Abstract:
A driver circuit provides electrical energy from a power source to a fluorescent lamp such as that used in a flat-panel or other liquid crystal display (LCD). The circuit includes a transformer having a primary winding and a secondary winding, with the ends of the secondary winding coupled to the fluorescent lamp. A first switch switchably provides a drive output signal to the transformer based upon a switch input signal. A current control loop adjusts the switch input in response to the current in one of the windings of the transformer, and a luminance control loop adjusts the switch input in response to the brightness of the light. A lamp current frequency control loop adjusts the polarity of the primary winding in response to a signal received from the transformer to thereby adjust the frequency of the lamp drive current applied to the fluorescent lamp.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This is a continuation-in-part of application Ser. No. 10/788,895 entitled “Fluorescent Lamp Driver System” filed Feb. 27, 2004 now U.S. Pat No. 7,312,780. 

   TECHNICAL FIELD 
   The present invention generally relates to optical displays, and more particularly relates to lamp drivers in optical displays. 
   BACKGROUND 
   Various types of optical displays are commonly used in a wide variety of applications including computer displays, televisions, cockpit avionics, night vision (NVIS) applications and the like. Included among these various types of optical displays are liquid crystal displays (LCDs) such as active matrix LCDs (AMLCDs). LCDs typically use a passive or active matrix display grid to form an image on the display surface. Such displays typically include any number of pixels on the display grid that are arrayed in front of a backlight. By controlling the light passing from the backlight through each pixel, color or monochrome images can be produced in a manner that is relatively efficient in terms of physical space and electrical power consumption. 
   Frequently, LCD backlights are implemented with fluorescent lamps or the like. A fluorescent lamp is any light source in which a fluorescent material transforms ultraviolet or other energy into visible light. Typically, a fluorescent lamp includes a glass tube that is filled with argon or other inert gas, along with mercury vapor or the like. When an electrical current is provided to the contents of the tube, the resulting arc causes the mercury gas within the tube to emit ultraviolet radiation, which in turn excites phosphors located inside the lamp wall to produce visible light. Fluorescent lamps have provided lighting for numerous home, business and industrial settings for many years. 
   Despite the widespread adoption of displays and other products that incorporate fluorescent light sources, however, designers continually aspire to improve the electrical efficiency of the light source, to extend the dimmable range of the light source, and/or to otherwise enhance the performance of the light source, as well as the overall performance of the display. In the avionics arena, in particular, there is a need to reduce power consumption while also improving the displayed image presented to the viewer across a wide range of luminance. Therefore, it is desirable to create an improved lamp driver system that provides a relatively wide luminance range and relatively precise brightness control while providing good electrical efficiency. 
   BRIEF SUMMARY 
   In various embodiments, a driver circuit provides electrical energy from a power source to a fluorescent lamp such as that used in a flat panel display, head-up display, liquid crystal display and/or the like. Power is provided to the lamp via a transformer with a primary and a secondary winding, with the ends of the secondary winding coupled to the fluorescent lamp. A high-side current steering circuit is configured to switchably provide a drive output coupling the power source to the transformer in response to a switch input. In various embodiments, a current control loop is configured to adjust the input to the high-side current steering circuit in response to the current in one of the windings of the transformer and/or a luminance control loop is configured to adjust the switch input in response to the brightness of the light. A lamp current frequency control loop may then be configured to adjust an electrical polarity of the primary winding to adjust the frequency of current applied to the lamp. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
       FIG. 1  is a block diagram of a dual-loop lamp control circuit; 
       FIG. 2  is a block diagram of one embodiment of a triple-loop lamp control circuit; and 
       FIG. 3  is a block diagram of an alternate embodiment of a triple-loop lamp control circuit. 
   

   DETAILED DESCRIPTION 
   The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
   According to various exemplary embodiments, a lamp driver circuit with at least three resonant loops provides for highly efficient and effective lamp operation. A current control loop and a luminance control loop are provided, along with a separate lamp current frequency control loop that controls the frequency of electrical current applied to the lamp. This “frequency loop” obtains a trigger signal from the transformer coupled to the lamp, or from another source as appropriate. The trigger signal is then processed with suitable analog and/or digital circuitry to provide appropriate electrical signals coupled to each end of the primary transformer winding. By separating the polarity of the applied power from the current and luminance control loops, the frequency of the drive signal applied across the lamp can be increased or otherwise adjusted. These adjustments in frequency can improve the efficiency of the light source, reduce undesirable electromagnetic interference (EMI) emissions, and/or produce other benefits. 
   The term “coupled” in the context of this document refers to the direct or indirect connection of two devices or objects in a physical, logical, electrical or other appropriate sense. While devices “coupled” together may electrically communicate or otherwise interoperate with each other, they need not be physically joined together. In particular, two objects that are “coupled” together may have one or more intervening objects (e.g. electrical components such as resistors, capacitors, digital or analog filters and/or the like) between them and need not be in direct physical or electrical contact with each other. 
   Referring now to  FIG. 1 , an exemplary two-loop lamp drive circuit  100  suitably delivers energy to a plasma in a fluorescent lamp  104  in a resonant manner. The arrangement of circuitry shown in the figure has two fundamental control loops: a current control circuit  162 , and an optical feedback circuit  164 . Lamp driver  100  is appropriately designed to obtain input power from a regulated, filtered power source  102 , such as a battery or other reference source. Various embodiments of drive circuit  100  may be able to regulate power delivered to lamp  104  from a widely ranging input supply, but for avionics applications that exhibit a large dimming ratio, better results may be achieved with a fairly tightly regulated input supply. 
   The main arc drive circuitry  100  suitably includes at least a current control circuit  162  and an optical feedback circuit  164  that control lamp current and lamp luminance, respectively. As shown in  FIG. 1 , in one embodiment an arc transformer  120  with a center-tapped primary winding  125  is fed current through the center tap by an inductor  124 . Two switches (e.g. N-channel FETs or the like)  108 ,  110  drive the outer legs  112 ,  114  (respectively) of the primary winding  126  on the arc transformer  120  in an alternating fashion to provide an AC signal on the secondary winding  128 . A “high side” current steering module  106  suitably provides drive current to transformer  120  via an inductor  124  and/or any other circuitry as appropriate. The arc transformer secondary winding  128  is coupled to the two end terminals of the fluorescent lamp  104  as appropriate. Since the power FETs  108  and  110  in the arc drive each carry relatively high levels of current in the embodiment shown, a high-current driver  131  and  133  on the gate of each switch quickly transitions the FET through the linear region as it is commanded between off and on states, optimizing efficiency of the drive system. 
   Source leads of the switches  108 ,  110  are shown connected together and through a current sense resistor  138  (e.g. a resistor of about 0.025-ohms or so) to signal return. Continuous current in sense resistor  138  is filtered and amplified in loop  162 ; this signal drives the positive input of a hysteretic comparator  134 . The output of the hysteretic comparator in the  FIG. 1  embodiment drives high side current steering circuit  106 . This drive signal is shown in  FIG. 1  as switch input signal  101 , which may be filtered and/or otherwise adjusted by filter circuitry  105  as appropriate and desired for the particular embodiment. 
   The two N-channel FET drivers  108  and  110  are driven by signal  135 , which in this embodiment is shown to coincide with drive signal  101 . Signal  135  is provided as a clock input to a D flip-flop with latching output. D flip-flop operation ensures only one N-channel FET is on at any time. In operation, the rising (or trailing) edge of any pulse arriving on signal line  135  can shift the signal  137  provided at the data (D) input of the device. In practice, signal  137  is provided from the inverting output (/Q) of the same device, thereby providing that switched  108  and  110  should remain in opposite (i.e. activated or non-activated) states, and that the states of each switch  108 ,  110  should change on any rising edge of signal  135 . As noted below, this same structure can receive an input  135  from other sources in circuit  100  to improve operation. Signal  135  can be obtained from the power switch  106 , from inductor  124 , from transformer  120  and/or for any other signal node existing between the voltage source  102  and lamp  104  as appropriate. Since flip-flop  130  in this embodiment is toggled by any rising voltage edge on signal  135 , many equivalent input signals  135  could be provided. Additionally, flip-flop  130  could be equivalently replaced with a trailing edge flip-flop, with a conventional latch circuit, with discrete components configured to provide latching functions, and/or with any other logical or electrical equivalent as appropriate. 
   Current control loop  162  regulates the flow of current through the plasma in the fluorescent lamp for a particular luminance desired to be produced from the lamp. The desired luminance is provided by an input drive signal  149  that is received from an external control source as appropriate. High-side current steering, controlled by a hysteretic comparator  134 , maintains the level of current for the given light output by periodically or aperiodically refreshing the current control source (e.g. transformer  120 ) with power from power supply  102 . Low-side current steering, also driven from the hysteretic comparator  134  in  FIG. 1 , determines the path excitation current flows in the current control circuitry  120  and lamp interface, and the direction that current flows within the lamp. Lamp current frequency can range from about 10 kHz to 100 kHz or more in this embodiment, depending on lamp characteristics, current control and lamp interface elements, current-loop voltage amplifier gain, comparator hysteresis, luminance level and/or other factors as appropriate. Current, after flowing through the plasma in lamp  104 , returns to the lamp interface and current control circuitry  120 , finally arriving back to the filtered input power source  102  after being measured, filtered, and/or otherwise processed as appropriate by current control circuit  162  before being presented to an input of hysteretic comparator  134 . 
   Generated light suitably exits the lamp at an angle that may be approximately normal to the outside glass surface. Some of this light impinges on a photodiode, photosensor and/or other photon-to-current converter  144  that is coupled to the arc drive circuitry via optical feedback circuit  164 . The optical feedback circuit  164  obtains an electrical signal from photon to current converter (e.g. photodetecting diode  144 ) that measures the luminous flux coming from the lamp  104 , and that outputs a proportional electrical current. This current can then be converted to a voltage and provided to an input of an error amplifier  148  to produce an optical amplifier that has relatively high gain at low luminance and exponentially decreasing gain at high luminance. The logarithmic amplifier  146  helps control stability in the optical control loop when higher levels of luminance and power are desired from the fluorescent lamp driver  100 . The error amplifier  148  in turn drives an input to the hysteretic converter  134  described above. Luminance command signals  149  to lamp driver  100  may be obtained and processed as appropriate. 
   The positive input terminal of the error amplifier  148  is generally maintained at or near zero (or some other reference) potential. The output of error amplifier  148  can be compared with the output of the current control loop amplifier  132  at hysteretic converter  134  as appropriate. This hybrid control arrangement causes the current control loop circuitry  162  to drive plasma in the fluorescent lamp, thereby generating an intensity of fluorescent light corresponding to a signal out of the optical amplifier  146  that has the effect of negating luminance commanded signals  149 . Hysteretic comparator  134  thus couples the current control loop  162  with the optical feedback loop  164 , and it is the complex interplay between the two loops and the fluorescent lamp, which determine the physical processes occurring with plasma in the lamp channel. 
   The effects of current control loop  162  and luminance control loop  164  therefore combine to produce a resonant drive signal  125  to transformer  120 , which in turn provides a drive signal to lamp  104  that is determined as a function of drive signal  125  and the polarity of winding  126 , which in turn is determined by the conducting or non-conducting states of switches  108  and  110 . In the embodiment shown in  FIG. 1 , the polarity of the voltage on winding  126  and the drive signal  125  are both determined in response to a common signal, since the input signal  135  used to toggle flip-flop  130  is effectively the same signal used to control the applied voltage at switch  106 . In various embodiments, however, these two signals can be separated so that changes in polarity of the voltage on winding  126  are not directly related to the application of the drive signal. Stated another way, the polarity of the voltage across winding  126  can be adjusted at a different rate than the rate at which the drive signal  125  is changed. 
     FIGS. 2 and 3 , for example, show two circuits and techniques whereby the polarity of the voltage across winding  126  is toggled in response to the conditions within the lamp reflected back through transformer  120  to the primary side, rather than from the input to switch  106 . This can be obtained by, for example, obtaining the input  135  to flip-flop  130  from an electrical node located between the output of high-side current steering module  106  and transformer  120 . Moreover, because electrical effects of lamp  104  are reflected in signals propagating across transformer  120 , obtaining the input to a low-side current control from the transformer  120  or signals coupled thereto can have the effect of adjusting the frequency of electrical current applied to the lamp in response to lamp operation. 
     FIG. 2 , for example, shows that a signal  202  obtained from the primary side of transformer  120  can be rectified, filtered, amplified and/or otherwise processed to produce a suitable input signal  135  to flip-flop  130 . In this embodiment, drive signal  125  applied to the center tap of primary winding  126  is also applied (as signal  202 ) to filter circuitry  203  as appropriate in a separate lamp current frequency control loop  205 . Effects of lamp operation are coupled to drive signal  125  through transformer  120 , thereby allowing signal  125  to additionally drive the current frequency control applied to lamp  104 . Loop  205  as shown in  FIG. 2  includes a rectifier/limiter  201 , filter/amplifier  203  and amplifier  204 . In other embodiments, rectifier  201  and/or amplifier  204  may be omitted or combined within filter  203 . 
   Filter  203  processes the received signal  202  by applying any suitable delay or other filter to produce an output with desired timing characteristics. Filter  203  may also incorporate a low or band pass filter to remove high-frequency noise from (at least) the edges of the input signal to produce an output signal  135  having a desired waveform and frequency. In various embodiments, filter  203  is an active filter that adjusts the frequency of signals  135  in response to the intensity of light produced by lamp  104 ; this may be accomplished by adjusting filter  203  in response to an output  209  from optical control circuit  164  or error amplifier  148 . In other embodiments, however, filter  203  is a more passive filter that does not obtain input from the light intensity loop, and signal  209  is omitted. Filter  203  may also incorporate an amplifier (e.g. one or more operational amplifiers) to amplify and/or attenuate input signals  202  as appropriate. 
   Rectifier/limiter  201  is any circuit or the like capable of further shaping signals  202 . Signals  202  may be rectified using a conventional diode rectifier, for example. The rectified signals may be further limited at any appropriate voltage to prevent overloading of amplifier  204  or other circuitry. In various equivalent embodiments, rectifier circuit  201  is eliminated, placed in front of filter  203 , incorporated within filter  203  and/or otherwise located within loop  205 . 
   Amplifier  204  is provided in any appropriate manner; in various embodiments, amplifier  204  is effectively a digital amplifier that provides a high or low reference (e.g. “rail”) voltage at the output in response to input signals. This digital-type output can be useful in providing a sharp clock signal to flip-flop  130  in some embodiments. Alternatively, filter  203  could incorporate any sort of analog amplifier as appropriate to equivalently encompass the function of amplifier  204 . 
   In many embodiments, it may be desirable to toggle the polarity of winding  126  at a rate that is relatively fast with respect to the rate at which signal  125  changes. This rate can be determined using conventional RC filter design techniques. Moreover, conventional low, band and/or high-pass filtering techniques using RC or other analog filtering components can be used to shape the edges of signal  202  as desired. In alternate embodiments, digital sampling and filtering techniques can be used. One or more amplifiers  104  (which may be an op amp or other amplification module) can also be provided to amplify and/or attenuate signals  202  so that they produce signals of  135  with appropriate magnitude for flip-flop  130 . As noted above, the signals  135  are generally provided to the “clock” input of flip-flop  130 , which suitably responds to rising and/or falling edges of signals  135  to toggle the outputs provided at the “Q” and “/Q” terminals of the device. 
     FIG. 3  shows an equivalent embodiment that contains a drive signal loop  205  that obtains a signal input  202  from an auxiliary winding  302  associated with transformer  120 . This auxiliary winding  302  may be wrapped around the core of transformer  120  on either the primary or secondary side of the device. In various embodiments, auxiliary winding  302  is wrapped around the primary side core of transformer  120  and contains enough windings to produce input signals  202  to drive control loop  205  as described above. The number of windings in winding  302  can be selected to produce output signals  135  with appropriate magnitude; alternately and/or additionally, the signals  202  obtained from winding  302  can be amplified or attenuated by amplifier  204  as appropriate. 
   Various embodiments of loop driver circuitry  100  therefore provide a drive control loop  205  that operates at a different rate from the signal  101  produced by current loop  162  and/or optical control loop  164 . Because the polarity of the voltage applied across winding  126  can be separated from the drive signal  125  itself in this manner, high frequency AC drive signals can be applied to lamp  104 , and/or performance of circuit  100  may be improved as appropriate. This adjustment in AC frequency may also be used to avoid undesirable RF emissions at particular frequencies (e.g. at a frequency that interferes with another component in a display system), or for any other purpose. 
   The concepts set forth above are generally referenced in the context of a “triple loop” driver circuit having a current control loop, a light intensity control loop and a drive control loop for ease of understanding. In practice, however, the concepts of a drive control loop may be implemented distinct from the current control and/or light intensity loops across a wide variety of alternate, yet equivalent, embodiments. 
   While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Various changes may be made in the function and arrangement of elements described in the exemplary embodiments without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.