WO2008061991A1

WO2008061991A1 - A method and circuit for driving an electroluminescent lighting device

Info

Publication number: WO2008061991A1
Application number: PCT/EP2007/062591
Authority: WO
Inventors: Joseph William Mcclean; Adrian Clifford Ward
Original assignee: Advanced Modular Solutions Limited
Priority date: 2006-11-21
Filing date: 2007-11-20
Publication date: 2008-05-29
Also published as: EP2314135A1; IES20070844A2

Abstract

The present invention relates to a method and circuit for driving an electroluminescent lighting device, and more specifically, an electroluminescent (EL) panel or cell. The present invention is adapted to compensate for any reduction in alternating current passing between the electrodes as a result of capacitive reactance increases due to aging of the EL panel. The shape of the alternating current supplied by the drive circuit is also sinusoidal, thereby significantly reducing audible noise from the EL panel.

Description

A METHOD AND CIRCUIT FOR DRIVING AN ELECTROLUMINESCENT LIGHTING DEVICE

The present invention relates to a method and circuit for driving an electroluminescent lighting device, and more specifically, an electroluminescent (EL) panel or cell.

EL panels are typically used in backlighting applications for devices including mobile telephones, wrist watches, computer monitors and the like. Such EL panels are generally fabricated as laminate structures comprising a dielectric layer coated with a phosphor layer, whereby this multilayered structure is sandwiched between a transparent electrode and an opaque electrode. Electrical connections to the electrodes are provided by an arrangement of connection terminals whereby on the passage of alternating electric current between the terminals the phosphor layer between the electrodes emits light through the transparent electrode. Application of an alternating voltage across the connection terminals causes an alternating electric current to flow between the connection terminals. The intensity of the emitted light is proportional to the level of alternating electric current flowing between the connection terminals, and the energy of the light emitted is proportional to the frequency of the alternating electric current flowing. EL panels are generally operated at alternating voltages in excess of about 120V and at frequencies of 200Hz to 2000Hz. The level of alternating current and the associated frequency determine the intensity and energy of the emitted light so that the higher the current and frequency, the higher the intensity and energy of the emitted light.

However, several problems exist with EL panels and the prior art systems for driving them. For example, the light intensity of an EL panel will decrease with service life. This phenomenon is known as "ageing" and is the result of the capacitive reactance of the EL panel increasing, whereby such an increase reduces the level of alternating current passing between the electrodes, and hence the intensity of the light emitted. A further problem relates to the level of audible noise that is emitted from an EL panel as a result of the alternating current flowing through the phosphor and dielectric layers. Such emissions are undesirable and result from the phosphor molecules vibrating back and forth at the same frequency as the alternating current flowing through them. This audible noise increases with the time rate of change of current (dl/dt). One approach to eliminating this audible noise is to supply a sinusoidal current waveform to drive the EL panel. However, drivers having the means required for producing such sinusoidal current waveforms are impractical as a result of the need for very large and bulky resonant LC circuits to be used in conjunction with them.

United States patent publication no. US2003/0052615 (Lynch et al.) discloses a circuit for driving an EL panel. In one of the embodiments described, the circuit includes a high voltage DC power supply generating a high voltage square wave having a frequency equal to that of the operating frequency of the EL panel (typically 200Hz to 2000Hz), a switching H-Bridge circuit and a square wave generator. This high voltage square wave is coupled to the EL panel through a reactive element, such as a resistor. The circuit described in this embodiment will, to some extent, limit the audible noise from an EL panel by converting the high voltage square waveform into an exponentially increasing and reducing waveform according to the RC time constant of the reactive element and the capacitance value of the EL, thereby reducing the time rate of change of current (dl/dt) in the EL panel. The circuit will, to some extent, partially compensate for the loss of brightness in the EL panel due to ageing. However, as a result of the presence of the series reactance, significant ohmic power losses will occur in all but the very lowest power EL panel applications, therefore limiting its use for driving EL panels of only a few square centimeters.

In a further embodiment described in United States patent publication no. US2003/0052615, the driving square waveform is shaped by an LC circuit and offset by an offset circuit before the H-Bridge. In this arrangement the H-Bridge, which is again connected to a high voltage DC power supply, is now operated as a linear amplifier instead of a switching amplifier. As the waveform has already been shaped there is no need for a series reactance before connecting to the EL panel. Although this embodiment eliminates the need for a reactive element, and therefore eliminates ohmic power losses, because the H-Bridge is now operating as a linear amplifier significant semi-conductor losses will occur in the H-Bridge. This again has the effect of limiting the use of this circuit for driving EL panels of only a few square centimeters. In addition, the circuit disclosed has no means to compensate for the loss of brightness in the EL panel due to ageing.

The prior art has thus primarily focused on using the capacitive properties of EL panels in conjunction with series reactance to shape the voltage waveform applied to the EL panel. Furthermore, such prior art has also focused on illuminating EL panels having area dimensions in the region of only a few cm² to a few hundreds of cm² as a result of ohmic losses in the series reactance. This effectively excludes the use of these drive systems with larger EL panel areas.

It is a therefore an object of the present invention to provide a method and a circuit for driving an EL panel which goes at least some way toward overcoming the above problems and/or which will provide the public and/or industry with a useful alternative.

It is acknowledged that the term 'comprise' may, under varying jurisdictions be provided with either an exclusive or inclusive meaning. For the purpose of this specification, and unless otherwise noted explicitly, the term comprise shall have an inclusive meaning - i.e. that it may be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components. Accordingly, the term 'comprise' is to be attributed with as broader interpretation as possible within any given jurisdiction and this rationale should also be used when the terms 'comprised' and/or 'comprising' are used.

Further aspects of the present invention will become apparent form the ensuing description which is given by way of example only.

According to the invention, there is provided a drive circuit for an electroluminescent (EL) panel, the drive circuit comprising:

an output terminal coupled to the EL panel;

a power processing circuit coupled to a high voltage power supply, a pulse width modulation circuit and the output terminal;

a waveform generating circuit coupled to the pulse width modulation circuit, the waveform generating circuit for generating a sinusoidal waveform having a variable amplitude and/or a variable frequency, the pulse width modulation circuit for converting the sinusoidal waveform to a pulse width modulated square waveform which, when applied to the power processing circuit, is amplified to a pulse width modulated square waveform with the same or substantially the same voltage amplitude as the output of the high voltage power supply,

the power processing circuit further adapted for recovering the sinusoidal wave output from the pulse width modulated square wave voltage output for supply to the output terminal, and

means coupled to the power processing circuit and the waveform generating circuit for maintaining a constant sinusoidal current flow at the output terminal.

The drive circuit of the present invention supplies an alternating current to the EL panel that remains constant over the lifetime of the EL panel, thereby regulating the light output from the EL panel to an essentially constant value so as to maintain constant brightness over its lifetime. The present invention is adapted to compensate for any reduction in alternating current passing between the electrodes as a result of capacitive reactance increases due to aging of the EL panel.

The shape of the alternating current supplied by the drive circuit is also sinusoidal, thereby significantly reducing audible noise from the EL panel. Additionally, as a result of the absence of series ohmic elements the power efficiency of the drive circuit is significantly increased and EL panels having areas of hundreds of thousands of square centimetres are able to be efficiently powered by the present invention. Such efficiencies are expected to be in the region of more than 90%, and approaching 94 to 96%. The use of a drive circuit which supplies a sinusoidal current to an EL panel without the need for a very large and bulky resonant LC circuit will thus effectively eliminate unwanted audible noise.

In another embodiment of the invention, the means for maintaining a constant sinusoidal current flow at the output terminal comprises a current control circuit coupled to a system control circuit and one of the power processing circuit and the output terminal, the system control circuit being further coupled to the waveform generating circuit.

In another embodiment of the invention, the current control circuit comprises a current sensing circuit and a sensed current processing circuit, the current sensing circuit for measuring the flow of alternating current in the EL panel, and the sensed current processing circuit for converted the sensed AC current to a DC voltage. Preferably, the system control circuit compares the DC voltage level with a reference voltage level and generates an error DC voltage which is supplied to the waveform generating circuit. The error DC voltage is thus proportional a level of deviation of sinusoidal current flow in the EL panel from a predefined level of sinusoidal current flow.

Preferably, the amount of the error DC voltage is proportional to an increase in the capacitive reactance of the EL panel due to aging of the EL panel.

In another embodiment of the invention, the waveform generating circuit, on receiving the error DC voltage, adapts the amplitude and/or the frequency of the sine wave according to the error DC voltage to maintain the current in the EL at a constant current.

Preferably, the DC voltage signal is an analog signal.

In another embodiment of the invention, the power processing circuit comprises a switching circuit coupled to a filtering circuit.

Preferably, the filtering circuit is a low pass LC filter.

Preferably, the filtering circuit only passes sinusoidal waves having a frequency the same as a frequency of the sinusoidal waveform generated by the waveform generating circuit.

In another embodiment of the invention, the switching circuit is a four quadrant synchronous buck converter operating in either full bridge or half bridge configuration.

Preferably, in a full bridge configuration the EL panel is connected between two arms of a bridge connected push pull transistor pairs driven in anti phase to develop a peak to peak differential voltage of twice the supply voltage.

Preferably, in the half bridge configuration the EL panel is connected between one arm of a push pull transistor pair and a capacitor network, whereby the voltage across the EL panel cannot exceed the supply voltage.

In an alternative embodiment of the invention, the switching circuit is a two quadrant full bridge producing half wave sinusoidal wave voltage outputs.

Preferably, the two quadrant full bridge comprises switch timing means.

In a further alternative embodiment of the invention, the switching circuit is a synchronous buck-boost converter operating in non-isolated or isolated flyback converter mode.

Preferably, the EL panel is connected directly to an energy storage inductor via a bidirectional current switch. Preferably, the energy storage inductor has an isolating winding.

Preferably, the buck-boost converter is operable in two or four quadrant modes.

In one embodiment of the invention, the output terminal is galvanically isolated from the high voltage power supply by means of an off line isolated power supply.

In an alternative embodiment of the invention, the output terminal is directly connected via either a simple rectification circuit or pre-regulator circuit.

In a still further embodiment of the invention, the power processing circuit is directly connected to a DC voltage source, such as one of a battery and a photo-voltaic cell and a DC power supply.

In another embodiment of the invention, the system control circuit is an analog circuit.

Preferably, the system control circuit forms, with the current control circuit, the PWM circuit and the power processing circuit, a closed loop feedback system, to continuously maintain a constant sinusoidal current flow at the output terminal.

In another embodiment of the invention, the system control circuit comprises a negative feedback amplifier.

In an alternative embodiment of the invention, the system control circuit comprises a nonlinear comparator amplifier to control the sinusoidal current flow at the output terminal between a higher and lower hysteresis band. In an alternative embodiment of the invention, the system control circuit is a digital circuit operable to use digital signal processing techniques to convert the output of the current control circuit to a digital format to control the sinusoidal current flow at the output terminal using digital control techniques.

In another embodiment of the invention, the current sensing circuit is adapted to output a voltage proportional to the drive current of the EL panel.

Preferably, the current sensing circuit comprises at least one resistor located adjacent to the output terminal and/or an output current path of the power processing circuit, said at least one resistor adapted for developing a voltage proportional to the current of the EL panel.

Preferably, the resistor is a discrete component.

Alternatively, the resistors are used for switching transistor internal resistance.

In an alternative embodiment of the invention, the current sensing circuit comprises at least one transformer.

In a further alternative embodiment of the invention, the current sensing circuit comprises a Hall effect device for measuring the magnetic field strength around a conductor carrying current flow to the output terminal

In another embodiment of the invention, the sensed current processing circuit comprises means for processing the alternating output voltage of the current sensing circuit into a DC voltage signal.

Preferably, the error DC voltage has an average, peak or quasi peak value of the DC voltage signal.

In another embodiment of the invention, the sensed current processing circuit comprises either passive or active integration techniques to produce a voltage that has an average value of the applied alternating voltage waveform derived from the current sensing circuit. In an alternative embodiment, in which the sensed current processing circuit comprises using one or more of:

single or multiple pole resistor/capacitor networks;

active filters, a discrete or integrated combination of operational amplifiers and resistor/capacitor feedback networks;

switched capacitor filters, an integrated amplifier and switched capacitor network;

digital signal processing (DSP) an analog to digital conversion process that computes mathematically the average or RMS value of the alternating output voltage;

a switch and capacitor that may be controlled to produce an output voltage equal to the applied input at any particular point in time;

a digitally controlled version of the analogue circuit, and

full or half wave rectification with output capacitor to produce a voltage equal to the peak value of the applied alternating output voltage.

According to a further aspect of the invention, there is provided a method for driving an electroluminescent (EL) panel, the method comprising using a drive circuit having an output terminal coupled to the EL panel and a power processing circuit coupled to a high voltage power supply and the output terminal, the method comprising the steps of:

generating a sinusoidal waveform having a variable amplitude and/or a variable frequency,

converting the sinusoidal waveform to a pulse width modulated square waveform,

applying the pulse width modulated square waveform to the power processing circuit to generate an amplified pulse width modulated square waveform with the same or substantially the same voltage amplitude as the output of the high voltage power supply,

recovering the sinusoidal wave output from the pulse width modulated square wave voltage output for supply to the output terminal, and

maintaining a constant sinusoidal current flow at the output terminal.

In another embodiment of the invention, the method comprises the further step of measuring the flow of alternating current in the EL panel, and converting the sensed AC current to a DC voltage.

In another embodiment of the invention, the method comprises the further step of comparing the DC voltage level with a reference voltage level and generating an error DC voltage which is supplied to the waveform generating circuit.

In another embodiment of the invention, the method comprises the further step of adapting the amplitude and/or the frequency of the sine wave generated by the waveform generator according to the error DC voltage to maintain the current in the EL at a constant current. According to a further aspect of the invention, there is provided a device incorporating a drive circuit configured according to a previous aspect.

According to a further aspect of the invention, there is provided a drive circuit for an electroluminescent (EL) panel, the circuit comprising an output terminal coupled to the EL panel and a power processing circuit coupled to a high voltage power supply and the output terminal, the circuit further comprising:

means for generating a sinusoidal waveform having a variable amplitude and/or a variable frequency,

means for converting the sinusoidal waveform to a pulse width modulated square waveform,

means for applying the pulse width modulated square waveform to the power processing circuit to generate an amplified pulse width modulated square waveform with the same or substantially the same voltage amplitude as the output of the high voltage power supply,

means for recovering the sinusoidal wave output from the pulse width modulated square wave voltage output for supply to the output terminal, and

means for maintaining a constant sinusoidal current flow at the output terminal.

According to a further aspect of the invention, there is provided a device incorporating a drive circuit as above.

Preferably, the device comprises an EL panel driven by the drive circuit.

The invention will hereinafter be more particularly described with reference to the accompanying drawings which show, by way of example only, embodiments of a driver for an electroluminescent lighting device according to the invention.

Fig.1 is a simplified diagrammatic of a characteristic EL panel;

Fig. 2 is a block schematic showing the components in a drive circuit for an EL panel according to the present invention;

Fig. 3 is a block schematic showing a circuit formed in accordance with an embodiment of the invention;

Fig. 4a is a schematic circuit drawing of a high voltage power source shown in Fig.3;

Figs. 4b to 4g are combined block schematic circuit drawings showing alternative configurations for implementing the high voltage power source of the present invention;

Figs. 5a to 5d are schematic circuit drawings of the additional components shown in Fig.3; Figs. 6a to 6c are combined block schematic circuit drawings showing alternative configurations for implementing a current sensing circuit according to the present invention, Figs. 7a to 7c are combined block schematic circuit drawings showing alternative configurations for implementing a system control circuit according to the present invention,

Figs. 8a to 8e are combined block schematic circuit drawings showing alternative configurations for implementing a power processing circuit according to the present invention;

Figs. 9a to 9f are combined block schematic circuit drawings showing alternative configurations for implementing a sensed current processing circuit according to the present invention;

Fig. 10 is a graph showing the output of the oscillator circuit at two alternate frequencies; and Fig. 11 is a graph showing the PWM switch output voltage and the demodulated sine wave output voltage that is applied to the EL panel.

Referring to the drawings, and initially to Fig. 1 , there is shown an EL panel 2 fabricated from a dielectric layer 17 coated with a phosphor layer 12 The multilayered structure shown is sandwiched between a transparent electrode 13 and an opaque electrode 14 Electrical connections to the electrodes are achieved by connection terminals 15 and 16 respectively

Fig. 2 is a block schematic showing the components in a drive circuit 1 for powering an EL panel 2 according to the present invention. The drive circuit comprises a power processing circuit 4 coupled to a high voltage power supply 5, a pulse width modulation circuit 11 and the output terminal 3. Also shown is a waveform generating circuit 6 coupled to the pulse width modulation circuit 11 , the waveform generating circuit 6 for generating a sinusoidal waveform having a variable amplitude and/or a variable frequency. The pulse width modulation circuit 11 converts the sinusoidal waveform to a pulse width modulated square waveform which, when applied to the power processing circuit 4, is amplified to a pulse width modulated square waveform with the same or substantially the same voltage amplitude as the output of the high voltage power supply 5. The power processing circuit

4 is further adapted for recovering the sinusoidal wave output from the pulse width modulated square wave voltage output for supply to the output terminal 3.

Also shown are means coupled to the power processing circuit 4 and the waveform generating circuit 6 for maintaining a constant sinusoidal current flow at the output terminal 3. Such means comprise a current control circuit 8 coupled to a system control circuit 7 and one of the power processing circuit 4 and the output terminal 3. The system control circuit 7 is further coupled to the waveform generating circuit 6.

The current control circuit 8 comprises a current sensing circuit 9 and a sensed current processing circuit 10, the current sensing circuit 9 for measuring the flow of alternating current (AC) in the EL panel 2, and the sensed current processing circuit 10 for converted the sensed AC current to a DC voltage signal. The DC voltage signal is an analog signal.

The system control circuit 7 compares the DC voltage level with a reference voltage level and generates an error DC voltage which is supplied to the waveform generating circuit 6. The error DC voltage is thus proportional a level of deviation of sinusoidal current flow in the EL panel from a predefined level of sinusoidal current flow. Conversely, the amount of the error DC voltage is proportional to an increase in the capacitive reactance of the EL panel 2 due to aging of the EL panel 2.

In operation, the waveform generating circuit 6, on receiving the error DC voltage, adapts the amplitude and/or the frequency of the sine wave according to the error DC voltage to maintain the current in the EL panel 2 at a constant current. Such adjustments to the amplitude and/or the frequency of the sine wave are occurring continuously and take account of any increases in the capacitive reactance of the EL panel 2 due to aging and thus compensate for any reduction in alternating current passing between the EL panel 2 electrodes. In this way, a constant sinusoidal current flow is maintained at the output terminal 3.

Fig. 3 is a block schematic showing a drive circuit 1 formed in accordance with an exemplary embodiment of the invention. In the embodiment shown, a high voltage DC power source 41 is connected to an H-Bridge 42. A low voltage sine wave having a frequency (for example, 1000Hz) that is the same as that of the operating frequency of the EL panel 46 is generated by an oscillator circuit 43. The output of the oscillator circuit 43 is connected to a low voltage/high frequency (for example, 125kHz) pulse width modulator (PWM) circuit 44. The PWM circuit 44 outputs a low voltage/high frequency pulse width modulated square wave having a modulation frequency the same as that of the oscillator circuit 43. The PWM square wave is then amplified to the voltage level of the high voltage DC power source 41 by the H-Bridge 42. This high voltage pulse width modulated square wave is connected to a low pass LC filter 45 which only passes the high voltage sine wave having a frequency that is the same as the operating frequency of the oscillator circuit 43. This high voltage sine wave is then connected to the EL panel 46. As a result of the waveform across the LC circuit being switching at a very high frequency (in this embodiment, 125kHz) the values of the inductance, L, and the capacitance, C, can be made very small. The values of the L and C can be made arbitrarily small by increasing the frequency of the PWM circuit 44.

The high frequency current flowing in the H-Bridge 42 is a pulse width modulated version of the sine wave current flowing in the EL panel 46. A current measuring circuit 48 measures the high frequency current flowing in the H-Bridge 42. The current measuring circuit 48 filters out an analog signal that is proportional to the sine wave current that is flowing in the EL panel 46. This analog signal is compared to a fixed reference level current and the difference is amplified by a negative feedback amplifier 49. This amplified output is connected into the PWM circuit 44 completing a closed loop feedback and ensuring that the current in the EL panel 46 is maintained at a constant value. The overall circuit arrangement ensures that the current flowing in the EL panel 46 is maintained constant over the lifetime of the EL panel 46, thereby ensuring that the brightness of the EL panel 46 also remains constant over that lifetime. Furthermore the circuit arrangement ensures that the current flowing in the EL 46 is sinusoidal, and this minimises audible noise.

The components referred to in the exemplary embodiment of Fig. 3 provide an example of components which may be used to implement the drive circuit of the present invention. Each sub-circuit, or block representing such sub-circuits, shown in Fig. 3 corresponds to a sub-circuit described above in Fig. 2. The following table provides a summary of this correspondence:

Alternative implementations, not limited to the implementation shown in Fig. 3, are also envisaged for the present invention, and suitable components and circuits for such implementations are detailed below. Additional details of the components shown in Fig. 3 are also described further.

The high voltage DC power source 41 is shown in Fig. 4a as an isolated single ended primary inductance converter (SEPIC) producing a maximum output power of 170W. This topology combines the advantages of both the boost and flyback topologies, having key benefits including single stage design; boost input circuit generates a very low input ripple current, reducing the complexity of the input filter requirements; the switch current has a peak value of line D and allows the use of a current transformer for current sensing, and the boost inductor and output transformer are integrated into a single device.

Referring to the circuit shown in Fig. 4a, the AC mains is applied to the power supply via FS1 , R62, a 280V 12J VDR and input filter L1 ,C1 ,C2,C3,C4. This voltage is rectified by

D13,D14,D15 and D16 and the resulting full wave rectified voltage applied to winding 6 of

TX2. This winding is the input inductor section of the transformer and is switched at

20OkHz by the PFC control IC U2. Input current is built up in this inductor and programmed to follow the wave shape of the rectified mains voltage. At switch turn off this current is forced to flow through the transformer section coupling capacitor C23 and into the transformer primary winding W3. The voltage developed across this winding is regulated to 200V and offset by the input voltage. The output voltages and currents and now set by the transformer turns ratio. As the transformer has been designed to have a very low leakage inductance ,and the inductor section W6 a high leakage inductance, the transformer magnetising current is "steered" to flow into the transformer and not the input circuit resulting in a greatly reduced switching frequency component of input current.

The power supply generates the following voltages: 2 x 275V for the class D amplifier output stage power rails; +/- 15V for the control system bias voltages, and +15V for the PFC bias voltage. The peak inverse voltage of the 275V output rails rectifier will be 800V, two 600V 1A silicon carbide (SiC) series connected rectifiers are used in this application. The SiC rectifiers have essentially no reverse recovery currents, greatly reducing their turn off power loss and reducing the turn on power loss in the primary switch. The output voltages are filtered by two 350V series connected 10OuF electrolytic capacitors.

As shown in Figs. 4b to 4e, the high voltage DC power source 41 can be mains isolated, or not mains isolated. Fig. 4b shows a mains isolated configuration in which the output stage is galvanically isolated from the mains supply by means of an off line isolated power supply. In Figs. 4c and 4d, non isolated configurations are shown in which the output stage is directly connected via either a simple rectification circuit or pre-regulator circuit. Fig. 4e shows a DC input in which the power processing circuit is directly connected to a DC voltage source, such as a battery, photo-voltaic cell or DC power supply. Fig. 4f and 4g show the primary and secondary side respectively of the power supply 41 shown in Fig. 4a.

With reference now to Figs. 5a and 5d, the oscillator circuit 43 is a current controlled oscillator, used to generate the required sine wave output signal to the Class D amplifier of the PWM circuit 44. This oscillator 43 is formed by U5A, U5B, C31 , C32, R27, R60, R28, C52 and R29. The current that flows in Q4 determines the voltage amplitude of the sine wave. This circuit is a second order filter with sustaining feedback. Figs. 5c and 5d show the circuit of Fig. 5a in more detail. Fig. 5c shows the peak current limit, dead time and "blink" function circuits. Fig. 5d show the oscillator, pulse width modulator and system control circuits.

The graph in Fig. 10 shows the output of the oscillator 43 at two alternate frequencies. On application of power the circuit 43 will generate an exponentially decaying sine wave at the set frequency. This voltage is amplified with sufficient gain by U5A to saturate its output and produce a rail to rail square wave. This square waveform is then applied to the second order filter to generate a sine wave output.

The EL panel 46 output current is controlled by the use of a closed loop system including the current measuring circuit/current sensor 48 and the negative feedback amplifier 49. In operation, the output load current flows through in the positive direction current sense resistors R113.R108 and in the negative direction R109.R114. The resulting sensed voltages are applied to an integrator network R107.C107 R108.C108 which produce an average value of the positive and negative peak sensed voltage.

These voltages are applied to a differential integrating amplifier comprising U5D, R42, R59, C40 and R43, R38, C39. The output of this stage is now positive and equal to the absolute sum of the average values of the applied input voltages. A further filter stage R36.C38 at the output of this amplifier reduces the ripple voltage content to an acceptable value. The time constants of the 3 integrators are set to ( 1/f Signal) 1.53ms to ensure fast response with a low overall ripple voltage content. This signal is the analog signal that is proportional to the sine wave current that is flowing in the EL panel 46.

The analog signal is compared to a current demand signal generated by resistor network R55, potentiometer R61 and R56. The maximum output voltage of this network can be pre-set to 3 values 1.2, 1.0, 0.8V by selecting the appropriate switch setting (SW2) to give a high, nominal or low value of maximum output current. Amplifier U5C and transistor Q4

form the negative feedback amplifier 49. The current demand signal applied to the positive amplifier input will cause the control transistor to increase the oscillator output signal until the sensed feedback voltage equals the demand voltage. The control loop regulates the output current at this value.

Fig. 6a shows one implementation of the current sensing circuit 9 which comprises the use of a current sense resistor. In this method resistors are placed in the output or switch current path, and will develop a voltage across them proportional to the current. The resistor may be a discrete component or a switching transistor internal resistance. Fig. 6b shows a further implementation of the current sensing circuit 9 in which a current sense transformer is used. In this arrangement, the resistor is replaced by the transformer for greater flexibility. Fig. 6c shows a still further implementation in which a Hall effect device is used. This method uses an integrated circuit designed to measure the magnetic field strength around a conductor carrying a current. The device produces an output voltage calibrated to be proportional to the current flowing in the conductor.

The output voltage from the oscillator circuit 43 is applied to the input of U6 (MAX4295) in the PWM circuit 44. This is a class D audio amplifier with complimentary output stages. The frequency is internally set to 125kHz, and is constant over the full duty cycle range. The PWM outputs are applied to U1 1A,U11 B. These 3 input AND gates and R49,R50,C48,C49 form a 100ns of dead time between switching cycles to ensure there is no cross conduction and shoot through current in the power amplifier output stage.

Fig. 7a shows an alternative implementation of the system control circuit 7 and involves the use of a closed loop feedback system employing a control reference, the output of the current sense signal processing stage and a linear control amplifier, to continuously control the output current via the power processing circuit 4.

Fig. 7b shows an implementation showing the use of a closed loop feedback system employing a control reference, the output of the sensed current signal processing circuit and a non-linear (comparator) amplifier, to control the output current between a higher and lower hysteresis band.

Fig. 7c shows a further implementation of the system control circuit 7 involving the use digital control means. The use of such DSP techniques may be used to convert the analog output of the sensed current processing circuit to a digital format to control the output current using digital control techniques.

The H-bridge 42, the LC circuit 45 and the mosfet high and low side driver 47 combine form a class D or PWM amplifier switching at the control IC (U6) frequency of 125kHz. The H-bridge output stage 42, 45 has been designed to produce a maximum of 350V RMS with 0.5A loading._The key of this design is in the use of SiC rectifiers D103, D105, D106, D110 to serve as free wheel diodes during the current commutating cycle of each half bridge stage. These diodes have essentially no reverse stored charge and thus no reverse current flows at turn off. The advantage of this is the reduction in both turn on losses of the complimentary MOSFET and turn off losses in the diode. More importantly it is also now possible to sense independently the current in both switching cycles to facilitate the current mode control system without additional circuitry.

The LC circuit 45 is a low pass LC filter. It filters the sine wave from the high frequency switching waveform generated by the H-Bridge 42 as shown in the graph in Fig. 11.

The graph in Fig. 11 shows PWM switch output voltage and the demodulated sine wave output voltage that is applied to the EL panel.

Unlike prior art where the RC or LC filters act on a square wave or exponential wave at the operation frequency (for example 1kHz), in the current invention the LC filter acts on a high frequency PWM switching square wave and filters out the operation frequency sine wave. This leads to the direct advantage of reducing the bulky RC or LC in the prior art by

a factor of switching frequency divided by the operation frequency, in this case by a factor of 125. (125kHz/1 kHz). This LC filter can be made arbitrarily small by increasing the PWM H-Bridge switching frequency.

The power processing circuit 4, shown in Fig. 3 as comprising an H-bridge 42 and a low pass LC circuit 45, may also be implemented in alternative ways. Figs. 8a and 8b show alternative implementations of the power processing circuit 4 involving the use of a four quadrant synchronous buck converter operating in either full (H bridge) or half bridge configuration. Shown in Fig. 8b is a bi-phase H-bridge using a two quadrant full bridge producing half wave outputs. Schematically, this is the same as the four quadrant version shown in Fig. 8a but includes switch timing to improve efficiency. In this arrangement the EL panel is connected between the arms of a bridge connected push pull transistor pair, these are driven in anti phase to develop a peak to peak differential voltage of twice the supply voltage. In the half bridge version shown in Fig. 8b the EL panel is connected between one arm of a push pull transistor pair and a capacitor network. The voltage across the lamp cannot exceed the supply voltage.

Fig. 8c and 8d show still further alternative implementations of the power processing circuit 4. Fig. 8c involving a boost derived converter in which the EL panel is connected directly to an energy storage inductor via a bi-directional current switch. Conversely, in the isolated version shown in Fig. 8d the energy storage inductor has an isolating winding included. This configuration is operable in both two and four quadrant modes. Fig. 8e shows a full H-bridge version of the power processing circuit 4.

Figs. 9a and 9f show alternative implementations of the sensed current processing circuit 10. Figs. 9a to 9c show signal integration implementations involving methods using either passive or active integration techniques to produce a voltage that has an average value of the applied alternating voltage waveform derived from the current sensor. Such integration methods may be implemented using single or multiple pole resistor/capacitor networks; active filters, a discrete or integrated combination of operational amplifiers and resistor/capacitor feedback networks; switched capacitor filters, an integrated amplifier and switched capacitor network; and digital signal processing (DSP) an analogue to digital conversion process that computes mathematically the average or RMS value of the input signal. Figs. 9d and 9e show sample and hold methods. Such methods may be implemented using an analog sample and hold method involving a switch and capacitor that may be controlled to produce an output voltage equal to the applied input at any particular point in time. Alternatively, digital sample and hold method, which is a digitally controlled version of the analog circuit, may be used. Fig. 9f shows a rectification and filtering method involving full or half wave rectification with output capacitor. This is essentially a simpler version of the sample and hold method used to produce a voltage equal to the peak value of the applied input signal.

It is to be understood that the invention is not limited to the specific details described herein which are given by way of example only and that various modifications and alternatives are possible without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS:

1. A drive circuit for an electroluminescent (EL) panel, the drive circuit comprising:

an output terminal coupled to the EL panel;

2. A drive circuit as claimed in Claim 1 , in which the means for maintaining a constant sinusoidal current flow at the output terminal comprises a current control circuit coupled to a system control circuit and one of the power processing circuit and the output terminal, the system control circuit being further coupled to the waveform generating circuit.

3. A drive circuit as claimed in Claim 2, in which the current control circuit comprises a current sensing circuit and a sensed current processing circuit, the current sensing circuit for measuring the flow of alternating current in the EL panel, and the sensed current processing circuit for converted the sensed AC current to a DC voltage.

4. A drive circuit as claimed in any Claim 3, in which the system control circuit compares the DC voltage level with a reference voltage level and generates an error DC voltage which is supplied to the waveform generating circuit.

5. A drive circuit as claimed in Claim 3 or 4, in which the amount of the error DC voltage is proportional to an increase in the capacitive reactance of the EL panel due to aging of the EL panel.

6. A drive circuit as claimed in Claims 4 or 5, in which the waveform generating circuit, on receiving the error DC voltage, adapts the amplitude and/or the frequency of the sine wave according to the error DC voltage to maintain the current in the EL at a constant current.

7. A drive circuit as claimed in Claim 6, in which the DC voltage is applied to the system control circuit as an analog signal.

8. A drive circuit as claimed in any previous claim, in which the power processing circuit comprises a switching circuit coupled to a filtering circuit.

9. A drive circuit as claimed in Claim 8, in which the filtering circuit is a low pass LC filter.

10. A drive circuit as claimed in Claim 9, in which the filtering circuit only passes sinusoidal waves having a frequency the same as a frequency of the sinusoidal waveform generated by the waveform generating circuit.

11. A drive circuit as claimed in Claims 8 to 10, in which the switching circuit is a four quadrant synchronous buck converter operating in either full bridge or half bridge configuration

12. A drive circuit as claimed in Claim 11 , in which, when in a full bridge configuration, the EL panel is connected between two arms of a bridge connected push pull transistor pairs driven in anti phase to develop a peak to peak differential voltage of twice the supply voltage.

13. A drive circuit as claimed in Claim 1 1 , in which, when in a half bridge configuration, the EL panel is connected between one arm of a push pull transistor pair and a capacitor network, whereby the voltage across the EL panel cannot exceed the supply voltage.

14. A drive circuit as claimed in Claims 8 to 10, in which the switching circuit is a two quadrant full bridge producing half wave sinusoidal wave voltage outputs.

15. A drive circuit as claimed in Claim 14, in which the two quadrant full bridge comprises switch timing means.

16. A drive circuit as claimed in Claims 8 to 10, in which the switching circuit is a synchronous buck-boost converter operating in non-isolated or isolated fly-back converter mode

17. A drive circuit as claimed in Claims 15 or 16, in which the buck-boost converter is operable in two or four quadrant modes.

18. A drive circuit as claimed in any previous claim, in which the EL panel is connected directly to an energy storage inductor via a bi-directional current switch.

19. A drive circuit as claimed in Claim 18, in which the energy storage inductor has an isolating winding.

20. A drive circuit as claimed in any previous claim, in which the output terminal is galvanically isolated from the high voltage power supply by means of an off-line isolated power supply.

21. A drive circuit as claimed in any of Claims 1 to 20, in which the output terminal is directly connected via either a simple rectification circuit or pre-regulator circuit.

22. A drive circuit as claimed in any of Claims 1 to 23, in which the power processing circuit is directly connected to a DC voltage source, such as one of a battery and a photovoltaic cell and a DC power supply.

23. A drive circuit as claimed in Claims 2 to 22, in which the system control circuit forms, with the current control circuit, the PWM circuit and the power processing circuit, a closed loop feedback system, to continuously maintain a constant sinusoidal current flow at the output terminal.

24. A drive circuit as claimed in Claims 2 to 22, in which the system control circuit comprises a negative feedback amplifier to continuously control the sinusoidal current flow at the output terminal via the power processing circuit.

25. A drive circuit as claimed in Claims 2 to 22, in which the system control circuit comprises a non-linear comparator amplifier to control the sinusoidal current flow at the output terminal between a higher and lower hysteresis band.

26. A drive circuit as claimed in Claims 2 to 22, in which the system control circuit is a digital circuit operable to use digital signal processing techniques to convert the output of the current control circuit to a digital format to control the sinusoidal current flow at the output terminal using digital control techniques.

27. A drive circuit as claimed in Claims 3 to 26, in which the current sensing circuit comprises at least one resistor located adjacent to the output terminal and/or an output current path of the power processing circuit, said at least one resistor adapted for developing a voltage proportional to the current.

28. A drive circuit as claimed in Claims 3 to 26, in which the current sensing circuit comprises at least one transformer.

29. A drive circuit as claimed in Claims 3 to 26, in which the current sensing circuit comprises a Hall effect device for measuring the magnetic field strength around a conductor carrying current flow to the output terminal

30. A drive circuit as claimed in Claim 3 to 26, in which the sensed current processing circuit comprises either passive or active integration techniques to produce a voltage that has an average value of the applied alternating voltage waveform derived from the current sensing circuit.

31. A drive circuit as claimed in Claim 29, in which the sensed current processing circuit comprises using one or more of: single or multiple pole resistor/capacitor networks;

a digitally controlled version of the analogue circuit, and

32. A drive circuit for an electroluminescent (EL) panel, the circuit comprising an output terminal coupled to the EL panel and a power processing circuit coupled to a high voltage power supply and the output terminal, the circuit further comprising:

means for recovering the sinusoidal wave output from the pulse width modulated square wave voltage output for supply to the output terminal, and means for maintaining a constant sinusoidal current flow at the output terminal.

33. A method for driving an electroluminescent (EL) panel, the method comprising using a drive circuit having an output terminal coupled to the EL panel and a power processing circuit coupled to a high voltage power supply and the output terminal, the method comprising the steps of:

converting the sinusoidal waveform to a pulse width modulated square waveform,

maintaining a constant sinusoidal current flow at the output terminal.

34. A method for driving an electroluminescent (EL) panel as claimed in Claim 33, comprising the further step of measuring the flow of alternating current in the EL panel, and converting the sensed AC current to a DC voltage.

35. A method for driving an electroluminescent (EL) panel as claimed in Claim 33 or 34, the method comprising comparing the DC voltage level with a reference voltage level and generating an error DC voltage which is supplied to the waveform generating circuit.

36. A method for driving an electroluminescent (EL) panel as claimed in Claims 32 to 35, the method comprising the further step of adapting the amplitude and/or the frequency of the sine wave generated by the waveform generator according to the error DC voltage to maintain the current in the EL at a constant current.

37. A device comprising a drive circuit as claimed in any one of claims 1 to 32.

38. A device as claimed in Claim 37, further comprising an EL panel driven by the drive circuit.

39. A drive circuit for an electroluminescent (EL) panel substantially as herein described with reference to and as illustrated in the accompanying drawings.

40. A method for driving an electroluminescent (EL) panel substantially as herein described with reference to and as illustrated in the accompanying drawings.