GB2354380A

GB2354380A - Power supply circuit for electroluminescent device

Info

Publication number: GB2354380A
Application number: GB9916396A
Authority: GB
Inventors: David Gary Burns
Original assignee: Ultra Electronics Ltd
Current assignee: Ultra Electronics Ltd
Priority date: 1999-07-13
Filing date: 1999-07-13
Publication date: 2001-03-21
Also published as: GB9916396D0

Abstract

A circuit for powering an electroluminescent (EL) device comprises a load, including an inductance (16) and an EL device (18), an inverter (10) connected to a DC supply to provide an AC supply to the load, and a control means to monitor the waveform of the AC signal to time the switching of the inverter. Hence, the switching of the inverter can be adjusted to adapt to, for example, changes in the circuit in order to maintain the load in resonance. The inverter may be a bridge or H-bridge type inverter.

Description

2354380 POWER SUPPLY This invention relates to a power supply circuit for

an electroluminescent device.

A known kind of power supply circuit for an electroluminescent device comprises an AC power supply connected to an oscillatory circuit including the electroluminescent device.

The oscillatory circuit is driven at its resonant frequency by the AC source such that the voltage supplied to the electroluminescent device is amplified. However, this kind of power supply circuit is capable of powering electroluminescent devices of limited size only.

According to one aspect, the invention provides a circuit for powering an electroluminescent device, the circuit comprising a load comprising an inductance and an electroluminescent device, an inverter arranged to connect a DC signal to the load and to switch the sense in which the DC signal is connected to the load in order to establish an AC signal in the load, and control means which monitors the waveform of the AC signal to time the switching of the inverter. Hence, the switching of the inverter can be adjusted to adapt to, for example, changes in the circuit in order to maintain the load in resonance. Preferably the electroluminescent device and the inductance are in series. The inverter may be a bridge inverter, and preferably it is an H bridge type inverter.

Advantageously, the control means is arranged to cause the inverter to switch the sense of the DC signal after the waveform crosses a threshold. Preferably, the threshold is, approximately, the mid point of the peak-topeak amplitude of the waveform. Preferably, the control means comprises current sensing means for sensing the current in the load.

In one embodiment, the circuit comprises initiating means for causing the initiation of oscillations in the load. Preferably, this is achieved by arranging a start-up device to cause switching of the inverter at a predetermined frequency to initiate oscillations of the load.

i I 2 Preferably, the inverter comprises transistor based switches which are switched by switching waveforms.

According to a second aspect, the invention provides a method of supplying power for an electroluminescent device comprising the steps of inverting a DC signal by switching the sense in which it is applied to a load comprising an inductance and an electroluminescent device to establish an AC waveform in the load, and timing the switching by referring to the AC waveform.

Advantageously, the switching is triggered by the AC waveform crossing a threshold. The threshold may be, approximately, the midpoint of the peakto-peak amplitude of the waveform.

By way of example only, an embodiment of the invention will now be described with reference to the accompanying figures in which:

Figure I illustrates a power supply circuit for an electroluminescent load; and Fip-ure 2 illustrates an inverter for use in a power supply circuit for an electroluminescent load.

The power supply circuit of Figure 1, generally indicated 1, comprises an inverter 10 for converting a DC input to an AC output. The DC input to the inverter 10 is in the form of either a positive 12 or negative 14 supply signal. The operation of the inverter 10 will be described in more detail later with reference to Figure 2. The AC output of the inverter 10 is supplied to an inductor 16 and an electroluminescent device 18 which are connected in series. The electroluminescent device 18 is connected to ground via a resistor 20. The resistor 20 and the inductor 16, together with the capacitance of the electroluminescent device 18, constitute an oscillatory circuit. This oscillatory circuit is driven into resonance by the output of the inverter 10, causing an amplification of the voltage across, and powering, the electroluminescent device 18.

3 The circuit I contains a feedback arrangement which maintains the oscillatory circuit formed by the inductor 16, the electroluminescent device 18 and the resistor 20 in resonance. A current sensor 22 detects the waveform of the current flowing into resistor 20. Zero crossing detector 24 monitors this waveform and determines the instances when the current waveform crosses its zero amplitude level, i.e. the level substantially intermediate the maximum and minimum values of the amplitude of the current waveform. The zero crossing detector 24 supplies control signals to the inverter 10 via driver 26 for maintaining the resonance. The circuit I further comprises a start detector 28, the fianction of which will be described later with reference to Figure 2.

With reference to Figure 2, the inverter 10 comprises an H bridge 30. The DC supply signal is connected to rail 32 which is connected to earth via parallel paths 34 and 36.

Each of paths 34 and 36 contains a respective pair of MOSFET transistors in series. The output load to be driven by the inverter 10 is connected between a point on path 34 intermediate its transistors 38 and 40 and a point on path 36 intermediate its transistors 42 and 44.

The transistors 38 to 44 are supplied with drive signals which cause them to switch on and off in such a manner as to generate an AC signal in the output load comprising the inductor 16, the electroluminescent device 18, and the resistor 20. Transistors 38 and 44 are supplied with drive waveform "A" and transistors 40 and 42 are supplied with drive waveform "B", which is the inverse of drive waveform "A". Hence, the switching of transistor pair 38 and 44 is in an antiphase relationship with the switching of transistor pair 40 and 42. Drive waveform "A" is the output of driver 26 in Figure 1, and drive waveform "B" is its inverse.

The drive waveforms "A" and "B" are switched between logic high and logic low levels by the zero crossing detector 24 in Figure 1, as will be described later. When a drive waveform is at logic high, its respective transistors are rendered conducting. When a drive waveform is at logic low, its respective transistors are switched off. When drive waveform "A" is high, drive waveform "B" is low, and transistors 38 and 44 are switched on and transistors 40 and 42 are switched off Current then flows through the output load 4 of the inverter in a first direction, via components 38,16,18,20 and 44. When drive waveform "A" is at logic level low, drive waveform "B" is at logic level high, and transistors 40 and 42 are conducting and transistors 38 and 44 are switched off. Current then flows through the output load of the inverter 10 in a second direction, which is opposite to the first direction, via components 42,20,18,16 and 40. In this way, the DC input is converted to an AC output.

When zero crossing detector 24 (Figure 1) detects a zero crossing event, it changes the logic level of drive waveforin "A" to its opposite state. Correspondingly, drive waveform "B" is also changed to its opposite state. It will be appreciated that the resulting effect is for the current direction in the output load of the inverter 10 to be reversed whenever a zero crossing event is detected in the oscillatory waveform. sensed by current sensor 22. This maintains the resonance of the oscillatory arrangement of the inductor 16, the electroluminescent device 18, and the resistor 20, the frequency of the inverter output effectively being adapted by the zero crossing detector 24 as necessary. The final amplitude of the output AC waveform can be adjusted by changing the level of the DC input signal to the inverter 10.

When the power supply circuit is first activated, or when it is restarted, oscillations must first be established in the oscillatory arrangement. To achieve this, a start detector 28 signals the zero crossing detector 24 to change the logic level of the drive waveforms "A" and "B" supplied to the inverter 10 at a predetermined frequency. This results in alternation of the current direction through the output load of the inverter at a frequency determined by the start detector 28. Once oscillations have been established in the oscillatory arrangement by the transistor switching action imposed by the start detector 28, control of the circuit is given to the zero crossing detector which changes the logic level of the drive waveforms "A" and "B" upon detection of a zero crossing event in the current waveform.

Claims

1. A circuit for powering an electroluminescent device, the circuit comprising a load comprising an inductance and an electroluminescent device, an inverter arranged to connect a DC signal to the load and to switch the sense in which the DC signal is connected to the load in order to establish an AC signal in the load, and control means which monitors the waveform of the AC signal to time the switching of the inverter.

2. A circuit according to claim 1, wherein the control means is arranged to cause the inverter to switch the sense of the DC signal after the waveform crosses a threshold.

3. A circuit according to claim 2, wherein said threshold is substantially the midpoint of the peak to peak amplitude of the waveform.

4. A circuit according to any one of claims I to 3, wherein the inverter comprises transistor based switches which are switched by switching waveforms.

5. A circuit according to any one of claims I to 4, wherein the inverter is a bridge inverter.

6. A circuit according to any one of claims I to 5, further comprising initiating means for causing the initiation of oscillations in the load.

7. A circuit according to claim 6, wherein the initiating means is arranged to cause switching of the inverter at a predetermined frequency to initiate oscillations of the load.

8. A method of supplying power for an electroluminescent device comprising the steps of inverting a DC signal by switching the sense in which it is applied to a load comprising an inductance and an electroluminescent device to establish an AC waveform. in the load, and timing the switching by referring to the AC waveform.

6

9. A method according to claim 8, wherein the switching of the DC signal is triggered by the AC waveforin crossing a threshold.

10. A method according to claim 9, wherein the threshold is substantially the midpoint of the peak to peak amplitude of the waveform.

11. A method according to any one of claims 8 to 10, further comprising the step of causing the initiation of oscillations in the load by switching the DC signal at a predetermined frequency to initiate oscillations of the load.

12. A circuit for powering an electroluminescent device, the circuit substantially as substantially hereiribefore described with reference to the accompanying figures.

13. A method of supplying power for an electroluminescent device, the method substantially as hereinbefore described with reference to the accompanying figures.