Control circuit of multi-phase motor

Abstract

The control circuit of the multiphase motor 57 comprises a plurality of inverter bridges, a plurality of output terminals and at least one isolation switch 62, 63. Each inverter bridge is arranged to provide an output voltage for a phase of the motor 57. Each output terminal is arranged to be coupled to a phase of the motor 57 to provide the output voltage to the phase of the motor 57. Each isolation switch 62, 63 is coupled between one of the inverter bridges and one of the output terminals to selectively isolate the output terminal from the inverter bridge. Each isolation switch 62, 63 comprises a gallium nitride (GaN) transistor.

Description

[0001] The present invention relates to a control circuit for a multi-phase electric motor, an electric motor device using such a control circuit, an electric steering device using such an electric motor device, a vehicle using such a steering device, and associated methods.

[0002] Power MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) are commonly used to implement inverter bridges that drive permanent magnet synchronous motors (PMSMs), as well as for phase isolation functions in automotive applications and beyond.

[0003] The disadvantage of using MOSFETs for phase isolation is due to the presence of an intrinsic body diode in the MOSFET. This allows the MOSFET to be switched in one direction with a low forward voltage drop (V SD , the forward voltage drop of the body diode, when carrying rated current, is typically less than 1V) to conduct current. This allows damping currents that may be generated by the voltage generated by the rotating motor and under bridge MOSFET short circuit fault conditions. These damping currents are undesirable and may result in a reduction in steering assistance to the user, for example in an Electric Power Steering (EPAS) system.

[0004] Such unwanted currents can only be fully controlled by using 3 or 4 MOSFET phase isolation devices, as shown in Figures 1 and 2 of the accompanying drawings, respectively. In each case, three pairs of MOSFETs 1 and 2, 3 and 4, and 5 and 6 selectively connect one phase of a three-phase EPAS motor 7 to either a DC bus 8 or ground 9. For demonstration purposes, MOSFET 1 is shown in each case as having failed short circuit.

[0005] Arrow 10 shows the potential uncontrolled current flow in such a short circuit fault situation, which may be controlled using MOSFET phase isolation switches. In the example of Figure 1, the MOSFET phase isolation switches 11, 12, 13 in each phase need to be able to block the unwanted current, remembering that each MOSFET can only block current flowing in one direction.

[0006] In the example of Figure 2, two MOSFETs 14, 15, 16, 17 are provided in series for each of the two phases of the motor 7. This provides the same level of control as Figure 1.

[0007] In addition, if the current in the two phases not connected to the short-circuited inverter MOSFET 2 is to be controlled, it is necessary to block the current flowing into and out of each of the faulty inverter branches separately. Such a solution would require a 6-phase isolation MOSFET (18, 19, 20, 21, 22, 23) implementation as shown in Figure 3.

[0008] Since the phase-isolation MOSFETs are power devices that are not only expensive but also introduce additional insertion losses in the motor control circuit, it is desirable to limit the number of devices required to implement the phase isolation function.

[0009] According to a first aspect of the present invention, there is provided a control circuit for a multi-phase motor, the control circuit comprising:

[0010] a plurality of inverter bridges, each inverter bridge being arranged to provide an output voltage to a phase of the electric motor;

[0011] a plurality of output terminals, each output terminal being arranged to be coupled to a phase of the motor to provide an output voltage to said phase of the motor; and

[0012] at least one isolation switch, each isolation switch coupled between one of the inverter bridges and one of the output terminals to selectively isolate the output terminal from the inverter bridge;

[0013] Each isolation switch contains a gallium nitride (GaN) transistor.

[0014] Thus, we have realized that Gallium Nitride (GaN) transistors can be used as isolation switches, avoiding the problems of prior art Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) involving the inherent parasitic body diode of a given MOSFET. By using GaN transistors as isolation transistors, the number of components can be reduced (because not as many components need to be used, because there is no need to compensate for the body diode), which reduces manufacturing efforts, and also reduces energy consumption and heat dissipation.

[0015] The GaN transistor may provide selective isolation of each isolation switch; that is, the GaN transistor may provide a switching function to selectively apply the output voltage of the inverter bridge to whose output the isolation switch is coupled.

[0016] Each inverter bridge that is not coupled to an isolation switch can be coupled to the output. Thus, some of the inverter bridges can be coupled directly to the output without an isolation switch.

[0017] The control circuit may be provided with a controller arranged to close each isolation switch in a normal operating mode so that each isolation switch does not isolate its output terminal from its inverter bridge and allows the output voltage from its inverter bridge to be applied to its output terminal. Thus, the controller may be arranged to apply a positive voltage between the gate terminal and the source terminal of each GaN transistor in a normal operating mode.

[0018] The controller may also be arranged to cause at least one of the isolating switches to isolate the output to which it is coupled from the inverter bridge to which it is coupled in the phase isolation mode of operation. The controller may be arranged to apply a voltage between the gate terminal and the source terminal of each of the isolating switches to be isolated in the phase isolation mode:

[0019] If the voltage across the GaN transistor is positive, zero voltage is applied; and

[0020] If the voltage across the GaN transistor is negative, a negative voltage is applied.

[0021] The negative voltage applied between the gate terminal and the source terminal of each GaN transistor may depend on the voltage across the GaN transistor.

[0022] Typically, the motor will be a three-phase motor and, therefore, there may be three inverter bridges and three outputs. In such cases, there may be two or three disconnect switches, depending on whether it is desired to control the current in the non-isolated phases. This type of control requires three disconnect switches (one per phase).

[0023] According to a second aspect of the invention there is provided a motor arrangement comprising a motor having a plurality of phases and the control circuit of the first aspect of the invention, wherein each phase is coupled to one of the outputs of the control circuit.

[0024] The electric motor may be a permanent magnet synchronous motor.

[0025] According to a third aspect of the present invention, there is provided an electric power steering device, the electric power steering device comprising a steering mechanism of a vehicle and the electric motor device of the second aspect of the present invention, wherein the electric motor is coupled to the steering device so as to drive the steering mechanism to change the steering direction of the vehicle. The steering device may be an electric power steering device that provides steering assistance to the driver, or may be a fully electric power steering device in which the electric motor alone drives the steering mechanism.

[0026] According to a fourth aspect of the present invention, there is provided a vehicle having the electric power steering apparatus of the third aspect of the present invention, wherein the electric power steering apparatus is arranged to change the steering of the vehicle so as to change the direction in which the vehicle moves.

[0027] According to a fifth aspect of the present invention, there is provided a method for controlling a control circuit of a multi-phase motor, the control circuit comprising:

[0028] a plurality of inverter bridges, each inverter bridge being arranged to provide an output voltage to a phase of the electric motor;

[0029] a plurality of output terminals, each output terminal being arranged to be coupled to a phase of the motor to provide an output voltage to said phase of the motor; and

[0030] at least one isolation switch, each isolation switch coupled between one of the inverter bridges and one of the output terminals to selectively isolate the output terminal from the inverter bridge;

[0031] Each isolation switch includes a gallium nitride (GaN) transistor;

[0032] The method comprises: in a normal operation mode, closing each isolation switch by turning on each GaN transistor; and in a phase isolation operation mode, opening at least one isolation switch by stopping each GaN transistor from conducting, to isolate at least one output terminal from an inverter bridge to which it is coupled.

[0033] The method may include applying a positive voltage between a gate terminal and a source terminal of each GaN transistor in a normal operating mode.

[0034] The method may include applying a voltage between a gate terminal and a source terminal of each of the isolation switches to be isolated in a phase isolation mode:

[0035] If the voltage across the GaN transistor is positive, zero voltage is applied; and

[0036] If the voltage across the GaN transistor is negative, a negative voltage is applied.

[0037] The negative voltage applied between the gate terminal and the source terminal of each GaN transistor may depend on the voltage across the GaN transistor.

[0038] Now, by way of example only, embodiments of the present invention are described below with reference to the accompanying drawings, in which:

[0039] FIG1 shows a circuit diagram of a motor device of an electric power steering system according to a first embodiment of the prior art;

[0040] FIG2 shows a circuit diagram of a motor device of an electric power steering system according to a second embodiment of the prior art;

[0041] 3 shows a circuit diagram of a motor device for an electric power steering system according to a third embodiment of the prior art;

[0042] Figure 4 A circuit diagram showing a motor device of an electric power steering system according to a first embodiment of the present invention;

[0043] Figure 5 A circuit diagram showing a motor device of an electric power steering system according to a second embodiment of the present invention;

[0044] Figure 6 Shown with Figure 4or a driver circuit for use with the circuit of 5; and

[0045] Figure 7 Shown is shown in Figure 4 or Figure 5 A graph of the reverse bias characteristics of a typical GaN transistor used in a circuit, plotted as current flow versus voltage drop across the transistor.

[0046] In the attached figure Figure 4 1 to 3. A motor arrangement for an electric power steering (EPAS) system according to a first embodiment of the invention is shown in FIG. Features equivalent to those discussed above with reference to FIGS. 1 to 3 have been given corresponding reference numerals, increased by 50.

[0047] The permanent magnet synchronous motor 57 is coupled to a steering column 71 via a gearbox 70 to provide steering assistance thereto. The steering column 71 is coupled to a steering mechanism 72 that changes the direction in which the wheels (not shown) of a vehicle 73 provided with the device point.

[0048] The motor 57 has three phases. To drive the motor 57 from a DC bus 58 (relative to ground 59), three inverter bridges are provided, each inverter bridge comprising a top switch 51, 53, 55 (selectively coupling a phase to the DC bus 58) and a bottom switch 52, 54, 56 (selectively coupling the phase to ground 59). These switches may be implemented as MOSFETs (metal oxide semiconductor field effect transistors).

[0049] To prevent unwanted current 60 from circulating in the event of a top or bottom switch failure (eg switch 51 exhibiting a short circuit failure), isolation switches 62, 63 are provided in both phases. Rather than using MOSFETs, these switches use Gallium Nitride (GaN) transistors.

[0050] GaN transistors are rapidly becoming a commercially viable replacement for MOSFETs. The GaN transistor structure is characterized by the absence of a parasitic bipolar junction and, as a result, a different reverse bias operating regime. With zero bias gate to source, there are no electrons beneath the gate region. As the drain voltage is reduced, a positive bias is developed on the gate relative to the drift region, injecting electrons beneath the gate. Once the gate threshold is reached, there are enough electrons beneath the gate to form a conducting channel. Since a threshold voltage is required to turn on a GaN transistor in the reverse direction, the forward voltage of the “diode” is higher than that of a silicon transistor. Importantly, by applying a negative V to the GaN device, the GaN device can be turned on in the reverse direction. GS Bias (as shown in the attached figure Figure 7 ), the forward voltage of this "diode" can be further increased to several volts.

[0051] Due to the lack of parasitic MOSFET body diode performance, in this embodiment, the use of GaN transistors for phase isolation reduces the number of semiconductor switches required for the entire solid-state phase isolation relay (SSPIR) implementation from 4 to 2, where it is only desired to limit the unwanted current in the event of a switch failure 10. The circuit of this embodiment is thus functionally equivalent to Figure 1 or 2 of the accompanying drawings.

[0052] In normal operation, each GaN transistor 62, 63 is turned on by a positive voltage Vgs between the gate and source terminals in a similar manner to a MOSFET.

[0053] Under fault conditions such as a shorted inverter MOSFET 51, the phase-isolated GaN switches 62, 63 can be turned off to prevent uncontrolled currents from circulating in the motor phases. This can be done by controlling the gate-source voltage of each GaN switch 62, 63 depending on the voltage across the switching element.

[0054] For positive voltages across the GaN switches 62, 63, the gate-source voltage can be maintained close to 0V. For negative voltages across the GaN switches 62, 63, the applied gate-source voltage is negative. The negative gate-source voltage to be applied can be determined by the voltage across the switches 62, 63, as described below with respect to Figure 6 Discussed.

[0055] In the attached figure Figure 5 A second embodiment of the present invention is shown, which is functionally equivalent to FIG. 3 of the accompanying drawings. Figure 4 Features of the circuit have been indicated by corresponding reference numerals, increased by 50.

[0056] In this embodiment, there are three three-phase isolation switches 111, 112, 113, all using GaN transistors. Since these devices can block current in either direction, the number of phase isolation semiconductor switches is reduced from 6 to 3 to allow control of the non-faulty phase (as shown in Figure 3 above). Therefore, the benefits of both embodiments are lower power component count, cost and lower insertion loss.

[0057] Figure 6 A GaN phase-isolated driver circuit 120 is shown that achieves the above performance; this can be used to control Figure 4 or Figure 5 For descriptive purposes only, we describe its use in controlling Figure 4 The switch 62 is used in the embodiment of the present invention, but the operating principle is the same for any of the phase isolating switches 62, 63, 111, 112, 113 described above.

[0058] In normal operation, switch SW1 is closed and driver circuit 120 applies a voltage (V ) between the gate and source of GaN switch 62 suitable for fully turning on GaN switch 62. gs ). Compared with R2, R1 is relatively large in resistance and therefore does not significantly affect the gate voltage. When the GaN switches 62, 63; 111, 112, 113 are closed, the current in each phase can be freely controlled in the motor 57, 107 through the three-phase inverter 51, 52, 53, 54, 55, 56; 101, 102, 103, 104, 105, 106.

[0059] When it is desired to turn off the GaN switch 62, in the event of a short circuit failure of one of the inverter MOSFETs 51, 52, 53, 54, 55, 56; 101, 102, 103, 104, 105, 106, the voltage output by the driver circuit 120 can be controlled to 0V and SW1 is opened.

[0060] Observe the voltage across the GaN switch 62 (i.e., the voltage V between the drain and source). ds ), for positive V ds Condition,V gs The diode R2 in series with SW1 keeps the voltage close to 0V. ds condition, the diode across SW1 is reverse biased and the gate voltage is determined by R1 and the drain voltage.

Claims

1. A control circuit for a multi-phase motor, the control circuit comprising: a plurality of inverter bridges, each inverter bridge being arranged to provide an output voltage to a phase of the electric motor; a plurality of output terminals, each output terminal being arranged to be coupled to a phase of the motor to provide the output voltage to the phase of the motor; and at least one isolation switch, each isolation switch coupled between one of the inverter bridges and one of the output terminals to selectively isolate the output terminal from the inverter bridge; wherein each of the isolation switches comprises a gallium nitride transistor, the gallium nitride transistor comprising a gate terminal, a source terminal and a drain terminal, wherein the control circuit is provided with a controller arranged to close each isolating switch in a normal operating mode so that each isolating switch does not isolate its output terminal from its inverter bridge and allows the output voltage from its inverter bridge to be applied to its output terminal, The controller includes a reference terminal and an output terminal, the reference terminal is connected to the source terminal of the gallium nitride transistor, and the output terminal is connected to the gate terminal of the gallium nitride transistor. wherein the switch (SW1) and the diode at both ends of the switch (SW1) are connected in parallel between the reference end and the source terminal of the gallium nitride transistor, and the resistor (R2) is connected between the output end and the gate terminal of the gallium nitride transistor, wherein the controller is arranged to apply a positive voltage between the gate terminal and the source terminal of each gallium nitride transistor in the normal operating mode and with the switch (SW1) closed, wherein the controller is arranged, in a phase isolation mode of operation, such that at least one of the isolation switches isolates the output to which it is coupled from the inverter bridge to which it is coupled, wherein the controller is arranged to apply a voltage between the gate terminal and the source terminal of each of the isolation switches to be isolated in phase isolation mode and with the switch (SW1) open: If the voltage across the gallium nitride transistor is positive, applying a zero voltage through the resistor (R2) in series with the diode across the switch (SW1); and If the voltage across the gallium nitride transistor is negative, a negative voltage is applied.

2. The control circuit of claim 1, wherein each gallium nitride transistor provides selective isolation of each isolation switch.

3. A control circuit according to claim 1 or claim 2, wherein each inverter bridge not coupled to an isolation switch is coupled to an output terminal.

4. The control circuit of claim 1 , wherein the controller is arranged such that the negative voltage applied between the gate terminal and the source terminal of each gallium nitride transistor is dependent on the voltage across the gallium nitride transistor.

5. A method for controlling a control circuit of a multi-phase motor, the control circuit comprising: a plurality of inverter bridges, each inverter bridge being arranged to provide an output voltage to a phase of the electric motor; a plurality of output terminals, each output terminal being arranged to be coupled to a phase of the motor to provide the output voltage to the phase of the motor; and at least one isolation switch, each isolation switch coupled between one of the inverter bridges and one of the output terminals to selectively isolate the output terminal from the inverter bridge; wherein each of the isolation switches comprises a gallium nitride transistor, the gallium nitride transistor comprising a gate terminal, a source terminal and a drain terminal; The control circuit is provided with a controller, The controller includes a reference terminal and an output terminal, the reference terminal is connected to the source terminal of the gallium nitride transistor, and the output terminal is connected to the gate terminal of the gallium nitride transistor. wherein the switch (SW1) and the diode at both ends of the switch (SW1) are connected in parallel between the reference end and the source terminal of the gallium nitride transistor, and the resistor (R2) is connected between the output end and the gate terminal of the gallium nitride transistor, The method comprises: in a normal operating mode, closing each isolation switch by turning on each gallium nitride transistor; and in a phase isolation mode of operation, isolating at least one output terminal from the inverter bridge to which it is coupled by disconnecting at least one isolation switch by causing each gallium nitride transistor to stop conducting, The method comprises applying a positive voltage between the gate terminal and the source terminal of each gallium nitride transistor in the normal operating mode and with the switch (SW1) closed, The method comprises applying a voltage between the gate terminal and the source terminal of each of the isolation switches to be isolated in a phase isolation mode and with the switch (SW1) open: If the voltage across the gallium nitride transistor is positive, applying a zero voltage through the resistor (R2) in series with the diode across the switch (SW1); and If the voltage across the gallium nitride transistor is negative, a negative voltage is applied. 6 . The method of claim 5 , wherein the negative voltage applied between the gate terminal and the source terminal of each gallium nitride transistor depends on the voltage across the gallium nitride transistor.

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