Patent Publication Number: US-2023158896-A1

Title: Active discharge of an electric drive system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European Patent Application No. EP21210185.1, filed on Nov. 24, 2021, currently pending. European Patent Application No. EP21210185.1 is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     This invention relates to the active discharge of an electric drive system. 
     BACKGROUND 
     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 dc 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  2  seconds to a voltage below 60 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. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic representation of portions of the electrical and drive system of an electric vehicle. 
         FIG.  2    is a schematic representation of components that participate in the discharge of a DC link capacitor. 
         FIGS.  3 - 5    are schematic representations of different implementations of discharge circuitry. 
         FIG.  6    is a schematic representation of components that participate in the discharge of a DC link capacitor. 
         FIG.  7    is a graphical representation of the time course of various voltage signals during discharge of a DC link capacitor. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     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. Furthermore, the particular features, structures or characteristics may be combined in any suitable combinations and/or subcombinations in one or more embodiments or examples. 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 200 and 1200 Volts), whereas the battery for a lower voltage domain may have a nominal voltage of between 5 and 50 volts. For example, the battery for a lower voltage domain may be a 12.6 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.  1    is a schematic representation of portions of the electrical and drive system  100  of an electric vehicle. Electrical and drive system  100  includes a high voltage domain that is powered by a primary battery  105  and a low voltage domain that is generally powered by a low voltage battery  110 . The electric drive system is in the high voltage domain and includes primary battery  105  reversibly coupled and decoupled between a high rail  115  and a low rail  120  by a switch  125 . The electric drive system also includes an inverter  130  and an electric motor  135 . In operation, under the direction of control electronics, inverter  130  converts the dc voltage supplied by battery  105  into an ac voltage and supplies electric motor  135  with power. ADC link capacitor  140  is coupled between rails  115 ,  120 . However, it should be appreciated that the DC link capacitor  140  may be included in the inverter  130 , as illustrated by the thick dashed lines. 
     Other components  145  of the vehicle are in the low voltage domain. Examples of components  145  can include on-board electronics and sensors, head- and tail-lights, the dashboard, and/or other components. In general, components  145  are powered by battery  110  coupled between rails  155 ,  160 . In some implementations, battery  110  is a 12.6 V automotive battery. In general, rails  115 ,  120  in the high voltage domain are isolated from rails  155 ,  160  in the low voltage domain. However, in some vehicle types, it may be possible for rail  160  to be coupled to rail  120  and for batteries  105 ,  110  to share a common return. 
     A step-down power converter  150  interfaces between the high voltage domain and the low voltage domain in that it is configured to convert the high voltage across rails  115 ,  120  into a lower voltage across rails  155 ,  160 . For example, in some implementations, step-down power converter  150  may convert 200 to 1200 volts in the high voltage domain to an output voltage of between 5 and 50 volts in the low voltage domain. In general, but not necessarily, power converter  150  does not convert power continuously during operation of the vehicle. Rather, power converter  150  operates as a backup or emergency power supply and only converts power in selected circumstances, e.g., in the event that battery  110  fails or discharges. Power converter  150  can be implemented in a number of different ways using any of a number of different power converter topologies. For example, power converter  150  can be implemented as an isolated flyback converter. 
     As discussed further below, power converter  150  also participates in the discharge of the DC link capacitor when battery  105  is decoupled from rails  115 ,  120  by switch  125 . 
     Switches  125  are either mechanical or solid state switches and coupled to connect and disconnect battery  105  from rails  115 ,  120 . Under normal conditions, battery  105  will be connected to rails  115 ,  120  when the vehicle that includes electrical and drive system  100  is in operation, e.g., moving or ready to move. Battery  105  will be disconnected from rails  115 ,  120  during shut-off or in the event of a sufficiently severe fault condition. 
     Upon connection of battery  105  to rails  115 ,  120 , both DC link capacitor  140  and inverter  130  will be biased by battery  105 . The voltage developed across DC link capacitor  140  will tend towards equality with the voltage provided by battery  105 . However, deviations from equality will occur since DC link capacitor  140  accepts and provides charge more quickly than battery  105 . In addition, the DC link capacitor  140  is generally placed physically closer to the power switches of inverter  130  and some distance from the battery  105 . The cable inductance could lead to transient voltage events. DC link capacitor  140  thus acts to smooth the voltage between rails  115 ,  120  across inverter  130 . 
     Inverter  130  can include a collection of phase legs that are each formed by a pair of switching devices coupled in series between rails  115 ,  120 . 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  130  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  120 , but the controller can be in either or both of the high voltage and low voltage domains. 
       FIG.  2    is a schematic representation of components that participate in the discharge of the DC link capacitor  140  when battery  105  is decoupled from rails  115 ,  120  by switch  125 . In addition to components that have already been described, discharge circuitry  205  is also illustrated. Further, low voltage domain components  145  include a control signal output  210  and a discharge drive signal output  215 . 
     Discharge circuitry  205  is coupled between rails  155 ,  160  in the low voltage domain. In the illustrated implementation, rails  155 ,  160  are coupled to terminals KL 30 , KL 31  under the DIN 72552 standard, with the positive terminal of battery  110  protected from reverse biasing by a decoupling diode. As discussed further below, discharge circuitry  205  can be implemented in a variety of different ways. Regardless of the particular implementation, when driven by a discharge drive signal DR 1 , discharge circuitry  205  acts as a load on the output of step-down power converter  150 . In other words, the discharge circuit  205  is coupled to the output of the step-down power converter  150 . During this time, step-down power converter  150  draws power from the high voltage domain to supply discharge circuitry  205 —and discharge DC link capacitor  140 . 
     In particular, in operation, a control signal triggers switch  125  to disconnect primary battery  105  from high voltage supply rails  115 ,  120 . 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  145  in the low voltage domain and output over a control signal output  210 . 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  125  to disconnect primary battery  105  from high voltage supply rails  115 ,  120 . 
     Components  145  can also output one or more discharge drive signals DR 1 , DR 2 , . . . over one or more discharge drive signal outputs  215 . It should be appreciated that the additional discharge drive signals DR 2  . . . DRN are optional and shown in dashed lines. The discharge drive signals DR 1 , DR 2 , DRN trigger discharge circuitry  205  to load the output of step-down power converter  150  and thereby draw power from the high voltage domain. In general, discharge drive signals DR 1 , DR 2 , . . . trigger discharge circuitry  205  after the control signal triggers switch  125  to disconnect primary battery  105  from high voltage supply rails  115 ,  120 . For example, discharge circuitry  205  can be triggered some microseconds to milliseconds after switch  125  is opened. However, in some applications, discharge circuitry  205  can be triggered seconds after switch  125  is opened. In some implementations, the control signal that opens switch  125  and one or more of discharge drive signals DR 1 , DR 2 , DRN are the same signal and output over a single terminal. 
     In response to discharge drive signals DR 1 , DR 2 , DRN, discharge circuitry  205  loads the output of step-down power converter  150 . Step-down power converter  150  draws power that was stored on DC link capacitor  140  (and possibly elsewhere in the high voltage domain) to supply discharge circuitry  205 . By drawing power from DC link capacitor  140 , step-down power converter  150  discharges DC link capacitor  140 , e.g., to levels sufficient to meet regulatory requirements. 
     As discussed above, discharge drive signals DR 1 , DR 2 , DRN generally trigger discharge circuitry  205  after the control signal triggers switch  125  to disconnect primary battery  105  from high voltage supply rails  115 ,  120 . However, even if by happenstance discharge drive signals DR 1 , DR 2 , DRN were to trigger discharge circuitry  205  while primary battery  105  is still inadvertantly connected to high voltage supply rails  115 ,  120 , this failure would not cascade. In more detail, step-down power converter  150  inherently limits the power that is supplied to discharge circuitry  205  and other components in the low voltage domain. Even if primary battery  105  remains connected to high voltage supply rails  115 ,  120  while discharge circuitry  205  attempts to discharge, step-down power converter  150  will limit the power provided to the low voltage domain and reduce the chance that discharge circuitry  205  and other elements are damaged. 
       FIG.  3    is a schematic representation of one implementation of discharge circuitry  205 . The illustrated implementation of discharge circuitry  205  includes a resistance  305  and switch  310  coupled in series between rails  155 ,  160 . The control terminal of switch  310  is coupled to be driven by a single discharge drive signal DR 1 . In the illustrated implementation of discharge circuitry  205 , switch  310  is shown as an NMOS transistor. 
     Other implementations using other transistor devices are possible—both for this implementation of discharge circuitry  205  and for the other implementations discussed below. In any case, the resistance of switch  310  in the on state is much smaller than the magnitude of the associated resistance  305 . 
     In operation, discharge drive signal DR 1  drives switch  310  into conduction and current is conducted through resistance  305 . In many implementations, discharge drive signal DR 1  drives switch  310  intermittently. For example, discharge drive signal DR 1  can be a pulse train that drives switch  310  into and out of conduction repeatedly, thereby avoiding excessive resistive heating of resistance  305 . Alternatively, resistance  305  can be configured to withstand continuous resistive heating. In any case, discharge circuitry  205  loads step-down power converter  150  and dissipates power according to the magnitude of the voltage difference between rails  155 ,  160  and the magnitude of the current flow through resistance  305  and switch  310 . 
       FIG.  4    is a schematic representation of another implementation of discharge circuitry  205 . The illustrated implementation of discharge circuitry  205  includes resistances  405 ,  410  and switches  415 ,  420 . Resistance  405  and switch  415  are connected in series to form a first conduction path between rails  155 ,  160 . Resistance  410  and switch  420  are connected in series to form a second conduction path between rails  155 ,  160 . The control terminal of switch  415  is coupled to be driven by a first discharge drive signal DR 1 . The control terminal of switch  420  is coupled to be driven by a second discharge drive signal DR 2 . The resistance of each switch  415 ,  420  when in the on state is much smaller than the magnitude of the associated resistance  405 ,  410 . 
     In operation, discharge drive signals DR 1 , DR 2  drive switches  415 ,  420  into conduction and current is conducted through discharge circuitry  205 —either continuously or intermittently. Further, switches  415 ,  420  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  415 ,  420  are driven at times to conduct simultaneously and at times alternatively—are also possible. 
     When switches  415 ,  420  are driven to conduct simultaneously, the net current through discharge circuitry  205  is higher, step-down power converter  150  will draw more power from DC link capacitor  140 , and DC link capacitor  140  will be discharged relatively more quickly. When switches  415 ,  420  are driven to conduct alternatively, power will be drawn from DC link capacitor  140  more slowly. However, resistive heating of resistances  405 ,  410  can be reduced. For example, assume that resistance  405  conducts for a first duration before switch  415  is switched out of conduction. Without current flow through switch  415  and resistance  405 , resistance  405  can cool—even while current is conducted by switch  420  and resistance  410 . Corresponding cooling of resistance  410  can be achieved by switching switch  420  out of conduction. In another example, both switches  415 ,  420  are driven to conduct simultaneously and the value of the resistances  405 ,  410  can be reduced as compared to resistance  305  of  FIG.  3   . For example, if both switches  415 ,  420  are driven to conduct simultaneously, the value of resistances  405 ,  410  can be substantially 50% of the value of resistance  305  of  FIG.  3    to conduct the equivalent power of the example discharge circuit shown in  FIG.  3   . 
     Furthermore, the shown implementation provides a redundancy for conduction/discharge path in case one path (combination of  405 / 415  or  410 / 420 ) fails. 
     Regardless of the particular driving scheme, discharge circuitry  205  loads step-down power converter  150 . The instantaneous power dissipation by discharge circuitry  205  is related to the magnitude of the voltage difference between rails  155 ,  160  and the instantaneous magnitude of the current(s) through resistances  405 ,  410  and switches  415 ,  420 . 
       FIG.  5    is a schematic representation of another implementation of discharge circuitry  205 . The illustrated implementation of discharge circuitry  205  includes resistances  505 ,  510 ,  515 ,  520 , switches  525 ,  530 , and a bridge node  535 . Resistances  505 ,  510  are both coupled between rail  155  and bridge node  535 . Resistance  515  and switch  525  are coupled to form a first conduction path between bridge node  535  and rail  160 . Resistance  520  and switch  530  are coupled to form a second conduction path between bridge node  535  and rail  160 . The control terminal of switch  525  is coupled to be driven by a first discharge drive signal DR 1 . The control terminal of switch  530  is coupled to be driven by a second discharge drive signal DR 2 . The resistance of each switch  525 ,  530  in the on state is much smaller than the magnitude of each of resistances  505 ,  510 ,  515 ,  520 . 
     In operation, discharge drive signals DR 1 , DR 2  drive switches  525 ,  530  into conduction and current is conducted through discharge circuitry  205 —either continuously or intermittently. Further, switches  525 ,  530  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  525 ,  530  are driven at times to conduct simultaneously and at times alternatively—are also possible. 
     When switches  525 ,  530  are driven to conduct simultaneously, current will flow through resistances  505 ,  510 ,  515 ,  520  in accordance with the ratios of their respective resistances. However, when only one of switches  525 ,  530  is driven to conduct, current will continue to flow through both resistances  505 ,  510  but exclusively through the respective one of resistances  515 ,  520 . Once again, a respective one of resistances  515 ,  520  can be provided with time to cool if needed. For example, both switches  525 ,  530  are driven to conduct simultaneously and the value of the resistances  505 ,  510 ,  515 ,  520  can be reduced as compared to resistance  305  of  FIG.  3   . For example, if both switches  525 ,  530  are driven to conduct simultaneously, the value of resistances  505 ,  510 ,  515 ,  520  can be substantially 25% of the value of resistance  305  of  FIG.  3    to conduct the equivalent power of the example discharge circuit shown in  FIG.  3   . 
     Furthermore, the shown implementation increases the level of redundancy compared to  FIG.  3    and  FIG.  4    and provides alternative discharge paths in case up to two resistors or one switch fails. 
     Regardless of the particular driving scheme, discharge circuitry  205  loads step-down power converter  150 . The instantaneous power dissipation by discharge circuitry  205  is related to the magnitude of the voltage difference between rails  155 ,  160  and the instantaneous magnitude of the current(s) through resistances  505 ,  510 ,  515 ,  520  and switches  525 ,  530 . 
     Further, by appropriate selection of the components and their arrangement in the conduction path(s), some implementations of discharge circuitry  205  may be configured to remain operational and conduct current even when battery  105  is not disconnected from the DC link capacitor  140  by switch  125 . This may be beneficial in inadvertent circumstances, for example, if switch  125  were to malfunction. Even if such configurations of discharge circuitry  205  were to remain coupled to battery  105 , discharge circuitry  205  would not be overloaded by the constant power provided by step-down converter  150 . With the connection to battery  105  remaining, DC link capacitor  140  may not discharge. However, neither the step-down converter  150  nor discharge circuitry  205  need be damaged or destroyed. 
     Other implementations of discharge circuitry  205  are also possible. For example, each of the implementations shown in  FIGS.  3 - 5    can be used in a series or parallel combination with the other. Various other resistive network and switching configurations can also be used. 
       FIG.  6    is a schematic representation of components that participate in the discharge of the DC link capacitor  140  when battery  105  is decoupled from rails  115 ,  120  by switch  125 , namely, the components of a control board  600 . In addition to discharging DC link capacitor  140 , the components of control board  600  can also diagnose the functionality of step-down power converter  150 , e.g., to ensure that step-down power converter  150  is available to discharge DC link capacitor  140 . 
     In more detail, control board  600  includes various control board electronics  605  that are in the low voltage domain. Control board electronics  605  can be a subset of components  145 . One or more sense lines  610 ,  615 ,  620 ,  625  can be coupled between control board electronics  605  and various nodes on control board  600 . In various combinations, different sense lines  610 ,  615 ,  620 ,  625  can provide indications about the supply of power to control board electronics  605  and proper functioning of step-down power converter  150 . 
     For example, sense lines  610 ,  615  are coupled to nodes A, B, i.e., across a sense resistance  630  on rail  155  at the output of step-down power converter  150 . Sense line  610  provides information regarding the sensed voltage at node A to the control board electronics  605 . Sense line  615  provides information regarding the sensed voltage at node B to the control board electronics  605 . Current flow along rail  155  can be measured according to a voltage difference between sense lines  610 ,  615  and indicate whether step-down power converter  150  is providing power to control board  600 . 
     As another example, sense lines  620 ,  625  are coupled to nodes C, D, i.e., across a decoupling diode  635  that protects a battery (e.g., battery  110 ) that supplies control board electronics  605  during normal operations. Sense line  620  provides information regarding the sensed voltage at node C to the control board electronics  605 . Sense line  625  provides information regarding the sensed voltage at node D to the control board electronics  605 . A comparison of the voltage on either of sense lines  620 ,  625  with the voltage on sense line  615  indicates whether control board electronics  605  are supplied by battery  110  or by step-down power converter  150 . 
     As yet another example, the voltage on either or both of sense lines  610 ,  615  can be used to identify a malfunction in step-down power converter  150 . For example, at a time when battery  110  supplies control board electronics  605 , if the voltage on either sense line  610 ,  615  drops below a threshold level, step-down power converter  150  can be identified as malfunctioning. As yet another example, at a time when step-down power converter  150  supplies control board electronics  605 , if the voltage on sense line  615  drops below a threshold level, this too can used to identify that step-down power converter  150  is malfunctioning, i.e., to identify that step-down power converter  150  is incapable of supplying control board electronics  605  with sufficient current. 
     In operation, discharge circuitry  205  can be used to confirm proper functioning of step-down power converter  150 , e.g., during system checks at vehicle start-up or even periodically during operation of the vehicle. As discussed above, discharge circuitry  205  acts as a load across the output of step-down power converter  150  when driven into conduction by one or more discharge drive signals DR 1 , DR 2 , . . . Loading step-down power converter  150  with discharge circuitry  205  will generate a voltage difference between sense lines  610 ,  615 . With the known load provided by discharge circuitry  205  and the voltage difference across sense resistance  630 , discharge circuitry  205  can be used to determine whether step-down power converter  150  is capable of supplying sufficient power to control board electronics  605  in the event of battery  110  failing. Further, since rail  155  includes a decoupling diode  640  that decouples discharge circuitry  205  from control board electronics  605 , the functioning of step-down power converter  150  can be confirmed even when control board electronics  605  are supplied by battery  110 . In particular, the voltage on the anode side of diode  640  (i.e., the voltage sensed by line  615 ) can vary without impairing the supply of power to control board electronics  605  so long as the voltage on the anode side of diode  640  does not rise approximately one diode drop above the voltage sensed by line  620 . 
       FIG.  7    is a graphical representation  700  of the time course of various voltage signals during discharge of a DC link capacitor, e.g., DC link capacitor  140 . Representation  700  includes an x-axis  705 , a y-axis  710 , and three traces  715 ,  720 ,  725 . Position along x-axis  705  indicates time and is scaled uniformly for all three traces  715 ,  720 ,  725 . The illustrated duration of x-axis  705  between switching start and end of the signal  725  is in the range of the intended discharge time, as discussed previously. Position along y-axis  705  indicates voltage and is scaled differently for the different traces  715 ,  720 ,  725 . 
     Trace  715  represents the output voltage of step-down power converter  150 , i.e., the voltage across rails  155 ,  160  in the vicinity step-down power converter  150  (e.g., on the anode side of diode  640 ). 
     Trace  720  represents the voltage across a DC link capacitor, e.g., DC link capacitor  140 . 
     Trace  725  represents an example discharge drive signal DR 1 . As shown, trace  725  is a relatively high frequency pulse train and drives the receiving discharge circuity  205  into and out of conduction. Discharge circuity  205  thus alternates between loading step-down power converter  150  and dissipating resistive heating. 
     In the illustrated implementation, step-down power converter  150  switches from stand-by mode into operation at a time  730  in response to the pulses in the discharge drive signal DR 1  shown trace  725 . Time  730  can be, e.g., the time when a shut down or fault signal is received by board electronics  605  or other components  145 . The output voltage of step-down power converter  150 —as represented by trace  715 —remains at a regulated output level  735 . Step-down power converter  150  draws power from the high voltage domain to maintain output level  735  even when discharge circuity  205  is loading its output. 
     Initially, the voltage across a DC link capacitor (as represented by trace  720 ) will decrease generally exponentially with time at a constant draw of power by discharge circuity  205 . However, the voltage across a DC link capacitor will eventually approach the regulated output level  735  of step-down power converter  150 . Eventually, the voltage across DC link capacitor  140  will drop too far for step-down power converter  150  to maintain output level  735 . In any case, the voltage across DC link capacitor  140  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  140  will be reduced below 60 volts, whereas in other contexts, the voltage across DC link capacitor  140  will be reduced below 30 volts. 
     In the illustrated implementation, at this time, the discharge drive signal DR 1  represented by trace  725  stops driving discharge circuity  205  and discharge through step-down power converter  150  ends. The voltage across DC link capacitor  140  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 DR 1  represented by trace  725  can either stop driving discharge circuity  205  sooner (i.e., before the voltage across DC link capacitor  140  becomes so low that step-down power converter  150  cannot regulate its output to output level  735 ) or later (i.e., after step-down power converter  150  cannot regulate its output to output level  735 ). 
     In some implementations, the discharge drive signal DR 1  represented by trace  725  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  140 . 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. Accordingly, other implementations are within the scope of the following claims.