Integrated precharging and discharging for electric vehicle drive system capacitors

A shared resistor performs precharging and discharging functions of capacitors in an electric vehicle drive system. In a precharge state, the shared resistor is connected between the capacitors and a DC source via a precharge relay. In a discharge state, the resistor is connected across the capacitors via a discharge transistor. Otherwise, the resistor is disconnected. A bypass switch is connected between the resistor and an input capacitor. The bypass switch is rendered conductive during the precharge state and during the discharge state. The discharge transistor is activated only during the discharge state. As a result, the invention uses less components by virtue of eliminating separate resistance elements for pre-charging and discharging and by eliminating discharge switches dedicated to separate resistances. The circuit integration and the placement of components outside the inverter module improves overall system cost and packaging size.

CROSS REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

BACKGROUND OF THE INVENTION

The present invention relates in general to drive systems for electric vehicles, and, more specifically, to circuitry for combining the functions of precharging of a capacitor upon energizing of the electric drive and discharging the capacitor upon deactivation of the electric drive.

Electric vehicles, such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs), utilize inverter-driven electric machines to provide traction torque. A typical electric drive system may include a DC power source (such as a battery pack or a fuel cell) coupled by contactor switches (i.e., relays) to an input capacitor for buffering the battery voltage. A DC-DC converter (also known as a variable voltage converter, or VVC) couples the input capacitor to a main DC linking capacitor that supports a high voltage DC bus. The VVC may bi-directionally direct a current flow between the input capacitor and the linking capacitor to regulate a voltage across one of the capacitors. A three-phase motor inverter is connected between the main buses with outputs of the inverter connected to a traction motor in order to convert DC bus power to an AC voltage coupled to the windings of a traction motor in order to propel the vehicle. During deceleration of the vehicle, the motor can be driven by the vehicle wheels and used to deliver electrical power to charge the battery during regenerative braking of the vehicle, with the DC-DC converter working in the opposite direction to convert the generated power to a DC voltage appropriate for charging the battery pack. In some vehicles, a generator driven by an internal combustion (gasoline) engine is provided to generate electric power to charge the battery. A second three-phase inverter typically connects the generator output to the high voltage DC bus.

Due to the high voltages present when the electric drive is in use, special precautions are necessary during activation and deactivation of the drive. During activation, for example, the contactors are opened at a time when the capacitors are discharged at about zero Volts. Closing the contactors with the capacitors in a discharged or low charged state would present a low impedance to the battery pack, resulting in a very high inrush current that could cause damage to the contactors and other components. One solution is to provide a constant resistance between a contactor and the capacitors. However, use of a current-limiting resistor in series with the contactors is undesirable after the initial precharging because of the associated voltage drop and power consumption it would cause during subsequent normal operation. Therefore, a separate circuit branch, or precharging circuit, is often used. The known precharging circuits utilize a switch and a resistor in series between the DC supply and the capacitors. Turning on the switch allows the capacitors to be charged through the resistor, and the presence of the resistor limits the inrush current to prevent damage to the switch. Once the capacitors are precharged, then i) the main contactors can be closed without receiving any inrush current and ii) the precharge switch can be opened so that the precharge resistor is disconnected.

During deactivation, it becomes necessary to discharge the capacitors. A shutdown of the electric drive system can result from a vehicle key-off, a high-voltage DC interlock fault, or a vehicle crash, for example. During shutdown, the battery is quickly isolated from the rest of the electric system by opening the mechanical contactors. This also isolates the electric charges present on the DC capacitors. Due to safety requirements, the HV capacitor charges should be quickly discharged within a specific time. For example, U.S. Federal Motor Vehicle Safety Standards (FMVSS) may require that the voltage on the DC link capacitor must be less than 60V within 5 seconds in certain circumstances.

The simplest conventional methods for discharging the link capacitor use a resistance placed across the capacitor to dissipate the charge. The resistor placement can be passive or active. A passive discharge resistor (PDR) is hard-wired in parallel with the link capacitor. The passive resistor must have a relatively large resistance to avoid excessive power loss during normal operation. Consequently, it could take one to two minutes to dissipate an HV charge down to a safe level. To discharge more quickly, an active discharge circuit uses a resistor in series with a transistor switch so that the charge can be selectably dissipated through a smaller resistance value.

The circuit components for the active discharge circuits and at least some components for a precharge circuit are typically included on a printed circuit board in an Inverter System Controller (ISC) module. Thus, the size, component count, and cost of an ISC module are all increased. It would be desirable to perform the precharge and discharge functions with fewer components so that size and cost of an ISC module can be reduced.

SUMMARY OF THE INVENTION

In one aspect of the invention, an electric drive system for a vehicle with a DC source comprises a link capacitor between a positive bus and a negative bus. A precharge contactor has an input adapted to be connected to the DC source and has an output. A resistance element connects the precharge contactor output to the positive bus. A discharge switch selectably connects the negative bus to a junction between the precharge contactor and the resistance element. When the precharge contactor is conducting and the discharge switch nonconducting, then the capacitor is precharged. When the discharge switch is conducting, then the capacitor is discharged. The same resistance element carries both the precharging current and the discharging current of the capacitor.

In a preferred embodiment, the drive system has a link capacitor and an input capacitor. The input capacitor has positive and negative terminals selectably coupled to the DC source by positive and negative contactors, respectively. A voltage converter couples the positive terminal to the positive bus. A bypass switch selectably couples the positive terminal to the positive bus. In a precharge state, the precharge contactor and the bypass switch are conductive while the discharge switch and the positive contactor are nonconductive. In a discharge state, the discharge switch and the bypass switch are conductive while the precharge, positive, and negative contactors are nonconductive.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1illustrates an electric drive system10of a known type which is useful with a powersplit hybrid drive, for example. A battery pack11is coupled by contactor relay switches12and13to a variable voltage converter (VVC)14having input capacitor15. A DC link capacitor16is connected to an output of VVC14establishing a positive bus17and a negative bus18. A motor inverter20couples a traction motor21to the DC voltage between busses17and18. Likewise, a generator inverter22couples an electrical generator23to the DC link. Inverters20and22are each comprised of a plurality of switching devices (such as insulated gate bipolar transistors, IGBTs) in a bridge configuration including three phase legs. The IGBTs inverters20and22, as well as the IGBTs in VVC14, are driven according to control signals (e.g., PWM switching signals) from a controller24in a conventional manner. Battery pack11may provide an output voltage of about 200V to 300V, while the DC link is normally operated at a higher voltage of about 600V to 800V, for example. Even though they are not usually at the same voltage, it is important to provide precharging and discharging for both capacitors.

FIG. 2illustrates a common arrangement for precharging applied to drive system10. A precharge circuit25has an input coupled across battery pack11and an output connected to positive terminals of link capacitor16and input capacitor15in order to supply a charge onto the capacitors during startup of electric drive10so that when contactors12and13are closed, they are not damaged by an inrush current. After capacitors15and16are precharged to the voltage of battery pack11, precharge circuit25is deactivated so that no power is lost during normal operation of drive system10.

FIG. 3shows a conventional arrangement for actively discharging link capacitor16. A similar circuit can also be used for discharging input capacitor15(not shown). An active discharge circuit26has a discharge resistor27in series with a discharge switch (e.g., transistor)28. Switch28has a control terminal for selectably turning the discharge switch on and off via a disable circuit29in response to a disable command signal from a controller (not shown). The controller may be comprised of a conventional Motor Generator Control Unit (MGCU) as known in the art. The function of disable circuit29is to perform a logical inversion of the disable command signal. Thus, when the disable command signal has a high logic level, an output of disable circuit29connected to the control terminal has a low voltage level so that switch28is turned off (and capacitor16is not discharged). The low voltage level can be obtained by shunting the control terminal to negative bus18, for example. When the disable command signal ceases (i.e., drops to a low logic level), the output of disable circuit29is automatically pulled up to a voltage sufficient to turn on discharge switch28and capacitor16is quickly discharged. In the event of a failure of the control unit, any command signals may be lost. Disable circuit29logically inverts command signal so that if there is a loss of command signals due to failure of the control unit then capacitor16is discharged. Therefore, protection against a high voltage on capacitor16is obtained even when the control unit fails.

FIG. 4illustrates an electric drive system30providing both precharging and discharging of capacitors using the known techniques. Electric drive30includes a battery pack31connected to an inverter system controller (ISC) module32for driving a traction motor33. External components interconnecting battery pack31with module32include a manual service disconnect (MSD) switch34and fuse35feeding DC power from battery pack31to main contactors (i.e., electronically-controlled relay switches)36and37and a precharge contactor38. A precharge resistor40is shown as being external of module32, but it could alternatively be mounted inside module32.

Module32includes an input capacitor41arranged to receive battery voltage when main contactors36and37are closed. Battery voltage is provided to an input of a VVC42having its output connected across a DC link capacitor43, creating a high-voltage rail between a positive bus44and a negative bus45. The high-voltage DC is converted to AC by an inverter46for driving three-phase motor33.

A bypass switch47is connected between capacitor41and positive bus44in order to bypass VVC42(e.g., when positive bus44is intended to operate at a voltage equal to battery voltage) as known in the art. Bypass switch47is also used during the precharge state of electric drive30as follows. Prior to entering the precharge state, main contactors36and37and precharge contactor38are open (nonconductive) and capacitors41and43are substantially discharged. To initiate precharging, precharge contactor38and main contactor37are closed (conductive) so that current flows through precharge resistor40in order to supply charging current directly to link capacitor43. Simultaneously, bypass switch47is activated so that it conducts charging current to input capacitor41. After sufficient charging, the voltages across capacitors41and43are substantially the same as the battery voltage. Precharge contactor38is then opened, and main contactors36is closed (contactor37is already closed during precharge and it remains closed). Drive circuit30is then ready for normal operation for driving motor33(and for transferring power from motor33back to battery pack31during regenerative braking).

For discharging capacitors41and43during a shutdown, active discharge circuits are provided which include a discharge resistor50and discharge switch51connected in series across capacitor43and a discharge resistor52and a discharge switch53connected in series across input capacitor41. Discharge switches51and53may be comprised of IGBTs or MOSFETs, for example. When a controller (not shown) determines that a shutdown is required, it initiates a discharge state by opening main contactors36and37. Then then controller renders discharge switches51and53conductive in order to dissipate charge from capacitors41and43in resistors52and50, respectively. Although one resistor symbol is shown for each discharge resistors50and52, each may include multiple resistor devices connected together to provide sufficient power dissipation capability.

The circuit inFIG. 4demonstrates that known methods for pre-charging and discharging of the capacitors utilize a relatively large number of components, most of which have been mounted within the inverter module. This creates higher component costs and increases the overall size and complexity of the associated module.

FIG. 5shows an electric drive system60incorporating an integrated pre-charging and discharging circuit for a reduced component count and simplified inverter module. Elements of drive system60inFIG. 5which are identical to components ofFIG. 4are indicated using the same reference numbers. Input capacitor41and DC link capacitor43receive DC power derived from battery pack31via main contactor relays36and38and via VVC42. An inverter system controller module61is constructed without any internal active discharge system circuit components. A combined precharge/discharge resistance element62connects precharge contactor38to positive bus44. Resistance element62preferably includes one or more fixed resistors to providing a resistance value and a power dissipation capacity that achieves desired charging and discharging rates. Other types of resistance elements can be utilized such as a FET driven in its transition zone.

The integrated precharge/discharge circuit of drive system60further includes a discharge switch63that selectively connects a junction between precharge contactor38and resistor62with negative bus45. A controller65is configured to provide control signals to contactors36,37, and38and to provide command signals to transistor driver circuits66and68for controlling bypass switch47and discharge switch63, respectively. In the illustrated embodiment, bypass switch47is shown as an IGBT with a bypass diode67. Diode67alleviates the need for activating the IGBT during discharge of the input capacitor41as described below. In the event that a bypass diode was not present, then it would become necessary to provide a drive command signal to activate bypass switch47during discharging of capacitor41.

When controller65determines that the drive system is being activated from an inactive state, then it triggers a precharge state. Prior to the precharge state, contactors36,37, and38are all nonconductive and capacitors41and43are substantially discharged. To begin the precharge state controller65renders precharge contactor38, main contactor37, and bypass switch47conductive, which results in a current flow as illustrated inFIG. 6. Current flows through resistance element62to positive bus44thereby charging link capacitor43directly. Current also flows from resistance element62to input capacitor41via bypass switch47. Controller65typically monitors a voltage on the DC link using a sensor (not shown). Once the DC link voltage reaches a level substantially equal to the battery voltage, then precharge contactor38is opened, thereby disconnecting resistance element62from the DC power. Bypass switch47is rendered nonconductive and main contactor36is closed, so that drive60is ready to provide normal operating current flow as shown inFIG. 7.

When inverter operation is commanded to shut down, controller65opens main contactors36and37to isolate battery pack31and then initiates a discharge state. In order to discharge capacitors41and43, controller65renders discharge switch63conductive via a command signal provided to driver circuit68. Driver circuit68converts the command signal to an appropriate current and voltage for driving the transistor of discharge switch63(e.g., utilizing the disable logic as shown inFIG. 2). As shown inFIG. 8, once discharge switch63is rendered conductive, discharge current flows from link capacitor43through resistance element62and discharge switch63. Another discharge current flows from input capacitor41through bypass diode67, resistance element62, and discharge switch63. DC link capacitor43typically has a higher voltage which causes it to discharge first through resistance element62. Once link capacitor43has discharged sufficiently to allow forward biasing of diode67then input capacitor41also begins to discharge. After the charges on capacitors41and43have depleted to a safe level, discharge switch63may be turned off by controller65.

The foregoing invention is able to use a common resistance for performing the charging and discharging functions. In a precharge state, the common resistance element is connected between each of the capacitors and a DC source via a precharge relay. In a discharge state, the resistance element is connected across each capacitor via a discharge transistor. Otherwise, the resistance element is disconnected. A bypass switch is connected between the resistance element and the input capacitor. The bypass switch is rendered conductive during the precharge state and during the discharge state. The discharge transistor is activated only during the discharge state. As a result, the invention uses less components by virtue of eliminating separate resistance elements for pre-charging and discharging and by eliminating discharge switches dedicated to separate resistances. The circuit integration and the placement of components outside the inverter module improves overall system cost and packaging size.