ELECTRIC MOTOR DRIVE WITH GALLIUM NITRIDE POWER SWITCHES HAVING LOW-SIDE SHORT CIRCUIT SAFE STATE

A switching circuit of a motor drive includes a high-side switch configured to selectively conduct current between a DC positive conductor and an output conductor, and a low-side switch configured to selectively conduct current between the output conductor and a DC negative conductor. The high-side switch comprises a depletion mode (D-Mode) gallium nitride (GaN) high-electron-mobility transistor (HEMT) and a Si-FET in a cascaded configuration, and the low-side switch comprises a D-Mode GaN HEMT. This arrangement can provide a safe state operation in which the switching circuit provides a default condition providing electrical continuity between the DC negative conductor and the output conductor and providing electrical isolation between the DC positive conductor and the output conductor in the event of a loss of control signals.

FIELD

The present disclosure relates generally to power switches in a drive circuit for generating an alternating current (AC) power to supply an electric motor. More specifically, the present disclosure relates to power switches that are configured to have a safe state in which the conductor supplying the electric motor is automatically short-circuited to a low-side conductor.

BACKGROUND

AC motor drives are used in a variety of applications to supply power to permanent-magnet synchronous motors. Conventional AC motor drives may be configured to short-circuit all motor phases in a safe state to protect the semiconductors from overvoltage. To accomplish such a safe state, switches in a high-side path of the motor drive are opened and switches in a low-side path are closed.

A phase switch circuit may be designed to ensure that each motor phase is driven to the safe state by causing switches in the low-side path to default to a closed or conductive condition and by causing switches in the high-side path to default to an open or non-conductive condition in the event of a sudden loss of control signals.

SUMMARY

The present disclosure provides a switching circuit including a high-side switch configured to selectively conduct current between a DC positive conductor and an output conductor, and a low-side switch configured to selectively conduct current between the output conductor and a DC negative conductor. At least one of the high-side switch or the low-side switch comprises a high-electron-mobility transistor (HEMT) and a second transistor in a cascaded configuration.

DETAILED DESCRIPTION

Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, a motor drive20for providing AC power to an electric motor22is disclosed. As best shown inFIG.1, the motor drive20includes a DC bus24comprising a DC negative conductor VBUS−and a DC positive conductor VBUS+having a high DC voltage with respect to the DC negative conductor VBUS−. The DC negative conductor VBUS−may also be called a “low-side path”, and the DC positive conductor VBUS+may also be called a “high-side path”. The DC bus24is preferably energized to about 400 VDC to 500 VDC, although other voltages may be used depending on the supply available and/or the requirements of a given application.

As also shown inFIG.1, a DC supply26energizes the DC bus24with the high DC voltage via DC supply leads28. The DC supply26may be, for example, a battery pack, a DC generator, or a rectifier producing the high DC voltage from an AC source such as a utility line, or from an AC generator. The motor drive20includes a first output terminal PHASE_A_OUT for connection of a first motor lead T1to supply AC power to the electric motor22, a second output terminal PHASE_B_OUT for connection of a second motor lead T2to supply AC power to the electric motor22, and a third output terminal PHASE_C_OUT for connection of a third motor lead T3to supply AC power to the electric motor22. In the example embodiment shown in the figures, a three-phase electric motor22is used; however, other numbers of phases may be used, such as a single-phase motor or one having six or more phases.

The motor drive20includes a first phase switch30afor selectively connecting the first output terminal PHASE_A_OUT to either of the DC positive conductor VBUS+or the DC negative conductor VBUS−of the DC bus24at any given time. The motor drive20also includes a second phase switch30bfor selectively connecting the second output terminal PHASE_B_OUT to either of the DC positive conductor VBUS+or the DC negative conductor VBUS−of the DC bus24at any given time. The motor drive20also includes a third phase switch30cfor selectively connecting the third output terminal PHASE_C_OUT to either of the DC positive conductor VBUS+or the DC negative conductor VBUS−of the DC bus24at any given time. Each of the phase switches30a,30b,30cincludes a high-side switch Shconfigured to selectively conduct current between the DC positive conductor VBUS+and a corresponding one of the output terminals PHASE_A_OUT, PHASE_B_OUT, PHASE_C_OUT. Each of the phase switches30a,30b,30calso includes a low-side switch Slconfigured to selectively conduct current between the DC negative conductor VBUS−and the corresponding one of the output terminals PHASE_A_OUT, PHASE_B_OUT, PHASE_C_OUT.

A controller32coordinates operation of the phase switches30a,30b,30cby providing control signals upon control lines34a,34b,34c.

FIG.2shows a schematic diagram of a switching circuit40in accordance with some embodiments of the present disclosure. The switching circuit40ofFIG.2may be used as any of the phase switches30a,30b,30cof the motor drive20. The switching circuit40ofFIG.2may be used in other applications, such as in an inverter to supply one or more different AC loads.

The switching circuit40includes a high-side switch Shconfigured to selectively conduct current between a DC positive conductor VBUS+and an output conductor PHASE_A_OUT. The switching circuit40also includes a low-side switch S1configured to selectively conduct current between the output conductor PHASE_A_OUT and a DC negative conductor VBUS−, where the DC positive conductor VBUS+at a higher voltage potential than the DC negative conductor VBUS−. At least one of the high-side switch Shor the low-side switch Slcomprises a high-electron-mobility transistor (HEMT)44and a second transistor46in a cascaded configuration. The high-side switch Shin the example circuit shown inFIG.2includes the high-electron-mobility transistor (HEMT)44and the second transistor46in the cascaded configuration, although in other embodiments, the high-electron-mobility transistor (HEMT)44and the second transistor46may be part of the low-side switch Sl.

The HEMT44of the high-side switch Shincludes a depletion mode (D-Mode) transistor in the example switching circuit40shown inFIG.2, More specifically, the HEMT44of the high-side switch Shincludes a depletion mode (D-Mode) gallium nitride (GaN) HEMT. Such a depletion mode transistor defaults to a conductive condition (i.e. to conduct current between the drain and the source thereof in the absence of a control voltage being applied to a gate thereof. Other types of HEMT may be used, such as a silicon carbide (SiC) transistor. The second transistor46may be a Silicon (Si)-based field effect transistor (FET), such as a metal-oxide field effect transistor (MOSFET), although the second transistor46may be a different type of device, such as a junction device, another type of FET, and/or a silicon controlled rectifier. The cascaded configuration may be a cascode, which may be defined as two-stage amplifier that consists of a common-emitter stage feeding into a common-base stage, or a similar or equivalent arrangement using other devices, such as FETs.

In some embodiments, and as shown inFIG.2, the cascaded configuration includes a gate G of the HEMT44connected to the output conductor PHASE_A_OUT. In some embodiments, and as shown inFIG.2, the cascaded configuration includes a drain terminal D of the HEMT44connected to the DC positive conductor VBUS+, a source terminal S of the HEMT44connected to a drain terminal D of the second transistor46, and a source terminal S of the second transistor46connected to the output conductor PHASE_A_OUT. In some embodiments, the cascaded configuration includes a gate G of the second transistor46connected to a control line36h,36lfor controlling operation of the high-side switch Shor the low-side switch Sl. More specifically, and as shown inFIG.2, the gate G of the second transistor46is connected to the high-side control line34hfor controlling operation of the high-side switch Sh. In some embodiments, and as shown inFIG.2, the cascaded configuration of the HEMT44and the second transistor46is configured to default to an open circuit configuration with the control line36h,36lconnected thereto being de-asserted. In other words, a positive control voltage may be applied to the high-side control line34hin order to cause the high-side switch Shto be in a conductive state.

The high-side switch Shis configured to selectively conduct current between the DC positive conductor VBUS+and an output conductor PHASE_A_OUT in response to assertion of a high-side control line34h. In the example circuit shown inFIG.2, the high-side control line34his connected to a gate terminal G of the second transistor46, although other arrangements are possible in which the high-side control line34his connected to a different terminal and/or a different device within the high-side switch Sh. A high-side driver36hwithin the controller32is configured to selectively assert the high-side control line34h, for example with a pulse width modulation (PWM) signal, to approximate an AC waveform on the output conductor PHASE_A_OUT.

The low-side switch Slin the example switching circuit40shown inFIG.2includes a switching transistor42, which is a depletion mode (D-Mode) gallium nitride (GaN) HEMT, connected directly between the output conductor PHASE_A_OUT and a DC negative conductor VBUS−. Other types of HEMT may be used, such as a silicon carbide (SiC) transistor. The low-side switch Slis configured to selectively conduct current between the output conductor PHASE_A_OUT the a DC negative conductor VBUS−in response to assertion of a low-side control line34l. In the example circuit shown inFIG.2, the low-side control line34lis connected to a gate terminal G of the switching transistor42, although other arrangements are possible in which the low-side control line34lis connected to a different terminal and/or a different device within the low-side switch Sl. A low-side driver36lwithin the controller32is configured to selectively assert the low-side control line34l, for example with a pulse width modulation (PWM) signal, to approximate an AC waveform on the output conductor PHASE_A_OUT. In some embodiments, such as the example shown inFIG.2, the low-side control line34lmay be asserted with a low voltage. In other words, the low-side switch Slmay be driven to a conductive state in response to a low or 0V control voltage on the low-side control line34l.

The present disclosure provides a switching circuit40using depletion mode switches, which are in a conductive state in the absence of a control signal, while also providing for safe-state operation with the output conductor PHASE_A_OUT conducted with a DC negative conductor VBUS−and isolated from a DC positive conductor VBUS+in the event of a loss of control signals34h,34i. The switching circuit40may be used as a phase switch30a,30b,30cof a motor drive20, with the output conductor PHASE_A_OUT providing power to an electric motor22. If the gates G of each of the transistors42,44,46in the switching circuit40are 0V with respect to their corresponding source terminals S, the switching circuit40provides a conductive path between the DC negative conductor VBUS−and the output conductor PHASE_A_OUT. Furthermore, when all the control signals34h,34lare in a low voltage state, the switching circuit40provides electrical continuity to conduct current between the DC negative conductor VBUS−and the output conductor PHASE_A_OUT while also providing electrical isolation, blocking current flow, between the DC positive conductor VBUS+and the output conductor PHASE_A_OUT.

The switching circuit40shown inFIG.2may be used as a phase switch,30a,30b,30cin the motor drive20ofFIG.1, or in a motor drive with a different configuration, such as a different number of phases. Alternatively or additionally, the switching circuit40, or portions thereof, may be used as an AC generator circuit to generate an AC waveform upon the output conductor for other applications, such as for providing AC power source to a load from a DC supply, or as part of a step-up or step-down DC/DC converter.