阅读说明：本技术 用于电动马达电路的驱动电路 (Drive circuit for an electric motor circuit ) 是由 D·布莱迪 于 2020-08-24 设计创作，主要内容包括：一种驱动电路,包括：LC感性网络,其具有能够操作以使网络在ST状态与活动状态之间切换的电源开关；逆变器和控制器,控制器用于控制逆变器的开关以及电源开关的开-关状态,其中,控制器操作这些开关以提供：PWM周期的非驱动部分,在其中,电源开关保持断开并且至少一个相的顶部开关和底部开关保持闭合,使得驱动电路以ST模式操作；紧接在非驱动部分之后或之前的PWM周期的驱动部分,在其中,电源开关是闭合的,并且其中,控制器被配置成采用PWM模式,在其中,所有底部开关或所有顶部开关同时从闭合状态移动到断开状态,并且进一步地,电源开关基本上在驱动部分开始时从断开状态移动到闭合状态并且基本上在驱动部分结束时从闭合状态移动到断开状态。(A drive circuit, comprising: an LC inductive network having a power switch operable to switch the network between an ST state and an active state; an inverter and a controller for controlling the on-off state of the switches of the inverter and the power switches, wherein the controller operates the switches to provide: a non-driving portion of the PWM cycle in which the power switches remain open and the top and bottom switches of at least one phase remain closed so that the drive circuit operates in ST mode; a drive portion of a PWM cycle immediately following or preceding the non-drive portion, in which the power switches are closed, and wherein the controller is configured to adopt a PWM mode in which all of the bottom switches or all of the top switches are simultaneously moved from a closed state to an open state, and further the power switches are moved from the open state to the closed state substantially at the start of the drive portion and from the closed state to the open state substantially at the end of the drive portion.)

1. A drive circuit for an electrical load, such as a multi-phase motor, the drive circuit comprising:

a two-port LC inductive network having a pair of input nodes and a pair of output nodes, the LC inductive network including a power switch operable to switch the LC inductive network between an ST state and an active state;

an inverter connected to the two output nodes of the LC inductive network and having a plurality of top switches and a plurality of bottom switches, the plurality of top switches selectively connecting respective phases of the load to a first one of the two output nodes of the LC inductive network and the plurality of bottom switches selectively connecting respective phases of the load to a second one of the two output nodes of the LC inductive network, and

a controller for controlling the on-off state of each of the switches of the inverter and the on-off state of the power switches, the controller in use operating the switches during each PWM cycle to provide:

a non-driving portion of the PWM cycle in which the power switch remains open and the top and bottom switches of at least one phase remain closed, such that the drive circuit operates in an ST mode,

a drive portion of the PWM cycle immediately following or immediately preceding the non-drive portion, in which drive portion the power switch is closed and the top and bottom switches are arranged such that for each phase the top and bottom switches do not remain open simultaneously, such that the motor operates substantially in NST mode, and

wherein the controller is configured to employ a PWM mode in which all of the bottom switches or all of the top switches move from a closed state to an open state simultaneously with a start of the driving part and all of the bottom switches or all of the top switches move from an open state to a closed state simultaneously with an end of the driving part,

and further, in the PWM mode, the power switch moves from an open state to a closed state substantially at the start of the driving section and moves from a closed state to an open state substantially at the end of the driving section.

2. A drive circuit according to claim 1, wherein the drive portion has a fixed duration within each cycle of the PWM pattern, whereby the positions at which the drive portion starts and ends in each cycle are fixed.

3. The drive circuit according to claim 1 or claim 2, wherein the drive portion of the PWM period immediately follows the non-drive portion within the PWM period, and the controller provides a second non-drive portion immediately following the drive portion in which the power switch remains open and the top and bottom switches of the inverter also remain open, so that the drive circuit operates in the ST mode.

4. A drive circuit according to claim 3, wherein the duration of the three portions fills the PWM period.

5. A drive circuit according to any preceding claim, wherein the LC inductive network comprises a quasi-Z source converter topology.

6. A drive circuit according to claim 3, wherein the controller sets the duty cycle of the top and bottom switches in each phase during the drive portion in dependence on the required output of the motor.

7. The drive circuit according to claim 1, wherein the controller is configured to move the power switch from the open state to the closed state shortly after a moment when all of the bottom switches or all of the top switches move from the closed state to the open state simultaneously with a start of the drive section.

8. The drive circuit of claim 7, wherein the controller is configured to move the power switches from a closed state to an open state shortly before a time when all of the bottom switches or all of the top switches move from an open state to a closed state simultaneously with an end of the drive section.

9. A drive circuit according to claim 7, wherein the controller is configured to cause the power switch to remain open (non-conductive) continuously during the or each non-driving portion of the PWM cycle to disconnect the phase switch from the power supply.

10. A drive circuit according to claim 8, wherein the controller is configured to cause the power switch to remain open (non-conductive) continuously during the or each non-driving portion of the PWM cycle to disconnect the phase switch from the power supply.

11. The drive circuit of claim 7, wherein the controller is configured to cause the power switch to open and close only once per PWM cycle to define transitions at the beginning and end of the drive section.

12. The drive circuit of claim 8, wherein the controller is configured to cause the power switch to open and close only once per PWM cycle to define transitions at the beginning and end of the drive portion.

13. The drive circuit of claim 9, wherein the controller is configured to cause the power switch to open and close only once per PWM period to define transitions at the beginning and end of the drive section.

14. A method for driving a drive circuit for an electrical load, such as a multi-phase motor, the drive circuit comprising:

a two-port LC inductive network having a pair of input nodes and a pair of output nodes, the LC inductive network including a power switch operable to switch the LC inductive network between an ST state and an active state;

an inverter connected to the two output nodes of the LC inductive network and having a plurality of top switches and a plurality of bottom switches, the plurality of top switches selectively connecting respective phases of the load to a first one of the two output nodes of the LC inductive network and the plurality of bottom switches selectively connecting respective phases of the load to a second one of the two output nodes of the LC inductive network, and

a controller for controlling the on-off state of each of the switches of the inverter and the on-off state of the power switches, the method comprising the step of causing the controller to operate the switches in each PWM cycle to provide:

a non-driving portion of the PWM cycle in which the power switch remains open and the top and bottom switches of at least one phase remain closed, such that the drive circuit operates in an ST mode,

a drive portion of the PWM cycle immediately following or immediately preceding the non-drive portion, in which drive portion the power switch is closed and the top and bottom switches are arranged such that for each phase the top and bottom switches do not remain open simultaneously, such that the motor operates substantially in NST mode, and

wherein the controller is configured to employ a PWM mode in which all of the bottom switches or all of the top switches move from a closed state to an open state simultaneously with a start of the driving part and all of the bottom switches or all of the top switches move from an open state to a closed state simultaneously with an end of the driving part,

and further, in the PWM mode, the power switch moves from an open state to a closed state substantially at the start of the driving section and moves from a closed state to an open state substantially at the end of the driving section.

Technical Field

The present invention relates to circuits for driving electric motors, and more particularly to Pulse Width Modulation (PWM) control of switches of Z-source converters for driving multiphase brushless motors.

Background

To control a brushless electric motor, it is necessary to determine the position of the motor rotor and then control the current through the motor phase windings to produce the desired torque. The position may be measured using a dedicated position sensor or by estimating the position from other parameters using a position sensorless control scheme.

PWM control is typically implemented using a closed loop current controller of the type shown in fig. 1. The modulated voltage is applied to each phase winding of the motor using Pulse Width Modulation (PWM) control in response to the requested target current, and the resulting current is measured or estimated. The individual phase currents are then used by a controller (typically a PI controller) to generate the required pulse width modulated phase voltages to achieve the target current. The estimated motor position is used to ensure that the phase voltages can be applied to the correct phases at the correct time.

A typical prior art circuit 100 for a 3-phase motor is shown in fig. 2. The driver comprises a 6-FET inverter 110 arranged to apply a desired voltage to motor phase terminals of the motor 120 in a PWM pattern to achieve a desired phase current. A filter 130 is provided across the battery power supply 140 to stabilize the input voltage. With six switches TA, TB, TC, BA, BB, BC, it is possible to have each of these switches open or closed, there being eight possible states of the switches of the inverter, namely two so-called zero-voltage states and six active states (active states). Space vector modulation is used to construct a PWM pattern by combining one or more active states with one or more zero states in a defined PWM pattern within each modulation period. Other non-SVM techniques are also taught in this document.

A known problem with the circuit of figure 2 is that the available motor phase voltage is lower than the supply voltage. In automotive applications where the power source is a battery, this problem may lead to performance degradation when the battery is depleted. An alternative circuit 200 that overcomes these limitations is shown in fig. 3. In this arrangement, the filter 130 is replaced by a two-port LC inductive network (inductive network)210 with connected inductors and capacitors, and a power switch 220 that can be opened or closed to control the transfer of power from the battery to the inductors of the LC network. Such an arrangement combined with an inverter is referred to in this document as a bidirectional quasi-Z source converter or a power quasi-Z source converter. For convenience, the term quasi-Z source converter will be used hereinafter to refer to such circuits in which power switches are provided.

The Z-source converter topology provides an additional state for the bridge in which a phase of the motor can be shorted by both the upper and lower switches of that phase and the power switch remaining open. This is called the Shoot Through (ST) state because in this state, current cannot flow through the FET diodes into the motor phase. The other 8 conventional states when the power switch is closed are referred to as non-breakdown (NST) states. Thus, the Z-source converter has two modes of operation: ST state or mode, also referred to as non-drive mode hereinafter in this specification; and NST state or mode, hereinafter also referred to as drive mode.

During ST mode, the power switch is open and the inverter is shorted, so that power is transferred into the inductor. During the NST mode, the Z-FET is closed, and the power supply and two inductors can transfer energy to the load and charge the capacitor, thereby boosting the voltage available for application to the motor phase. The amount of voltage boost depends in part on the length of time spent in ST mode during each PWM cycle — a longer time will provide a higher voltage but allow a shorter NST time during which the motor can be driven using the higher voltage.

Another technique available when designing a motor drive circuit with a quasi-Z source converter including power switches in order to protect the inverter switches is to inject a short ST state around the time of switching the inverter switches by turning off the power switches just before the time of switching the inverter switches and then closing at or just after the time of switching the inverter switches. This provides a form of negative dead time.

Disclosure of Invention

According to a first aspect of the present invention there is provided a drive circuit for an electrical load, such as a multi-phase motor, the drive circuit comprising:

a two-port LC inductive network having a pair of input nodes and a pair of output nodes, the network comprising a power switch operable to switch the LC inductive network between an ST state and an active state;

an inverter connected to the two output nodes of the LC inductive network and having a plurality of top switches and a plurality of bottom switches, the plurality of top switches selectively connecting respective phases of the load to a first one of the two output nodes of the LC inductive network and the plurality of bottom switches selectively connecting respective phases of the load to a second one of the two output nodes of the LC inductive network,

and a controller for controlling the on-off state of each of the switches of the inverter and the on-off state of the power switches, the controller in use operating the switches in each PWM period to provide:

a non-driving portion of the PWM cycle in which the power switch remains open and the top and bottom switches of at least one phase remain closed, such that the drive circuit operates in an ST mode,

a drive portion of the PWM cycle immediately following or immediately preceding the non-drive portion, in which drive portion the power switch is closed and the top and bottom switches are arranged such that for each phase the top and bottom switches do not remain open simultaneously, such that the motor operates substantially in NST mode, and

wherein the controller is configured to employ a PWM mode in which all of the bottom switches or all of the top switches move from a closed state to an open state simultaneously with a start of the driving part and all of the bottom switches or all of the top switches move from an open state to a closed state simultaneously with an end of the driving part,

and further, in the PWM mode, the power switch moves from an open state to a closed state substantially at the start of the driving section and moves from a closed state to an open state substantially at the end of the driving section.

The drive portion may have a fixed duration within each cycle of the PWM mode. Thus, the positions at which the drive portion starts and ends in each cycle may be fixed.

The driving portion of the PWM period immediately follows the non-driving portion within the PWM period, and the controller may provide a second non-driving portion immediately following the driving portion in which the power switch remains off and the inverter top switch and the inverter bottom switch also remain off so that the driving circuit operates in the ST mode.

The duration of these two portions may fill the PWM period.

In an arrangement with three sections, the drive section is sandwiched between the non-sections of operation during the PWM period.

The duration of the three portions may fill the PWM period.

In the case where the third portion is provided, the sum of the durations of the first, second and third portions may be equal to the duration of the PWM period.

The LC network may include a quasi-Z source converter topology. The quasi-Z source converter topology may include a pair of inductors and a pair of capacitors. Each inductance or capacitance may comprise a series of inductors or a series of capacitors, or a single capacitor or a single inductor.

The duty cycle of the top switch and the bottom switch in each phase during the drive portion may be set according to a desired output of the motor. This in effect creates a separate PWM modulation of the inverter bridge within the driving portion of the entire PWM cycle, each driving portion representing one cycle of the sub-modulation. For a minimum amount of switching, each phase will have one on-time and one off-time within each drive section.

The power switch may move from the open state to the closed state shortly after a moment when all of the bottom switches or all of the top switches move from the closed state to the open state simultaneously with the start of the driving part. This provides protection for each phase because the power supply is effectively isolated from each phase when the inverter switches are switched, thereby preventing any shoot-through fault current.

The power switch may move from the closed state to the open state shortly after a moment when all of the bottom switches or all of the top switches move from the open state to the closed state simultaneously with an end of the driving part. This provides protection for each phase because the power supply is effectively isolated from each phase when the inverter switches are switched, which again prevents any breakdown fault current in the motor phases.

By using the edge-aligned PWM pattern in fixed PWM edges, which are edges aligned with the beginning and end of the driving portion of the PWM period, the number of switching operations of the power switches in each PWM period can be reduced compared to the prior art center-aligned PWM pattern for Z-source converter topologies. This is because the power switches of the LC network must first be turned off before the state of the top and bottom inverter switches changes, and aligning some edges of the PWM pattern with the beginning and end of the drive section reduces the number of times the power switches must be opened and closed.

Most preferably, the fixed edges of the PWM waveforms of all top switches are aligned with the beginning of the drive section and the fixed edges of the PWM waveforms of all bottom switches are aligned with the end of the drive section.

During the or each non-driving portion of the PWM period, the power switch may be continuously kept open (non-conductive) to disconnect the phase switch from the power supply.

Thus, the controller may be configured such that the power switch changes from open to closed only once and from closed to open only once during each PWM cycle. These variations may be aligned with the end of the drive section.

It is within the scope of the invention to insert an additional short duration period during the drive portion of the PWM mode in which the power switches remain open and then closed again before and during a change in the state of the switches of the inverter, as an alternative to using a dead time period in which all inverter switches remain open. This can prevent damage to the switches of the inverter.

For a three-phase motor, inserting an extra, short duration power switch open time will result in up to three additional open-close cycles of the power switch per PWM period, providing a total of four open-close cycles per PWM period.

However, keeping the power switch closed throughout the second section results in an optimal solution in which the power switch is opened and closed only once in each PWM period to define transitions at the beginning and end of the drive section.

For a leading-edge aligned PWM scheme in which the leading edges of the top switches are all aligned with the start of the drive section and the timing of the trailing edges of the top switches are modulated, the power switches of the LC network will have been opened at the start of the drive section and can then be closed after the leading edges have occurred or in synchronism with the leading edges of the PWM. Later, the power switch may be turned off shortly before the end of the driving portion.

In the presence of a non-driving portion starting at the beginning of each PWM cycle and another non-driving portion starting at the end of each PWM cycle, during continuous use, these non-driving portions will operate together to define one continuous non-driving operating period in ST mode, distributed across adjacent PWM cycles, that intersects with a driving portion in which a load, such as a motor, is actively driven.

During one or more non-drive portions, the controller may cause all top switches and all bottom switches of the inverter to close, thereby shorting the motor load across each phase.

The controller may control the inverter switches and the power switches by outputting respective voltage signals that are fed to the drive stage. The driver stage may convert these signals into respective voltages that are applied to the gates of each switch.

The controller may process a count signal from a counter that counts up from zero at the beginning of the PWM period to a maximum count value at or near the end of the PWM period. The controller may determine a duty cycle of the PWM modulation of the inverter switches in the second time portion, the duty cycle setting a time position of a varying edge of the PWM signal of each phase by a count value of the edge. The switches of the inverter may be switched when the count is reached.

One node of the input port of the LC network may be connected to the positive terminal of a voltage source (such as a battery) and the other node may be connected to the negative terminal of the voltage source or to ground.

The controller may open and close each top switch and each bottom switch of the inverter only once during each PWM cycle. This may be performed during the drive portion of the PWM cycle.

During the ST mode of operation, the switches of the inverter may each remain closed at all times.

Where the circuit is used in an automotive application, the voltage source may comprise a battery of the vehicle or a power bus (power bus) powered from one or more batteries of the vehicle.

According to a second aspect of the present invention there is provided a method of driving a drive circuit for an electrical load, such as a multi-phase motor, the drive circuit comprising:

a two-port LC inductive network having a pair of input nodes and a pair of output nodes, the LC inductive network including a power switch operable to switch the LC inductive network between an ST state and an active state;

an inverter connected to the two output nodes of the LC inductive network and having a plurality of top switches and a plurality of bottom switches, the plurality of top switches selectively connecting respective phases of the load to a first one of the two output nodes of the LC inductive network and the plurality of bottom switches selectively connecting respective phases of the load to a second one of the two output nodes of the LC inductive network,

and a controller for controlling the on-off state of each of the switches of the inverter and the on-off state of the power switch, the method comprising the step of causing the controller to operate the switches in each PWM period to provide:

a non-driving portion of the PWM cycle in which the power switch remains open and the top and bottom switches of at least one phase remain closed, such that the drive circuit operates in an ST mode,

a drive portion of the PWM cycle immediately following or immediately preceding the non-drive portion, in which drive portion the power switch is closed and the top and bottom switches are arranged such that for each phase the top and bottom switches do not remain open simultaneously, such that the motor operates substantially in NST mode, and

wherein the controller is configured to employ a PWM mode in which all of the bottom switches or all of the top switches move from a closed state to an open state simultaneously with a start of the driving part and all of the bottom switches or all of the top switches move from an open state to a closed state simultaneously with an end of the driving part,

and further, in the PWM mode, the power switch moves from an open state to a closed state substantially at the start of the driving section and moves from a closed state to an open state substantially at the end of the driving section.

Drawings

An embodiment of the invention will now be described, by way of example only, with reference to and as illustrated in the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a prior art motor circuit for driving a multi-phase brushless electric motor;

FIG. 2 is a prior art voltage source inverter circuit for applying PWM signals to a motor;

FIG. 3 is a prior art quasi-Z source converter motor circuit for applying a PWM signal to the motor;

fig. 4(a) is a block diagram of an exemplary control loop for a circuit including a quasi-Z source converter according to a first aspect of the present invention;

FIG. 4(b) shows a more detailed portion of the circuit of FIG. 4 (a);

FIG. 5 is a diagrammatic view of a 3-phase motor that may be driven by the circuits of FIGS. 4(a) and 4 (b);

FIG. 6 shows six normal PWM states that may be applied to the motor during the NST mode of operation, but without aligning the edges as required by the present invention;

FIG. 7 illustrates the switching pattern of the inverter switches and Z-FETs when the motor circuit of FIGS. 4(a) and 4(b) is in use;

FIG. 8 shows an alternative switching pattern that may be applied by the motor circuits of FIGS. 4(a) and 4 (b); and is

Fig. 9(a) -9(c) illustrate three exemplary controllers that may be implemented in the circuits shown in fig. 4(a) and 4 (b).

Detailed Description

As shown in fig. 4(a), the motor circuit for controlling a multiphase motor according to the present invention includes an embodiment of a Z-source converter 300. Converter 300 takes as input the required phase voltages output by the PI controller and applies appropriate PWM waveforms to the respective phases of the motor based on these required phase voltages. Fig. 9(a) -9(c) illustrate three exemplary embodiments of a controller that may be implemented within the circuit of fig. 4 (a).

In the disclosed embodiment, the Z-source converter has a quasi-Z-source converter topology, but the invention may be applied to other LC network topologies.

The quasi-Z source converter as shown in fig. 4(b) comprises a two-port inductive LC network 310, an inverter bridge 320 and a controller for controlling the state of the individual switches of the circuit. The input of the circuit 300 is connected to the supply and ground terminals of the battery 330 and the output of the inverter is connected to the multi-phase motor 340. The controller 350 controls the switching of the circuit,

the circuit allows the voltage applied to each phase to be varied, which in turn allows the torque produced by the motor to be controlled. Due to the presence of the Z-network, the voltage may be higher than the voltage connected to the battery supply, which makes the circuit particularly suitable for automotive applications where the state of the battery may sometimes be depleted. This also allows the motor to be operated at a more optimal operating point for a given motor design.

The z-network comprises a two-port network, which means that the z-network has two input nodes and two output nodes. The input nodes are connected to two terminals of a battery power supply. The particular topology of the network may be as shown in the example of fig. 3. Between the input and output ports are two capacitances (capacitances) C1 and C2 and two inductors L1 and L2, each of which comprises a separate set of capacitors. The network also includes a power switch 360, such as a field effect transistor. The power switch may be caused to open and close to change the mode of operation of the inductive network between the ST mode and the non-ST mode.

The inverter bridge 320 includes a set of switches forming a three-phase bridge 18 for use with a three-phase motor. For motors with more than three phases, the bridge can also have more phases. Each arm of the bridge comprises a pair of switches in the form of a top transistor TA, TB, TC and a bottom transistor BA, BB, BC connected in series between two output nodes of the Z-network. Thus, there are three top switches connected to one output node and three bottom switches connected to another output node.

The Z-source converter circuit 300 may be used to drive a three-phase brushless motor, such as the three-phase brushless motor shown in fig. 5. The three-phase brushless motor comprises a three-phase brushless motor 1 provided by way of example, comprising a rotor 2 having, for example, six embedded magnets 4 therein, in which case the magnets are arranged to provide six poles alternating between north and south poles about the rotor. Thus, the rotor defines three straight or d-axes evenly spaced about the rotor and three orthogonal or q-axes spaced from one another between the d-axes. The d-axis is aligned with the poles of the magnet 4, where the flux lines from the rotor are in the radial direction; and the q-axis is interposed between the d-axes where the lines of magnetic flux from the rotor are in the tangential direction.

The stator 6 comprises a nine-slot copper wound element having three sets of three teeth 8A, 8B, 8C each with a common winding forming a respective phase. Thus, there are three electrical cycles in each complete rotation of the rotor, and the three teeth 8A, 8B, 8C in any phase are always in the same electrical position as each other.

Three motor windings, not shown, generally labeled as phases A, B and C, are connected in a star network. The phase windings are wound around the stator teeth 8A, 8B and 8C, respectively. The motor windings 12, 14, 16 are each tapped from between a complementary pair of respective top and bottom transistors of the inverter. The transistors are turned on and off in a controlled manner by a controller 350 comprising processing means such as a microprocessor and optional memory to provide pulse width modulation of the potential applied to each phase winding and hence also of the current flowing through the winding. This in turn controls the strength and orientation of the magnetic field generated by the windings.

The PWM controller 350 includes a counter that repeatedly counts up from zero to a set maximum value, the time taken to reach the maximum value being equal to one PWM period. The start of the count is aligned with the start of the PWM period. Once the maximum value is reached, the counter is reset and the count is repeated.

Referring to fig. 6, during motoring, each motor phase in a three-phase system should only be connected to the positive supply voltage or to ground, and thus there are eight possible states available to the control circuit. Each of these states is available during a drive mode of the motor (so-called NST mode). Using 1 to represent one of the phases at positive voltage and 0 to represent the phase connected to ground, state 1 can be represented as [100], indicating that phase a is 1, phase B is 0 and phase C is 0, state 2 is represented as [110], state 3 is represented as [010], state 4 is represented as [011], state 5 is represented as [001], state 6 is represented as [101], state 0 is represented as [000] and state 7 is represented as [111 ]. Each of the states 1 to 6 is an on state in which current flows through all of the windings 2, 4, 6, through one of the windings in one direction and through the other two windings in the other direction. The two states not shown are state 0 and state 8, neither of which applies any drive to the motor, but can be safely used. State 0 is a zero volt state where all windings are connected to ground, and state 7 is a zero volt state where all windings are connected to the supply rail.

States 1, 2, 3, 4, 5, and 6 are also referred to herein as states + A, -C, + B, -A, + C, and-B, respectively, because they each represent a state in which the application of a voltage to the winding is positive or negative for a respective one of the phases. For example, in the + a state, the a phase is connected to the supply rail and the other two phases are connected to ground, and in the-a state, these connections are reversed.

Another state may exist because the circuit includes a Z network. In this eighth state (inverter state), the power switches are open and one or all of the phases of the motor are shorted between the supply rail and ground by keeping both the top and bottom switches of that phase closed. When connected in this case, the motor is considered to be operated in a breakdown (ST) mode or a non-drive mode, and when operated in one of the other 8 states 0 to 7, the motor is considered to be operated in a non-breakdown (NST) mode or a drive mode.

When the inverter of the circuit is controlled by the controller to produce pulse width modulation of the switches, each phase will typically be turned on and off once per PWM period. The relative length of time spent in each state will determine the magnitude and direction of the magnetic field generated in each winding and hence the magnitude and direction of the total torque applied to the rotor. These time lengths may be calculated by various modulation algorithms.

In this embodiment, the following specific switching patterns of the inverter switches and the power switches are applied by the PWM controller. This mode is shown in figure 7 of the drawings. Only one of the three phases (phase a) is shown, with the traces of TA and BA corresponding to the top and bottom switches in the phase a arm of the inverter.

During the first part of the PWM period, which is the non-driven part of the PWN cycle within the meaning of the claimed invention, which starts when the count is zero and ends when the value of the count reaches a predefined value stored in the memory, the Z-FET remains off and the inverter is operated in ST mode, whereby all three phases are short-circuited. This is achieved by turning on both the top switch TA and the bottom switch BA of phase a, as shown in fig. 7. At this point, the power switch remains open (off and non-conductive) to disconnect the two sets of capacitors from each other so that they do not lose their charge.

During a second portion of the PWM cycle, which is subsequent to the first portion, which defines the driving portion of the PWM cycle within the meaning of the claimed invention, the inverter is operated in NST mode, allowing the motor to be driven. In the drive section, the leading edge of the PWM signal for the bottom switch is aligned with the beginning of the drive section, and the trailing edge is shifted according to the duty cycle of the PWM signal as demanded by the controller. The trailing edge of the PWM signal for the top switch is aligned with the end of the drive section and the leading edge is shifted according to the duty cycle of the PWM signal as demanded by the controller. During this mode, the power switch remains open to connect the load to the battery. The available modulation range is determined by how long the NST mode lasts within the PWM period. As shown, this pattern occupies about half of the total PWM period.

It should also be noted that the power switch remains off for a short period of time when the inverter switch changes state, by keeping the power switch off for a short time after the end of the first non-driving portion, and by briefly turning off the power switch during the switching of the inverter switch at the end of the driving portion. This ensures that the switch is not damaged during switching, since in a practical embodiment the switch does not immediately change state.

During a third portion (which is a non-driving portion) following the second portion of the PWM cycle, the power switches are again kept open to isolate the load from the battery, and the top and bottom switches TA and BA of the inverter are operated in ST mode, whereby all phases are shorted.

In the alternative shown in fig. 8, an additional short ST state is added at the time of the trailing edge of the PWM mode to prevent damage to the switch, and a small period of dead time DT is also included by slightly extending the ST time to just after the start of the drive section and just before the end of the drive section. This may be required when the inverter switches cannot be operated at small or zero dead time. Typically, this is the case where the switch is not opened or closed as quickly as the PWM cycle. These additional ST periods require the power switches to be additionally opened and closed three times during each PWM cycle. Applicants have appreciated that this may be preferable to applying states 0 and 7 because the ST period effectively generates a negative dead time.