GB2404772A - Control of an electroluminescent display matrix - Google Patents

Abstract

An electroluminescent display 1 with a plurality of display elements arranged in rows X and columns Y to form a matrix. Each element is connected to a row-conductor associated with its respective row and to a column-conductor associated with its respective column such that the element may be illuminated by applying a voltage across the conductors. The display further comprises a driver comprising a half H-bridges H1 etc. which are controlled by controller 3 so as sequentially to supply drive voltages from rail 7. Full drive voltage is supplied to a row-conductor of a row that is to be driven and one third drive voltage is supplied to the remaining row-conductors. The column-conductor(s) associated with the elements to be illuminated are grounded. Two thirds voltage is applied to the conductors associated with the remainder of the columns.

Description

2404772

Control of an Electroluminescent Display Matrix

The present invention relates to electroluminescent displays, and in particular to such displays in the form of a display matrix.

Electroluminescent displays have selectively illuminable regions for displaying information. Such displays have the advantage over competing technologies that they can be large, flexible and are relatively inexpensive.

Although electroluminescent lamps were known in the 1950's, these had a short lifetime and it was not until the 1980's that a flexible electroluminescent device was developed. However, this was used as an LCD backlight and only recently have practical electroluminescent displays become available.

Electroluminescent displays generally comprise a layer of phosphor material, such as a doped zinc sulphide powder, between two electrodes. It is usual for at least one electrode to be composed of a transparent material, such as indium tin oxide (ITO), provided on a transparent substrate, such as a polyester or polyethylene terephthalate (PET) film. The display may be formed by depositing electrode layers and phosphor layers onto the substrate, for example by screen printing, in which case opaque electrodes may be formed from conductive, for example silver-loaded, inks. Examples of electroluminescent devices are described in WO 00/72638 and WO 99/55121.

An electroluminescent display of the general type described above is illuminated by applying an alternating voltage of an appropriate frequency between the electrodes of the lamp to excite the phosphor. Commonly, the phosphors used in electroluminescent displays require a voltage of a few hundred volts.

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Typically, such electroluminescent displays may have a capacitance in the range lOOpF to lyF.

Since only a small current is required, this comparatively high drive voltage can easily be produced from a low voltage DC supply by a circuit such as the well known "flyback converter".

This comprises an inductor and an oscillating switch arranged in series. In parallel with the oscillating switch, a diode and a capacitor are arranged in series. The switch oscillates between an open state and a closed state. In the closed state, a current flows from the DC supply through the inductor and the switch. When the switch is opened, the current path is interrupted, but the magnetic field associated with the inductor forces the current to keep flowing. The inductor therefore forces the current to flow through the diode to charge the capacitor. The diode prevents the capacitor discharging while the switch is closed. The capacitor can therefore be charged to a voltage that is higher than the DC supply voltage, and current at this voltage can be drawn from the capacitor.

In order to supply an alternating current to a load from a flyback converter, an H-bridge may be provided in parallel with the capacitor. In general, an H-bridge comprises two parallel limbs, each limb having a first switch in series with a second switch. On each limb between the first and second switches, there is a node, and the load is connected between the respective nodes of the limbs. Current can flow through the load in one direction via the first switch of one limb and the second switch of the other limb and in the other direction via the other two switches. The switches of the H-bridge are operated so that current flows through the load first in one direction and then in the other.

Where multiple electro-luminescent segments are provided to form a display, the segments may be controlled by having a single high voltage rail of

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constant voltage that is selectively switched across the segments that are required to light. This is achieved by using a half H-bridge transistor configuration to drive a common, usually front, electrode and a number of half H-bridges to drive each of the multiple segments. The common electrode will be switched at a frequency in the region of a few tens of hertz to a few kilohertz. Segments that are not required to light will be driven with the same signal as the common electrode such that they see no net voltage. Segments required to light will be driven at the same frequency but in anti-phase with the common electrode such that they see an alternating voltage of peak-to-peak value that is twice that of the high voltage rail. This enables simple control of which segments light by control of the phase of their driving signals.

Although this is a practicable arrangement for simple displays such as the well-known seven-segment displays used to display digits in clocks and calculators and for many other applications, problems arise when it is desired to provide a matrix-type display. Such displays contain multiple rows and columns of display elements. It is well known to use LEDs to provide matrix displays. It is clearly impracticable to provide a separate signal directly to each element in the matrix and so the usual arrangement is for all the LEDs in a given column to be connected to a common conductor and likewise for the rows. Thus, each LED may be illuminated by applying a voltage across the conductors associated with its row and column.

It will be appreciated that if (as is usual) more than one element is to be illuminated, this cannot be achieved by simply applying a voltage across the pairs of conductors corresponding to each element because this would result in unwanted elements also being illuminated.

The matrix is therefore driven by selecting one row

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at a time and then driving the LEDs in that row that are to be illuminated while leaving the non-selected rows floating.

This arrangement is effective for LEDs because, as diodes, they act as a resistive load when forward biased. However, electro-luminescent display elements behave as capacitors. When this standard multiplexing system is applied to an electro-luminescent display matrix, the rows and columns left disconnected will float to half of the total supply voltage on average. This results in the display segments in the non-selected rows seeing half of the total drive voltage. Therefore this method produces a poor contrast ratio between on and off segments.

According to the present invention there is provided an electoluminescent display comprising a plurality of display elements arranged in rows and columns to form a matrix, each element being connected to a row-conductor associated with its respective row and to a column-conductor associated with its respective column such that the element may be illuminated by applying a voltage across said conductors, the display further comprising a driver arranged sequentially to supply a first (highest) drive voltage to a row-conductor of a row that is to be driven and a third drive voltage to the remaining row-conductors, the third drive voltage being lower than the first drive voltage, whilst grounding the column-conductor(s) associated with the elements to be illuminated and applying a second voltage to the conductors associated with the remainder of the columns, the second voltage being intermediate the first and third voltages.

Thus, the elements that are illuminated (ON) are driven at the first voltage because they are connected to a conductor at this voltage and one at ground.

The remaining (OFF) elements are connected either to ground and to the third voltage; to the second

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voltage and the third voltage; or to the first voltage and the second voltage. Consequently, they are driven at the difference between the first and second voltages; the second and third voltages and the third voltage respectively. It will be appreciated that each of these drive voltages is significantly lower than the first drive voltage. Consequently, the invention provides significantly better contrast than is provided using the prior art (LED-type) arrangement.

It should be appreciated that the terms "row" and "column" are simply used to identify generally perpendicular arrangements in the display. It is immaterial whether or not the "rows" are arranged horizontally.

Although in some applications a degree of variation in the brightness of the OFF elements is not critical, preferably they are all driven to substantially the same level of brightness by providing them with at least approximately the same magnitude of OFF drive voltage. (It does not matter whether the polarity is the same or opposite.) To achieve this, the difference between the first and second voltages and between the second and third voltages is preferably equal to the third voltage.

This is achieved if the first voltage is the full supply voltage, the second voltage is two-thirds of this and the third voltage is one-third. (Ground is an arbitrarily defined zero voltage, which may or may not be at earth potential. The drive voltages may be positive or negative with respect to it) . Thus, it will be seen that each element that is to be illuminated has the full supply voltage applied across it whilst the remaining units have one-third of this voltage applied. This provides a 3:1 ratio compared to the 2:1 ratio of the prior art.

This provides another aspect of this invention, which is to provide an electroluminescent matrix display, the display being arranged such that each

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element that is to be illuminated has the full supply voltage applied across it whilst the remaining elements have substantially one-third of this voltage applied.

The invention also extends to a method of driving an electroluminescent matrix display wherein each element that is to be illuminated has the full supply voltage applied across it whilst the remaining units have substantially one-third of this voltage applied.

Electroluminescent displays may be driven by a varying DC signal, but this shortens their life. Preferably, therefore, the polarity of the elements is repeatedly reversed during operation. If in the arrangement discussed above the supply voltage is taken to be positive, when the polarity is reversed, the selected rows are connected to ground and the non-selected rows are driven at two thirds of supply voltage. Active columns are driven at full supply and inactive columns are driven at one third of full supply voltage.

Since the display contains a plurality of rows, it will be appreciated that the rows may be driven sequentially in the known manner so that the whole display may be controlled.

The display is preferably controlled by means of a driver that provides the appropriate voltage to each row and column of the display in order to provide a desired display output. The necessary four state supply can be achieved with a standard fly-back power supply adapted to have outputs of VO = OV, VI = Vpp/3, V2 = 2Vpp/3, V3 = Vpp where Vpp is the maximum output voltage.

These three voltage levels can be generated from an adapted flyback power supply. In place of the inductor in a standard flyback power supply, an inductor with four (i.e. two extra) connections is used such that each terminal provides one of the desired voltages. The four connections are arranged such that the number of turns in the inductor (and therefore the inductance) is the

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same between the first and second terminals, the second and third terminals and the third and fourth terminals. The first and fourth terminals of this inductor are connected where the normal inductor is connected in the standard flyback power supply. Typically, the supply is completed by the provision of a diode and capacitor in series that are connected between each of the intermediate connections on the inductor and ground.

Such a power supply is in itself regarded as inventive and so, viewed from another aspect, the invention provides a power supply for an electroluminescent matrix display comprising a flyback converter having an inductor with four output connections, wherein each terminal provides one of four desired output voltages. The voltages are preferably as described above.

When the flyback power supply is operated as normal, a voltage is produced on the second terminal equal to one third of the voltage on the fourth terminal and a voltage is produced on the third terminal equal to two thirds of the voltage on the fourth terminal. This power supply can be used to produce the three required voltages for driving the electroluminescent display matrix.

The three voltage levels could also be generated from an adapted energy recovery power supply, in which the transformed inductor has three separate secondary windings, with different number of turns in the ratio 1:2:3 and each with its own independent secondary switch.

However, switching three separate high voltage supplies to each output presents a comparatively high cost in terms of the component count. This would be undesirable for any reasonable size of electroluminescent display matrix.

Preferably, therefore, a power supply with a single output is provided and the desired voltages are achieved

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by selectively connecting elements to the power supply such that an amount of charge is applied to the elements sufficient to provide the desired voltages across them. This approach is based upon the fact that each element behaves as a capacitor, consisting as it does of an electroluminescent material sandwiched between two electrodes. Since for a capacitor Q = CV where Q is charge, C is capacitance and V is voltage, it follows that for a given capacitance a desired voltage can be achieved by supplying a certain amount of charge. In any display, the Capacitance of the elements is known and in most cases each element will have a similar value. Consequently, the amount of charge that has to be supplied can be calculated.

The desired amount of charge may be supplied by controlling the operation of a fly-back converter. As discussed above, fly-back converters comprise an inductor that is selectively grounded by means of a transistor switch so as to provide a high voltage output pulse. Preferably the transistor receives control pulses from a pulse width modulator that determine the timing and/or length of the output pulses and consequently the flow of charge from the power supply. The pulse width modulator may in turn be controlled, by means of a processor, based upon the desired display output.

Because of the problems described previously, it is not possible to supply charge to individual elements. However, the elements may be considered as groups. For the control system described in this invention, the segments in the display are preferably split into four groups. The segments in each of these groups are always connected together in parallel. Therefore, each of these groups can be treated mathematically as a single capacitor with a total capacitance equal to the sum of the capacitances of all the display segments in that group. The four groups are as follows: the lit elements; the non-lit elements in the same row as lit

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elements; the non-lit elements in the same column as lit elements; and the non lit elements that are in different rows and columns from the lit elements.

In a given display cycle, the groups would not normally all be driven simultaneously. Preferably the active elements are driven first, together with the one the other groups that requires the highest charge. Subsequently the group requiring the next highest charge is also driven and finally all elements are driven. Elements being driven have their two conductors connected so as to have the power supply voltage or some proportion of the power supply voltage connected across them so charge will flow into them. Non-driven elements have their two conductors connected so that no charge can flow into them.

Although this arrangement has been described in relation to electroluminescent displays, it is equally applicable to the driving of other arrays of capacitive elements. Thus, viewed from another aspect, there is a provided a method of driving an array of capacitive elements, comprising dividing the elements into four groups and supplying charge to the separate groups such that the segments in each of these groups are always connected together in parallel, whereby a desired voltage is produced across each element. Typically each element is a display element in a matrix display and the method facilitates the driving of the matrix display. Preferably the matrix is driven as described above.

Preferably the charging process is controlled by determining the load capacitance at each stage in the cycle. This may be calculated by determining which segments are connected in parallel and which in series across the power supply and also taking into account the effect of any smoothing capacitor provided across the supply. From this the amount of charge required to achieve the desired voltages across each element may be determined and also the charging sequence and operation

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of the power supply required to achieve this. Again, such data is preferably pre-determined and stored in a look-up table.

Although it is possible to drive electroluminescent displays using varying DC, in order to maximise performance and prevent a DC bias being created across any segment, each segment is preferably given an equal, number of positive and negative polarity pulses.

The sequence of negative and positive polarity pulses given to a segment has an effect on the brightness of' the light it emits. For example, a segment which is driven with alternating positive and negative polarity pulses will be brighter than a segment which is driven with eight positive pulses followed by eight negative pulses.

Consider, for example, an eight row multiplexed display. One possible drive sequence would be for each row to be driven in order at one polarity, the polarity reversed and then each row driven in order again. In this case, inactive segments which are in a complete column of inactive segments would be driven with eight positive polarity pulses followed by eight negative polarity pulses. Inactive segments that are in a column that includes active segments would be driven with a different sequence of positive and negative polarity pulses, dependant on which segments in that column are active. This is because inactive segments in the fourth group (as defined above) are charged in the opposite polarity to segments in the second and third groups. This variation in polarity sequence may result in uneven contrast across inactive segments and consequently in a "shadowing" effect in between, for example, text digits. Any pre-defined and frequently repeated sequence of positive and negative polarity pulses could therefore result in an uneven level of contrast between inactive segments.

Therefore, it is preferred not to run a fixed and

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repeated polarity sequence for every cycle. Instead, the polarity sequence for each cycle may be chosen at random, e.g. from a list of random numbers. The same sequence is then run with the polarity exactly reversed in order to give each segment the same number of negative and positive polarity pulses. By cycling through the polarity sequences in the random list, each inactive segment in the matrix will be driven with a pseudo-random sequence and therefore have the same brightness as the other inactive segments. This results in an even contrast across all inactive segments. This random driving system can be used independently of the other inventive concepts described herein and forms a still further aspect of this invention.

Running the whole system at a high frequency is preferred because it will improve the contrast ratio due to the low sensitivity of EL phosphors to high frequencies. The inactive segments will be driven at a high frequency with the OFF pulses and the active segments will be driven at a lower frequency with the ON pulses. The high frequency will be equal to the lower frequency multiplied by the number of rows, or groups of rows which are used in the multiplex cycle.

Whilst the invention has primarily been discussed in terms of a complete display device, it will be appreciated that display panels and their respective drivers may be produced separately. Consequently, the invention extends to drivers for electroluminescent displays that operate according to the foregoing methods.

Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings:

Figure 1 is a schematic view of a matrix array of electroluminescent elements;

Figure 2 is a schematic representation of a control system for such an array;

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Figure 3 is a schematic representation of a power supply for use in combination with the control system of figure 2;

Figure 4 is a schematic illustration of a four element display indicating different groups of elements; and

Figures 5 and 6 illustrate the use of exemplary charging sequences.

An array 1 of electroluminescent elements 2 is provided to form a display, as shown in Figure 1. Each element 2 Comprises a phosphor electroluminescent material sandwiched between a transparent electrode at the front and a further electrode at the rear. The front electrodes are in turn connected to a series of column-conductors Y: - Yn and the rear electrodes are likewise connected to a series of row-conductors Xj-X,^

Turning now to figure 2 the array is shown at 1 with only two rows and three columns illustrated for clarity. Each row Yj etc and column X, etc. is connected to the output of a modified half H-bridge H1-H5. Each of these comprises a pair of MOSFET transistor switches 4, 5 and a tri-state controller 6. Switches 4 have one pole connected to high voltage supply 7 and switches 5 have one pole connected to ground 8. The other poles of the pairs of switches are connected together and to the respective row or column via conductors 9 etc.

The switches 4, 5 are connected to tri-state controllers 6, which are in turn each connected to an output from controller 3, which provides them with control signals. These determine whether switch 4 or 5 is to be closed, thereby applying a high voltage to the row/column or grounding it. There is also a third setting in which both switches are left open so that the row/column floats.

The high voltage supply is in the range 0-250 volts. This is provided from the output Vout of power supply 10 as shown in figure 3.

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Turning to that figure, the power supply 10 comprises a fly-back converter 11, a pulse width modulator 12 and a processor 13.

The fly-back converter is formed from a coil 14 that is connected at one end to a low voltage supply VDL1 and at the other to MOSFET 15 and diode 16. The transistor is arranged selectively to connect coil 14 to ground. The other terminal of diode 16 leads to the power supply output V0UT. By selectively disconnecting the coil from ground, high voltage pulses are provided in the well-known manner. These are fed via diode 16 to a capacitive load, which may thereby be charged to a far higher voltage than VDU.

A smoothing capacitor Cs is provided across the output. This is selected to have significantly less capacitance than the load to ensure that it controls voltage overshoot upstream of the H-bridges without significantly increasing the energy required to charge the load.

A Discharge circuit 17 is provided to discharge the load and smoothing capacitor Cs to ground as required. This reduces power dissipation in the high-voltage array switches and produces a controlled discharge path.

The operation of the fly-back circuit 11 is controlled by means of pulse width modulator 12. This receives inputs 18, 19 indicating the desired pulse width and number of pulses respectively and in response provides an output signal at 20 that controls transistor 15 in the fly-back converter 11. The output from the pulse width modulator is fed to the fly-back converter via AND gate 21. The other input to this gate is an inverted input from the DISCHARGE signal. This has the result of disabling the output of pulse width modulator 12 during the discharge period to avoid wasting energy.

The pulse width modulator 12 and discharge circuit 17 are controlled by means of processor 13. (Controller 3 of figure 2 is also provided by means of processor 13

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and so it also provides N outputs (N being the sum of the number of rows and columns) to control tri-state controllers 6.)

The processor receives input data at 22 indicating which segments are active (i.e. lit or ON). Depending on this data, the outputs discussed above are provided. By controlling the modified half H-bridges HI etc. the processor determines which rows and columns are connected to ground, high voltage or left to float. By controlling the fly-back converter 11 the amount of charge that is supplied via the high voltage line 7 to the rows/columns connected to it is also determined.

The result of this is that in each cycle, different amounts of charge may be supplied to different elements to enable them to be charged to different voltages. This is arranged so that active elements see the maximum voltage across them and inactive ones see one third of this voltage, even though only one power supply with a single output is provided. At the start of a cycle, as the voltage is ramped up on the fly-back power supply, the active rows are connected to V0UT and the active columns to ground, while inactive rows and columns are selectively connected to V0UT or ground, or left floating so as to produce the required voltage on each row and column at the end of the cycle.

The processor determines what charge is required to be supplied as follows. The segments in the display matrix are split into four groups, as illustrated schematically in Figure 4, which shows a display with the segments arranged in one of four groups. The segments which are positioned at the intersection of active rows and active columns are to be driven to the full supply voltage while all other segments are to be driven to 1/3 of the full supply voltage. The four groups of segments are those at the intersection of active rows with active columns (XAYJ , segments at the intersection of active rows with inactive columns (XAYT),

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segments at the intersection of inactive rows with active columns (X1YA) and segments at the intersection of inactive rows with inactive columns (X2YT) . Each of these four groups can then be treated as a single capacitor.

The groups of inactive segments which are connected across the power supply in parallel with the active segments and the smoothing capacitor is determined by whether the inactive rows and columns are connected to V0UT, ground, or left floating, as illustrated in Table 1.

Inactive Columns (Y) connected to:

GND

Vout

Floating

Inactive Rows (X) connected to:

Vout

XaY, + XIYA + _L X,Y, ~r

X,YA —{—

X,Ya =J=

GND

XAY, —J—

XiYi —J—

XaY, =!= X,Y, =j=

Floating

XAYi —J—

X,Y, =}= X,Ya =j=

XaY, =j= X,Y, =4= X,YA "T"

Table 1

Connections are selected in stages. Each stage puts a certain amount of charge into each group of segments. The amount of charge required at each stage in order to achieve the same voltage across each group of inactive segments is determined by the relative capacitances of each group of segments.

In order to achieve this, the first stage is to connect the largest set of inactive segments, and then progressively increase the number of inactive segment areas connected in series. The charging sequence used in this embodiment has three stages, with charges Ql, Q2 and Q3 respectively being delivered in each stage. The largest inactive segment area (segment group 1) is

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connected for the first stage. For the second stage, the next largest inactive segment area (segment group 2) is connected in series. For the final stage, all three sets of inactive segments (groups 1, 2 and 3) are connected in series. The capacitances of the inactive segment groups 1, 2 and 3 are CI, C2 and C3 respectively. The voltage achieved on each set of inactive segments is given by:

1 G

v. - 0

k, = -L

C

a c c c

^ + 0 -£■ + 0

C+C '+C c +c +c c +c +c

L-1 ^ 1 -/M. L S K--p ^y"AA

c c a rz +o s

L2 c +c +c c +c +c fi ^ K AA ^ y-'S V 'y ^ ^AA ^ S

+ CAa + Q

where:

j___L j_

r ~~ r r

K /I y i 2

_L_j_ j_

c ~ c c c

K"y *"l ^2 l'3

C^ is the capacitance of the active segments that are at the intersection of the active rows and active columns. Cs is the capacitance of the smoothing capacitor. As Yl = V2 = V3 = Vm.JX / 3, the charges that are to be provided at each stage Qir Q2 and Q, are then determined.

Rather than calculate these values in real time, in this embodiment the processor contains a look-up table giving the charges to be supplied and the appropriate connections for each stage corresponding to each possible number of active segments.

The processor next operates the half H-bridges HI etc. to allow charge to be supplied to the active

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elements and those in the first group of inactive elements. At the same time, it causes pulse width modulator 12 to deliver control pulses to MOSFET 15 of the correct number and width to deliver charge Q, . Subsequently, the process is repeated such that charge Q;, is delivered to the active elements and the first and second groups of inactive elements. Finally, charge Qt is delivered to all elements.

After the charging sequence has been completed, the display matrix is discharged. A different row, or group of rows is then selected and the appropriate charging sequence is run determined by the segments which are required to be lit in that row or group of rows.

Figures 5 and 6 illustrate two examples where a twenty-eight segment matrix is used to display two different patterns. In each case, the pattern is shown at the top. At the bottom there are shown the appropriate connections and charging sequences that are to be used in each case. Note that in each case the active segments are connected in parallel with the other segments that are being supplied.

In figure 5, there are three active elements (shown shaded) that are to be driven in the first row of the display. The active rows and columns are identified by the subscript A. It may be seen that there are 12 elements in the XjYj group, nine in the XxYA group and four in the XAYt group. It can therefore be seen that it is necessary to put most charge into the XxY: group, the next most into the XxYa group and the least into the X^ group. This means that the sequence and connections shown at the bottom of the figure may be used in order to provided the desired charge on each element.

In figure 6 the second row is now to be driven. There are five active elements in this row (the same notation applies) . there are six elements in the XjYj group, fifteen in the XxYa group and two in the XaYt group. It can therefore be seen that it is necessary to

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put most charge into the XTYA group, the next most into the XjY2 group and the least into the XaYj group. Thus a different sequence of connections and charging is required. This is shown in a comparable manner to the previous figure at the bottom of figure 6.

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Claims (17)

Claims

1. An electoluminescent display comprising a plurality of display elements arranged in rows and columns to form a matrix, each element being connected to a row-conductor associated with its respective row and to a column-conductor associated with its respective column such that the element may be illuminated by applying a voltage across said conductors, the display further comprising a driver arranged sequentially to supply a first drive voltage to a row-conductor of a row that is to be driven and a third drive voltage to the remaining row-conductors, the third drive voltage being lower than the first drive voltage, whilst grounding the column-conductor (s) associated with the elements to be illuminated and applying a second voltage to the conductors associated with the remainder of the columns, the second voltage being intermediate the first and third voltages.

2. A display as claimed in claim 1, wherein the first voltage is the full supply voltage, the second voltage is two-thirds of the full supply voltage and the third voltage is one-third of the full supply voltage.

3. An electroluminescent matrix display, the display being arranged such that each element that is to be illuminated has the full supply voltage applied across it whilst the remaining elements have substantially one-third of this voltage applied.

4. A display as claimed in claim 1, 2, or 3, wherein the polarity of the elements is repeatedly reversed during operation.

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5. A display as claimed in any preceding claim, wherein rows of elements are driven sequentially so that the whole display may be controlled.

6. A display as claimed in any preceding claim comprising a flyback power supply, the supply having an inductor with four output terminals, each terminal providing one of the desired voltages.

7. A display as claimed in any of claims 1 to 5 comprising a power supply having a single output, the desired voltages being achieved by selectively connecting elements to the power supply such that an amount of charge is applied to the elements sufficient to provide the desired voltages across them.

8. A display as claimed in claim 7, wherein the power supply comprises a flyback power supply.

9. A display as claimed in claim 8, wherein the fly-back power supply comprises an inductor that is selectively grounded by means of a transistor, the transistor being controlled by pulses from a pulse width modulator that determine the timing and/or length of the output pulses from the flyback power supply.

10. A display as claimed in any of claims 7, 8 or 9, wherein for a given charging cycle, the elements are grouped, the segments in each of each group being connected together in parallel.

11. A display as claimed in claim 10, wherein the groups are: (i) the lit elements; (ii) the non-lit elements in the same row as lit elements; (iii) the non-lit elements in the same column as lit elements; and (iv) the non lit elements that are in different rows and columns from the lit elements.

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12. A display as claimed in claim 11, wherein the lit elements are driven first, together with the one the other groups that requires the highest charge, then the group requiring the next highest charge is also driven and finally all elements are driven.

13. A display as claimed in any of claims 7 to 12, comprising determining the load capacitance at each stage in the drive cycle and from this determining the amount of charge required to achieve the desired voltages across each element and the charging sequence and operation of the power supply required to achieve this.

14. A display as claimed in any preceding claim wherein the polarity sequence for each stage in the drive cycle is be chosen at random, the same sequence then being run with the polarity exactly reversed in order to give each segment the same number of negative and positive polarity pulses.

15. A power supply for an electroluminescent matrix display comprising a flyback converter having an inductor with four output connections, wherein each terminal provides one of four desired output voltages.

16. A method of driving an electroluminescent matrix display wherein each element that is to be illuminated has the full supply voltage applied across it whilst the remaining elements have substantially one-third of this voltage applied.

17. A method of driving an array of capacitive elements, comprising dividing the elements into four groups and supplying charge to the separate groups, the arrangement being such that the elements in each of these groups are

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always connected together in parallel, whereby a desired voltage is produced across each element.