INVERTER BUS WITH INTEGRATED BLEED RESISTOR

An electric drive system includes a power electronics module with a prefabricated DC bus bar assembly. The prefabricated DC bus bar assembly includes a molded bleed resistor which serves two different functions. First, it dissipates energy from a capacitor when the battery is disconnected. Secondly, it physically supports the positive bus bar and the negative bar with respect to one another.

TECHNICAL FIELD

The present disclosure relates to inverters for electrified vehicles. More particularly, it relates to an inverter having a bleed resistor.

BACKGROUND

Electrified vehicles utilize batteries which provide the electrical energy in a Direct Current (DC) form. The powertrains utilize motors to convert electrical energy into mechanical torque to propel the vehicle. Several types of motors, such as permanent magnet synchronous motors, require that the electrical energy be in Alternating Current (AC) form. Inverters are used to convert the DC electrical power from the battery to AC electrical power at the frequency and phase that is required for the motor. A contactor switch may be provided between the battery and the inverter to de-energize the inverter when the vehicle is not in use, for the protection of service personnel among others. However, the inverter may include a capacitor which also stores electrical energy and is not disabled by the contactor switch. Bleed resistors may be used to gradually dissipate the energy from the capacitor.

SUMMARY

An electric drive system includes two rigid bus bars, a power electronics module, and a molded bleed resistor. The power electronics module is configured to transform direct current power carried by the bus bars into alternating current power. The power electronics module includes a capacitor connected between the two bus bars. The molded bleed resistor is bonded directly to the bus bars such that the molded bleed resistor rigidly fastens the two bus bars to one another. The bleed resistor is configured to dissipate energy from the capacitor. The electric drive system may include a variable voltage converter and an inverter. If present, the variable voltage converter is configured to deliver the direct current power at a second voltage different than a battery voltage. The inverter is then configured to convert the power at the second voltage into alternating current power. The bus bars may connect the variable voltage convert to the inverter. The bleed resistor may encircles both of the two bus bars. The bleed resistor may be formed from a polymer or a ceramic and fine carbon particles. The electric drive system may also include a contactor switch electrically connected to one of the two bus bars and a battery electrically connected to the contactor switch and to the other bus bar.

A method of fabricating a bus bar assembly includes holding two rigid conductive bus bars in a predetermined position relative to one another and injecting substance between the bus bars. For example, the bus bars may be held in the predetermined position by placing them into a mold. The bus bars are not in contact with each other in the predetermined position. The substance bonds to the bus bars and hardens, thereby rigidly attaching the two bus bars together in the predetermined relative position. The substance contains fine carbon particles such that it forms a bleed resistor. The substance may also contain a polymer or a ceramic. The mold may be shaped such that the bleed resistor encircles both of the two bus bars.

A method of assembling an electric drive system includes connecting a DC bus bar assembly to a power electronics module. The power electronics module includes a capacitor and an inverter. The DC bus bar assembly includes a positive bus bar and a negative bus bar rigidly fastened to one another by a molded bleed resistor. The molded bleed resistor is bonded to the positive bus bar and the negative bus bar prior to connection to the power electronics module. The bleed resistor may encircle both the positive bus bar and the negative bus bar. The bleed resistor may contain a polymer or a ceramic and fine carbon particles. The method may also include electrically connecting a contactor switch to one of the bus bars and electrically connecting a battery to the contactor switch and to the other bus bar. Alternatively, the method may include electrically connecting the positive bus bar to an output of a variable voltage converter.

DETAILED DESCRIPTION

Referring now toFIG.1, a block diagram of an exemplary electric vehicle (“EV”)12is shown. In this example, EV12is a plug-in hybrid electric vehicle (PHEV). EV12includes one or more electric machines14(“e-machines”) mechanically connected to a transmission16. Electric machine14is capable of operating as a motor and as a generator. Transmission16is mechanically connected to an engine18and to a drive shaft20mechanically connected to wheels22. Electric machine14can provide propulsion and slowing capability while engine18is turned on or off. Electric machine14acting as a generator can recover energy that may normally be lost as heat in a friction braking system. Electric machine14may reduce vehicle emissions by allowing engine18to operate at more efficient speeds and allowing EV12to be operated in electric mode with engine18off under certain conditions.

A traction battery24(“battery) stores energy that can be used by electric machine14for propelling EV12. Battery24typically provides a high-voltage (HV) direct current (DC) output. Battery24is electrically connected to a power electronics module26. Power electronics module26is electrically connected to electric machine14and provides the ability to bi-directionally transfer energy between battery24and the electric machine14. For example, battery24may provide a DC voltage while electric machine14may require a three-phase alternating current (AC) voltage to function. Power electronics module26may convert the DC voltage to a three-phase AC voltage to operate electric machine14. In a regenerative mode, power electronics module26may convert three-phase AC voltage from electric machine14acting as a generator to DC voltage compatible with battery24.

Battery24is rechargeable by an external power source36(e.g., the grid). Electric vehicle supply equipment (EVSE)38is connected to external power source36. EVSE38provides circuitry and controls to control and manage the transfer of energy between external power source36and EV12. External power source36may provide DC or AC electric power to EVSE38. EVSE38may have a charge connector40for plugging into a charge port34of EV12. Charge port34may be any type of port configured to transfer power from EVSE38to EV12. A power conversion module32of EV12may condition power supplied from EVSE38to provide the proper voltage and current levels to battery24. Power conversion module32may interface with EVSE38to coordinate the delivery of power to battery24. Alternatively, various components described as being electrically connected may transfer power using a wireless inductive coupling.

Wheel brakes44are provided for slowing and preventing motion of EV12. Wheel brakes44are part of a brake system50. Brake system50may include a controller to monitor and control wheel brakes44to achieve desired operation.

The various components discussed may have one or more associated controllers to control and monitor the operation of the components. The controllers can be microprocessor-based devices. The controllers may communicate via a serial bus (e.g., Controller Area Network (CAN)) or via discrete conductors. For example, a system controller48(i.e., a vehicle controller) is present to coordinate the operation of the various components.

As described, EV12is in this example is a PHEV having engine18and battery24. In other embodiments, EV12is a battery electric vehicle (BEV). In a BEV configuration, EV12does not include an engine.

Referring now toFIGS.2and3, schematic diagrams of components of power electronics module26in an electric drive system of EV12is shown. As described above, power electronics module26is coupled between battery24and motor14. Power electronics module26converts DC electrical power provided from battery24into AC electrical power for providing to motor14. In this way, power electronics module26drives motor14with power from battery24for the motor to propel EV12. During regenerative braking, power electronics module26converts the AC power induced within motor14to DC power to charge battery24.

Power electronics module26includes a DC-DC converter51and an inverter52. As known to those of ordinary skill, inverters convert DC power to multi-phase AC power (three-phase being most common). DC-DC converters can boost (increase) or buck (decrease) the DC voltage available to the inverter from what is available from the battery. DC power from the battery is delivered on a negative bus bar54and a positive bus bar56. Bus bars54and56form a first DC bus. DC power from the converter51is delivered to inverter52by negative bus bar54and positive bus bar58. Bus bars54and58form a second DC bus. Inverter52delivers AC power to the motor via AC terminals60. Some embodiments may omit the DC-DC converter51.

Converter51and inverter52include a plurality of power switch units. Each power switch unit includes a power switch62arranged anti-parallel with a diode64. Converter51also includes an inductor66and a capacitor67. Inverter52also includes a capacitor68. The switches62may be, for example, Silicon Carbide (SiC) or Insulated Gate Bipolar Transistors (IGBTs).

FIG.4schematically illustrates additional components of an electric drive system. The negative bus bar54is electrically connected to a negative terminal of battery24. A contactor switch70is electrically connected between the positive battery terminal and the positive bus bar56. Opening the contactor switch70disconnects DC power stored in the battery from the power electronics module26. In alternative embodiments, the contactor switch may be connected between the battery and the negative bus bar.

Although opening the contactor switch disconnects the battery, a substantial amount of electrical power may be retained in the capacitors67and68. To dissipate this energy, bleed resistors72and74are installed between the negative bus bar54and the positive bus bar of the respective first and second DC busses. The resistance of the bleed resistors is selected to be high enough that it doesn't dissipate an excessive amount of energy during normal operation but low enough that the energy stored in the capacitors67and68is dissipated at a sufficient rate when contactor switch70is open.

FIG.5is a pictorial view indicating the physical arrangement of a DC bus with a bleed resistor and a capacitor. The first DC bus includes negative bus bar54, positive bus bar56, and bleed resistor72and is connected to capacitor67. The second DC bus includes negative bus bar54, positive bus bar58, and bleed resistor74and is connected to capacitor68. Terminals76are provided to connect to other components of the respective converter or inverter. The bus bars are rigid metal components. The bleed resistor is bonded to opposing surfaces of the bus bars. The bleed resistor may be made, for example, from either a polymer or a ceramic infused with fine carbon particles to provide the desired level of electrical resistance. In addition to providing the desired resistance, the bleed resistor fastens the bus bars to one another and rigidly positions them with respect to one another. This reduces the chance of one of both bus bars being accidentally bent and coming into contact with the other one. As discussed below, it also facilitates assembly. The bleed resistor is separated from the capacitor and other power electronics components such that heat generated at the bleed resistor will not heat the other components.

FIG.6is a cross-section through the bleed resistor in one possible embodiment. In this embodiment, the bleed resistor72or74encircles both the negative bus bar54and the positive bus bar56or58. This embodiment provides additional bonding area to increase the fastening capability of the bleed resistor. The bleed resistor may be formed from a polymer or ceramic base material infused with fine carbon particles. The quantity of fine carbon particles is set to establish the desired resistance value.

FIG.6is a flowchart for a manufacturing process to fabricate and install a power electronics module having a molded bleed resistor. At80, the positive bus bar56or58and the negative bus bar54are fabricated. The bus bars are each fabricated from electrically conductive materials. The bus bars may be fabricated, for example, using stamping processes. At82, the bleed resistor72or74is molded with the bus bars to create the bus bar assembly. An over-molding process may be utilized wherein the positive and negative bus bars are placed in a mold before the fine carbon particle infused plastic or ceramic material is injected into the mold. After molding, the bleed resistor rigidly supports the positive bus bar and the negative bus bar in position with respect to one another. As such, the bus bar assembly can be easily handled or shipped between assembly locations. At84, the DC bus bar assembly is attached mechanically and electrically to the capacitor. The bus bars are also attached to other components depending on whether this is the first DC bus or the second DC bus. At86, the negative bus bar is connected to the negative terminal of the battery. At88, the contactor is electrically connected in series between the positive terminal of the battery and the positive bus bar. In alternative embodiments, a contactor switch may be installed between the negative terminal of the battery and the negative bus bar either instead of or in addition to the contactor switch between the positive battery terminal and the positive bus bar. In the case of the second DC bus, the positive bus bar would instead be electrically connected to the output of the variable voltage converter.