AC-AC converter

Abstract

An AC-AC converter circuit 100 comprises a serially connected first set 102 and second set 112 of power semiconductor devices 104, 108, 114, 118. Each set includes a pair of series connected power semiconductor devices, each device comprising a control node 106, 110, 116, 120 for controlling its state. The converter is configured to accept as an input an AC input signal applied across the first and second sets. The converter is controllable by way of control signals applied to the respective control nodes to generate an AC output signal between a first node 128 between the power semiconductor devices of the first set and a second node 130 between the power semiconductor devices of the second set. The converter may be connected between a mains power supply and an induction coil in a device. The device may include a heating element adjacent to the induction coil, where the converter provides a high frequency signal to the induction coil to heat the heating element. The device may be a hair straightener.

Description

BACKGROUND Power converters of various types exist, including those that accept AC as an input and output AC. The output AC can be of a different frequency and / or voltage compared to the input AC, and / or can be galvanically isolated from the input AC. The output AC can also be converted to DC in a further circuit. SUMMARY According to an aspect of the invention there is provided a converter circuit for accepting an AC input signa! and generating an ,AC output signal, the converter circuit comprising: a first set of power semiconductor devices comprising: a first power semiconductor device having a first control node for receiving a first control signal for controlling a state of the first power semiconductor device; and a second power semiconductor device connected in series with the first power semiconductor device, the second power semiconductor device having a second control node for receiving a second control signal for control ling a state of the second power semiconductor device; a second set of power semiconductor devices connected in series with the first set of power semiconductor devices, the second set of power semiconductor devices comprising: a third power semiconductor device having a third control node for receiving a third control signal for controlling a state of the third power semiconductor device; and a fourth power semiconductor device connected in series with the third power semiconductor device, the fourth power semiconductor device having a fourth control node for receiving a fourth control signal for controlling a state of the fourth power semiconductor device; wherein the first powder semiconductor device, the second power semiconductor device, the third power semiconductor device, and the fourth power semiconductor device are controllable by way of the respective first, second, third and fourth control nodes such that the converter circuit is operable to: accept as an input an AC input signal applied between: a terminal of the first power semiconductor device on a side of the first power semiconductor device electrically opposite to the second power semiconductor device; and a terminal of the fourth power semiconductor device on a side of the fourth power semiconductor device electrically opposite to the third power semiconductor device; and generate an AC output signal between: a first node disposed between the first power semiconductor device and the second power semiconductor device; and a second node disposed between the third power semiconductor device and the fourth power semiconductor device. This may provide a simple and / or cost effective circuit for AC-AC conversion, and may also avoid the need for one or more separate isolated power supplies. The converter circuit may include a control circuit connected to the first, second, third, and fourth control nodes, wherein the control circuit is configured to output the first control signal, the second control signal, the third control signal, and the fourth control signal, for controlling the respective states of the first power semiconductor device, second power semiconductor device, third power semiconductor device, and fourth power semiconductor device. The converter circuit may comprise’ a first half-bridge circuit comprising: drive circuitry for amplifying the first control signal and the second control signal; and the first power semiconductor device and the second power semiconductor device, wherein the first and second control nodes are connected to receive the respective amplified first and second control signals; and a second half-bridge circuit comprising: drive circuitry for amplifying the third control signal and the fourth control signal; and the third power semiconductor device and the fourth power semiconductor device, wherein the third and fourth control nodes are connected to receive the respective amplified third and fourth control signals. The use of half-bridge circuits may simplify the design, and / or reduce the cost, of the AC-AC converter. The half-bridge circuits may take the form of one or more integrated half-bridge circuits. Optionally, either or both of the half-bridge circuits can be packaged together. Any necessary drive circuitry' can optionally be integrated in the same package(s) or provided separately. Alternatively, any or all of the power semiconductor devices and / or drive circuitry can comprise discrete components. The converter circuit may be configured to alternate between: a first, mode, in which: the first power semiconductor device and the second power semiconductor device are alternately conducting relative to each other; and when the first power semiconductor device is conducting, the fourth power semiconductor device is conducting, and when the second power semiconductor device is conducting, the third power semiconductor device is conducting; and a second mode, in which: the third power semiconductor device and the fourth power semiconductor device are alternately conducting relative to each other; and when the third power semiconductor device is conducting, the second power semiconductor device is conducting, and when the fourth power semiconductor device is conducting, the first power semiconductor device is conducting. The converter circuit may be configured such that: in the first mode, the third power semiconductor device and the fourth power semiconductor device are conducting; and in the second mode, the first power semiconductor device and the second power semiconductor device are conducting. Alternating between the first and second modes, and controlling which power semiconductor devices are conducting in each mode, may allow for a compact AC-AC converter circuit. The converter circuit may be configured to operate in: the first mode when a voltage at the first input node is positive; and the second mode when a voltage at the second input node is positive. The converter circuit may comprise an output network connected to the first output node and the second output node, the output network being configured to block DC between the first output node and the second output node. The output network may comprise a serially-connected capacitance configured to block the DC. The output network may comprise a resonant circuit configured such that an output current flows serially through the resonant circuit in: a first direction when the converter circuit is operating in the first mode; and an opposite direction when the converter circuit is operating in the second mode. It will be appreciated that there may be a brief period during transition between the first and second modes, and vice versa, where the output current is flowing in the opposite direction to that described, but in general, the current flow is substantially as defined in the preceding paragraph. The use of a resonant circuit may reduce switching power losses, and enable generation of a sinusoidal output current. The converter circuit may comprise a transformer, a primary' winding of which is connected between the first, node and the second node. The converter circuit may comprise a rectification circuit connected to be driven by a secondary winding of the transformer. The converter circuit may comprise an active rectifier and LC circuit, connected to be driven by the secondary' winding of the transformer. Each of the first power semiconductor device, second power semiconductor device, third power semiconductor device, and fourth power semiconductor device may comprise any suitable type of transistor, including, but not limited to, gated transistors, including field effect transistors (FETs) (including GaN FETS and MOSFETS) and insulated gate bipolar junction transistors (IGBTs). According to a further aspect of the invention, there is provided a device comprising: the converter circuit of a preceding aspect., connectable to a mains power supply, and an induction coi I connected between the first output node and the second output node. The device may comprise a heating element disposed adjacent to the induction coil, the device being configured such that, when the converter circuit is connected to a mains power supply and operative, it converts the mains power supply to a higher frequency induction drive current, and supplies the induction drive current to the induction coil, thereby to cause the induction coil to inductively heat the heating element. The induction coil may be an inductive charging coil for inductive charging of a further device. According to a further aspect of the invention, there is provided a method of operating a converter circuit according to any one of the preceding claims, the method comprising providing the first control signal, the second control signal, the third control signal, and the fourth control signal to the respective first control node, second control node, third control node, and fourth control node, so as to cause the convertor circuitry to alternate between operating in: a first mode, in which: the first power semiconductor device and the second power semiconductor device are alternately conducting relative to each other; and when the first power semiconductor device is conducting, the fourth power semiconductor device is conducting, and when the second power semiconductor device is conducting, the third power semiconductor device is conducting; and a second mode, in which: the third power semiconductor device and the fourth power semiconductor device are alternately conducting relative to each other; and when the third power semiconductor device is conducting, the second power semiconductor device is conducting, and when the fourth power semiconductor device is conducting, the first power semiconductor device is conducting. For example, in the first mode, the third power semiconductor device and the fourth power semiconductor device can continuously be conducting, and in the second mode, the first power semiconductor device and the second power semiconductor device can continuously be conducting. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic of a circuit, comprising a converter circuit according to an embodiment of the invention; Figure 2 shows a schematic of a resonant circuit and load for use with the converter circuit of Figure 1; Figure 3 shows a schematic of a further resonant circuit and load for use with the converter circuit of Figure 1, Figure 4 shows a schematic of a further circuit, comprising a further converter circuit according to an embodiment of the invention; Figures 5 to 8 are schematics of the converter circuit of Figure 4, in various modes and states of operation; Figure 9 is a schematic of a half-bridge rectifier comprising a set of power semiconductor devices, for use with a power converter according to an embodiment of the invention; Figure 10 is a graph showing various voltages and currents within the circuit of Figure 4, for one period of input AC voltage; Figure 11 is a graph showing various voltages and currents within the circuit of Figure 4 when the input voltage reaches a peak value of the positive half cycle of the AC input voltage; Figure 12 is a graph showing various voltages and currents within the circuit of Figure 4 when the input voltage reaches a peak value of the negative half cycle of the AC input voltage, Figure 13 is a schematic of a further circuit, comprising a converter circuit according to a further embodiment of the invention, Figure 14 is a schematic of a further circuit, comprising a converter circuit according to a further embodiment of the invention, Figure 15 is a schematic of a further circuit, comprising a power supply according to a further embodiment of the invention, Figure 16 is a flowchart showing a method according to an embodiment of the present invention; and Figure 17 is a schematic of a device in the form of a hair straightener incorporating a converter circuit, according to an embodiment of the present invention. DETAILED DESCRIPTION Referring to Figure 1, there is shown a converter circuit 100. As described in more detail below with reference to Figures 4 to 8, the converter circuit 100 is configured for accepting an AC input at a first frequency and generating an AC output at a second frequency different to the first frequency. The converter circuit 100 comprises a. first, set 102 of power semiconductor devices. The power semiconductor devices can take any suitable form, such as a field effect transistor (FET) (such as a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) or a Gallium Nitride Field Effect Transistor (GaN FET)), an insulated gate bipolar junction transistor (IGBT), or any other suitable type of transistor(s). In Figure 1, the power semiconductor devices are illustrated as circled switches with control nodes. Examples of the use of specific power semiconductor devices are shown in later embodiments. The first set 102 includes a first power semiconductor device 104 having a first control node 106 for receiving a first control signal for controlling a state of the first power semiconductor device 104. The first set 102 also includes a second power semiconductor device 108 connected in series with the first power semiconductor device 104, the second power semiconductor device 108 having a second control node 110 for receiving a second control signal for controlling a state of the second power semiconductor device 108. The converter circuit 100 also comprises a second set 112 of power semiconductor devices connected in series with the first set 102 of power semiconductor devices. The second set 112 of power semiconductor devices includes a third power semiconductor device 114 having a third control node 116 for receiving a third control signal for controlling a state of the third power semiconductor device 114. The second set 112 also includes a fourth power semiconductor device 118 connected in series with the third power semiconductor device 114, the fourth power semiconductor device 118 having a fourth control node 120 for receiving a fourth control signal for controlling a state of the fourth power semiconductor device 118. In use, the respective first, second, third and fourth control nodes 106, 110, 116, 120 are connected to receive corresponding control signals from a control circuit 126. The control circuit 126 can take the form of a microcontroller or other processing device or circuit capable of outputting the required control signals, but other control circuits will suggest themselves to the skilled person. In use, the converter circuit is connected to accept as an input an AC input signal applied between: a terminal 122 (in this case, the drain) of the first power semiconductor device 104 on a side of the first power semiconductor device 104 electrically opposite to the second power semiconductor device 108; and a terminal 124 (in this case, the drain) of the fourth power semiconductor device 118 on a side of the fourth power semiconductor device 118 electrically opposite to the third power semiconductor device 114. When appropriately controlled by the control circuit 126, the converter circuit 100 generates an AC signal between: a first node 128 disposed between the first power semiconductor device 104 and the second power semiconductor device 108; and a second node 130 disposed between the third power semiconductor device 114 and the fourth power semiconductor device 118. An output circuit can be connected between the first node 128 and the second node 130. The characteristics of the AC signal output between the first node 128 and the second node 130 may depend on the electrical characteristics of any such output circuit when driven by the first and second nodes 128, 130. For example, and as shown in Figure I, the output circuit can be an output network in the form of a resonant circuit 132, which in turn can be connected to a load 134. It will be appreciated that, depending upon the implementation, the load 134 can be connected directly across the first node 128 and the second node 130. Optionally, the load 134 itself can include one or more reactive components such that the load 134 itself has resonant characteristics. In some embodiments, reactance can be at least partly provided by parasitic or incidental inductance(s) and / or capacitance(s). Any output network, such as the load and / or any resonant circuit, can be configured to impede or block transmission of DC current between the first and second nodes 128, 130. For example, the output network can comprise a serially-connected capacitance configured to block DC. Examples of such capacitances are described below with reference to Figures 2 and 3. Figure 2 shows one example of a resonant circuit in the form of an impedance transformation network 132. The impedance transformation network 132 comprises a first inductor 136 connected to the first node 128. A first capacitor 142 is connected between the other side of the first inductor 136 and the second node 130. The impedance transformation network 132 also includes a second capacitor 138 connected in series with the first inductor 136 and a first output node 140 of the impedance transformation network 132, for blocking DC. In Figure 2, the load 134 takes the form of an inductive heating element drive circuit driving an inductive heating element, an equivalent circuit for which is represented in Figure 2 by a resistive component 144 in series with an inductive component 146. The values of the first inductor 136 and the first capacitor 142 can be chosen such that the current through the load wall be constant, irrespective of the values of the load impedance of inductive component 146 and resistive component 144. In Figure 3, the load 134 is represented by a simple resistance 166, and the resonant circuit 132 is a series inductor 148 and capacitor 150. While the circuits of Figures 2 and 3 are shown with individual components, it will be appreciated that these can be considered simplified representations of more complex circuitry. Also, additional inductances, capacitances, and resistances, whether parasitic or related to isolation and / or noise reduction, are not shown for clarity. Turning to Figure 4, there is shown an embodiment of the converter circuit 100, a driving circuit 152 and the resonant circuit 132 and load 134 of Figure 3. The driving circuit 152 includes an AC power source 154, which can be, for example, a mains AC power supply. An inductor 156 is connected in series with the AC power source 154, and a capacitor 158 is connected across the power supply 154, to smooth input current ripple and reduce the amount of high frequency noise passing back into the AC power source. In the embodiment of Figure 4, the first, second, third, and fourth pov / er semiconductor devices 104, 108, 114, 118 take the form of first, second, third, and fourth n-channel MOSFET devices 104, 108, 114, 118. The first and second MOSFET devices 104, 108 are connected in the same orientation as each other, with their drains at the upper side as shown in Figure 4. The third and fourth MOSFET devices 114, 118 are connected in the same orientation as each other (and with opposite orientation to the first and second MOSFET devices 104, 108), with their drains at the lower side as shown in Figure 4. Figure 9 show's an example of one implementation of the first set 102 of MOSFET power semiconductor devices. The first set 102 comprises a first half-bridge integrated circuit 102, comprising drive circuitry configured to receive control signals received from the control circuit 126. The drive circuitry comprises a first amplifying circuit 176 for amplifying the first control signal and providing it to the first control node 106, and a second amplifying circuit 178 for amplifying the second control signal and providing it to the second control node 110. The first and second amplifying circuits 176, 178 are powered by a DC power supply Vcc. The first half-bridge integrated circuit 102 can be provided as a single integrated circuit package, or other packages and / or combinations of discrete components) can be used to suit the particular implementation. A second half-bridge integrated circuit (not shown) can similarly comprise the third and fourth MOSFETs 114, 118, and similar drive circuitry (not shown) for amplifying the third control signal and the fourth control signal, the third and fourth control nodes 116, 120 being connected to receive the respective amplified third and fourth control signals. In other embodiments, the power semiconductor devices (and any associated gate drive circuitry) can be packaged in one or more integrated circuit packages. Each package can contain one or more power semiconductor devices, and optionally an associated drive circuit for the or each power semiconductor device. For example, a half bridge integrated circuit can be provided in one package, and gate drive circuitry can be provided in one or more other packages. Alternatively, at least one of the power semiconductors and any associated gate drive circuury can be packaged together. In the embodiment of Figure 15, the control circuit 126 can comprise a digital signal controller. The skilled person will appreciate that other integrated circuit packages, or indeed different discrete or integrated circuits, can be used to suit the particular implementation. Figures 5 to 9 show a sequence of states of the converter circuit 100, which are controlled by way of control signals from the control circuit 126 controlling the state of the first, second, third, and fourth MOSFETs 104, 108, 114, 118. Figures 5 and 6 show two states of a first mode of the converter circuit 100. In both states (i.e., in Figures 5 and 6), the third and fourth MOSFETs 114, 118 are ON, meaning that they conduct current between source and drain with minimal resistance. In this first mode, the first and second MOSFETs 104, 108 are controlled such that they are alternately conducting relative to each other. That is, while the first MOSFET 104 is controlled to be ON, the second MOSFET 108 is controlled to be off, and vice versa. Figures 7 and 8 show two states of a second mode of the converter circuit 100. In both states (i.e., in Figures 7 and 8), the first and second MOSFETs 104, 108 are ON, meaning that they conduct current between source and drain with minimal resistance. In this second mode, the third and fourth MOSFETs 114, 118 are controlled such that they are alternately conducting relative to each other. That is, while the third MOSFET 114 is controlled to be ON, the fourth MOSFET 118 is controlled to be off, and vice versa. Operation of the converter circuit 100 of Figure 4 will now be described in detail with reference to Figures 5 to 8. Control circuit 126 controls the converter circuit to be in: the first mode when the voltage at the terminal 122 (i.e., one AC input to the converter circuit 100) is higher than the voltage at the terminal 124 (i.e., the other AC input to the converter circuit 100), as shown in Figures 5 and 6; and the second mode when the voltage at the terminal 122 is lower than the voltage at the terminal 124, as shown in Figures 7 and 8. The voltage at the terminal 122 and / or the terminal 124 can be determined by the control circuit 126 or by any other suitable mechanism. The voltage can be determined directly by sampling the voltage at either of both of the terminals 122, 124, or indirectly by sampling a voltage or current at some other suitable point, or by receiving a signal indicative of the voltage from another circuit (not shown). In Figure 5, the voltage at the terminal 122 is higher than the voltage at the terminal 124. The first MOSFET 104, third MOSFET 114 and fourth MOSFET 118 are ON, meaning that they can conduct current between source and drain with minimal resistance. The second MOSFET 108 is OFF, meaning it substantially blocks current flow between source and drain. As shown by arrow 164, current flows from the AC power source, through the inductor 156, to the terminal 122. Because the first MOSFET 104 is ON and the second MOSFET 108 is OFF, current passing through the first MOSFET 104 is directed through the first node 128 into the resonant circuit 132, through the inductor 148, then into the load 134 and through the resistance 166. Current returns from the load 134 into the resonant circuit 132, where it charges the capacitor 150. As the potential difference across the capacitor 150 rises, current flows out the other side into the second node 130, through the fourth MOSFET 118, and back tow?ards the AC power source 154. The states of the first MOSFET 104 and the second MOSFET 108 are then alternated, as shown in Figure 6. That is, the first MOSFET 104 is switched from ON to OFF, and the second MOSFET 108 is switched from OFF to ON. As shown by arrow 168, charge stored in the capacitor 150 causes a current to flow from the capacitor 150 back through the resistance 166, then through the inductor 148 into the first node 128. The current then passes through the second MOSFET 108 and third MOSFET 114, the second node 130, and returns to the capacitor 150. The states of the first MOSFET 104 and the second MOSFET 108 are then alternated again, returning the state of the converter circuit 100 to that shown in Figure 5. The state of the converter circuit 100 is repeatedly alternated between the states shown in Figures 5 and 6 while the voltage at the terminal 122 remains higher than the voltage at the terminal 124. Once the voltage at the terminal 122 becomes lower than the voltage at the terminal 124, the converter circuit switches from the first mode to the second mode. In Figure 7, the voltage at the terminal 122 is lower than the voltage at the terminal 124. The first MOSFET 104, second MOSFET 108, and fourth MOSFET 118 are ON, meaning that they can conduct current between source and drain with minimal resistance. The third MOSFET 114 is OFF, meaning it substantially blocks current flow’ between source and drain. As shown by arrow 170, current flows from the AC power source to the terminal 124. Because the fourth MOSFET 118 is ON and the third MOSFET 114 is OFF, current is directed through the second node 130 into the resonant circuit 132, into the capacitor 150. As the potential difference across the capacitor 150 rises, current flows out the other side into the load 134 and through the resistance 166. Current returns from the load 134 into the resonant circuit 132, where it passes through the inductor 148. Although the second MOSFET 108 is ON, current cannot pass through it because the third MOSFET 114 is OFF. Accordingly, current must pass through the first MOSFET 104, and back towards the AC power source 154 through the terminal 122 and the inductor 156. The states of the third MOSFET 114 and the fourth MOSFET 118 are then alternated, as shown in Figure 8. That is, the third MOSFET 114 is switched from OFF to ON, and the fourth MOSFET 118 is switched from ON to OFF. As shown by arrow 172, charge stored in the capacitor 150 causes a current to flow from the capacitor 150 back to the second node 130. Because the fourth MOSFET 118 is now OFF, the current reaching the second node 130 is directed through the third MOSFET 114 and the second MOSFET 108, then through the inductor 148 and the resistance 166, before returning to the capacitor 150. The state of the third MOSFET 114 and the fourth MOSFET 118 are then alternated again, returning the state of the converter circuit 100 to that shown in Figure 7. The state of the converter circuit 100 is repeatedly alternated between the states shown in Figures 7 and 8 while the voltage at the terminal 122 remains lower than the voltage at the terminal 124. Once the voltage at the terminal 122 again becomes higher than the voltage at the terminal 124, the converter circuit switches from the second mode back to the first mode, and the process continues as described above in relation to Figures 5 and 6. It may take a brief period of time for current flow to adjust when changing between the first and second modes. However, it will be appreciated that current flows serially through the resonant circuit in: a first direction for a majority of the time when the converter circuit is operating in the first mode; and an opposite direction for a majority of the time when the converter circuit is operating in the second mode. In the embodiment of Figures 4 to 8, the rate at which the converter circuit 100 alternates between the states shown in Figures 5 and 6, and between the states shown in Figures 7 and 8, is considerably higher than the frequency of the AC power source. For example, if the AC power source is a mains power supply at 50 or 60 Hz, the converter circuit may be controlled by the control circuit 126 to alternate between the states of Figures 5 and 6, and the states of Figures 7 and 8, at at least several kHz. For example, the alternating rate may be at least 10 kHz, 100 kHz, or even 1 MHz or more, depending upon the application. The values of the resonant circuit components are selected to accord with the desired output frequency of the converter circuit. For example, the switching frequency of the transistors can be slightly faster than the resonant frequency of the resonant circuit to achieve optimal operation, i.e., low switching power loss, and sinusoidal output waveform. However, the switching frequency can be faster or slower than the resonant frequency in other implementations. Figure 10 is a set of graphs showing, from top to bottom: * gate control signals QI, Q2, Q3, Q4 supplied to the gates 106, 110, 116, 120 of the respective first MOSFET 104, second MOSFET 108, third MOSFET 114, and fourth MOSFET 118, as described in detail above; ® the voltage Vab across the first node 128 and second node 130; * the input current I / w (i.e., the current from the input voltage source); and ® the load current Icoii (i.e,, the current that flows through resistance 166), The graphs of Figure 10 show these values across one period of input voltage (i.e., 2 milliseconds of 50Hz input AC voltage source), for the circuit of Figure 4 operating as described above with reference to Figures 5 to 8. Arrow 222 (positive input voltage) indicates the first mode described above with reference to Figures 5 and 6, and arrow 224 indicates the second mode described above with reference to Figures 7 and 8. Figure 11 is a graph showing zoomedrin detail of the second io fourth graphs of Figure 10 around the point when the input voltage reaches a positive peak 226 (i.e., the first quarter of one period). Figure 12 is a graph showing zoom edrin detail of the second to fourth graphs of Figure 10 around the point when the input voltage reaches a negative peak 228 (i.e., the third quarter of one period). By switching the four transistors as described above, square voltage waveforms are generated across 128 and 130, as shown in Figures 11 and 12. When this square waveform is applied to a resonant load, as described above for example, it will generate AC current as shown in the bottom plots. It should be noted that Figure 10 is simplified representation of the various signals, whereas Figures 11 and 12 show modelled values. For clarity, Figure 10 shows a lower frequency of gate signal alternation than is shown in Figures 11 and 12. Figure 13 shows an isolated AC-DC converter circuit 174. The AC-DC converter circuit 174 includes the converter circuit 100, a transformer 202, and a rectification circuit 204. The output of the resonant circuit 132 is connected across a primary winding 206 of the transformer 202. An input of the rectification circuit 204 is connected across a secondary winding 208 The rectification circuit 204 includes a bridge rectifier comprising first, second, third, and fourth diodes 210, 212, 214, 216. A filter capacitor 218 is connected across the output of the bridge rectifier to smooth its output. The bridge rectifier and the filter capacitor operate in a known manner, converting the high-frequency AC signal from the secondary winding 208 into DC for supply to the load 134. Figure 14 shows bidirectional isolated AC-AC / DC converter circuit 175. The AC-AC / DC converter circuit 175 includes the converter circuit 100 and the transformer 202 of Figure 13. However, instead of the rectification circuit 204, the bidirectional isolated AC-AC / DC converter circuit 175 includes a further converter circuit 220. The further converter circuit 220 shares many elements with the converter circuit 100 described above, and like elements are indicated with the same reference signs in both the converter circuits 100, 220. The AC-AC / DC converter circuit 175 can be used as a solid state transformer or a bidirectional charger, for example. The AC-AC / DC converter circuit 175 is configured to accept AC at the input to the converter circuit 100, and to output AC across the load 134. In at least some implementations, the AC-AC / DC converter circuit 175 is configured to output AC at the same frequency as the AC provided at the input. For example, if the AC power supply source 154 is a 50 or 60 Hz mains power supply, then the output across the load 134 is also at 50 or 60 Hz. The output voltage is determined by the winding ratio between the primary and secondary windings 206, 208, as is well understood by the skilled person. The AC-AC / DC converter circuit 175 can be operated in reverse, where the load 134 is replaced by an AC powder source, and the AC power source 154 is replaced by a load. In at least some embodiments, the use of a converter circuit, such as converter circuit 100 or 220, may enable the use of a simplified power supply for one or more other components of the converter circuit and supporting circuitry' / For example. Figure 15 shows an AC-AC converter 300 comprising the converter circuit 100, the first set 102 of power semiconductors devices, the second set 112 of power semiconductor devices, and the control circuit 126. The converter circuit 100 is as described in Figure 4, bui similar principles may be applied to other converter circuits falling within the scope of the claims. The control circuit 126 comprises a microcontroller 126 configured to output control signals to control the first, second, third, and fourth MOSFETs 104, 108, 114, 118 by way of the corresponding first, second, third, and fourth control nodes 106, 110, 1 16, 120. The microcontroller 126 requires a 5 V DC power supply. The first set 102 and the second set 112 both incorporate drive circuitry as described above in relation to Figure 9, noting that the reference signs used for the first set 102 in Figure 9 are repeated for the corresponding components of the second set 112. The amplifying circuits 176, 178 require a 15 V DC power supply. The AC-AC converter 300 comprises a DC power supply 180. The DC power supply comprises a rectifier circuit 182 comprising first and second diodes 184, 186 connected to the respective terminals of the AC power supply 154. The rectified outputs of the first and second diodes 184, 186 are fed via a first smoothing capacitor 188 to a first buck converter 190. The output of the first buck converter 190 is fed via a second smoothing capacitor 192 to a second buck converter 194. The design of buck converters is well understood, and so the first and second buck converters 190, 194 are not described in detail. The output of the first buck converter 190 is 15 V DC, which is connected to power the first and second amplifying circuits 176, 178. The output of the second buck converter J 94 is 5 V DC, which is connected via a third smoothing capacitor 196 to power the microcontroller 126. The first buck converter 190, the second buck converter 194, and the first, second, and third smoothing capacitors 188, 192, 196 are all connected to a common ground 198. The common ground 198 is also connected between the first set 102 and the second set 112. The reference level of each transistor’s gate voltage is the voltage at the transistor’s source. Prior art AC-AC converters can have transistors arranged such that their sources have different voltage potentials, requiring isolated power supplies to provide different reference voltages for gate drive circuits. The arrangements described herein need not use isolated power supplies, which can reduce complexity and / or cost. Different implementations of the converter circuit can be incorporated into devices, according to various embodiments of the invention. For example, such a device can include a converter circuit that is connectable to a mains power supply, and an induction coil connected between the first output node and the second output node. The induction coil can be for causing inductive heating of an inductively coupled heating element disposed adjacent to the induction coil. The device is configured such that, when the converter circuit is connected to a mains power supply and operating, it. converts the mains power supply to a higher frequency induction drive signal and supplies it to the induction coil, thereby to cause the induction coil to inductively heat the heating element. Such device may be implemented on any air heating devices such as air heater or hair dryer. In another embodiment, the device may be implemented in hair straightener as shown and illustrated in figure 17. Figure 17 shows a schematic view of a device incorporating a converter circuit, in the form of a hair straightener 500. The hair straightener 500 comprises a handle 572. A first arm 574 and a second arm 576 are connected to the handle 572 at a hinge 578, such that the arms can pivot away from each other. The first arm 574 includes a first hair contact plate 522 and the second arm 576 includes a second hair contact plate 524. The first plate 522 and the second plate 524 are formed from a heat-conductive material, and may include a low-friction coating. A spring (not shown) biases the arms away from each other, such that a user can position a tress of hair (not shown) between the first plate 522 and the second plate 524, and then squeeze the first and second arms 574 and 576 together to capture the tress of hair between the first plate 522 and the second plate 524. The tress of hair is then drawn through the hair straightener 500 to style the hair. The hair straightener 500 comprises a first inductive coil 502 and a second inductive coil 504. The first inductive coil 502 and the second inductive coil 504 are connected to be driven by a drive circuit 520 under the control of a controller 530. The drive circuit 520 includes a converter circuit, such as the converter circuit 300 of Figure 15, driving the impedance matching circuit 132 of Figure 2. The controller 530 comprises a control circuit, such as the control circuit 126 of Figure 15, for sending control signals to the control nodes of the first, second, third, and fourth power semiconductor devices 104, 108, 114, 118 via amplifying circuits 176 and 178. Mains power is supplied to hair straightener 500 by way of a power cord 514 that terminates at a mains plug 516, which is configured to be plugged into a mains voltage socket. The mains power is provided as an input to the drive circuit 520. The controller 126 and the amplifying circuits 176, 178 are powered by DC power supplies 190, 194, which are in turn powered by the mains, for example as shown in Figure 15. The first inductive coil 502 is configured and arranged to inductively heat the first plate 522, and the second inductive coil 504 is configured and arranged to inductively heat the second plate 524, while the hair straightener 500 is in operation. The load 134 of Figure 4 is an equivalent circuit for the first and second inductive coils 502, 504 and the respective first and second plates 522, 524. Any other suitable arrangement or configuration of the inductive coils and heating elements may be used. For example, each heating element may be inductively heated by more than one indicative coil. In other implementations having an induction coil, the induction coil can be configured to allow inductive charging of a further device. For example, the converter circuit can be a mobile phone (i,e., cellphone) charger for inductively charging a mobile phone. Turning to Figure 16, there is shown a method 400 of operating a converter circuit. The converter circuit can, for example, be in accordance with any converter circuit described or claimed herein. The method comprises providing the first control signal, the second control signal, the third control signal, and the fourth control signal to the respective first control node, second control node, third control node, and fourth control node, so as to cause the convertor circuitry to alternate between operating in: a first mode (402), in wdiich: the third power semiconductor device and the fourth power semiconductor device are conducting; and the first power semiconductor device and the second power semiconductor device are alternately conducting relative to each other; and a second mode (404), in which: the first power semiconductor device and the second power semiconductor device are conducting; and the third power semiconductor device and the fourth power semiconductor device are alternately conducting relative to each other. Although the invention has been described with reference to a number of specific embodiments, the skilled person will appreciate that the invention may be implemented in many other forms.

Claims

1. A converter circuit for accepting an AC input signal and generating an AC output signal, the converter circuit comprising:a first set of power semiconductor devices comprising:a first power semiconductor device having a first control node for receiving a first control signal for controlling a state of the first power semiconductor device; anda second power semiconductor device connected in series with the first power semiconductor device, the second power semiconductor device having a second control node for receiving a second control signal for controlling a state of the second power semiconductor device;a second set of power semiconductor devices connected in series with the first set of power semiconductor devices, the second set of power semiconductor devices comprising:a third power semiconductor device having a third control node for receiving a third control signal for controlling a state of the third power semiconductor device; anda fourth power semiconductor device connected in series with the third power semiconductor device, the fourth power semiconductor device having a fourth control node for receiving a fourth control signal for controlling a state of the fourth power semiconductor device;wherein the first power semiconductor device, the second power semiconductor device, the third power semiconductor device, and the fourth power semiconductor device are controllable by way of the respective first, second, third and fourth control nodes such that the converter circuit is operable to:accept as an input an AC input signal applied between:a terminal of the first power semiconductor device on a side of the first power semiconductor device electrically opposite to the second powder semiconductor device; anda terminal of the fourth power semiconductor device on a side of the fourth power semiconductor device electrically opposite to the third power semiconductor device; andgenerate an AC output signal between:a first node disposed between the first power semiconductor device and the second power semiconductor device; anda second node disposed between the third power semiconductor device and the fourth power semiconductor device.

2. The converter circuit of claim 1, further including a control circuit connected to the first, second, third, and fourth control nodes, wherein the control circuit is configured to output the first control signal, the second control signal, the third control signal, and the fourth control signal, for controlling the respective states of the first powder semiconductor device, second power semiconductor device, third power semiconductor device, and fourth power semiconductor device.

3. The converter circuit of claim 1 or claim 2, comprising:a first half-bridge circuit comprising:drive circuitry for amplifying the first control signal and the second control signal; andthe first power semiconductor device and the second power semiconductor device, wherein the first and second control nodes are connected to receive the respective amplified first and second control signals; anda second half-bridge circuit comprising:drive circuitry for amplifying the third control signal and the fourth control signal; andthe third powder semiconductor device and the fourth powder semiconductor device, wherein the third and fourth control nodes are connected to receive the respective amplified third and fourth control signals.

4. The converter circuit of any preceding claim, configured to alternate between:a first mode, in which:the first power semiconductor device and the second power semiconductor device are alternately conducting relative to each other; andwhen the first power semiconductor device is conducting, the fourth power semiconductor device is conducting, and when the second power semiconductor device is conducting, the third power semiconductor device is conducting; and a second mode, in which:the third power semiconductor device and the fourth power semiconductor device are alternately conducting relative to each other; andwhen the third power semiconductor device is conducting, the second power semiconductor device is conducting, and when the fourth power semiconductor device is conducting, the first powder semiconductor device is conducting.

5. The converter circuit of claim 4, configured to operate in:the first mode when a voltage at the first input node is positive; and the second mode when a voltage at the second input node is positive.

6. The converter circuit of any preceding claim, wherein the output network comprises a resonant circuit configured such that an output current flows serially through the resonant circuit in:a first direction when the converter circuit is operating in the first mode, andan opposite direction when the converter circuit is operating in the second mode.

7. The converter circuit of any preceding claim, comprising:control node drive circuitry for amplifying the first, second, third, and fourth control signals; andat least one non-isolated power supply connected for powering the control node drive circuitry.

8. The converter circuit of claim 7, wherein the at least one non-isolated power supply is connected to be powered by the AC input, signal.

9. The converter circuit of any preceding claim, comprising a transformer, a primary' winding of which is connected between the first node and the second node.

10. The converter circuit of claim 9, comprising a rectification circuit connected to a secondary winding of the transformer.

11. The converter circuit of any preceding claim, wherein each of the first power semiconductor device, second power semiconductor device, third power semiconductor device, and fourth power semiconductor device comprises:a field effect transistor (FET); oran insulated gate bipolar junction transistor (IGBT);12. A device comprising:the converter circuit of any preceding claim, connectable to a mains porver supply, andan induction coi I connected between the first output node and the second output node.

13. The device of claim 12, comprising a heating element disposed adjacent to the induction coil, the device being configured such that, when the converter circuit is connected to a mains power supply and operating, it converts the mains power supply to a higher frequency induction drive signal and supplies it to the induction coil, thereby to cause the induction coil to inductively heat the heating element.

14. The device of claim 12, wherein the induction coil is an inductive charging coil for inductive charging of a further device.

15. A method of operating a converter circuit according to any one of the preceding claims, the method comprising providing the first control signal, the second control signal, the third control signal, and the fourth control signal to the respective first control node, second control node, third control node, and fourth control node, so as to cause the converter circuitry' to alternate between operating in:a first mode, in which:the first power semiconductor device and the second power semiconductor device are alternately conducting relative to each other, andwhen the first power semiconductor device is conducting, the fourth power semiconductor device is conducting, and when the second power semiconductor device is conducting, the third power semiconductor device is conducting; and a second mode, in which:5 the third power semiconductor device and the fourth power semiconductordevice are alternately conducting relative to each other; andwhen the third power semiconductor device is conducting, the second power semiconductor device is conducting, and when the fourth power semiconductor device is conducting, the first power semiconductor device is conducting.10

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