Patent Publication Number: US-2007103089-A1

Title: Circuit for driving cold cathode tubes and external electrode fluorescent lamps

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefits of U.S. Provisional Patent Application Ser. No. 60/570,533, filed May 11, 2004 and Ser. No. 60/573,319, filed May 21, 2004 and is a continuation of U.S. patent application Ser. No. 11/127,605, filed May 11, 2005. The disclosure of these applications are hereby incorporated by reference in their entirety, including all figures, tables, and drawings. 
    
    
     BACKGROUND OF THE INVENTION  
      Increasing amounts of energy are being consumed everyday while energy costs continue to skyrocket. Recent emphasis on convenience and safety can be attributed to some of the demand for increased energy. For example, vending machines are routinely placed on street corners, in public buildings and near gas stations to conveniently offer their wares anytime of day. Banks now provide access to their services to customers 24 hours a day through Automatic Teller Machines (ATMs). ATMs are well lit to draw attention to their location however it is also important that these ATMs be well lit for safety. Many of these devices are lit with fluorescent lights. Fluorescent lights are also increasingly being used in industrial settings and in the home. Fluorescent lamps use a considerable amount of energy and produce a lot of heat. Further, when these lamps are provided with too much current, light output weakens and becomes irregular. Many circuits have been designed in an attempt to drive fluorescent lamps and cold cathode fluorescent lamps more efficiently (U.S. Pat. No. 5,495,405; U.S. Pat. No. 5,854,543; U.S. Pat. No. 5,930,121; U.S. Pat. No. 5,959,412; U.S. Pat. No. 6,118,221; and U.S. Patent Application Publication No. US 2004-0056610 A1). Cold cathode tubes and external electrode fluorescent lamps consume less energy and are more efficient and reliable. Replacing fluorescent lamps with cold cathode tubes or external electrode fluorescent lamps that are driven by an energy efficient circuit would reduce overall energy consumption and save considerable money for many consumers.  
      All patents, patent applications, provisional patent applications and publications referred to or cited herein, are incorporated by reference in their entirety to the extent they are not inconsistent with the explicit teachings of the specification.  
     SUMMARY OF THE INVENTION  
      The invention is a circuit for driving cold cathode tubes (CCFL) and external electrode fluorescent lamps (EEFL) that consumes very little energy. The subject circuit unlike conventional CCFL and EEFL drivers cannot be shifted and therefore is an efficient, reliable driver. A high voltage inductor or a high voltage/high frequency transformer in combination with a current limiting capacitor, the lamp and a low resonance value capacitor are tuned to an oscillation frequency set by a steady state square wave oscillator generator. The circuit has a thermal shutdown feedback circuit to monitor temperature and a lamp detection feedback/lamp disconnect driver to insure the circuit drives the lamps reliably. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a circuit diagram showing a preferred embodiment of the circuit of the subject invention using alternating current input.  
       FIG. 2  is a circuit diagram of another preferred embodiment of the circuit of the subject invention using alternating current input.  
       FIG. 3  is a schematic diagram of the preferred embodiment of the circuit shown in  FIG. 2 .  
       FIG. 4  is a circuit diagram of another preferred embodiment of the circuit of the subject invention using direct current input.  
       FIG. 5  is a circuit diagram of another preferred embodiment of the circuit of the subject invention using direct current input.  
    
    
     DETAILED DESCRIPTION OF INVENTION  
      The invention is a circuit for driving cold cathode fluorescent lamps (CCFL) and external electrode fluorescent lamps (EEFL) that uses significantly less power then known lamp driving circuits. The circuit can be configured for a single channel or for multiple channels.  
      The circuit of the subject invention must be supplied with a clean source of direct current (DC).  FIGS. 1-3  show preferred embodiments of the subject circuit powered by alternating current (AC).  FIGS. 4-5  show preferred embodiments of the subject circuit powered by DC. One skilled in the art recognizes that there are many ways to obtain a clean DC source.  FIG. 3  shows a particularly preferred method for obtaining such a source. The circuit shown in  FIG. 3  uses an AC line source of 110 volts (V) or 220 V AC. To clean the input of harmonic noise, the circuit includes a common mode line filter  10 , a varsistor, a filter capacitor, a differential line filter  12 , two high voltage diodes and three filter capacitors. The common mode line filter and the differential line filter are set to have inductances that suit the running frequency. The varsistor and filter capacitors are set to the AC line voltage. The high voltage diodes are about 700 V. A voltage selection switch  14  allows the circuit to use 110 V or 220 V AC. For the circuit of the subject invention to be effective, it is only necessary that the source be filtered by capacitance and inductance to clean the source of harmonic noise. This can most often be accomplished for DC input with a differential line filter.  
      A steady state square wave half bridge oscillator generator  16  is used to set a running frequency for the subject circuit. The oscillator generator is preferably an integrated circuit (IC). A DC voltage down converter  18  provides power to the IC at start-up and continues to power the system as the circuit runs. The oscillator generator has a low side and high side outputs for the high side MOSFET  20  and a low side MOSFET  22  which complete a half bridge. The MOSFETs should be high voltage of about, at least, 500 V. The circuit is protected from overheating by a thermal shutdown feedback  24  circuit which monitors the temperature of the power switching MOSFETS and is coupled to one of the heat sinks of a MOSFET. The thermal circuit shuts off the DC power if the circuit overheats.  
      A DC blocking capacitor  26  insures no DC enters the AC supply rail. The DC blocking capacitor is a high switching ballast inverter, for example, a high frequency, metalized, polypropylene film capacitor is useful in the subject invention. The AC rail supplies one or more channels. A channel comprises a lamp driver and element, a non-inductive or non-capacitive loading lamp detection feedback and a lamp disconnect driver. The lamp driver and element include a high voltage inductive device with a current limiting resonance capacitor  32 , the lamp unit  34  and a low resonance value capacitor  36  two which are in series are set and tuned to the oscillation frequency of the oscillator generator to achieve resonance. The high voltage inductive device can be a high voltage inductor  28  or a high voltage/high frequency transformer  30 . The high voltage inductor&#39;s inductance must be calculated for the running frequency and has a dielectric breakdown strength of least 2000 V root mean square. The high voltage/high frequency transformer has turns calculated for the inductance to suit the running frequency, a core that will not saturate and a turn ratio defined and tuned for the frequency and output voltages. In an exemplified embodiment the running frequency was set at 43 kilohertz (KHz). This frequency was chosen because it achieves optimum lamp brightness in the chosen lamp while using little power. A standard circuit to drive a compact fluorescent lamp uses 120 watts. The circuit shown in  FIG. 1  uses only 28 watts. This circuit however uses a less efficient high voltage inductor to boost voltage. A more preferred embodiment of the circuit of the subject invention is shown in  FIGS. 2 and 3 . In this embodiment, a high voltage/high frequency transformer is used to achieve the necessary voltage. The transformer is more efficient than the inductor and thus the circuit uses only 19 watts.  
      Each channel of the subject circuit further includes a non-inductive or non-capacitive loading lamp detection feedback  38  and a lamp disconnect driver  40 . This lamp feedback/disconnect prevents false disconnects at start-up due to cold bulbs. The lamp detection feedback comprises a current sensing resistor which is a low resistance element to minimize loss, a fast recovery diode to convert power to DC, a filter, a resistor/capacitor (RC) buffer circuit, a voltage comparator and a latching silicon control rectifier (SCR). The voltage comparator is powered by a high frequency step down transformer and power converter  41  and provides a fixed reference point that addresses each channel. The lamp disconnect driver is a low input voltage/low current relay that is a mechanical device for disconnecting the lamp upon a signal from the lamp detection feedback. This insures that if one lamp is disconnected, the single lamp is shut off and the remainder of the lamps on the rail will continue to run. The disconnect driver further disconnects the entire channel eliminating all power to the channel which prevents the user from being shocked. The circuit also eliminates noise from the disconnected channel.  
      The subject circuit is reliable since it runs at a constant frequency and cannot be shifted. The exemplified embodiments show the circuit of the subject invention driving three channels and lamps. It is important to note the circuit effectively drives one and more than three channels. The lamps do not flicker. The thermal shutdown circuit prevents the circuit from failing, burning the circuit case, a fire risk and destroying the circuit. The lamp detection feedback and lamp disconnect driver prevent premature lamp disconnect and automatically reconnect and refire the lamps in the event of power loss or lightening strike.  
       FIGS. 4 and 5  show alternative embodiments of the circuit of the subject invention. In these embodiments, there is a DC source input. The circuit is simpler in that it is unnecessary to convert from AC to DC input. The circuit in  FIG. 4  uses an initial DC voltage step up and step down lamp igniter and variable driver circuit  42  to step-up the voltage. The lamp igniter is a high frequency up/down power converter that converts line voltage to maximize the efficiency of the conversion. A high voltage inductor  28  in the lamp driver provides the final voltage increase. The circuit in  FIG. 5  has a high voltage/high frequency step-up transformer  30  in the lamp driver which makes an additional voltage step-up unnecessary.  
      It is understood that the foregoing examples are merely illustrative of the present invention. Certain modifications of the articles and/or methods employed may be made and still achieve the objectives of the invention. Such modifications are contemplated as within the scope of the claimed invention.