GB2462007A - Power adaptor for LED light source - Google Patents

Abstract

A power adaptor 20 for a lighting unit having a solid state light source 50a,50b,50c, such as LEDs, comprises an input 22 for connection to a power supply and produces an output signal suitable for driving the solid state light source. The power adaptor controls the average power of the output signal such that the peak power of the output signal is reduced, relative to the average power of the output signal, as the average power of the output signal is increased. The output signal may have a power waveform that is variable to control the average power of the output signal, where the power waveform includes a periodic component having a frequency that it increased and an amplitude that is decreased as the average power of the output signal is increased. The power adaptor may be housed in a lighting unit having a connector adapted to connect to a fitting for a conventional filament light bulb.

Description

Title -Improvements 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 adjusts the current in order to control the intensity of the light output.

In WO 2006/01 8604, a number of arrangements of power adaptors and dimming 1 5 controllers are described for lighting systems having both incandescent and solid state lighting units. These existing power adaptors are able to control the intensity of the light output and/or a colour characteristic of a solid state lighting unit by producing driving signals for the LEDs using conventional pulse width modulation (PWM), such that changes in the duty cycle (or on-duration) of the LED drive current give rise to corresponding changes in the average current made available to the LEDs via the power adaptor.

In WO 2007/0261 70, a power adaptor for a solid state lighting unit is described that provides pulsed driving signals to the LEDs, such that the power level of each driving signal varies as a non-linear function of the amount of power available to the power adaptor. In particular, the pulsed driving signal has a triangular waveform, and the power level is varied using a DC offset for that waveform. This arrangement provides improved dimming resolution at low powers, and hence low brightnesses, which is advantageous because the human eye is most sensitive to changes of light intensity at low brig htnesses.

Nevertheless, a pulsed driving signal may not be able to achieve maximum brightness for a solid state light source because the average current required to attain maximum brightness of a solid state light source is often equal, or near, the peak current for that 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 present invention, there is provided a power adaptor for a lighting unit having a solid state light source, the power adaptor comprising an input adapted for connection to a power supply, and the power adaptor being adapted to produce an output signal suitable for driving the solid state light source, wherein the power adaptor is adapted to control the average power of the output signal, such that the peak power of the output signal is reduced relative to the average power of the output signal, as the average power of the output signal is increased.

According to a further aspect of the present invention, there is provided a power adaptor for a lighting unit having a solid state light source, the power adaptor comprising an input adapted for connection to a power supply, and the power adaptor being adapted to produce an output signal suitable for driving the solid state light source, the output signal having a power waveform that is variable to control the average power of the output signal, wherein the power waveform includes a periodic component having a frequency that is increased and an amplitude that is decreased as the average power of the output signal is increased.

The power adaptor according to the present invention is advantageous principally because the increase in the frequency of the periodic component, and the decrease in the amplitude of the periodic component, reduces the peak power of the output signal relative to the average power of the output signal (eg the RMS power), as the average power of the output signal is increased. This reduction of the peak power relative to the average power enables a greater light intensity to be achieved for a particular solid state light source, and also reduces the harmonics that are drawn from the mains supply.

Since the voltage drawn from the input is regulated by the characteristics of the solid state light source, the reduction of the peak power relative to the average power will therefore reduce the peak current relative to the average current at the solid state light source. A skilled person will be aware that although this invention is described below in relation to the regulation of the power being supplied to the solid state light source, this invention is also advantageous in relation to power 1 0 adaptors that regulate the current being supplied to the solid state light source. In this case, the output signal would include a current waveform that is variable to control the average current of the output signal, wherein the current waveform includes a periodic component having a frequency that is increased and an amplitude that is decreased as the average current of the output signal is increased.

The power waveform of the output signal preferably also includes a constant component, ie a DC level, that varies the height of the periodic waveform component relative to zero power. Hence, both the periodic component and the constant component of the power waveform are preferably variable, in order to control the average power of the output signal.

The output signal may comprise the power waveform superimposed upon a power signal drawn from the input. In particular, the power waveform preferably has the form of a distortion applied to a power signal drawn from the input. The power signal drawn from the input may be full-wave rectified.

The power waveform preferably has a frequency that is greater than the frequency of the power drawn from the input. Furthermore, the amplitude of the power waveform is preferably sufficiently low that it does not cause distortion of the power drawn from the input that is greater than allowed by the relevant harmonic standard.

The constant component of the power waveform may be dependent on the average power available at the input. For example, the constant component may be proportional to the average power available at the input. The constant component may also be dependent on a lighting profile stored in a memory of the power adaptor. The constant component is preferably controlled by a controller of the power adaptor.

Within a range of the lowest average powers of the output signal, the rate of change of the average power of the output signal, relative to the rate of change of the constant component of the power waveform, preferably increases as the average power increases in that range.

The periodic component may be any periodic waveform, but is preferably a triangular waveform.

The increase in frequency of the periodic waveform component, and the decrease in the amplitude of the periodic waveform component, as the average power of the output signal is increased, may be achieved using any of the control circuitry well known to a person skilled in the art for this purpose, such as a programmable integrated circuit. By the "frequency" of the periodic component is meant the number of peaks of the waveform for a given unit of time, eg one second. By the "amplitude" of the periodic component is meant the power difference between each peak and its adjacent trough of the power waveform.

The input of the power adaptor is preferably adapted for connection to a mains power supply. In presently preferred embodiments, the power adaptor comprises a power transfer module that is coupled to the input and provides the output signal suitable for driving the solid state light source, and a controller that is able to deliver a control signal to the power transfer module. The power adaptor preferably draws 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.

The control signal of the controller is preferably able to control the power drawn from the input by the power transfer module, and hence determine the power waveform of the output signal. In particular, the control signal is preferably a voltage signal having the same waveform as the power waveform of the output signal.

The voltage waveform of the control signal is preferably therefore variable to control the average power of the output signal, wherein the power waveform includes a periodic component having a frequency that is increased, and an 1 0 amplitude that is decreased, as the average voltage of the control signal is increased. In addition, the voltage waveform of the control signal preferably includes a constant voltage component, ie a DC level, that varies the height of the periodic component relative to ground. Hence, both the periodic component and the constant component are preferably variable, in order to control the average 1 5 voltage of the control signal.

The constant component of the control signal may be dependent on the average power available at the input, which is preferably monitored by the controller. For example, the constant component may be proportional to the average power available at the input. The constant component may also be dependent on a lighting profile stored in a memory of the controller. The controller may generate the periodic component of the control signal separately from the constant component, and those components may then be combined to form the control signal that is transmitted to the power transfer module.

The periodic component of the control signal may be any periodic waveform, but is preferably a triangular waveform. The constant component of the control signal preferably determines the height of the periodic component relative to ground, such that the peaks of the voltage waveform are effectively shifted in height relative to ground, thereby providing more or less average voltage to the power transfer module.

Hence, for medium to high power levels, the control signal includes the complete periodic waveform component. However, in order to provide greater dimming resolution at low output powers, and hence low light intensities, the control signal only includes a positive portion of the periodic waveform component for a range of the lowest average powers of the output signal, such that the rate of change of the average power of the output signal, relative to the rate of change of the constant component of the control signal, increases as the average power increases in that range.

1 0 The increase in frequency of the periodic component of the control signal, and the decrease in the amplitude of the periodic component of the control signal, as the average voltage of the control signal is increased, may be achieved using any of the circuits well known to a person skilled in the art for this purpose, and in particular may include an RC filter. By the "frequency" of the periodic component is meant the number of peaks of the waveform for a given unit of time, eg one second. By the "amplitude" of the periodic component is meant the power difference between each peak and its adjacent trough of the control signal waveform.

The power adaptor preferably draws 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 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 is preferably determined at least partially by a reference signal, which may include a control signal provided by the controller, as discussed above.

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 1 5 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 power transfer module and/or the controller are 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.

The power transfer module may include a power factor correction circuit.

However, the power factor correction circuit of the power transfer module preferably differs from a conventional power factor correction circuit in that it does not include a feedback loop from the LED, and hence is an open loop circuit in relation to control of the LED. In this case, the power correction factor circuit 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 pre-determined average power is transferred as an output power to the LED such that the LED is controlled by the power drawn from the input, rather than the current supplied to the LED. The pre-determined average power is preferably determined at least partially by a reference signal, which may include a control signal of the controller, and preferably also a current sense resistor. The current drawn from the input is preferably controlled by an electronic switch, such as a MOSFET, which is preferably in series with a current sense resistor.

Alternatively, the power transfer module may include 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 including a second inductor. The resonant circuit preferably comprises a load 1 0 connected in parallel across the first capacitor, wherein the load comprises the second inductor and an output for driving the solid state light source, 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. An example of this form of power transfer module is described in GB 2449616 B. The power transfer module preferably includes a transformer and one or more diodes at its output from which the output power signal is delivered to the light source. The transformer isolates the light source from the mains supply, and the one or more diodes 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.

Where the control circuitry is powered by a connection with the output of the power transfer module, said connection is preferably made by means of an auxiliary winding on the transformer at the output of the power transfer module. In this case, the power adaptor preferably includes a variable resistance circuit at the output of the power transfer module that enables there to be sufficient power available at the output of the power transfer module to power the control circuitry, 1 0 even when the solid state light source has been fully dimmed. In particular, the variable resistance circuit preferably increases in resistance as the power supplied to the solid state light source is reduced. The variable resistance circuit is preferably therefore provided with a control signal, which is preferably derived from the control signal supplied by the controller to the power transfer module. Most 1 5 preferably, the path for this control signal includes an isolating component, such as an opto-isolator, in order to isolate the solid state light source from the mains supply.

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 includes an opto-isolator in order to maintain isolation of the light source from the mains supply. This fault detection circuit is a feedback circuit, but it preferably draws no power from the output of the power transfer module 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 is preferably adapted to provide a control signal that varies the reference signal provided to the power transfer module, 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 reference 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, 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.

Since each power transfer module is not a power regulator, if a power reducing device reduces the power available to the input of the power adaptor, the output power will reduce in proportion. Each power transfer module would normally draw a reduced average power from the input, and hence provide a reduced average power at the output, as the power available from the input of the power adaptor reduces. However, each power transfer module may be designed to maintain the current drawn from the input at an increased value than is required for maximum light output of the associated LED, when a maximum amount of power is available from the input of the power adaptor, which is then in turn reduced by the controller to provide only the power required for maximum output and no more. As the external power reducing device lowers the available input power, the controller may then vary the reference signal provided to each power transfer module so that the current drawn from the input is increased, and the average power of the output therefore stays equal to the power required by the LED for maximum brightness.

This allows the LED to have a maximum brightness, even when the power available from the input of the power adaptor has been reduced until the power available from the input of the power adaptor would normally equal the power required by the LED for maximum brightness. Any reduction in the power available to the power transfer module below this threshold will necessarily result in a dimming of the LED.

The controller is preferably adapted to detect the average voltage, eg the rms voltage, of the mains supply, and select an appropriate proportional relationship between the current being drawn at the input of the power adaptor and the voltage at the input of the power adaptor, in order to maintain the same pie-determined average power for two or more different average voltages of the mains supply.

1 5 Where the power transfer module and/or the controller includes a multiplier that determines the proportional relationship between the current and the voltage that would result in a pre-determined average power, the controller preferably provides a reference signal to the multiplier that is dependent upon the average voltage of the mains supply. Most preferably, the controller is adapted to detect whether the mains supply has an rms voltage of approximately 11 OV or an rms voltage of approximately 230V, and supply an appropriate reference signal to the multiplier.

In particular, the reference signal for the 11 OV rms voltage may be approximately 200% of the reference signal for the 230V rms voltage.

Conventionally, the current supplied to an LED must be limited by a resistor in series with the LED, in order to prevent damage to the LED. However, since the power adaptor according to the invention may be adapted to provide a predetermined average power to the LED, and the potential difference of the LED is substantially constant, the current will be regulated without any need for a series resistor or current sense regulation at the output.

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.

In presently preferred embodiments, the power transfer module is adapted to draw power immediately, once a voltage is present at the input of the power adaptor, such that the power transfer module enables sufficiently continuous and smooth conduction to maintain a TRIAC device fired during the remainder of a mains cycle. Any integrated circuit of the power transfer module is preferably therefore maintained in a standby mode when no voltage is present at the input of the power adaptor, for example when a TRIAC device causes an off period of the mains cycle, and is adapted to draw power immediately and continuously once a voltage is present. Most preferably, the controller is adapted to detect whether a voltage is present at the input of the power adaptor, and send a control signal to the integrated circuit of the power transfer module to switch that integrated circuit to a standby mode when no voltage is detected at the input, and to an operational mode when a voltage is detected at the input. By "standby mode" is meant that the integrated circuit is supplied with sufficient power to be operational, but the power transfer module does not draw sufficient power from the mains supply to drive the solid state light source. By "operational mode" is meant that the power transfer module draws sufficient power from the mains supply to drive the solid state light source.

Hence, where a power reducing device is adapted to cause the voltage at the input of the power adaptor to be switched on and off at least once each cycle, for example using a triode alternating diode switch (TRIAC), the controller is preferably adapted to switch the integrated circuit of the power transfer module to a standby mode whenever no voltage is detected at the input, and to an operational mode whenever a voltage is detected at the input. The integrated circuit of the power transfer module may therefore be switched between standby and operational modes many times each second, for example 100 times each second for a 50Hz mains supply. This arrangement maintains high efficiency at low cost at all TRIAC firing times from the mains supply.

Where the power adaptor is adapted to drive a plurality of solid state light sources, and hence comprises a plurality of output channels, the controller is preferably able to independently switch the integrated circuits of the plurality of power transfer modules to a standby mode. In particular, the controller is preferably adapted to inactivate a solid state light source by switching the integrated circuit of the associated power transfer module to a standby mode.

The power adaptor may therefore comprise an output for a high efficiency light source, such as a white light source, as well as one or more sources of lower efficiency, such as coloured light sources. In this embodiment, the power adaptor may be adapted to switch the one or more lower efficiency outputs (eg coloured light sources) to a standby mode, whilst providing power to the high efficiency output (eg white light source), at a pre-determined power level, such as a maximum power level. The arrangement would result in very high efficiency at the maximum power level.

This arrangement also enables a lighting system in which the light unit provides white light at a maximum power level, and the coloured light effects, such as a warmer white light, at lower power levels. In particular, the high efficiency light 1 0 source is preferably a high colour temperature white light source, and the one or more lower efficiency light sources preferably includes a coloured light source, such as an amber light source, for mixing with the white light source to create a more desirable, eg warmer, "white" light. This arrangement may therefore encourage lower power levels to be utilised, which may therefore save energy without any loss of efficiency at a maximum power level.

Where the control circuitry of the power adaptor is supplied by an integrated constant current power supply, this power supply preferably includes a shut-off feature, such that unnecessary power is not supplied whilst the lower efficiency outputs are inactive.

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.

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 that forms part of the lighting system of Figure 1; Figures 3a-3d are schematic illustrations of the waveform of control signals of the power adaptor at 25% power level (Figure 3a), 50% power level (Figure 3b), 75% power level (Figure 3c) and 100% power level (Figure 3d); and Figures 4a-4d are measurements of the power of an output signal of the power adaptor at 25% power level (Figure 4a), 50% power level (Figure 4b), 75% power level (Figure 4c) and 100% power level (Figure 4d).

Figure 1 shows a lighting system according to a first embodiment of 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 rheostat, and comprises a power adaptor 20 and a solid state lighting unit 50. The solid state lighting unit 50 comprises three coloured emitters 50a,50b,50c in a colour array, one each of red, green and blue LEDs. The power adaptor 20 is supplied with electrical power from the mains circuit, and is adapted to control the electrical power supplied to the solid state lighting unit 50. The power adaptor 20 and solid state lighting unit 50 may be enclosed within separate housings, or within a common housing of a type as described in WO 2006/0 1 8604.

Referring now to Figure 2, the power adaptor 20 comprises an input 22 for drawing electrical power from the mains circuit, and three power transfer modules 40a,40b,40c for providing electrical power to each of the three LEDs 50a,50b,50c in the solid state lighting unit 30. Each power transfer module 40a,40b,40c is connected to the input 22 via a filter 24 and a rectifier 26, so that a sinusoidal voltage waveform drawn from the mains circuit is supplied to each power transfer module 40a,40b,40c as a full-wave rectified waveform.

A voltage monitoring circuit 32 is connected to the input 22 via the filter 24 and rectifier 26, and acts to monitor the average power available at the input 22. In this embodiment, the voltage monitoring circuit 32 is simply a potential divider that 1 5 provides a reduced amplitude representation of the signal drawn through the filter 24 and rectifier 26.

The power adaptor 20 is also provided with a controller 30, which includes a Programmable Interface Chip (PlC). The controller 30 has an input that receives a signal from the voltage monitoring circuit 32, and three outputs for supplying separate control signals to each of the power transfer modules 40a,40b,40c via a corresponding series resistor. Each control signal is calculated using the signal received from the voltage monitoring circuit 32, and hence the average power available at the input 22 of the power adaptor 10. The control signal(s) determine the power supplied by the power transfer modules 40a,40b,40c to the LEDs 50a,50b,50c, and hence determine the colour and/or intensity of the light emitted by the solid state lighting unit 30.

Each power transfer module 40a,40b,40c comprises a power factor correction circuit 42a,42b,42c including integrated circuit L6562 operating in power transition mode, which is manufactured by ST Microelectronics of 39 Chemin du Champ des Filles, C. P.21, CH 1228 PIan-Les-Ouates, Geneva, Switzerland. The power factor correction circuit 42a,42b,42c is adapted to draw a current that is substantially in phase with the voltage of the input power signal, and provide an output power signal of constant average power with a current substantially in phase with the voltage. The power factor correction circuit 42a,42b,42c therefore includes a multiplier that determines the current to be drawn, for a given input voltage, in order to maintain a constant average power output with the current and voltage waveforms substantially in phase with each other. The power factor correction circuit 42a,42b,42c also acts to reduce the harmonic distortion of the current drawn from the mains supply.

The power factor correction circuits 42a,42b,42c differ from a standard power factor correction circuit in that there is no need for a feedback loop from the output to the integrated circuit. The efficiencies of the power factor correction circuits 42a,42b,42c are therefore increased relative to a conventional power factor correction circuit.

The power factor correction circuits 42a,42b,42c determine the current to be drawn at the input using the difference between the voltage of the electrical power supply and a reference voltage provided to a pin of the integrated circuit L6561. In order to vary the power output of the power adaptor 20, the controller 30 is therefore adapted to control the reference voltage provided to the power factor correction circuit 42a,42b,42c.

In particular, the controller 30 is adapted to output control signals that are a function of (i) the power presently available at the input 22 and (ii) a lighting profile stored in a memory of the controller 30. The memory stores a series of lighting profiles, one of which is selected by the controller as a function of a power historically available at the input 22. This enables a user to select a particular lighting profile by operating the power reduction device in a particular manner, as described in WO 2006/0 1 8604.

The reference voltage of each power transfer module 40a,40b,40c is equal to the signal from the voltage monitor 32, which is supplied via corresponding series resistors, off-set by a control signal supplied by the controller 30, which is supplied via corresponding series resistors. The controller 30 is therefore able to vary the reference voltage of the power factor correction circuit 42a,42b,42c, and hence vary the current drawn from the mains circuit, by varying the control signal that is sent to each power transfer module 40a,40b,40c.

Each power transfer module 40a,40b,40c would normally reduce the power supplied to the associated LED 30a,30b,30c as the power available at its input reduces, and hence the power available from the mains circuit reduces. However, each power transfer module 40a,40b,40c is configured to attempt to provide greater power to the LED 50a,50b,50c than is required for maximum light output, when the power reducing device 10 enables maximum power to be drawn from the mains circuit, and the controller 30 is configured to decrease the power drawn from the input 22 by the power transfer module 40a,40b,40c, so that the output of the power transfer module 40a,40b,40c is in fact equal to the power required by the LED 50a,50b,50c for maximum brightness.

Where a lighting profile requires an LED 50a,50b,50c to have a maximum brightness, even when the power reducing device 10 has reduced the power available from the mains circuit, the controller 30 outputs a control signal that increases the power drawn by the power transfer module 40a,40b,40c, so that the output of the power transfer module 40a,40b,40c remains equal to the power required by the LED 50a,50b,50c for maximum brightness. In this way, the controller 30 is able to maintain the LED 50a,50b,50c at maximum brightness until the power available from the mains circuit equals the power required by the 50a,50b,50c for maximum brightness. Any reduction in the power available to the power transfer module 40a,40b,40c below this threshold will result in a dimming of the LED 50a,50b,50c.

The controller 30 is also adapted to detect the rms voltage of the mains supply, and select an appropriate proportional relationship between the current being drawn at the input 22 of the power adaptor 20 and the voltage at the input 22 of the power adaptor 20, in order to maintain the same pre-determined average power for two different rms voltages. In particular, the controller 30 is adapted to detect whether the mains supply has an rms voltage of approximately 11 OV or an rms voltage of approximately 230V, and supply an appropriate reference signal to the multipliers of the power factor correction circuits 42a,42b,42c. In particular, the reference signal for the 11 OV rms voltage is approximately 200% of the reference signal for the 230V rms voltage.

The output of each of the power factor correction circuits 42a,42b,42c passes though a transformer that isolates the LED 50a,50b,50c from the mains circuit, and a diode that ensures that no negative current flows through the LED 50a,50b,50c.

1 0 A bulk storage capacitor is shown in parallel with the LED 50a,50b,50c, which reduces the variation in the voltage being supplied to the LED 50a,50b,50c.

However, this bulk storage capacitor is not a necessary part of the power adaptor and is only used to lower the peak currents and ripple, and hence allow a higher efficiency through the LED as it approaches DC.

The controller 30 determines the average power delivered by each power transfer module 40a,40b,40c to its corresponding LED 50a,50b,50c. The potential difference across the LED 50a,50b,50c will be regulated by the characteristics of the LED 50a,50b,50c to a particular average value, and the average current will be that determined by the power transfer module 40a,40b,40c to be necessary in order to deliver the required power to the LED 50a,50b,50c. In particular, each LED 50a,50b,50c will have a particular forward potential difference, which may fall anywhere within a tolerance range due to temperature or manufacturing variations.

Each LED 50a,50b,50c will therefore regulate the voltage of the output from the associated power transfer module 40a,40b,40c to its particular forward potential difference. Each power transfer module 40a,40b,40c will provide an average current to the connected LED 50a,50b,50c that is determined to be required to achieve the desired power, set by the controller 30.

In addition, the first power transfer module 40a is adapted to provide power to the power factor correction circuits 42a,42b,42c of each of the power transfer modules 40a,40b,40c, as well as the integrated circuits of the controller 30. The first power transfer module 40a differs from the remaining power transfer modules 40b,40c in that the transformer includes an auxiliary winding 44 that supplies power, via a rectifying diode and a bulk capacitor, to the power factor correction circuits 42a,42b,42c of the power transfer modules 40a,40b,40c, as well as the controller 30.

In this arrangement, if the output of the power factor correction circuit 42a of the first power transfer module 40a is reduced to zero, in order to fully dim the associated LED 50a, the power available from the auxiliary winding 44 to power the power factor correction circuits 42a,42b,42c and the controller 30 will also reduce to zero. For this reason, the modified power transfer module 40a includes a variable resistance circuit 47 that enables the associated LED 50a to be fully dimmed, whilst maintaining sufficient power at the auxiliary winding 44 to power the power factor correction circuits 42a,42b,42c and the controller 30. In particular, the variable resistance circuit 47 is adapted to have an increased resistance as the power supplied by the power factor correction circuit 42a to the associated LED 50a is reduced, such that the LED 50a is fully dimmed even when there remains sufficient power at the output of the power transfer module 42a to power the power factor correction circuits 42a,42b,42c and the controller 30. The variable resistance circuit 47 is also adapted to have a negligible resistance when the power supplied to the LED 50a is at medium to high powers. This arrangement therefore does not affect the efficiency of the power adaptor at medium to high powers, where efficiency is important, and this arrangement also removes the need for an additional power supply.

The variable resistance circuit 47 is provided with a control signal that is derived from the control signal supplied by the controller 30 to the power factor correction circuit 42a, along input path 30a, in order to control the resistance of the variable resistance circuit 47, in the manner described above. The path of the control signal for the variable resistance circuit 47 includes an opto-isolator 46, so that the associated LED 50a is isolated from the mains supply.

The controller 30 of the power adaptor 20 is adapted to take into account the increased power required by the first power transfer module 40a, when the associated LED 50a is fully dimmed, in order to maintain power supply to the power factor correction circuits 42a,42b,42c and the controller 30 of the power adaptor 20.

The controller 30 is also adapted to detect whether a voltage is present at the input 22 of the power adaptor, and send a control signal to the integrated circuits of the power factor correction circuits 42a,42b,42c to switch those integrated circuits to a standby mode when no voltage is detected at the input, and to an operational mode when a voltage is detected at the input. Each of the power factor correction circuits 42a,42b,42c is therefore adapted to draw power immediately, once a voltage is present at the input 22 of the power adaptor 20.

Each power transfer module 40a,40b,40c preferably also includes a fault detection circuit 50a,50b,50c that is connected between the output of the transformer and diode arrangement, and a disable pin on the integrated circuit of the power factor correction circuit 42a,42b,42c. The fault detection circuit 48a,48b,48c does not draw any power during normal operating conditions. However, in the event that an LED 50a,50b,50c stops conducting, the associated fault detection circuit 50a,50b,50c causes that power transfer module 40a,40b,40c to shut-off. The fault detection circuit 48a,48b,48c includes an opto-isolator, so that the LEDs 50a,50b,50c are isolated from the mains supply.

As discussed above, the power factor correction circuits 42a,42b,42c are adapted to draw a current that is substantially in phase with the voltage of the input power signal, and provide an output power signal of constant average power with a current substantially in phase with the voltage. In addition, the power factor correction circuits 42a,42b,42c are adapted to determine the current to be drawn at the input using the difference between the voltage of the electrical power supply and a reference voltage provided to a pin of the integrated circuit L6561. Hence, in order to vary the power output of each of the power transfer modules 40a,40b,40c, the controller 30 is adapted to control the reference voltage provided to each power factor correction circuit 42a,42b,42c. In particular, the controller 30 supplies control signals that vary the reference voltages of the power transfer modules 40a,40b,40c, the control signals being calculated using the signal received from the voltage monitoring circuit 32, and hence the average power available at the input 22 of the power adaptor.

The control signals from the controller 30 each comprise a voltage waveform having a periodic component and a constant component. Each control signal provides a reference voltage to the power transfer module 40a,40b,40c that determines the power drawn from the input 22 by that power transfer module 40a,40b,40c, and hence the power of the associated output signal.

The power transfer modules 40a,40b40c therefore each generate an output signal having a power waveform that is determined by the voltage waveform of the control signal, and this power waveform is applied as a distortion to the power drawn by the power transfer modules 40a,40b,40c from the input 22. The power waveform therefore comprises a periodic component and a constant component, which are both varied by the control signal of the controller 30 in order to determine the average power drawn from the input 22 by the power transfer module 40a,40b,40c, and hence the average power supplied to the associated LED 50a,50b,50c.

The periodic component of the control signal has a substantially triangular shape.

The controller 30 varies the constant component of the control signal, such that the peaks of the substantially triangular waveform are effectively shifted in height relative to ground, thereby providing more or less power to the LED5 50a,50b,50c, as more or less of the triangular waveform is exposed relative to ground.

Hence, for medium to high power levels, the control signal includes the complete triangular waveform of the periodic component. However, in order to provide greater dimming resolution at low output powers, and hence low light intensities, the control signal only includes the positive portion of the periodic component for a range of the lowest average voltages of the control signal, and hence a range of the lowest average powers of the output signal. In this way, the rate of change of the average voltage of the control signal, and hence the rate of change of the average power of the output signal, relative to the rate of change of the constant component of the control signal, increases as the average voltage of the control signal, and hence the average power of the output signal, increases in that range.

In addition, in order to reduce the peak power of the output signal relative to the average power (eg the RMS power), the controller 30 is adapted to (i) increase the frequency of the periodic component of the control signal, and (ii) reduce the amplitude of the periodic component of the control signal, as the constant component of the control signal is increased, and hence the average power being supplied to the associated LED 50a,50b,50c is increased. In particular, Figures 3a-3d schematically illustrate the control signals for generating output signals at 25% power level (Figure 3a), 50% power level (Figure 3b), 75% power level (Figure 3c) and 100% power level (Figure 3d).

Since the average power required to attain maximum brightness of a solid state light source is often equal, or near, the peak power for that light source, this decrease in the peak power of the output signal relative to the average power (eg the RMS power) enables a greater brightness to be achieved for a particular solid state light source. In addition, this decrease in the peak power of the output signal relative to the average power (eg the RMS power) reduces the harmonics that are drawn from the mains supply.

As discussed above, the output signal of each power transfer module 40a,40b,40c is a combination of the power waveform determined by the control signal from the controller, and the power drawn from the mains supply through the filter 24 and rectifier 26. The power waveform is therefore superimposed upon the full-wave rectified waveform of the power drawn from the input 22 by the power transfer module 40a,40b,40c.

In particular, Figures 4a-4d are measurements of the power of the output signal a power transfer module 40a,40b,40c at 25% power level (Figure 4a), 50% power level (Figure 4b), 75% power level (Figure 4c) and 100% power level (Figure 4d).

The 1/frequency value is indicated on each figure by the arrow 60. In particular, this value is 974.2ps for the 25% power level, 863.9ps for the 50% power level, 661.7ps for the 75% power level, and 349.2ps for the 100% power level.

Claims (25)

1. Claims 1. A power adaptor for a lighting unit having a solid state light source, the power adaptor comprising an input adapted for connection to a power supply, and the power adaptor being adapted to produce an output signal suitable for driving the solid state light source, wherein the power adaptor is adapted to control the average power of the output signal, such that the peak power of the output signal is reduced relative to the average power of the output signal, as the average power of the output signal is increased.
2. 2. A power adaptor as claimed in Claim 1, wherein the output signal has a power waveform that is variable to control the average power of the output signal, wherein the power waveform includes a periodic component having a frequency that is increased and an amplitude that is decreased as the average power of the 1 5 output signal is increased.
3. 3. A power adaptor as claimed in Claim 1 or Claim 2, wherein the output signal comprises the power waveform superimposed upon a power signal drawn from the input.
4. 4. A power adaptor as claimed in Claim 3, wherein the power waveform has the form of a distortion applied to a power signal drawn from the input.
5. 5. A power adaptor as claimed in Claim 4, wherein the power signal drawn from the input is full-wave rectified.
6. 6. A power adaptor as claimed in any one of Claims 3 to 5, wherein the power waveform has a frequency that is greater than the frequency of the power drawn from the input.
7. 7. A power adaptor as claimed in any Claim 6, wherein the amplitude of the power waveform is sufficiently low that it does not cause distortion of the power drawn from the input that is greater than allowed by the relevant harmonic standard.
8. 8. A power adaptor as claimed in any one of Claims 2 to 7, wherein the power waveform of the output signal includes a constant component that varies the height of the periodic waveform component relative to zero power.
9. 9. A power adaptor as claimed in Claim 8, wherein the constant component is dependent on the average power available at the input.
10. 10. A power adaptor as claimed in any Claim 9, wherein the constant component is proportional to the average power available at the input.
11. 11. A power adaptor as claimed in any Claim 9 or Claim 10, wherein the constant component is also dependent on a lighting profile stored in a memory of the power adaptor.
12. 12. A power adaptor as claimed in any one of Claims 8 to 11, wherein the constant component is controlled by a controller of the power adaptor.
13. 13. A power adaptor as claimed in any one of Claims 8 to 12, wherein within a range of the lowest average powers of the output signal, the rate of change of the average power of the output signal, relative to the rate of change of the constant component of the power waveform, increases as the average power increases in that range.
14. 14. A power adaptor as claimed in any one of Claims 2 to 13, wherein the periodic component is a triangular waveform.
15. 15. A power adaptor as claimed in any one of Claims 2 to 14, wherein the input of the power adaptor is adapted for connection to a mains power supply.
16. 16. A power adaptor as claimed in Claim 15, wherein the power adaptor comprises a power transfer module that is coupled to the input and provides the output signal suitable for driving the solid state light source, and a controller that is able to deliver a control signal to the power transfer module.
17. 17. A power adaptor as claimed in Claim 16, wherein the control signal of the controller is able to control the power drawn from the input by the power transfer module, and hence determine the power waveform of the output signal.
18. 18. A power adaptor as claimed in Claim 17, wherein the control signal is a voltage signal having the same waveform as the power waveform of the output signal.
19. 19. A power adaptor as claimed in any one of Claims 16 to 18, wherein the control signal only includes a positive portion of the periodic waveform component for a range of the lowest average powers of the output signal, such that the rate of change of the average power of the output signal, relative to the rate of change of the constant component of the control signal, increases as the average power increases in that range.
20. 20. A lighting system comprising a power adaptor as claimed in any preceding claim and a lighting unit including one or more solid state light sources.
21. 21. A lighting system as claimed in Claim 20, wherein the lighting unit is provided with a plurality of solid state light sources.
22. 22. 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 1 to 19, and one or more solid state light sources.
23. 23. A lighting unit as claimed in Claim 22, wherein the lighting unit 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.
24. 24. A lighting unit as claimed in Claim 23, wherein the connector is adapted to connect to a fitting for a conventional filament light bulb.
25. 25. A lighting unit as claimed in Claim 24, wherein the lighting unit includes a bayonet or threaded connector.