GB2591768A

GB2591768A - Bidirectional switch control

Info

Publication number: GB2591768A
Application number: GB2001552.5A
Authority: GB
Inventors: Taghizadeh Hassan
Original assignee: Sprint Electric Ltd
Current assignee: Sprint Electric Ltd
Priority date: 2020-02-05
Filing date: 2020-02-05
Publication date: 2021-08-11
Anticipated expiration: 2040-02-05
Also published as: GB202001552D0; GB2591768B

Abstract

A switch control circuit for controlling one bidirectional switch (Fig.9) is configured, upon receiving an OFF signal, to turn off a positive direction of the switch after a time t+OFF and turn off a negative direction of the switch after a time t-OFF; it is further configured, upon receiving an ON signal, to turn on the positive direction of the switch after a time t+ON and turn on a negative direction of the switch after a time t-ON. The positive and negative direction switch ON/OFF times may be dependent on the direction of current flow (Fig.10) and allow for safe 4-step commutation between two switches. A multi-switch control circuit comprises a plurality of switch control circuits, each controlling a respective single bidirectional switch, and a master control unit. When power is to be commutated from a first switch with input voltage V1 to a second switch with input voltage V2 the master control unit sends an OFF signal to the first switch control circuit and an ON signal to the second switch control unit; the respective switch control units may control the positive and negative direction turn ON/OFF times based on the input voltage difference V2-V1 with respect to zero. The switch control circuits are modular and may be used in applications such as AC-AC conversion.

Description

BIDIRECTIONAL SWITCH CONTROL

Field of the Invention

The present invention relates to control circuits for bidirectional switches and particularly, although not exclusively, to control of bidirectional switches in AC to AC power conversion systems.

Background

Some power converters such as AC choppers and AC to AC motor drives (Matrix Converters) require power switching devices that are capable of carrying current in both directions through the switch and blocking voltage of both polarities across the switch. These bidirectional switches, which are also known as 4-quadrant switches, can be implemented using a pair of 2-quadrant (unidirectional) semiconductor devices such as IGBTs or MOSFETs with antiparallel diodes. The device pair are commonly connected in series but with opposite polarities to form a bidirectional switch as shown in Figure 1. This is known as a 'Common Emitter' configuration but other topologies are possible.

Since the bidirectional switch has full blocking capability for both directions of current flow there is no natural freewheeling path for inductive load current through the switch. Therefore, both devices S, and S2 Oof the bidirectional switch (BS) need to be actively controlled in order to provide a path for the load current C\I at all times. This makes the commutation (the transfer of current from one bidirectional switch to another) of bidirectional switches much more complicated than unidirectional switches, needing several steps to ocomplete reliably and safely.

This state-diagram is typically implemented in a logic circuit within a Programmable Logic Device (PLD) C\J such as a Field Programmable Gate Array (FPGA).

As the number of states increases, more complex logic is required to implement the state-diagram which in turn requires more complex hardware components and circuitry. A complex logic circuit also requires more development effort for it to be reliable.

A 4-step commutation method from a bidirectional switch Sa to a bidirectional switch Sb is shown in Figure 2. The binary number in each state shows the state of all the power devices (Sal Sa2 -Sbl Sb2). For a 2-phase to single-phase matrix converter, where there are two bidirectional switches and therefore four individual power devices, each state is represented by a 4-bit binary number specifying the switching state of the power devices. Al represents an ON state of the switch and a 0 represents an OFF state of the switch.

The present invention has been devised in light of the above considerations.

Summary of the Invention

An aspect of the present invention provides a switch control circuit for controlling a bidirectional switch, the switch control circuit configured to: (i) when an OFF signal is received, turn off a positive direction after a time t+OFF, and turn off a negative direction of the bidirectional switch after a time t-OFF, and (ii) when an ON signal is received, turn on the positive direction of the bidirectional switch after a time t+oN, and turn on the negative direction of the bidirectional switch after a time toN.

A switch control circuit controls one single bidirectional switch. Thus the commutation is performed at switch level, rather than at a system level. A simple ON or OFF signal is all that is needed from a master control unit in a system and the switch level control circuits can then implement the commutation safely. The logic circuits (switch control circuits) controlling the switches are simpler and modular. Using the example of a three-phase matrix converter, a conventional logic circuit would need to control a total of 21 states. Utilising the present invention reduces the number of states to be controlled to 12. Further, the number of transitions to be utilised in a conventional logic circuit is 48, whereas there are only 24 distinct transitions when the present invention is utilised. Further, each state in a conventional circuit has 6 bits, whereas utilising the present invention, only 2 bits per state are required. Each of these effects of using the present invention in the three-phase matrix converter means that the components required in a PLO are simpler and fewer. Implementing the present invention therefore requires simpler code and results in less logic cells in the hardware implementation. So, smaller and cheaper programmable devices can be used for switching and the provision of control of switching is more efficient.

The present invention enables switch-level control of the process which reduces the complexity of the C\I20 system-level control of commutation from one bidirectional switch to another. A system implementing the switch-level control of the present invention is modular and the approach can be simply scaled to systems O using many switches, for example, multi-phase, multi-switch systems. A system utilising the present invention can be made more simply, thereby reducing time for development and design validation. Such a C\I system is also more reliable because a simpler system will have fewer problems in the field which, given that failures in the commutation process can result in catastrophic failures in the system, is of the utmost importance. The system may also be cheaper to implement as the reduced complexity means a higher performance, less expensive logic device can be used compared to that required for a conventional implementation.

A bidirectional switch can control positive and negative current/voltage. The aspects of the bidirectional switch that control each direction can be controlled separately, for example using two series unidirectional switches.

The times may be set to satisfy either of these inequalities: (a) LOFF < ttON < t+OFF < t-ON, Or (b) t+OFF < t-ON < t-OFF < t+ON.

In this way, when two bidirectional switches are utilised in a system, each controlled by a switch control circuit, and a first switch is turned on and a second switch is turned off, then the following process may occur: -a first one of the directions of the second switch is turned off, - then the second one of the directions of the first switch is turned on, - then the second one of the directions of the second switch is turned off, - then the first one of the directions of the first switch is turned on.

Which direction is turned off first (i.e. which of (a) and (b) are followed) may depend on the direction of the output current. When the output current is positive, the timings may satisfy inequality (a) and when the output current is negative, the timings may satisfy inequality (b).

The times may be set to satisfy either of these inequalities: (c) t-ON < t-OFF < t+ON < t+OFF or (d) t+ON < t+OFF < t-ON < t-OFF.

In this way, when two bidirectional switches are utilised in a system, each controlled by a switch control circuit, and a first switch is turned on and a second switch is turned off, then the following process may occur: - a first one of the directions of the second switch is turned on, - then the first one of the directions of the first switch is turned off, - then the second one of the directions of the second switch is turned on, O -then the second one of the directions of the first switch is turned off. C\120

* Which direction is turned on first (i.e. which of (c) and (d) are followed) may depend on the sign of the O voltage across the input terminal. When the input voltage across the switches is positive (V2-Vi>0, the input voltage of the second switch is higher than the input voltage of the first switch), the timings may C\J satisfy inequality (c) and when the input voltage across the switches is negative (V2-Vi<0, the input voltage of the second switch is less than the input voltage of the first switch), the timings may satisfy inequality (d).

The shortest of t-OFF and t+OFF may be 0 seconds or may be between 100ns and 1000ns. The shortest of t OFF and t+OFF may be less than 1s. Thus the commutation may begin immediately after the ON and OFF signals are received at the respective switch control circuits.

A dead time delay time, td, may be predefined and a minimum difference between each of the times t+OFF, LON, t-OFF, and t.ch may be td. This can reduce the chance of actions happening out of order and reduce the chance of short or open circuits occurring by introducing a delay between actions. For example, the times may satisfy one of the following sets of inequalities: (a) t-OFF +td-S tioN, t+ON ÷td 5 t+OFF, t+OFF +td t-oN Or (b) t+OFF +td S t-ON, t-ON +td t-OFF, t-OFF +tdS t+ON, Or (C) tON +td S t-OFF +td S t+ON +td S t+OFF, Or (d) t+ON +td t+OFF +td S LON +td 5 t-OFF.

The times may be (a) t-OFF = OS, t+ON = td, t+OFF = 2td, and tON = 3tu, Or (b) t+OFF = Os, LON = td t-OFF = 2td, and t+ON = 3tu, Or (C) t-ON = OS, t-OFF =td, t+ON.2td, t+OFF = Std Or (d) t+ON = OS, t+OFF =td, t-ON = 2td, t-OFF = Std. This gives sufficient time between the actions while making the switch time as short as possible by using the minimum separation between each action.

The bidirectional switch may comprise a positive unidirectional switch and a negative unidirectional switch connected in series. In this case, turning on the positive direction may comprise closing the positive unidirectional switch and turning on the negative direction may comprise closing the negative unidirectional switch. Turning off the positive or negative direction may comprise opening the respective positive or negative unidirectional switch. Opening or closing of the switch may utilise a gate voltage where the unidirectional switch is formed of a transistor. The unidirectional switches may be formed IGBTs or MOSFETs. The bidirectional switch may further comprise a positive diode connected around the negatives switch and a negative diode connected around the positive switch.

In another aspect, the present invention provides a multi-switch control circuit comprising a plurality of switch control circuits as described above, each for controlling a respective bidirectional switch and a master control unit for selectively sending the ON and OFF signals to the switch control circuits.

The master circuit may also receive signals from a current or voltage sensor associated with the switches to be switched between. The master circuit may send an indication of the current or voltage signal to the switch control circuits 0 When power is to be switched to a first switch which is off from a second switch which is on, the master C\I20 control unit may be configured to send an ON signal to a first one of the plurality of the switch control N. circuits which controls the first switch and an OFF signal to a second one of the switch control circuits 0 which controls the second switch.

C\J The ON and OFF signals may be sent simultaneously.

The master control unit may be provided by software and/or programmable logic.

In another aspect, the present invention provides a multi-switch system comprising a multi-switch control circuit as described above and a plurality of bidirectional switches, each of the switch control circuits being configured to control a respective one of the bidirectional switches.

In another aspect, the present invention provides an AC to AC power conversion system comprising a multi-switch system as described above.

In another aspect, the present invention provides a method of controlling a bidirectional switch by a switch control circuit comprising the steps of: (i) when an OFF signal is received by the switch control circuit, turning off the positive direction of the bidirectional switch after a time t+OFF, and turning off the negative direction of the bidirectional switch after a time tOFF; and (ii) when an ON signal is received by the switch control circuit, turning on a positive direction of the bidirectional switch after a time hoN, and turning on a negative direction of the bidirectional switch after a time Lori.

In another aspect, the present invention provides a method of commuting power from a first bidirectional switch to a second bidirectional switch in multi-bidirectional switch system comprising the steps of: sending, from a master control unit, an OFF signal to a first switch control circuit controlling the first bidirectional switch, and sending an ON signal to a second switch control circuit controlling the second bidirectional switch; and (i) when the OFF signal is received by the first switch control circuit, the first switch control circuit turns off the positive direction of the first bidirectional switch after a time tfrOFF, and turns off the negative direction of the first bidirectional switch after a time t-OFF, and (ii) when the ON signal is received by the second switch control circuit, the second switch control circuit turns on a positive direction of the bidirectional switch after a time t-ou, and turns on a negative direction of the bidirectional switch after a time toN.

The invention has been developed for control of the bi-directional semiconductor switches in a three-phase AC-to-AC Matrix Converters but could equally be used in other types of power electronic converter. It could also be utilised in other types of switches such as electro-mechanical relays or even solenoid valves for controlling gas or liquid flow. In this application, the lower switching speed requirements would mean that the state machine logic could be effectively implemented in software rather than a programmable logic device.

The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. In particular, the methods described C\I above may also include steps which the control circuits and/or systems are configured to carry out.

O Summary of the Figures

Embodiments and experiments illustrating the principles of the invention will now be discussed with C\J reference to the accompanying figures in which: Figure 1 shows a typical configuration for a bidirectional switch implementation.

Figure 2 shows a state diagram for 4-step commutation of the circuit shown in Figure 3.

Figure 3 shows a two-phase to single-phase Matrix converter.

Figure 4 shows switching states of the power devices for two-phase to single phases matrix converter when a) lo>0 and b)10<0 Figure 5 shows switching states of the power devices for two-phase to single phases matrix converter during the first step of 4-step current based commutation technique when a) lo>0 and b) lo<0.

Figure 6 shows switching states of the power devices for two-phase to single phases matrix converter during the second step of 4-step current based commutation technique when a) lo>0 and b) l<0.

Figure 7 shows switching states of the power devices for two-phase to single phases matrix converter during step 3 of 4-step current based commutation technique when a) 10>0 and b) lo<0.

Figure 8 shows switching states of the power devices for two-phase to single phases matrix converter during step 4 of 4-step current based commutation technique when a) 6>0 and b) lo<0.

Figure 9 shows a typical configuration for a bidirectional switch.

Figure 10 shows a proposed switch-level state diagram based on current-based 4-step commutation technique.

Figure 11 shows a proposed state diagram for current based 4-step commutation of the circuit shown in Figure 3.

Figure 12 shows a 3-phase Matrix converter.

Figure 13 shows a state diagram for current-based four-step commutation technique of one output leg of a 3-phase matrix converter.

C\I20 Figure 14a, Figure 14b, and Figure 14c show a proposed switch-level 4-step commutation state-diagram for leg 'a' of a 3 phase matrix converter.

Detailed Description of the Invention

C\I Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art All documents mentioned in this text are incorporated herein by reference.

An example of a commutation of current in bidirectional switches will now be described. The commutation is achieved using a 4-step process. The method can be described using a 2-phase to single-phase Matrix Converter shown in Figure 3, which has 2 bidirectional switches, BS, and BS. The objective of switch commutation is to transfer the current from one switch to the other, all the while observing two rules: 1. The voltage across the bidirectional switches, vab, must not be short circuited.

2. The output inductive load should not be open circuited.

To ensure correct sequencing through the commutation process, knowledge of either the direction of the output current, /0 through the switches, or the direction of the voltage across the switches, vab, is required. The 4-step commutation technique is described here where the direction of the output current is known. The operating principles for voltage-based commutation are analogous, but follow different rules for the safe states. Typically only one commutation method (current or voltage) will be implemented in a system, but it is possible for a system to be built that is capable of both methods. The method can be chosen by a user or by the system depending on the circumstances and/or current/voltage measurements.

In current commutation, an incorrect reading of the output current sign for current base commutation will cause an open circuit. This can occur, for example, during zero crossing of the output current due to limited accuracy of the sensor and/or noise on the measurement signal and/or current sensor failure.

There are methods to mitigate open circuits for example using a clamp circuit.

In voltage commutation, an incorrect reading of the input voltage sign will cause input short circuit. This can occur, for example, during zero crossing of the value due to limited accuracy of the sensor and/or noise on the measurement signal and/or voltage sensor failure. Mitigation for input short is more challenging than for current commutation and therefore current based commutation is typically implemented.

Safe switching states for current commutation are those which won't lead to output open circuit or input short circuit and are shown in Table 1 where a "1" represents the closed state of the device and "0" represents the Open state.

Table I

State 5" 5a2 Sbl Sb2 Sign L, 1 1 1 0 0 2 0 0 1 1 3 1 0 0 0 ± 4 0 I 0 0 -0 0 1 0 ± 6 0 0 0 1 - 7 1 0 I 0 + 8 0 1 0 I -States 1 and 2 are the stable states where both power devices in the bidirectional switch are on or both are off. States 3-8 are transition states which are only used during current commutation from one switch to another. During the stable states 1 and 2, the output current can flow in either direction. During transition states 3-8 the output current can only flow in one direction as indicated in the last column of the table. For safe operation the commutation from one stable state to another stable state needs to be performed in a defined sequence of steps through the transition states dependent on the current direction.

Assume that the current switching state is state 1 and bidirectional switch BS a is closed and bidirectional switch BSI, is open as shown in Figure 4(a). If the output current is positive in the direction shown in Figure 4(a), the load current is conducted through 5,, and the antiparallel diode of device 5,2 (D2).

When the output current needs to be commutated to the bidirectional switch B5b it is not possible to simply turn BSa OFF and turn BSb ON since the devices are not ideal and need a finite time to transition between states. Attempting this (turning Sand S" OFF and Si" and Sb2 ON simultaneously) could open-

S

circuit the load current causing an overvoltage that could damage the devices. Instead a path for the load current is provided at all time during commutation. This is known as 4-step commutation.

The four steps for commutating current from BS, to BSI, are summarised below: Step 1) The commutation starts by turning off the device that is not conducting in BS,. The device that is turned off depends on the output current direction: Sto if 10 > 0 and is Sit, if 10 < O. Therefore, the switching state in this step would be 3 or 4 (referring to Table 1), depending on the current direction, as shown in Figure 5.

Step 2) After a predefined dead-time delay td the device that will conduct the output current in BSI, will be turned on.: 5b1 if to > 0 (state 5) and is 51,2 if lb < 0 (state 6). This provides the path for the inductive load current when the second device in BSc, is turned oft The commutation of current from BS, to BSb occurs in this step if Val, < 0 (Vab = Va -MO, as shown in Figure 6, otherwise commutation will occur in step 3. It should be noted that both devices of ach cannot be simultaneously turned on at this step as it may cause a short circuit on the input side.

O Step 3) Now that a current path is provided for the inductive load current, the second device of BS, (Sat if 1" > 0 (state 7) or Su, if 10 < 0 (state 8)) can be turned off safely after a predefined short delay of td °1/4120 known as dead-time. The current commutation from BSc, to 115b occurs in this step if vat, > O. Step 4) The commutation is completed in this step by turning on the second device of BSb (51,2 if Jo > 0 or S,1 if /b < 0). In this step the switching state is set to the stable state 2. C\I

Although the 4-step current commutation requires four steps to safely commutate, the actual current commutation between two bidirectional switches always takes place at step 2 or 3 depending on the polarity of the voltage across the bidirectional switches as shown by V bb in Figure 3.

In the proposed approach the commutation is implemented at switch-level. The switching states of the power devices of the bidirectional switch are identified for each step of the 4-step commutation process and then a switch-level process is developed. There are four feasible states for the power devices of a bidirectional switch, BS,' shown in Table 2, where two of them are stable states corresponding to ON and OFF states of the bidirectional switch (states 1 and 4 respectively), and two others are transition states which are only used during commutation period (states 2 and 3). During ON state of the switch, the current, can flow in either directions through the switch. When in state 2, the current can only be positive. Conversely, in state 3, the current can be only negative. When in state 4, where both switches are off, the current should be zero.

Table2, Feasible

The following observations can be made: * For a bidirectional switch BS, that needs to be turned on: o If Ix is positive: * Sn turns on after td (Dead-time delay) * Sn turns on after 3ta O If i is negative: * Sn turns on after td * Sn turns on after 3t4 * For a bidirectional switch BS, that needs to be turned off: * If./," is positive: C\I ^ Sx2 turns off immediately 15* Sn turns off after 2t4 O If c is negative: Sn turns off immediately N* Sn turns off after 2t11 C\J From the above, a switch-level state diagram can be developed which represents the switching states of the power devices Sn and 5,2 during the commutation period for the bidirectional switch BS,.

When the voltage commutation, is performed, the methods described may be adjusted to satisfy the below safe switching states. In voltage commutation, the output phase must stay connected to an input phase. The safe switching states in Table 3 ensure that this occurs. The safe switching states are those which won't lead to output open circuit or input short circuit. This means the output phase is connected to one of the input phases (to avoid an open circuit) while input voltage is blocked by at least one device (a switch or an antiparallel diode or a series connection of both). State

Sign /1., s.b +/-Table 3, Sale combirigtoris of Ss,thtei te for ci State Sal a2 Sbl Sb2 Sign Vab 0 0 +1- 2 o 0 +1-+ 3 1 1 1 0 4 1 1 0 1 0 1 1 1 7 0 8 1 0 0 1 The remainder of the description addresses current commutation, but the methods are equally applicable to voltage commutation.

The proposed method, where each switch has its own switch-level state diagram, can now be applied to the 2-phase to single-phase converter circuit shown in Figure 3, resulting in the two switch-level state odiagrams in Figure 11. The system state process in Figure 2 has now been replaced by two of the 01/4110 proposed switch-level state process of Figure 10, one for each bidirectional switch in the circuit. The state diagram in Figure 2 and the proposed state diagram in Figure 11 give identical states for all power Ois3/4."** devices at any time but the proposed switch-level state process is modular in design and thus easier to implement and test. Thus the switch control circuits needed to implement this process are modular and simple in comparison to the circuit used in a system level approach. C\i15

After the logic circuit is developed for the switch-level state process shown in Figure 10, it can be applied to the other bi-directional switches in the system.

The advantages of this modular approach becomes more apparent as the complexity (number of switches) increases.

A 3-phase Matrix Converter consists of 9 bidirectional switches in a 3x3 matrix configuration, as shown in Figure 12, and controls them to convert a fixed frequency 3-phase input AC supply directly into a variable frequency AC output voltage The Matrix Converter is typically fed by a voltage source and therefore the input terminals (A, B and C in Figure 12) must not be short circuited. This means only one switch in each output phase leg of the converter should be ON at any time. Furthermore, the output phase load current, which is typically inductive, should not be opened. As with a 2-phase matrix converter, a 4-step commutation process has to be employed on each leg of the converter to safely control commutation of the incoming and outgoing switches in the leg The state diagram that is used to represent the switching states of the power devices in each leg of the converter is shown in Figure 13, in this case for the output phase leg a. Since there are three bidirectional switches (six power devices) in each leg of the converter, each state is represented by a 6 digit binary number to indicate the state of each device. There are 3 stable states where one of the bidirectional switches of the converter leg is ON and two other switches are OFF, indicated by the green states in Figure 13. The 4-step commutation which occurs between the ON switch (outgoing switch) and one of the OFF switches Oncoming switch) is similar to a two-phase to single-phase matrix converter. However, the number of states (shown in Figure 13) to implement 4-step commutation for each leg of 3-phase Matrix Converter is almost three times the number of states needed for a 2-phase Matrix Converter (shown in Figure 2). Furthermore, each state is represented by a six digit binary number compared with a four digit binary number that was required in a two phase matrix converter. Therefore, the logic circuit to implement 4-step commutation for a 3-phase matrix converter is considerably more complex than the logic circuit required for a 2-phase matrix converter.

It should be further noted that three of the state diagrams shown in Figure 13 would be required to implement the 4-step commutation for the 3-phase to 3-phase Matrix Converter -one for each output leg.

The proposed switch-level implementation of 4-step commutation can now be applied to a 3-phase Matrix Converter. There is no need to create a new state diagram and logic circuit design for the implementation.

0\120 Instead, the same state diagram as Figure 10 is replicated across the additional switches in the leg. Since there are three switches in each leg of the 3-phase matrix converter, three switch-level state diagrams are required. 4-step commutation, based on the proposed method, is shown for output phase leg 'a' of a 3-N. phase matrix converter in Figure 14. C\J

The states of the power devices generated by the proposed method are identical to the conventional method, however the proposed technique has a modular approach and uses the same identical switch-level state diagram for any additional switches added to the converter. This makes the design scalable and hence easier to expand reducing the time for both development and design validation.

The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

Throughout this specification, including the claims which follow, unless the context requires otherwise, the word "comprise" and "include", and variations such as "comprises", "comprising", and "including" will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

It must be noted that, as used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent "about," it will be understood that the particular value forms another embodiment. The term "about" in relation to a C \I 20 numerical value is optional and means for example +/-10%. C\I

Claims

Claims: 1) A switch control circuit for controlling one bidirectional switch, the switch control circuit configured to: (i) when an OFF signal is received, turn off a positive direction after a time t+OFF, and turn off a negative direction of the bidirectional switch after a time t-OFF, and (ii) when an ON signal is received, turn on the positive direction of the bidirectional switch after a time hoN, and turn on the negative direction of the bidirectional switch after a time tON.
2) A switch control circuit according to claim 1, wherein either (a) t -OFF < t+ON < t+OFF < t-ON, or (b) t +OFF <t-ON < t-o < t+ON or (c) tON < t-OFF < t+ON < t+OFF, or (d) t+ON < t+OFF < tON < t-OFF.
3) A switch control circuit according to claim 1, wherein when the output current is in the positive direction, then (a) t-OFF < t+ON < t+OFF < tON, and when the current in the switch is in the negative direction, then (b) t+OFF < t-ON < t-OFF < t+ON.
4) A switch control circuit according to any preceding claim, wherein the shortest of t_OFF and t +OFF iS seconds.
5) A switch control circuit according to any preceding claim, wherein a dead time delay time, td, is predefined and a minimum difference between each of the times t +OFF, tON, t-OFF, and t+ON is td.
6) A switch control circuit according to claim 5, wherein either (a) t t+ --OFF = OS, t+ON =td, OFF =2td, and toN = 3td, or (b) t +OFF = OS, tON =td, t OFF =ad, and t+ON = 3t1.
7) A switch control circuit according to any preceding claim, wherein the bidirectional switch comprises a positive unidirectional switch and a negative unidirectional switch connected in series.
8) A multi-switch control circuit comprising: a plurality of switch control circuits according to any of the preceding claims, each switch control circuit for controlling a respective bidirectional switch; and a master control unit for selectively sending the ON and OFF signals to the switch control circuits.
9) A multi-switch control circuit according to claim 8, wherein when power is to be commutated from a first switch to a second switch, the master control unit is configured to send an OFF signal to a first one of the plurality of the switch control circuits which controls the first switch and an ON signal to a second one of the switch control circuits which controls the second switch.
10) A multi-switch control circuit according to claim 9, wherein the input voltage of the second switch is V2, and the input voltage of the first switch is Vi, and wherein when V2-VI>0, then (c) .-ON < t.-OFF < t+ON < t+OFF and when V2-Vi<O, then (d) t+ON < t+OFF < t-ON < t-OFF.
11) A multi-switch control circuit for a bidirectional switch according to claim 8,9 or 10, wherein the ON and OFF signals are sent simultaneously.
12) A multi-switch control circuit according to any of claims 8 to 11, wherein the master control unit is provided by software and/or programmable logic.
13) A multi-switch system comprising a multi-switch control circuit according to any of claims 8 to 12, and a plurality of bidirectional switches, each of the switch control circuits being configured to control a respective one of the bidirectional switches.
14) An AC to AC power converter comprising a multi-switch system according to claim 13.
15) A three-phase matrix converter comprising a multi-switch system according to claim 13, wherein there are nine bidirectional switches and nine switch control circuits, each control circuit being configured to control a respective one of the switches 16) A method of controlling a bidirectional switch by a switch control circuit comprising the steps of: (i) when an OFF signal is received by the switch control circuit, turning off the positive direction of the bidirectional switch after a time OFF, and turning off the negative direction of the bidirectional switch -after a time t OFF, * and (ii) when an ON signal is received by the switch control circuit, turning on a positive direction of the bidirectional switch after a time t+ON, and turning on a negative direction of the bidirectional switch after a time t-oN.17) A method according to claim 16, wherein either (a) t OFF < t+ON < t+OFF < tON or (b) t+OFF <t-ON < t OFF < t+ON, or (c) t ON +td t-OFF +td t+ON +td t+OFF, or (d) t +ON +td t+OFF +td t ON +td t-OFF.18) A method according to claim 16, wherein when the output current is in the positive direction, then (a) t-OFF < t+ON < t+OFF < tON, and when the current in the switch is in the negative direction, then (b) t+OFF < tON < t-OFF < t+ON.19) A method according to any one of claims 16 to 18, wherein the shortest of t-OFF and t+OFF iS 0 seconds.20) A method according to any one of claims 16 to 19, wherein a dead time delay time, td, is predefined and a minimum difference between each of the times f +OFF, tON, t OFF, and t+ON is td.21) A method according to claim 20, wherein either (a) tOFF = Os, t+ON =td, t+OFF =21d, and toN = 3td, or (b) hoFF = Os, t-ON =td, t-OFF =2td, and hoN = 3td, or (C) t-ON = Os, t-OFF =td, t+ON =2td, t+OFF = 3td or (d) LON = Os, t+OFF =td, t-ON = 2td, t-OFF = 3td.22) A method according to any one of claims 16 to 21, wherein the bidirectional switch comprises a positive unidirectional switch and a negative unidirectional switch connected in series.23) A method of commuting power from a first bidirectional switch to a second bidirectional switch in multi-bidirectional switch system comprising the steps of: sending, from a master control unit, an OFF signal to a first switch control circuit controlling the first bidirectional switch, and sending an ON signal to a second switch control circuit controlling the second bidirectional switch; and performing, by the first switch control circuit, the method of claim 16; and performing, by the second switch control circuit, the method of claim 16.24) A method according to claim 23, wherein the input voltage of the second switch is V2, and the input voltage of the first switch is V1, wherein when V2-Vi>0, then (c) t.-ON < t OFF < t+ON < t+OFF and when V2-V1<0, then (d) .+ON < t+OFF < tON < t-OFF.25) A method according to claim 23 or 24, wherein the ON and OFF signals are sent simultaneously.26) A method according to any of claims 23 to 25, wherein the master control unit is provided by software and/or programmable logic. 20 27) A method of controlling an AC to AC power converter comprising: commutating power from a first bidirectional switch to a second bidirectional switch in a multi-bidirectional switch system using a method according to any of claims 23 to 26.28) ) A method of controlling an AC to AC power converter according to claim 27 wherein the AC to AC power converter is a three-phase power converter.