Patent Publication Number: US-9888533-B2

Title: Power adaptors

Description:
This application is a continuation application from U.S. Ser. No. 13/066,244, Improvements Relating to Power Adaptors, by David Thomas Summerland, et al., filed Apr. 8, 2011, which was a continuation application of PCT/GB2009/051334, filed Oct. 8, 2009, which claims priority and benefit of UK application 0818411.1, filed Oct. 8, 2008, UK application 0821297.9, filed Nov. 21, 2008, UK application 0822691.2, filed Dec. 12, 2008 and UK application 0909139.8, filed May 28, 2009 which are hereby incorporated by reference for all purposes. 
    
    
     This invention relates to power adaptors, and in particular power adaptors suitable for providing a change in effective voltage between the input and the output of the power adaptor. 
     In domestic applications, the voltage available from the mains supply is typically 120V-240V AC, at frequencies of 50 or 60 Hz. Where a power adaptor is adapted to drive a low voltage load, such as a 10V load, the ratio between the voltage available at the supply and the voltage required to drive the load is between 10 and 20. With such a large ratio, conventional switched-mode power adaptors used to drive low voltage loads become very inefficient because the switching operates at small duty ratios with very short conduction times and current waveforms having high peak to average ratios. 
     It is common therefore in prior art power adaptors to include a magnetic transformer with a turns ratio suitable to create a step down in voltage and a corresponding step up in current. In some conventional power supplies, this magnetic transformer is provided at the input to the power adaptor, such that the entire power adaptor operates at lower voltage. In such cases, the magnetic transformer operates at a supply frequency (50 or 60 Hz) and is relatively bulky and expensive. In other cases, the magnetic transformer is integrated as part of the switching circuit, allowing it to operate at the switching frequency of the electronic components in the power adaptor. Such circuits therefore have the advantage that the magnetic transformer can be made smaller. 
     Nevertheless, all of these prior art arrangements require a magnetic transformer, which is rather bulky and inefficient. 
     There has now been devised an improved power adaptor which overcomes or substantially mitigates the above-mentioned and/or other disadvantages associated with the prior art. 
     According to the invention, there is provided a power adaptor comprising an input for connection to an AC power supply, and a resonant circuit coupled to the input that provides an output suitable for driving a load, wherein the capacitance and inductance of the resonant circuit are selected to provide a pre-determined change in effective voltage, and a corresponding pre-determined change in effective current, between the input and the output of the power adaptor. 
     The power supply according to the invention is advantageous principally because the power adaptor provides a pre-determined change in effective voltage, and a corresponding pre-determined change in effective current, between the input and the output of the power adaptor, without any need for a magnetic transformer. The power adaptor may therefore be adapted to drive a low voltage load from a higher mains AC supply, without the need for a magnetic transformer, and without the need for an electronic switching circuit operating at small duty ratios (short on-times relative to switching periods) and having current waveforms with high peak to average ratios. In particular, the power adaptor may be adapted to connect to a high voltage AC mains supply (eg 110V or 230V AC, at frequencies of 50 Hz or 60 Hz), and provide an output suitable for driving a low voltage solid state light source (eg 10-20V). 
     The capacitance and inductance of the resonant circuit may be selected to provide a pre-determined decrease in effective voltage, and a corresponding pre-determined increase in effective current, between the input and the output of the power adaptor. Alternatively, the capacitance and inductance of the resonant circuit are selected to provide a pre-determined increase in effective voltage, and a corresponding pre-determined decrease in effective current, between the input and the output of the power adaptor. 
     In presently preferred embodiments, the capacitance and inductance of the resonant circuit are selected to provide a pre-determined ratio between the effective voltage at the input of the power adaptor and the effective voltage at the output of the power adaptor, and a pre-determined ratio between the effective current at the input of the power adaptor and the effective current at the output of the power adaptor. 
     The resonant circuit is preferably configured to provide an output having an increased or decreased effective current relative to the effective current through a resonant inductor, such that the output has a correspondingly decreased or increased effective voltage relative to the effective voltage of the power supply. The output preferably has an effective current that is increased or decreased relative to the current through the resonant inductor by a factor of at least two, and most preferably by a factor of at least five. In addition, the effective voltage decrease or increase of the output relative to the AC power supply is preferably by a factor of at least two, and most preferably by a factor of at least five. 
     The pre-determined changes in effective voltage and effective current between the input and the output of the power adaptor are preferably achieved with no assistance from a magnetic transformer. Indeed, the power adaptor may be devoid of any magnetic transformers, other than signal or power supply transformers, which may be present in the power adaptor. 
     The pre-determined changes in effective voltage and effective current between the input and the output of the power adaptor may be achieved with no assistance from a transformer. However, the power adaptor may be provided with a piezoelectric transformer that isolates the output from the input of the power adaptor. In this arrangement, the piezoelectric transformer may provide a further pre-determined change in effective voltage, and a further pre-determined change in effective current, between the input and the output of the power adaptor. 
     Where the power adaptor is provided with a piezoelectric transformer, the piezoelectric transformer may be arranged to provide at least some of the capacitance of the resonant circuit. In one embodiment, the piezoelectric transformer provides all of the capacitance of the resonant circuit. The inclusion of a piezoelectric transformer therefore offers several advantages when incorporated into a power adaptor having a resonant circuit. Hence, according to a further aspect of the invention, there is provided a power adaptor comprising an input for connection to an AC power supply, a resonant circuit coupled to the input that provides an output suitable for driving a load, and a piezoelectric transformer that isolates the output from the input of the power adaptor. 
     The output is preferably suitable for driving a constant current load, such as a solid state light source. The resonant circuit is preferably therefore configured to provide an output having a substantially constant voltage, which is pre-determined for a particular effective voltage of the AC power supply. In particular, the resonant circuit is preferably configured to either boost (ie raise) or buck (ie lower) the actual voltage received at the input of the power adaptor, in order to provide a substantially constant voltage, which is pre-determined for a particular effective voltage of the AC power supply, for the majority of the input AC cycle. 
     This arrangement of the present invention therefore enables the output to have a substantially constant voltage, which is pre-determined for a particular effective voltage of the AC power supply, for a greater proportion of the input AC cycle than that provided by a magnetic transformer. This arrangement therefore enables the power adaptor to have a lower bulk storage capacitance than power adaptors that utilise magnetic transformers to provide a pre-determined change in effective voltage. 
     This arrangement of the present invention is suitable for driving fixed loads. However, the power adaptor may also be adapted to drive variable loads. In particular, the power adaptor may be adapted to turn off the output, when the load is insufficient to be driven by the output provided by the power adaptor, and then turn on the output when the load is sufficient to be driven by the output provided by the power adaptor. 
     In a particularly preferred arrangement for providing an output that is suitable for driving a constant current load, the resonant circuit is an LCL series-parallel resonant circuit. 
     By “LCL series-parallel resonant circuit” is meant a resonant circuit comprising a first inductor and a first capacitor in series, and a parallel load leg including a second inductor. The first inductor and first capacitor are preferably connected in series between two input terminals of the resonant circuit, and the resonant circuit preferably comprises a load leg connected in parallel across the first capacitor, wherein the load leg comprises the second inductor and an output for driving the load, which are connected in series. In particular, the LCL resonant circuit preferably has input terminals and output terminals with a first inductor L 1 , connected from a first input terminal through a common point with second inductor L 2 , to a first output terminal, the second input terminal being directly connected to the second output terminal, and a capacitor C 1 , connected between the common point between the two inductors and the direct connections between second terminals of input and output. The input terminals are preferably adapted to be driven from a high frequency inverter. Any of the first inductor, the first capacitor and the second inductor may comprise a single inductive or capacitive component or a combination of such components. 
     The resonant circuit is preferably adapted such that at one of its resonant frequencies, the power adaptor provides a constant current output, at a given effective input voltage, and the resonant circuit is preferably driven at that resonant frequency or a sub-harmonic thereof, or sufficiently near to that resonant frequency or a sub-harmonic thereof for the power adaptor to be suitable for use with a constant current load, such as a solid state light source. In particular, the first and second inductors are preferably selected such that the reactance X L1  of the first inductor and the reactance X L2  of the second inductor are substantially equal in magnitude, and are substantially equal in magnitude to the reactance X C1  of the first capacitor. In particular, X L1 ≈X L2 ≈−X C1  in presently preferred embodiments. 
     When the chosen components satisfy these conditions, at a given input voltage, the current delivered to a load will be constant, independent of the load connected to the power adapter. Furthermore, variation of the input voltage would directly control the magnitude of the constant current delivered to the load. When driving a constant voltage load, such as LEDs, the power delivered to the load would therefore be directly proportional to the input voltage, without requiring any feedforward or feedback control. 
     Where the LCL series-parallel resonant circuit is adapted to provide a constant current output, the capacitance of the LCL series-parallel resonant circuit is preferably selected with a reactance X C1  to match a required load resistance R L  and a required, relatively higher, input resistance R in  for the resonant circuit. The first capacitor is preferably selected using the following equation:
 
 X   C1 =√{square root over ( R   in   R   L )}  (1)
 
where X C1  is the reactance of the first capacitor. The reactance of the first capacitor is preferably therefore equal to the square-root of the product of the required load resistance R L  and the required input resistance R in  for the resonant circuit.
 
     The output of the power adaptor is preferably therefore adapted to be connected to a load of apparent impedance R L  the value of the first and second inductors, and the first capacitor, L 1 , L 2  and C 1 , being chosen such that at least one frequency, the reactances of L 1 , L 2  and C 1  are approximately similar in magnitude and that at least one frequency the apparent impedance seen at the input terminals R in  is transformed by the LCL resonant circuit to be approximately equal to the square of the reactance of the capacitor X C1  divided by the apparent impedance of the load, R L . 
     This selection of the inductance and capacitance of the resonant circuit therefore provides a pre-determined change in effective voltage between the input and the output of the power adaptor. 
     In presently preferred embodiments, the power adaptor according to the invention comprises an input for connection to a mains AC power supply, and the resonant circuit provides an output suitable for driving a solid state light source. As discussed above, the resonant circuit is preferably an LCL series-parallel resonant circuit. 
     The use of an LCL series-parallel resonant circuit is particularly advantageous when the power adaptor is adapted to provide an output suitable for driving a solid state light source. In particular, the LCL series-parallel resonant circuit may be adapted to provide a constant current output suitable for driving a solid state light source, which is not dependent upon the load, and does not require any form of feedback or complex control. A power adaptor including an LCL series-parallel resonant circuit may therefore provide a much more efficient transfer of power from the mains power supply to the solid state light source, in comparison to prior art power adaptors, and the power adaptor may be more compact and have a lower manufacturing cost than prior art adaptors. In addition, the power at the output of the power adaptor would typically reduce as the input power reduces, and hence a power adaptor including an LCL series-parallel resonant circuit is suitable for use with conventional power reducing devices associated with the mains power supply. Hence, according to a further aspect of the invention, there is provided a power adaptor for a solid state light source, the power adaptor comprising an input for connection to a mains power supply, and an LCL series-parallel resonant circuit coupled to the input that provides an output suitable for driving the solid state light source. 
     The resonant circuit is preferably adapted to provide, at a given input voltage, a constant current output. The power delivered to the output preferably therefore varies with variation of the voltage at the input, with no need for any control. In particular, the magnitude of the constant current is preferably proportional to the input voltage. Furthermore, the resonant circuit is preferably adapted to provide, at a given input voltage, a constant current output that is independent of the load. In order to achieve these characteristics, the resonant circuit is preferably adapted such that one of its resonant frequencies provides these properties, and the resonant circuit is preferably driven at that resonant frequency, or sufficiently near to that resonant frequency for the power adaptor to be suitable for use with a solid state light source. 
     Nevertheless, it has been found that by driving the resonant circuit at a sub-harmonic of the resonant frequency, the power factor and/or efficiency of the power adaptor may be improved. Most preferably, the resonant circuit is driven at a sub-harmonic of 1/x, where x is an odd number, for example, 1/3, 1/5 or 1/7. Driving the resonant circuit at a sub-harmonic of the resonant frequency has the advantage that the switching frequency and switching losses of the resonance drive circuit may be reduced, thereby improving the efficiency of the power adaptor. In most prior art resonant circuits, driving the circuit at a sub-harmonic would reduce the power. However, the LCL series-parallel resonant circuit may be adapted to have one of its resonant frequencies at 0 Hz, as discussed in more detail below, which allows low frequency currents to pass through to the load. Hence, the current passing through the resonant circuit and the power delivered to the load does not change substantially if the circuit is driven at a sub-harmonic of the resonant frequency. 
     The LCL series-parallel circuit maybe adapted to have three resonant frequencies, a first resonant frequency at 0 Hz, ie DC current, a second resonant frequency that provides, at a given input voltage, a constant current output that is independent of the load, and a third resonant frequency that provides, at a given input voltage, a current that varies with load. These resonant frequencies are preferably achieved by selecting the first inductor, the second inductor and the first capacitor, such that the reactances of those components are substantially equal. The third resonant frequency may be adapted to provide a significantly greater power at the output, relative to the second resonant frequency. A controller of the power adaptor may therefore be adapted to switch between the different resonant frequencies in order to utilise their different characteristics. For example, a controller of the power adaptor may be adapted to switch between the second and third resonant frequencies to compensate for a change of input voltage, eg between 230V and 110V AC. Further examples of such control include loading a TRIAC in the lighting system at critical points, and altering the power factor and/or regulation of the power adaptor. 
     As the voltage at the input varies sinusoidally, the current drawn from the input by an LCL series-parallel resonant circuit, configured as described above, will inherently follow a square shape. However, the waveform of the current drawn from the input by the resonant circuit may be modified by a controller of the power adaptor. The power adaptor may therefore include a controller adapted to determine the waveform of the current drawn from the input by the resonant circuit. In particular, the controller may be adapted to modify the waveform of the current that would inherently be drawn by the resonant circuit, such that the waveform of the current drawn from the input is more similar in shape to the waveform of the voltage at the input. In particular, the current drawn by the resonant circuit may have a waveform that is generally sinusoidal, but with flattened peaks. 
     The resonant circuit is preferably driven by a resonance drive circuit, which provides a resonance drive signal to the resonant circuit. The resonance drive signal is preferably an alternating signal, and is preferably provided by an oscillator that may control two or four electronic switches, eg FETs. The resonance drive signal typically has the form of a square wave. The purpose of the drive circuit is to excite the resonant circuit with an alternating voltage, the alternating voltage typically consisting of blocks of positive and negative voltage. The electronic switches are typically connected together in the form of a full bridge inverter (4 switches) or a half bridge inverter (2 switches). 
     As discussed above, the power adaptor may be adapted to modify the waveform of the current that would inherently be drawn by the resonant circuit, and in particular modify the shape and/or size of that waveform. In particular, a resonance drive signal may be provided to the resonant circuit, wherein the resonance drive signal is adapted to determine the desired input current waveform. For instance, the resonance drive signal may be adapted in a variety of ways including, but not limited to, any of the following including combinations thereof: (i) introducing a dead-band between half-cycles or full cycles of the alternating drive signal, (ii) varying the frequency of the drive signal, and (iii) missing cycles of the alternating drive signal. 
     Where the resonance drive signal is adapted by missing cycles of the alternating drive signal, these missing cycles may be arranged in a discontinuous arrangement, in a single continuous group, or in a plurality of continuous groups, for each mains supply cycle. Where the missing cycles are arranged in a plurality of continuous groups, the number of continuous groups for each mains supply cycle is preferably selected to be appropriate for the output power, and hence may be variable with the output power. 
     As discussed below, the power adaptor may be adapted to control the light output from the solid state light source. In this embodiment, the resonance drive signal is preferably variable, for example by a controller, in order to determine the light output from the solid state light source. The resonance drive signal is preferably also adapted to optimise the power factor and/or efficiency of the power adaptor. 
     Alternatively, where the power adaptor is configured such that the light output from the solid state light source is only controllable by varying the power available at the input of the power adaptor, the resonance drive signal may be predetermined, preferably to optimise the power factor and/or efficiency of the power adaptor. 
     Any controller of the power adaptor, as discussed above, is preferably adapted to control the resonant drive signal provided to the resonant circuit, in order to determine the waveform of the current drawn from the input by the resonant circuit. This controller of the power adaptor may be provided by an integrated circuit, such as a microprocessor, an analogue electronic circuit, or any combination of analogue and digital electronics. Indeed, the controller of the power adaptor may be an application specific, integrated circuit, which may be manufactured at very low cost. In this configuration, the oscillator of the drive circuit may also form part of the integrated circuit, or may be a separate circuit. 
     The determination of the frequency at which the resonant circuit is driven may be used to calibrate the power adaptor for improved efficiency. Alternatively, the frequency at which the resonant circuit is driven may be varied during use, in order to vary the power being supplied to the solid state light source. 
     The output for driving the solid state light source may be isolated from the resonant circuit, particularly for applications in which users would have access to the solid state light source and/or associated circuitry. In this case, the power adaptor preferably comprises a piezoelectric transformer to provide this isolation. 
     The resonant circuit may also include a pair of potential dividing capacitors, to which the first capacitor is connected. Alternatively, where the resonance drive circuit contains four electronic switches (eg FETs) arranged to create two switching legs (eg a “H-bridge”), as a single phase inverter, the pair of capacitors could be replaced by a single capacitor. These capacitors are preferably Y capacitors. 
     In another embodiment, the resonance drive circuit comprises two electronic switches (eg FETs) connected between the LCL series-parallel resonant circuit and ground, ie two “low-side” switches. These two low-side switches preferably each alternate between ON and OFF, which a first switch being ON whilst a second switch is OFF, and vice versa. This arrangement is particularly advantageous where the switches are driven by a low voltage controller, such as an integrated circuit. 
     In this embodiment, the first resonant inductor of the LCL series-parallel resonant circuit preferably comprises two inductors, one connected to one end of the first capacitor, and the other connected to the other end of the first capacitor. In this arrangement, one of these two inductors will be active in the positive half cycle of the supply, and the other of these two inductors will be active in the negative half cycle of the supply. In one embodiment, these two inductors are wound about a common core, such that the first resonant inductor of the LCL series-parallel resonant circuit is a three terminal inductor. 
     The power adaptor may draw current from the input as a function of the voltage at the input in order that the power adaptor appears as a resistive load to the mains supply. This is preferably achieved by: (i) minimising the capacitance at the input of the power adaptor, (ii) drawing a current waveform from the input that is substantially in phase with the voltage waveform at the input, and/or (iii) drawing current that is substantially proportional to the voltage. These features reduce current distortion and harmonic currents drawn from the mains supply, and increase the efficiency and power factor of the power adaptor by removing the capacitive load presented to the mains supply. Indeed, these features enable the power adaptor and connected solid state light source to be presented to the mains supply as a conventional filament light source. 
     Alternatively, the power adaptor may draw power from the input as a function of the voltage at the input, such that the power adaptor does not appear as a resistive load to the mains supply. 
     The solid state light source is preferably a Light Emitting Diode (LED), or a series of two or more LEDs. The power adaptor preferably includes one or more diodes at its output, eg a diode bridge, to ensure that no reverse currents are present that could damage the solid state light source. 
     Any control circuitry of the power adaptor may be powered by an integrated power supply. Alternatively, the control circuitry of the power adaptor may be powered by a connection to one of the inductors of the resonant circuit, for instance a connection to a winding coupled to that inductor. 
     Where the power adaptor includes an integrated power supply, the integrated power supply preferably draws power directly from the mains power supply, most preferably via the input of the power adaptor. In particular, the integrated power supply is preferably a constant current power supply, such as a switch mode constant current regulator, which preferably does not cause excessive inrush and is low in cost. The control circuitry is preferably adapted to shut itself down during the off periods of a mains cycle, for example when the power adaptor is connected to a TRIAC or similar device, so that the constant current device can be low in power and hence the efficiency high. 
     The power adaptor preferably also includes a fault detection circuit that disables the resonant circuit, preferably by removing the oscillating drive signal, in the event that the load is removed, which may be caused by failure or disconnection of the light source, for example. The fault detection circuit preferably connects an output of the resonant circuit with the controller. This fault detection circuit is a feedback circuit, but it preferably draws minimal power from the output of the resonant circuit during normal operation, and hence should not be confused with an active feedback circuit that regulates the power output. The fault detection circuit would be active during a fault condition only, and is not essential for controlling the output power during normal use. 
     The power adaptor may include a filter at its input for reducing harmonic currents drawn from the mains supply. The filter may comprise a small non-electrolytic capacitor-inductor network. The power adaptor preferably also includes a rectifier at its input that converts the input waveform to one of constant polarity. Most preferably, the rectifier is a full wave rectifier that reverses the negative (or positive) portions of the alternating current waveform. Nevertheless, there is no need for the power adaptor to provide a steady DC signal at the input of the LCL series-parallel resonant circuit, and hence a bulk storage capacitor (also known as a reservoir capacitor or smoothing capacitor) is preferably not provided between the input of the power adaptor and the LCL series-parallel resonant circuit. Hence, the power adaptor is preferably substantially free of bulk storage capacitance between the input of the power adaptor and the resonant circuit. Indeed, the power adaptor is preferably substantially free of electrolytic capacitors. This enables the supply to be designed with minimal reactance, minimal inrush current, and long life with reduced size and cost relative to prior art power adaptors for solid state lighting systems. A bulk storage capacitor may be provided at the output of the power adaptor, but this is not essential for the functioning of the power adaptor with a conventional solid state light source. 
     The power adaptor according to the invention is preferably suitable for use in a lighting system that utilises any power reducing device for determining the power available at the input of the power adaptor. In particular, the power reducing device may be a variable resistor, such as a Variac, or a rheostat. The power adaptor may also be adapted to function in lighting systems that include a dimmer control utilising SCR phase control or a triac in order to reduce the power available at the input of the power adaptor. In this case, however, the power adaptor may be adapted to draw a minimum current from the mains supply to keep the SCR stable during the full mains cycle, unless the lighting unit is switched off, to ensure the continued functioning of the dimmer control. 
     A further advantage of the power adaptor according to the invention is that no monitoring of the voltage at the input, for example by a controller of the power adaptor, is necessary. Hence, the power adaptor according the invention may be devoid of any means for monitoring the voltage at the input, and in particular the power adaptor may be adapted such that the controller does not receive a signal from the input. 
     The power adapter may include a controller able to deliver a control signal to the resonant circuit for reducing power drawn from the input. However, in other embodiments, the power adaptor does not include a controller having such a feature. In particular, the power adaptor may be adapted so that the light output from the solid state light source is only controllable by varying the power available at the input of the power adaptor. In particular, the power available at the input of the power adaptor may be varied using an external device, such as an external power reducing device, associated with the mains supply. This embodiment is particularly suitable for use with a lighting unit including an integral power adaptor, which would be suitable for incorporation into a conventional lighting circuit. In order to maximise the efficiency of the power adaptor, the power adaptor is preferably adapted to transfer all power available at the input, save for unavoidable losses, to the output of the power adaptor. 
     According to a further aspect of the invention, there is provided a lighting system comprising a power adaptor as described above and a lighting unit including at least one solid state light source. 
     The lighting unit will typically be provided with a plurality of solid state light sources. In order to achieve different colours of light output, the lighting unit may include solid state light sources that emit light of different colours, for example LEDs that emit light of red, green and blue colour. Furthermore, the lighting unit may also include LEDs of amber, cyan and white colour in order to raise the colour rendering index. 
     The power adaptor and the lighting unit may have a common housing, or may be housed separately. Indeed, the power adaptor may be adapted to provide power to a plurality of lighting units, each lighting unit including a plurality of solid state light sources. Furthermore, the lighting system may include a plurality of such power adaptors. The lighting system may also include a power reducing device, such as a variable resistor, a rheostat or a dimmer control that utilises SCR phase control. 
     The power adaptor according to the invention is particularly suitable for use with a lighting unit including an integral power adaptor, which would be suitable for incorporation into a conventional lighting circuit. Hence, according to a further aspect of the invention, there is provided a lighting unit suitable for direct connection to a mains supply, the lighting unit comprising a power adaptor as described above and one or more solid state light sources, in which the light output from the one or more solid state light sources is controllable by varying the power available at the input of the power adaptor. In order to maximise the efficiency of the power adaptor, the power adaptor is preferably adapted to transfer all power available at the input, save for unavoidable losses, to the output of the power adaptor. 
     The lighting unit preferably comprises a housing for accommodating the power adaptor and the one or more solid state light sources, and a connector for connecting the input of the power adaptor to the mains supply. The connector is preferably adapted to connect to a fitting for a conventional filament light bulb. In particular, the lighting unit may include a bayonet or threaded connector. In one embodiment, the light output from the one or more solid state light sources is only controllable by varying the power available at the input of the power adaptor. 
     According to a further aspect of the invention, there is provided an electronic impedance matching circuit incorporating an LCL resonant circuit with input terminals and output terminals with a first inductor L 1 , connected from a first input terminal through a common point with second inductor L 2 , to a first output terminal, the second input terminal being directly connected to the second output terminal, and a capacitor C 1 , connected between the common point between the two inductors and the direct connections between second terminals of input and output, the input terminals being driven from a high frequency inverter, the output terminals being connected to a load of apparent impedance RL, the value of the components L 1 , L 2  and C 1  being chosen such that at least one frequency, the reactances of L 1 , L 2  and C 1  are approximately similar in magnitude and that at least one frequency the apparent impedance seen at the input terminals is transformed by the LCL resonant circuit to be approximately equal to the square of the reactance of the capacitor divided by the apparent impedance of the load, RL. 
     The frequency of operation of the high frequency inverter is preferably close to the frequency where the reactances of L 1 , L 2  and C 1  are approximately similar. 
     The electronic impedance matching circuit is preferably adapted to drive one or more LEDs with a voltage requirement which is substantially lower than the input supply voltage. 
     Alternatively, however, the electronic impedance matching circuit may be adapted to charge a battery with a voltage requirement which is substantially lower than the input supply voltage, or may be adapted to drive an electric motor at more than one speed by dynamically matching the varying apparent load impedance to the supply voltage. 
    
    
     
       A preferred embodiment of the invention will now be described in greater detail, by way of illustration only, with reference to the accompanying drawings, in which 
         FIG. 1  is a schematic diagram of a power adaptor according to the invention; 
         FIG. 2  is a schematic diagram of a resonant circuit, including a resonance controller and a resonance drive circuit, that forms part of the power adaptor of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of the resonant circuit of  FIG. 2 , including an alternative resonant drive circuit; 
         FIG. 4  is a schematic diagram of a second alternative to the circuit shown in  FIG. 2 ; 
         FIG. 5  is a schematic diagram of a third alternative to the circuit shown in  FIG. 2 ; 
         FIG. 6  is a schematic diagram of a fourth alternative to the circuit shown in  FIG. 2 ; 
         FIG. 7  is a schematic diagram of a fifth alternative to the circuit shown in  FIG. 2 ; and 
         FIG. 8  is a schematic diagram of a lighting system according to the invention. 
     
    
    
       FIG. 1  shows a power adaptor  20  according to the invention. The power adaptor  20  comprises an input  22  for drawing electrical power from the mains circuit, and an output  24  for providing electrical power to the three LEDs  60   a , 60   b , 60   c  of the solid state lighting unit  50 . The power adaptor  20  includes a filtering and rectifying circuit  30  at the input  22 , such that the AC voltage waveform drawn from the mains circuit is supplied to the remainder of the power adaptor circuitry as a full-wave rectified waveform (DC+). 
     The power adaptor  20  also includes a low power, auxiliary power supply  32 , and a resonant circuit  34  including a resonance controller  40  and a resonance drive circuit  42 , which are described in more detail below with reference to  FIG. 2 . The low power, auxiliary power supply  32  provides a low power DC output (+V) for powering the integrated circuits of the resonance controller  40  and the resonance drive circuit  42 . This provides a stable power supply to the integrated circuits of the power adaptor to ensure stable functioning of those circuits. It is noted that in other embodiments, the integrated circuits of the power adaptor are powered by connections to additional windings coupled to one of the inductors of the resonant circuit, and hence the auxiliary power supply  32  is omitted. 
     The resonant circuit  34 , including the resonance controller  40  and the resonance drive circuit  42 , is shown in  FIG. 2 . The resonance controller  40  includes a control circuit and is adapted to control the resonance drive circuit  42 . In particular, the resonance controller  40  has an output for supplying a control signal to the resonance drive circuit  42 , which determines the form of the current drawn from the input by the resonant circuit  34 . It is noted that in other embodiments, the resonance drive circuit  42  is self-oscillating, and the control circuit is omitted altogether. 
     The resonant circuit  34  has the form of an LCL series-parallel resonant circuit (L 1 , C 1  and L 2 ). The resonance drive circuit  42  is adapted to drive the LCL series-parallel resonant circuit with a square wave driving signal. This square wave signal is generated by two electronic switches, eg FETs, connected to a first end of the resonant circuit, and associated drive circuitry  44 . The FETs are controlled by the resonance controller  40 . The output of the resonant circuit  34  is rectified using a diode bridge, and then smoothed by a capacitor (C 5 ) at the output of the rectifier, so as to form an output suitable for driving the LEDs  60   a , 60   b , 60   c . The capacitors C 2  and C 3  create a connection point for the second end of the resonant circuit, substantially midway in voltage between DC+ and 0V. 
     Alternatively, the resonance drive circuit  42  contains four electronic switches (eg FETs) arranged to create two switching legs (in a “H-bridge”), as a single phase inverter, as illustrated in  FIG. 3 . In this embodiment, the capacitors C 2  and C 3  have been be replaced by a single capacitor (C 2 ) connected between DC+ and 0V. The circuit cannot operate with no capacitance across the DC supply, as a small amount of capacitance is required to protect the switches from overvoltage damage during switching transients. 
     The LCL series-parallel resonant circuit is configured such that at a chosen frequency, the reactance of L 1  (X L1 ), the reactance of C 1  (X C1 ) and the reactance of L 2  (X L2 ) are substantially equal. In this configuration, the LCL series-parallel resonant circuit has two non-zero resonant frequencies. The frequency at which the reactances are equivalent will be one of the two non-zero resonant frequencies. When driving the resonant circuit at this frequency, the resonant circuit supplies a constant current to the output, and hence to the LEDs  60   a , 60   b , 60   c , regardless of the load. The magnitude of the constant current is proportional to the input voltage. This resonant frequency is 
     
       
         
           
             
               
                 
                   
                     ω 
                     1 
                   
                   = 
                   
                     + 
                     
                       1 
                       
                         
                           
                             L 
                             S 
                           
                           ⁢ 
                           
                             C 
                             P 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The resonance controller  40  and the resonance drive circuit  42  is therefore adapted to excite the LCL series-parallel resonant circuit close to this resonant frequency, ω 1 . As a consequence of driving the resonant circuit close to the resonant frequency, the switching losses in the electronic switches are reduced, and hence the efficiency of the circuit is improved. Further advantages include the reduction of conducted and radiated electromagnetic interference, and hence the reduction of the expense of necessary filtering and screening components. 
     The normal characteristic of this configuration of the LCL series-parallel resonant circuit is to draw a power which is directly related to input voltage. Without any control, as the voltage at the input  22  varies sinusoidally, the AC current drawn from the input  22  would follow a square shape. However, it is possible to use the on-time modulation and/or the frequency of the switches to reduce the power drawn from the input  22  in the proximity of each zero crossing, and therefore to improve the input current harmonics. In addition, the optional capacitor (C 5 ) on the output of the rectifier smoothes the power delivered to the LED such that the light output will contain less fluctuation. 
     A fault detection circuit is preferably provided that includes a connection between the output of the LCL series-parallel resonant circuit and a disable pin on the PIC of the resonance controller  40 , through resistor R 1 , and a connection with 0V through resistor R 2 . The fault detection circuit draws minimal power. However, in the event that an LED  60   a , 60   b , 60   c  stops conducting, the associated fault detection circuit quickly detects a rise in voltage at the output of the resonant circuit and causes the resonance controller  40  to shut-off its output to the resonant drive circuit  42 , and hence cause the drive signal to be removed from the resonant circuit  34 . In  FIG. 2 , the fault detection circuit is shown connected between L 2  and the diode bridge. However, please note that this circuit could also be connected between the positive end of the diode bridge and the positive terminal of the output  24 . 
     The amount of power delivered to the LEDs  60   a , 60   b , 60   c  can be varied with the variation of the input mains supply voltage, which makes it suitable for use with a power reducing device  10 . 
       FIG. 4  shows a further alternative to the circuit shown in  FIG. 2 , in which the resonance controller  40  has been omitted. In this embodiment, the resonance drive circuit  42  consists simply of two electronic switches, eg FETs, connected to a first end of the resonant circuit, and associated drive circuitry  44  that is any form of analogue or digital circuit capable of providing a suitable drive signal to the electronic switches. Furthermore, this embodiment does not include any fault detection circuit (R 1  and R 2  in  FIGS. 2 and 3 ), or any capacitor (C 5   FIGS. 2 and 3 ) at the output of the rectifier. 
     The power adaptors described above in relation to  FIGS. 1-4  are each adapted to connect to a high voltage power supply (eg 110V or 230V AC, at frequencies of 50 Hz or 60 Hz), and provide an output suitable for driving a low voltage load, such as a solid state light source (eg 10-20V). In particular, the LCL series-parallel resonant circuit of each power adaptor is adapted to provide an output having a significantly decreased voltage, and a significantly increased current, relative to the power supply, without any need for a magnetic transformer. 
     The LCL series-parallel resonant circuits of the power adaptor described in relation to  FIGS. 1-4  each have a first terminal and second terminal connected to a full bridge inverter with four switching devices or a half bridge inverter with two switching devices and voltage dividing capacitors. A first inductor L 1 , and first capacitor C 1  are connected in series from the first terminal to the second terminal. The load leg of the circuit is connected in parallel with the first capacitor C 1 , the load leg comprising a second inductor L 2  in series with a rectifying means to supply unidirectional current to the load while current in the resonant circuit alternates at high frequency. 
     In such a circuit, the voltage across the first capacitor C 1  determines the current which is driven through the load leg. It would be expected therefore that if the reactance of the first inductor L 1  was increased, a greater voltage would be dropped across that component and the voltage across the load leg would be more closely matched to the lower voltage required. 
     It has been discovered that this is not the case, but it is possible to choose values for the resonant components L 1 , L 2  and C 1  such that the current in the load leg is significantly higher than the current in the first inductor L 1 . 
     The current in the load leg of this circuit at any frequency is given by: 
     
       
         
           
             
               
                 
                   
                     i 
                     
                       L 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                     
                   
                   = 
                   
                     
                       
                         X 
                         
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                       ⁢ 
                       V 
                     
                     
                       
                         
                           R 
                           L 
                         
                         ⁢ 
                         
                           X 
                           
                             L 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                       
                       + 
                       
                         
                           R 
                           L 
                         
                         ⁢ 
                         
                           X 
                           
                             C 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                       
                       + 
                       
                         j 
                         ⁡ 
                         
                           ( 
                           
                             
                               
                                 X 
                                 
                                   L 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   1 
                                 
                               
                               ⁢ 
                               
                                 X 
                                 
                                   C 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   1 
                                 
                               
                             
                             + 
                             
                               
                                 X 
                                 
                                   L 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                 
                               
                               ⁢ 
                               
                                 X 
                                 
                                   L 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   1 
                                 
                               
                             
                             + 
                             
                               
                                 X 
                                 
                                   L 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                 
                               
                               ⁢ 
                               
                                 X 
                                 
                                   C 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   1 
                                 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Where X L1 , X L2 , X C1  are the reactances of the resonant components L 1 , L 2  and C 1 , respectively, V is the excitation voltage, R L  is the effective resistance of the load and j is the reactive component. 
     When X L1 =X L2 =−X C1  the above equation simplifies to 
     
       
         
           
             
               
                 
                   
                     i 
                     
                       L 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                     
                   
                   = 
                   
                     V 
                     
                       - 
                       
                         j 
                         ⁡ 
                         
                           ( 
                           
                             X 
                             
                               C 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     In this configuration, the current in the load is independent of the load, and is proportional to the input supply voltage. 
     The further step of decreasing X C1  results in an increase in the load current for the same voltage. However, a surprising aspect of this invention is that at resonance, the input resistance of the circuit is 
     
       
         
           
             
               
                 
                   
                     R 
                     in 
                   
                   = 
                   
                     
                       X 
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                       2 
                     
                     
                       R 
                       L 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Rearranging,
 
 X   C1 =√{square root over ( R   in   R   L )}  (6)
 
     Hence it is possible to choose a value of X C1  to match a given load R L  to a required (higher) value of R in  such that the current drawn at the input is small and the current delivered to the load is high. 
     Thus, this embodiment of the invention can drive a low voltage LED string from a higher voltage supply by correct choice of capacitors and inductors. The circuit also benefits from the constant current aspects of this circuit. 
     As an example an LED string of forward voltage 12V with a current requirement of 1 A is to be driven from a 230V AC power supply. The apparent resistance of the load R L  is 12Ω. The power of the load is 12 W so the power of the input (assuming no losses) is 12 W. If the half bridge inverter with split capacitors is used to drive the resonant circuit, the effective voltage applied on the resonant circuit is ±115V. The required input resistance is therefore approximately 1100Ω. The value of X C1  is therefore 115Ω, which corresponds to a capacitance of 20 nF at a frequency of 70 kHz. The corresponding values of L 1  and L 2  would be 260 μH. 
     A further embodiment of the power adaptor according to invention is shown in  FIG. 5 . This embodiment is similar to the previous embodiments, in that it comprises a half-bridge inverter (M 1 , M 2 ), an LCL series-parallel resonant circuit (L 1 , C 1 , L 2 ), a pair of potential-dividing capacitors (C 2 , C 3 ), and a schottky diode bridge (D 1 -D 4 ) and a capacitor (C 4 ) at its output. The output is connected to one or more LEDS (two LEDs, LED  1  and LED  2 , are shown in  FIG. 5 ), which are connected in series. 
     However, this embodiment differs from the previous embodiments in that the capacitor (C 1 ) of the LCL series-parallel resonant circuit is defined by a piezoelectric transformer. The piezoelectric transformer comprises four piezoelectric transformer elements, which are formed of a ceramic material, such as PZT (lead zirconate titanate). 
     The LCL series-parallel resonant circuit including the piezoelectric transformer is adapted to provide an output having a significantly decreased voltage, and a significantly increased current, relative to the power supply. In particular, the power adaptor is adapted to connect to a high voltage power supply (eg 110V or 230V AC, at frequencies of 50 Hz or 60 Hz), and provide an output suitable for driving a low voltage solid state light source (eg 10-20V). 
     The piezoelectric transformer also isolates the output of the power adaptor from the input of the power adaptor. 
     A further embodiment of the power adaptor according to invention is shown in  FIG. 6 . This embodiment is similar to the previous embodiments, in that it comprises an LCL series-parallel resonant circuit (L 1 , C 1 , L 2 ), a pair of potential-dividing capacitors (C 2 , C 3 ), and a schottky diode bridge (D 1 -D 4 ) and a capacitor (C 4 ) at its output. The output is connected to two LEDs (LED  1  and LED  2 ), which are connected in series. 
     However, this embodiment differs from the previous embodiments in that the resonance drive circuit comprises two FETs (M 1 , M 2 ) connected between the LCL series-parallel resonant circuit and ground, ie two “low-side” switches. These two low-side switches each alternate between ON and OFF, which a first switch being ON whilst a second switch is OFF, and vice versa, so as to create a square-wave driving signal. 
     Furthermore, the first resonant inductor (L 1 ) of the LCL series-parallel resonant circuit of the previous embodiments has been replaced by two inductors (L 1   a  and L 1   b ), one connected to one end of the capacitor C 1 , and the other connected to the other end of the capacitor C 1 . In this arrangement, one of these two inductors (L 1   a ) will be active in the positive half cycle of the resonant frequency at the output, and the other of these two inductors (L 1   b ) will be active in the negative half cycle of the resonant frequency at the output. 
     This embodiment is particularly advantageous in arrangements in which the switches (M 1 , M 2 ) are driven by a low voltage controller, such as an integrated circuit. 
     A further embodiment of the power adaptor according to invention is shown in  FIG. 7 . This embodiment is identical to the embodiment shown in  FIG. 6 , save for the inclusion of a piezoelectric transformer in an arrangement that corresponds to the arrangement shown in  FIG. 5 . This embodiment combines the advantages discussed above in relation to  FIGS. 5 and 6 . 
     Finally,  FIG. 8  shows a lighting system according to the invention. The lighting system is connected to a mains circuit including a mains supply L,N and a power reducing device  10 , such as a TRIAC, and comprises a power adaptor  20  according to the invention and a solid state lighting unit  50 . The solid state lighting unit  50  comprises three LEDs  60   a , 60   b , 60   c  connected in series. The power adaptor  20  is supplied with electrical power from the mains circuit, and is adapted to provide electrical power to the LEDs  60   a , 60   b , 60   c  of the solid state lighting unit  50 .