GB2467731A

GB2467731A - Driving a resonant circuit according to input voltage

Info

Publication number: GB2467731A
Application number: GB0822693A
Authority: GB
Inventors: David Thomas Summerland
Original assignee: Holdip Ltd; E Light Ltd
Current assignee: Holdip Ltd
Priority date: 2008-12-12
Filing date: 2008-12-12
Publication date: 2010-08-18
Anticipated expiration: 2028-12-12
Also published as: GB0822693D0; GB2467731B

Abstract

A method is disclosed for driving a resonant circuit (34, figure 1) of an electronic device, including the steps of monitoring the voltage being supplied to the resonant circuit (34), and providing a driving signal to the resonant circuit (34) having a frequency that varies according to the voltage being supplied to the resonant circuit (34). An integrated electronic circuit 40, an electronic device and a power adaptor adapted to perform this method are also disclosed. The integrated circuit may comprise a module 44 for driving electronic switches to drive the resonant circuit at a range of different frequencies. The circuit is suitable for providing a constant voltage load, such as an LED 24, with varying current from a varying input voltage.

Description

Title -Improvements relating to Resonant Circuits This invention relates to resonant circuits, and in particular to the driving of a resonant circuit.

Resonant circuits are electric circuits that have a very low impedance at one or more resonant frequencies. Resonant circuits typically include one or more inductors in series, or in parallel, with one or more capacitors.

It is generally desirable to drive a resonant circuit at one of its resonant frequencies, or sufficiently close to one of its resonant frequencies to obtain a desired output. The resonant frequency of the resonant circuit may be pre-determined by either theory or experiment, and the resonant circuit may be driven at that frequency. However, it is often desirable to vary the frequency at which the resonant circuit is driven during use, in order to take account of variation of the resonant frequency of the resonant circuit during use.

A typical arrangement that addresses this problem comprises a controller that is adapted to receive feedback from the resonant circuit, for example a measurement of the current through a resonant inductor (eg using a sense resistor), in order to dynamically vary the driving frequency to a determined resonant frequency.

However, these prior art arrangements require additional circuitry that reduces the efficiency of the electronic device in which the resonant circuit is incorporated, and also increases the cost of manufacture of the electronic device.

There has now been devised an improved method of driving a resonant circuit of an electronic device, and an improved electronic device including a resonant circuit, which overcome or substantially mitigate the above-mentioned and/or other

disadvantages associated with the prior art.

According to a first aspect of the invention, there is provided a method of driving a resonant circuit of an electronic device, which method comprises the steps of: (a) monitoring the voltage being supplied to the resonant circuit; and (b) providing a driving signal to the resonant circuit having a frequency that varies in accordance with variation of the voltage being supplied to the resonant circuit.

According to a further aspect of the invention, there is provided an electronic device including a resonant circuit, wherein the electronic device comprises a circuit for monitoring the voltage being supplied to the resonant circuit, and a circuit for providing a driving signal to the resonant circuit having a frequency that varies in accordance with variation of the voltage being supplied to the resonant circuit.

The method and electronic device according to the invention are advantageous principally because the frequency of the driving signal to the resonant circuit may be varied to take account of variation of the voltage being supplied to the resonant circuit, without any need for feedback from the resonant circuit, for example a measurement of the current through a resonant inductor (eg using a sense resistor). The electronic device within which the resonant circuit is incorporated may therefore be of higher efficiency and lower cost than prior art devices.

In presently preferred embodiments, the voltage is monitored during each cycle of the supply to the resonant circuit, and the frequency of the driving signal is varied in accordance with variation of the voltage being supplied to the resonant circuit during each cycle of that supply. The electronic device is preferably therefore adapted to monitor the voltage during each cycle of the supply to the resonant circuit, and the electronic device is preferably adapted to vary the frequency of the driving signal in accordance with variation of the voltage being supplied to the resonant circuit during each cycle of that supply. In particular, the supply to the resonant circuit may be an AC supply or a rectified AC supply, and hence the electronic device may be adapted to provide an AC supply or a rectified AC supply to the resonant circuit. The electronic device may therefore be adapted for connection to an AC power supply, such as a mains power supply.

The frequency of the driving signal is preferably determined using a measurement of the voltage being supplied to the resonant circuit, and hence the electronic device is preferably adapted to measure the voltage being supplied to the resonant circuit, and determine the frequency of the driving signal using that measurement. In addition, the frequency of the driving signal may be determined using one or more other variables of the resonant circuit (eg time elapsed since start-up, measured temperature, predicted temperature, etc).

The electronic device is preferably adapted to provide a driving signal having a frequency approximately equal to a resonant frequency, or a sub-harmonic thereof, of the resonant circuit for the voltage being supplied to the resonant circuit. In particular, the electronic device is preferably adapted to determine a resonant frequency of the resonant circuit, for the voltage being supplied to the resonant circuit, and the electronic device is adapted to provide a driving signal having that resonant frequency, or a sub-harmonic thereof.

The electronic device may comprise stored data regarding resonant frequencies of the resonant circuit for a range of different supply voltages. For example, the stored data may include look-up tables of resonant frequencies for a range of different supply voltages. Alternatively, or in addition, the electronic device may comprise stored data regarding the relationship between the resonant frequency of the resonant circuit and the voltage being supplied to the resonant circuit, for example a mathematical function derived from theory and/or experiment.

The driving signals may be pulsed or alternating signals, but are preferably alternating signals. The circuit for providing a driving signal preferably comprises one or more electronic switches, and most preferably two or more electronic switches. In particular, the circuit for providing a driving signal to the resonant circuit preferably comprises an oscillator circuit that drives one or more electronic switches, and a controller that determines the frequency of the oscillator circuit.

The controller and/or the oscillator circuit are preferably provided by an integrated circuit, which is most preferably a programmable integrated circuit.

In presently preferred embodiments, each electronic switch includes a capacitor connected across the switch, in order to increase its transit time.

The circuit for monitoring the voltage being supplied to the resonant circuit preferably comprises a controller adapted to receive a signal from an input to the resonant circuit. The controller is preferably provided by an integrated circuit, which is most preferably a programmable integrated circuit. In particular, an input of the electronic device, which is adapted for connection to a power supply, is preferably connected to a pin of the integrated circuit, which is preferably at high impedance.

As discussed above, the electronic device according to the invention preferably comprises an integrated circuit, which is most preferably a programmable integrated circuit. The integrated circuit preferably comprises a module for monitoring the voltage being supplied to a resonant circuit, and a module for driving one or more electronic switches to provide a driving signal to the resonant circuit having a frequency that varies in accordance with variation of the voltage being supplied to the resonant circuit.

According to a further aspect of the invention, there is provided an integrated electronic circuit comprising a module for monitoring the voltage being supplied to a resonant circuit, and a module for driving one or more electronic switches to provide a driving signal to the resonant circuit having a frequency that varies in accordance with variation of the voltage being supplied to the resonant circuit.

The module for monitoring the voltage being supplied to a resonant circuit is preferably adapted to monitor the voltage being supplied to a resonant circuit as described above in relation to the method and device of the invention. Similarly, the module for driving one or more electronic switches to provide a driving signal to the resonant circuit preferably drives one or more electronic switches to provide a driving signal as described above in relation to the method and device of the invention.

Furthermore, the integrated electronic circuit according to the invention is preferably adapted to connect to the other components of the electronic device described above.

The method, electronic device and integrated circuit according to the invention are particularly advantageous in relation to power adaptors. In presently preferred embodiments, the electronic device is a power adaptor, which comprises 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.

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 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. The constant current load is preferably a solid state light source.

In a particularly preferred arrangement for providing an output that is suitable for driving a constant current load, the resonant circuit is preferably 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 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. 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 XL1 of the first inductor and the reactance XL2 of the second inductor are substantially equal in magnitude, and are substantially equal in magnitude to the reactance X1 of the first capacitor. In particular, XL1 XL2 -Xci 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 LED5, 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 Xci to match a required load resistance RL and a required, relatively higher, input resistance Rfor the resonant circuit. The first capacitor is preferably selected using the following equation: Xc1 = ..jRiflRL (1) where Xci 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 RL and the required input resistance for the resonant circuit.

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 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 is preferably 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.

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 FET5) 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.

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.

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.

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 power adaptor according to the invention; Figure 2 is a schematic diagram of a resonant circuit, including a controller and a resonance drive circuit, that forms part of the power adaptor of Figure 1; Figure 3 is a schematic diagram of the resonant circuit of Figure 2, including an alternative resonant drive circuit; Figure 4 is a schematic diagram of a second alternative to the circuit shown in Figure 2; Figure 5 is a schematic diagram of a third alternative to the circuit shown in Figure 2; Figure 6 is a schematic diagram of a fourth alternative to the circuit shown in Figure 2; and Figure 7 is a schematic diagram of a lighting system according to the invention.

Figures 1 to 7 illustrate electronic devices, and in particular power adaptors, according to the invention. Each power adaptor comprises a controller 40 adapted to monitor the voltage being supplied to the resonant circuit, and provide a driving signal to the resonant circuit having a frequency that varies in accordance with variation of the voltage being supplied to the resonant circuit. The controller 40 is a programmable integrated circuit.

Although the controller 40 may be adapted to perform these functions in a variety of ways, as discussed above, an example of a controller 40 suitable for use with the power adaptors of Figures 1 to 7 is described below.

The controller 40 is adapted to receive a signal from an input to the resonant circuit, so as to monitor the voltage being supplied to the resonant circuit. In particular, the DC+ line of the power adaptor (see Figures 1-3), which is adapted for connection to a power supply via input 22 and the filtering and rectifying circuit 30, is connected to a pin of the controller 40, which is at high impedance.

The power adaptors described below are each adapted for connection to a mains AC power supply, and include a filtering and rectifying circuit 30, such that the resonant circuit receives a rectified AC supply. The controller 40 is therefore adapted to monitor the voltage being supplied to the resonant circuit during each cycle of the supply to the resonant circuit, and the frequency of the driving signal is varied in accordance with variation of the voltage being supplied to the resonant circuit during each cycle of that supply.

The controller 40 is adapted to determine a resonant frequency of the resonant circuit, for the voltage being supplied to the resonant circuit, and the controller 40 is adapted to provide a driving signal having that resonant frequency, or a sub-harmonic thereof. In particular, the controller 40 is adapted to measure the voltage being supplied to the resonant circuit, and determine the resonant frequency of the resonant circuit using that measurement. The resonant frequency may also be determined using one or more other variables of the resonant circuit (eg time elapsed since start-up, measured temperature, predicted temperature, etc).

In order to determine the resonant frequency of the resonant circuit using the voltage measurement (and any other variables), the controller 40 utilises look-up tables of resonant frequencies for a range of different supply voltages (and any other variables), which are stored in its memory.

Figure 1 shows a power adaptor 20 according to the invention. The power adaptor 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 controller 40 and a resonance drive circuit 42, which are described in more detail below with reference to Figure 2.

The low power, auxiliary power supply 32 provides a low power DC output (+V) for powering the integrated circuits of the 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 controller 40 and the resonance drive circuit 42, is shown in Figure 2. As discussed above, the controller 40 is a programmable integrated circuit, which is adapted to control the resonance drive circuit 42. In particular, the 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.

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 controller 40. The output of the resonant circuit 34 is rectified using a diode bridge, and then smoothed by a capacitor (CS) at the output of the rectifier, so as to form an output suitable for driving the LEDs 60a,60b,60c. The capacitors 02 and 03 create a connection point for the second end of the resonant circuit, substantially midway in voltage between DC+ and OV.

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 3. 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.

The LCL series-parallel resonant circuit is configured such that at a chosen frequency, the reactance of Li (XL1), the reactance of Ci (Xci) and the reactance of L2 (XL2) 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 60a,60b,60c, regardless of the load. The magnitude of the constant current is proportional to the input voltage. This resonant frequency is 1 (2)

VLSCP

The controller 40 and the resonance drive circuit 42 are therefore adapted to excite the LCL series-parallel resonant circuit close to this resonant frequency, w1.

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 (CS) 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 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 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 2, 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.

The power adaptors described above in relation to Figures 1-3 are each adapted to connect to a high voltage power supply (eg 11 OV or 230V AC, at frequencies of 50Hz or 60Hz), 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 Figures 1-3 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 Li, and first capacitor Ci 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 Ci, the load leg comprising a second inductor L2 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 Ci determines the current which is driven through the load leg. It would be expected therefore that if the reactance of the first inductor Li 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 Li, L2 and Ci such that the current in the load leg is significantly higher than the current in the first inductor Li.

The current in the load leg of this circuit at any frequency is given by: i -ThX;1 � xx + (3) Where XL1, XL2, X1 are the reactances of the resonant components Li, L2 and i 0 Ci, respectively, V is the excitation voltage, RL is the effective resistance of the load and j is the reactive component.

When XLI = XL2 = -Xc1 the above equation simplifies to

V

is J(X) (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 X1 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 (5) Rearranging, = 6 Hence it is possible to choose a value of Xci to match a given load RL to a required (higher) value of 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 1 2V with a current requirement of 1A is to be driven from a 230V AC power supply. The apparent resistance of the load RL is 120. The power of the load is 12W so the power of the input (assuming no losses) is 12W. If the half bridge inverter with split capacitors is used to drive the resonant circuit, the effective voltage applied on the resonant circuit is �11 5V.

The required input resistance is therefore approximately 1100 0. The value of Xci is therefore 1150, which corresponds to a capacitance of 2OnF at a frequency of kHz. The corresponding values of L1 and L2 would be 260 pH.

A further embodiment of the power adaptor according to invention is shown in Figure 4. This embodiment is similar to the previous embodiments, in that it comprises a half-bridge inverter (Ml, M2), an LCL series-parallel resonant circuit (Li, Ci, L2), a pair of potential-dividing capacitors (C2, C3), and a schottky diode bridge (Di -D4) and a capacitor (C4) at its output. The output is connected to one or more LEDS (two LEDs, LED 1 and LED 2, are shown in Figure 4), which are connected in series.

However, this embodiment differs from the previous embodiments in that the capacitor (Ci) 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 ii OV or 230V AC, at frequencies of 50Hz or 60Hz), 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 Figure 5. This embodiment is similar to the previous embodiments, in that it comprises an LCL series-parallel resonant circuit (Li, Ci, L2), a pair of potential-dividing capacitors (C2, C3), and a schottky diode bridge (Di-D4) and a capacitor (C4) 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 (Mi,M2) 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 (Li) of the LCL series-parallel resonant circuit of the previous embodiments has been replaced by two inductors (Lia and Li b), one connected to one end of the capacitor Ci, and the other connected to the other end of the capacitor Ci. In this arrangement, one of these two inductors (Li a) will be active in the positive half cycle of the resonant frequency at the output, and the other of these two inductors (Li 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 (Mi,M2) 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 Figure 6. This embodiment is identical to the embodiment shown in Figure 5, save for the inclusion of a piezoelectric transformer in an arrangement that corresponds to the arrangement shown in Figure 4. This embodiment combines the advantages discussed above in relation to Figures 4 and 5.

Finally, Figure 7 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 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 LED5 60a,60b,60c of the solid state lighting unit 50.

Claims

Claims 1. A method of driving a resonant circuit of an electronic device, which method comprises the steps of: (a) monitoring the voltage being supplied to the resonant circuit; and (b) providing a driving signal to the resonant circuit having a frequency that varies in accordance with variation of the voltage being supplied to the resonant circuit.
2. A method as claimed in Claim 1, wherein the voltage is monitored during each cycle of the supply to the resonant circuit, and the frequency of the driving signal is varied in accordance with variation of the voltage being supplied to the resonant circuit during each cycle of that supply.
3. A method as claimed in Claim 2, wherein the supply to the resonant circuit is an AC supply or a rectified AC supply.
4. A method as claimed in any preceding claim, wherein the frequency of the driving signal is determined using a measurement of the voltage being supplied to the resonant circuit.
5. A method as claimed in Claim 4, wherein the frequency of the driving signal is determined using one or more other variables of the resonant circuit.
6. A method as claimed in any preceding claim, wherein the driving signal has a frequency approximately equal to a resonant frequency, or a sub-harmonic thereof, of the resonant circuit for the voltage being supplied to the resonant circuit.
7. An method as claimed in any preceding claim, wherein a driving signal is provided to the resonant circuit, which has a frequency that varies in accordance with variation of the voltage being supplied to the resonant circuit, without any need for feedback from the resonant circuit.
8. An electronic device including a resonant circuit, wherein the electronic device comprises a circuit for monitoring the voltage being supplied to the resonant circuit, and a circuit for providing a driving signal to the resonant circuit having a frequency that varies in accordance with variation of the voltage being supplied to the resonant circuit.
9. An electronic device as claimed in Claim 8, wherein the electronic device is adapted to monitor the voltage during each cycle of the supply to the resonant circuit, and the electronic device is adapted to vary the frequency of the driving signal in accordance with variation of the voltage being supplied to the resonant circuit during each cycle of that supply.
10. An electronic device as claimed in Claim 9, wherein the electronic device is adapted to provide an AC supply or a rectified AC supply to the resonant circuit.
11. An electronic device as claimed in any of Claims 8 to 10, wherein the electronic device is adapted to measure the voltage being supplied to the resonant circuit, and determine the frequency of the driving signal using that measurement.
12. An electronic device as claimed in Claim 11, wherein the frequency of the driving signal is determined using one or more other variables of the resonant circuit.
13. An electronic device as claimed in any one of Claims 8 to 12, wherein the electronic device is adapted to provide a driving signal having a frequency approximately equal to a resonant frequency, or a sub-harmonic thereof, of the resonant circuit for the voltage being supplied to the resonant circuit.
14. An electronic device as claimed in Claim 13 wherein the electronic device is adapted to determine a resonant frequency of the resonant circuit, for the voltage being supplied to the resonant circuit, and the electronic device is adapted to provide a driving signal having that resonant frequency, or a sub-harmonic thereof.
15. An electronic device as claimed in Claim 14, wherein the electronic device comprises stored data regarding resonant frequencies of the resonant circuit for a range of different supply voltages.
16. An electronic device as claimed in Claim 14 or Claim 15, wherein the electronic device comprises stored data regarding the relationship between the resonant frequency of the resonant circuit and the voltage being supplied to the resonant circuit.
17. An electronic device as claimed in any one of Claims 8 to 16, wherein the circuit for providing a driving signal to the resonant circuit comprises an oscillator circuit that drives one or more electronic switches, and a controller that determines the frequency of the oscillator circuit.
18. An electronic device as claimed in Claim 17, wherein the controller and/or the oscillator circuit are provided by an integrated circuit.
19. An electronic device as claimed in any one of Claims 8 to 18, wherein the circuit for monitoring the voltage being supplied to the resonant circuit comprises a controller adapted to receive a signal from an input to the resonant circuit.
20. An electronic device as claimed in any one of Claims 8 to 19, wherein the electronic device is adapted to vary the frequency of the driving signal, in accordance with variation of the voltage being supplied to the resonant circuit, without any need for feedback from the resonant circuit.
21. An electronic device as claimed in any one of Claims 8 to 20, wherein the circuit for providing a driving signal comprises one or more electronic switches.
22. An electronic device as claimed in Claim 21, wherein each electronic switch includes a capacitor connected across the switch, in order to increase its transit time.
23. An electronic device as claimed in any one of Claims 8 to 22, wherein the electronic device is a power adaptor, which comprises 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.
24. A power adaptor as claimed in Claim 23, wherein the resonant circuit is an LCL series-parallel resonant circuit.
25. A power adaptor as claimed in Claim 24, wherein the resonant circuit is 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 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.
26. A power adaptor as claimed in Claim 25, wherein the constant current load is a solid state light source.
27. A lighting system comprising a power adaptor as claimed in any one of Claims 23 to 26 and a lighting unit including a solid state light source.
28. A lighting system as claimed in Claim 27, wherein the lighting unit is provided with a plurality of solid state light sources.
29. A lighting unit suitable for direct connection to a mains supply, the lighting unit comprising a power adaptor as claimed in any one of Claims 23 to 26 and a solid state light source.
30. An integrated electronic circuit comprising a module for monitoring the voltage being supplied to a resonant circuit, and a module for driving one or more electronic switches to provide a driving signal to the resonant circuit having a frequency that varies in accordance with variation of the voltage being supplied to the resonant circuit.
31. An integrated electronic circuit as claimed in Claim 30, wherein the integrated electronic circuit is adapted to monitor the voltage during each cycle of the supply to the resonant circuit, and the integrated electronic circuit is adapted to vary the frequency of the driving signal in accordance with variation of the voltage being supplied to the resonant circuit during each cycle of that supply.
32. An integrated electronic circuit as claimed in Claim 31, wherein the integrated electronic circuit is adapted to monitor an AC supply or a rectified AC supply to the resonant circuit.
33. An integrated electronic circuit as claimed in any of Claims 30 to 32, wherein the integrated electronic circuit is adapted to measure the voltage being supplied to the resonant circuit, and determine the frequency of the driving signal using that measurement.
34. An integrated electronic circuit as claimed in Claim 33, wherein the frequency of the driving signal is determined using one or more other variables of the resonant circuit.
35. An integrated electronic circuit as claimed in any one of Claims 30 to 34, wherein the electronic device is adapted to provide a driving signal having a frequency approximately equal to a resonant frequency, or a sub-harmonic thereof, of the resonant circuit for the voltage being supplied to the resonant circuit.
36. An integrated electronic circuit as claimed in Claim 35 wherein the integrated electronic circuit is adapted to determine a resonant frequency of the resonant circuit, for the voltage being supplied to the resonant circuit, and the integrated electronic circuit is adapted to provide a driving signal having that resonant frequency, or a sub-harmonic thereof.
37. An integrated electronic circuit as claimed in Claim 36, wherein the integrated electronic circuit comprises stored data regarding resonant frequencies of the resonant circuit for a range of different supply voltages.
38. An integrated electronic circuit as claimed in Claim 36 or Claim 37, wherein the integrated electronic circuit comprises stored data regarding the relationship between the resonant frequency of the resonant circuit and the voltage being supplied to the resonant circuit.
39. An integrated electronic circuit as claimed in any one of Claims 30 to 38, wherein the integrated electronic circuit is adapted to vary the frequency of the driving signal, in accordance with variation of the voltage being supplied to the resonant circuit, without any need for feedback from the resonant circuit.