This invention relates to a battery operated power supply for an electroluminescent (EL) lamp and, in particular, to the combination of a voltage boost driving an inverter for powering an EL lamp.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer may include a phosphor powder or there may be a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current. Because an EL lamp is a capacitor, alternating current must be applied to the electrodes to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage, the current through the EL lamp ceases, and the lamp stops producing light.
In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by an inverter that converts direct current into alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. The frequency of the alternating current through an EL lamp affects the life of the EL lamp, with frequencies between 200 hertz and 1000 hertz being preferred. Ionic migration occurs in She phosphor at frequencies below 200 hertz. Above 1000 hertz, the life of the phosphor is inversely proportional to frequency.
The prior art discloses using any one of several types of inverters in which the energy stored in an inductor is supplied to an EL lamp as a small current at high voltage. The voltage on the lamp is pumped up by a series of pulses from the inverter. The direct current produced by inverter must be converted into an alternating current in order to power an EL lamp. U.S. Pat. No. 4,527,096 (Kindlmann) discloses a switching bridge for this purpose. U.S. Pat. No. 5,313,117 (Kimball) discloses an alternative design that produces an AC voltage directly.
A typical EL lamp draws approximately one milliampere of current per square inch of light emitting surface at full brightness. In the prior art, most of the applications for EL lamps required a lamp having an area of one to three square inches. There are many applications for EL lamps in which the area of the lamp exceeds five square inches and existing power supplies are not very efficient, in terms of luminance per watt, at providing sufficient power for an EL lamp; i.e. providing the same luminance at the same efficiency as smaller inverters.
Larger inverters, that is, inverters producing higher voltage, greater current, or both, tend to be less efficient because of greater switching losses. Typically, a higher input voltage to an inverter causes an EL lamp to produce more light. Any power conversion circuit imposes a loss and adding a boost ahead of an inverter would not be expected to be more efficient than the inverter alone because of the loss, typically ten to fifteen percent, in the boost.
A high power inverter for EL lamps uses transistors that can conduct larger current, which increases the size of the semiconductor die and, therefore, the cost of the inverter. Preferably, the cost of a boost and a low power inverter is less than the cost of a high power inverter alone.
In view of the foregoing, it is therefore an object of the invention to provide an efficient inverter for powering large area lamps, e.g. lamps having an area greater than five square inches.
Another object of the invention is to provide a power supply for operating large area lamps with less power than in the prior art.
A further object of the invention is to provide a power supply for powering large area lamps that is more cost effective than inverters of the prior art.