WO2017082885A1

WO2017082885A1 - Active snubber system and method of operating the same

Info

Publication number: WO2017082885A1
Application number: PCT/US2015/060000
Authority: WO
Inventors: Pierluigi Tenca; Piniwan Thiwanka Bandara Wijekoon; Nicolas Lapassat; Elise SAVARIT
Original assignee: General Electric Company
Priority date: 2015-11-10
Filing date: 2015-11-10
Publication date: 2017-05-18

Abstract

An active snubber system for at least one switching device includes at least one sensing device configured to measure at least one characteristic associated with the switching device. Active snubber system also includes at least one auxiliary device including first and second auxiliary terminals coupled to first and second switch terminals, respectively. Active snubber system further includes at least one controller coupled to the sensing device, and configured to transmit a control signal to a third auxiliary terminal to regulate a resistance of the auxiliary device. Control signal is at least partially based on a function of a value of the at least one characteristic. Controller facilitates at least one of regulation of a current transmitted from the first to the second switch terminal by diverting at least a portion of the current through the auxiliary device, and regulation of a voltage between the first and the second switch terminal.

Description

ACTIVE SNUBBER SYSTEM AND METHOD OF

OPERATING THE SAME

BACKGROUND

[0001] The field of the disclosure relates generally to snubbers for electrical power systems, and, more particularly, to active snubber circuit systems configured to control the time rate of change of the voltage (dv/dt) across semiconductor devices such as insulated-gate bipolar transistors (IGBTs).

[0002] In a variety of power system applications, including high-voltage power converters, semiconductor switch devices, such as IGBTs, are controlled by gate drive command signals, such as those generated by pulse-width modulation, and the resulting switching of the collector-emitter voltage also yields a reference voltage which is fed back into the controller in certain applications. To mitigate switching-synthesized high voltages spikes across IGBTs, it is desirable in certain applications to couple IGBTs in series with known snubbers coupled across the main line terminals being switched. Design goals in those known snubber applications include achieving, to the extent possible, matched voltage division, i.e., voltage sharing, among the serially coupled IGBTs.

[0003] At least some of the known snubbers use large power rating resistors, diodes, and capacitors which tend to reduce efficiency and are unsuitable for space-constrained applications. Furthermore, power converters using such known snubbers often require matched components rated to withstand at least the highest expected switching-synthesized transient voltage spike. Many of these known snubbers also require capacitors between the gate and emitter terminals of switch devices, making maintenance and retrofitting more difficult, and tending to make switching unsynchronized due to stray gate capacitances, especially in high switching speed applications. Also, such known snubbers often require sophisticated gate drive controllers. In such known snubbers, achieving dynamic voltage sharing and closely synchronized dv/dt across switching device terminals is thus challenging without use of expensive, bulky, and inefficient components that are expensive to produce and difficult to maintain. BRIEF DESCRIPTION

[0004] In one aspect, an active snubber system for at least one switching device is provided. The at least one switching device includes at least a first switch terminal, a second switch terminal, and a third switch terminal. The active snubber system includes at least one sensing device configured to measure at least one characteristic associated with the at least one switching device. The active snubber system also includes at least one auxiliary device. The auxiliary device includes a first auxiliary terminal coupled to the first switch terminal, a second auxiliary terminal coupled to the second switch terminal, and a third auxiliary terminal. The active snubber system further includes at least one controller coupled to the at least one auxiliary device and to the at least one sensing device. The at least one controller is configured to transmit a control signal to the third auxiliary terminal to regulate a resistance of the auxiliary device. The control signal is at least partially based on a function of a value of the at least one characteristic. The controller facilitates at least one of regulation of a current transmitted from the first switch terminal to the second switch terminal by diverting at least a portion of the current through the auxiliary device, and regulation of a voltage between the first switch terminal and the second switch terminal.

[0005] In another aspect, a method of operating an active snubber system for at least one switching device is provided. The at least one switching device includes at least a first switch terminal, a second switch terminal, and a third switch terminal. The method includes selecting at least one characteristic associated with the at least one switching device. The method also includes selecting a reference function at least partially based on a function of a value of the at least one characteristic. The method further includes selecting at least one sensing device configured to measure the at least one characteristic. The method further includes selecting at least one auxiliary device including a first auxiliary terminal, a second auxiliary terminal, and a third auxiliary terminal. The method also includes selecting at least one controller configured to transmit a control signal to the third auxiliary terminal, the control signal at least partially based on a function of a value of the at least one characteristic, where the controller facilitates at least one of regulating a current transmitted from the first switch terminal to the second switch terminal by diverting at least a portion of the current through the auxiliary device, and regulating a voltage between the first switch terminal and the second switch terminal. The method further includes coupling the first auxiliary terminal and the second auxiliary terminal to the first switch terminal and the second switch terminal, respectively, where the coupling facilitates defining a current path between the first switch terminal and the second switch terminal, and where a resistance of the current path is at least partially based on a function of a value of the at least one characteristic. The method also includes coupling the controller to the third auxiliary terminal and to the at least one sensing device. The method further includes coupling the at least one sensing device to the at least one switching device and to the controller.

DRAWINGS

[0006] These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

[0007] FIG. 1 is a schematic view of an of exemplary power converter circuit with the active snubber system;

[0008] FIG. 2 is a schematic view of an exemplary active snubber system used in the power converter shown in FIG. 1;

[0009] FIG. 3 is a schematic view of an alternative active snubber system used with the power converter shown in FIG. 1; and

[0010] FIG. 4 is a schematic view of another alternative active snubber system used with the power converter shown in FIG. 1.

[0011] Unless otherwise indicated, the drawings provided herein are meant to illustrate features of embodiments of this disclosure. These features are believed to be applicable in a wide variety of systems comprising one or more embodiments of this disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the embodiments disclosed herein. DETAILED DESCRIPTION

[0012] In the following specification and the claims, reference will be made to a number of terms, which shall be defined to have the following meanings.

[0013] The singular forms "a", "an", and "the" include plural references unless the context clearly dictates otherwise.

[0014] "Optional" or "optionally" means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.

[0015] Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as "about", "approximately", and "substantially", are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, and such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.

[0016] As used herein, the term "reference function" refers to a predetermined mathematical description of how the value of the voltage across the terminals of a switching device is intended to vary or not vary over time in a power system, such as a power converter.

[0017] The active snubber systems described herein are suited to mitigate switching-synthesized high voltage spikes across semiconductor switching devices such as insulated-gate bipolar transistors (IGBTs) in power converters, including power converters with serially coupled IGBTs. Specifically, the active snubber systems described herein do not require a high number of large passive components including power resistors, power diodes, and large energy storage capacitors. Further, specifically, the active snubber systems described herein mitigate, reduce and/or smooth the time rate of change of voltage spikes across the collector and emitter terminals (dv_ce/dt), and achieve dynamic voltage sharing across power converter switching devices such as IGBTs, including applications with serially coupled IGBTs. Further, such active snubber systems are more power efficient, smaller, and less expensive to manufacture and maintain than other known snubber systems. Moreover, the active snubber systems described herein do not include capacitors located between the gate and emitter terminals of semiconductor switching devices such as IGBTs.

[0018] The active snubber systems described herein also facilitate regulating the total current sum of the switch current, the current in its anti-parallel diode, and the current in the auxiliary device. Regulating this total current, which includes, without limitation, the "tail current" of the main switch and/or the recovery current of its anti-parallel diode, is achieved by the active snubber systems described herein during and/or after turning off IGBT main switches. Specifically, the controllers used in the active snubber systems described herein regulate the total current, which includes the tail current of the main switch and the recovery current of its antiparallel diode, during and/or after turn off for a relatively long time as compared to the commutation process. Such active snubber systems are used for the aforementioned aims in power converters where space is at a premium, sophisticated gate drive controllers are not available, and where matched semiconductor switching devices and matched snubbers are not desirable or feasible.

[0019] FIG. 1 is a schematic view of a power converter circuit 100 with an active snubber system. In the exemplary embodiment, an active snubber 102 is coupled to a semiconductor switching device 104, e.g., an IGBT, which switches a main power line 106. In the exemplary embodiment, a diode 108, i.e., an antiparallel diode, is coupled to and between a collector terminal 110 and an emitter terminal 112 of IGBT switching device 104. Collector terminal 110 and emitter terminal 112 are also referred to herein as a first switch terminal and a second switch terminal, respectively, of switching device 104. In an alternative embodiment, not shown, diode 108 is not coupled to and between collector terminal 110 and emitter terminal 112. An IGBT gate terminal 114 is coupled to output 116 of a gate driver device 118 which takes in a switching control signal 120 from a controller in the power converter, not shown. Gate terminal 114 is also referred to herein as a third switch terminal. Gate driver device 118 turns-on and turns-off IGBT switching device 104 and thereby controls the states of main power line 106. Further, in the exemplary embodiment, an emitter voltage line 122 provides feedback to gate driver device 118 via a gate driver input 124 and to other portions, not shown, of power converter circuit 100 through a wire 126. In the exemplary embodiment, the various features and component elements of power converter circuit 100 which provide gate driving signals to gate terminal 114, some of which are not shown in FIG. 1, are collectively referred to as a gate drive circuit of the switching device. In an alternative embodiment, not shown, emitter voltage line 122 and wire 126 are not present. Active snubber 102 is coupled to IGBT collector terminal 110 via a snubber-collector line 128 and is coupled to IGBT emitter terminal 112 via a snubber-emitter line 130.

[0020] In the exemplary embodiment, one active snubber system 132 is coupled across a single IGBT switching device 104. Also, in some embodiments, not shown, one active snubber system 132 is coupled to any number of IGBT switching devices 104, i.e., where IGBT switching devices are coupled in series, i.e., serially coupled, along main power line 106 being switched. Alternatively, in some embodiments, individual IGBT switching devices 104 serially coupled along main power line 106 have one active snubber system 132 coupled to collector terminals 110 and to emitter terminals 112 of some switching devices 104, while other switching devices 104 serially coupled along main power line 106 in the same power converter circuit 100 do not have active snubber system 132 so coupled. Likewise, in some embodiments, not shown, one or a plurality of active snubber systems 132 are coupled across IGBT switching devices 104 that are arranged in power converter circuit 100, for example, and without limitation, in parallel, in series, and in any combinations thereof, along a single or a plurality of main power lines 106.

[0021] Further, in the exemplary embodiment, a gate drive-to-active snubber line 134 is coupled to and between the line for switching control signal 120 and active snubber 102. In an alternative embodiment, not shown, coupling gate drive-to- active snubber line 134 to and between the line for switching control signal 120 and active snubber 102 is not present. As described further below, gate drive-to-active snubber line 134 provides a synchronization signal 135 to enable operation of the active snubber system 132. Moreover, in the exemplary embodiment, the synchronization signal carried by gate drive-to-active snubber line 134 is switching control signal 120 provided to gate terminal 114 of switching device 104. Synchronization signal originates in the gate drive circuit of the switching device and, as discussed further below, is transmitted and received by active snubber system 132 a finite amount of time ahead of the switching event of switching device 104 initiated by gate terminal 114 receiving output 116 from gate driver device 118.

[0022] Also, in the exemplary embodiment, a communication line 136 is used to set circuit parameters, including, without limitation, voltage, current, resistance, impedance, capacitance, inductance, frequency, phase, delay time, and gain, of component elements of active snubber 102. In an alternative embodiment, not shown, communication line 136 is not present. Using communication line 136 is advantageous where active snubber 102 is implemented, at least in part, as a digital system where circuit parameters of component elements of active snubber 102 are electronically controllable as a part of, for example, a system implemented using a computer and associated hardware and software. Communication line 136 allows the operator of power converter circuit 100 and active snubber system 132 to control the aforementioned parameters from a remote location, for example.

[0023] In operation, switching control signal 120 initiates a switching of main power line 106. Switching of main power line 106 results in a synthesized transient voltage spike across collector terminal 110 and emitter terminal 112 of switching device 104. Active snubber system 132, described in further detail below, mitigates the undesirable effects of the transient voltage spikes by providing an alternative current path around switching device 104. The dv_ce/dt of the transient voltage spike is thereby controllable by active snubber system 132 to conform to a reference function. Furthermore, in operation, whether or not there is a higher voltage potential on collector terminal 110 than on emitter terminal 112, i.e., where the current on the main power line is an alternating current, the description and operation of active snubber system 132 is the same as discussed herein. Moreover, in the exemplary embodiment, a function of the synthesized voltage between, i.e., across, collector terminal 110 and emitter terminal 112 is a measurable and controllable operational parameter of each switching device 104 as coupled to main power line 106, including, without limitation, as a plurality of switching devices. [0024] FIG. 2 is a schematic view of an exemplary active snubber system 132 used in various configurations of power converter circuit 100 including, without limitation, those shown in FIG. 1. In the exemplary embodiment, at least one capacitor 200 having a capacitance value C_sense includes a first capacitor terminal 202 coupled to snubber- collector line 128 and a second capacitor terminal 204 coupled to snubber-emitter line 130.

[0025] Also, in the exemplary embodiment, at least one auxiliary device 206 includes a first auxiliary terminal 208 coupled to snubber-collector line 128 and a second auxiliary terminal 210 coupled to snubber-emitter line 130. At least one auxiliary device 206 is configured to function as a controllable resister and includes, without limitation, such devices as non-linear controllable resistors, varistors, and transistors such as IGBTs, metal-oxide semiconductor field-effect transistors (MOSFETs), and bipolar junction transistors (BJTs). Also in the exemplary embodiment, auxiliary device 206 includes a third auxiliary terminal 212 configured to receive a control signal 216 to regulate the resistance, including, without limitation, the instantaneous resistance, of auxiliary device 206. Further, in the exemplary embodiment, control signal 216 is transmitted to its destination in active snubber system 132 on an electrical line in FIGS. 1-4.

[0026] Further, in the exemplary embodiment, auxiliary device 206 includes at least one current sensing device 214 coupled to and between second auxiliary terminal 210 and second capacitor terminal 204. In an alternative embodiment, not shown, at least one current sensing device 214 is coupled to and between first auxiliary terminal 208 and first capacitor terminal 202. Current sensing device 214 is configured to measure an instantaneous current, icsense, through at least one capacitor 200, which represents a characteristic, including, without limitation, a physical quantity associated with switching device 104. Also, in the exemplary embodiment, current sensing device 214 is coupled to a controller 215. Further in the exemplary embodiment, controller 215 is configured to receive and convert the measured instantaneous current through at least one capacitor 200 into an absolute value of the instantaneous current, |ic_Sense|, and further convert into a voltage value, VOU_T, proportional to icsense- As such, in the exemplary embodiment, a control signal 216 is the output of controller 215 which is transmitted to third auxiliary terminal 212. In another alternative embodiment, not shown, current sensing device 214 includes controller 215 integrated within it, and control 216 is transmitted as an output from integrated current sensing device 214 and controller 216 to third auxiliary terminal 212.

[0027] In the exemplary embodiment, at least one resistor 218, i.e., a two terminal component, is serially coupled to and between first capacitor terminal 202 and first auxiliary terminal 208. Alternatively, in some embodiments, not shown, any number resistors 218 having any resistance value are serially coupled to and between first capacitor terminal 202 and first auxiliary terminal 208. Further, alternatively, in some embodiments, not shown, no resistors 218 are coupled to and between first capacitor terminal 202 and first auxiliary terminal 208. Likewise, in some alternative embodiments, not shown, any number of resistors 218 having any resistance value are serially coupled to and between second capacitor terminal 204 and second auxiliary terminal 210. Further, in some other exemplary embodiments, not shown, any number of resistors 218 having any resistance value are serially coupled to and between first auxiliary terminal 208 and first capacitor terminal 202, and also to and between second auxiliary terminal 210 and second capacitor terminal 204. Moreover, in the exemplary embodiment, the value of the resistance of resistor 218 is selected such that, for example, and without limitation, the values of the instantaneous voltage across collector terminal 1 10 and emitter terminal 1 12, and across Csense, do not differ from one another so much that the differences in voltage affect the ability of the active snubber system 132 to control the dv_ce/dt of the transient voltage spike to conform to a reference function. Inclusion or non-inclusion of at least one resistor 218 in active snubber system 132 is guided by the particular application in power systems including, without limitation, power converter circuit 100.

[0028] Further, in the exemplary embodiment, control signal 216 is a voltage, Vout, which is transmitted to third auxiliary terminal 212. Moreover, in the exemplary embodiment, the resistance, including, without limitation, the instantaneous resistance, RAU_X, of auxiliary device 206 between first auxiliary terminal 208 and second auxiliary terminal 210 is proportional to the magnitude of the voltage, V_out, of control signal 216. As such, in the exemplary embodiment, RAU_X is a function of icsense, including, without limitation, a first-order time derivative of the current transmitted from the first switch terminal to the second switch terminal. In an alternative embodiment, not shown, auxiliary device 206 includes additional terminals other than first 208, second 210, and third 212 auxiliary terminals configured to provide additional functions such as enabling or disabling the regulation of resistance of auxiliary device 206 in response to control signal 216. Also, in some alternative embodiments, not shown, control signal 216 includes signals other than a voltage level, including, without limitation, current levels, digital signals, and optical signals.

[0029] Also, in the exemplary embodiment, control signal 216, i.e., V_out, is provided by the output of controller 215. Specifically, in the exemplary embodiment, controller 215 is configured to output control signal 216 as a voltage, V_out, which is proportional to the magnitude of the current through C_sense, i.e., |icsense|- Moreover, the aforementioned proportionalities include, without limitation, direct and indirect proportionalities as demanded by the specific applications and component devices chosen for particular applications.

[0030] In the exemplary embodiment, active snubber system 132 is coupled to switching device 104 by coupling snubber-collector line 128 to first auxiliary terminal 208 and to first capacitor terminal 202. Snubber-collector line 128 is coupled to collector terminal 110 of switching device 104. Further, in the exemplary embodiment, active snubber system 132 is coupled to switching device 104 by coupling snubber-emitter line 130 to and between second auxiliary terminal 210 and second capacitor terminal 204. Moreover, in the exemplary embodiment, snubber-emitter line 130 is coupled to emitter terminal 112 of switching device 104.

[0031] In addition, in the exemplary embodiment, current sensing device 214 and controller 215 are alternately enabled and disabled by a synchronization signal including, without limitation, at least one of an optical signal and an electrical signal, transmitted on gate drive-to-active snubber line 134. Likewise, in the exemplary embodiment, gate-to-active snubber line 134 includes at least one of an optical and an electrical line configured to carry the synchronization signal. Further, in the exemplary embodiment, synchronization signal is transmitted to its destination in active snubber system 132 on gate-to-active snubber line 134 in FIG. 1, FIG. 2, and FIG. 3. Specifically, gate drive-to-active snubber line 134 is coupled to enable terminals 220 of current sensing device 214 and controller 215. More specifically, in the exemplary embodiment, current sensing device 214 and controller 215 are enabled to measure and convert the instantaneous current through C_sense 200 in response to synchronization signal on gate drive-to-active snubber line 134. For example, such functionality of current sensing device 214 and controller 215 is enabled when the synchronization signal on gate drive-to-active snubber line is present, and such functionality is disabled when synchronization signal is absent. In an alternative embodiment, such functionality of current sensing device 214 and controller 215 is enabled when the synchronization signal on gate drive -to-active snubber line is absent, and such functionality is disabled when synchronization signal is present. Including the alternating enable and disable control of current sensing device 214 and controller 215, and gate drive -to-active snubber line 134 to transmit synchronization signal provides additional benefits in the exemplary embodiment, including, without limitation, more efficient use of power, enhanced response time, and higher speed operation due to the ability of the synchronization signal to reach active snubber system 132 ahead of the inherent delay of switching device 104 and other associated circuitry.

[0032] In operation, during the turned-on state of switching device 104, current in main power line 106 flows with little to no resistance through switching device 104. When an event initiated by power converter circuit 100 occurs, including, without limitation, turn-off switching of main power line 106 by switching device 104, an undesirable transient voltage spike develops across collector terminal 110 and emitter terminal 112. The transient voltage spike, v_ce, which is a synthesized voltage occurring from, for example, inductive loads on main power line 106 being switched, synthesizes build-up of charge in C_sense 200, the magnitude of which depends on the magnitude of the voltage spike, v_ce. The voltage across C_sense 200 will, at all pre-turn-off and post-turn-off times, be equal to v_ce since it is coupled in parallel to collector terminal 110 and emitter terminal 112.

[0033] Immediately prior to the turn-off event in the exemplary embodiment, little or no current will flow through C_sense 200 and there will be little or no synthesized voltage across first capacitor terminal 202 and second capacitor terminal 204. In the exemplary embodiment, controller 215 is configured, for example, to output control signal 216 as V_out which increases with the magnitude of the instantaneous current through Csense 200. Also, in the exemplary embodiment, auxiliary device 206 is configured, for example, to decrease its resistance, including, without limitation, its instantaneous resistance, with increasing values of V_out- As such, the resistance of auxiliary device 206 will be quite high and little or no current will flow through it immediately prior to the turn- off event.

[0034] Also, in operation of the exemplary embodiment, compared with the state of active snubber system 132 prior to the transient voltage spike, immediately after the turn-off event, current through C_sense 200 is relatively high and controller 215 will have its highest control signal 216 output V_out value. Therefore, compared with the state of active snubber system 132 prior to the transient voltage spike, in the exemplary embodiment, the resistance of auxiliary device 206 will decrease below a predetermined value and will be at its lowest. Specifically, in the operation of the exemplary embodiment, current will flow immediately after the turn-off event in a shunt path between collector terminal 110 and emitter terminal 1 12 at the highest rate. Moreover, in the operation of the exemplary embodiment, as the magnitude of the transient voltage spike varies over a short time, the voltage and current associated with at least one capacitor 200 will also vary over a short time, and will, through control signal 216 of controller 215, regulate the resistance of auxiliary device 206.

[0035] In addition, in the exemplary embodiment, current sensing device 214 is enabled, i.e., is turned-on and sensing the instantaneous current through C_sense 200, immediately upon the initiation of switching control signal 120 from the controller of power converter circuit 100 to gate terminal 114 via gate driver 118. Thus, the synchronization signal carried from switching control signal 120 to current sensing device 214 and controller 215 readies the operation of active snubber system 132 some finite amount of time, i.e., on account of the delay time due to additional power converter circuit 100 elements through which switching control signal 120 must pass, prior to switching control signal 120 reaching gate terminal 114. Therefore, prior to development of a transient voltage spike across collector terminal 110 and emitter terminal 112 of switching device 104, current sensing device 214 of active snubber system 132 is sensing the instantaneous current through C_sense 200. Therefore, in the alternative embodiment, a more synchronized response to transient voltage spikes is achieved, along with the aforementioned benefits. [0036] Also, in the exemplary embodiment, selection of the value of at least one capacitor 200 and the value of at least one resistor 218 facilitates determining and controlling a time constant of active snubber system 132. The time constant is a parameter providing the desired response time of active snubber system 132, depending on the particular power converter or other power system application. The selection of the capacitance value of at least one C_sense 200 and the value of at least one resistor 218 at least partially defines a time constant to facilitate following the reference function by the active snubber system. More specifically, in the exemplary embodiment, the selection of the values of C_sense 200, along with the particular device, tuning, and/or reference function, i.e., set point, associated with auxiliary device 206, controller 215, and current sensing device 214, facilitates flexible and cost-effective control of the actual properties and behavior of the transient voltage spike according to the desired function, including, without limitation, the dvce/dt of the synthesized voltage across collector terminal 110 and emitter terminal 112.

[0037] FIG. 3 is a schematic view of an alternative active snubber system 132 used in various configurations of power converter circuit 100 including, without limitation, those shown in FIG. 1. In the exemplary embodiment, a voltage sensing device 300 is configured to measure an instantaneous voltage between first capacitor terminal 202 and second capacitor terminal 204, i.e., vcsense- Specifically, in the exemplary embodiment, voltage sensing device 300 is coupled to and between first and second terminals of at least one capacitor, C_sense 200, by way of at least one voltage sensing line 302. Further, in the exemplary embodiment, vcsense represents a characteristic, including, without limitation, a physical quantity associated with switching device 104. Also, in the exemplary embodiment, voltage sensing device 300 is coupled to controller 215. Further, in the exemplary embodiment, controller 215 is configured to convert the measured instantaneous voltage into an absolute value of the instantaneous voltage, |vcsense|, and further convert it into voltage value, VOU_T, proportional to vcsense- As such, in the exemplary embodiment, control signal 216 is the output of controller 215 which is transmitted to third auxiliary terminal 212. In another alternative embodiment, not shown, voltage sensing device 300 includes controller 215 integrated within it, and control signal 216 is transmitted as an output from voltage sensing device 200 to third auxiliary terminal 212. [0038] In the exemplary embodiment shown in FIG. 3, and as shown in FIG. 2 and described above, the value of the resistance of auxiliary device 206 between first auxiliary terminal 208 and second auxiliary terminal 210 is proportional to the value of V_out of control signal 216. In addition, in the exemplary embodiment, voltage sensing device 300 and controller 215 are alternately enabled and disabled by a synchronization signal including, without limitation, at least one of an optical or electrical signal, transmitted on gate drive -to-active snubber line 134. Specifically, gate drive-to-active snubber line 134 is coupled to enable terminals 220 of voltage sensing device 300 and controller 215. More specifically, in the exemplary embodiment, voltage sensing device 300 and controller 215 are enabled to measure and convert the instantaneous voltage across Csense 200 in response to synchronization signal on gate drive-to-active snubber line 134. For example, such functionality of voltage sensing device 300 and controller 215 is enabled when synchronization signal on gate drive-to-active snubber line 134 is present, and such functionality is disabled when synchronization signal is absent. In an alternative embodiment, such functionality of voltage sensing device 300 and controller 215 is enabled when synchronization signal on gate drive-to-active snubber line 134 is absent, and such functionality is disabled when synchronization signal is present. Including the alternating enable and disable control of voltage sensing device 300 and controller 215, and gate drive- to-active snubber line 134 to transmit synchronization signal provides additional benefits in the exemplary embodiment, including, without limitation, those described above with reference to FIG. 2.

[0039] In operation, during the turned-on state of switching device 104, current in main power line 106 flows with little to no resistance through switching device 104, and there is little or no voltage across C_sense 200. When an event initiated by the power converter circuit 100 occurs, including, without limitation, turn-off switching of main power line 106, an undesirable transient voltage spike will attempt to develop across collector terminal 1 10 and emitter terminal 1 12, as described above with reference to FIG. 2.

[0040] Immediately prior to the turn-off event, little or no current will flow through C_sense 200 and there will be little or no potential difference between first capacitor terminal 202 and second capacitor terminal 204. In the alternative embodiment, controller 215 is configured, for example, to output control signal 216 as V_out, which increases with |vc_Sense|, and, likewise increases with the value of v_ce, and thereby regulates the resistance of auxiliary device 206, as described above with reference to FIG. 2.

[0041 ] In addition, in the alternative embodiment, voltage sensing device 300 is enabled, i.e., is turned-on and sensing the instantaneous voltage across C_sense 200, immediately upon initiation of switching control signal 120 directed from a power converter controller, not shown, to gate terminal 1 14 via gate driver 1 18. As described above with reference to FIG. 2, prior to the development of a transient voltage spike across collector terminal 1 10 and emitter terminal 1 12 of switching device 104, voltage sensing device 300 is measuring the value of vcsense and controller 215 is ready to function to convert vcsense into |vc_Sense| and further into VOUT-

[0042] Also, in the exemplary embodiments, selection of the capacitance value of at least one C_sense 200 and the value of at least one resistor 218 facilitates determining and controlling a time constant of the active snubber system 132 circuit, as described above with reference to FIG. 2. The actual properties and behavior of the transient voltage spike is thereby controllable by active snubber system 132 to conform to a reference function, including, without limitation, a function that includes the total time derivative of the synthesized voltage across collector terminal 1 10 and emitter terminal 1 12 of switching device 104, i.e., dv_ce/dt.

[0043] FIG. 4 is a schematic view of another alternative active snubber system 132 used in various configurations of the power converter circuit 100 including, without limitation, those shown in FIG. 1. As in the exemplary embodiments of FIG. 2 and FIG. 3, a sensing device 400 is coupled to at least one capacitor 200 to measure the instantaneous current and/or voltage associated with C_sense 200, both of which are characteristics, including, without limitation, physical quantities associated with at least one switching device 104. A modified controller 401 is coupled to sensing device 400, and is configured to output control signal 216 which is, for example, a voltage value, V_out, proportional to a physical quantity associated with C_sense 200. The characteristic, including, without limitation, the physical quantity associated with C_sense 200 in the exemplary embodiment, as well as those exemplary embodiments discussed above with reference to FIGS. 1-3, includes, without limitation, at least one of voltage, current, resistance, impedance, inductance, frequency, phase, delay time, and gain, and first-order time derivatives thereof.

[0044] In the exemplary embodiment, and as applicable to the description above with reference to FIGS. 2 and 3, sensing device 400 and modified controller 401 both include circuitry associated with sensing device 400 and modified controller 401, respectively, configured to receive the synchronization signal transmitted on gate drive-to- active snubber line 134 to enable terminals 220 of sensing device 400 and modified controller 401. The circuitry associated with sensing device 400 and modified controller 401 is further configured to enable the functionality of sensing device 400 and modified controller 401 when the synchronization signal is present, and is further configured to disable the functionality of sensing device 400 and modified controller 401 when the synchronization signal is not present. Specifically, the circuitry associated with sensing device 400 and modified controller 401 is configured to receive, transduce, transmit, and process the synchronization signal into further electrical signals to carry out the enabling and disabling of the functionality of sensing device 400 and modified controller 401. In the exemplary embodiments, alternative schemes to enable and disable this functionality including, without limitation, those discussed above with reference to FIGS. 2 and 3, are also implementable by circuitry associated with sensing device 400 and modified controller 401.

[0045] In operation, in the exemplary embodiment, sensing device 400 is enabled, i.e., is turned-on and sensing at least one physical property, including, without limitation, a physical quantity associated with at least one capacitor 200, immediately upon the initiation of switching control signal 120 directed from a power converter controller, not shown, to gate terminal 1 14 via gate driver 118. Further details on the operation of the exemplary embodiment, and the benefits thereof, are as described above with reference to FIGS. 2 and 3.

[0046] Specifically, in the exemplary embodiment, modified controller 401 includes a converter 402 configured to receive measured icsense and/or vc_ense and convert one or both of those values into their absolute values and further convert one or both of their absolute values into VOU_T, as described above with reference to FIGS. 2 and 3. Control signal 216 is transmitted as a voltage value function, i.e., VOU_T, on a line coupled to a non-inverting terminal 403 of at least one operational amplifier 404, including, without limitation, a high gain differential amplifier. Also, in the exemplary embodiment, operational amplifier 404 is within modified controller 401. In other alternative embodiments, not shown, operational amplifier 404 is located outside modified controller 401 , but within active snubber system 132. Operational amplifier 404 includes an inverting terminal 406 coupled to first capacitor terminal 202 or to second capacitor terminal 204. Also, in the exemplary embodiment, modified controller 401 also includes a voltage source 412 is serially coupled to and between inverting terminal 406 and the selected capacitor terminal. Voltage source 412 is configured to reference the instantaneous value of the voltage of non-inverting terminal 403. In some alternative embodiments, not shown, voltage source 412 is not present. In other alternative embodiments, not shown, voltage source 412 is present in active snubber system 132, but is not located within modified controller 401.

[0047] Also, in the exemplary embodiment, an output terminal 408 of operational amplifier 404 transmits a modified control signal 410, including, without limitation, a differentially amplified control signal, to third auxiliary terminal 212. In the exemplary embodiment and as described above with reference to FIGS. 2 and 3, the value of the resistance of auxiliary device 206 is proportional to modified control signal 410, thereby controlling current transmitted through the auxiliary device 206.

[0048] Alternatively, in some embodiments, a reference voltage provided by voltage source 412 provides a constant or time-varying voltage parameter used by active snubber system 132 to control the function of the voltage across collector terminals 1 10 and emitter terminals 1 12 of switching devices 104 in power converter circuit 100, including, without limitation, dv_ce/dt, as further described below.

[0049] Further, in the above-described exemplary embodiments, regulating a current transmitted from the first switch terminal to the second switch terminal includes diverting a portion of it through auxiliary device 206. The tail current of the switching device 104 itself, however, is a characteristic that is possible to be measured, but not itself possible to be regulated directly. Nevertheless, owing to the controllable shunt path in parallel to switching device 104 represented by auxiliary device 206 of the above- described active snubber systems 132, it is possible to regulate the current through auxiliary device 206 such that the sum of the currents, including, without limitation, a tail current of at least one, or all switching devices 104 of the plurality of switching devices 104, a current through the at least one diode 108, and a current through at least one of first auxiliary terminal 208, second auxiliary terminal 210, and third auxiliary terminal 212 of at least one auxiliary device 206, including, without limitation, sums of currents which are different from switching device 104 to switching device 104 in a plurality of switching devices 104, are controlled differently from auxiliary device 206 to auxiliary device 206. Hence, a desired sum of currents for all switching devices 104 of a plurality of switching devices 104, including, without limitation, a plurality of serially coupled switching devices 104, represents a predetermined set point for which active snubber systems 132 facilitate following reference function, thus facilitating equivalent dynamic voltage sharing among all switching devices 104 of a plurality of switching devices 104.

[0050] The above-described active snubber systems control a function of the voltage between the collector and emitter terminals of the switching device(s) present in a power converter circuit. Specifically, the function is dictated by the component values of the active snubber system, including, without limitation, the capacitance value of C_sense and the values of the resistance of the resistor, the gain of the operational amplifier, and/or the voltage of the voltage source. Further, specifically, some or all of these component values change with the switching state of the switching device(s) and/or with the state of the transient voltage spikes across the collector and emitter terminal(s) of the switching device(s) in a power converter circuit. Moreover, the aforementioned states of switching device(s) and the values of component elements of the active snubber system result in the variable control of the resistance of the auxiliary device. This current path is thus a controllable current path around the switching devices to mitigate undesirable effects of switching-synthesized, transient voltage spikes. Also, the specific component values of the active snubber system, including, without limitation, those values which are implemented through analog, digital, or combinations thereof of such circuitry and devices, establish a reference function of the voltage across the collector and emitter terminals of the switching device(s). In a power converter circuit, for example, selecting and controlling the specific component values facilitates inducing the actual behavior of the power converter with active snubber system to follow the desired reference function.

[0051] The above-described active snubber systems mitigate undesired effects of switching-synthesized high voltage spikes across semiconductor switching devices such as IGBTs in power converters, including power converters with serially coupled IGBTs. Specifically, the exemplary active snubber systems dispense with the need for a high number of large passive components including power resistors, power diodes, and large energy storage capacitors. Further, specifically, the above-described active snubber systems reduce and/or smooth the dv_ce/dt of transient voltage spikes, thus mitigating its detrimental effects in power converter circuits and other power systems. The above-described active snubber systems achieve dynamic voltage sharing across power converter switching devices with improved power efficiency using smaller devices that are less expensive to manufacture and maintain. Further, the above-described active snubber systems enable the retrofitting of existing power converters that do not possess sophisticated gate drive controllers, yet still need to be employed for high speed switching applications. Moreover, the above-described active snubber systems do not include capacitor(s) located between the gate and emitter terminals of semiconductor switching devices, and also reduce or eliminate the need for matching the parameters of semiconductor switching devices and individual snubber components.

[0052] The above-described active snubber systems also facilitate regulating the current in IGBTs. Regulating this transient current, including, without limitation, the "tail current", is achieved by the active snubber systems described herein during and/or after turning off IGBT main switches. Specifically, the controllers used in the above-described active snubber systems regulate the tail current during and/or after turn off for a relatively long time as compared to the commutation process. Such active snubber systems are used for the aforementioned aims in power converters where space is at a premium, sophisticated gate drive controllers are not available, and where matched semiconductor switching devices and matched snubbers are not desirable or feasible. [0053] An exemplary technical effect of the methods, systems, and apparatus described herein includes at least one of: (a) mitigating switching-synthesized high voltage spikes across semiconductor switching devices such as IGBTs in power converters, including power converters with serially coupled IGBTs; (b) dispensing with the need for a high number of large passive components including power resistors, power diodes, and large energy storage capacitors; (c) reducing and/or smoothing the dv_ce/dt of transient voltage spikes; (d) achieving dynamic voltage sharing across power converter switching devices such as IGBTs, including applications with serially coupled IGBTs with improved power efficiency using smaller devices that are less expensive to manufacture and maintain; (e) enabling the retrofitting with improved active snubber systems of existing power converters that do not possess sophisticated gate drive controllers, yet still need to be employed for high speed switching applications; (f) providing an improved active snubber system that does not include capacitor(s) located between the gate and emitter terminals of semiconductor switching devices such as IGBTs; and (g) mitigating the need for matching the parameters of semiconductor devices and the components of snubber systems.

[0054] Exemplary embodiments of methods, systems, and apparatuses for active snubber systems are not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the methods, systems, and apparatuses may also be used in combination with other systems requiring lowering and/or smoothing of the dv_ce/dt across power converter semiconductor switching devices due to transient switching-synthesized voltage spikes, and the associated methods, and are not limited to practice with only the systems and methods described herein. Rather, the exemplary embodiment can be implemented and utilized in connection with many other applications, equipment, and systems that may benefit from using autonomous active snubber systems to improve the performance, reliability, power efficiency, maintainability, and manufacturability of power converters and other power systems in various applications. [0055] Although specific features of various embodiments of the disclosure may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the disclosure, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing.

[0056] Some embodiments involve the use of one or more electronic or computing devices. Such devices typically include a processor or controller, such as a general purpose central processing unit (CPU), a graphics processing unit (GPU), a microcontroller, a reduced instruction set computer (RISC) processor, an application specific integrated circuit (ASIC), a programmable logic circuit (PLC), and/or any other circuit or processor capable of executing the functions described herein. The methods described herein may be encoded as executable instructions embodied in a computer readable medium, including, without limitation, a storage device and/or a memory device. Such instructions, when executed by a processor, cause the processor to perform at least a portion of the methods described herein. Where data, signals, and other transmissions are to be transmitted by way of communication devices, methods, and systems, those devices, methods, and systems, including, without limitation, serial, fiber-optic, telephonic, cellular, radio, infrared, satellite, internet, Ethernet, wire-line, are understood to be known and enabled for use in this disclosure to persons having ordinary skill in the art. The above examples are exemplary only, and thus are not intended to limit in any way the definition and/or meaning of the term processor.

[0057] This written description uses examples to disclose the embodiments, including the best mode, and also to enable any person skilled in the art to practice the embodiments, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.

Claims

WHAT IS CLAIMED IS:

1. An active snubber system for at least one switching device, the at least one switching device including at least a first switch terminal, a second switch terminal, and a third switch terminal, said active snubber system comprising: at least one sensing device configured to measure at least one characteristic associated with the at least one switching device; at least one auxiliary device, said auxiliary device comprising a first auxiliary terminal coupled to the first switch terminal, a second auxiliary terminal coupled to the second switch terminal, and a third auxiliary terminal; and at least one controller coupled to said at least one auxiliary device and to said at least one sensing device, said at least one controller configured to transmit a control signal to said third auxiliary terminal to regulate a resistance of said auxiliary device, the control signal at least partially based on a function of a value of the at least one characteristic, wherein said controller facilitates at least one of: regulation of a current transmitted from the first switch terminal to the second switch terminal by diverting at least a portion of the current through said auxiliary device; and regulation of a voltage between the first switch terminal and the second switch terminal.

2. The active snubber system in accordance with Claim 1, wherein the first switch terminal is at a higher voltage potential than the second switch terminal.

3. The active snubber system in accordance with Claim 1, wherein said at least one sensing device comprises at least one capacitor comprising a first capacitor terminal coupled to the first switch terminal and a second capacitor terminal coupled to the second switch terminal.

4. The active snubber system in accordance with Claim 1, wherein the at least one characteristic includes at least one of: the voltage between the first switch terminal and the second switch terminal; a time derivative of the voltage between the first switch terminal and the second switch terminal; the current transmitted from the first switch terminal to the second switch terminal; and a time derivative of the current transmitted from the first switch terminal to the second switch terminal.

5. The active snubber system in accordance with Claim 1 further comprising at least one two terminal component coupled between the first switch terminal and the second switch terminal, wherein said at least one two terminal component comprises at least one of a resistive component and an inductive component.

6. The active snubber system in accordance with Claim 3 further comprising at least one resistor serially coupled to and between at least one of: said first capacitor terminal and the first switch terminal; and said second capacitor terminal and the second switch terminal.

7. The active snubber system in accordance with Claim 1, wherein the at least one switching device is a plurality of switching devices.

8. The active snubber system in accordance with Claim 4, wherein a second switch terminal of a first switching device of the plurality of switching devices is serially coupled to a first switch terminal of a second switching device of the plurality of switching devices.

9. The active snubber system in accordance with Claim 3, wherein the at least one characteristic includes an instantaneous current through said at least one capacitor, said at least one sensing device comprises at least one current sensing device configured to measure the instantaneous current, said controller further configured to convert the instantaneous current into the control signal, the control signal at least partially based on at least one of: a voltage value function of a value of the instantaneous current; a voltage value function of a value of an absolute value of the instantaneous current; a current value function of a value of the instantaneous current; and a current value function of a value of an absolute value of the instantaneous current.

10. The active snubber system in accordance with Claim 3, wherein the at least one characteristic includes an instantaneous voltage across said first capacitor terminal and said second capacitor terminal, said at least one sensing device comprises at least one voltage sensing device configured to measure the instantaneous voltage, said controller further configured to convert the instantaneous voltage into the control signal, the control signal at least partially based on at least one of: a voltage value function of a value of the instantaneous voltage; a voltage value function of a value of an absolute value of the instantaneous voltage; a current value function of a value of the instantaneous voltage; and a current value function of a value of an absolute value of the instantaneous voltage.

11. The active snubber system in accordance with Claim 3, wherein said controller comprises at least one high gain differential amplifier comprising: a non-inverting terminal coupled to a line transmitting the control signal; an inverting terminal coupled to said second capacitor terminal; and an output terminal coupled to said third auxiliary terminal, said output terminal configured to transmit a modified control signal to said third auxiliary terminal, the modified control signal at least partially based on a function of a value of the at least one characteristic.

12. The active snubber system in accordance with Claim 11 further comprising a voltage source serially coupled to and between said inverting terminal and said second capacitor terminal, said voltage source configured to synthesize at least one of a constant voltage and a time-varying voltage, said voltage source further configured to reference an instantaneous value of a voltage of said non-inverting terminal.

13. The active snubber system in accordance with Claim 1 further comprising a gate drive -to-active snubber line, wherein at least one of said at least one sensing device and said at least one controller comprises an enable terminal coupled to said gate drive-to-active snubber line, said gate drive -to-active snubber line configured to transmit a synchronization signal from a gate drive circuit of the switching device to said enable terminal, said enable terminal further coupled to circuitry associated with at least one of said at least one controller and said at least one sensing device configured to receive the synchronization signal, wherein said circuitry associated with at least one of said at least one sensing device and said at least one controller is further configured to enable a functionality of at least one of said at least one sensing device and said at least one controller when the synchronization signal is present, and disable the functionality when the synchronization signal is absent.

14. The active snubber system in accordance with Claim 13, wherein said gate drive-to-active snubber line comprises at least one of an electrical line and an optical line, the synchronization signal includes at least one of an electrical signal and an optical signal, and said circuitry associated with at least one of said at least one sensing device and said at least one controller comprises circuitry to receive, transduce, transmit, and process the synchronization signal into further electrical signals.

15. A method of operating an active snubber system for at least one switching device, the at least one switching device including at least a first switch terminal, a second switch terminal, and a third switch terminal, said method comprising: selecting at least one characteristic associated with the at least one switching device; selecting a reference function at least partially based on a function of a value of the at least one characteristic; selecting at least one sensing device configured to measure the at least one characteristic; selecting at least one auxiliary device including a first auxiliary terminal, a second auxiliary terminal, and a third auxiliary terminal; selecting at least one controller configured to transmit a control signal to the third auxiliary terminal, the control signal at least partially based on a function of a value of the at least one characteristic, wherein the controller facilitates at least one of: regulating a current transmitted from the first switch terminal to the second switch terminal by diverting at least a portion of the current through the auxiliary device; and regulating a voltage between the first switch terminal and the second switch terminal; coupling the first auxiliary terminal and the second auxiliary terminal to the first switch terminal and the second switch terminal, respectively, wherein said coupling facilitates defining a current path between the first switch terminal and the second switch terminal, and wherein a resistance of the current path is at least partially based on a function of a value of the at least one characteristic; coupling the controller to the third auxiliary terminal and to the at least one sensing device; and coupling the at least one sensing device to the at least one switching device and to the controller.

16. The method in accordance with Claim 15 further comprising: selecting a capacitance value of at least one capacitor, the at least one capacitor including a first capacitor terminal and a second capacitor terminal, wherein the capacitance value at least partially defines a time constant to facilitate following the reference function by the active snubber system; coupling the first capacitor terminal and the second capacitor terminal to the first switch terminal and the second switch terminal, respectively; and coupling the at least one sensing device to the at least one capacitor, wherein the at least one capacitor, the at least one auxiliary device, and the at least one sensing device are on separate parallel circuit branches of the active snubber system.

17. The method in accordance with Claim 16 further comprising: selecting a gain value of at least one high gain differential amplifier to facilitate following the reference function by the active snubber system; coupling a non-inverting terminal of the at least one high gain differential amplifier to the controller to receive the control signal transmitted by the controller; coupling an inverting terminal of the at least one high gain differential amplifier to one of the first capacitor terminal and the second capacitor terminal; and coupling an output terminal of the at least one high gain differential amplifier to the third auxiliary terminal, wherein the output terminal transmits a modified control signal to the third auxiliary terminal.

18. The method in accordance with Claim 17 further comprising: selecting a voltage source, including at least one of a constant voltage source and a time-varying voltage source, the voltage source configured to reference an instantaneous value of the voltage of the non-inverting terminal to facilitate following the reference function by the active snubber system; and serially coupling the voltage source between the inverting terminal and one of the first capacitor terminal and the second capacitor terminal.

19. The method in accordance with Claim 16 further comprising: selecting a resistance value of at least one resistor to facilitate following the reference function by the active snubber system; and serially coupling the at least one resistor to and between at least one of: the first capacitor terminal and the first switch terminal; and the second capacitor terminal and the second switch terminal.

20. The method in accordance with Claim 15, wherein selecting a reference function at least partially based on a function of a value of the at least one characteristic comprises selecting at least one of: a voltage between the first switch terminal and the second switch terminal; a time derivative of the voltage between the first switch terminal and the second switch terminal; a current transmitted from the first switch terminal to the second switch terminal; and a time derivative of the current transmitted from the first switch terminal to the second switch terminal.

21. The method in accordance with Claim 15 further comprising controlling a function of the value of a synthesized voltage between the first switch terminal and the second switch terminal of the at least one switching device to facilitate following the reference function by the active snubber system, wherein the at least one switching device is a plurality of switching devices, and wherein a measurable and controllable operational parameter of each switching device of the plurality of switching devices includes the function of the value of the synthesized voltage between the first switch terminal and the second switch terminal.

22. The method in accordance with Claim 21, wherein said controlling a function of the value of the synthesized voltage comprises controlling at least one switching device of the plurality of switching devices with a specific reference function.

23. The method in accordance with Claim 21 further comprising coupling at least one antiparallel diode to at least one switching device of the plurality of switching devices, wherein said controlling a function of the value of a synthesized voltage comprises regulating a sum of currents comprising a sum of at least one of the following: a tail current of at least one switching device of the plurality of switching devices; a current through the at least one antiparallel diode; and a current through at least one of the first auxiliary terminal, the second auxiliary terminal, and the third auxiliary terminal.

24. The method in accordance with Claim 23, wherein said controlling a function of the synthesized voltage further comprises controlling each switching device of the plurality of switching devices to facilitate at least one of: regulating the sum of currents; and equalizing the voltage between the first switch terminal and the second switch terminal.

25. The method in accordance with Claim 16, wherein: said selecting at least one characteristic comprises selecting at least one of an instantaneous current through the at least one capacitor and an instantaneous voltage between the first capacitor terminal and the second capacitor terminal; said selecting at least one sensing device comprises selecting at least one of a current sensing device and a voltage sensing device; and said selecting at least one controller comprises the control signal at least partially based on a voltage value function of at least one of: a value of the instantaneous current through the at least one capacitor; a value of an absolute value of the instantaneous current through the at least one capacitor; a value of the instantaneous voltage between the first capacitor terminal and the second capacitor terminal; and a value of an absolute value of the instantaneous voltage between the first capacitor terminal and the second capacitor terminal.