GB2379098A

GB2379098A - Tunable capacitive load driver for electroluminescent lamp

Info

Publication number: GB2379098A
Application number: GB0120520A
Authority: GB
Inventors: Mark Purcell
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-08-23
Filing date: 2001-08-23
Publication date: 2003-02-26
Anticipated expiration: 2021-08-23
Also published as: WO2003019987A1; GB0120520D0; GB2379098B

Abstract

An efficient power supply for driving a capacitive load, such as an electroluminescent lamp, which can be tuned either electronically or mechanically to resonate at different frequencies, and which can automatically compensate for ageing and temperature characteristics of the load. The driver consists of a parallel resonant LC oscillator circuit 14 that uses the load 7 as the capacitance part of the oscillator, and a variable inductor 8 as the inductance part of the oscillator. The electronically tunable oscillator can form a voltage controlled oscillator which can be included within a phase locked loop 4 to automatically lock itself to a known frequency irrespective of capacitive load. Part of the phase locked loop control voltage can be fed into the voltage output control stage 3 to compensate for the reduction of light output due to the ageing characteristics of an electroluminescent lamp. This is done by gradually increasing the output voltage as the EL capacitance falls so as to keep the light output constant. In addition changes in capactitance due to variations in ambient temperature can be conpensated for, and the frequency hence colour of the emitted light automatically kept constant. The output voltage to the load 7 is isolated and fully floating, and can be referenced to a fixed voltage (usually ground) to prevent excessive voltage excursions. The driver exhibits inherent soft start and soft shutdown characteristics to eliminate high voltage spikes on power up or power down. The inductance and capacitance of the parallel resonant circuit acts as a filter to remove any high frequency content.

Description

TUNABLE AND EFFICIENT CAPACITTVE LOAD DRIVER This invention relates to an efficient power supply for driving a capacitive load, such as an electroluminescent lamp, which can be tuned either electronically or mechanically to resonate at different frequencies, and which can automatically compensate for ageing and temperature characteristics of the load.

A piece of electroluminescent (EL) material is essentially capacitive. To excite the EL into emitting light one has to drive a high AC voltage wavefonn, preferably sinewave, across the EL material. The voltage across the EL predominantly determines the brightness, and the frequency predominately defines the colour of emitted light.

There are several circuits available to drive capacitive panels efficiently. One common method is to use high voltage inverters, typically via a half or full bridge drive circuit. These circuits have several disadvantages from both a safety and electromagnetic interference (EMI) point of view. If the transistors were to be destroyed, they might blow short circuit; this can expose the panel to the maximum voltage and represents a safety hazzard. The inverters also typically use high voltage pulse width modulating, which can be a significant source of EMI. Extra complexity using opto-isolation is usually necessary to have fully floating outputs. These circuits can also exhibit hard startup and shutdown characteristics which can also cause problems.

EL lamps or capacitive panels can also be driven efficiently by making the panel the capacitor part of a resonant LC oscillator circuit. This traditionally means having to select the right inductance for the capacitance of the panel (which varies proportionally with area), in order to keep the frequency and hence colour of an EL panel constant. These circuits can have isolated output statges and as such are safer than their high voltage inverter counterparts, but they do not compensate for a reduction in capacitance as one would get from an ageing piece of EL material and hence the frequency would gradually increase over time. In an EL panel this would show itself as a change in colour. They also do not compensate for the loss in light output as an EL panel ages, or compensate for changes in capactitance due to temperature variations. These circuits do, however, have softer startup and shutdown charateristics than the high voltage inverters.

The object of this invention is to provide a power supply driver for electroluminescent (EL) lamps or other capacitive loads based on a resonant LC oscillator circuit that is very efficient, and that can either be manually tuned, or can automatically tune itself to oscillate at a preset or variable frequency for a range of panel sizes. For a piece of EL material this keeps the colour that the EL panel emits constant independent of panel size, and this in turn allows a single driver to be designed that can be used with a range of EL or capacitive plate sizes. The tuning mechanism involves turning a normal transformer, which usually has a fixed inductance as seen by the load, into a variable inductor which can either be varied electronicically or mechanically.

In addition this invention allows the driver to automatically compensate for the ageing charateristics of an EL panel or capacitive load by increasing the voltage applied to it while keeping the frequency constant. It also compensates for the variation of capacitance with temperature.

The invention has soft-start and soft-shutdown charateristics that prevent high voltage spikes from appearing on the output during power up or power down cycles. It also has a fully floating and isolated output stage which can be referenced to a known voltage (usually ground) to prevent excessive output voltage excursions. The output drive waveform is inherently filtered from high frequency noise by the inductor and capacitor of the LC circuit to reduce EMI to a minimum, and is symmetrical about the centre of the waveform.

The invention can be powered by a low voltage DC source, although variations of the invention may use higher voltages. The output voltage can be varied by changing the DC supply voltage. By providing an efficient AC to DC converter the unit may be powered from a range of AC power sources including mains voltages.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings in which: FIGURE 1 shows a block diagram of the driver.

FIGURE 2 shows the oscillator section with the variable inductor.

With reference to Figure 1: The driver itself consists of a parallel LC resonant voltage controlled oscillator 14 which uses a capacitive load or EL lamp 7 as the capactitance part of an LC resonant circuit. The inductor part 8 of the resonant circuit is made variable by means of an external control voltage 5, which changes the current 16 flowing through a control winding 10 to vary the inductance. The circuit can be powered by an AC power source 1 which converts the AC input to low voltage DC voltage 2, or can be directly powered by low voltage DC. The output voltage that the resonant circuit produces is directly proportional to the DC supply voltage 15 to the oscillator, which is also made variable 4. The resonant output frequency is tapped 17 from the low voltage side of the transformer 8 and phase compared in the PLL chip 4 with a presettable or variable reference frequency 13. The PLL chip will charge and discharge an RC loop filter 5 to create a control voltage which is turned into a current 16 and fed into the control winding 10 to vary the frequency of oscillation to match the reference frequency 13; hence the output frequency becomes independent of the size of the capacitive load. Part of the control voltage is fed into the output voltage control circuit 3 to increase the output voltage as the capacitance decreases; this provides ageing compensation for EL lamps and keeps the light output constant over the EL lamp's lifespan. Changes in capacitance and light output due to temperature variations are compensated for by automatically keeping the frequency constant.

With reference to Figure 2 : The voltage controlled resonant LC oscillator is formed by replacing the single transformer of a Baxandall type parallel resonant LC oscillator with two separate transformers 8 in which all currents are equaL The control windings 10 add a DC magnetic field bias to both cores of these inductors, partially saturating them and hence changing their inductance. The control winding is wrapped around one transformer core, then the other, in such a way that the voltages that would be induced on the control windings cancel; this means the control winding does not load the circuit in any way. The resulting oscillator has a symmetrical output and can be tuned to oscillate over a wide range of frequencies.

Claims

CLAIMS 1) An efficient driver for capacitive loads such as an electroluminescent lamps, using a parallel resonant LC oscillator that consists of an electronically or manually variable inductor as the inductance part of the oscillator, and the load itself as the capacitance part of the oscillator.
2) A driver as claimed in Claim 1 where a control winding or windings, a permanent magnet or movable slug is used to vary the inductance part of the oscillator.
3) A driver as claimed in Claim 2 where a pair or number of pairs of equally wound transformers are used as the variable inductance part of the oscillator.
4) A driver as claimed in Claim 2 where a bobbin or multiple bobbin type transformers are used as the variable inductance part of the oscillator.
5) A driver as claimed in Claim 2 where a three legged type transformer or multiple transformers are used as the variable inductance part of the oscillator.
6) A driver as claimed in any preceding claim that is included in a phase locked loop circuit to keep the output frequency constant with varying load.
7) A capacitive load driver substantially as herein described and illustrated in the accompanying drawings.