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Controlled Discharge Circuit of the Electroluminescent Lamp Driver

IP.com Disclosure Number: IPCOM000012184D
Published in the IP.com Journal: Volume 3 Issue 5 (2003-05-25)
Included in the Prior Art Database: 2003-May-25
Document File: 3 page(s) / 131K

Publishing Venue

Siemens

Related People

Juergen Carstens: CONTACT

Abstract

The Electroluminescent Lamp (EL) is a semiconductor crystal placed between the plates of a “capacitor”. When high voltage is applied across the electrodes of an EL lamp, the resulting electrical field excites the atoms to a higher energy state. When the electrical field is removed, the atoms fall back to a lower energy state, emitting photons as visible light. The color can be controlled by selecting the phosphor type, by adding fluorescent dyes in the phosphor layer, by using a color filter over the lamp, or a combination of these processes. The brightness increases approximately with the square of the applied voltage and increases almost linear with the frequency. Higher voltage and/or frequency affect the lamp life, with higher frequencies generally decreasing the lifetime more than increased voltage. The lamp is basically a strip of plastic that is coated with a phosphorous material, which emits light when a high voltage which was first applied across it, is removed or reversed. Long periods of the DC voltage to the material tend to break down the material and reduce its lifetime. With these considerations in mind, the ideal signal to drive an EL lamp would be a high voltage sine wave. It is not easy to generate a high voltage sine wave form from a low DC voltage, but with different approaches a signal similar to a sine wave could be created. The EL lamp behaves almost like a capacity. It has to be charged and then discharged in some way. How the charge and the discharge are carried out is a technical challenge. As long as we have only one load with a specific characteristic, the components of the charge and the discharge circuit could be optimized for this particular lamp. On the other hand it is a difficult to find a solution, which can drive a display with several lamps (each lamp connected to a separate channel) without any change in the driving voltage of them and without any interference and cross talk between them. In our case the range of the EL lamps was very large, between 1cm2 (1nF) and 100cm2 (100nF), which leads to several technical problems.

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Controlled Discharge Circuit of the Electroluminescent Lamp Driver

Idea: Tibor Olah, AT-Vienna; Siegfried Krainer, AT-Vienna; Herbert Huber, AT-Vienna

The Electroluminescent Lamp (EL) is a semiconductor crystal placed between the plates of a "capacitor". When high voltage is applied across the electrodes of an EL lamp, the resulting electrical field excites the atoms to a higher energy state. When the electrical field is removed, the atoms fall back to a lower energy state, emitting photons as visible light. The color can be controlled by selecting the phosphor type, by adding fluorescent dyes in the phosphor layer, by using a color filter over the lamp, or a combination of these processes. The brightness increases approximately with the square of the applied voltage and increases almost linear with the frequency. Higher voltage and/or frequency affect the lamp life, with higher frequencies generally decreasing the lifetime more than increased voltage.

The lamp is basically a strip of plastic that is coated with a phosphorous material, which emits light when a high voltage which was first applied across it, is removed or reversed. Long periods of the DC voltage to the material tend to break down the material and reduce its lifetime. With these considerations in mind, the ideal signal to drive an EL lamp would be a high voltage sine wave. It is not easy to generate a high voltage sine wave form from a low DC voltage, but with different approaches a signal similar to a sine wave could be created.

The EL lamp behaves almost like a capacity. It has to be charged and then discharged in some way. How the charge and the discharge are carried out is a technical challenge. As long as we have only one load with a specific characteristic, the components of the charge and the discharge circuit could be optimized for this particular lamp. On the other hand it is a difficult to find a solution, which can drive a display with several lamps (each lamp connected to a separate channel) without any change in the driving voltage of them and without any interference and cross talk between them. In our case the range of the EL lamps was very large, between 1cm2 (1nF) and 100cm2 (100nF), which leads to several technical problems.

First the low DC supply has to be up converted. The polarity of the voltage connected to the EL lamp has to be changed with a certain frequency (in our case it was 800Hz) to create a periodically changing electrical field. Since the lamp behaves like a capacitor, after the charge period there is a high voltage across it. The sine wave would be the ideal waveform for the EL lamp from the lifetime point of view. By simply reversing the polarity would lead to problems from the lifetime and power dissipation points of view. The large discharge current and the abruptly changing electrical field might drastically decrease the lifetime of the EL lamp. In the conventional solutions a part of the power used by the lamp driver is dissipa...