Bridge rectifier for diode-rectified alternating current generator

Published: 23rd March 2011
Views: N/A

Bridge rectifier is used to rectify current output from alternative current sources, such as an alternating current generator. Bridge rectifiers for motor vehicle alternators are well known in the art and generally include two metal parts used as heat sinks that are electrically insulated from each other. As a result of the current which is transmitted therethrough, the bridge rectifier becomes heated due to the internal power loss on each individual diode. Thus, the bridge rectifier must be properly cooled in order to handle the maximum required current while still being tolerant to increased temperatures due to internal power losses.

Each of the metal parts or carrier plates include semiconductor diodes which are arranged to polarize the two metal parts into respective positive and negative direct voltage output terminals. The diodes are then connected to respective phase windings of an output winding of the alternating current generator.

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 typically 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.

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.

Various difficulties or problems have occurred using this standard diode rectifier. For example, since the diode rectifier is mounted to an alternating current generator which is used with a motor, there are space limitations within the motor, for example, which limit the size of the diode rectifier. One prior art solution to this problem is constructing or fabricating the carrier plates which are connected to the rectifier diodes into a shape which is more than a half circle approximating the circular shape of the alternating current generator. The carrier plates are constructed as a positive heat sink and a negative heat sink and the two heat sinks are arranged coaxially in separate planes spaced apart by an axial distance from one another. See, for example, U.S. Pat. No. 4,952,829 to Armbruster et al., incorporated herein by reference.

Another problem which has been experienced with diode rectifiers includes the need to carefully match the diode characteristics in order to avoid imbalance in the amount of current conducted by the individual diodes. If thermal imbalance is experienced, certain diodes will increase current flow which may result in thermal runaway. Thermal runaway involves a diode which is unable to regulate its current flow and temperature. In this situation, the diode conducts increased current and experiences increased temperature until the individual diode is no longer able to conduct such a high current or experience such a high temperature, and the diode becomes destroyed. Frequently, thermal runaway results in the destruction of an individual diode, and the destroyed diode becomes short circuited thereby rendering the entire bridge rectifier inoperative.

Another problem which has been encountered in bridge rectifiers is that the bridge rectifiers must not only be able to withstand normal battery charging current, but must also be able to supply current, perhaps as much as ten times the normal charging current. These increased current situations may occur, for example, when the motor vehicle is being started. Bridge rectifiers, as discussed, are typically unable to absorb or conduct these types of excess currents and are also unable to rapidly dissipate the resulting heat. Thus, the heat generated within the bridge rectifier may destroy the individual diodes. 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.

A further attempt at increasing the current capacity and heat dissipating characteristics of the bridge rectifier includes the 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.

FIGS. 1a-1b are illustrations of a similar bridge rectifier as depicted in Steele et al. In FIG. 1a, combined alternator cover and carrier plate 2 includes carrier plate or heat sink 4 connected to alternator cover 6 (only partially depicted to expose underlying plate 4). Carrier plate 4 includes receiving bore holes 8 which are formed for receiving the diodes. Carrier plate 4 includes alternator mounting holes 10 for mounting carrier plate 4 to the alternator cover 6 via standard connection means such as a bolt or screw connection. Alternator cover 6 includes three main alternator air passages which interact with the twelve air passages 14 in corner plate 4, thereby cooling radiating fins 13. As depicted in FIG. 1b (alternator cover 6 omitted for simplicity), carrier plate 4 is of a rectangular shape (in side view) having the air passages 14 completely disposed within carrier plate 4.

FIG. 2 is an illustration of the positioning of the bridge rectifier 1 within a standard alternating current generator generally designated with reference letter G. As depicted in FIG. 2, the completely assembled bridge rectifier 1 which includes carrier plate 4 and cover 1a is connected to alternator cover 6 via any standard connection means, such as screws 7. Reference numeral 3 denotes the bottom of carrier plate 4, while reference numeral 5 denotes the top of carrier plate 4. Bridge rectifier 1 is also connected to regulator 9. As mentioned previously, the standard bridge rectifier shown in Steele et al. and FIGS. 1a-1b are well known in the art and may also be purchased from Wetherill Associates, Inc. of Royersford, Pa. as part no. 31-113 including cover part no. 46-1858.

While there have been, as described above, several attempts to increase the current and heat capacity of the bridge rectifier, none of these prior attempts have been completely satisfactory. That is, none of these prior art attempts have increased the current and heat capacity of the bridge rectifier in an economical manner.

As a result of our dissatisfaction with existing bridge rectifiers, we have discovered a significant problem which is the basis of the poor performance characteristics of prior art bridge rectifiers. This problem resides in the poor performance characteristics of the carrier plate, and the resulting defects or failures which arise therefrom. In addition, we have also recognized that while the surface area of the carrier plate is restricted by the circular shape of the alternating current generator, the depth of the carrier plate is not. Also, we have discovered new configurations of the carrier plate which more evenly distribute the electric current and resulting heat and more efficiently cool the carrier plate by facilitating increased passage or flow of air through the carrier plate.

The electrical current through a diode creates heat. The heat affects negatively the current capability of a diode. Thus, temperature/current capability are in opposite relation. Over a given temperature a diode is destroyed. Therefore on an alternator which generates high current, it is most necessary to cool the diodes by heat sinks and improve the heat dissipation of the heat sinks by increasing the mass and flowing air over/through them.

The present invention satisfies the above features and benefits by providing an improved bridge rectifier for an alternating current generator including a first heat sink having a first set of diodes, and an insulating layer disposed on the first heat sink. In addition, the improved bridge rectifier includes a second heat sink disposed on the insulating layer. The second heat sink includes a base section including first and second areas, a second set of diodes, and diode receiving holes in the base section and receiving the second set of diodes therein. The second heat sink also includes a plateau section disposed on the first area of the base section, and air passages disposed in and extending through the base and plateau sections. The improved bridge rectifier further includes connectors, connecting the first heat sink and the second heat sink together with the insulating layer disposed therebetween, a cover connected to the base section and covering the second area of the base section, and a capacitor connected to the cover and to the second heat sink.

The second heat sink includes at least one of the following: air passages which include external air passage slots opening an external portion of the air passages in the plateau section for enhanced cooling of the second heat sink, or air passages which include internal air passage slots opening an internal portion of the air passages in the plateau section for enhanced cooling of the second heat sink, or air passages which are repositioned to maximize exposure of the air passages to receive air from the alternating current generator, or air passages which include bottom channels formed in the second heat sink for receiving air from the bottom of the second heat sink for enhanced cooling of the second heat sink, or diode receiving holes which are separated by a substantially same distance between each other for balancing thermal stress resulting in uniform heat conduction by the second heat sink, or a second area of the second heat sink includes ridges to increase a surface area of the second area for enhanced cooling of the second heat sink.

Report this article Ask About This Article

More to Explore