Patent Description:
Electric drive systems are found in electric vehicles such as, e.g., electric cars and trucks, hybrid electric cars and trucks, and electric trains and trams. Electric vehicles generally include an inverter that converts a battery or other de output into an ac signal for driving an electric motor. In these vehicles, an energy storage capacitance is commonly used as an intermediate buffer between the battery and the inverter. These capacitances can be referred to as "DC link capacitors". These capacitances provide low-inductive current paths to the inverter output stage, and to store energy.

An electric drive system in a battery-powered electric vehicle will typically be shut down several thousand times over its operational lifespan. During a shutdown, the battery is isolated from the rest of the electric drive system. However, without further measures, the intermediate DC link capacitor will retain a charge after being disconnected from the battery. For safety reasons, regulatory agencies often require that this charge be dissipated reasonably soon after shutdown. Vehicle manufacturers may also have discharge requirements. For example, a typical requirement would have the DC link capacitor discharged within <NUM> seconds to a voltage below <NUM> volts.

In some cases, a discharge switch and a resistor can be coupled across the DC link capacitor. After disconnection from the battery, this discharge switch is switched into conduction and the DC link capacitor is discharged through the resistor.

<CIT> describes a drive unit that includes a motor , an inverter, a first electric storage device, a step up-down converter, a first capacitors and a second capacitors, a DC-DC converter, and a relay. The DC-DC converter is driven, while a target duty ratio of the step up-down converter is set such that a total loss of the step up-down converter and the DC-DC converter becomes greater than a maximum loss value of the step up-down converter, and the step up-down converter is controlled, when the relay is turned off to discharge charge of the first capacitor and the second capacitor.

<CIT> describes 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.

<CIT> describes a device and method for discharging an intermediate circuit capacitor in a voltage converter. For this purpose, the discharging process of the intermediate circuit capacitor is controlled by a discharge controller in such a way that the intermediate circuit capacitor is discharged by an electrical load with a predetermined discharge current. The discharge current is preferably at least approximately constant during the discharge period.

<CIT> describes a power supply redundant system in which power supply systems of different voltages coexist, which allows electric power to be supplied to a load even if a ground fault or short circuit occurs therein.

<CIT> describes an energy storage arrangement for onboard network of hybrid motor vehicle.

<CIT> describes a vehicle control device that includes a drive motor and an engaging/disengaging mechanism having a function to permit and cut off torque transmission between the drive motor and wheels.

In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention.

Reference throughout this specification to "one embodiment", "an embodiment", "one example" or "an example" means that a particular feature, structure or characteristic described in connection with the embodiment or example is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment", "in an embodiment", "one example" or "an example" in various places throughout this specification are not necessarily all referring to the same embodiment or example. Particular features, structures or characteristics may be included in an integrated circuit, an electronic circuit, a combinational logic circuit, or other suitable components that provide the described functionality. In addition, it is appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale.

As discussed above, regulatory agencies often require that the charge retained on a DC link capacitor after disconnection from the high-voltage battery be dissipated reasonably soon after shut down.

In implementations of the present disclosure, a power converter that converts the high voltage across a DC link capacitor to a lower voltage participates in the discharge of the DC link capacitor. Although such power converters can play a variety of different roles in an electric vehicle, a common role is a backup power supply for low-voltage components of the vehicle.

In more detail, many electric vehicles have two or more voltage domains. The first is the high voltage/high power domain of the electric drive system, i.e., the circuitry that provides power for driving the electric motor. For example, the DC link capacitors and inverters discussed above operate in this domain and generally are able to operate with voltages up to several hundred volts. Other voltage domains are lower voltage/lower power and may provide power for vehicle components such as on-board electronics and sensors, head- and tail-lights, the dashboard, and others. In general, different voltage domains are supplied by different batteries. For example, the battery for the electric drive system may have a nominal voltage of several hundred volts (e.g., between <NUM> and <NUM> Volts), whereas the battery for a lower voltage domain may have a nominal voltage of between <NUM> and <NUM> volts. For example, the battery for a lower voltage domain may be a <NUM> volt automotive battery.

For various reasons, the different voltage domains are generally separated from one another. One exception to this separation are power converters that convert voltages in the high voltage/high power domain into lower voltages. A common example is a backup power supply (or, alternatively, an emergency power supply) for the low-voltage components. In particular, in the event that the battery for the lower voltage domain fails or discharges, a backup power supply can provide power drawn from the drive system battery to the low voltage domain and help ensure continued safe operation of the vehicle.

Since such power converters draw power from the high voltage drive system battery, they can participate in active discharge of high voltages across the DC link capacitor. Further, since the output voltages of such power converters are necessarily lower than the voltages in the high voltage domain, components (e.g., bleeder resistors) that are rated for lower voltages can be used for the active discharge. Also, in many cases, the signals that trigger active discharge (e.g., a signal to shut down the vehicle) generally originate in a low voltage domain. By conducting the active discharge in a low voltage domain, those signals need not be conveyed to the high voltage/high power domain.

<FIG> is a schematic representation of portions of the electrical and drive system <NUM> of an electric vehicle. Electrical and drive system <NUM> includes a high voltage domain that is powered by a primary battery <NUM> and a low voltage domain that is generally powered by a low voltage battery <NUM>. The electric drive system is in the high voltage domain and includes primary battery <NUM> reversibly coupled and decoupled between a high rail <NUM> and a low rail <NUM> by a switch <NUM>. The electric drive system also includes an inverter <NUM> and an electric motor <NUM>. In operation, under the direction of control electronics, inverter <NUM> converts the dc voltage supplied by battery <NUM> into an ac voltage and supplies electric motor <NUM> with power. A DC link capacitor <NUM> is coupled between rails <NUM>, <NUM>. However, it should be appreciated that the DC link capacitor <NUM> may be included in the inverter <NUM>, as illustrated by the thick dashed lines.

Other components <NUM> of the vehicle are in the low voltage domain. Examples of components <NUM> can include on-board electronics and sensors, head- and tail-lights, the dashboard, and/or other components. In general, components <NUM> are powered by battery <NUM> coupled between rails <NUM>, <NUM>. In some implementations, battery <NUM> is a <NUM> V automotive battery. In general, rails <NUM>, <NUM> in the high voltage domain are isolated from rails <NUM>, <NUM> in the low voltage domain. However, in some vehicle types, it may be possible for rail <NUM> to be coupled to rail <NUM> and for batteries <NUM>, <NUM> to share a common return.

A step-down power converter <NUM> interfaces between the high voltage domain and the low voltage domain in that it is configured to convert the high voltage across rails <NUM>, <NUM> into a lower voltage across rails <NUM>, <NUM>. For example, in some implementations, step-down power converter <NUM> may convert <NUM> to <NUM> volts in the high voltage domain to an output voltage of between <NUM> and <NUM> volts in the low voltage domain. In general, but not necessarily, power converter <NUM> does not convert power continuously during operation of the vehicle. Rather, power converter <NUM> operates as a backup or emergency power supply and only converts power in selected circumstances, e.g., in the event that battery <NUM> fails or discharges. Power converter <NUM> can be implemented in a number of different ways using any of a number of different power converter topologies. For example, power converter <NUM> can be implemented as an isolated flyback converter.

As discussed further below, power converter <NUM> also participates in the discharge of the DC link capacitor when battery <NUM> is decoupled from rails <NUM>, <NUM> by switch <NUM>.

Switches <NUM> are either mechanical or solid state switches and coupled to connect and disconnect battery <NUM> from rails <NUM>, <NUM>. Under normal conditions, battery <NUM> will be connected to rails <NUM>, <NUM> when the vehicle that includes electrical and drive system <NUM> is in operation, e.g., moving or ready to move. Battery <NUM> will be disconnected from rails <NUM>, <NUM> during shut-off or in the event of a sufficiently severe fault condition.

Upon connection of battery <NUM> to rails <NUM>, <NUM>, both DC link capacitor <NUM> and inverter <NUM> will be biased by battery <NUM>. The voltage developed across DC link capacitor <NUM> will tend towards equality with the voltage provided by battery <NUM>. However, deviations from equality will occur since DC link capacitor <NUM> accepts and provides charge more quickly than battery <NUM>. In addition, the DC link capacitor <NUM> is generally placed physically closer to the power switches of inverter <NUM> and some distance from the battery <NUM>. The cable inductance could lead to transient voltage events. DC link capacitor <NUM> thus acts to smooth the voltage between rails <NUM>, <NUM> across inverter <NUM>.

Inverter <NUM> can include a collection of phase legs that are each formed by a pair of switching devices coupled in series between rails <NUM>, <NUM>. In general, the switching devices will be insulated-gate bipolar transistors (IGBT) or other power semiconductor devices. Other power semiconductor switches could include gallium nitride (GaN), silicon (Si), or silicon carbide (SiC) based transistors. Further, metal-oxide field-effect transistors (MOSFET) or bipolar junction transistors (BJT) may also be used.

The switching of the switching devices in inverter <NUM> is driven by gate driver circuitry under the control of a controller. The gate driver circuitry is in the high voltage domain and referenced to rail <NUM>, but the controller can be in either or both of the high voltage and low voltage domains.

<FIG> is a schematic representation of components that participate in the discharge of the DC link capacitor <NUM> when battery <NUM> is decoupled from rails <NUM>, <NUM> by switch <NUM>. In addition to components that have already been described, discharge circuitry <NUM> is also illustrated. Further, low voltage domain components <NUM> include a control signal output <NUM> and a discharge drive signal output <NUM>.

Discharge circuitry <NUM> is coupled between rails <NUM>, <NUM> in the low voltage domain. In the illustrated implementation, rails <NUM>, <NUM> are coupled to terminals KL30, KL31 under the DIN <NUM> standard, with the positive terminal of battery <NUM> protected from reverse biasing by a decoupling diode. As discussed further below, discharge circuitry <NUM> can be implemented in a variety of different ways. Regardless of the particular implementation, when driven by a discharge drive signal DR1, discharge circuitry <NUM> acts as a load on the output of step-down power converter <NUM>. In other words, the discharge circuit <NUM> is coupled to the output of the step-down power converter <NUM>. During this time, step-down power converter <NUM> draws power from the high voltage domain to supply discharge circuitry <NUM>-and discharge DC link capacitor <NUM>.

In particular, in operation, a control signal triggers switch <NUM> to disconnect primary battery <NUM> from high voltage supply rails <NUM>, <NUM>. The control signal itself can be triggered, e.g., by shut down of the vehicle or a sufficiently severe fault condition. In general, the control signal will originate from components <NUM> in the low voltage domain and output over a control signal output <NUM>. For example, the control signal can originate from, e.g., user control/interface components and/or safety components in the low voltage domain. If needed, the control signal can be stepped up or transferred across a galvanic isolation barrier to trigger the driving of switch <NUM> to disconnect primary battery <NUM> from high voltage supply rails <NUM>, <NUM>.

Components <NUM> can also output one or more discharge drive signals DR1, DR2,. over one or more discharge drive signal outputs <NUM>. It should be appreciated that the additional discharge drive signals DR2. DRN are optional and shown in dashed lines. The discharge drive signals DR1, DR2,. DRN trigger discharge circuitry <NUM> to load the output of step-down power converter <NUM> and thereby draw power from the high voltage domain. In general, discharge drive signals DR1, DR2,. trigger discharge circuitry <NUM> after the control signal triggers switch <NUM> to disconnect primary battery <NUM> from high voltage supply rails <NUM>, <NUM>. For example, discharge circuitry <NUM> can be triggered some microseconds to milliseconds after switch <NUM> is opened. However, in some applications, discharge circuitry <NUM> can be triggered seconds after switch <NUM> is opened. In some implementations, the control signal that opens switch <NUM> and one or more of discharge drive signals DR1, DR2,. DRN are the same signal and output over a single terminal.

In response to discharge drive signals DR1, DR2,. DRN, discharge circuitry <NUM> loads the output of step-down power converter <NUM>. Step-down power converter <NUM> draws power that was stored on DC link capacitor <NUM> (and possibly elsewhere in the high voltage domain) to supply discharge circuitry <NUM>. By drawing power from DC link capacitor <NUM>, step-down power converter <NUM> discharges DC link capacitor <NUM>, e.g., to levels sufficient to meet regulatory requirements.

As discussed above, discharge drive signals DR1, DR2,. DRN generally trigger discharge circuitry <NUM> after the control signal triggers switch <NUM> to disconnect primary battery <NUM> from high voltage supply rails <NUM>, <NUM>. However, even if by happenstance discharge drive signals DR1, DR2,. DRN were to trigger discharge circuitry <NUM> while primary battery <NUM> is still inadvertantly connected to high voltage supply rails <NUM>, <NUM>, this failure would not cascade. In more detail, step-down power converter <NUM> inherently limits the power that is supplied to discharge circuitry <NUM> and other components in the low voltage domain. Even if primary battery <NUM> remains connected to high voltage supply rails <NUM>, <NUM> while discharge circuitry <NUM> attempts to discharge, step-down power converter <NUM> will limit the power provided to the low voltage domain and reduce the chance that discharge circuitry <NUM> and other elements are damaged.

<FIG> is a schematic representation of one implementation of discharge circuitry <NUM>. The illustrated implementation of discharge circuitry <NUM> includes a resistance <NUM> and switch <NUM> coupled in series between rails <NUM>, <NUM>. The control terminal of switch <NUM> is coupled to be driven by a single discharge drive signal DR1. In the illustrated implementation of discharge circuitry <NUM>, switch <NUM> is shown as an NMOS transistor. Other implementations using other transistor devices are possible-both for this implementation of discharge circuitry <NUM> and for the other implementations discussed below. In any case, the resistance of switch <NUM> in the on state is much smaller than the magnitude of the associated resistance <NUM>.

In operation, discharge drive signal DR1 drives switch <NUM> into conduction and current is conducted through resistance <NUM>. In many implementations, discharge drive signal DR1 drives switch <NUM> intermittently. For example, discharge drive signal DR1 can be a pulse train that drives switch <NUM> into and out of conduction repeatedly, thereby avoiding excessive resistive heating of resistance <NUM>. Alternatively, resistance <NUM> can be configured to withstand continuous resistive heating. In any case, discharge circuitry <NUM> loads step-down power converter <NUM> and dissipates power according to the magnitude of the voltage difference between rails <NUM>, <NUM> and the magnitude of the current flow through resistance <NUM> and switch <NUM>.

<FIG> is a schematic representation of another implementation of discharge circuitry <NUM>. The illustrated implementation of discharge circuitry <NUM> includes resistances <NUM>, <NUM> and switches <NUM>, <NUM>. Resistance <NUM> and switch <NUM> are connected in series to form a first conduction path between rails <NUM>, <NUM>. Resistance <NUM> and switch <NUM> are connected in series to form a second conduction path between rails <NUM>, <NUM>. The control terminal of switch <NUM> is coupled to be driven by a first discharge drive signal DR1. The control terminal of switch <NUM> is coupled to be driven by a second discharge drive signal DR2. The resistance of each switch <NUM>, <NUM> when in the on state is much smaller than the magnitude of the associated resistance <NUM>, <NUM>.

In operation, discharge drive signals DR1, DR2 drive switches <NUM>, <NUM> into conduction and current is conducted through discharge circuitry <NUM>-either continuously or intermittently. Further, switches <NUM>, <NUM> can be driven at the same time so that they conduct simultaneously or at different times so that they conduct alternatively. Hybrid driving schemes-- in which switches <NUM>, <NUM> are driven at times to conduct simultaneously and at times alternatively-are also possible.

When switches <NUM>, <NUM> are driven to conduct simultaneously, the net current through discharge circuitry <NUM> is higher, step-down power converter <NUM> will draw more power from DC link capacitor <NUM>, and DC link capacitor <NUM> will be discharged relatively more quickly. When switches <NUM>, <NUM> are driven to conduct alternatively, power will be drawn from DC link capacitor <NUM> more slowly. However, resistive heating of resistances <NUM>, <NUM> can be reduced. For example, assume that resistance <NUM> conducts for a first duration before switch <NUM> is switched out of conduction. Without current flow through switch <NUM> and resistance <NUM>, resistance <NUM> can cool-even while current is conducted by switch <NUM> and resistance <NUM>. Corresponding cooling of resistance <NUM> can be achieved by switching switch <NUM> out of conduction. In another example, both switches <NUM>, <NUM> are driven to conduct simultaneously and the value of the resistances <NUM>, <NUM> can be reduced as compared to resistance <NUM> of <FIG>. For example, if both switches <NUM>, <NUM> are driven to conduct simultaneously, the value of resistances <NUM>, <NUM> can be substantially <NUM>% of the value of resistance <NUM> of <FIG> to conduct the equivalent power of the example discharge circuit shown in <FIG>.

Furthermore, the shown implementation provides a redundancy for conduction / discharge path in case one path (combination of <NUM> / <NUM> or <NUM> / <NUM>) fails.

Regardless of the particular driving scheme, discharge circuitry <NUM> loads step-down power converter <NUM>. The instantaneous power dissipation by discharge circuitry <NUM> is related to the magnitude of the voltage difference between rails <NUM>, <NUM> and the instantaneous magnitude of the current(s) through resistances <NUM>, <NUM> and switches <NUM>, <NUM>.

<FIG> is a schematic representation of another implementation of discharge circuitry <NUM>. The illustrated implementation of discharge circuitry <NUM> includes resistances <NUM>, <NUM>, <NUM>, <NUM>, switches <NUM>, <NUM>, and abridge node <NUM>. Resistances <NUM>, <NUM> are both coupled between rail <NUM> and bridge node <NUM>. Resistance <NUM> and switch <NUM> are coupled to form a first conduction path between bridge node <NUM> and rail <NUM>. Resistance <NUM> and switch <NUM> are coupled to form a second conduction path between bridge node <NUM> and rail <NUM>. The control terminal of switch <NUM> is coupled to be driven by a first discharge drive signal DR1. The control terminal of switch <NUM> is coupled to be driven by a second discharge drive signal DR2. The resistance of each switch <NUM>, <NUM> in the on state is much smaller than the magnitude of each of resistances <NUM>, <NUM>, <NUM>, <NUM>.

When switches <NUM>, <NUM> are driven to conduct simultaneously, current will flow through resistances <NUM>, <NUM>, <NUM>, <NUM> in accordance with the ratios of their respective resistances. However, when only one of switches <NUM>, <NUM> is driven to conduct, current will continue to flow through both resistances <NUM>, <NUM> but exclusively through the respective one of resistances <NUM>, <NUM>. Once again, a respective one of resistances <NUM>, <NUM> can be provided with time to cool if needed. For example, both switches <NUM>, <NUM> are driven to conduct simultaneously and the value of the resistances <NUM>, <NUM>, <NUM>, <NUM> can be reduced as compared to resistance <NUM> of <FIG>. For example, if both switches <NUM>, <NUM> are driven to conduct simultaneously, the value of resistances <NUM>, <NUM>, <NUM>, <NUM> can be substantially <NUM>% of the value of resistance <NUM> of <FIG> to conduct the equivalent power of the example discharge circuit shown in <FIG>.

Furthermore, the shown implementation increases the level of redundancy compared to <FIG> and provides alternative discharge paths in case up to two resistors or one switch fails.

Regardless of the particular driving scheme, discharge circuitry <NUM> loads step-down power converter <NUM>. The instantaneous power dissipation by discharge circuitry <NUM> is related to the magnitude of the voltage difference between rails <NUM>, <NUM> and the instantaneous magnitude of the current(s) through resistances <NUM>, <NUM>, <NUM>, <NUM> and switches <NUM>, <NUM>.

Further, by appropriate selection of the components and their arrangement in the conduction path(s), some implementations of discharge circuitry <NUM> may be configured to remain operational and conduct current even when battery <NUM> is not disconnected from the DC link capacitor <NUM> by switch <NUM>. This may be beneficial in inadvertent circumstances, for example, if switch <NUM> were to malfunction. Even if such configurations of discharge circuitry <NUM> were to remain coupled to battery <NUM>, discharge circuitry <NUM> would not be overloaded by the constant power provided by step-down converter <NUM>. With the connection to battery <NUM> remaining, DC link capacitor <NUM> may not discharge. However, neither the step-down converter <NUM> nor discharge circuitry <NUM> need be damaged or destroyed.

Other implementations of discharge circuitry <NUM> are also possible. For example, each of the implementations shown in <FIG> can be used in a series or parallel combination with the other. Various other resistive network and switching configurations can also be used.

<FIG> is a schematic representation of components that participate in the discharge of the DC link capacitor <NUM> when battery <NUM> is decoupled from rails <NUM>, <NUM> by switch <NUM>, namely, the components of a control board <NUM>. In addition to discharging DC link capacitor <NUM>, the components of control board <NUM> can also diagnose the functionality of step-down power converter <NUM>, e.g., to ensure that step-down power converter <NUM> is available to discharge DC link capacitor <NUM>.

In more detail, control board <NUM> includes various control board electronics <NUM> that are in the low voltage domain. Control board electronics <NUM> can be a subset of components <NUM>. One or more sense lines <NUM>, <NUM>, <NUM>, <NUM> can be coupled between control board electronics <NUM> and various nodes on control board <NUM>. In various combinations, different sense lines <NUM>, <NUM>, <NUM>, <NUM> can provide indications about the supply of power to control board electronics <NUM> and proper functioning of step-down power converter <NUM>.

For example, sense lines <NUM>, <NUM> are coupled to nodes A, B, i.e., across a sense resistance <NUM> on rail <NUM> at the output of step-down power converter <NUM>. Sense line <NUM> provides information regarding the sensed voltage at node A to the control board electronics <NUM>. Sense line <NUM> provides information regarding the sensed voltage at node B to the control board electronics <NUM>. Current flow along rail <NUM> can be measured according to a voltage difference between sense lines <NUM>, <NUM> and indicate whether step-down power converter <NUM> is providing power to control board <NUM>.

As another example, sense lines <NUM>, <NUM> are coupled to nodes C, D, i.e., across a decoupling diode <NUM> that protects a battery (e.g., battery <NUM>) that supplies control board electronics <NUM> during normal operations. Sense line <NUM> provides information regarding the sensed voltage at node C to the control board electronics <NUM>. Sense line <NUM> provides information regarding the sensed voltage at node D to the control board electronics <NUM>. A comparison of the voltage on either of sense lines <NUM>, <NUM> with the voltage on sense line <NUM> indicates whether control board electronics <NUM> are supplied by battery <NUM> or by step-down power converter <NUM>.

As yet another example, the voltage on either or both of sense lines <NUM>, <NUM> can be used to identify a malfunction in step-down power converter <NUM>. For example, at a time when battery <NUM> supplies control board electronics <NUM>, if the voltage on either sense line <NUM>, <NUM> drops below a threshold level, step-down power converter <NUM> can be identified as malfunctioning. As yet another example, at a time when step-down power converter <NUM> supplies control board electronics <NUM>, if the voltage on sense line <NUM> drops below a threshold level, this too can used to identify that step-down power converter <NUM> is malfunctioning, i.e., to identify that step-down power converter <NUM> is incapable of supplying control board electronics <NUM> with sufficient current.

In operation, discharge circuitry <NUM> can be used to confirm proper functioning of step-down power converter <NUM>, e.g., during system checks at vehicle start-up or even periodically during operation of the vehicle. As discussed above, discharge circuitry <NUM> acts as a load across the output of step-down power converter <NUM> when driven into conduction by one or more discharge drive signals DR1, DR2,. Loading step-down power converter <NUM> with discharge circuitry <NUM> will generate a voltage difference between sense lines <NUM>, <NUM>. With the known load provided by discharge circuitry <NUM> and the voltage difference across sense resistance <NUM>, discharge circuitry <NUM> can be used to determine whether step-down power converter <NUM> is capable of supplying sufficient power to control board electronics <NUM> in the event of battery <NUM> failing. Further, since rail <NUM> includes a decoupling diode <NUM> that decouples discharge circuitry <NUM> from control board electronics <NUM>, the functioning of step-down power converter <NUM> can be confirmed even when control board electronics <NUM> are supplied by battery <NUM>. In particular, the voltage on the anode side of diode <NUM> (i.e., the voltage sensed by line <NUM>) can vary without impairing the supply of power to control board electronics <NUM> so long as the voltage on the anode side of diode <NUM> does not rise approximately one diode drop above the voltage sensed by line <NUM>.

<FIG> is a graphical representation <NUM> of the time course of various voltage signals during discharge of a DC link capacitor, e.g., DC link capacitor <NUM>. Representation <NUM> includes an x-axis <NUM>, a y-axis <NUM>, and three traces <NUM>, <NUM>, <NUM>. Position along x-axis <NUM> indicates time and is scaled uniformly for all three traces <NUM>, <NUM>, <NUM>. The illustrated duration of x-axis <NUM> between switching start and end of the signal <NUM> is in the range of the intended discharge time, as discussed previously. Position along y-axis <NUM> indicates voltage and is scaled differently for the different traces <NUM>, <NUM>, <NUM>.

Trace <NUM> represents the output voltage of step-down power converter <NUM>, i.e., the voltage across rails <NUM>, <NUM> in the vicinity step-down power converter <NUM> (e.g., on the anode side of diode <NUM>).

Trace <NUM> represents the voltage across a DC link capacitor, e.g., DC link capacitor <NUM>.

Trace <NUM> represents an example discharge drive signal DR1. As shown, trace <NUM> is a relatively high frequency pulse train and drives the receiving discharge circuity <NUM> into and out of conduction. Discharge circuity <NUM> thus alternates between loading step-down power converter <NUM> and dissipating resistive heating.

In the illustrated implementation, step-down power converter <NUM> switches from stand-by mode into operation at a time <NUM> in response to the pulses in the discharge drive signal DR1 shown trace <NUM>. Time <NUM> can be, e.g., the time when a shut down or fault signal is received by board electronics <NUM> or other components <NUM>. The output voltage of step-down power converter <NUM>--as represented by trace <NUM>--remains at a regulated output level <NUM>. Step-down power converter <NUM> draws power from the high voltage domain to maintain output level <NUM> even when discharge circuity <NUM> is loading its output.

Initially, the voltage across a DC link capacitor (as represented by trace <NUM>) will decrease generally exponentially with time at a constant draw of power by discharge circuity <NUM>. However, the voltage across a DC link capacitor will eventually approach the regulated output level <NUM> of step-down power converter <NUM>. Eventually, the voltage across DC link capacitor <NUM> will drop too far for step-down power converter <NUM> to maintain output level <NUM>. In any case, the voltage across DC link capacitor <NUM> can be reduced to an acceptably safe level. The specific voltage of the safe level will generally depend on the operational context. For example, in some contexts, the voltage across DC link capacitor <NUM> will be reduced below <NUM> volts, whereas in other contexts, the voltage across DC link capacitor <NUM> will be reduced below <NUM> volts.

In the illustrated implementation, at this time, the discharge drive signal DR1 represented by trace <NUM> stops driving discharge circuity <NUM> and discharge through step-down power converter <NUM> ends. The voltage across DC link capacitor <NUM> will continue to decrease-albeit at a slower rate-due to parasitic and other power consumption in the high voltage domain.

In other implementations, the discharge drive signal DR1 represented by trace <NUM> can either stop driving discharge circuity <NUM> sooner (i.e., before the voltage across DC link capacitor <NUM> becomes so low that step-down power converter <NUM> cannot regulate its output to output level <NUM>) or later (i.e., after step-down power converter <NUM> cannot regulate its output to output level <NUM>).

In some implementations, the discharge drive signal DR1 represented by trace <NUM> is not a continuous pulse train but a constant ON signal or rather, e.g., a PWM or frequency-modulated signal that varies with the voltage across DC link capacitor <NUM>.

A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made.

Example <NUM>. The discharge system of example <NUM>, wherein the discharge drive circuitry is configured to drive the discharge circuitry to intermittently load the step-down power converter.

Example <NUM>. The discharge system of example <NUM> or <NUM>, further comprising fault detection circuitry, wherein the discharge drive circuitry is configured to respond to identification of a fault by driving the discharge circuitry to load the step-down power converter.

Example <NUM>. The discharge system of any one of examples <NUM> to <NUM>, wherein the step-down power converter is a backup or emergency power converter that is coupled to supply the user interaction component with power in an event of a failure of a battery to supply the user interaction component with power.

Example <NUM>. The discharge system of any one of examples <NUM> to <NUM>, wherein the step-down power converter is configured to step down an input voltage between <NUM> to <NUM> volts to an output voltage of between <NUM> and <NUM> volts.

Example <NUM>. The discharge system of any one of examples <NUM> to <NUM>, wherein the discharge circuitry comprises a resistance coupled in series with a switch across the output of the step-down power converter.

Example <NUM>. An electric vehicle comprising:.

Example <NUM>. The electric vehicle of example <NUM>, further comprising a switch disposed to reversibly couple and decouple the electric motor coupled to receive power from a positive supply rail, wherein the switch is controlled by a control signal output from circuitry coupled to be supplied with power by the step-down power converter.

Example <NUM>. The electric vehicle of any one of examples <NUM> to <NUM>, further comprising:.

Example <NUM>. The electric vehicle of example <NUM>, wherein:.

Example <NUM>. The electric vehicle of example <NUM> or <NUM>, wherein:
the output of the step-down power converter and the output of the second battery are both coupled to voltage supply rails for electrical components of the electric vehicle.

Example <NUM>. The electric vehicle of example <NUM>, wherein the electrical components of the electric vehicle include one or more of on-board electronics, on-board sensors, head- lights, tail-lights, or a dashboard.

Example <NUM>. A discharge system for an electric vehicle, the discharge system comprising:.

Example <NUM>. The discharge system of example <NUM>, further comprising a decoupling diode disposed along a first of the supply rails, wherein the decoupling diode is disposed between a coupling of the discharge circuitry to the first of the supply rails and a coupling of the battery to the first of the supply rails.

Example <NUM>. The discharge system of any one of examples <NUM> to <NUM>, wherein the output voltage of the step-down power converter is lower than a sum of a nominal voltage of the battery and a diode drop of the decoupling diode.

Example <NUM>. The discharge system of any one of examples <NUM> to <NUM>, further comprising a sense line, wherein the sense line is coupled to the first of the supply rails between the output of the step-down power converter and the decoupling diode.

Example <NUM>. The discharge system of example <NUM>, wherein the sense line is further coupled to the electronics, wherein the electronics are configured to reversibly drive the discharge circuitry and, based on a voltage sensed over the sense line, determine if the step-down power converter is operational.

Example <NUM>. The discharge system of any one of examples <NUM> to <NUM>, wherein further comprising a second sense line, wherein the second sense line is coupled to an output of the battery.

Claim 1:
A discharge system for an electric vehicle that has two voltage domains, wherein a higher of the voltage domains is for providing power for driving an electric motor of the electric vehicle and a lower of the voltage domains is for providing power for components of the electric vehicle, the discharge system comprising:
a user interaction input component configured to receive input that originated from a human user, wherein the input indicates that the electric vehicle is to shutdown;
a step-down power converter (<NUM>) configured to step down an input voltage in the higher voltage domain to an output voltage in the lower voltage domain, wherein the input voltage is higher than the output voltage, wherein the step-down power converter (<NUM>) is a backup or emergency power converter that is coupled to output the output voltage across supply rails (<NUM>, <NUM>) in the lower voltage domain and supply the user interaction component with power in the lower voltage domain in an event of a failure of a battery (<NUM>) in the lower voltage domain to supply the user interaction component with power;
discharge circuitry (<NUM>) coupled to the output of the step-down power converter (<NUM>) between the supply rails (<NUM>, <NUM>) in the lower voltage domain, wherein the discharge circuitry is reversibly driveable to load the step-down power converter (<NUM>);
the battery (<NUM>) coupled across the supply rails (<NUM>, <NUM>) in the lower voltage domain; and
discharge drive circuitry (<NUM>) configured to drive the discharge circuitry to load the step-down power converter (<NUM>) in response to the input received by the user interaction input component indicating that the electric vehicle is to shutdown, wherein the discharge drive circuitry (<NUM>) comprises electronics (<NUM>) coupled to be supplied with power from the supply rails (<NUM>, <NUM>) in the low voltage domain, wherein the electronics include an output terminal coupled to provide a discharge drive signal (DR1) to the discharge circuitry, wherein the discharge circuitry is reversibly driveable by the discharge drive signal to allow current to flow between the supply rails in the lower voltage domain.