Systems and methods for discharging bus voltage using semiconductor devices

Systems and methods are provided for discharging a high-voltage bus using semiconductor devices. A discharge system for a first voltage rail and a second voltage rail comprises a first semiconductor device coupled to a first voltage rail and a second semiconductor device coupled between the first semiconductor device and a second voltage rail. A control circuit is coupled to the first semiconductor device and the second semiconductor device. In response to a discharge condition, the control circuit is configured to activate the first semiconductor device and gradually activate the second semiconductor device, such that the energy potential between the first voltage rail and the second voltage rail is gradually dissipated through the semiconductor devices.

TECHNICAL FIELD

Embodiments of the subject matter described herein relate generally voltage discharge circuits, and more particularly, embodiments of the subject matter relate to discharge circuits suitable for use in discharging high-voltage bus capacitance found in electric and hybrid vehicles.

BACKGROUND

In recent years, advances in technology, as well as ever evolving tastes in style, have led to substantial changes in the design of automobiles. One of the changes involves the power usage and complexity of the various electrical systems within automobiles, particularly alternative fuel vehicles, such as hybrid, electric, and fuel cell vehicles.

In most hybrid vehicles, energy storage devices, such as capacitors, are often used to improve efficiency by capturing energy within the powertrain system or supplying additional power during periods of operation when a primary energy source cannot supply the required power quickly enough. For example, regenerative braking may be used to capture energy by converting kinetic energy to electrical energy and storing the electrical energy in a bank of capacitors for later use. In order to accommodate high-voltage operation within automobiles, capacitor banks or supercapacitors are often used because they have the ability to quickly store energy and can be discharged at a much higher rate than other energy sources. However, capacitors may retain a charge after power is removed from a circuit or an automobile is turned off. Therefore, high-voltage capacitors should be properly discharged after turning off a vehicle or before accessing the equipment housing the capacitors.

Discharging a capacitor is typically accomplished by placing a discharge or bleed resistor across the capacitor or bus terminals in parallel. In addition to requiring additional components, these designs also require discharge resistors with the ability to handle high average power dissipation. These resistors generally occupy a larger surface area and often require additional harnesses, connectors, and heat sinks, which prevent the discharge resistors from being built on a circuit board. In addition to the increased spatial requirements, these discharge circuits are not utilized during most normal operating modes.

BRIEF SUMMARY

An apparatus is provided for a discharge system for a first voltage rail and a second voltage rail. An energy potential exists between the first voltage rail and the second voltage rail. The discharge system comprises a first semiconductor device coupled to the first voltage rail and a second semiconductor device coupled between the first semiconductor device and the second voltage rail. A control circuit is coupled to the first semiconductor device and the second semiconductor device. The control circuit is configured to activate the first semiconductor device in response to a discharge condition, and gradually activate the second semiconductor device in response to the discharge condition, such that the energy potential is gradually dissipated through the semiconductor devices.

In another embodiment, an apparatus is provided for an electrical system for use in a vehicle. The electrical system comprises a capacitance between a first voltage rail and a second voltage rail. The electrical system further comprises an inverter module having a phase leg including a first transistor coupled to the first voltage rail and a second transistor coupled between the first transistor and the second voltage rail. A control circuit is coupled to the inverter module. The control circuit is configured to apply a constant voltage to the gate terminal of the first transistor, the constant voltage being greater than the threshold voltage of the first transistor. The control circuit is further configured to apply a control voltage to the gate terminal of the second transistor. The control voltage is initially less than the threshold voltage of the second transistor, and the control circuit is configured to gradually increase the control voltage to a voltage greater than the threshold voltage, such that the second transistor is gradually activated and energy stored by the capacitance is gradually dissipated through the transistors.

A method is provided for discharging an energy potential between a first voltage rail and a second voltage rail using an inverter phase leg coupled between the first voltage rail and the second voltage rail. The inverter phase leg is controlled by a gate driver circuit. The method comprises detecting a discharge condition and setting gate driver circuit controlling the inverter phase leg to a discharge mode in response to the discharge condition, wherein the energy potential is gradually dissipated through the inverter phase leg.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the subject matter or the application and uses of such embodiments. Although the embodiments of the subject matter are discussed herein in the context of vehicle drive systems, the subject matter may apply to alternative implementations in other applications. As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Any implementation described herein as exemplary is not necessarily to be construed as preferred or advantageous over other implementations. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.

The following description refers to elements or nodes or features being “coupled” together. As used herein, unless expressly stated otherwise, “coupled” means that one element/node/feature is directly or indirectly joined to (or directly or indirectly communicates with) another element/node/feature, and not necessarily mechanically. Thus, although the schematics shown depict one exemplary arrangement of elements, additional intervening elements, devices, features, or components may be present in an embodiment of the depicted subject matter. In addition, certain terminology may also be used in the following description for the purpose of reference only, and thus are not intended to be limiting. For example, terms such as “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context.

Technologies and/or concepts described herein relate generally to systems and methods for discharging high-voltages that exist in electric circuits, architectures, and systems, such as, for example, electric and hybrid vehicle drive systems. Various functionality and features of automotive drive systems are well known and so, in the interest of brevity, many conventional aspects will only be mentioned briefly herein or will be omitted entirely without providing the well known details.

FIG. 1illustrates an electrical system100suitable for use in a vehicle, in accordance with one embodiment. The electrical system100includes, without limitation, an energy source102, an inverter module104, a motor106, and a controller108. The inverter module104provides AC power to the motor106from the energy source102under control of the controller108. In an exemplary embodiment, at least one capacitor110is provided between the energy source102and inverter module104for capturing energy within the electrical system100, as will be understood. It should be understood thatFIG. 1is a simplified representation of the electrical system100, andFIG. 1is not intended to limit the subject matter described herein.

In an exemplary embodiment, the energy source102is coupled to the inverter module104and capacitor110via a high-voltage bus112. The high-voltage bus112may be realized as a pair of conductive elements, such as wires, cables, or busbars. A first conductive element of the bus112corresponds to a positive reference voltage and a second conductive element corresponds to a negative reference voltage, wherein the difference between the positive reference voltage and the negative reference voltage is considered to be the voltage of the bus112. In various embodiments, the high-voltage bus112has a voltage that may range from 300 volts to 500 volts or higher during normal operation of the electrical system100. Although not illustrated inFIG. 1, in practice, there may be a switch coupled between the energy source102and the high-voltage bus112, which may in turn be opened to decouple the energy source102and allow high-voltage stored on the capacitor110and/or within the electrical system100to be discharged, as will be appreciated in the art.

Depending on the embodiment, the energy source102may be realized as a battery or battery pack, a fuel cell or fuel cell stack, one or more capacitors (e.g., an ultracapacitor or capacitor bank), or another suitable voltage source. AlthoughFIG. 1depicts a single energy source102, in practice, numerous energy sources may be present. The motor106may be realized as an electric motor, a generator, a traction motor, or another suitable motor known in the art. The motor106may be an induction motor, a permanent magnet motor, or another type of motor suitable for the desired application.

In an exemplary embodiment, the inverter module104includes at least one phase leg. As described in greater detail below in the context ofFIG. 2, an inverter phase leg includes a pair of switches, each switch having a freewheeling diode associated therewith, and an output node between sets of switches and diodes. It should be understood that although the inverter module104may be described herein in the context of an individual phase leg, in practice, the inverter module104may include any number of phase legs. In an exemplary embodiment, the output node of an inverter phase leg is coupled to a phase of the motor106, wherein the inverter104is configured to convert DC voltage from the high-voltage bus112(e.g., DC voltage provided by energy source102) into an AC voltage for powering the motor106, as is commonly understood in the art.

In an exemplary embodiment, the controller108is in operable communication and/or electrically coupled to the inverter104. The controller108is responsive to commands received from the driver or operator of the vehicle (e.g., via an accelerator pedal) or alternatively, commands received from the electronic control system (not shown) within the vehicle. The controller108provides commands to the inverter104to control the output at the output node of the inverter phase leg by employing high frequency pulse width modulation (PWM) of the switches, as is understood in the art. Depending on the embodiment, the controller108may be realized as hardware, software, firmware, or various combinations thereof.

Referring now toFIG. 2, in an exemplary embodiment, a discharge system200suitable for use in the electrical system100includes, without limitation, a high-voltage bus (e.g., bus112) having a pair of voltage rails202,204, an inverter phase leg206, and a control circuit208. A capacitance, such as at least one capacitance element210, may be coupled electrically parallel to the inverter phase leg206between the voltage rails202,204, such that a stored energy potential, VC, exists between the voltage rails202,204. In an exemplary embodiment, the control circuit208is configured to discharge the stored energy potential using the inverter phase leg206in response to a discharge condition, as described in greater detail below.

In an exemplary embodiment, the inverter phase leg206includes a pair of semiconductor devices212,216and a pair of diodes214,218coupled between the voltage rails202,204with an output node220located between the semiconductor devices212,216. A first semiconductor device212is coupled to the first voltage rail202and the output node220. A first freewheeling diode214is coupled between the first voltage rail202and the output node220. In an exemplary embodiment, the first semiconductor device212and diode214are antiparallel, meaning they are electrically in parallel with reversed or inverse polarity. The antiparallel configuration allows for bidirectional current flow while blocking voltage unidirectionally, as will be appreciated in the art. In this configuration, the direction of current through the first semiconductor device212is opposite to the direction of allowable current through the freewheeling diode214. A second semiconductor device216is coupled between the output node220(e.g., the second semiconductor device216may be coupled to the first semiconductor device212) and the second voltage rail204. A second freewheeling diode218is coupled between the output node220and the second voltage rail204, such that the second semiconductor device216and freewheeling diode218are antiparallel. In practice, the output node220may be coupled to a winding of a motor (e.g., motor106) for driving a phase of the motor with inverter phase leg206, as will be appreciated in the art. It should be further appreciated that althoughFIG. 2depicts a single inverter phase leg206, in practice, multiple inverter phase legs may be present.

In an exemplary embodiment, the semiconductor devices212,216are realized as transistors. Preferably, the semiconductor devices212,216are realized as insulated-gate bipolar transistors (IGBTs), although in some embodiments, the semiconductor devices212,216may be realized as field-effect transistors (FETs).FIG. 2illustrates an exemplary configuration of the semiconductor devices212,216and diodes214,218for N-channel transistor semiconductor devices212,216. For clarity and ease of explanation, the subject matter will be described herein in terms of an N-channel configuration, however, it will be appreciated in the art that the subject matter may be implemented using P-channel devices in a similar manner.

In an exemplary embodiment, the first semiconductor device212is a transistor having a first gate terminal222and an associated threshold voltage, νTH1. The first semiconductor device212allows current flow (i.e., the semiconductor device212is turned on) when a voltage applied to the first gate terminal222exceeds the threshold voltage, νTH1. Similarly, the second semiconductor device216has a second gate terminal224and an associated threshold voltage, νTH2, wherein the second semiconductor device216allows current flow when voltage applied to the second gate terminal224exceeds the threshold voltage, νTH2. In accordance with one embodiment, the semiconductor devices212,216are identical transistor devices (e.g., same manufacturer and model) such that νTH1and νTH2are substantially equal.

In an exemplary embodiment, the control circuit208includes a gate driver circuit226coupled to the gate terminals222,224of the respective semiconductor devices212,216. The control circuit208is configured to utilize high frequency pulse width modulation (PWM) to alternately activate (i.e., turn on) the semiconductor devices212,216to produce an AC voltage at the output node220, as will be understood. In this regard, the gate driver circuit226may include normal gate drive circuitry228,230that can be selectively and controllably coupled to the gate terminals222,224of a respective semiconductor device212,216. Although not illustrated, the normal gate drive circuitry228,230may be configured to employ high frequency PWM under the control of another device (e.g., controller108), as will be appreciated in the art. In an exemplary embodiment, the normal gate drive circuitry228,230is coupled to the gate terminals222,224via switches232,234. In this configuration, when the switches232,234are in a state such that the normal gate drive circuitry228,230is coupled to the semiconductor devices212,216, the gate driver circuit226may be understood as being in a normal operating mode.

In an exemplary embodiment, the control circuit208includes a controller236coupled to the switches232,234. The controller236is configured to detect a discharge condition and set the gate driver circuit226to a discharge mode in response to the discharge condition. As used herein, a discharge condition should be understood as a situation where it is desirable to discharge a voltage (e.g., VC) that may be stored within an electrical system to protect against electrostatic discharge or other negative effects. For example, a discharge condition may be an attempt to access a unit or compartment containing a high-voltage component, a vehicle crash or accident, or turning off of a vehicle housing the electrical system. Although not illustrated, the controller236may be configured to detect the discharge condition using one or more sensors or receive an input signal indicative of a discharge condition from another vehicle module, such as an electronic control unit. As described below, in the discharge mode, the gate driver circuit226is configured to gradually dissipate the energy potential between the voltage rails202,204(i.e., VC) to a safe level within a specified period of time using the inverter phase leg206without damaging the semiconductor devices212,216.

In an exemplary embodiment, the gate driver circuit226includes discharge gate drive circuitry238,240coupled to the switches232,234. The controller236may be configured to set the gate driver circuit226to discharge mode by activating (or changing the state of) switches232,234in order to couple the discharge gate drive circuitry238,240to the gate terminals222,224of the respective semiconductor device212,216. In this configuration, when the switches232,234are in a state such that the discharge gate drive circuitry238,240is coupled to the semiconductor devices212,216, the gate driver circuit226may be understood as being in the discharge mode. In an exemplary embodiment, the first discharge gate drive circuitry238is configured to activate the first semiconductor device212and the second discharge gate drive circuitry240is configured to gradually activate the second semiconductor device216, such that the energy potential between the voltage rails202,204is gradually dissipated through the semiconductor devices212,216. In an alternate and equivalent embodiment, the second discharge gate drive circuitry240may be configured to activate the second semiconductor device216and the first discharge gate drive circuitry238configured to gradually activate the first semiconductor device212, such that the energy potential between the voltage rails202,204is gradually dissipated through the semiconductor devices212,216.

Referring now toFIG. 3,FIG. 4, andFIG. 5, and with continued reference toFIG. 2, in an exemplary embodiment, the controller236is configured activate the switches232,234in response to detecting a discharge condition at time t0. The first discharge gate drive circuitry238is configured to apply a constant voltage to the gate terminal222of the first semiconductor device212as shown inFIG. 3. The constant voltage is greater than the threshold voltage, νTH1, for the semiconductor device212, such that the semiconductor device is capable of conducting current (i.e., turned on). Preferably, the constant voltage is only slightly greater than the threshold voltage, νTH1, such that the first semiconductor device212operates in a sub-saturation mode, which may alternatively be referred to as the linear or ohmic mode. In this sub-saturation mode, the first semiconductor device212has a higher resistance than it would otherwise have in the saturation mode at higher gate voltages. In accordance with one embodiment, the constant voltage exceeds the threshold voltage by an amount ranging from approximately 2.5% to 5% of the threshold voltage. For example, for a threshold voltage of 4 Volts, the constant voltage may be 0.1 to 0.2 Volts above the threshold. As the gate voltage is increased, the semiconductor device212discharges more energy and will increase in temperature, as will be appreciated in the art. Thus, the constant voltage should be adjusted to meet the desired discharge time while keeping the temperature of the semiconductor device212low enough to prevent a failure.

As shown inFIG. 4, in an exemplary embodiment, the second discharge gate drive circuitry240is configured to apply a control voltage to the gate terminal224of the second semiconductor device216. At time t0, the control voltage is initially less than the threshold voltage, νTH2, of the second semiconductor device216, such that the second semiconductor device216is not activated (i.e., it is off). The second discharge gate drive circuitry240is configured to gradually increase the control voltage, such that the second semiconductor device216is gradually activated and the stored energy is gradually dissipated through the semiconductor devices212,216. In this regard, the second discharge gate drive circuitry240gradually increases the control voltage to a voltage greater than the threshold voltage, νTH2, by time t2. In accordance with one embodiment, the second discharge gate drive circuitry240maintains a constant control voltage after time t2. As used herein, “gradually activated” means that the control voltage is increased in an incremental manner, such that a gradually activated semiconductor device responds by gradually allowing an increased amount of current to flow from source to drain as the gate voltage increases and the semiconductor device approaches saturation.

In an exemplary embodiment, the capacitance element210may be realized as a capacitor (or a bank of capacitors) or another electrical load in a vehicle that is coupled to the voltage rails202,204(e.g., high-voltage bus112). The capacitance element210stores and/or retains an electrical energy potential or voltage, VC, even when not connected to an energy source.

As shown inFIG. 5, at time t1, when the control voltage on the second gate terminal224crosses the threshold voltage, νTH2, the energy potential between the two voltage rails202,204(i.e., the energy stored in capacitance element210) begins to be dissipated through the two semiconductor devices212,216which are turned on. In the situation where the capacitance element210comprises a capacitor or another capacitive load, the voltage between the two voltage rails202,204, VC, decays exponentially. In accordance with one embodiment, the resistance of the second semiconductor device216decreases as the control voltage increases over time t1to t2, such that the voltage discharge curve shown inFIG. 5resembles an RC circuit with a varying resistance, as will be appreciated in the art. In an exemplary embodiment, the controller236is configured to detect when the discharge condition no longer exists and switch the gate driver circuit226back to the normal operating mode by switching the switches232,234.

Referring back toFIG. 4, in an exemplary embodiment, the second discharge gate drive circuitry240increases the control voltage linearly (e.g., a ramp function) as shown inFIG. 4. Alternatively, the second discharge gate drive circuitry240may increase the control voltage logarithmically, quadratically, exponentially, or in another manner suitable for the particular discharge system. Preferably, the second discharge gate drive circuitry240is configured such that the control voltage smoothly crosses the threshold voltage to protect against a potentially damaging immediate discharge through the semiconductor devices212,216. In accordance with one embodiment, the initial control voltage at time t0is determined by subtracting a tolerance value, from the threshold value, νTH2to ensure reliable operation of the discharge system200. The tolerance value may be based on the various tolerances associated with the semiconductor device216. For example, the tolerance value may be based on the threshold voltage range data provided in a manufacturer data sheet for the device, operating temperature variations, and other environmental factors that may affect device performance. Similarly, the final control voltage at time t2may be determined by adding a tolerance value to the threshold value, νTH2.

In an exemplary embodiment, the voltage levels for the constant voltage and control voltage along with the time period from t0to t2are adjusted such that the stored energy potential is sufficiently dissipated to a desired level within a specified time period. For example, in an automotive application, the voltage between the voltage rails202,204may be between 300 to 400 volts and potentially higher. In an exemplary embodiment, the discharge system200is configured to discharge a voltage of 300 to 400 volts to a lower level of around 40 volts or less within three seconds. Furthermore, it should be appreciated that the voltage applied to the gate terminal222of the first semiconductor device212need not be constant, and in fact, in one or more alternative embodiments, the same discharge gate function may be used for both semiconductor devices212,216.

Although not illustrated, the control circuit208may include additional circuitry or functionality to protect the discharge system200during fault conditions. For example, if an energy source is connected across the voltage rails202,204, the control circuit208may be able to detect a failure to discharge and apply zero (or negative) voltage to the gate terminals222,224to prevent the semiconductor devices212,216from overheating. The control circuit208may be configured to wait a period of time before attempting to resume discharge of the high-voltage bus.

In accordance with one embodiment, the controller236is configured to control normal operation of the inverter phase leg206and/or the gate driver226, for example, by providing signals to modify the PWM duty cycle of the normal gate drive circuitry228,230, as will be appreciated in the art. In this regard, in accordance with another embodiment, the controller236may be configured to discharge the voltage rails202,204without use or inclusion of discharge gate circuitry238,240or switches232,234. For example, in response to detecting a discharge condition, the controller236may modify the duty cycle of the first normal gate drive circuitry228for the first semiconductor device212such that the first semiconductor device212is turned on (e.g., applying a constant voltage great enough to cause the device to operate in a saturation mode). The controller236may then modify the duty cycle of the second normal gate drive circuitry230such that the second semiconductor device216is repeatedly turned on for very short periods of time (e.g., pulsed). The normal gate drive circuitry230may repeatedly apply a voltage pulse at the gate terminal224of the second semiconductor device216that has a limited duration such that the second semiconductor device216does not operate in a saturation mode. For example, the normal gate drive circuitry230may be configured to turned on or pulse the second semiconductor device216for approximately 500 nanoseconds to one microsecond. Because the gate driver226takes a finite amount of time to reach the gate voltage required for the device to operate in the saturation mode, if the duration of the discharge pulse is chosen for a short enough time period, the second semiconductor device216operates in the sub-saturation mode (e.g., in the linear or ohmic region) such that it gradually dissipates energy from the high-voltage bus202,204.

It will be appreciated in the art that the duration of the voltage pulse should be adjusted to obtain the desired discharge characteristics without damaging the semiconductor device216. However, in some embodiments, the gate driver226may already include cross-conduction or de-saturation detection circuitry which may protect the semiconductor devices212,216if the width of the discharge pulse is chosen to be too large. The controller236and/or normal gate drive circuitry230may be configured to turn on (e.g., operate in saturation mode) the second semiconductor device216to complete the discharge once the voltage on the bus202,204is sufficiently discharged to a level that is safe for both devices212,216.

One advantage of the system and/or method described above is that the discharge system allows a high-voltage bus to be discharged without requiring additional discharge components, such as discharge resistors or relays. Furthermore, discharge system may be implemented in a manner that allows for a fast discharge of the bus while also minimizing the power absorption or stress on the semiconductor devices. Additionally, the systems and methods described above may be utilized in different types of automobiles, different vehicles (e.g., watercraft and aircraft), or in other electrical systems altogether, as it may be implemented in any situation where a high-voltage bus needs to be reliably discharged.