TECHNICAL FIELD 
     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. 
     BACKGROUND 
     Hybrid electric vehicles (HEV&#39;s), plug-in hybrid electric vehicles (PHEV&#39;s), battery electric vehicles (BEV&#39;s), fuel cell vehicles and other known electrified 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. High voltage current is typically supplied to the electric machines by one or more batteries that store electrical power. 
     Semiconductor switching devices are known for supporting bidirectional power flow in many products. Switching units of the semiconductor device perform switching operations according to a drive signal produced by a controller to control a load. For example, electrified vehicles often include inverter systems having a plurality of semiconductor switching units, such as power MOSFET&#39;s or insulated gate bipolar transistors (IGBT&#39;s), that undergo switching operations to power one or more AC drive motors from a DC storage battery, or alternatively, to charge the DC storage battery from an 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. 
     SUMMARY 
     A method according to an exemplary aspect of the present disclosure includes, among other things, controlling a vehicle using switching loss information of a semiconductor switching device, the switching loss information 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 that is communicated to cool the semiconductor switching device. 
     In a further non-limiting embodiment of either of the foregoing method, 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 multitude 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 includes 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. 
     In a further non-limiting embodiment of the foregoing method, operation of 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 an upper bridge and a lower bridge of the circuit. 
     In a further non-limiting embodiment of any of the foregoing methods, alternately freewheeling the inductor includes alternating between switching a first switching unit ON and OFF to freewheel in the upper bridge and switching a second switching unit ON and OFF to freewheel 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 the 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 includes switching a first diode and a second diode ON. 
     In a further non-limiting embodiment of any of the foregoing methods, the conduction cycle includes charging an inductor with energy from a capacitor, freewheeling the inductor in either an upper bridge or a lower bridge of the circuit, and discharging the energy from the inductor into the capacitor. 
     In a further non-limiting embodiment of any of the foregoing methods, the conduction and switching cycle includes charging an inductor with energy from a capacitor, freewheeling the inductor alternately between an upper bridge and a lower bridge of the circuit, and discharging the energy from the inductor into the capacitor. 
     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 measuring device configured to measure a voltage of a first energy storage device of a semiconductor circuit, a second measuring 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 measuring devices. 
     In a further non-limiting embodiment of the foregoing device, the control unit is configured to communicate the switching loss to a control system of an electrified vehicle. 
     In a further non-limiting embodiment of either of the foregoing devices, the semiconductor circuit includes a plurality of switching units configured in a 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 control unit is configured to operate the semiconductor circuit in each of a conduction cycle and a conduction and switching cycle in order to calculate the switching loss. 
     The embodiments, examples and alternatives of the preceding paragraphs, the claims, or the following description and drawings, including any of their various aspects or respective individual features, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are 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 that accompany the detailed description can be briefly described as follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a powertrain of an electrified vehicle. 
         FIG. 2  illustrates a topology of a semiconductor switching device circuit. 
         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 operating 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 a circuit operating cycle. 
         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 a circuit operating cycle. 
         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 a circuit operating cycle. 
         FIG. 7  is a graphical representation of a conduction cycle and a conduction and switching cycle of a semiconductor switching device circuit. 
     
    
    
     DETAILED DESCRIPTION 
     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 cycle ratios, and operating currents and voltages in order to measure switching loss. The circuit of the semiconductor switching device may be operated in a conduction cycle and a conduction and switching cycle in order 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 to control a vehicle system or operation. These and other features are discussed in greater detail herein. 
       FIG. 1  schematically illustrates a powertrain  10  for an electrified vehicle  12 , such as a HEV. Although depicted as a HEV, it should be understood that the concepts described herein are not limited to HEV&#39;s and could extend to other electrified vehicles, including but not limited to, PHEV&#39;s, BEV&#39;s, and fuel cell vehicles. 
     In one embodiment, the powertrain  10  is a powersplit system that employs a first drive system that includes a combination of an engine  14  and a generator  16  (i.e., a first electric machine) and a second drive system that includes 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 make up an electric drive system  25  of the powertrain  10 . The first and second drive systems generate torque to drive 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 through a power transfer unit  18 . In one 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 transfer unit  18  when acting as a generator to convert kinetic energy to electrical energy. The generator  16  can alternatively function as a motor to convert electrical energy into kinetic energy, thereby outputting 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  can 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 vehicle drive wheels  30  through a second power transfer unit  32 . The second power transfer unit  32  may include a gear set having a plurality of gears  34 A,  34 B,  34 C,  34 D,  34 E, and  34 F. Other power transfer units may also be suitable. The gears  34 A- 34 F 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 transfer 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 . 
     The motor  36  can also be employed to drive the vehicle drive wheels  30  by outputting torque to a shaft  46  that is 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  can be employed as motors to output torque. For example, the motor  36  and the generator  16  can each output electrical power to a high voltage bus  48  and the battery  50 . The battery  50  may be a high voltage battery that is capable of outputting electrical power to operate the motor  36  and the generator  16 . Other types of energy storage devices and/or output devices can also be incorporated for use with the electrified vehicle  12 . 
     The motor  36 , the generator  16 , the power transfer unit  18 , and the power transfer unit  32  may generally be referred to as a transaxle  42 , or transmission, of the electrified vehicle  12 . Thus, when a driver selects a particular shift position, the transaxle  42  is appropriately controlled to provide the corresponding gear for advancing the electrified vehicle  12  by providing traction to the vehicle drive wheels  30 . 
     The powertrain  10  may additionally 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 to perform the necessary control functions for operating the electrified vehicle  12 . In one embodiment, the control system  44  is a combination vehicle system controller and powertrain control module (VSC/PCM). Although it is 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 shift positions is occurring. The control system  44  may also communicate with a battery control module of the battery  50 , or other control devices. 
     Additionally, 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 , such as for supporting bidirectional power flow. In one embodiment, the controller  54  is an inverter system controller combined with a 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 . For example, the semiconductor switching device  62  may undergo switching operations to power the motor  36  using energy from the battery  50 , or alternatively, to charge the battery  50  via the generator  16 . It should be appreciated that the exemplary semiconductor switching device  62  could alternatively be used as part of a battery charging system, a switched mode power supply, an industrial drive, a home appliance, or any other appliance that utilizes semiconductor switching devices. 
     The semiconductor switching device  62  includes a plurality of switching units  64  and diodes  66 . In one non-limiting embodiment, the switching units  64  and diodes  66  are arranged 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 one non-limiting embodiment, the upper bridge  88  includes a first switching unit  64 - 1  (also labeled IGBT1), a first diode  66 - 1 , a second switching unit  64 - 2  (also labeled IGBT2) and a second diode  66 - 2 , and the lower bridge  88  includes a third switching unit  64 - 3  (also labeled IGBT3), a third diode  66 - 3 , a fourth switching unit  64 - 4  (also labeled IGBT4) and a fourth diode  66 - 4 . The first switching unit  64 - 1  and the fourth switching unit  64 - 4  are configured as active switches, whereas the diodes  66 - 2  and  66 - 3  are configured as passive switches, in one embodiment. 
     The semiconductor switching device  62  may additionally incorporate a capacitor  68  (i.e., a first energy storage device) and an inductor  70  (i.e., a second energy storage device). In one embodiment, the capacitor  68  is a near ideal capacitor or a fixed film capacitor that has a predetermined amount of energy and voltage in its initial (steady) stage. A voltage source  69  supplies the energy to the capacitor  68 . In one non-limiting embodiment, the voltage source  69  is the high voltage battery of an electrified vehicle (see, for example, battery  50  of  FIG. 1 ). 
     As discussed in greater detail below, an exemplary method of 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 difference in voltage (ΔV) across the capacitor  68  between the initial and final states can provide a numerical value of energy loss in terms of a combined switching, conduction and stray loss. A corresponding test can be performed to duplicate the current profile in the inductor  70 , which can be used in conjunction with the combined switching, conduction and stray loss 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 circuit  60  for measuring switching losses associated with one or more switching units  64  of the semiconductor switching device  62 . The switching loss measurement system  72  may include a control unit  74 , a first measurement device  76  and a second measurement device  78  that are in communication with the control unit  74 , and optionally, a volt meter  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 sensor, such as a hall type probe or sensor. 
     In use, the first measurement device  76  measures a voltage across the capacitor  68 . The volt member  80  may be a digital volt meter for displaying the voltage measured by the first measurement device  76 . The second measurement device  78  measures current through the inductor  70 . The voltage and current readings of the first measurement device  76  and the second measurement device  78  may be stored, evaluated and/or processed by the control unit  74 . In one embodiment, the control unit  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 plotting 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 control unit  74  is programmed with the necessary logic (including any necessary algorithms, etc.) for recording and analyzing 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 control unit  74  can operate the circuit  60  of the semiconductor switching device  62  in both a conduction cycle and a conduction and switching cycle in order 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 include three phases. Phase 1 involves charging the inductor  70  with energy from the energy storage device  68 . Phase 2 involves performing a plurality of switching cycles in one or both of the upper bridge  86  (Phase 2A) or a lower bridge  88  (phase 2B) of the circuit  60 . Phase 3 involves discharging the energy from the inductor  70  back into the capacitor  68 . Each of these phases is discussed in greater detail below with reference to  FIGS. 4-7 . 
       FIGS. 4A and 4B  illustrate Phase 1 of 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  will be equal to the voltage V supplied by the voltage source  69 . Energy is communicated along a current path  84  (shown schematically with arrows in  FIG. 4A ) during a time period between time t1 and time t2 (see  FIG. 4B ) in order to charge the inductor  70 . Between time t1 and t2, the switching unit  64 - 1  and the switching unit  64 - 4  are switched “ON” in order to charge the inductor  70 . The voltage V c1  of the capacitor  68  at any given time (t) may be measured and analyzed by the switching loss measurement system  72  (see  FIG. 3 ) and can be expressed by the following equation:
 
 V   C1 ( t )= L   1   dI ( t )/ dt+I ( t )*( R   IGBT1   +R   IGBT4   +R   L1 )  (1)
 
     where 
     C1=the capacitor  68   
     L1=the inductor  70   
     IGBT1=the first switching unit  64 - 1   
     IGBT4=the fourth switching unit  64 - 4   
     Next, as illustrated in  FIGS. 5A, 5B and 5C , a plurality of switching cycles may be performed in either 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  may freewheel in one or both of the upper bridge  86  and the lower bridge  88  between a time t2 and a time t3 (see  FIG. 5C ) by running 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 performed alternately between the upper bridge  86  and the lower bridge  88  for a specific number of cycles between time t2 and time t3. The number of cycles may vary depending upon design specific parameters. The duration between time t2 and time t3 is dependent upon various factors such as switching speed, switching frequency and inductor characteristics. Either the capacitor  68  or the inductor  70  may supply the necessary energy for performing the switching cycles. 
     Referring to  FIG. 5A , the switching unit  64 - 1  and the diode  66 - 2  are switched “ON” in order to allow the inductor  70  to freewheel in the upper bridge  86 . Referring to  FIG. 5B , the switching unit  64 - 4  and the diode  66 - 3  are switched “ON” in order to allow the inductor  70  to freewheel in the lower bridge  88  between time t2 and time t3. In other words, during Phases 2A and 2B, the switching units  64 - 1  and  64 - 4  may be alternately turned ON and OFF to induce a switching action and hence change the freewheeling loop of the inductor  70  current. Each time the switching units  64 - 1  and  64 - 4  are switched between ON and OFF, the energy for the switching losses (turn-on, reverse recovery and turn-off losses) can be measured at the capacitor  68  and the inductor  70  via the switching loss measurement system  72 . These measurements are communicated to the control unit  74  for further processing. 
     The current I of the inductor  70  at any given time t during Phase 2A or 2B may be measured and calculated by the control unit  74  of the switching loss measurement system  72  and can be expressed by the following equation:
 
 I ( t )= I ( t   2 )* e   −(t)/τ   (2)
 
     where 
     τ=L1/(R L1 +R D2 +R IGBT1 ) 
     Phase 3 of the circuit  60  operation is illustrated in  FIGS. 6A and 6B  and occurs between time t3 and time t4. During this phase, the inductor  70  discharges its energy to the capacitor  68 . In one embodiment, energy is transferred along a current path  94  back to the capacitor  68 . The inductor  70  charges the capacitor  68  via the diodes  66 - 2  and  66 - 3 , which are turned ON during Phase 3. Each switching unit  64  is turned OFF during Phase 3. 
     For a given time t between time t3 and time t4, the voltage V of the capacitor  68  can be expressed by the following equation:
 
 V ( t )= L*dI ( t )/ dt   (3)
 
     Accordingly, a difference ΔV c1−c  between the voltage at time=0 and time=t4 can be expressed by the following equation:
 
Δ Vc   1−C   =Vc   1 ( t   0 )− Vc   1 ( t   4 )  (4)
 
     In one non-limiting embodiment, the conduction cycle of the circuit  60  is a compilation of Phase 1, either Phase 2A or Phase 2B, and Phase 3. Operation in the conduction cycle enables the extraction of a conduction loss E C  associated with the semiconductor switching device  62 . The conduction loss E C  can be represented by the following equation:
 
 E   c =0.5 *C   1 *[( V   C1(t0)   2   −V   C1−C(t4)   2 )]  (5)
 
     In another non-limiting embodiment, the conduction and switching cycle consists of Phase 1, Phase 2A, Phase 2B and Phase 3. The alternating repetition of Phases 2A and 2B determines the switching cycle count of the circuit  60 . Each switching cycle count will involve a turn-on loss, a reverse recovery loss, and a turn-off loss for two switching units  64  (i.e., switching units  64 - 1  and  64 - 4 ). 
     Referring to  FIG. 7 , the conduction cycle and the conduction and switching cycle are represented in graphical form. Curve  96  indicates the conduction cycle, and curve  98  indicates the conduction and switching cycle. It may 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 t2 to t3, there is a drop in capacitor  68  voltage, indicating that the capacitor  68  is providing the necessary energy for the switching processes that occur during phases 2A and 2B. Therefore, the conduction and switching loss energy can be calculated as:
 
 E   C+SW =0.5* C   1 *[( V   C1(t0)   2   −V   C1−C+SW(t4)   2 )]  (6)
 
     Hence, the total switching loss energy can be calculated by subtracting the conduction loss from the combined conduction and switching loss as shown by the following equation:
 
 E   SW =( E   C+SW )−( E   C )  (7)
 
     Assuming that the switching loss is equal between two switching units  64 , the switching loss per cycle per switching unit  64  can be calculated as:
 
 E=E   SW /2 N   (8)
 
     In one embodiment, the control unit  74  of the switching loss measurement system  72  is programmed with each of equations (1) through (8) and any other necessary hardware and software for calculating switching loss information in the manner described above. The switching loss information calculated using the system and method of this disclosure can be used to control various operations of an electrified vehicle. For example, in on one non-limiting embodiment, the switching loss information can be used to modify an amount of thermal cooling that is communicated to cool the semiconductor switching device  62 , among other control operations. 
     Although the different non-limiting embodiments are illustrated as having specific components or steps, the embodiments of this disclosure are not limited to those particular combinations. It is possible to use some of the components or features from any of the non-limiting embodiments in combination with features or components from any of the other non-limiting embodiments. 
     It should be understood that like reference numerals identify corresponding or similar elements throughout the several drawings. It should be understood that although a particular component arrangement is disclosed and illustrated in these exemplary embodiments, other arrangements could also benefit from the teachings of this disclosure. 
     The foregoing description shall be interpreted as illustrative and not in any limiting sense. A worker of ordinary skill in the art would understand that certain modifications could come within the scope of this disclosure. For these reasons, the following claims should be studied to determine the true scope and content of this disclosure.
A method according to an exemplary aspect of the present disclosure includes, among other things, controlling a vehicle using switching loss information of a semiconductor switching device, the switching loss information derived from a conduction loss and a combined conduction and switching loss.