GB2333911A

GB2333911A - Capacitor drive circuit

Info

Publication number: GB2333911A
Application number: GB9802121A
Authority: GB
Inventors: Hakan Andersson
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 1998-01-30
Filing date: 1998-01-30
Publication date: 1999-08-04
Anticipated expiration: 2018-01-30
Also published as: GB2333911B; GB9802121D0

Abstract

A method of providing AC charging of a capacitive element 10, having first and second electrodes el,e2, from a DC source 1 comprises first and second cycles: the first cycle comprising a first charging step in which the first electrode el is connected 6a to be charged to a first voltage, a connection step in which the first electrode el is connected 5c, 6c to the second electrode e2 , thereby equalising the voltage on each electrode to be half the said first voltage, and a first discharge step in which the voltage from the first electrode e1 is connected 6b to be discharged; and the second cycle comprising a second charging step in which the second electrode e2 is connected 5a to be charged to a second voltage, a connection step in which the second electrode e2 is connected 5c, 6c to the first electrode e1 , thereby equalising the voltage on each electrode to be half the second voltage, and a second discharge step in which the voltage from the second electrode e2 is connected 5b to be discharged. The capacitive element may be an electroluminescent lamp. The first and second voltages may be provided by converter 2, 3.

Description

CAPACITOR DRIVE CIRCUITRY The present invention relates to capacitor drive circuitry, for example to drive circuitry for driving electroluminescent lamps.

DESCRIPTION OF THE RELATED ART Electroluminescent (EL) lamps or panels are essentially capacitors with one transparent electrode and a special phosphorous material as a dielectric.

When a strong AC voltage is applied the phosphor glows.

A strong AC voltage (100-150V) is normally not present in portable electronic equipment, such as mobile telephone units, and must consequently be generated from a low voltage DC source if EL lamps or displays are to be used in such equipment.

This requirement can be realised by use of an inverter, a switched-mode inductor based circuit. This high voltage process may be controlled by logic which is clocked by oscillators. The EL lamp to be energised is powered by a repeatedly charged inductor. The inductor receives current from a DC source and discharges into the capacitance of the EL lamp. With each cycle, the voltage across the lamp is increased.

When the voltage has become sufficient the lamp is discharged and the polarity of the inductive charging is reversed. The result is a symmetrical low frequency (100-400Hz) alternating strong (100-150V) AC voltage at the lamp's input.

Three different types of known EL drivers will be briefly surveyed.

The first driver, for example as described in US patent 5,313,141, and shown in Figure 1 of the accompanying drawings, uses the following switching principle: state 1: U1 and U4 are closed, whilst U3 is open and U2 is oscillating (a boost regulator); state 2: U1 is oscillating and U4 is open whilst U2 and U3 are closed (a BuckBoost regulator - positive to negative converter); and finally, by alternating between states 1 and 2 at a low frequency (100-400 Hz), a saw-tooth wave is created.

The second driver, shown in Figure 2, functions as a dual boost regulator that builds up the voltage in the EL periodically. When a sufficient voltage level is reached the charged electrode discharges and the other electrode voltage is then built up. This procedure will repeatedly continue at a frequency between 100-400 Hz. Consequently, the EL panel's terminals are switched to be alternately connected to ground and Dl's cathode and then reversed.

The third driver configuration, shown in Figure 3, works as a single boost regulator. Since the EL panel is connected across the diode and the inductor it receives only a rectified cycle of positive voltage.

The switches TR1 and TR2 control the charging of the inductor and discharging of the EL panel respectively.

TR1 oscillates to build up the voltage in EL and TR 2 discharges the EL voltage out once per cycle.

SUMMARY OF THE PRESENT INVENTION According to one aspect of the present invention there is provided a method of providing AC charging of a capacitive element, having first and second electrodes, from a DC source, the method comprising first and second cycles: the first cycle comprising a first charging step in which the first electrode is charged to a first voltage, a connection step in which the first electrode is connected to a second electrode, thereby equalising the voltage on each electrode to be half the said first voltage, and a first discharge step in which the voltage from the first electrode is discharged; and the second cycle comprising a second charging step in which the second electrode is charged to a second voltage, a connection step in which the second electrode is connected to the first electrode, thereby equalising the voltage on each electrode to be half the second voltage, and a second discharge step in which the voltage from the second electrode is discharged.

According to a second aspect of the present invention there is provided apparatus for providing AC charging of a capacitive element having first and second electrodes, from a DC source, the apparatus comprising: first cycle means for charging the first electrode to a first voltage, thereafter connecting the first electrode to the second electrode, thereby equalising the voltage on each electrode to be half the said first voltage, and then discharging the voltage from the first electrode; second cycle means for charging the second electrode to a second voltage, thereafter connecting the second electrode to the first electrode thereby equalising the voltage on each electrode to be half the second voltage, and then discharging the voltage from the second electrode.

In one embodiment the discharging of the voltage from the first electrode coincides in time with the charging of the second electrode to the second voltage.

In another embodiment, the discharging of the first electrode overlaps in time with the charging of the second electrode. In a further, preferred, embodiment these discharging and charging steps are separated in time.

BRIEF DESCRIPTION OF THE 9ROWING Figures 1, 2 and 3 show respective circuit diagrams of three known electroluminescent lamp drivers; and Figure 4 shows a circuit diagram of an EL lamp driver in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior art electroluminescent drivers have been described with reference to Figures 1, 2 and 3 above, and an embodiment according to the first aspect of the present invention will now be described with reference to Figure 4.

A DC supply voltage 1 is connected in series with an inductor 2 and a diode 3. A switch 4 serves to connect the other end of the inductor 2 to ground, such that oscillating opening and closing of the switch 4 causes the inductor to charge from the DC supply 1 and to discharge through the diode 3.

An electroluminescent lamp 10 has two electrodes el and e2. The first electrode el is connected to two high voltage switches; the first, 5, having three poles a, b and c, and the second, 7b, serving to enable connection of the electrode el to ground or to a battery 9. Similarly, the second electrode e2 is connected to two high voltage switches; one, 6, having three poles, a, b and c and the other 7a, serving to enable connection of terminal e2 to terminal 9. Of the high voltage switches 5 and 6, the c terminals are connected to one another via a resistor 8, the a terminals are each connected to the diode 3, and the b terminals are connected to ground.

The circuit of Figure 4 circuit operates according to the second aspect of the invention as will now be described.

The principle underlying embodiments of the invention is to recycle charged power of the EL lamp when alternately charging both electrodes of the EL lamp, by periodically levelling out the voltage on the two electrodes.

In the preferred embodiment, the start-up routine is as follows: Period 1) At the beginning of the process it is easy to pump up or increase the voltage of electrode 1 (el, by the inductor controlled by switch 4) because the electrode has little voltage. Consequently, the first step is big, for example it may be up to around 60V.

Period 2) The voltages on the electrodes are levelled out to half the first voltage step, for example to 30V each, by connection through a high voltage switch (poles Sc, 6c).

Period 3) The remaining voltage of el is discharged by switch 7b to ground.

Period 4) The second electrode (e2) is pumped further by a voltage lower than the first pump voltage, say 56V. This gives the second electrode a voltage of 30 + 56V = 86V by closing switch poles 6a-5b (the second pump-up sequence becomes harder because electrode e2 is already at 30V, so only a 56V increase is possible).

Period 5) The electrode voltages are again levelled out, now to 43V each, through the high voltage switch poles 4c-6c.

Period 6) The remaining voltage of e2 is discharged via switch 7a to ground via terminal 9.

Period 7) el is once again pumped up by a further voltage, e.g. 53V, to 43 + 53V = 96V by closing switch poles 5a-6b.

Period 8) The electrode voltages are levelled out to 48V each through the high voltage switch poles, 5c6c.

Period 9) The remaining voltage of el is discharged via a switch 7b to ground voltage.

Period 10) e2 is once again pumped up, this time by another lower voltage, say 52V, to 48 + 52V = 100V by closing poles 6a-5b.

Period 11) The electrode voltage is again levelled out, this time to S0V each through the high voltage switch poles 5c-6c.

Period 12) The remaining voltage of e2 is discharged via switch 7a to ground voltage.

Period 13) El is once again pumped up by 50V to 50 + 50V = 100V by closing 5a-6b.

Thus, finally a stable situation is reached which will continue having about 100V peak voltage.

Clearly, if the first electrode has a voltage level of 100V and the second 0V, when the high voltage switch-connection, which is across the electrodes of the lamp, conducts a balancing process takes place resulting in both electrodes having 50V each. With this process the remaining 50V of the first electrode is discharged by a switch either to ground or to batter depending on logic I/O level. Thereafter each pumping cycle brings the respective electrode up to the derived 100V potential.

In an alternative embodiment, periods 3 and 4 (and periods 9 and 10) described above can overlap in time.

For example, the voltage of the first electrode el can be discharged to ground at the same time as the second electrode e2 is charged up (i.e. they coincide in time). Alternatively, the charging of the second electrode e2 can be started part way through the discharge of the first electrode el, so that the discharging of the first electrode and the charging of the second electrode overlap in time.

The benefit with this type of driver is that less time is spent pumping the EL lamp, and so this results in lower power consumption.

Claims

1. A method of providing AC charging of a capacitive element, having first and second electrodes, from a DC source, the method comprising first and second cycles: the first cycle comprising a first charging step in which the first electrode is charged to a first voltage, a connection step in which the first electrode is connected to a second electrode, thereby equalising the voltage on each electrode to be half the said first voltage, and a first discharge step in which the voltage from the first electrode is discharged; and the second cycle comprising a second charging step in which the second electrode is charged to a second voltage, a connection step in which the second electrode is connected to the first electrode, thereby equalising the voltage on each electrode to be half the second voltage, and a second discharge step in which the voltage from the second electrode is discharged.

2. A method as claimed in claim 1, wherein the first discharge step and the second charging step are performed separately.

3. A method as claimed in claim 1, wherein the first discharge step coincides in time with the second charging step.

4. A method as claimed in claim 1 wherein the first discharge step overlaps in time with the second charging step.

5. A method as claimed in claim 1, wherein the first and second cycles are performed alternately for a predetermined number of repetitions.

6. A method as claimed in claim 5 wherein the first discharge step and second charging step of a repetition are performed separately.

7. A method as claimed in claim 5, wherein the first discharge step of a repetition coincides in time with the corresponding second discharge step of that repetition.

8. A method as claimed in claim 5, wherein the first discharge step of a repetition overlaps in time with the corresponding second charging step of that repetition.

9. A method as claimed in claim 5,6,7 or 8 wherein the second discharge step of a repetition coincides in time with the corresponding first charging step of the next repetition.

10. A method as claimed in claim 5,6,7, or 8, wherein the second discharge step of a repetition overlaps in time with the corresponding first charging step of the next repetition.

11. A method as claimed in any one of the preceding claims, wherein the capacitive element is an electroluminescent lamp.

12. Apparatus for providing AC charging of a capacitive element having first and second electrodes, from a DC source, the apparatus comprising: first cycle means for charging the first electrode to a first voltage, thereafter connecting the first electrode to the second electrode, thereby equalising the voltage on each electrode to be half the said first voltage, and then discharging the voltage from the first electrode; and second cycle means for charging the second electrode to a second voltage, thereafter connecting the second electrode to the first electrode thereby equalising the voltage on each electrode to be half the second voltage, and then discharging the voltage from the second electrode.

13. Apparatus as claimed in claim 12, comprising control means for operating alternately the first and second cycle means for a predetermined number of repetitions.

14. Apparatus as claimed in claim 12, comprising control means for operating the first and second cycle means such that discharging the voltage from the first electrode coincides in time with charging the second electrode to the second voltage.

15. Apparatus as claimed in claim 12, comprising control means for operating the first and second cycle means such that discharging the voltage from the first electrode overlaps in time with charging the second electrode to the second voltage.

16. Apparatus as claimed in any one of claims 12 to 15, wherein the capacitive element is an electroluminescent lamp.

17. A method of providing charging of a capacitive element substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.

18. Apparatus for providing charging of a capacitive element substantially as hereinbefore described with reference to, and as shown in, Figure 4 of the accompanying drawings.