improve noise immunity. Hysteresis can prevent false MOSFET on/off switching due to ripple or other high frequency oscillations on the supply line. For example, 25 mV of hysteresis is equivalent to 5% of the monitor pin thresholds and is common for UV and OV protection devices.

For their own protection or to reduce ignition loading, some automotive accessory circuits must be disconnected from the supply line during startup or shutdown. Due to the large transients involved, these circuits may require more hysteresis than the protection device can provide by itself. For such applications, the increased hysteresis requirement can be satisfied by matching the protection device with a supply monitor that has adjustable hysteresis. This article walks through how to design a wide voltage range automotive circuit protector.

Figure 1. Power path control with wide voltage monitor hysteresis

Automotive UV/OV and Overcurrent Monitor with Circuit Protection

The architecture shown in Figure 1 protects electronics that are sensitive to undervoltage, overvoltage, and overcurrent transients present on an automotive supply. Figure 1 is an example of a wide voltage range automotive circuit protector. In this circuit, an LTC4368  from Analog Devices serves as the specialized UV and OV protection device and is responsible for connecting the load to the supply. The role of the window comparator is managed by an LTC2966.

The LTC2966 monitors reverse voltage, undervoltage, and overvoltage conditions. Monitoring thresholds and hysteresis levels are configured by the resistor networks on the INH and INL pins and the voltages on the RS1 and RS2 pins. OUTA is the UV window comparator output and OUTB is the OV window comparator output. The polarity of these outputs can be selected to be inverting or noninverting with respect to the inputs via the PSA and PSB pins. In Figure 1, they are configured to be noninverting. The OUTA and OUTB outputs from the LTC2966 are pulled up to the REF pin of the LTC2966 and are fed directly to the UV and OV pins of the LTC4368.

The LTC4368 provides reverse current and overcurrent protection. The size of the current sense resistor, R11, determines the reverse current and overcurrent levels. The LTC4368 decides if the

Apr
30

Designing Dual 48-V/12-V Battery Automotive Systems

The future of 48V/12V battery systems in automobiles now lurks just around the corner. Most major automobile manufacturers across the globe have been working on proving out their systems for the past few years, and it’s evident that their implementation will be relatively near term. This is a necessary and crucial step in the long and arduous journey to the fully autonomous passenger vehicle, which doesn’t require a human at the controls and has true autonomous driving.

Nevertheless, this doesn’t mean the 12-V battery is going away—there are far too many legacy systems in the installed vehicle base for this to occur. What it does mean is that autonomous cars will have both a 12-V battery and a 48-V battery (Fig. 1).

1. Next-generation cars will be powered by a 12-V and a 48-V battery.1. Next-generation cars will be powered by a 12-V and a 48-V battery.

A vehicle’s internal systems will either run off the 48-V lithium-ion (Li-ion) battery or the 12-V sealed lead-acid (SLA) battery—but not both. In addition to having two separate charging circuits for these individual batteries due to their respective chemistries, there must also be a mechanism that enables charge to move between them without causing any damage to the batteries or any system within the vehicle. An added benefit is that having two batteries also allows for redundancy should one of them fail during operation.

While this certainly complicates the design of the various electrical subsystems within the vehicles, there are some advantages to be gained. According to some auto manufacturers, a 48-V-based electric system results in a 10% to 15% gain in fuel economy for internal combustion engine vehicles, thereby reducing CO2 emissions.

Moreover, future vehicles that use a dual 48V/12V system will enable engineers to integrate electrical booster technology that operates independently of the engine load, thereby improving acceleration performance. Such compressors are already in the advanced stages of development and will be placed between the induction system and the intercooler, using the 48V rail to spin-up the turbos.

Globally, fuel-economy regulations have been tightening, while autonomous-driving capability with connectivity continues to proliferate in new automobiles. Accordingly, the 12-V automobile electric system has reached its usable power limit. As if these changes aren’t already enough, there has been a significant increase in automotive electronic systems. These changes, coupled with related demands for power, have created a new spectrum of engineering opportunities. Clearly, the 12V lead-acid battery automotive system with its 3kW power limit must be supplemented.

Furthermore, new automobile standards impact how these systems need to work. A newly proposed automotive standard, known as LV 148, combines a secondary 48-V bus with the existing automotive 12-V system. The 48-V rail includes an integrated starter generator (ISG) or belt start generator, a 48-V Li-Ion battery, and a bidirectional dc-dc converter, which can deliver tens of kilowatts of available energy from the 48- and 12V batteries. This technology