System and method for measuring switching loss associated with semiconductor switching devices

Abstract

Method comprising: a.) Operating a circuit of a semiconductor switching device (62) in a conduction cycle; wherein the conduction cycle comprises: - charging an inductor (70) with energy from a capacitor (68); - letting the inductor (70) run freely in either an upper bridge (86) or a lower bridge (88) of the circuit, and - discharging the energy from the inductor (70) into the capacitor (68); and Calculating a conduction loss associated with the circuit; b.) Operating the circuit in a conduction and switching cycle; wherein the conduction and switching cycle comprises: - charging the inductor (70) with energy from the capacitor (68); - alternately allowing the inductor (70) to run freely between the upper bridge (86) and the lower bridge (88) of the circuit, and - discharging the energy from the inductor (70) into the capacitor (68); and Calculating a combined conduction and switching loss associated with the circuit; c.) Subtract the conduction loss from the combined conduction and switching loss to calculate a switching loss of the circuit.

Description

This disclosure relates to a semiconductor switching device and, more particularly, but not exclusively, to a system and method for measuring switching loss associated with one or more switching units of a semiconductor switching device.Hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs), fuel cell vehicles, and other known electrically operated vehicles differ from conventional motor vehicles in that they are powered by one or more electric machines (i.e., electric motors and / or generators) instead of or in addition to an internal combustion engine. Typically, high voltage current is supplied to the electric machines by one or more batteries that store electrical power.Semiconductor switching devices are known to support bidirectional power flow in many products. Switching units of the semiconductor device perform switching operations according to a drive signal generated by a controller to control a load. For example, electrically powered vehicles often include inverter systems having a plurality of semiconductor switching units, such as power MOSFETs or insulated gate bipolar transistors (IGBTs), that undergo switching operations to power one or more AC drive motors from a AC storage battery or, alternatively, charge the AC storage battery from a AC source, such as a generator.It may become necessary to calculate switching losses associated with the switching units of a semiconductor switching device. For example, switching loss information may be important for controlling other vehicle systems and operations.DE 10 2005 036 317 A1 teaches a method for determining the power loss of an electronic switch, in which the current flowing through the switch is measured directly and multiplied by a previously known collector-emitter voltage.US 2010 / 0225290 A1 describes a critical-conduction resonant-transient boost power circuit having improved efficiency. The improved efficiency is achieved by providing a measurement circuit that measures when the current in the coil of the boost power circuit is near zero or when the voltage at its switching node is near zero.U.S. Pat. No. 9,043,066 B2 discloses a vehicle having an electric drive device and a discharge circuit and a control method therefor.There is therefore a need to eliminate or at least reduce the disadvantages of known systems without having to enter into compromise to the extent of known solutions. In particular, there is a need to provide a method for detecting switching losses in a semiconductor switching device.The object is achieved by the subject matter of the independent claim. Advantageous embodiments are specified in the dependent patent claims and the following description.A method according to an exemplary aspect of the present disclosure may include, among other things, controlling a vehicle using switching loss information of a semiconductor switching device, wherein the switching loss information is derived from a conduction loss and a combined conduction and switching loss.In a further non-limiting embodiment of the foregoing method, the step of controlling includes modifying an amount of thermal cooling transferred to cool the semiconductor switching device.In a further non-limiting embodiment of either of the foregoing methods, the method includes calculating the conduction loss and the combined conduction and switching loss by charging an inductor with energy from a capacitor, performing a plurality of switching cycles, and discharging the energy from the inductor into the capacitor.In a further non-limiting embodiment of any of the foregoing methods, the method includes measuring voltages and currents associated with the capacitor and the inductor during each of the charging, performing, and discharging steps.In a further non-limiting embodiment of any of the foregoing methods, the method comprises deriving the switching loss information by subtracting the conduction loss from the combined conduction and switching loss.A method according to another exemplary aspect of the present disclosure includes, among other things, operating a circuit of a semiconductor switching device in a conduction cycle, calculating a conduction loss associated with the circuit, operating the circuit in a conduction and switching cycle, calculating a combined conduction and switching loss associated with the circuit, and subtracting the conduction loss from the combined conduction and switching loss to calculate a switching loss of the circuit.The conduction cycle preferably includes charging an inductor with energy from a capacitor, free-wheeling the inductor in either an upper bridge or a lower bridge of the circuit, and discharging the energy from the inductor into the capacitor.The conduction and switching cycle preferably includes charging an inductor with energy from a capacitor, alternatively freewheeling the inductor between an upper bridge and a lower bridge of the circuit, and discharging the energy from the inductor into the capacitor.In a further non-limiting embodiment of the foregoing method, performing each of the conduction cycle and the conduction and switching cycle includes charging an inductor with energy from a capacitor, performing a plurality of switching cycles, and discharging the energy from the inductor into the capacitor.In a further non-limiting embodiment of either of the foregoing methods, the charging step includes switching a first switching unit and a second switching unit of the circuit between ON and OFF and measuring a voltage across the capacitor before and after the switching step.In a further non-limiting embodiment of any of the foregoing methods, the performing step includes alternately freewheeling the inductor between a top bridge and a bottom bridge of the circuit.In a further non-limiting embodiment of any of the foregoing methods, the alternately freewheeling the inductor includes alternating between switching a first switching unit to ON and OFF for freewheeling in the upper bridge and switching a second switching unit to ON and OFF for freewheeling in the lower bridge.In a further non-limiting embodiment of any of the foregoing methods, the method includes measuring voltages and currents associated with the capacitor and the inductor during each of the charging, performing, and discharging steps.In a further non-limiting embodiment of any of the foregoing methods, the method includes deriving the switching loss based on the voltages and currents measured during each of the charging, performing, and discharging steps.In a further non-limiting embodiment of any of the foregoing methods, the discharging step comprises turning a first diode and a second diode ON.A semiconductor switching device according to an exemplary aspect of the present disclosure includes, among other things, a switching loss measurement system including a first measurement device configured to measure a voltage of a first energy storage device of a semiconductor circuit, a second measurement device configured to measure a current of a second energy storage device of the semiconductor circuit, and a control unit configured to derive a switching loss associated with the semiconductor circuit based on voltage and current inputs from the first and second measurement devices.In a further non-limiting embodiment of the foregoing apparatus, the controller is configured to communicate the switching loss to an electrified vehicle control system.In a further non-limiting embodiment of either of the foregoing devices, the semiconductor circuit includes a plurality of switching units configured in an H-bridge arrangement.In a further non-limiting embodiment of any of the foregoing devices, the first energy storage device is a capacitor and the second energy storage device is an inductor.In a further non-limiting embodiment of any of the foregoing devices, the controller is configured to operate the semiconductor circuit in each of a conduction cycle and a conduction and switching cycle to calculate the switching loss.The embodiments, examples and alternatives of the preceding paragraphs, the claims and the following description, and the drawings, including any of their various aspects or respective individual features, may be taken independently or in any combination. Features described in connection with an embodiment may be applied to all embodiments, provided such features are not incompatible.The various features and advantages of this disclosure will become apparent to those skilled in the art from the following detailed description. The drawings attached to the detailed description may be briefly described as follows. FIG. 1 schematically illustrates a powertrain of an electrified vehicle. FIG. 2 illustrates a topology of a circuit of a semiconductor switching device. FIG. 3 schematically illustrates a switching loss measurement system for measuring switching losses associated with one or more switching units of a semiconductor switching device. FIG. 4A illustrates a first phase of an operation cycle of a circuit of a semiconductor switching device. FIG. 4B is a graphical representation of current and voltage profiles of energy storage devices of a semiconductor switching device during the first phase of an operating cycle of a circuit. FIG. 5A illustrates a first portion of a second phase of an operating cycle of a circuit. FIG. 5B illustrates a second portion of a second phase of an operating cycle of a circuit. FIG. 5C is a graphical representation of current and voltage profiles of energy storage devices of a semiconductor switching device during the second phase of an operating cycle of a circuit. FIG. 6A illustrates a third phase of an operating cycle of a circuit. FIG. 6B is a graphical representation of current and voltage profiles of energy storage devices of a semiconductor switching device during the third phase of an operating cycle of a circuit. FIG. 7 is a graph showing a conduction cycle and a conduction and switching cycle of a circuit of a semiconductor switching device.This disclosure relates to a system and method for measuring switching loss associated with one or more switching units of a semiconductor switching device. An H-bridge switching topology may be operated at various predetermined switching frequencies, duty cycles, and operating currents and voltages to measure switching loss. The circuit of the semiconductor switching device may be operated in a conduction cycle and a conduction and switching cycle to determine a conduction loss and a combined conduction and switching loss of the semiconductor device. The switching loss is calculated by subtracting the conduction loss from the combined conduction and switching loss. The switching loss information may be used for controlling a vehicle system or operation. These and other features are discussed in more detail herein.FIG. 1 schematically illustrates a powertrain 10 for an electrically operated vehicle 12, such as an HEV. It should be understood that although depicted as HEVs, the concepts described herein are not limited to HEVs, but could be extended to other electrified vehicles including, but not limited to, PHEVs, BEVs, and fuel cell vehicles.In one embodiment, the powertrain 10 is a power distribution system employing a first drive system including a combination of an engine 14 and a generator 16 (i.e., a first electric machine) and a second drive system including at least a motor 36 (i.e., a second electric machine), the generator 16, and a battery 50. For example, the motor 36, the generator 16, and the battery 50 may form a first electric drive system 25 of the powertrain 10. The first and second drive systems generate torque for driving one or more sets of vehicle drive wheels 30 of the electrified vehicle 12, as discussed in greater detail below.The engine 14, such as an internal combustion engine, and the generator 16 may be connected by a power transfer unit 18. In a non-limiting embodiment, the power transfer unit 18 is a planetary gear set. Of course, other types of power transfer units including other gear sets and transmissions may be used to connect the engine 14 to the generator 16. The power transfer unit 18 may include a ring gear 20, a sun gear 22, and a carrier assembly 24. The generator 16 is driven by the power transmission unit 18 when it functions as a generator for converting kinetic energy into electric energy. The generator 16 may alternatively function as a motor for converting electrical energy to kinetic energy to thereby output torque to a shaft 26 connected to the carrier assembly 24 of the power transfer unit 18. Because the generator 16 is operatively connected to the engine 14, the speed of the engine 14 may be controlled by the generator 16.The ring gear 20 of the power transfer unit 18 may be connected to a shaft 28 that is connected to the vehicle drive wheels 30 through a second power transfer unit 32. The second power transmission unit 32 may include a gear set including a plurality of gears 34A, 34B, 34C, 34D, 34E, and 34F. Other power transmission units may also be suitable. The gears 34A- 34F transfer torque from the engine 14 to a differential 38 to provide traction to the vehicle drive wheels 30. The differential 38 may include a plurality of gears that enable the transmission of torque to the vehicle drive wheels 30. The second power transfer unit 32 is mechanically coupled to an axle 40 through the differential 38 to distribute torque to the vehicle drive wheels 30.Also, the motor 36 may be utilized to drive the vehicle drive wheels 30 by outputting torque to a shaft 46 also connected to the second power transfer unit 32. In one embodiment, the motor 36 and the generator 16 are part of a regenerative braking system in which both the motor 36 and the generator 16 may be employed as motors for outputting torque. For example, the motor 36 and the generator 16 may each output electrical power to a high voltage bus 48 and the battery 50. The battery 50 may be a high voltage battery capable of outputting electric power for operating the motor 36 and the generator 16. Other types of energy storage devices and / or output devices for use with the electrified vehicle 12 may also be included.The engine 36, the generator 16, the power transfer unit 18, and the power transfer unit 32 may be generally referred to as a transaxle 42 or a transmission of the electrified vehicle 12. Therefore, when a driver selects a particular gear position, the transaxle 42 is adequately controlled to provide the corresponding gear for propelling the electrified vehicle 12 by providing traction to the drive wheels 30.The powertrain 10 may also include a control system 44 for monitoring and / or controlling various aspects of the electrified vehicle 12. For example, the control system 44 may communicate with the electric drive system 25, the power transfer units 18, 32, or other components to monitor and / or control the electrified vehicle 12. The control system 44 includes electronics and / or software for performing the control functions necessary for operation of the electrified vehicle 12. In one embodiment, the control system 44 is a combination vehicle system controller and vehicle system controller / powertrain control module (VSC / PCM). Although shown as a single hardware device, the control system 44 may include multiple controllers in the form of multiple hardware devices or multiple software controllers within one or more hardware devices.A controller area network (CAN) 52 allows the control system 44 to communicate with the transaxle 42. For example, the control system 44 may receive signals from the transaxle 42 to indicate whether a transition between gear positions is occurring. The control system 44 may also communicate with a battery control module of the battery 50 or other control devices.In addition, the electric drive system 25 may include one or more controllers 54, such as an inverter system controller (ISC). The controller 54 is configured to control specific components within the transaxle 42 such as the generator 16 and / or the motor 36, for example, to assist bidirectional power flow. In one embodiment, the controller 54 is an inverter system controller combined with an inverter system controller / variable voltage converter (ISC / VVC).FIG. 2 illustrates a circuit 60 of a semiconductor switching device 62. in one embodiment, the semiconductor switching device 62 is part of an inverter system for an electrified vehicle, such as the electrified vehicle 12 of FIG. 1. It should be appreciated that the example semiconductor switching device 62 may alternatively be used as part of a battery charging system, a switched mode power supply, an industrial drive, a household appliance, or any other device that uses semiconductor switching devices.The semiconductor switching device 62 includes a plurality of switching units 64 and diodes 66. In a non-limiting embodiment, the switching units 64 and the diodes 66 are configured as IGBT / diode pairs. However, other configurations are also contemplated.The circuit 60 may be configured in an H-bridge arrangement that includes an upper bridge 86 and a lower bridge 88. Each of the upper bridge 86 and the lower bridge 88 may include two pairs of switching units 64 and diodes 66.In a non-limiting embodiment, the upper bridge 88 includes a first switching unit 64-1 (also referred to as IGBT1), a first diode 66-1, a second switching unit 64-2 (also referred to as IGBT2), and a second diode 66-2, and the lower bridge 88 includes a third switching unit 64-3 (also referred to as IGBT3), a third diode 66-3, a fourth switching unit 64-4 (also referred to as IGBT4), and a fourth diode 66-4. In one embodiment, the first switching unit 64-1 and the fourth switching unit 64-4 are configured as active switches, while the diodes 66-2 and 66-3 are configured as passive switches.The semiconductor switching device 62 may also include a capacitor 68 (i.e., a first energy storage device) and an inductor 70 (i.e., a second energy storage device). In one embodiment, capacitor 68 is a near-ideal capacitor or a fixed foil capacitor that has a predetermined amount of energy and voltage in its initial (stationary) stage. A voltage source 69 supplies the capacitor 68 with energy. In a non-limiting embodiment, the voltage source 69 is a high voltage battery of an electrified vehicle (see, for example, battery 50 of FIG. 1 ).As discussed in more detail below, an example method for measuring switching loss may include operating the circuit 60 by transferring the energy from the capacitor 68 to the inductor 70, performing a series of switching cycles (switching state), and returning the energy to the capacitor 68. The voltage difference (ΔV) across the capacitor 68 between the initial and final states may provide a numerical energy loss value with respect to combined switching, conduction, and leakage. A corresponding test may be performed to duplicate the current profile in the inductor 70, which may be used in conjunction with the combined switching, conduction, and leakage to determine a total switching loss associated with the semiconductor switching device 62.Referring to FIG. 3, a switching loss measurement system 72 may be connected to the switching loss measurement circuit 60 associated with one or more switching units 64 of the semiconductor switching device 62. The switching loss measurement system 72 may include a controller 74, a first measurement device 76, and a second measurement device 78 in communication with the controller 74, and optionally a voltmeter 80. In one embodiment, the first measurement device 76 is a passive probe or sensor, and the second measurement device 78 is a current probe or current sensor, such as a Hall probe or sensor.In use, the first measurement device 76 measures a voltage across the capacitor 68. the voltmeter 80 may be a digital voltmeter for displaying the voltage measured by the measurement device 76. The second measurement device 78 measures the current through the inductor 70 The voltage and current measurements of the first measurement device 76 and the second measurement device 78 may be stored, evaluated, and / or processed by the controller 74. In one embodiment, the controller 74 is an oscilloscope that can display the voltage and current information measured by the switching loss measurement system 72 in graphical form, such as by recording voltage / current over time.In another embodiment, the switching loss measurement system 72 may be an integrated component of the semiconductor switching device 62. The controller 74 is programmed with the necessary logic (including any necessary algorithms, etc.) to record and analyze the voltage and current readings from the switching loss measurement system 72 to derive a switching loss associated with the switching units 64 of the semiconductor switching device 62. In one embodiment, the switching loss measurement system 72 is part of an inverter system that communicates switching loss information to the control system 44 of the electrified vehicle 12. The control system 44 may then use the switching loss information to control various aspects of the vehicle.In one embodiment, the controller 74 may operate the circuit 60 of the semiconductor switching device 62 in both a conduction cycle and a conduction and switching cycle to measure a switching loss associated with the semiconductor switching device 62. In one embodiment, the conduction cycle and the conduction and switching cycle each comprise three phases. Phase 1 includes charging the inductor 70 with energy from the energy storage device 68. phase 2 includes performing a plurality of switching cycles in one or both of the top bridge 86 (phase 2A) or a bottom bridge 88 (phase 2B) of the circuit 60. phase 3 includes discharging the energy from the inductor 70 back into the capacitor 68.FIGS. 4A and 4B illustrate phase 1 of the operation of the circuit 60. in this phase, the capacitor 68 charges the inductor 70. at time (t)=0, the voltage V c1 of the first capacitor 68 corresponds to the voltage V supplied by the voltage source 69. Energy is transferred along a current path 84 (shown schematically in FIG. 4A with arrows) during a period between time t 1 and time t 2 (see FIG. 4B ) to charge the inductor 70. Between time t 1 and t 2, the switching unit 64-1 and the switching unit 64-4 are turned "ON" to charge the inductor 70. The voltage V c1 of the capacitor 68 at a given time (t) may be measured and analyzed by the switching loss measurement system 72 (see FIG. 3 ) and expressed by the following equation: where C 1 the capacitor 68 L 1 the inductor 70 IGBT 1 the first switching unit 64-1 IGBT 1 the fourth switching unit 64-4Next, as illustrated in FIGS. 5A, 5B, and 5C, a plurality of switching cycles may be performed in one or both of the upper bridge 86 (phase 2A) and the lower bridge 88 (phase 2B) of the circuit 60. In other words, the inductor 70 in one or both of the upper bridge 86 and the lower bridge 88 may freewheel between a time t 2 and a time t 3 (see FIG. 5C ) by flowing current along a current path 90 (see FIG. 5A ) and / or a current path 92 (see FIG. 5B ). In one embodiment, a plurality of switching cycles are alternately performed between the upper bridge 86 and the lower bridge 88 for a specific number of cycles between time t 2 and time t 3. The number of cycles may vary depending on design specific parameters. The duration between time t 2 and time t 3 depends on various factors such as switching speed, switching frequency, and inductor characteristics. Either capacitor 68 or inductor 70 may provide the energy necessary to perform the switching cycles.Referring to FIG. 5A, the switching unit 64-1 and the diode 66-2 are switched to "ON" to allow free-wheeling of the inductor 70 in the top bridge 86. Referring to FIG. 5B, the switching unit 64-4 and the diode 66-3 are switched to "ON" to enable free-wheeling of the inductor 70 between time t2and time t3in the lower bridge 88. In other words, during phase 2A and 2B, the switching units 64-1 and 64-4 may be alternately switched to ON and OFF to induce a switching operation and, as a result, to change the free-wheeling loop of the current of the inductor 70. Each time the switching units 64-1 and 64-4 are switched between ON and OFF, the energy for the switching losses (on, reverse recovery, and off losses) at the capacitor 68 and the inductor 70 can be measured by the switching loss measurement system 72. These measurements are communicated to the controller 74 for further processing.The current I of the inductor 70 at a given time t during phase 2A or 2B may be measured and calculated by the controller 74 of the switching loss measurement system 72 and expressed by the following equation:Phase 3 of the operation of circuit 60 is illustrated in FIGS. 6A and 6B and occurs between time t 3 and time t 4. During this phase, the inductor 70 discharges its energy into the capacitor 68. The inductor 70 charges the capacitor 68 via the diodes 66- 2 and 66- 3 that are turned ON during phase 3. Each switching unit 64 is turned OFF during phase 3.For a given time t between time t 3 and time t 4, the voltage V of the capacitor may be expressed by the following equation:Accordingly, the difference ΔV c1-c between the voltage at time = 0 and time = t4 can be expressed by the following equation:In a non-limiting embodiment, the conduction cycle of circuit 60 is a compilation of phase 1, either phase 2A or phase 2B, and phase 3. The conduction loss Ec can be represented by the following equation:In another non-limiting embodiment, the conduction and switching cycle consists of phase 1, phase 2A, phase 2B, and phase 3. The alternate repetition of phase 2A and 2B determines the switching cycle count of circuit 60.Referring to FIG. 7, the conduction cycle and conduction and switching cycle are shown in graphical form. Curve 96 indicates the conduction cycle and curve 98 indicates the conduction and switching cycle. It can be assumed that the current profile 100 of the inductor 70 is replicated in both the conduction cycle 96 and the conduction and switching cycle 98. From time t2to t3, there is a drop in the voltage in capacitor 68 indicating that the capacitor is providing the necessary energy for the switching processes that take place during phase 2A and 2B. Therefore, the conduction and switching loss energy can be calculated as:As a result, the total switching loss energy can be calculated by subtracting the conduction loss from the combined conduction and switching loss as represented by the following equation:Assuming that the switching loss between two switching units 64 is equal, the switching loss per cycle per switching unit 64 can be calculated as:In one embodiment, the controller 74 of the loss-of-shift measurement system 72 is programmed with each of equations (1) through (8) and any other necessary hardware and software for calculating loss-of-shift information in the manner described above. The switching loss information calculated using the system and method of this disclosure may be used to control various operations of an electrified vehicle. For example, in one non-limiting embodiment, the switching loss information may be used, among other control operations, to modify an amount of thermal cooling transferred to cool the semiconductor switching device 62.Although the various non-limiting embodiments are illustrated as having specific components or steps, the embodiments of this disclosure are not limited to these specific combinations. It is possible to use some of these components or features of any of the non-limiting embodiments in combination with features or components of any of the other non-limiting embodiments.It should be understood that like reference numerals identify corresponding or similar elements throughout the drawings. It should be understood that although a particular arrangement of components is disclosed and illustrated in these example embodiments, other arrangements could benefit from the teachings of this disclosure.The foregoing description is to be interpreted as illustrative and not as restrictive. One of ordinary skill in the art would understand that certain modifications could fall within the scope of this disclosure. For these reasons, the following claims should be tested to determine the true scope and content of this disclosure.

Claims

A method comprising: a.) Operating a circuit of a semiconductor switching device (62) in a conduction cycle; the conduction cycle comprising: - charging an inductor (70) with energy from a capacitor (68); - free-wheeling the inductor (70) in either an upper bridge (86) or a lower bridge (88) of the circuit, and - discharging the energy from the inductor (70) into the capacitor (68); and calculating a conduction loss associated with the circuit; b.) Operating the circuit in a conduction and switching cycle; the conduction and switching cycle comprising: - charging the inductor (70) with energy from the capacitor (68); - alternately free-wheeling the inductor (70) between the top bridge (86) and the bottom bridge (88) of the circuit, and - discharging the energy from the inductor (70) into the capacitor (68); and calculating a combined conduction and switching loss associated with the circuit; c.) subtracting the conduction loss from the combined conduction and switching loss to calculate a switching loss of the circuit.The method of claim 1, wherein performing each of the conduction cycle and the conduction and switching cycle comprises: - charging an inductor (70) with energy from a capacitor (68), - performing a plurality of switching cycles, and - discharging the energy from the inductor (70) into the capacitor (68).The method according to claim 2, wherein the charging step comprises: - switching a first switching unit (64-1) and a second switching unit (64-4) of the circuit between ON and OFF, and - measuring a voltage across the capacitor (68) before and after the switching step.The method of claim 2, wherein the performing step comprises alternately free-wheeling the inductor (70) between an upper bridge (86) and a lower bridge (88) of the circuit.The method of claim 4, wherein the alternately freewheeling the inductor (70) comprises alternating between switching a first switching unit (64-1) to ON and OFF for freewheeling in the upper bridge (86) and switching a second switching unit (64-4) to ON and OFF for freewheeling in the lower bridge (88).The method of claim 2, further comprising: measuring voltages and currents associated with the capacitor (68) and the inductor (70) during each of the charging, performing, and discharging steps.The method of claim 6, further comprising: deriving the switching loss based on the voltages and the currents measured during each of the charging, performing, and discharging steps.The method of claim 2, wherein the discharging step comprises turning ON a first diode (66-2) and a second diode (66-3).

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