GB2300529A - Driver circuit for electroluminescent devices - Google Patents

Abstract

The driver circuit comprises a timing circuit for supplying high frequency pulses S1 to a transistor Q1 and lower frequency pulses S2 to a transistor Q2. The high frequency pulses enable an inductor L1 to charge the electroluminescent device to a high voltage from a low voltage battery supply before the voltage is reset to ground potential by pulses S2 fed to the transistor Q2.

Description

TITLE Low Cost High Efficient Driver For Electroluminescent Devices BACKGROUND OF INVENTION The phenomenon of electroluminescence in phosphor was first discovered and studied by Destriau in 1947. Devices based on this phenomenon were not used widely until the 1990s. As Liquid Crystal Display devices now being used widely in commercial and domestic type of battery operated products, electroluminescent displays - consisting of a thin sheet of light emitting coatings - become the choice as back lighting devices. However, one of the major problems in using electroluminescent displays in battery operated devices is to deduce a high voltage (100 volts) AC driving signal of around 400 Hz from the battery.

In 1984, Anthony D'Onofrio and Jack Gunzy proposed a DC to AC inverter using astable multivibrator which uses magnetic components operating at frequency of around 3 - 5 kHz in order to convert magnetic energy into electrical energy for a capacitive electroluminescent lamp. At this frequency, a large size and expensive inductive device is required which prohibits this design to be used in a portable environment. Later, in 1985, Peter J Kindlmann proposed an alternative method which consists of a full bridge switching circuits for a similar application. Kindlamann's design requireS an external crystal and additional frequency dividing circuit which increases the component count of the design and makes it impractical for low cost implementation.

The proposed design is aimed to overcome the shortfalls of the above two methods. With the introduction of a low cost ASIC to provide a self generated timing signal and a simple DC to AC step up converter, a suitable driving voltage source can be obtained for lighting capacitive electroluminescent devices at their optimal operating condition.

SUMMARY OF INVENTION The present invention is a driver circuit which can be used to drive capacitive electroluminescent devices. The circuit consists of all the necessary timing, trigger and driving signals required to be used in a low voltage application, such as those served as back lighting for small to medium sized portable LCD based equipment. Some examples of those applications include time-pieces, pagers, wire and wireless communication handsets and stations, digital diaries, calculators, remote control units for consumer electronics, electronic personal organizers and many other electronic related instruments and devices.

The driver circuit consists of an inverter, used to step up the DC voltage as applied by a battery source. An anti-phase trigger signal is used to allow current to charge up the small size inductor to a high enough voltage to be used for driving the electroluminescent devices.

This voltage can be as high as 500 volts at no load with a DC supply voltage of 5 volts or 100 to 200 volts at no load with a DC supply voltage of 0.9 to 1.5 volts. A high frequency signal controls the charging action preferably in a range of 30 - 50 pulses per cycle to produce a high enough voltage to be used for lighting the EL device.

With suitable choice of the timing signals, a number of successive charging actions can be achieved to obtain a voltage which is high enough for the electroluminescent device to be operated in its optimal condition.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a circuit diagram, which consists of a two terminal inductor (L), a diode (D), three resistors (R1 R2 and R3), two npn transistors (Q1 and Q2) and a timing/trigger sub-circuit in a form of an Application Specific Integrated Circuit (ASIC) labelled IC1.

Figure 2 is the timing/trigger signal source as generated by the IC1, which comprises two clocking signals, namely: 1. a 32 kHz square wave with a 95% duty cycle 2. a 400 Hz square wave with a 50% duty cycle These two clocking signals have a nominal voltage level of 1 volt or just sufficient to turn on the two npn transistors (Q1 and 02). The 400 Hz signal (S2) is fed to the base of transistor Q2 and the signal S1 in a form of an inverted S2 being modulated by the 32kHz signal is fed into the base of the other transistor Q1.

Figure 3 is the typical no load voltage across the output of the proposed driving circuit (Vo).

Figure 4 is the typical voltage across a lcm x 2.5cm electroluminescent lamp.

DESCRIPTION OF THE PREFERRED EMBODIMENT The electroluminescent (EL) device drive circuit which is shown in FIG.1 is to be used to drive a capacitive EL lamp that provides backlighting for LCD display units. This circuit is primarily designed to be operative with a minimum of a single battery cell of 1.5 volt. The ASIC is a CMOS logic IC producing the necessary timing and trigger signals for turning on the two npn transistors (Q1 and Q2) In the drive circuit, inductor (L), which is a two terminal inductor, has one terminal electrically connected to the diode (D) and the collector terminal of the npn transistor (Q1) at a common point and the other terminal connected electrically to battery +V. In the first 29.6875 second, transistor (Q1) is at its non-conducting state.The next 1.9625 second the transistor (Q1) is turned on by the trigger signal (S1).

This sudden turn on charges a voltage across the inductor (L) and the forward bias of the diode allows the voltage at (Vo) to be equal to the charged voltage. The charging sequence will be carried on by the remaining 39 similar turn on pulses of the trigger signal (S1). These charging pulses will force the cathode of the diode (D) to a high voltage respect to ground. This voltage can reach a nominal 200 volts. The forty charging sequences take a total of 40x(29.6875+1.9625) seconds, i.e. 1,250 seconds to complete. The next 1,250 seconds, transistor (Q2) will conduct and cause the charged up voltage to reset to ground level. The total charge up and reset sequence will take a total of 2,500 seconds to complete. This complete cycle is then repeated again and again to form a drive signal of 200volt at 400Hz at no load condition. A resistance of a typical capacitive EL lamp of surface area of 2.5cm2 will cause the drive voltage to drop to around 80 volts, which is an optimal driving signal for the EL lamp.

Claims (5)

1. A driver circuit for capacitive electroluminescent device comprising a small sized inductor connected to a collector terminal of a first npn transistor and through a diode to the capacitive electroluminescent device; a resistor connected between the capacitive device and a collector terminal of a second npn transistor; and a timing and trigger circuit for driving the first transistor to conduction at a high frequency for a number of pulses for each one pulse for which the timing and trigger circuit drives the second transistor to conduction.

2. A driver circuit according to claim 1, wherein the output voltage to the capacitive device under a no load condition is about 200 volts at 400 Hertz.

3. A driver circuit according to claim 1, wherein the output voltage to the capacitive device under a no load condition is about 100 volts at 400 Hertz.

4. A driver circuit according to claim 2 or claim 3, wherein the said output voltage is obtained from a DC voltage source of from 0.9 to 1.5 volts.

5. A driver circuit for an electroluminescent device substantially as described herein with reference to the drawings.