Patent Application
20120230076
This invention generally relates to static voltage balancing among series-connected power switching devices, which preferably comprise one or more insulated gate bipolar transistors (IGBTs) or similar (e.g., JFETs, MOSFETs, SiC transistors, etc.). More particularly, the invention relates to a method of controlling sharing of voltage among such devices, to a reference signal controller, a reference signal generator, an active voltage control circuit, and to a power switching circuit such as a DC-AC inverter or a power converter for motor control.
Insulated gate bipolar transistors (IGBTs) are used for a wide range of power applications, from power supplies, computers and locomotives, to high voltage transmission lines. In particular, IGBTs are advantageously used to control large currents by the application of low level voltages or currents, some IGBTs having ratings of, e.g., 1600V and 1200 A.
The use of a single high voltage IGBT in a system for switching medium or high voltages is generally undesirable, since a suitable IGBT may be costly and/or have slow switching speed. Such systems are generally more easily constructed using multiple IGBTs arranged in a series topology. In an example inverter, IGBTs may be stacked and placed between power supply rails to form a phase leg as shown for example in
Nevertheless, difficulties remain even when a series connection of IGBTs is used, for example in relation to reliability and/or cost. If the overall voltage is not shared equally among the series IGBTs in the off (i.e., substantially non-conducting) state, lifetime of the IGBTs supporting a greater share of the overall voltage may be reduced. Even where additional circuitry is provided to improve the control of the IGBTs, this generally does not ensure perfect equality of shared voltage amongst the IGBTs and adds to the cost, power consumption, size and/or complexity of a power switching circuit comprising the series IGBTs.
Voltage balancing of an IGBT series connection generally focuses on dynamic voltage balancing, i.e., voltage balancing during switching transients. For example, snubber circuits may be used to assist and/or delay the switching of individual devices. However, these circuits generally increase the overall switching time of the series connection, may add to circuit size and/or cost and/or may result in losses that are difficult to recover. Moreover, circuits focused on dynamic voltage balancing generally do not achieve perfect equality of voltage sharing in the interval between switching events.
Thus improvements in static voltage balancing remain desirable, even in the presence of dynamic voltage balancing circuitry, to improve voltage balancing of series IGBTs when they are controlled to be in the off-state. This is the case since, for example, where large IGBT current and voltage occurs during a switching transient, the first IGBT to turn off may end up supporting a disproportionately large share of the overall voltage during the off state. The unequal voltage sharing may result in higher collector-emitter voltage in one or more of the IGBTs during the off-state such that the IGBT(s) have reduced reliability and/or operate beyond their safe operating limits. Thus, it may be advantageous to apply static balancing to redistribute the voltage more equally across series-connected IGBTs.
Static voltage balancing may be improved by paralleling a large voltage-divider resistor across each IGBT. However, there are several disadvantages of such paralleling. One disadvantage is that the paralleled resistor may cause extra power losses. A second disadvantage is that the value of the voltage-divider resistor generally has to be chosen very carefully: if the value is too big, the voltage sharing may be poor; conversely, if the value is too small, the power losses may be too high. A further disadvantage is that the use of extra components such as voltage-divider resistors and capacitors may reduce the robustness of the system. Further still, for IGBTs having a current tail following turn off (this depends on their design), the tail current effectively gives them a high leakage current for a period after turn off, further complicating the choice of voltage-divider resistor.
Active voltage clamping circuits, for example using a zener diode voltage reference, are available. Such circuits may prevent series IGBTs operating beyond their safe operating limits by limiting IGBT collector voltage. However, the individual IGBTS may nevertheless operate under sufficiently different current-voltage conditions in the nominal off-state of the series connection that reliability of the series connection of IGBTS as a whole may be significantly degraded.
Thus, there remains a need for improved static voltage balancing among series-connected power switching devices. Such improvement may concern, inter alia, increased reliability of IGBT switching, scalability (e.g., allowing a greater number of IGBTs to be connected in series), cost, size, complexity, efficiency (i.e. low power dissipation, e.g., by reducing switching losses), and/or quality of output voltage and current waveforms. etc. Similar improvements may be advantageous for voltage balancing among series connected JFETs, MOSFETs, Silicon Carbide transistors, etc.
For use in understanding the present invention, the following disclosures are referred to by way of useful background information:
According to a first aspect of the invention, there is provided a method of controlling sharing of voltage among series-connected power switching devices, wherein at least one said device is an insulated gate bipolar transistor (IGBT), the method comprising: controlling the IGBT dependent on a reference signal and collector or emitter voltage of the IGBT such that during an off period of said IGBT said reference signal limits an absolute value of collector-emitter voltage of said IGBT to be within a range; and control to temporarily change during said limiting said reference signal from an initial value to a temporary clamp value to reduce said range, said change when each of said devices is in a substantially non-conducting state.
Generally, the IGBT is considered throughout this specification to be an re-channel IGBT. However, as the skilled person reading this specification will appreciate, the invention is further applicable to p-channel IGBT control.
The range may be restricted by the reference signal only at one end, e.g., the limiting may merely set an upper or lower limit to the collector-emitter voltage, e.g., by determining a threshold beyond which the collector or collector-emitter voltage cannot pass; thus the limiting does not necessarily mean that the collector or collector-emitter voltage is at a threshold set by the reference signal. Furthermore, the reduction in the range may merely narrow the range by reducing a maximum level which the collector or collector-emitter voltage may not exceed (i.e., in embodiments without influencing any minimum collector-emitter voltage).
Preferably, the temporary change control is implemented by a control circuit acting on a reference signal, or by a reference signal generator that generates the reference signal controlled to have the temporary change ab initio. The reference signal including the temporary change may be input to an AVC or CAVC circuit, for example as VREF of
The temporary change control may be applied to reference signal(s) of one or more, e.g., all, of the series power switching devices in the off state, .e.g., all such devices in an inverter leg, or in an off side of an inverter leg. The change may be applied shortly after a last one of these devices has been turned off.
The reference signal may have a waveform such as shown in various figures of the present specification, i.e., substantially pulse-like to provide transitions to and from the off period, the limiting and temporary change occurring during the off period. However, in an embodiment the reference signal may have any waveform shape providing it comprises at least an off period for maintaining the off state of the IGBT, however long this period has lasted and/or will last. (“Off period” and “clamping period” are used interchangeably throughout this specification and correspond to a period throughout which the device is being controlled to be in a substantially (e.g., completely, or conducting less than ˜1 to ˜2% of the on state current—disregarding any change of off-state current during the temporary change) non-conducting state, i.e. off state, e.g., as shown by the ‘Off’ period between reference signal transitions in
Optionally, the change is a voltage reduction of said reference signal and said initial value is a maximum voltage value of said reference signal during said limiting, for example where the IGBT is n-channel. However, the change and initial value may alternatively be an increase and a minimum, for example where the IGBT is p-channel. (The skilled person reading this specification will appreciate that that reference signal and temporary change may depend on, e.g., a number and/or arrangement of inverting amplifiers and/or non-inverting amplifiers between the reference signal and IGBT control terminal (gate)).
More specifically, the temporary change is preferably a reduction of the reference signal from, and subsequent return to, a particular value such as the initial value. The initial value is preferably a value for maintaining an off state of the IGBT during an increase in the shared voltage, e.g., due to a transient or ripple, more preferably while limiting the IGBT to a safe operating region. The shared voltage may for example be at least a portion of a supply rail of an inverter for DC-to-AC conversion, a supply rail of an electric car, or of a High Voltage Direct Current (HVDC) line generally used for long-distance transmission, etc.
The series-connected power switching devices may comprise one or more IGBTs, for example may be a chain of IGBTs or may be one or more IGBTs in a chain with one or more other power switching devices, e.g., MOSFET(s), JFETs, SiC transistors, etc. (References throughout this specification to an IGBT device may be considered to refer to a single IGBT or to an IGBT module comprising an IGBT and a freewheel diode, which is typically found in parallel with the IGBT in a module, to reduce damage such as due to flyback).
The temporary clamp value preferably limits the collector-emitter voltage to a voltage substantially equal to (e.g., within ˜95-˜105% or ˜98-˜102% or exactly equal to) said shared voltage (e.g., Vss) divided by the number of series-connected power switching devices sharing that voltage. Additionally or alternatively, the initial value of the reference signal is preferably at least about 10%, 20% or 30% greater or less than the temporary clamp value; the initial and temporary clamp values being relative to the value of the reference signal when the device is controlled to be fully on and/or relative to the IGBT emitter voltage—see use of emitter voltage as voltage reference in
The control dependent on the reference signal and collector and/or emitter voltage of the IGBT may be provided by controlling a gate-emitter voltage or gate current of the IGBT on the basis of comparison of a voltage dependent on the reference signal and a voltage dependent on collector voltage of the IGBT. Such gate control may involve the passing of a small collector-emitter current; however this is generally negligible. Examples of such IGBT control are shown by the Active Voltage Clamping (AVC) circuit of
The initial value of the reference signal (e.g., VCLAMP as shown in the reference signal profiles of
To understand advantages of embodiments, it is noted that where a power switching device, e.g., IGBT, in a series connection supports for example Vss/n+m %, this may mean that another of the power switching devices supports Vss/n−m %. A difference of 2m % (20% where a safety margin of 10% is used in relation to voltage clamping) then exists between the voltages supported by the two devices. A device supporting the higher voltage between its collector and emitter terminals may thus be stressed more than the other and this may be detrimental to reliability of the whole series IGBT connection. In an embodiment, the temporary change, e.g., reduction, in the reference signal may however push the associated power switching device away from a state where it would be supporting more than Vss/n, e.g., where it is clamped at Vss/n+m %, such that the voltage shares of the two devices becomes more equal. Thus, an advantage of an embodiment may be to improve reliability and/or lifetime of the series connection as a whole.
Further in this regard, an embodiment may enhance scalability of a power switching circuit comprising the series chain of power switching devices. For example, where a leg of such a circuit has 100 IGBTs in series and unequal voltage distribution in the off state occurs such that 10 of these IGBTs are each supporting 10% more voltage than if the overall voltage were distributed equally, this may allow one of the IGBTs to support effectively zero volts (albeit in a substantially non-conducting state). Similarly, if 50 of the 100 IGBTs are operating at +10%, five of them may be operating at substantially 0V. Generally, the greater the number of power switching devices provided in the series connection, the greater the number of devices that may be operating at substantially 0V. Thus, even when clamping (e.g., using AVC or CAVC as described below) is implemented to prevent each IGBT voltage exceeding the overall voltage divided by the total number of series IGBTs+m % margin, where the number of IGBTs is large such voltage limiting may nevertheless allow significant differences in current-voltage operating conditions between individual IGBTs. This may be detrimental to reliability of the series connection as whole and thus may impose a limit on the number of IGBTs and overall supply voltage. By improving equality of voltage sharing, an embodiment may advantageously increase or substantially eradicate this limit.
It is further noted that, since unequal voltage sharing of series-connected IGBTs may mean that the devices are in different states when turn-on is triggered, the turn-on times among the devices may be different. However, the temporary change, e.g., reduction of the reference signal in an embodiment may reduce residual charge in the IGBT to substantially equalize (e.g., to within 1, 2 or 5% difference) turn-on time of said IGBT and turn-on time of another of the power switching devices (e.g., another said IGBT). This may for example occur where two or more of the power switching devices are each controlled by a reference signal (respective or in common) having a said temporary change.
Regarding timing, the temporary change may occur anywhere within a finite said off period, for example may begin at, end at or extend over a mid-point of the off period, or may occur towards either end of the off period. The exact timing relative to the start of the off period may be fixed in advance, or may be determined on the basis of monitoring, e.g. of the IGBT collector current. For example, the exact timing of the start of the change may be determined on the basis of detecting within the off (clamping) period the tail current (e.g., value and/or rate of change) and/or a temporary increase in collector current due to leakage subsequent to the start of the off period (see
It is further noted regarding timing that there may be a plurality of the temporary changes of a particular reference signal at different respective times during a single off period. As for the case of a single temporary change within the off period, the exact timing and number of these temporary changes may depend on the nature of the overall supply voltage, e.g., inverter supply rail, electric car supply rail, HVDC, etc., and/or may be determined on the basis of monitoring (e.g., voltage, current within the series connection, and/or temperature).
Regarding duration, and considering application of an embodiment for example in an inverter, a said off period may last for about, e.g., 5, 8, 10, 20, 50, 100 or 200 microseconds. The temporary change preferably lasts for less than about 10% of the off period, more preferably less than 5%. The duration of the temporary change for any off period duration may be for example be less or equal to about 1 or 2 microseconds, more preferably less than or equal to about 5, 8 or 10 microseconds.
Returning to discussion of the control mechanism, each of the series-connected power switching devices (including the IGBT) may have a separate control loop for active clamping (e.g., AVC or CAVC). Each of a plurality of such devices may be controlled dependent on the same reference signal. Additionally or alternatively, a plurality of the reference signals may be provided for controlling respective groups of one or more of the devices, at least one of the provided reference signals being temporarily changed during it's off (clamping) period. Where a plurality of reference signals for the series connections each have a said temporary change, the reference signals preferably synchronized to have coincident off and/or temporary change periods. Preferably, the temporarily changes are performed substantially simultaneously (e.g., within about 1 microsecond of each other) for each of the reference signals. However, the temporary changing of one or more of the reference signals may be performed at a different time during a synchronized (i.e., same start and end time) off period, relative to the temporarily changes of other(s) of the reference signals. Thus, the reference signals may be controlled so that their respective temporary changes are synchronized, or they may be controlled individually so that the relative timings of the respective temporary changes are changed cycle by cycle.
In an embodiment, a said temporary change may be applied only to a selected one or more of the series connected devices, e.g., the IGBT(s), in a particular turn-off/turn-on cycle, depending on monitoring of conditions within or ambient to the series connection, such as voltage (e.g., device collector and/or collector-emitter voltage), current (e.g., device collector current), temperature (device and/or ambient), etc. For example, only device(s) that are supporting greater than Vss/n (Vss being the shared voltage and not necessarily the full voltage between supply rails) and/or whose VCE voltage is being actively suppressed by an associated reference signal (e.g., having a said initial value) may have temporary change(s) applied to their corresponding reference signal(s). As for all instances of monitoring described herein, this monitoring may allow IGBT gate drives to be controlled to react to changing conditions such as device degradation and/or temperature. Thus, monitoring may be applied to a selection of or all power switching devices in a single or multiple-leg inverter; in embodiments such an inverter may have more than, e.g., 50 or 100 or even more such devices. Each device may be monitored individually to determine whether and/or when a temporary change should be applied to a reference signal provided in respect of that device.
Thus, where a plurality of the reference signals are provided, one or more may be selected to have the temporary change applied to it, the selection on the basis of monitoring at least one voltage in the series connection, and the temporarily changing performed on the or each of the selected reference signals during an off (clamping) period which is preferably synchronized across the reference signals. Such a monitored voltage may for example be a collector-emitter voltage or collector voltage of any power switching device in the series connection.
The method may be advantageous for improving reliability under rapidly and/or widely changing conditions, e.g., of load current or temperature. As an example, the method may comprise determining, on the basis of monitoring the shared voltage, load current or a temperature such as temperature of a the IGBT or ambient temperature, any one or more of: said initial value; said temporary clamp value; depth or height of the temporary change of the reference signal (i.e., difference between the initial value and temporary clamp value); duration of said off period; duration of said temporary change; start time of said temporary change; frequency of occurrence and/or number of occurrences of said temporarily changing of said reference signal within a said off period. Such determination may be performed in respect of the, each or any one or more said reference signal(s), depending on whether an embodiment provides one or more reference signals.
Additionally or alternatively in an embodiment providing one or more reference signals, there may be provided the method, comprising determining, on the basis of monitoring the shared voltage or a temperature such as temperature of a said IGBT or ambient temperature, any one or more of: for each of the reference signals, whether to perform a said temporary change during its said off period; and for each of the reference signals when to perform a said temporary change during its said off period. Similarly as above, such determination may be performed in respect of the, or each or any one or more of a plurality of, said reference signal(s). Where an embodiment makes such a determination as to whether to perform the temporary change, if the determination is negative the embodiment may automatically check again after a fixed period, e.g., 10 us, whether to apply the temporary change at a later time.
The invention further provides processor control code (i.e. a non-transitory computer-readable medium of program instructions) to implement the above-described method comprising any one or more of the above optional features, for example on an embedded processor. The code may be provided on a carrier such as a disk, CD- or DVD-ROM, programmed memory such as read-only memory (Firmware), or on a data carrier such as an optical or electrical signal carrier. Code (and/or data) to implement embodiments of the invention may comprise source, object or executable code in a conventional programming language (interpreted or compiled) such as C, or assembly code, code for setting up or controlling an ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array), or code for a hardware description language such as Verilog (Trade Mark) or VHDL (Very high speed integrated circuit Hardware Description Language). As the skilled person will appreciate such code and/or data may be distributed between a plurality of coupled components in communication with one another.
According to a second aspect of the invention, there is provided a reference signal controller for applying a temporary change to a reference signal, said reference signal for voltage clamping an IGBT such that during an off period of said IGBT said reference signal limits collector-emitter voltage of the IGBT to be within a range, the reference signal controller arranged to temporarily change during said limiting said reference signal from an initial value to a temporary clamp value, said temporary change for controlling sharing of voltage among series-connected power switching devices including said IGBT when said devices are each in a substantially non-conducting state.
As for the above-described method, the devices may comprise a plurality of IGBTs, e.g., may be a chain of series-connected IGBTs. The reference signal controller may be implemented as a stand-alone unit (
Regarding timing, the controller is preferably configured to trigger, e.g., using a timer, the temporary change at substantially a mid-point, or to extend over a mid-point, of the reference signal off (clamping) period, or nearer either end of the off period. Such triggering may be in response to, for example, monitoring conditions such as, e.g., voltage and/or current within the series connection, and/or temperature; such monitoring may further be used to determine the depth and/or length of the temporary change. The reference signal controller preferably comprises trigger circuitry, e.g., a tail current monitor, configured to ensure that the temporary change is substantially, e.g., exactly, at the end of a tail current period of said IGBT, e.g., where the IGBT is NPT- or even PT-type with tail current. Additionally or alternatively, the reference signal controller may comprise trigger circuitry (e.g., comprising a timer) to trigger a said temporary change that extends to the end of the off period immediately prior to turning on said IGBT by the reference signal.
A timer may be provided in the reference signal controller to control duration of the temporary change to be less than ˜10% of the off period, preferably less than ˜5% of the off period, and/or to be less than or equal to about 2 microseconds, more preferably less than or equal to about 5 microseconds. (A timer may similarly be provided in the reference signal generator to determine off period duration which may be, e.g., about 5, 10, 20, 50, 100 or 200 microseconds).
A power switching circuit, e.g., inverter for AC-to-DC conversion, comprising the reference signal controller and the series-connected power switching devices and preferably AVC or CAVC substantially as in
There may further be provided a reference signal generator configured to generate a reference signal for an active voltage control circuit, said reference signal generator comprising the above reference signal controller arranged to apply said temporary change to said generated reference signal. Such a generator may be used to replace the generator shown in
There may further be provided a reference signal generator configured to generate a reference signal for an active voltage control circuit, said reference signal for voltage clamping an IGBT such that during a off period said reference signal when input to a said active voltage control circuit limits collector-emitter voltage of said IGBT to be within a range, the reference signal having an off period comprising a temporary change of said reference signal from an initial value to a temporary clamp value, said temporary change for controlling sharing of voltage among series-connected power switching devices including said IGBT when each of said devices is in a substantially non-conducting state. Such a reference signal generator may be for example a digital to analogue converter digitally controlled and/or programmed to provide an output waveform having the profile of the reference signal comprising the temporary change (for example as shown in any of
There may further be provided an active voltage control circuit, e.g., AVC or CAVC circuit for example as shown in
There may further be provided a power switching circuit comprising any above-described reference signal controller or reference signal generator and further comprising the series-connected power switching devices including the IGBT, wherein the power switching devices preferably comprise a plurality of IGBTs, the power switching circuit comprising a plurality of active voltage control circuits each arranged to control a respective said IGBT dependent on said reference signal.
There may further be provided a power switching circuit such as in inverter, comprising any above-described reference signal controller or reference signal generator, and said series-connected power switching devices including the IGBT, wherein said power switching devices comprise groups of one or more IGBTs and a plurality of active voltage control circuits each to control a respective said IGBT, the power switching circuit arranged to provide a plurality of said reference signals, each said reference signal to control the active voltage control circuits of a said group, the reference signal controller to temporarily change at least one said reference signal during a said off period.
Such a power switching circuit may comprise a voltage monitor to monitor at least one voltage in said series connection, a selector to select one or more said reference signals on the basis of said monitoring, said reference signal controller arranged to temporarily change the or each said selected reference signal during a said off period. The monitored voltage may be for example at a collector-emitter or collector of a said IGBT.
Any one or more of the instances of monitoring referred to above in relation to the method, reference signal controller, reference signal generator, active voltage control circuit and power switching circuit and their optional features may be performed using communications to and/or from units for performing the monitoring. Thus, a controller may determine one or more feature(s), such as on which reference signal(s) (i.e., for which of the power switching devices) the temporarily changing will be performed during a given off/clamping period, the initial value of reference signal(s), the temporary clamp value(s), depth of the temporary change (s), duration of the clamping period(s) and/or of the temporary change (s); and/or frequency of occurrence and/or number of occurrences of temporarily change (s) within off/clamping period(s). This may be achieved by the controller polling monitoring units coupled to the series connection, e.g., to the power switching devices and/or points within the series coupling of such devices. The controller may determine the feature(s) to have different values in respect of different power switching devices, depending on differences in states of the devices indicated by the monitoring. In an embodiment, the controller may however be substituted with distributed control.
According to a further aspect of the present invention, there is provided a power switching device controller for controlling sharing of voltage among series-connected power switching devices, wherein at least one said device is an insulated gate bipolar transistor (IGBT), the apparatus comprising: means for controlling the IGBT dependent on a reference signal and collector or emitter voltage of the IGBT such that during an off period of said IGBT said reference signal limits an absolute value of collector-emitter voltage of said IGBT to be within a range; and means for control to temporarily change during said limiting said reference signal from an initial value to a temporary clamp value to reduce said range, said change when each of said devices is in a substantially non-conducting state.
Any of the above reference signal controller, reference signal generator, active voltage control circuit, power switching circuit, inverter, or power switching device controller may be provided, wherein said IGBT is replaced with a JFET, MOSFET or SiC transistor, and said collector-emitter voltage is a drain-source voltage.
Any two or more of the above aspects, with or without any one or more of the optional features of the preferred embodiments, may be combined in any permutation. Further aspects of the invention comprise apparatuses corresponding to the above method embodiments and methods corresponding to the above reference signal controller reference signal generator, active voltage control circuit and power switching circuit embodiments. Still further aspects provide a reference signal controller, reference signal generator, active voltage control circuit or power switching circuit as described and/or illustrated herein.
Illustrative embodiments are defined in the appended dependent claims.
For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:
a, 5b, 5c, 5d, 5e show, respectively,: (a) waveforms associated with two IGBTs in series to both of which are applied the same reference signal with temporary clamp, i.e., temporary change in value of the reference signal, during the off-state; (b) a zoom-in of the turn-off of
a and 8b show: (a) an example inverter comprising series-connected IGBTs; and (b) a single leg of the inverter of
a and 9b show waveforms of clamping of two IGBTs in series without temporary clamping, i.e., without a temporary change, the setups of
The following introduces unclamped and clamped IGBT switching, before proceeding to describe static voltage balancing by applying a specially designed temporary clamping reference signal during IGBT off-state, i.e., a reference signal comprising a temporary change otherwise referred to as a ‘temporary clamp’.
Techniques for controlling IGBT switching include voltage, current, dv/dt, and/or di/dt open or closed loop feedback control. An example of closed loop control is shown by the schematic of an Active Voltage Control (AVC) technique in
In more detail, different amounts of residual charge in the IGBTs at turn-off may cause divergence in the IGBT states during the off-period as shown by the traces, which diverge and then settle to respective values in
Moreover, the difference between the respective values of the IGBT collector-emitter voltages at the end of the off period indicates unequal voltage sharing, which may reduce lifetime of the series connection of IGBTs. The device supporting higher voltage may suffer higher operating temperature and/or greater physical stress, which may lead for example to ageing of the device and/or cracking of the device packaging. Consequently, the lifetime of the two IGBTs combined may be reduced.
In comparison to AVC as described above, Cascade Active Voltage Control (CAVC) generally enhances the stability of the feedback system, increases the preciseness of voltage following, and/or reduces the switching power losses. In the CAVC of
In relation to an embodiment, the profile of the reference signal VREF in such AVC techniques is of particular interest.
(A reference signal used in any embodiment may for example have a range of from −6V to 11.5V. The amplification ratio for VCE may be ˜100).
The above clamping techniques applied to series-connected IGBTs may however allow one or more of the IGBTs to have a low voltage and be fully off while others of the IGBTs are in a higher voltage off state. Moreover, at least one of the power switching devices may be clamped, i.e., limited by the reference signal, during an increase in the overall shared voltage for example due to a transient or ripple, while other(s) may be operating in an unlimited state, e.g., supporting substantially 0V. Where the IGBTs are in such different states, their respective control loops may be saturated differently; since it is generally difficult to pull a control loop out of saturation, the different states will generally remain for the remainder of a clamping period. Furthermore, the resulting different operational states of the IGBTs may cause considerable problems at turn on, as the IGBTs are in different states closer or further away from turn on into a high current. In such a case the voltage sharing may become extremely poor, with increased likelihood of an overvoltage in one or more of the series IGBTs. This generally is not satisfactory for reliable high power equipment. Even just unequal voltage sharing, wherein not all of the series-connected IGBTs operate in a true off state, may degrade at least long term reliability of the IGBTs. Such issues relating to different states before turn-on may arise with various clamping methods for series power switching devices such as IGBTs, MOSFETs, etc.
The use of parallel sharing resistors potentially offers a solution to the issues outlined above, but has disadvantages such as cost. Adapting AVC or CAVC to have a clamping voltage that follows the overall shared voltage (Vss) such that all IGBTs in a series connection always support exactly Vss/n (n=the total number of IGBTs in the series connection) is difficult due to transients in Vss.
In an embodiment of the invention, static voltage balancing may be achieved by applying during IGBT off-state a specially designed temporary clamping reference signal, i.e., a reference signal comprising a temporary change such that where the upper value of the reference (
By applying a temporary clamp, i.e., temporary change such as reduction, to the reference signal, voltage sharing may be temporarily re-imposed as the IGBTs with a higher voltage have their voltage reduced, which may advantageously cause those IGBTs with a lower voltage to have their voltage raised, thereby sharing out the actual supply voltage more evenly.
The ‘temporary clamp’ is shown in
The temporary clamp may be triggered on the basis of monitoring an excursion in a control loop such as the AVC or CAVC control loop of
In an embodiment, the temporary clamp is generally of such a short duration that it is unlikely to coincide with a supply voltage transient and should it do so the current rise rate will generally limit the peak current reached before the reference reverts to its previous higher value. Preferably, the temporary clamp period is also relatively short to avoid inaccuracies in the voltage division causing a significant current in the IGBT string.
Preferably, the temporary clamp is a dip in the off-state reference value. The value of the reduced reference signal during the temporary clamp is preferably set by the exact division of the supply voltage by the number of series-connected power switching devices, e.g., IGBTs.
Temporary clamp periods of around 5 microseconds for a high current device may be applied at multiple times during the VREF clamp signal with benefit. For example, the VREF signal of
Furthermore, for different IGBTs the temporary clamp may be applied at preferential times relative to the initial turn off ramp.
Losses associated with the temporary clamp are generally minimal. Even if the temporary clamp turns on the IGBTs with the higher than desired voltages in an embodiment, very little current flows, since those IGBTs with a voltage below the desired level remain fully off.
The clamp is preferably temporary within the Off period shown in
a and 5b show that the temporary clamp can push the different VCEs of the series-connected power switching devices together. Specifically, the temporary clamping pushes the higher VCE lower causing the lower VCE higher, as they all follow the reference. Thus, and as indicated above, the temporary clamp may advantageously cause the voltages across those IGBTs with a lower voltage to have their voltage raised thereby sharing out the actual supply voltage more evenly.
Looking at
The position of the temporary reduction may for example be shortly after turn-off or shortly before turn-on of the power switching device(s). Generally, it is preferable to apply the temporary reduction to all series-connected power switching devices desired to be controlled off. For example, if the position is shortly before turn-on (e.g., within ˜10 us of turn-on), all of the devices may thus have a more similar ‘history’ and thus, for example, internal distribution of charge, prior to the turn-on. If shortly after turn-off (e.g., within ˜10 us of turn-off and/or earlier than ˜10 us before turn-on), it may however be preferable to apply the temporary reduction to all except, e.g., one, of the series-connected power switching devices desired to be controlled off, to be more certain that the series combination as a whole remains off, i.e., in a highly resistive, low current state. A controller may at any time be used to determine (for example on the basis of monitoring VCE voltages across the series connection) to which device(s) the reduction is applied; however this may be restricted to a middle portion of an Off period so that the above applies by default in respect of temporary reductions applied at positions shortly after turn-off and/or in positions shortly before turn-on.
More specifically, the position of the temporary clamp may be different according to the type of IGBT. For a Non-Punch Through (NPT) device, as the tail current time is quite long, the temporary clamping is preferably placed within and/or at the end of the tail current time. For a Soft Punch Through (SPT) device whose tail time is short, the temporary clamp may be placed very near to the ramp. The length of the temporary clamp may also depend on the device characteristics as well. If the device's power rating is small, which generally means it will respond to the reference signal very fast, the temporary clamp can be short, e.g., up to 2 microseconds, otherwise it should be longer, e.g., up to 5 or 10 microseconds. The value of the temporary clamp reduction is usually set as a small percentage of the proportional share of the operating voltage of the string. This may ensure that each of the devices connected in series maintains a similar operating voltage within a small band according to the original design.
Embodiments may be implemented in low voltage chips, computers, locomotives, high voltage transmission lines, motor control and inverters such as for renewable energy sources, e.g., wind turbines.
Feedback from the IGBTs, for example indicating detection of clamping, to a remote monitoring station enables performance to be monitored and advantageously early power transistor failure to be detected.
No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1103806.4 | Mar 2011 | GB | national |
