Electronic Design

Higher-Voltage Vehicles Generate New Demands For Power Management

Power semiconductors and smart-power ICs will play critical roles in the auto industry's transition to dual, 42-/14-V electrical systems.

During the twentieth century, the power required for vehicles grew continuously as car manufacturers added both standard and optional equipment to meet the increasing consumer demands. Because of this ongoing rise in power consumption, automakers and suppliers were forced to begin evaluating higher supply voltages and alternative power supply architectures. As a result of these efforts, members of industry proposed a new standard for automotive electrical systems, one based on a voltage, 42 V, which is three times higher than that used in today's vehicles.

The need for this higher voltage is being driven by the expectation that automotive electrical loads will continue to rise sharply in the future as additional electrical equipment is designed into the vehicle (see the table). These new electrical loads will push power requirements beyond the capabilities of existing 14-V electrical systems. In fact, the peak current of any one of the anticipated loads is sufficient to create a problem in an existing vehicular power supply. On the other hand, these current levels would be reduced by one third in a vehicle that's built with a 42-V system

Even with a transition to higher-voltage systems, the addition of two or more loads that demand high power simultaneously creates a problem that can only be solved by incurring considerably increased cost. As a result, future automotive electrical systems will demand1:

  • energy and power management strategies,
  • high sustained power, and
  • the ability to cope with power peaks while ensuring a stable supply.

Increasing the vehicle voltage will allow the addition of higher power loads without requiring more expensive and heavier wiring. But some loads, such as the existing vehicle lighting, would be better served by a voltage lower than today's 14 V. Nevertheless, the existing 14-V components, like power semiconductors and the hardware designed to make lower-voltage loads possible, can't be changed without a transition period.

To smooth the transition to 42 V, the industry is examining dual, 42-/14-V vehicle power supply systems. In one possible architecture, 42-V power from the main battery (36 V nominal) is stepped down to 14 V by a dc-dc converter. The dc-dc converter then stores its energy in a second battery (12 V nominal), which is then used to run the lower-power 14-V loads (Fig. 1). Both starting and charging are accomplished with 42 V, but all existing 14-V hardware and systems such as lighting could still operate from 14 V. The bidirectional dc-to-dc converter shown in the figure provides a redundant power source for improved fault tolerance in drive-by-wire systems including brake-, throttle-, steer- and accelerate-by-wire.

Restricting the transients on both the 42-V and the 14-V supply lines is essential for achieving lower cost for power electronics. That means staying below 60 V for the 42-V side and below 20 V for the 14-V side to minimize semiconductor costs. Vehicle manufacturers have some options when it comes to implementing dual-voltage systems. They may opt for the dual-battery architecture shown, or else adopt a single-battery solution.

The latter approach would involve a direct move to 42 V with distributed 14-V supplies used to power the smaller loads. In either case, a key consideration is limiting the voltage so that semiconductor manufacturers and electronics suppliers can provide similar hardware to a variety of end-customers.

Once the higher voltage is established, actuators and systems that consume high power can be designed to utilize the higher voltage. New system approaches, such as an integrated alternator, starter, and flywheel, exemplify how the design of products for future vehicles can be radically different from those based on existing methodologies.

A variety of semiconductors, including power FETs, smart-power ICs, CMOS microcontrollers, memory, and numerous discrete devices, are affected by a change in the vehicle's power supply system. Those devices that interface directly to the supply without overvoltage protection or a secondary circuit have the highest impact on cost. These include power switches, power rectifiers, and transitional voltage ICs. The proposed 42-V system voltage and overvoltage limits provide a narrower guard band of less than 1.5 times the nominal system voltage for semiconductor devices that are specified at the proposed overvoltage limits.

Transitional voltage ICs are the link between the 42-V system and the applications that will still require 14 V or less for many years to come. These power-management applications center around body electronics and will primarily include lighting. In today's vehicles, over 100 lamps need the 14-V system due to limitations in tungsten filament technology. Without a changeover to high-intensity discharge (HID) or neon lamps, transitional voltage semiconductors will be required to isolate lamps from the 42-V supply.

The dual-battery architecture has a large, bidirectional dc-dc converter that controls and charges both voltage buses and batteries (Fig. 1, again). This architecture will become costly and impractical as additional high-power loads are implemented. At this time it's believed that a decentralized architecture will provide the most benefits to a 42-V system.

In one such decentralized architecture, a 42-V integrated starter alternator (ISA) supplies 42 V directly to 42-V loads and indirectly to distributed 14-V and lower-voltage loads (Fig. 2). Many medium-power step-down converters, which are from several watts to a few hundred watts, will be utilized to supply power to 14-V loads located throughout the vehicle. These devices can be placed either remotely, at either the load or the control electronics, or in a central module that provides power conversion for several loads.

For example, ON Semiconductor's CS51022 pulse-width-modulation controller may be combined with the MTP16N25E power FET and other discrete semiconductors to create a medium-power point-of-use dc-dc converter for applications of up to 100 W. This circuit is ideally suited for powering high-power 14-V loads in a 42-V system. Lower-power loads—those less than approximately 20 W—can be powered by integrated supplies. This architecture has the additional benefit of providing a tightly controlled 14-V bias, which will extend the lamps' lifetime.

For 14-V automotive loads that extend up to a few hundred watts, it's possible to employ a dc-dc converter reference design based on the CS51022 (Fig. 3). This circuit could be applied in a distributed system, eliminating the "single point of failure" associated with centralized power systems.

The heart of the design uses a CS51022 enhanced current-mode PWM controller. It includes many features like externally programmable undervoltage lockout, programmable soft start, frequency operation at up to 1 MHz (although this design uses 150 kHz), overvoltage protection, and 1-A sink/source of gate drive. Among its other features are low start-up current, current slope compensation, a 5-V reference, and an externally programmable dual-threshold overcurrent protection. Under extreme overcurrent conditions, that last feature can reinitiate the soft-start sequence. The controller additionally features a sleep mode in which the device's current consumption is less than 100 µA.

The system topology is set up as a forward converter. In this topology, power is delivered to the output while the primary switch (Q2) is conducting. When Q2 is off, the stored energy in L2 delivers power to the load. The transformer windings must have zero volt-seconds (on average). Capacitor C18 (820 pF) provides the path to reset the transformer core (Fig. 4).

This dc-dc converter can supply up to 84 W to the load, making it suitable for many automotive applications including lamp drivers. The CS51022A has a voltage of up to 20 V and a temperature range that's between −40° and 85°C. The shunt regulator formed by the zener diode D2 and bipolar transistor Q1 enhances the system voltage capability for this 42-V circuit.

During normal operation this shunt regulator is turned off (back biased) by tying VOUT to the output of the shunt regulator. A future version of this controller will integrate many of the external components in this design, including the shunt regulator. Plus, the IC will be able to withstand a 60-V input voltage, so it won't require the external shunt.

The next generation of vehicles stands to benefit from power management implemented with power semiconductors and smart-power ICs provided that the system voltage is restricted to permit lower minimum breakdown voltages. These advantages are even more pronounced in a dual-voltage architecture. Cost reductions for existing products can be maintained and unnecessary cost increases for new products with higher-voltage requirements can be avoided by minimizing the system voltage transients. By working closely with automobile manufacturers and automotive electronic suppliers, semiconductor manufacturers will be able to provide power-management products that meet the 42-V specifications for the next generation of vehicles.

Reference:

  1. John M. Miller, Paul R. Nicastri, Shahram Zarei, "Automotive Power: Future System Architectures Face Formidable Hurdles," www.pcim.com/miller1/index.html.
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