Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
2000-10-06
2001-11-13
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S025000, C315S219000
Reexamination Certificate
active
06317347
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to LCD back lighting. More particularly, the invention is directed to a voltage feed push-pull resonant inverter that permits improved dimming performance.
BACKGROUND OF THE INVENTION
There has been an ever increasing demand for LCD displays within the past few years. Such displays are being employed by all sort of computer devices including flat display monitors, personal wireless devices and organizers, and large public display boards.
One disadvantage with the commercially available LDC displays is the dynamic range of the dimming control. Typically, LCD displays employ a backlighting arrangement which includes a cold cathode flourescent lamp, CCFL, that provides light to the displayed images. The dimming range of the available LCD backlight ranges from 100% to about 30% of the total lumen value. However, as the demand for better quality displays is increasing, and as many flat panel display monitors are taking the place of conventional CRT monitors, there is a need to increase the dimming range from 100% to about 10% or even 5% of the total lumen value. The increased dimming range provides for a remarkably improved image quality.
Another advantage of a wider dimming range is power savings. In many portable systems, such as wireless devices and personal digital assistants such as personal organizers, there is a considerable need for designs that save more battery power over longer periods of time before recharging. An expanded dimming range allows for more power conservation in the battery.
One of the reasons for the limited dimming range in the current LCD backlight systems is the driver circuitry that drives the CCFL lamp. Typically, the lamp is driven by a DC voltage source via an inverter circuit. Two major topologies for driving CCFL lamp include a half bridge inverter and a buck power stage plus current-feed push-pull inverter. In both topologies a transformer is employed to increase the input voltage source from few volts (in the order of 6-20 volts) to few hundreds of volts (in the order of 600 to 1800 volts). The circuitry coupled to the primary windings of the transformer is employed to provide a voltage signal as a voltage source, while the circuitry coupled to the secondary winding of the transformer drives the lamp.
One shortcoming of a half bridge inverter stems from the use of the low input voltage source that drives the inverter. Due to this low input voltage source, the half bridge inverter needs a high turn-ratio transformer to step up the output voltage. However, the efficiency of the driver decreases with the increase in the amplitude of the current in the primary circuit.
Furthermore, because the secondary circuit includes a ballast capacitor that is coupled to the lamp in series, the output current provided by the secondary winding in addition to a real component would include a reactive component, which is not used for illuminating the lamp. As a result the efficiency decreases.
One disadvantage with the buck power stage current-feed push-pull inverter is the use of two power stages; the buck stage and the push-pull inverter stage. Each stage contributes to the decrease in efficiency. For example, if the efficiency of the buck stage is about 90% and the efficiency of the current feed push-pull inverter is also about 90%, the combined efficiency of the system reduces to about 81%.
In addition, in order to achieve dimming, it is necessary to vary the voltage provided at the buck stage. However, the primary portion of the buck power stage plus current feed push-pull inverter employs a magnetizing inductance, and as a result it behaves as a voltage source. This behavior, in turn, causes the secondary portion of the inverter to act as a voltage source also. During dimming, as the voltage source level decreases, the impedance of the lamp increases, causing variation in the voltage level. As the dimming increases beyond a certain point in the total output lumen of the lamp, a flickering in the lamp becomes noticeable.
Thus, there is a need for an LCD backlight driving circuit that overcomes the disadvantages of the topologies described above, and provides a substantially wider dimming range as compared with those topologies.
SUMMARY OF THE INVENTION
According to one embodiment, the present invention relates to an inverter circuit for supplying power to a lamp. The circuit comprises a DC voltage supply source. The circuit also comprises a transformer having a primary and a secondary winding. The secondary winding is coupled to the lamp and the primary winding is coupled to the DC supply source. The circuit also comprises a pair of inductors coupled in series to opposite ends of the primary winding. A capacitor couples the inductors together. The circuit is configured such that the pair of inductors, in conjunction with the capacitor, alternately resonate to provide an AC current supply source to the lamp.
The circuit further comprises a pair of switches which alternately turn “on” and “off”, thereby causing the pair of inductors to alternately resonate. A control driver is coupled to the pair of switches and is configured to alternately turn on and off the pair of switches. Advantageously, the control driver comprises a PWM control driver. Advantageously, the primary winding includes a transformer tap that defines a first section and a second section, and the DC supply source is coupled to the primary winding.
The above description sets forth rather broadly the more important features of the present invention in order that the detailed description thereof that follows may be understood, and in order that the present contributions to the art may be better appreciated. Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.
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patent: 5495405 (1996-02-01), Fujimura e tal.
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patent: 5822201 (1998-10-01), Kijima
patent: 6008589 (1999-12-01), Deng et al.
patent: 6040662 (2000-03-01), Asayama
Nguyen Matthew
Philips Electronics North America Corporation
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