Control circuit for a multi-phase motor

Abstract

A control circuit for a multi-phase motor (57) comprises a plurality of inverter bridges, a plurality of outputs, 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 is arranged to be coupled to one phase of the motor (57) to provide the output voltage to that phase of the motor (57). Each isolation switch (62, 63) is coupled between one of the inverter bridges and one of the outputs, so as to selectively isolate the output from the inverter bridge. Each isolation switch (62, 63) comprises a Gallium Nitride (GaN) transistor.

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 for a phase of the motor; a plurality of outputs, each output being arranged to be coupled to one phase of the motor to provide the output voltage to that phase of the motor; at least one isolation switch, each isolation switch being coupled between one of the inverter bridges and one of the outputs, so as to selectively isolate the output from the inverter bridge, in which the at least one isolation switch is a Gallium Nitride (GaN) transistor comprising a gate, a drain, and a source; a controller comprising a first output, and a second output coupled to the gate of the GaN transistor; a switch and a diode connected in parallel between the first output and the source of the GaN transistor; and wherein during a normal mode of operation of the controller, the switch is closed and the diode is forwarded biased and the controller applies a positive voltage between the gate and the source of the GaN transistor so that the GaN transistor does not isolate its output from its inverter bridge when the switch is closed, and wherein during a phase isolation mode of operation of the controller, the switch is opened and the diode is reverse biased and the controller applies one of a zero or a negative voltage between the gate and the source of the GaN transistor so that the GaN transistor isolates its output from the inverter bridge when the switch is opened.

2. The control circuit of claim 1, in which each GaN transistor provides the selective isolation of each isolation switch.

3. The control circuit of claim 1, in which each inverter bridge which is not coupled to an isolation switch is coupled to an output.

4. The control circuit of claim 1, in which the controller is arranged such that the negative voltage applied between the gate and the source of each GaN transistor may be dependent upon a voltage across the GaN transistor.

5. The control circuit of claim 1, wherein the controller applies the zero voltage between the gate and the source of the GaN transistor when the GaN transistor has a positive drain to source voltage.

6. The control circuit of claim 1, wherein the controller applies the negative voltage between the gate and the source of the GaN transistor when the GaN transistor has a negative drain to source voltage.

7. A motor apparatus comprising a motor having a plurality of phases and a control circuit, wherein the control circuit comprises: a plurality of inverter bridges, each inverter bridge being arranged to provide an output voltage for a phase of the motor; a plurality of outputs, each output being arranged to be coupled to one phase of the motor to provide the output voltage to that phase of the motor; at least one isolation switch, each isolation switch being coupled between one of the inverter bridges and one of the outputs, so as to selectively isolate the output from the inverter bridge, in which the at least one isolation switch is a Gallium Nitride (GaN) transistor comprising a gate, a drain, and a source, and wherein each phase is coupled to one of the outputs of the control circuit; a controller comprising a first output, and a second output coupled to the gate of the GaN transistor; a switch and a diode connected in parallel between the first output and the source of the GaN transistor; and wherein during a normal mode of operation of the controller, the switch is closed and the diode is forwarded biased and the controller applies a positive voltage between the gate and the source of the GaN transistor so that the GaN transistor does not isolate its output from its inverter bridge when the switch is closed, and wherein during a phase isolation mode of operation of the controller, the switch is opened and the diode is reverse biased and the controller applies one of a zero or a negative voltage between the gate and the source of the GaN transistor so that the GaN transistor isolates its output from the inverter bridge when the switch is opened.

8. The motor apparatus of claim 7, in which each GaN transistor provides the selective isolation of each isolation switch.

9. The motor apparatus of claim 7, in which each inverter bridge which is not coupled to an isolation switch is coupled to an output.

10. A method of controlling 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 for a phase of the motor; a plurality of outputs, each output being arranged to be coupled to one phase of the motor to provide the output voltage to that phase of the motor; at least one isolation switch, each isolation switch being coupled between one of the inverter bridges and one of the outputs, so as to selectively isolate the output from the inverter bridge, in which the each isolation switch comprises a Gallium Nitride (GaN) transistor comprising a gate, a drain, and a source; a controller comprising a first output, and a second output coupled to the gate of the GaN transistor; a switch and a diode connected in parallel between the first output and the source of the GaN transistor; and the method comprising, in a normal mode of operation of the controller: closing the switch to forward bias the diode; and applying a positive voltage between the gate and the source of the GaN transistor so that the GaN transistor does not isolate its output from its inverter bridge when the switch is closed; and the method further comprising, in a phase isolation mode of operation of the controller: opening the switch to reverse bias the diode; and one of applying a zero or a negative voltage between the gate and the source of the GaN transistor so that the GaN transistor isolates its output from the inverter bridge when the switch is open.

11. The method of claim 10, in which the negative voltage applied between the gate and the source of each GaN transistor is dependent upon a voltage across the GaN transistor.

Description

(1) There now follows, by way of example only, description of embodiments of the invention, described with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a circuit diagram for a motor apparatus for an electric power assisted steering system in accordance with a first embodiment of the prior art;

(3) FIG. 2 shows a circuit diagram for a motor apparatus for an electric power assisted steering system in accordance with a second embodiment of the prior art;

(4) FIG. 3 shows a circuit diagram for a motor apparatus for an electric power assisted steering system in accordance with a third embodiment of the prior art;

(5) FIG. 4 shows a circuit diagram for a motor apparatus for an electric power assisted steering system in accordance with a first embodiment of the invention;

(6) FIG. 5 shows a circuit diagram for a motor apparatus for an electric power assisted steering system in accordance with a second embodiment of the invention;

(7) FIG. 6 shows a driver circuit for use with the circuits of FIG. 4 or 5; and

(8) FIG. 7 shows a graph showing the reverse bias characteristic of a typical Gallium Nitride transistor used in the circuits of FIG. 4 or 5, plotted as a graph of current flowing vs voltage drop across the transistor.

(9) A motor apparatus for an electric power assisted steering (EPAS) system in accordance with a first embodiment of the invention is shown in FIG. 4 of the accompanying drawings. Equivalent features to those discussed with reference to FIGS. 1 to 3 above have been given corresponding reference numerals, raised by 50.

(10) A permanent magnet synchronous motor 57 is coupled via a gearbox 70 to steering column 71 to provide steering assistance thereto. The steering column 71 is coupled to a steering mechanism 72 which changes the direction in which road wheels (not shown) of a vehicle 73 in which the apparatus is provided are pointing.

(11) The motor 57 has three phases. In order to drive the motor 57 from a DC bus 58 (relative to ground 59), three inverter bridges are provided, each 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 that phase to ground 59). These switches can be implemented as MOSFETs (metal oxide semiconductor field effect transistors).

(12) In order to prevent unwanted currents 60 circulating in case of a top or bottom switch failing (e.g. switch 51 exhibiting a short-circuit failure), isolation switches 62, 63 are provided in two of the phases. Rather than using MOSFETs, these switches instead employ Gallium Nitride (GaN) transistors.

(13) GaN transistors are rapidly becoming a commercially viable alternative to MOSFETs. The GaN transistor structure is characterised by the absence of the parasitic bipolar junction and as a result has a different reverse bias operation mechanism. With zero bias gate to source, there is an absence of electrons under the gate region. As the drain voltage is decreased, a positive bias on the gate is created relative to the drift region, injecting electrons under the gate. Once the gate threshold is reached, there are sufficient electrons under the gate to form a conductive channel. As it takes threshold voltage to turn on the GaN transistor in the reverse direction, the forward voltage of the “diode” is higher than a silicon transistor. Significantly, by applying a negative V.sub.GS bias to the GaN device (as shown in FIG. 7 of the accompanying drawings) this “diode” forward voltage can be further increased to several volts.

(14) Due to the lack of the parasitic MOSFET body diode behaviour, using GaN transistors for phase isolation reduces the number of semiconductor switches required for the complete solid state phase isolation relay (SSPIR) implementation from 4 to 2 in this embodiment, where it is merely desired to limit the unwanted current 10 in case of a switch failure. As such, the circuit of this embodiment is equivalent in function to that of FIG. 1 or 2 of the accompanying drawings.

(15) 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.

(16) Under fault conditions such as a short circuit inverter MOSFET 51, the phase isolation GaN switches 62, 63 can be turned off to prevent uncontrolled currents circulating in the motor phases. This can be carried out by controlling the gate-source voltage of each GaN switch 62, 63 depending on the voltage across the switching element.

(17) For a positive voltage across the GaN switch 62, 63, the gate-source voltage may be maintained close to 0V. For a negative voltage across the GaN switch 62, 63, the applied gate-source voltage is negative. The negative gate-source voltage to be applied may be determined by the voltage across the switch 62, 63, as discussed below with respect to FIG. 6.

(18) FIG. 5 of the accompanying drawings shows a second embodiment of the invention, which is equivalent in function to that of FIG. 3 of the accompanying drawings. Features equivalent to that of the circuit of FIG. 4 have been shown with corresponding reference numerals, raised by 50.

(19) In this embodiment, there are three phase isolation switches 111, 112, 113 all employing GaN transistors. As these devices can block current in either direction, the number of phase isolation semiconductor switches in order to allow control of the non-failed phases (as in FIG. 3 above) reduces from 6 to 3. Both embodiments therefore have the benefit of having lower power component count, cost and lower inserted losses.

(20) A GaN phase isolation driver circuit 120 to achieve the behaviour above is shown in FIG. 6; this can be used to control the GaN transistors of the circuits of either FIG. 4 or FIG. 5. For the sake of description only, we describe its use in controlling switch 62 of FIG. 4, but the principle of operation would be the same for any of the phase isolation switches 62, 63, 111, 112, 113 described above.

(21) In normal operation switch SW1 is closed and the driver circuit 120 applies a voltage between the gate and source of the GaN switch 62 (V.sub.gs) appropriate to fully turn on the GaN switch 62. R1 resistance is relatively large in comparison to R2 and therefore it does not significantly affect the gate voltage. With the GaN switches 62, 63; 111, 112, 113 closed in each phase current can be freely controlled in the motor 57, 107 with the three phase inverter 51, 52, 53, 54, 55, 56; 101, 102, 103, 104, 105, 106.

(22) In case of a short circuit failure of one of the inverter MOSFETS 51, 52, 53, 54, 55, 56; 101, 102, 103, 104, 105, 106 when it is desired to turn off the GaN switch 62, the voltage output by the driver circuit 120 can be controlled to 0V and SW1 opened. Looking at the voltage across the GaN switch 62 (that is, the voltage between the drain and the source V.sub.ds), for positive V.sub.ds conditions, V.sub.gs is held close to 0V by R2 in series with the diode across SW1. For negative V.sub.ds conditions, with the diode across SW1 being reverse biased, the gate voltage is determined by R1 and the drain voltage.