Patent Application
20130049843
1. Technical Field
Exemplary embodiments of the invention relate generally to a system and method for improving the reliability and efficiency of an electronic device such as an inverter. Moreover, such exemplary embodiments may relate to an improved drive circuit for voltage controlled power switches.
2. Discussion of Art
Power electronic devices may be used in a wide variety of systems and devices for delivering power to a load. For example, traction vehicles such as locomotives, employ electric traction motors for driving wheels of the vehicles. In some of these vehicles, the motors are alternating current (AC) motors whose speed and power are controlled by varying the frequency and the voltage of AC electric power supplied to the field windings of the motors. Commonly, the electric power is supplied at some point in the vehicle system as DC power and is thereafter converted to AC power of controlled frequency and voltage amplitude by a power electronic device such as an inverter. Power electronic devices may also be used in a variety of other applications, such as industrial power electronics, and stationary power conversion, among others. The power electronic device may include a set of semiconductor-based voltage controlled power switches (VCPS) such as reverse blocking insulated gate bipolar transistors (IGBTs), reverse conducting insulated gate bipolar transistors (RC-IGBTs), bi-mode insulated gate transistors (BIGTs), and the like. RC-IGBTs and BIGTs are reverse conductive power switches (RCPSs) which form a subgroup within the VCPS.
Briefly, in accordance with an exemplary embodiment of the invention, there is provided a power electronic module that includes a power switch module and a drive circuit operatively coupled to the power switch module. The exemplary drive circuit is configured to enable and disable a forward conduction mode operation of the switch module. The drive circuit disables forward conduction mode operation of the power switch module when the power switch module is operating in reverse conduction mode.
In another exemplary embodiment of the invention, there is provided a power system for a vehicle. The exemplary power system includes a first power switch module coupled to a DC rail voltage and configured to switch on and off in an alternation with a second switch module to produce an output AC waveform. The exemplary power system also includes a drive circuit operatively coupled to the first switch module and configured to enable and disable a forward conduction mode operation of the first switch module. The drive circuit disables forward conduction mode operation of the first power switch module when the first power switch module is operating in reverse conduction mode.
In another exemplary embodiment of the invention, there is provided a method comprising receiving a command from a control circuit to activate a forward conduction mode operation of a power switch module. The exemplary method also includes determining whether the power switch module is operating in reverse conduction mode. If the power switch module is operating in reverse conduction mode, forward conduction mode operation of the power switch module is disabled. If the power switch module is not operating in reverse conduction mode, forward conduction mode operation of the power switch module is enabled.
These and other features, aspects, and advantages of the invention 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:
Referring again to
As noted above, the inverters 208 used to generate the AC waveform may include VCPSs. A VCPS employs at least two power terminals and one or two control terminals. There are different naming conventions for the power terminals depending on the VCPS type, examples are anode and cathode or collector and emitter. When positive current through these power terminals is conducted from anode to cathode or from collector to emitter it is referred to as forward conduction. When positive current through the power terminals is conducted from cathode to anode or from emitter to collector it is referred to as reverse conduction. The same holds for the voltage across the power terminals where a positive voltage from anode to cathode or from collector to emitter is referred to as forward polarization and a positive voltage from cathode to anode or from emitter to collector is referred to as reverse polarization. In accordance with embodiments, the drive circuitry used to drive the switches may be configured to determine the polarity of the voltage across or the current through the power terminals of the switch. The drive signal produced by the drive circuitry may depend, at least in part, on the detected polarity. Depending on the type of switch used in the inverter, various benefits may be realized by controlling the drive signal based on the detected polarity.
Each IGBT 302 and 304 is driven by a gate driver 314 operatively coupled to the gate of the corresponding IGBT 302 and 304. A control circuit 316 may be operatively coupled to the gate drivers 314 to coordinate the switching of the IGBTs 302 and 304. The control circuit 316 may cause the IGBTs 302 and 304 to pulse on and off in an alternating fashion to produce an AC waveform at the phase output 318. To prevent a short circuit between the upper rail voltage 310 and the lower rail voltage 312, the drive signals for the IGBTs 302 and 304 are coordinated so that the IGBTs are not both turned on at the same time. For example, a time delay may be imposed between the time that one IGBT is switched off and the time that the other IGBT is switched on. This time delay is referred to herein as an interlock time and may be approximately 20 to 30 micro seconds, for example. Although only one leg 300 is shown, it will be appreciated that the inverter module may include two, three, or more legs, each providing an output AC waveform for a particular phase. For example, three legs may be used to produce a three-phase AC output waveform. A standard switching scheme is described further in relation to
The reverse conduction mode operation of the BIGTs 402 and 404 is determined by the polarity of the voltage across the emitter and collector of the BIGT 402 or 404. For example, the lower BIGT 404 will operate in reverse conduction mode if the voltage at the phase output 318 is lower than the voltage at the lower rail 312. When in reverse conduction mode, the BIGTs 402 and 404 provide a conductive path for freewheeling current that may result when the other BIGT in the leg 400 is switched off. The BIGTs 402 and 404 can operate in reverse conduction mode regardless of whether the gate voltage is positive or negative. During the reverse conduction mode operation of the lower BIGT 404, current is conducted from the lower rail 312 to the phase output 318.
The control circuit 316 may be operatively coupled to the gate driver 314 to coordinate the switching of the BIGTs 402 and 404. The control circuit 316 may cause the BIGTs 402 and 404 to pulse the forward conduction mode operation of the BIGTs 402 and 404 on and off in an alternating fashion to produce an AC waveform at the phase output 318. As described above in reference to
As shown in
For purposes of the present discussion, when current through a power switch module is positive, the corresponding power switch module is referred to as operating in forward conduction mode. When the voltage across the power terminals of the power switch is polarized in forward direction and the current through the power switch module is about zero the power switch module is referred to as operating in blocking mode. When the voltage across the power terminals of the power switch module is polarized in the forward direction, the voltage across the control terminals determine if the power switch module is in forward conduction or blocking mode. The control voltage level for enabling forward conduction mode is referred to as turn on level. The control voltage level for disabling forward conduction mode is referred to as turn off level. It will be appreciated that the polarity of the current through the power switch module determines whether the switch module is in forward conduction mode or reverse conduction mode. For example, in the case of the lower IGBT module 322, if the current is flowing from the phase output 318 to the lower rail 312 (current positive), the lower IGBT module 322 is operating in forward conduction mode. If current is flowing from the lower rail 312 to the phase out 318 (current negative), the lower IGBT module 322 is operating in reverse conduction mode. It should be noted that, depending on the voltage across the collector and emitter of the switch module, the IGBT module may operate in reverse conduction mode even if forward conduction mode operation is enabled by the gate voltage applied to the IGBT.
During the first half of the period, the voltage across the power terminals of the power switch module is polarized in forward direction and it operates alternating in forward conduction mode and blocking mode depending on the control voltage level. For example, current through the lower IGBT 304 is positive when the gate voltage is at +15V, and drops to zero when the gate voltage changes to −15V. The upper diode 306 conducts the resulting freewheeling current (not shown). During the first half of the period, the current in the lower power switch module remains at zero. During the second half of the period, the lower power switch module operates alternating in reverse conduction mode and blocking mode. When the upper power switch module is switched off during the second half of the period, the lower power switch module goes into reverse conduction mode and conducts the current from the upper power switch module which has been switched off. The control voltage in the standard switching scheme of
Problems can arise in the second half of the period, when the lower power switch module is conducting the freewheeling current. For example, in some circumstances, the upper power switch module may turn on improperly due to a spurious triggering or cosmic particle. If this occurs, a short circuit between the phase output 318 and the upper rail 310 voltage is induced, creating an intra-module commutation, which means that the current through the power switch module changes polarity. For example, with regard to the lower IGBT module in
To avoid a short circuit, the drive circuits may be configured to keep the corresponding control voltage at turn off level depending on the polarity of the current through the power switch module. Referring to the lower power switch module of
In the case of a reverse conductive power switch module (RCPS module) such as the BIGT switch modules of
To improve the efficiency of the RCPS inverter, a drive circuit incorporating the gate driver 314 may be configured to disable the forward conduction mode operation of the RCPS module depending on the polarity of the current through the module. Referring to the lower BIGT 404 as an example, a drive circuit incorporating the gate driver 314 may be configured to determine the polarity of the current in the lower BIGT. If the current polarity indicates that the lower BIGT is operating in reverse conduction mode, the lower BIGT may be commanded by the drive circuit to disable forward conduction mode, even if the external trigger from the control circuit 316 is commanding the lower BIGT 404 to enable forward conduction mode. In this way, the gate voltage applied to the lower BIGT 404 will be different, e.g. lower, than the stationary on-value when operating in reverse conduction mode, resulting in more efficient operation of the inverter. The gate driver 314 for the upper BIGT 402 may be configured in the same manner.
The control voltage signal, Vctrl, is represented by the dotted line 604. The dashed line 606 represents the current in the switch module when operating in forward conduction mode. The solid line 608 represents the current in the power switch module when operating in reverse conduction mode. As discussed in relation to
As discussed above in relation to
As shown in
In an embodiment, the current sensing circuit 702 includes a voltage sensor 720 for measuring the voltage between the first emitter terminal 714 and the second emitter terminal 716, and an integrator 722 to determine an estimate of the collector-emitter current based on the measured voltage. Due to the parasitic inductance 718, the voltage between the first emitter terminal 714 and the second emitter terminal 716 may be approximately equal to the instantaneous rate of change in the emitter-collector current, dIce/dt. Thus, the emitter-collector current can be estimated by integrating the voltage measured using the integrator 722.
The output of the integrator 722 is an estimate of the collector-emitter current in the BIGT 402 or 404. The output of the integrator 722 can be coupled to the comparator 704, which may determine whether the estimated current is greater than a reference current, Iref. In an embodiment, the reference current, Iref, may be zero, so that the output of the comparator 704 is directly related to the polarity of the estimated collector-emitter current. For example, if the estimated collector-emitter current is positive, meaning that the BIGT 402 or 404 is operating in forward conduction mode, the output of the comparator 704 may be set to logical one. If the estimated collector-emitter current is negative, meaning that the BIGT 402 or 404 is operating in reverse conduction mode, the output of the comparator 704 may be set to logical zero.
The output of the comparator 704 and the output of the control circuit 316 may be coupled to the input of the AND-gate 706. The output of the AND-gate is coupled to the input of the gate driver 314. Accordingly, the gate driver 314 will receive a command to enable forward conduction operation of the BIGT 402 or 404 if the control circuit 316 is commanding the BIGT 402 or 404 to turn on and the estimated collector-emitter current in the BIGT 402 or 404 indicates the that the BIGT 402 or 404 is not operating in reverse conduction mode.
As noted above, the same drive circuit could be used, for example, in an IGBT inverter, in which case, the BIGT 402 or 404 would be replaced by a power switch module that includes an IGBT 302 or 304 and anti-parallel diode 306 or 308. Similar to the BIGT inverter, the gate driver 314 will receive a command to turn on the IGBT 302 or 304 if the control circuit 316 is commanding the IGBT 302 or 304 to turn on and the estimate collector-emitter current in the IGBT switch module indicates the that diode 306 or 308 of the switch module is not conducting.
As shown in
Depending on the voltage to emitter at the collector terminal 710, the current from the current source 808 will follow a path through the diode 810 to the collector terminal 710 or to the comparator 804. The current input to the comparator is referred to as the sense current, Isense. When the BIGT 402 or 404 is operating in reverse conduction mode, the voltage at the collector terminal 710, Vc, will be negative. The negative voltage at the collector terminal 710 forward biases the diode 810 so that the source current, Is, follows a path through the diode 810 to the collector terminal 710 and Isense will be at or close to zero. When the BIGT 402 or 404 is operating in forward conduction mode or blocking mode, the voltage at the collector terminal 710, Vc, will be positive. The positive voltage at the collector terminal 710 negatively biases the diode 810 such that the source current, Is, is conducted to the comparator 804 and Isense equals a non-zero number approximately equal to the source current. To summarize, the input to the comparator will be less than the source current, Is, when operating in reverse conduction mode and approximately equal to the source current when operating in forward conduction mode or blocking mode.
The comparator 804 may compare the input current to the source current, Is, which may be a known value that can be determined based on the design considerations for a particular circuit. If the input current equals the source current (forward conduction mode), the comparator 804 may enable the forward conduction mode operation by, for example, sending a logic one to the AND-gate. If the input current is less than the source current (reverse conduction mode), the comparator 804 may disable the forward conduction mode operation by, for example, sending a logic zero to the AND-gate. The output of the control circuit 316 is also coupled to the input of the AND-gate 706 or 806. The output of the AND-gate is coupled to the input of the gate driver 314. Accordingly, the gate driver 314 will receive a command to activate forward conduction mode operation of the BIGT 402 or 404 if the control circuit 316 is commanding the BIGT 402 or 404 to turn on and the detected collector-emitter voltage across the BIGT 402 or 404 indicates the that the BIGT 402 or 404 is not operating in reverse conduction mode.
As shown in
If the voltage level across the lower resistor R2910 is greater than the voltage reference, the comparator 804 may enable the forward conduction mode operation by, for example, sending a logic one to the AND-gate. If the voltage level across the lower resistor R2910 is lower than the voltage reference (reverse conduction mode), the comparator 804 may disable the forward conduction mode operation by, for example, sending a logic zero to the AND-gate. The output of the control circuit 316 is also coupled to the input of the AND-gate 906. The output of the AND-gate 906 is coupled to the input of the gate driver 314. Accordingly, the gate driver 314 will receive a command to activate forward conduction mode operation of the BIGT 402 or 404 if the control circuit 316 is commanding the BIGT 402 or 404 to turn on and the detected collector-emitter voltage across the BIGT 402 or 404 indicates the that the BIGT 402 or 404 is not operating in reverse conduction mode.
At block 1004, a determination may be made regarding whether the power switch module is operating in reverse conduction mode. In embodiments, determining whether the power switch module is operating in reverse conduction mode includes determining a polarity of current through the power terminals of the power switch module. For example, determining whether the power switch module is operating in reverse conduction mode may include measuring a voltage across a parasitic inductance between a first emitter terminal and a second emitter terminal and integrating the voltage to estimate the current through the emitter terminal. In another example, determining whether the power switch module is operating in reverse conduction mode includes detecting the voltage level across the power terminals of the power switch module.
If, at block 1004, the switch module is operating in reverse conduction mode, the process flow may advance to block 1006 and the forward conduction mode operation of the power switch module may be disabled. Accordingly, forward conduction mode operation of the switch module is disabled when the polarity of the current is negative in a direction from the collector terminal to the emitter terminal.
If, at block 1004, the switch module is not operating in reverse conduction mode, the process flow may advance to block 1008 and the forward conduction mode operation of the power switch module may be enabled. Accordingly, the forward conduction mode operation of the power switch module will only be activated if the control circuit is commanding the activation of the forward conduction mode operation and the forward conduction mode operation is not disabled.
It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, other means of sensing current in the switch could be employed, such as a shunt or a giant magnetoresistive device. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. While the dimensions and types of materials described herein are intended to illustrate embodiments of the invention, they are by no means limiting and are exemplary in nature. Other embodiments may be apparent upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.
In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” “3rd,” “upper,” “lower,” “bottom,” “top,” “up,” “down,” etc. are used merely as labels, and are not intended to impose numerical or positional requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. §112, sixth paragraph, unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure.
As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising,” “including,” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property.
Since certain changes may be made in the above-described control method, without departing from the spirit and scope of the invention herein involved, it is intended that all of the subject matter of the above description or shown in the accompanying drawings shall be interpreted merely as examples illustrating the inventive concept herein and shall not be construed as limiting the invention.
