BACKGROUND OF THE INVENTION
The subject matter disclosed herein relates to voltage converter systems. More specifically, the present disclosure generally relates to using different types of switches in a voltage converter system to reduce losses and improve efficiency.
Silicon carbide (SiC) is a semiconductor that is increasingly being used in power electronic devices such as metal-oxide-semiconductor field-effect transistor (MOSFETs). SiC power electronic devices generally have relatively low switching losses at relatively high switching rates (e.g., kilohertz (kHz) range), operate at relatively high junction temperatures, and operate at relatively high voltages as compared to other power electronic devices that do not employ silicon carbide within the respective device. As such, SiC power electronic devices have gained interest in recent years in view of their switching performance and high temperature operation capabilities. However, since the costs of manufacturing SiC power electronic devices are not comparable to other power electronic devices, other silicon-based power electronic devices are used as a low cost alternative to using systems having SiC power electronic devices.
BRIEF DESCRIPTION OF THE INVENTION
In one embodiment, a voltage converter may include a first set of silicon (Si)-based power devices coupled to a first direct current (DC) voltage source and a second set of Si-based power devices coupled to a second DC voltage source. The voltage converter may also include a first set of silicon-carbide (SiC)-based power devices coupled to the first set of Si-based power devices and to the second set of Si-based power devices. Each SiC-based power device of the first set of SiC-based power devices may switch at a higher frequency as compared to each Si-based power device of the first and second sets of the Si-based power electronic devices.
In another embodiment, an apparatus that converts a direct current (DC) voltage signal into an alternating current (AC) voltage signal may include a first set of silicon-carbide (SiC)-based power electronic devices coupled to a first direct

current (DC) voltage source and a second set of silicon-carbide-based power electronic devices coupled to a second direct current (DC) voltage source. The apparatus may also include a first set of silicon-based power electronic devices coupled to the first set of SiC-based power electronic devices and to the second set of SiC-based power electronic devices, wherein each SiC-based power electronic device of the first and second sets of SiC-based power electronic devices is configured to switch at a higher frequency as compared to each silicon-based power electronic device of the first set of the Si-based power electronic devices.
In yet another embodiment, a voltage converter may include a first set of silicon (Si)-based power devices coupled to a first DC voltage source and a second set of Si-based power devices coupled to a second DC voltage source. The voltage converter may also include a first set of silicon-carbide (SiC)-based power devices coupled to the first set of Si-based power devices and to the second set of Si-based power devices. The voltage converter may also include a processor that may control switching of each SiC-based power device of the first set of SiC-based power devices and each Si-based power device of the first and second sets of the Si-based power devices, such that one SiC-based power device of the first set of SiC-based power electronic devices is conducting current in series with one Si-based power electronic device of the first or second set of the silicon-based power electronic devices at any given time.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, aspects, and advantages of the present 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:
FIG. 1 is a schematic diagram of a three-level hybrid converter system, in accordance with an embodiment;

FIG. 2 is graph of voltage signals output by the hybrid converter system along with a timing diagram of gate signals provided to switching devices in the hybrid converter system of FIG. 1, in accordance with an embodiment;
FIG. 3 illustrates a bar graph that compares energy losses for different types of converter systems, in accordance with an embodiment;
FIG. 4 is a schematic diagram of another three-level hybrid converter system, in accordance with an embodiment; and
FIG. 5 is a schematic diagram of a five-level hybrid converter system, in accordance with an embodiment.
DETAILED DESCRIPTION
One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. +
When introducing elements of various embodiments of the present disclosure, the articles "a," "an," and "the" are intended to mean that there are one or more of the elements. The terms "comprising," "including," and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.
Silicon (Si) power electronic devices are widely used in various power conversion systems (e.g., rectifiers, inverters) to convert voltage signals from alternating

current (AC) voltage signals to direct current (DC) voltage signals, and vice versa. However, silicon-based power electronic devices, such as silicon insulated-gate bipolar transistors (IGBTs), may lose an increasing portion of their energy as heat loss during high-frequency switching. As such, the performance of silicon-based power electronic devices maybe limited to some switching frequency (e.g., 1 kHz).
In contrast to silicon-based power electronic devices, silicon carbide-based power electronic devices, such as silicon carbide metal-oxide-semiconductor field-effect transistors (MOSFETs), may exhibit significantly lower switching losses as compared to silicon-based power electronic devices. As such, SiC power electronic devices may operate more efficiently than Si power electronic devices when switching frequently (e.g., > 1kHz) and at higher temperatures.
Although the switching losses for Si power electronic devices may be larger than the switching losses for SiC power electronic devices, the conduction losses (or the voltage drop) across the Si power electronic devices may remain relatively constant or increase at a slower rate as compared to the conduction losses for the SiC power electronic devices. That is, due to the structure of Si power electronic devices, such as Si IGBTs, the voltage drop across the Si power electronic device may generally be lower as compared to the SiC power electronic device, such as the SiC MOSFET, for the same current rating.
With the foregoing in mind, in one embodiment, both Si power electronic devices and SiC power electronic devices may be used together in a circuit to perform power conversion operations more efficiently. That is, a presently disclosed hybrid converter system generally employs Si power electronic devices that switch within a certain low frequency range (e.g., 0 - 1 kHz, line frequency, fundamental frequency) along with SiC power electronic devices that switch at a higher frequency range (e.g. > 1 kHz). By switching the Si power electronic devices at a line or fundamental frequency and using SiC power electronic devices to switch at high frequencies, the hybrid converter system may produce a high

quality voltage output that maximizes the benefit of the low switching loss properties of the SiC power electronic devices. Moreover, the hybrid converter system may coordinate gate signals provided to the Si power electronic devices and the SiC power electronic devices, such that the Si power electronic devices may carry zero current while they are switching. As such, the Si power electronic devices may have no switching loss.
By way of introduction, FIG. 1 illustrates a schematic diagram of a three-level hybrid converter system 10 that incorporates both Si power electronic devices and SiC power electronic devices to convert DC voltage signals to AC voltage signals, in accordance with an embodiment of the present approach. The three-level hybrid converter system 10 (hybrid converter system 10) may be characterized as an active neutral point clamped multilevel converter with hybrid switch assembly that uses both Si power electronic devices and SiC power electronic devices. It should be noted that the schematic diagram of FIG. 1 may represent one phase leg of a multi-phase converter system. As such, the three-level hybrid converter system 10 may be employed on one or more legs of such a multi-phase converter system.
In one embodiment, the Si power electronic devices and the SiC power electronic devices of the hybrid converter system 10 may be Si IGBTs 12 and SiC MOSFETs 14, respectively. The Si IGBTs 12 may include various types of IGBTs of different ratings (e.g.,1.7kV, 3.3kV, 4.5kV, or 6.5kV IGBT) that uses Si as the semiconductor material to switch between conductive to non-conductive states. In the same manner, the SiC MOSFETs may include various types of MOSFETs of different ratings that uses SiC as the semiconductor material to switch between conductive to non-conductive states. Although the following descriptions of various hybrid converter systems will be discussed with regard to illustrated the Si IGBTs 12 and the SiC MOSFETs 14, it should be noted that, in other embodiments, any suitable type of Si power electronic devices and SiC power electronic devices may be used in lieu of the Si IGBTs 12 and the SiC MOSFETs 14.

In some embodiments, multiple Si IGBTs 12 may be grouped together as part of a module 16. For example, in the hybrid converter system 10, two Si IGBTs 12 may be electrically coupled in series with each other and provide three interconnection nodes (e.g., 11, 13, 15) where the module 16 may be coupled to other electrical components. The interconnection nodes may be located at a collector side of one of the Si IGBTs 12, at an emitter side of one of the Si IGBTs 12, and in between two Si IGBTs 12.
In the same manner, multiple SiC MOSFETs 14 may be grouped together as part of a module 18, such that two SiC MOSFETs 14 may be electrically coupled in series with each other. Moreover, the module 18 may also have three interconnection nodes (e.g., 13, 17, 19) where the module 18 may be coupled to other electrical components. The interconnection nodes of the module 18 may be located at a drain side of one of the SiC MOSFETs 14, at a source side of one of the SiC MOSFETs 14, and in between two SiC MOSFETs 14.
Both the module 16 and the module 18 may be standardized, interchangeable components that may be used to build the hybrid system 10, in certain embodiments. As such, the module 16 and the module 18 may each be manufactured individually and be made available for assemblers to create different hybrid converter systems having different voltage and current ratings using standard building components.
With this in mind, each Si IGBT module 16 of the hybrid converter system 10 may be coupled across a DC voltage source (e.g., DC voltage source 20, DC voltage source 22). The intersection node (e.g., 13, 19) or output of each Si IGBT module 16 may then be coupled in series with the SiC MOSFET module 18. For instance, the interconnection node in between two Si IGBTs 12 of the modules 16 may be coupled to a source side and a drain side of the SiC MOSFETs of the module 18.
The AC output voltage of the hybrid converter system 10 may be provided at the output terminals (e.g., 21, 23), which are connected to the interconnection node (e.g., 17) between the SiC MOSFETs of the module 18 and to the interconnection

node (e.g., 15) between the voltage source 20 and the voltage source 22. In some embodiments, the voltage source 20 and the voltage source 22 both provide the same amount of DC voltage. As such, the Si IGBTs 12 and the SiC MOSFETs 14 may be switched on and off in a controlled manner to convert a DC voltage signal provided via the voltage sources 20 and 22 to an AC voltage signal output by the hybrid converter system 10. The AC voltage signal output may then be provided to various types of AC powered devices, such as AC motors and the like, to perform various types of operations.
In one embodiment, the switching of the Si IGBTs 12 and the SiC MOSFETs 14 may be controlled by gate signals provided to gates of the Si IGBTs 12 and the SiC MOSFETs 14. As such, the hybrid converter system 10 may include a hybrid converter control system 24, which may provide gate signals to each of the Si IGBTs 12 and the SiC MOSFETs 14 in the hybrid converter system 10 to control operation of the hybrid converter system 10.
The hybrid converter control system 24 may generally include a processor 26 that determines appropriate gate signals to provide to the Si IGBTs 12 and the SiC MOSFETs 14 of the hybrid converter system 10 to produce a desired AC voltage output signal using the DC voltage sources 20 and 22. The processor 26 may be any type of computer processor or microprocessor capable of executing computer-executable instructions (e.g., software code, programs, applications). The processor 26 may also include multiple processors that may cooperate to perform the operations described below.
Generally, as discussed above, the processor 26 may execute software applications that include programs to determine gate signals to provide to the Si IGBTs 12 and the SiC MOSFETs 14, such that the resulting AC voltage output corresponds to a desired voltage signal. For example, FIG. 2 illustrates an example timing diagram 30 of gate signals provided to respective gates of the Si IGBTs 12 and the SiC MOSFETs 14 for the embodiment of the hybrid converter system 10 of FIG. 1.

In certain embodiments, the processor 26 may provide gate signals to the Si IGBTs 12 and the SiC MOSFETs 14 such that one Si IGBT 12 will be in series with one SiC MOSFET 14 at any given time. Additionally, the processor 26 may send gate signals to the Si IGBTs 12 to cause the Si IGBTs 12 to switch at a fundamental line frequency (e.g., 60 Hz) and send gate signals to the SiC MOSFETs 14 to switch at a higher frequency (e.g., > 1 kHz) to synthesize the desired AC voltage output waveform. Accordingly, as shown in FIG. 2, the gate signals (e.g., Gl, G2, G3, G4) provided to the Si IGBTs 12 change less frequently as compared to the gate signals (e.g., G5, G6) provided to the SiC MOSFETs 14. As a result, AC voltage output 40 (reference wave) may correspond to a desired sine wave, as depicted in FIG. 2. FIG. 2 also depicts a carrier wave 42. The intersection of the carrier wave 42 and the AC voltage output 40 generally forms a square waveform or a pulse width modulation (PWM) gate waveform, which can be used to control the Si IGBTs 12 and the SiC MOSFETs 14.
With the foregoing in mind, to produce the AC voltage output 40, the processor 26 may coordinate the gate signals provide to the Si IGBTs 12 and the SiC MOSFETs 14 such that one Si IGBT 12 will be activated together in electrical series with one SiC MOSFET 14 at any given time and conduct current in series with each other, as discussed above. For example, referring to FIG. 1, switch Tl (e.g. Si IGBT 12) and switch T5 (e.g., SiC MOSFET 14) may be in series with each other during some time interval. To ensure that just one Si IGBT 12 is in series with a SiC MOSFET 14 at a given time (e.g., one Si IGBT 12 is conducting current in series with a Si MOSFET 14), the processor 26 may remove (e.g., stop or discontinue) a gate signal (e.g., at time ti) from one Si IGBT 12 when turning the one Si IGBT 12 off and provide a gate signal to another Si IGBT 12 to turn the other Si IGBT 12 on with a short delay (e.g., dead time, approximately lus, is provided in which both IGBTs are off to avoid a potential shoot through). In one embodiment, the processor 26 may remove and provide the respective gate signals when the AC voltage output 40 crosses zero (at time ti).

However, when the gate signal is removed from a respective Si IGBT 12 and the Si IGBT turns off or enters a non-conductive state, the corresponding SiC MOSFET 14 that was coupled in series with the respective Si IGBT 12 will already be turned off. That is, the corresponding SiC MOSFET 14 may be in a non-conductive state sooner than its corresponding Si IGBT 12. As such, when the gate signal of the corresponding Si IGBT 12 is removed, the current in the Si IGBT 12 is already zero due to the SiC MOSFET 14 already being off. As a result, the Si IGBT 12, which traditionally has higher switching losses as compared to SiC MOSFETs, has little or no loss during turn off.
In the same manner, when a gate signal for another Si IGBT 12 is provided to turn on the Si IGBT 12, a corresponding SiC MOSFET 14 will not be turned on until corresponding Si IGBT 12 is completely on and in a full conductive (e.g. on) state. At this time (e.g., time ti), the current in the respective Si IGBT 12 is still zero until the voltage across the Si IGBT 12 is almost zero. As such, the tum-on loss in the Si IGBT 12 is also minimized, resulting in little or no (e.g., zero) switching loss.
It should be noted, that by using the hybrid converter system 10 described above, the converter system is more efficient than a converter system that uses just SiC power electronic devices. For example, FIG. 3 illustrates a bar graph 50 that compares energy losses for a two-level converter system using just SiC power electronic devices (e.g., bar 52), a three-level converter system using just SiC power electronic devices (e.g., bar 54), and a three-level hybrid converter system that corresponds to the hybrid converter system 10 of FIG. 1 (e.g., bar 56).
The graph 50 of FIG. 3 compares the losses that occur in a two-level converter system using just SiC power electronic devices (e.g., bar 52), a three-level converter system using just SiC power electronic devices (e.g., bar 54), and a three-level hybrid converter system that corresponds to the hybrid converter system 10 of FIG. 1 (e.g., bar 56) employed in a 1.5 MW / 4.16 kV high-speed medium voltage drive. Moreover, the effective switching frequency at the AC voltage output terminal is approximately 20 kHz. The simulation associated with