Electrical loads for vehicles continue to escalate. At the same time, the overall package size available for the electrical generator continues to shrink. Consequently there is a need for a higher power density system and method of generating on-board electricity.
The increase in the demand for current to be produced by an alternator leads to a need for corresponding adaptation of the system for current regulation, and particularly of a system whereby the three-phase alternating current produced by an alternator is rectified, into a direct current, which can be stored in a battery of a vehicle or be used directly by the electrical circuit of the vehicle which is supplied with a direct current (DC) voltage.
Rectification of the three-phase alternating current is generally obtained by means of a rectifier bridge having six power diodes. Three of these diodes are the positive diodes, and are connected between the phase terminals of the stator windings of the alternator and the positive terminal B+ of the alternator which is connected to the battery and the electrical circuit of the vehicle. Three further diodes, namely the negative diodes, are connected between electrical ground or earth of the vehicle and the aforementioned phase terminals of the stator windings.
The diodes constitute the rectifier bridge and are subjected to high current. Hence, it is necessary to cool them in the most effective way possible. To this end, it is known to arrange the diodes on metal plates, which are arranged on the outside of the alternator and which constitute a dissipator for the heat produced by the diodes. The diodes are grouped on two carrier plates, one of which is reserved for the positive diodes, and the other for the negative diodes.
The rectifier diodes are connected to respective carrier plates, and these carrier plates are used as heat sinks for these diodes as well. The rectifier diodes are inserted by pressure in receiving bore holes of the carrier plate or heat sink, or are soldered to the carrier plate using appropriate solder alloys. The end wires connected to the rectifier diodes enable the rectifier diodes to be connected to external sources.
However, under certain particularly severe operating conditions, it has become apparent that this cooling of the diodes, and in general terms the cooling of the whole of the current regulating means, can be insufficient to ensure long-term reliability of the alternator.
The heat sinks are typically constructed in the shape of a circle or crescent and are fastened in the same plane to the alternating current generator.
It is important that the bridge rectifiers must not only be able to withstand normal battery charging current, but must also be able to supply high electrical load currents demanded by the vehicle and do so at high, elevated ambient temperatures. These demanding situations may occur, for example, when the vehicle is stuck in stop and go traffic on a hot night. Under such conditions the electrical loads are high due to headlights, air-conditioning, engine cooling fan, brake lights and various other miscellaneous electrical loads. The underhood temperatures are also high due to the outside ambient air temperature and the stop and go driving schedule. Bridge rectifiers, as discussed, are limited in their ability to reliably function under such high current and high temperature conditions. In order for bridge rectifiers to handle these types of excessive currents and heat, it becomes necessary to utilize a bridge rectifier which has higher current handling capability. Due to the space limitations of the alternating current generator, it then becomes very difficult to provide such a bridge rectifier from a feasibility standpoint as well as at an economical cost.
Increasing the current capacity and heat dissipating characteristics of the bridge rectifier has included mounting of semiconductor diode chips onto first and second metallic heat sinks which are electrically insulated from each other by a thin sheet of electrical insulating material. The diode chips are then covered by a protective insulating coating after connection to the respective heat sink. One of the metallic heat sinks includes a finned area which is subjected to cooling air when the bridge rectifier is mounted to the generator. The heat sink with the plurality of fins includes twelve air passages. This type of bridge rectifier is shown in U.S. Pat. No. 4,606,000 to Steele et al., incorporated herein by reference.
With time and vibration the electrical integrity of the electronic circuitry can fail due to fatigue from mechanical stress. This produces a loss of electrical output from the machine. Further, several parts are required to provide the output connection and therefore the piece or part cost and assembly cost is high.
There is a need to improve the performance characteristics of prior art bridge rectifiers. In addition, there is a need for a carrier plate which increases the dissipation of heat from the diode and more efficiently cool the diode by facilitating increased surface area of the heat sink available to a diode resulting in an increased rate of dissipation of heat from the diode through the carrier plate in which the diode is disposed. Furthermore, a more robust package assembly capable of withstanding vibration while providing an effective thermal dissipation that is also cost effective is accordingly desired.