NZ500773A - Resonant power supply for a capacitative electroluminescent panel - Google Patents

Abstract

An electroluminescent display power supply is supplied with DC power at input terminals 101, 102. Terminal 101 is connected to the center-tap of three terminal inductor113, the outer terminals of which are connected across the display 115. The three terminal inductor unit and the display together form an electrical inductive capacitive resonant circuit having a resonant frequency. The controllable transistor switches 104, 105 are connected respectively between the second input terminal and the first output terminal and the second input terminal and the second output terminal. Controller circuitry is connected to each of the switching transistors to cause each of the switches to open and close alternately and in opposite phase to a state of the other switching device at a rate corresponding to an operating frequency at or near the resonant frequency of the resonant circuit. This ensures the supply of alternating current to the electroluminescent display that is substantially sinusoidal.

Description

Power Supply for Electroluminescent Display TECHNICAL FIELD OF THE INVENTION This invention relates to a power supply for an electroluminescent display. The invention also relates to the use of inductive power transfer to provide power across and extended gap between a primary inductive trackway and a secondary pickup device, for lighting using electroluminescent panels.

BACKGROUND Inductive power transfer, although contactless, has in most applications in the prior art using primary pathways required that the configurations shall include ferromagnetic cores and that the secondary or pickup shall be quite closely placed in proximity to, or about the primary conductor. For example, Kelley m IS 4833337 uses elongated ferrite inverted "U" cores and a ferrite member fixed to the primary pathway as well. Boys & Green (W092/17929) use "E" cores with one primary'conductor located inside each space between the three limbs of the "E". Bolger (US 3914562) teaches a 120 Hz primary inductive cable along a roadway, the cable having iron laminations along its entire length. These laminations face corresponding laminations within the moving vehicles that draw power from the tracks. These are expensive, heavy constructions which will exhibit magnetic attraction forces and any magnetostrictive effects within the cores will tend to cause noise. For transferring power to moving road vehicles, avoidance of core structures (at least in the primary pathway) and a wider tolerance m positioning is clearly useful.

INTELLECTUAL PROPERTY OFFICE OF N.Z. 2 9 NOV 2001 RECEIVED d541xs 101/JP/tt Inductive power transfer systems in which various portions of the system are tuned to resonance are somewhat liable to instability should one or more resonant circuits assume a different resonant frequency to that of the system mean. Means to enhance stability are always useful, given that resonance is in most cases the preferred way to optimise the transfer of inductive power.

There are may applications in attention-gathering fields (for example advertising) in which it will be useful to extend the gap over which a useful field can be transmitted under inductive power transfer principles. Advantages of doing this include the concealment of the power sources so that panels appear to magically light up without a visible connection Hence the use of inductive power transfer, which itself may involve high frequencies, as a way of driving electroluminescent panels across a gap and without bare wires or contacts in a useful venture.

Electroluminescent panels have been available since at least 1957 as a source of lighting or of display and advertising material, yet they have proven to be difficult to drive at an acceptable level of brightness and at the same time retain a reasonably long life. Panels require a relatively high frequency (well above mains frequency) in order to glow at a useful level. Prior art driving circuits such as dedicated chips rely on inverters to develop AC power at typically 800-1200 Hz, and up to typically 50 V peak-to-peak. Because the output of those inverters is substantially a square-wave waveform, the phosphors of the panels are not excited optimally and brightness is not remarkable. Attempts to get more light with higher driving voltage usually results in breakdown of the dielectric and a failure of the panel, or a markedly curtailed life. There may be thermal runaway effects involved.

OBJECT It is an object of this invention to provide an improved power supply for electroluminescent panels. Another object is to provide a power supply for electroluminescent panels using inductively transferred electric power. d541xs 101/JP/tt STATEMENT OF INVENTION In one aspect the invention provides a power supply for an electroluminescent display. The power supply includes first and second input terminals that are connected and used to a supply of electrical energy, such as a substantially direct current supply. A three terminal 5 inductive unit is provided having a first outer terminal and second outer terminal and a centre-tap. The first and second outer terminals also provide output terminals for connection of the power supply to the electroluminescent display. The centre-tap is connected to the first input terminal.

The three terminal inductive unit and the electroluminescent display together comprise an 10 electrically resonant circuit having a natural resonant frequency.

Two switching devices are provided, being a first and second controllable switching device. Each switching device is capable of being disposed in a closed state or an open state. The first switching device is connected between the second input terminal and the first outer terminal. The second switching device is connected between the second input terminal and 15 the second outer terminal.

Control means are provided coupled to each of the switching devices. The control means is capable of causing each switching device to open and close alternately and in opposite phase to the state of the other switching device. The switching occurs at a rate corresponding to an operating frequency which is at or near the resonant frequency of the resonant circuit.

The output terminals thereby provide a substantially sine wave supply of alternating current to the electroluminescent display.

In a preferred embodiement the switching devices each have a control terminal and have current implification properties. On the control terminal of the first switching device the supply is electrical energy from the second outer terminal. The control terminal of the 25 second switching device is supplied with electrical energy from the first outer terminal.

In another preferred aspect of the invention, the supply of substantially direct current electrical energy is derived from the secondary resident pickup circuit into which electrical energy has been induced from a primary conductor. d541xs 101/JP/tt An intermediate resonant loop is preferably provided, being resonant at a selected frequency and being positioned between the primary conductor and the secondary resonant pickup circuit. The resonant loop provides magnetic coupling between the primary conductor and the secondary resonant pickup circuit so that electrical energy from the primary conductor 5 may be induced into the secondary resonant pickup circuit via the intermediate resonant loop.

DRAWINGS The following is a description of a preferred form of the invention, given by way of example 10 only, with reference to the accompanying diagrams.

Fig 1: is a circuit diagram used to drive an electroluminescent panel with inductively transferred power, using an intermediate loop.

Fig 2: is a circuit diagram showing the intermediate loop or circuit.

Fig 3: is a circuit diagram used to drive an electroluminescent panel with inductively 15 transferred power, without using an intermediate loop.

PREFERRED EMBODIMENT Application example 1: electroluminescent panel.

The invention will be described in relation to a particular application; driving an 20 electroluminescent panel incorporated into a the handle of a beer tap as used in bars, where the panel serves as a background for advertising material. Clearly, the invention can be applied to other situations.

The circuit for driving an electroluminescent panel includes four sections.- (a) DC to AC conversion producing a sine wave output at an optimal frequency, capable of driving the 25 panel more effectively, (b) power pickup means, (c) secondary control means operating on rectified power, and (d) panel disabling means. Refer to Fig 1. d541xs 101/JP/tt Section (a) of the circuit is a DC to sine wave converter which is adapted for a capacitative load such as that of an electroluminescent panel 115. An electroluminescent panel is a device having usually one transparent conductive plane, a dielectric layer including one or more phosphors capable of emitting light when excited, and a second conductive plane, so comprising a capacitor. The light emitted from an energised panel could be regarded as a result of the existence of a "lossy dielectric" because a change in the voltage field between the planes is required to excite the phosphor. The panels used are made with flexible plastics materials by the New Zealand manufacturer of the advertising displays, using DuPont phosphors and chemicals. A typical panel has an area of about 50cm2 and a capacitance of about lOnF.

We found that conventional excitation with approximately square wave waveforms from purpose-designed integrated-circuit (IC) excitation devices (e.g. the Supertex HV803) resulted in about 120 lux of emitted light, which is visible but not dramatic. Attempts to drive the panels harder, with more voltage, in order to get more light resulted in failure of either or both the panel and the IC driver and perhaps this is a result of the "impulse" nature of the step changes of a square drive being poorly matched to the requirements of the phosphor m terms of perceptible light. Perhaps thermal runaway effects occur during the failure process. In order to exercise the dielectric of a panel, and so excite the phosphors as efficiently as possible, it seems preferable to apply an alternating current (AC) waveform having a constant rate of change such as a sine wave. We have confirmed that panels driven with a sine wave alternating current using a circuit according to this invention can radiate a considerably increased amount of light over an apparently long period before degradation or failure occurs. Accordingly, we have constructed a sine-wave power supply circuit capable of producing a reasonably pure sine wave when used with a capacitative load (including electroluminescent panels) yet having high conversion efficiency and a low parts count.

Referring to Fig 1,101 is a positive power input and 102 is a negative power input. 103 is an inductance of small physical size (total 13 x 13 x 9mm) wound from 3500 turns of 0.05 mm insulted copper wire on an E-I ferrite coil former. It has an inductance of about 1.2 Henry. This part converts a voltage source such as our secondary pickup coil combination including a smoothing capacitor into a current source. d541xs 101/JP/tt The active components in the circuit comprise 104 and 105, two NPN transistors in a cross-coupled type of circuit. Suitable transistors are rated for 200 V Vceo, 250 mA, and have a B of about 40. (Steering diodes 108 and 116 simply provide for circuit disabling - see section (d)). The emitters of the transistors are connected to supply line 102; their bases are 5 connected at the first junction (between 106 and 107) of a resistor chain comprising 106 (47K), 107 (82K), and 109 (330K); or 110, 111, and 112 for the other- transistor. The top ends of these resistor chains are connected to the collectors of the opposite transistors. Also connected between the collectors are the ends of 113, a centre-tapped inductor otherwise like 103 which has the centre-tap connected to 103, and optionally one or more capacitors such as 10 114, used for frequency-adjusting purposes to each the about 1200 Hz desired frequency, and 115, one or more electroluminescent panels. 113 serves as a resonating inductance and as a DC feed or splitter.

Note that the desired frequency is not related to the "first frequency" used for power distribution which is typically in the range of from 10 kHz up to perhaps 30 kHz, depending 15 on switching device ratings, harmonic considerations, and the like.

The operating frequency is trimmed if necessary by adjustment of capacitor 114 to run at about 1200 Hz with the about 10 nF preferred electroluminescent panel. The magnetising inductance of the centre tapped inductor, which serves as a DC feed and as part of a resonant circuit, is about 1.2 Hy. In use the circuit is fed at about 50V DC which results in about 400 20 Lux of greenish light being emitted by the panel 115. For special effects such as flashing, the circuit can be switched off by disabling the base drives, actually by grounding the junction of the diodes 113 and 108 so interrupting signal and base current. On raising the diode voltage (above about 15V, so that the diodes become non-conductmg) the circuit rapidly (in about 2-3 cycles) builds up to its fully amplitude of oscillation. The controller 25 128 provides this output. (We have not discussed in detail any actual control signals for enabling or disabling the circuit. These may be internally generated by a simple free-running multivibrator for example, or may be picked up from an external source perhaps through the demodulation of control signals carried either through the primary conductor cable or as electromagnetic fields). Thus the circuit can be programmed to make the panel flash on and 30 off, or be dimmed with a rapid enable/disable sequence. Brightness control is also possible by varying the applied voltage, as long as the ratings of the panel and other parts are not exceeded The applied voltage could be varied by varying the reference voltage fed to the d541xs 101/JP/tt controller 127 for the shorting switch 124. Another mode of use of the comparator 128 is to inhibit the panel-driving converter when the bus voltage is under 40 volts, or enable it when the voltage is over 40V. This has the effect in the target application of causing the panel to flash brightly and perhaps briefly, rather than fade into dimness, if the coupling to the pickup 5 coil is reduced. In the target application this enhances human attention-gathering by the panel while in other applications this is a fail-safe feature for inadvertently reduced power transfer.

The power section will now be described. Integration of this circuit with an inductive pickup secondary circuit (sections (b) and (c)) is preferably as follows: A pickup coil 120, resonant 10 with capacitor 121, can collect inductive power in the form of a changing magnetic field and convert it into AC. Typically we generate 10-40 kHz AC in a primary conductor or primary coil 119 to provide a changing magnetic field. The circulating current in the resonant tank circuit 120-121 is rectified in the bridge rectifier 122 and passed through an inductance 123 of typically 560uH, intended to limit the peak current passing through a control switch 124 15 which is capable of shorting out the pickup coil (ignoring two diode voltage drops, of course.). If the shorting switch is in a high-impedance state, rectified current flows through the steering diode 125 and along the bus 101 to charge up the smoothing capacitor 126, across which the supply voltage is developed. In this circuit, one comparator determines whether the bus 101 voltage is over 50 volts, in which case its output line 129 goes high and 20 turns ON the switch 124, or under 49 volts, when its output goes low. This output controls the switch 124.

Novel means for enhancing the gap length 100 over which inductively coupled inductive power transfer or ICIPT can be transferred are provided as follows (see both Fig 3 and Fig 2). An at least several times increase of the distance over which effective amounts of 25 inductive power can be transferred is attainable. This aspect of the invention comprises placement of a simple intermediate resonant circuit 117,118 having (in this example at least) no control means or active components or the like between a primary source of a changing magnetic field and the ultimate consumer or consumers. In Fig 3 this novel resonant circuit is represented by the capacitor 118 and the mductance 117. In Fig 2 the novel intermediate 30 resonant circuit is represented by the capacitor 118, the lumped inductance 117, where power is transferred into the circuit from a source of inductive power passing through the inductor 119 which may be a straight conductor or a coil, and the elongated conductor 117A. In d541xs 101/JP/tt practice, the elongated conductor might energise a number of pickups such as the four tank circuits, one labelled 120 (inductor) and 121 (capacitor), each corresponding to a circuit such as that of Fig3 and another labelled 120' and 121'.

The physical configuration of the intermediate circuit may be varied according to the application. Electrically it may be represented as inductor 117 and capacitor 118 in Fig 3.

The intermediate pickup coil can develop higher circulating currents or at least a higher magnetic flux than a primary energised conductor, and so transfer power over a greater gap length 100. An interesting observation about the intermediate resonant circuit 117/118 is / that the overall stability of a system having extra resonant, tuned circuits of this type is 10 improved. If one circuit (say 120,121) is detuned away from resonance, its power factor changes in use so that for example the current leads the voltage. In a second circuit (say 117 and 118) inductively coupled to the first the power factor is reversed so that the voltage now leads the current. In situations where one inductively coupled link exists, the reversal leads to instability, but in situations where a second inductively coupled link to a third circuit (119) 15 also exists, the power factor relationship is again reversed and so the intermediate link leads to an improvement in frequency stability. Of course, this power supply may be used to drive an electroluminescent panel or similar device without the aid of an intermediate loop. Fig 1 shows such a power supply, practically identical to that of Fig 3 but with the omission of the loss 117 and 118. Note that in Figs 1 and 3 the primary conductive pathway 119 need not 20 have an actual discrete inductance, if sufficient current is flowing in a straight wire to provide an adequate flux.

Advantages of this power supply include that it renders an electroluminescent panel a much brighter, and hence more useful device. Furthermore, the increased brightness for a given peak voltage obtained with sine-wave driving seems to result in a greatly increased panel 25 life, although the exact improvement of lifetime remains to be defined. We had been causing panels and drivers to fail when testing the prior-art-square-wave driver circuit at higher voltages.

Application example 2: beer tap handles.

This example integrates all the inventions described in this specification. Electrically, the 30 beer tap handle circuit is that shown in Fig 1, where the luminescent panel 114 is d541xs 101/JP/tt incorporated in the handle of a beer tape to act as an advertising accessory and attract the attention of consumers. In this instance, the reduced coupling that occurs when the handle is operated by being pulled away from a rest position has the effect of causing the hitherto steadily lit panel to not simply go dim, but enter a flashing mode wherein the brightness of 5 each flash (which is of a reasonably long duration, depending on the size of capacitor 126) is comparable to the steady illumination (typically 400 lux using the circuit of this invention) of the handle in its rest position.

The components (apart from 119, 117, and 118) of Fig 1 are preferably installed in a concealed manner within each handle. Using surface-mount electronics size is not a 10 problem. Refer to Fig 2 for the physical appearance of an illuminated beer-tap handle for use in a bar.

The pickup coil (120) comprises perhaps 20 turns on a C-shaped core which may be cut from a toroidal core. In the example this is oriented vertically (i.e. along the axis of the handle) in order to collect flux emanating from the intermediate resonant loop 118-117. In Fig 2, the 15 rectangle 117A may be physically within a panel that passes close to each beer handle circuit (here suggested by the tank circuits 120,121 etc). In one corner a concentration of flux pickup means 117 is provided and this is in use oriented close to a source of magnetic field such as a coil 119 driven by a resonant power supply converter (not shown) with a sine wave at typically 40 kHz in this application. The tuning capacitor 118 is generally located close to 20 this coil. The primary coil 119 may be incorporated in a clip over the holder for the panel. In this particular application it is useful to be able to detach the panel including the coil 117/117A/118 and as there are no direct connection to it, this can be done easily. In fact the componentry can be totally concealed. The panel can be detachable in this application to permit access to the tape mechanism and for cleaning. The detachable panel may be cleaned, 25 for it has no active, sensitive parts or exposed electrical connections.

When in place, this panel is held so that its particular pickup area 117 is m proximity to the energising primary coil, and its border is near one or more pickup coils, 120,120' and so on; one on each beer tap.

Advantages of this system containing an intermediate resonant circuit for powering 30 electroluminescent panels in a consumer-accessible and often wet region include (1) that d541xs 101/JP/tt there is no need to replace relatively obtrusive primary coils adjacent to each tap, with wiring, and rigidly held in close relationship to toroids around the beer handles, so affecting the ability to clean the taps, and (2) that the distance over which effective coupling occurs is greater, so that precise positioning is not required and so that electroluminescent panels 5 remain lit during movement of the beer tap handles. Several beer taps may be driven from the one intermediate circuit.

We have not yet explored the operation of a intermediate circuit under high power operating conditions or where several intermediate circuits are to be driven simultaneously from a single primary conductor. Intermediate resonant loops circuits may also need to include 10 control circuits to limit the total circulating power. One possible example of a control "circuit" is a saturable ferrite core within the tuning inductance. Another is back-to-back Zener diodes connected across the tuning capacitor, selected so as to break down when the circulating voltage exceeds a predetermined limit.

INDUSTRIAL APPLICATION (1) Improved drive circuitry for electroluminescent panels increases the range of possible applications for such panels. Prior-art drivers resulted in about 120 lux of emitted light, which is visible but not dramatic. 400 Lux was available with the circuit of this invention. (2) Intermediate resonant circuits, by increasing the gap over which a given amount of power 20 can be transferred inductively, can increase the number of applications for IPT. They can reduce the amount of primary current required, and/or reduce the size of a secondary pickup inductor, or they may reduce the requirements for precise alignment of the pickup coil with the primary conductor.

Finally, it will be appreciated that various alterations and modifications may be made to the 25 foregoing without departing from the scope of this invention as set forth in the following claims.

INTELLECTUAL PROPERTY OFFICE OF N.Z. 2 9 NOV 2001 RECEIVED d541xs 101/JP/tt

Claims (2)

1. Claims 9 1 A power supply for an electroluminescent display, the supply including (a) a first input terminal and a second input terminal for supplying substantially direct current electrical energy; (b) a three terminal inductive unit having a first outer terminal and a second outer terminal and a centre-tap, the first and second outer terminals also providing output terminals for supplying power to the electroluminescent display, and the centre-tap being connected to the first input terminal, the three terminal inductive unit and the electroluminescent display together comprising an electrically resonant circuit having a natural resonant frequency; (c) a first controllable switching device and a second controllable switching device, each switching device having a closed state or an open state, the first switching device being connected between the second input terminal and the first outer terminal, and the second switching device being connected between the second input terminal and the second outer terminal; and (d) controlling means coupled to each of the switching devices for causing each switching device to open and close alternately and in opposite phase to a state of the other switching device at a rate corresponding to an operating frequency at or near the natural resonant frequency of the resonant circuit, whereby a substantially sinnsoidal supply of alternating current is supplied to the electroluminescent display.
2. 2 A power supply as claimed in claim 1, wherein the switching devices each have a control terminal and have current amplification properties, the control terminal of the first switching device being supplied with electrical energy from the second outer terminal, and the control terminal of the second switching device being supplied with electrical energy from the first outer terminal D54IXS 101 /1 P/tt A power supply as claimed m claim 2 wherein the supply includes interruption means to interrupt the supply of power to the electroluminescent display, the interruption means including means to lower the electric potential of the control terminal of each switching device to thereby prevent the switching device from operating A power supply as claimed in claim 1 including a second inductive unit connected m series between the centre-tap and the first input terminal to thereby provide substantially constant supply of current to the power supply A power supply as claimed in claim 1 wherein a capacitive unit is connected between the output terminals, the capacitive unit having a capacitance value which is selected to provide a selected natural resonant frequency to the resonant circuit A power supply as claimed in claim 1 wherein the supply of substantially direct current electrical energy is derived from a secondary resonant pick-up circuit into which electrical energy has been induced from a primary conductor A power supply as claimed in claim 6, further including an intermediate resonant loop, the intermediate resonant loop being resonant at a selected frequency and being positioned between the primary conductor and the secondary resonant pick-up circuit whereby the intermediate resonant loop provides magnetic coupling between the primary conductor and the secondary resonant pick-up circuit so that electrical energy from the primary conductor may be induced into the secondary resonant pickup circuit via the intermediate resonant loop. A power supply for an electroluminescent display substantially as described herein with reference to the accompanying drawings. Ol/lP/tt