GB2464211A

GB2464211A - Power adaptor having resonant circuit for solid state lighting

Info

Publication number: GB2464211A
Application number: GB0917586A
Authority: GB
Inventors: David Thomas Summerland
Original assignee: Holdip Ltd
Current assignee: Holdip Ltd
Priority date: 2008-10-08
Filing date: 2009-10-08
Publication date: 2010-04-14
Anticipated expiration: 2029-10-08
Also published as: GB2464211B; GB0818412D0; GB0917586D0

Abstract

A power adaptor 20 for a lighting unit 50 having one or more solid state light sources comprises an input for connection to a mains power supply and a at least one power transfer module coupled to the input and providing an output suitable for driving the solid state light source. The power transfer module includes a resonant circuit (34, fig. 2) having a first inductor (L1, fig. 3), a capacitor (C1, fig. 3) and a second inductor (L2, fig. 3) in series with the output to drive the light source, where L2 has a lower inductance than L1. The solid state light sources may be light emitting diodes LEDs.

Description

Title -Improvements relating to Lighting Systems This invention relates to lighting systems, and in particular to power adaptors for solid state light sources.

Recently, solid state light sources, such as light emitting diodes (LED5), have been incorporated into conventional lighting systems, particularly those found in domestic settings. LEDs are current-driven devices whose brightness is substantially proportional to their forward current. Conventionally, therefore, a solid state light source would be driven by a power adaptor that regulates the current through the light source, and may adjust the current in order to control the intensity of the light output, typically using Pulse Width Modulation (PWM).

A conventional method of regulating LED current is to drive the LED with a current-regulated power supply. In particular, current-regulated power supplies typically monitor the voltage across a current-sense resistor, and use PWM to vary the voltage applied to the LED such that the current driving the LED is regulated to a known value.

However, known power adaptors for solid state light sources that are adapted to connect solid state light sources to the mains supply, and in particular function with conventional power reducing devices, such as dimmers, may be inefficient, bulky and expensive to manufacture.

US 7,256,554 discloses power adaptors including feed-forward power drivers, in which an energy transfer element, such as an inductor, is used to store energy, and then release that energy to the solid state light source, using a discontinuous mode switching operation. However, these power adaptors may be inefficient and bulky, and must be controlled by a switch controller in order to be suitable for driving a solid state light source. In addition, the output current delivered by these power adaptors would be dependent upon the load, and hence the number of LEDs connected in series in the solid state light source.

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 a first aspect of the invention, there is provided a power adaptor for a lighting unit having one or more solid state light sources, the power adaptor comprising an input for connection to a mains power supply, and at least one power transfer module that is coupled to the input and provides an output suitable for driving the solid state light source, the power transfer module including a resonant circuit having a first inductor and a first capacitor, wherein a second inductor is connected in series with the output for driving the solid state light source, the second inductor having a lower inductance than the first inductor.

The power adaptor according to the invention is advantageous principally because the provision of a second inductor in series with the output for driving the solid state light source provides a much more efficient transfer of power from the mains supply to the solid state light source, in comparison to prior art power adaptors.

The power adaptor may also be more compact and have a lower manufacturing cost than prior art adaptors. Furthermore, the second inductor having a lower inductance than the first inductor enables a reduction in the value of the capacitance between the resonant circuit and the input, without affecting the resonant frequency.

The resonant circuit preferably comprises the first inductor and the first capacitor in series, with a load leg connected in parallel across the first capacitor, wherein the load leg comprises the second inductor and the output for driving the solid state light source, in series. The resonant circuit is preferably therefore an LCL series-parallel resonant circuit, in which 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 Li, connected from a first input terminal through a common point with second inductor L2, to a first output terminal, the second input terminal being directly connected to the second output terminal, and a capacitor Ci, 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.

i 0 In order to maximise the efficiency of the resonant circuit, the inductive reactance XL of the first inductor and the capacitive reactance X of the first capacitor are preferably substantially equal. In particular, the inductive reactance XL of the first inductor and the capacitive reactance Xc of the first capacitor preferably differ by less than +1-25%. is

Furthermore, the inductances of the first and second inductors are preferably selected to maximise the efficiency of the power adaptor, whilst maintaining the resonant characteristic of the circuit. In particular, reducing the inductance of the second inductor relative to the first inductor causes the current through the second inductor to be greater than the current through the first inductor, thereby reducing losses in the first inductor. However, if the inductance of the second inductor is reduced too much, the resonant characteristic of the circuit is lost. For example, the inductance of the second inductor may be between 50% and 90% of the inductance of the first inductor.

The resonant circuit is preferably driven by a driver circuit, which provides an alternating driver signal to the resonant circuit. The alternating signal is preferably provided by two electronic switches, eg FETs, and typically has the form of a square wave.

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 second capacitor to provide this isolation. The second capacitor is preferably connected in series with the first and second inductors, between the second inductor and the output for driving the solid state light source. The second capacitor is preferably a Y capacitor.

The resonant circuit preferably also includes a pair of potential dividing capacitors, to which the second capacitor is connected. These dividing capacitors are preferably Y capacitors.

The power adaptor preferably includes a controller adapted to deliver a control signal to the driver circuit, which controls the driver signal provided to the resonant circuit. In particular, the controller is preferably able to control electronic switches of the driver circuit, thereby controlling the power drawn from the input by the power adaptor. The controller may also be adapted to receive a voltage signal from the input, such that the control signal may be based upon that voltage signal.

Alternatively, the power adaptor may be adapted to transfer all power available at the input, save for unavoidable losses, to the output of the power adaptor. In particular, the power adapter may be adapted so that the output of the power adaptor is only controllable by external devices, such as external power reducing devices connected to the mains supply. This arrangement would be suitable for a lighting unit including an integral power adaptor, which would be suitable for incorporation into a conventional lighting circuit.

The power adaptor draws current from the input as a function of the voltage at the input in order that the power adaptor appears as a variable resistor to the mains supply. The power adaptor according to the invention is therefore also advantageous because the solid state light source would act like a conventional filament light bulb, in use. The intensity of light output from the solid state light source can therefore be controlled by an external power reducing device.

Alternatively, the intensity of light output from the solid state light source can be controlled by a combination of an external power reducing device, and the internal controller that is able to control the power drawn from the input of the power adaptor. For example, the internal controller may be adapted to cause a reduction of the power drawn from the input of the power adaptor when a maximum amount of power is available, and then lessen that reduction as the amount of power available falls, thereby causing the solid state light source to follow a non-linear dimming curve.

A further advantage of the power adaptor according to the invention is that the solid state light source, which is typically a Light Emitting Diode (LED), may be provided with a predetermined average power in order to achieve a desired intensity of light output. The power adaptor therefore enables a particular intensity of light output to be achieved, regardless of the forward potential difference of the LED. Most preferably, the average power provided to the LED is substantially equal to the average power drawn at the input of the power adaptor. The power adaptor and connected LED would therefore act like a conventional tungsten bulb when connected to a power reducing device, in that the intensity of its light output would inherently reduce as the power drawn from the input reduces, and hence the power adaptor does not regulate a constant output to the LED.

The power adaptor draws current from the input as a function of the voltage at the input in order that the power adaptor appears as a variable resistor to the mains supply. This is preferably achieved by: (i) minimising the capacitance at the input of the power adaptor, (ii) drawing a sinusoidal current waveform from the input that is substantially in phase with the sinusoidal voltage waveform at the input, and (iii) drawing current that is proportional to the voltage, such that the current falls as the voltage falls. 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 lamp.

The solid state light source is preferably a Light Emitting Diode (LED), or a series of two or more LEDs. Since the potential difference across an LED, or a series of two or more LEDs, is substantially constant, the power adaptor preferably controls the current drawn from the input in order to maintain the average power drawn from the input substantially constant at a pre-determined value. The pre-determined average power may be determined at least partially by a control signal provided by the controller.

The power adaptor is preferably adapted to draw a current from the input that is substantially proportional to the voltage at the input. In particular, the power transfer module is preferably adapted to draw a sinusoidal current waveform from the input that is substantially in phase with the sinusoidal voltage waveform at the input, and the power transfer module and/or the controller preferably includes a multiplier that determines the proportional relationship between the current and the voltage that would result in a pre-determined average power being drawn from the input. The controller is preferably adapted to sense the voltage waveform at the input of the power adaptor, and determine the required current to be drawn from the input based on that voltage waveform. The power adaptor preferably includes a voltage monitor that provides a reduced amplitude representation of the voltage at the input of the power adaptor.

Where a user has access to the solid state light source that is driven by the power adaptor, the power transfer module preferably includes either a transformer or a capacitor at the output of the resonant. In addition, the power transfer module preferably includes one or more diodes at its output to ensure that no reverse currents are present that could damage the light source.

The control circuitry of the controller and/or the power transfer module may be powered by an integrated power supply or by a connection with the output of the power transfer module.

Where the control circuitry of the controller and/or the power transfer module is powered by an integrated power supply, this power supply preferably draws power directly from the mains power supply, most preferably via the input of the power adaptor. In particular, the 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 transfer module preferably also includes a fault detection circuit that disables that module 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 no 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 transfer module may be adapted to drive a single LED, or a series of LEDs. The power adaptor preferably includes a plurality of power transfer modules that generate separate outputs. Most preferably, the power available from the input is identical for each of the power transfer modules, and the outputs drive separate LEDs and/or series of LEDs. It should be noted that in the absence of any control signal from the controller, the output power would reduce as the input power reduces and hence make the LED appear like a conventional filament bulb.

The controller may be adapted to provide a signal that controls the driving signal of the resonant circuit, and hence varies the pre-determined average power drawn from the input. Where the power adaptor includes a plurality of power transfer modules, the controller is preferably able to independently vary the control signals provided to those power transfer modules. This feature enables the intensity of light emitted by particular LEDs and/or series of LEDs to be independently varied.

In particular, the controller is preferably adapted to receive commands from a user interface, which may take the form of a power reducing device in series with the power adaptor, serial data via a hard wired or wireless controller, and/or controls provided on the power adaptor itself. The controller is preferably programmed with one or more lighting profiles that determine the manner in which the reference signals provided to the power transfer modules are varied in response to commands from a user interface. The controller preferably varies the reference signal by outputting a control signal, which preferably offsets the reference signal by a DC voltage.

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 power transfer module, 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 power transfer module. 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 transfer module, 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 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 is also able to function in lighting systems that include a dimmer control utilising SCR phase control in order to reduce the power available at the input of the power adaptor. In this case, however, the power transfer module 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.

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 preferably includes solid state light sources that emit light of different colours, and most preferably LEDs that emit light of red, green and blue colour. Furthermore, the lighting unit may also include LED5 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.

In order to provide a low cost and long life power adaptor, the power adaptor preferably has negligible capacitance. However, the power adaptor is preferably also adapted to keep a TRIAC of a dimming device in conduction during use.

Hence, the power adaptor is preferably adapted to have a sufficiently fast response time that high frequency components of the AC power-related signal received at the input, such as those created by a TRIAC, are not filtered out.

Hence, according to a further aspect of the invention, there is provided a lighting system comprising at least one LED, and at least one power adaptor coupled to the at least one LED and configured to provide DC power to the at least one LED, wherein the power adaptor is configured to receive from an AC power source an AC power-related signal having higher frequency components that a standard AC line voltage and to provide said DC power based on the AC power-related signal, wherein the power adaptor is configured to have a sufficiently fast response time that the higher frequency components are not filtered out and are transferred to the output in the same ratio as the lower frequency components of the AC power-related signal.

The higher frequency components that are transferred to the output may be those introduced into the AC power-related signal by a power reducing device and/or mains-borne signalling. In particular, the power reducing device may be a silicon switching device, such as a triode alternating diode switch (TRIAC), a silicon-controlled rectifier (SCR), or one or more MOSFETs.

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 Figure 1 is a schematic diagram of a lighting system according to the invention; Figure 2 is a schematic diagram of a power adaptor according to the invention that forms part of the lighting system of Figure 1; Figure 3 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 Figure 2; Figure 4 is a schematic diagram of the resonant circuit of Figure 3, including an alternative resonant drive circuit; Figure 5 is a schematic diagram of a alternative to the circuit shown in Figure 3; and Figure 6 is a schematic diagram of a further alternative to the circuit shown in Figure 3.

Figure 1 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 and a solid state lighting unit 50. The solid state lighting unit 50 comprises three LED5 60a,60b,60c 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 60a,60b,60c of the solid state lighting unit 50.

Referring now to Figure 2, 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 60a,60b,60c 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 Figure 3.

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 Figure 3. 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 (Li, Ci and L2). 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 FET5 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 (05) at the output of the rectifier, so as to form an output suitable for driving the LEDs 60a,60b,60c. The capacitors C2 and C3 create a connection point for the second end of the resonant circuit, substantially midway in voltage between DC+ and OV. An isolation capacitor (C4) may also be provided between L2 and the diode bridge, in order to isolate the switching leg from the diode bridge, and hence the LED5 60a,60b,60c.

Alternatively, the resonance drive circuit 42 contains four electronic switches (eg FET5) arranged to create two switching legs (in a "H-bridge"), as a single phase inverter, as illustrated in Figure 4. In this embodiment, the capacitors 02 and 03 have been be replaced by a single capacitor (02) connected between DC+ and OV. 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. In addition, a second isolation capacitor (06) is provided to isolate the additional switching leg from the diode bridge.

The resonant circuit 34 has the form of an LCL series-parallel resonant circuit (Li, Ci and L2), in which the inductive reactance XL of Li and the capacitive reactance Xc of Ci are equal, in order to generate the resonant frequency and maxim ise current to the load. However, this circuit differs from conventional LCL series-parallel resonant circuits in that L2 has a lower inductance than LI. In particular, in conventional LCL series-parallel resonant circuits, Li and L2 would have the same inductance. In this example, however, Ci has a capacitance of 4.7nF, Li has an inductance of imH, and L2 has an inductance of 700pH.

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 (C5) 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 PlC of the resonance controller 40, through resistor Ri, and a connection with OV through resistor R2. The fault detection circuit draws minimal power. However, in the event that an LED 60a,60b,60c 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 Figure 3, the fault detection circuit is shown connected between L2 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 60a,60b,60c can be varied with the variation of the input mains supply voltage, which makes it suitable for use with a power reducing device 10.

Figure 5 shows an alternative to the circuit shown in Figure 3, in which the isolation capacitor (C4 in Figure 3) has been omitted.

Figure 6 shows a further alternative to the circuit shown in Figure 3, in which the resonance controller 40 has been omitted. In this embodiment, the resonance drive circuit 42 consists simply of two electronic switches, eg FET5, 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 (Ri and R2 in Figures 3 and 4), any isolation capacitor (04 in Figures 3 and 4), or any capacitor (05 Figures 3 and 4) at the output of the rectifier.

Claims

Claims 1. A power adaptor for a lighting unit having one or more solid state light sources, the power adaptor comprising an input for connection to a mains power supply, and at least one power transfer module that is coupled to the input and provides an output suitable for driving the solid state light source, the power transfer module including a resonant circuit having a first inductor and a first capacitor, wherein a second inductor is connected in series with the output for driving the solid state light source, the second inductor having a lower inductance than the first inductor.
2. A power adaptor as claimed in Claim 1, wherein the resonant circuit comprises the first inductor and the first capacitor in series, with the load connected in parallel across the first capacitor, wherein the load comprises the second inductor and the output for driving the solid state light source, in series.
3. A power adaptor as claimed in Claim 2, wherein the inductive reactance XL of the first inductor and the capacitive reactance Xc of the first capacitor are substantially equal.
4. A power adaptor as claimed in any preceding claim, wherein the inductances of the first and second inductors are selected to maximise the efficiency of the power adaptor, whilst maintaining the resonant characteristic of the circuit.
5. A power adaptor as claimed in any preceding claim, wherein the inductance of the second inductor may be between 50% and 90% of the inductance of the first inductor.
6. A power adaptor as claimed in any preceding claim, wherein the resonant circuit is driven by a driver circuit, which provides an alternating driver signal to the resonant circuit.
7. A power adaptor as claimed in Claim 6, wherein the alternating signal is provided by two electronic switches, and has the form of a square wave.
8. A power adaptor as claimed in Claim 5 or Claim 6, wherein the power adaptor includes a controller adapted to deliver a control signal to the driver circuit, which controls the driver signal provided to the resonant circuit.
9. A power adaptor as claimed in any preceding claim, wherein the output for driving the solid state light source is isolated from the resonant circuit by a second capacitor.
10. A power adaptor as claimed in Claim 9, wherein the second capacitor is connected in series with the first and second inductors, between the second inductor and the output for driving the solid state light source.
11. A lighting system comprising a power adaptor as claimed in any preceding claim and a lighting unit including at least one solid state light source.
12. A lighting system as claimed in Claim 11, wherein the lighting unit comprises a plurality of solid state light sources.
13. A lighting system comprising at least one LED, and at least one power adaptor coupled to the at least one LED and configured to provide DC power to the at least one LED, wherein the power adaptor is configured to receive from an AC power source an AC power-related signal having higher frequency components that a standard AC line voltage and to provide said DC power based on the AC power-related signal, wherein the power adaptor is configured to have a sufficiently fast response time that the higher frequency components are not filtered out and are transferred to the output in the same ratio as the lower frequency components of the AC power-related signal.
14. A lighting system as claimed in Claim 13, wherein the higher frequency components that are transferred to the output are those introduced into the AC power-related signal by a power reducing device and/or mains-borne signalling.
15. A lighting system as claimed in Claim 14, wherein the power reducing device is a silicon switching device.
16. A lighting system as claimed in Claim 15, wherein the silicon switching device has an output inductor.
17. A lighting system as claimed in any one of Claims 13 to 16, wherein the power adaptor comprises a resonant circuit that is driven by a resonance driver circuit, wherein a power supply for the resonance driver circuit has negligible capacitance.