Electronic half bridge module

A metal oxide semiconductor (MOSFET) half bridge module for use in a 42 volt internal combustion engine starter/alternator circuit. The module is a compact, high power handling device which has an extremely low inductance. This low inductance module supports the current and current slew rates necessary to properly operate in an internal combustion engine starter/alternator circuit across a wide temperature range. The module has a thermally conductive base. A plurality of lower circuit boards are adjacently positioned within the base along the same plane. At least one semiconductor device has a first surface, the first surface of the semiconductor device is mounted to one of the plurality of circuit boards. A common terminal has planar portion which is coupled to each of the plurality of lower circuit boards. An upper circuit board is in electrical contact with the plurality of lower circuit boards and includes a connector providing external access for drive signals for the semiconductor device.

BACKGROUND OF THE INVENTION
 The present invention relates to an electronic bridge module, specifically
 to a high power metal oxide semiconductor half bridge module for an
 internal combustion engine starter/alternator circuit in which the module
 has a very low inductance.
 Starting an internal combustion engine and rectifying the voltage generated
 by an alternator once the engine is running places high demands on the
 electronic circuitry required to support these functions. A typical
 starter/alternator requires minimum voltage drop at its input and an
 immediate torque current to mechanically "crank" the engine. In order to
 minimize voltage drop and supply immediate current, the start/alternator
 circuit path must have a low resistance and an extremely fast current slew
 rate (di/dt). The circuit must perform these functions under extreme
 temperature conditions such as might be found in a typical automotive
 environment. For example, the circuit must be capable of supplying a 650
 amp current at -25.degree. C. in a 42 volt system such that the current
 slew rate is faster than 1500 amps per microsecond.
 An essential component of the starter/alternator circuit is a half bridge
 rectifier. The half bridge rectifier must be capable of supporting the
 characteristics described above. A typical half bridge rectifier is
 produced as a module employing metal oxide field effect transistor
 (MOSFET) devices. In order to achieve the desired performance
 characteristics and avoid device overvoltage and avalanche conditions, the
 low voltage silicon used in the MOSFET devices must provide a resistance
 path of less than 2 milliohms at 25.degree. C., and a maximum module
 inductance of 12 nH. Of course, the module must be of a size small enough
 to be mounted at a spot convenient to its placement within the
 starter/alternator circuit, typically as part of a bus bar assembly
 located in a motor vehicle's engine compartment.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a high power half
 bridge module for a starter/alternator circuit in which the module is
 small, has a very low inductance, and can support the power requirements
 needed to start an internal combustion engine in a wide temperature range.
 The present invention provides a rectifying module in which there is a
 thermally conductive base. A plurality of lower circuit boards are
 adjacently positioned within the thermally conductive base along a same
 plane. At least one semiconductor device has a first surface, the first
 surface of the semiconductor device is mounted to one of the plurality of
 lower circuit boards. A common terminal has a planar portion in which the
 planar portion is coupled to each of the plurality of lower circuit
 boards. An upper circuit board is in electrical contact with the plurality
 of lower circuit boards in which the upper circuit board includes a
 connector providing external access for drive signals for the
 semiconductor device.
 Another aspect of the module of present invention further includes at least
 one contact pin. The contact pin is used to provide the electrical contact
 between the upper circuit board and one of the plurality of lower circuit
 boards.
 As another aspect of the module of present invention, the contact pin
 includes a curved portion to form a resilient contact.
 As still another aspect, the module of present invention further comprises
 two voltage terminals in which each of the two voltage terminals are
 coupled to a different one of the plurality of the lower circuit boards.
 Another aspect of the module of present invention provides that the two
 voltage terminals extend from the plurality of lower circuit boards at
 differing distances.
 As still yet another aspect of the module of present invention, the at
 least one semiconductor device further includes a second surface located
 opposite the first surface in which the second surface is coupled to one
 of the plurality of lower circuit boards by at least one wire bond.
 As another aspect of the module of present invention, the at least one wire
 bond is "S"-shaped such that an attachment point on the one lower circuit
 board is angularly displaced from an attachment point on the second
 surface.
 Another aspect of the present invention provides a module in which the at
 least one semiconductor device is a MOSFET device and the attachment point
 of the second surface is a source region.
 As another aspect of the present invention, the module forms a half bridge
 rectifier.
 As a further aspect of the present invention, the module is implemented in
 a 42 volt internal combustion engine starter/alternator circuit.
 The present invention provides, as still another aspect, that the total
 package inductance of the module is less than approximately 8.4
 nanoHenries.
 As a further aspect of the module of the present invention, one of the
 voltage terminals is a negative terminal and the module further comprises
 a resistor and capacitor connected in a parallel arrangement between the
 negative terminal and the common terminal.
 Another aspect of the invention provides that, the total thickness of the
 module is less than approximately 0.75 inches.
 The present invention also provides a half bridge rectifying module for use
 in an internal combustion engine starting/alternator circuit, in which the
 module has a thermally conductive base. A plurality of lower circuit
 boards are adjacently positioned within the thermally conductive base
 along a same plane. At least one MOSFET semiconductor device has a first
 surface, the first surface of the MOSFET semiconductor device is mounted
 to one of the plurality of lower circuit boards. A common terminal has a
 planar portion coupled to each of the plurality of lower circuit boards.
 An upper circuit board is in electrical contact with the plurality of
 lower circuit boards and includes a connector providing external access
 for drive signals for the MOSFET semiconductor device.
 As still another aspect, the present invention provides a rectifying module
 in which there is a thermally conductive base. A lower circuit board is
 positioned within the thermally conductive base. At least one MOSFET
 semiconductor device has a first surface, which is a drain surface,
 mounted to the lower circuit board, and a second surface opposite the
 first surface which is coupled to the lower circuit board by at least one
 "S"-shaped wire bond, in which the attachment point on the lower circuit
 board is angularly displaced from an attachment point on the second
 surface. A common terminal has a planar portion, the planar portion is
 coupled to the lower circuit board. A plurality of voltage terminals are
 coupled to the lower circuit board and extend at different distances
 therefrom. An upper circuit board is in electrical contact with the lower
 circuit board, the upper circuit board includes a connector providing
 external access for drive signals for the MOSFET semiconductor device.
 Other features and advantages of the present invention will become apparent
 from the following description of the invention which refers to the
 accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION
 Initially, it is noted that like reference numerals on the different
 figures refer to like elements. As shown in FIG. 1., an example of a low
 inductance half bridge module 2 in accordance with the present invention
 is assembled with a base 4, two MOSFET circuit boards 6 and 8, an upper
 printed circuit board (shown as reference 42 on FIG. 4) and a cover (shown
 as reference 48 on FIG. 5). MOSFET circuit boards 6 and 8 are positioned
 within the base 4 such that they lie adjacent to one another and are
 arranged in a longitudinally planar relationship to one another. A
 negative voltage terminal 10 is mounted to MOSFET circuit board 6. Mounted
 to MOSFET circuit board 8 is a positive voltage terminal 12. A common
 terminal 14 is mounted such that it is in electrical contact with both
 circuit boards. A number of MOSFET devices 16 are mounted to each of
 MOSFET circuit boards 6 and 8. The base and cover may be fabricated from
 any strong, thermally conductive material such as AlSiC. MOSFET circuit
 boards 6 and 8 are each patterned to define a source bus 18, a drain bus
 20, gate connection pads 22 and source sense pads 24. In addition, MOSFET
 circuit board 8 is patterned to support a thermistor (not shown) mounted
 to the thermistor contact pads 26 for temperature sensing.
 The MOSFET circuit boards 6 are arranged such that the source contact bus
 18 of MOSFET circuit board 6 is in electrical contact with the negative
 terminal 10. The drain contact bus 20 of that same circuit board is in
 electrical contact with the common terminal 14. Similarly, the drain
 contact bus 20 of MOSFET circuit board 8 is in electrical contact with the
 positive terminal 12, and the source contact bus 18 of that same circuit
 board is in electrical contact with the common terminal 14. The negative
 terminal 10 and the positive terminal 12 can be laminated with an
 insulating material in the non-electrical contact regions such that they
 can be located close together within the module 2. Close placement of the
 positive and negative terminals saves space and has been advantageously
 found to provide a lower total induction for the module.
 MOSFET circuit board 8 of the present invention is shown in FIG. 2. MOSFET
 circuit board 8 and MOSFET circuit board 6 are identically patterned
 except for the addition of thermistor contact pads 26 on MOSFET circuit
 board 8. As such, the description of MOSFET circuit board 8 should be
 though of as also describing MOSFET circuit board 6. Of course, MOSFET
 circuit board 6 and MOSFET circuit board 8 can instead be manufactured as
 a single circuit board.
 Each MOSFET device 16 is mounted to the MOSFET circuit board 8 such that
 the drain contact (not shown) is in electrical contact with drain contact
 bus 20. The source contact 28 of each MOSFET device 16 is connected to the
 source contact bus 18 by a plurality of wire bonds 30. Each wire bond 30
 is shaped such that its contact point on the source contact bus 18 is not
 directly opposite its contact point on the MOSFET device 16, but rather is
 slightly offset, i.e., it is angularly displaced, thereby causing the wire
 bond to take the form of an "S" shape. Although a straight wire bond can
 be used, offset contact mounting provides a lower induction than does the
 straight wire bond. For example, testing has unexpectedly shown that the
 "S" shape results in a 1 nanoHenry reduction in total module induction.
 The gate contact 32 of each MOSFET device 16 is attached to a respective
 gate connection pad 22 via a wire bond 34, and the source contact 28 is
 additionally connected to a respective source sense pad 24 via a wire bond
 36. This allows for a convenient way to establish the gate to source
 voltage necessary to switch the MOSFET device 16.
 FIG. 3 shows a perspective view of the common terminal 14 of the present
 invention. The common terminal 14 is preferably arranged to have a shorter
 shape planar portion 38 positioned under an upper portion 40. The shorter
 shape planar portion 38 provides physical and electrical contact with
 circuit boards 6 and 8. The shorter shape portion 38 of the terminal 14
 combined with the contact region being positioned under the upper portion
 40 has been found to provide a lower induction than taller terminals and
 terminals in which the base is positioned outside the upper portion, i.e.,
 in an "S" shape. Also, the dual heights, i.e., lengths of the negative
 terminal 10 and the positive terminal 12 allow for a fully laminated bus
 in the case where the module 2 is as part of a bus bar assembly. This is
 accomplished such that, when assembled, the negative terminal 10 and the
 positive terminal 12 extend from their respective MOSFET circuit board 6
 and 8 at differing lengths.
 FIG. 4 shows a side view of the uncovered module. In particular, upper
 circuit board 42 is shown electrically connected to MOSFET modules 6 and 8
 by contact pins 44. Contact pins 44 are used to provide an electrical path
 from gate connection pads 22 and source sense pads 24 to the upper circuit
 board 42. Although shown as curved so as to provide resilient contact
 pressure, the contact pins 44 may also be straight. An external connector
 46 is mounted to the upper circuit board to provide an external point of
 connection for the MOSFET gates and source senses.
 FIG. 5 shows a view of the module 2 assembly. Cover 48 is arranged such
 that it mates with base 4 and provides external access to the negative
 terminal 10, the positive terminal 12, the common terminal 14 and the
 external connector 46.
 Module inductance is minimized when the path from the common terminal 14 to
 the negative and positive terminals is kept as short as possible. As shown
 in FIGS. 1 and 2, path length in the inventive device is minimized by
 using 2 pairs of 3 MOSFET devices 18 while still providing the current
 carrying capabilities necessary to start an internal combustion engine.
 Module inductance is further minimized in the inventive device by placing
 the gate connection pads 22 and source sense pads 24 as close together as
 possible and by keeping the length of the wire bonds as short as possible.
 The module 2 is assembled as follows. First, the MOSFET devices 16 are
 attached to the MOSFET circuit boards 6 and 8. Additional devices (not
 shown), such as the thermistor, are also attached to the MOSFET circuit
 board a this time. Next, the wire bonds 30, 34 and 36 are made from the
 MOSFET devices 16 to their respective contact points on the MOSFET circuit
 boards 6 and 8. The assembled MOSFET circuit boards 6 and 8 are tested to
 ensure proper operation and are then affixed to the base 2. Terminals 10,
 12 and 14 are attached along with upper circuit board 42 (via contact pins
 44) to the assembled MOSFET circuit boards 6 and 8. The cover 48 is
 attached to the base, and the module is then potted.
 As shown in FIG. 6, the above described embodiment of the invention has
 been tested as providing a maximum total package inductance of
 approximately 8.4 nanoHenries. The module is shown in FIG. 6 as capable of
 supporting a current slew rate in excess of 3700 amps per microsecond.
 This low inductance is needed to reduce overvoltage shorts in the device
 during periods of hard high frequency switching. In addition, the
 inventive module has been tested as providing a power delivery of 17
 kilowatts, and is capable of supporting 400 amps at 36 volts (i.e., a
 transient voltage drop of less than 10 volts) in starting mode, and 400
 amps at 52 volts in alternator mode. As such, the module of the present
 invention is suitable for use in a 42 volt environment, particularly in a
 42 volt internal combustion engine starter/alternator circuit.
 The inventive half bridge module 2 provides a compact (the total height,
 i.e., thickness, of the described embodiment of the module is less than
 0.75 inches), high power device which has an extremely low inductance.
 This low inductance allows the module 2 to support the current and current
 slew rates necessary to properly operate in an internal combustion engine
 starter/alternator circuit. Transient voltage drop in the module during
 engine starting is kept to a minimum.
 Although the present invention has been described in relation to particular
 embodiments thereof, many other variations and modifications and other
 uses will become apparent to those skilled in the art.