Overcoming load-dump tyranny
Power electronics in future automobiles will perform two functions: simple on/off switching (now done by relays and manual switches) and controlling loads with logic, inverters, and dc-dc converters. Together, the two classes of power electronics involved will not only accommodate higher voltages but also overcome basic defects in the conventional 12-V system, such as widely varying system voltage and destructively high voltage transients.
The voltage in a 12-V system actually ranges from about 9 V to 16 V, depending on the alternator output current, battery age and state of charge, and other factors. Loads are sized to function properly at the lowest system voltage; thus, when the voltage is higher, they draw more current than necessary. Load components therefore need to be rated for continuous operation at the highest current.
Then there is the notorious load-dump transient, a voltage spike that appears on the system when a fully loaded alternator suddenly loses its load--for example, when a charging battery is inadvertently disconnected. The voltage behind the alternator's armature reactance then suddenly shows up on the system, a 40-V, 100-ms transient if the alternator is protected by avalanche diodes, and 80 V or more if it is not. The switches and load components therefore have to be rated for temporary overvoltages at least four times the nominal system voltage.
This requirement is an expensive one, especially for semiconductor switches. A load dump may never occur during the life of a car, yet components have to be ready to handle it. The result is that load devices have to be grossly overrated both for continuous current and transient voltage.
In future cars, however, power electronic converters will pro vide an interface between the alternator and the distribution system, making it unnecessary to overrate components. The alternator in such a car can be allowed to generate an unregulated output that varies with engine speed; power-conditioning circuitry will take that output and turn it into a constant, transient-free system voltage for distribution. It will then be unnecessary to overrate either the load components or the semiconductor devices that control them.
Semiconductor manufacturers are doing their part in developing advanced power devices for cars. For example, Siemens AG, in Munich, now offers smart power switches expressly for automotive service. These MOSFET devices not only do high-side switching, disconnecting the load from the supply voltage bus instead of from ground, but they also shut down if their temperature rises excessively. They also can protect themselves against overcurrents, and act as resettable fuses with the aid of associated logic, either monolithically integrated with the MOSFET or packaged with it as a hybrid integrated circuit.
Until recently, a big impediment to acceptance of power MOSFETs in automobiles has been their high on-state drain-to-source resistance, known as Rds(on). Typically, Rds(on) has been 100 m(omega) for a device switching 10 A, giving a 1-V forward drop at a junction temperature of 125 šC and dissipating 10 W--not particularly efficient. Now, however, Siliconix Inc., Santa Clara, Calif., offers power MOSFETs, made by trench technology, that have an Rds(on) of only 16 m(omega) at a substantially higher junction temperature, 175 šC. Other manufacturers are beginning to offer similar devices.
Further, as noted, the cost of power electronic converters is rapidly approaching the low levels that make them practical in cars. From 50 cents per watt in 1990, the cost has dropped to 1520 cents per watt for converters with 1003000-W ratings. When costs fall to 510 cents per watt, wide penetration of the automotive market can be expected.