Abstract:
The present invention relates to a circuit for obtaining a wide dimming ratio from a Royer inverter and, more particularly, to a circuit that allows a back light to have wide dimming ratio without having unwanted current spikes that would significantly reduce the life of the back light bulbs. The invention is directed to a DC-to-AC inverter circuit for obtaining a wide dimming ratio in a display back light. The DC-to-AC inverter circuit consists of a voltage source, a Royer inverter circuit and switch circuit to turn on the Royer inverter. The Royer inverter is configured to receive a DC pulse modulated (PWM) signal that is coupled a transformer to produce an output AC PWM signal that is sent to the display back light. The configuration creates an imbalance in the Royer circuit that in turn prevents current spikes that occur when rapidly turning on the Royer inverter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a circuit for obtaining a wide dimming ratio from a Royer inverter and, more particularly, to a circuit that allows a back light in an electric device to have a wide dimming ratio without having unwanted current spikes that would significantly reduce the life of the back light bulbs. 
     Royer inverter circuits for converting a DC electrical signal to an AC electrical signal are well known. For example, these circuits are used when illuminating back lights used in conventional displays found in electronic devices. 
     Liquid crystal displays, such as those used in calculators and avionics instrumentation, are well known. Some LCD displays utilize aback light consisting of cold cathode fluorescent tubes (CCFTs) to form the display. In certain avionics scenarios, especially in dark conditions within the cockpit (such as when it is dark outside the cockpit), LCD displays become overly bright and the pilot wishes to dim the LCD displays to allow his/her eyes to adjust to the outside conditions. 
     Traditionally, back lights are dimmed in one of two ways. The first way is to pulse width modulate (PWM) the back light. This can deliver wide dimming ratios but larger starting transients can cause reduced bulb life and other problems such as large current transients in the system. When turn on is slowed to reduce the transients the dimming ratios are also reduced resulting in dimming ratios typically less than 30 to 1 where dimming ratios are defined as the time available for illumination divided by the actual illumination time. The second method is to reduce the current in the bulbs. This method produces limited dimming of the back light and typically results in dimming ratios of less than 20 to 1. 
     Expanding on the first method discussed above, a pulse width modulated (PWM) signal is used to turn on the circuit for driving the back light that establishes a period of time during which the back light may be turned on. This period is set long enough to provide a wide range of dimming. Dimming is accomplished when the back light is on for a time less than the full period available. This new way of illuminating back lights presented new problems. In particular, the PWM signal turns on the DC-to-AC inverter circuit at a fast rate that causes undesired current spikes in the inverter circuit. These current spikes carry over to the output signal of the inverter circuit and significantly shorten the life of the back light bulbs or back light CCFTs. The current spikes also propagate out of the power supply lines and can cause various system problems. 
     Accordingly, there exists a need for a display to have a wide dimming range without shortening the life of the back light bulbs or causing system power supply noise. The present invention fills these and other needs, and overcomes the short-comings of the prior art. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide rapid turn-on of a Royer inverter circuit without current spikes so that wide dimming of a back light is achieved. 
     To accomplish this and other related objects, the invention is directed to a DC-to-AC inverter circuit for obtaining a wide dimming ratio in a display back light. The DC-to-AC inverter circuit consists of a voltage source, a Royer inverter circuit and switch circuit to turn on the Royer inverter. In one embodiment, the Royer inverter is comprised of a pair of matched transistors that receive a DC pulse modulated (PWM) signal and that are coupled to a transformer to produce an output AC PWM signal that is sent to the display back light. A resistor combination is coupled to the emitter of one of the matched transistors to create an imbalance in the Royer circuit, thereby preventing or minimizing current spikes that occur when rapidly turning on the Royer inverter. 
     In a second embodiment, the transistors of the pair are mismatched. In particular, the transistors are selected to have sufficiently different saturation characteristics that permit rapid start-up of the circuit without excessive current spikes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description of the drawings of the preferred embodiment of the invention, in which: 
     FIG. 1 is a perspective view of an electronic instrument, such as a panel mounted avionics device, equipped with a liquid crystal display in accordance with the present invention; 
     FIG. 2 is an exploded perspective view of the parts of the display; 
     FIG. 3 is a functional block diagram of the principle electronic components used in the display of the present invention; 
     FIG. 4 is a schematic diagram illustrating the basic electronic components of the DC-to-AC inverter circuit of the present invention; and 
     FIG. 5 is a schematic diagram illustrating the electrical circuit elements of the DC-to-AC inverter, including the Royer inverter circuit of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference initially to FIG. 1, numeral  10  generally designates an instrument that is used in an avionics, marine or recreational application and that is equipped with a display generally designated by numeral  12 . Preferably, display  12  is a liquid crystal display, but could be of other known types of displays. The instrument includes a generally rectangular cabinet or housing  14  having a front panel  16  on which the display  12  is located. The front panel  16  is also equipped with various controls  18  including buttons, knobs and the like that would be consistent with an electrical instrument having such a display. 
     The preferred physical construction of display  12  is best shown in FIG. 2. A printed circuit board  20  is connected with a light block or back light  22  by means of a plurality of standard connections. The light block (back light)  22  contains a plurality of cold cathode flourescent tubes (CCFTs)  24  that emit light when energized. A special diffuser panel  26  is disposed on the front face of the CCFT back light  22 . The diffuser  26  is constructed such that it is able to transmit through it the light that is emitted by the back light  22 . Other arrangements or components may be employed and the diffuser  26  is not critical to the present invention. 
     The display  12  also includes a liquid crystal display (LCD) module  28  that preferably takes the form of a transmissive or backlit thin film transistor (TFT) display, but could also include double super twist nematic (DSTN) and supertwist nematic displays among others. The LCD module  28  is connected with the board  20  through standard electrical connections and is controlled in a manner to be subsequently described. An anti-reflective lens  30  covers the front face of the LCD module  28  and a suitable frame  32  is provided to connect the components illustrated in FIG. 2 together as a single unit that is installed in the cabinet or housing  14  of instrument  10 . The components in FIG. 2 are constructed and shaped to be assembled together by crimping and twisting the various tabs and other parts as would be readily understood. 
     FIG. 3 is functional block diagram of the principle electronic components used to operate the display. A central processing unit (CPU)  36  is associated with a random access memory (RAM)  38  and a read only memory (ROM)  40 . A LCD controller  42  actuates pixels (liquid crystals) of the LCD module  28  under the control of the CPU  36 . One method for controlling and refreshing pixel and display data is disclosed in U.S. patent application Ser. No. 09/354,886 which is incorporated herein by reference. In response to a pulse width modulated (PWM) signal from the CPU  36 , a DC to AC inverter circuit  46  preferably converts the direct current PWM signal into a corresponding alternating current PWM signal to activate the back light  22 . Preferably, the DC PWM signal would have square wave like properties with a peak of approximately 5 volts and have a turn on frequency of approximately 60 Hz. As would be understood, other methods for refreshing the LCD module  28  and generating the back light signal exist and would be consistent with this invention. In accordance with the present invention, a user interface  50  receives operator input through traditional input devices such as knobs, dials, rheostats and the like for adjusting the brightness of the back light  22 . The CPU  36  receives the input data and stores this information in the RAM  38 . A preset display refresh frequency and the back light frequency is stored in ROM  40 . 
     FIG. 4 is a schematic block diagram illustrating the basic electronic components of the DC to AC inverter circuit  46 . The DC to AC inverter circuit  46  comprises an input  52  to receive the DC PWM signal from the CPU  36  (FIG. 3) and an output  54  to send the AC PWM signal to the back light  22  (FIG.  3 ). Contained within the DC to AC inverter circuit  46  is a DC voltage source  56 , switch circuit  58  and a Royer inverter circuit  60 . The switch circuit  58  receives the DC PWM signal  46  and determines if it is high or low. The DC PWM signal is preferably a square wave signal ranging from zero to 5 volts. The switch circuit  58  turns the Royer inverter circuit  60  on and off at the same frequency of the DC PWM signal. The voltage source  56  provides a constant voltage to facilitate operation of both the switch circuit  58  and the Royer inverter circuit  60 . 
     FIG. 5 is a schematic diagram illustrating the electronic circuit elements of the DC to AC inverter circuit. The switch circuit  58  includes transistors Q 1  and Q 2  configured with resistors R 1 , R 2 , R 3 , and R 4  such that when an input DC PWM signal goes high, it triggers the base of transistor Q 1  and thereby turns on the Royer inverter circuit  60 . The base of npn transistor Q 1  is coupled to the input  52  via resistor R 1  and is further coupled to its emitter via resistor R 2 . The emitter of transistor Q 1  is grounded and coupled the terminal end of resistor R 2 . The emitter of pnp transistor Q 2  is coupled the constant voltage source  56  and to its base via resistor R 4 . The base of Q 2  is further coupled to the collector of Q 1  via resistor R 3 . This configuration allows a 0 to 5V DC PWM signal to turn on and off the voltage source  56  that feeds the Royer inverter circuit  60  via resistor and R 6 . 
     In the present invention, the Royer inverter circuit  60  includes a transformer T with a core  62  having a primary winding  64 , secondary winding  66  and a transistor base winding  68 . The primary winding  64  is provided with a center tap  70  that divides the primary winding into two electrically identical halves,  64   a  and  64   b , and that is coupled to a constant voltage source  56  through inductor L. In a preferred embodiment, transistors Q 3  and Q 4  are matched and their collectors are coupled to the primary winding ends  72  and  74 , respectively. The emitter on Q 3  is coupled to resistors R 7  and R 8  and the emitter on Q 4  is connected to ground. The capacitors C 1  and C 2  help set the Royer oscillating frequency. Zener diodes Z 1  and Z 2  protect the circuit components, particularly, Q 3  and Q 4  from turn-on and turn-off transients. The first base winding lead  76  is coupled to the base of Q 3  with the second base winding lead  78  coupled to the base of Q 4 . With resistors R 7  and R 8  coupled to transistor Q 3 , this configuration creates a mismatched transistor pair and, in conjunction with impedance L, begins the oscillation in the Royer inverter circuit. Values for resistors R 7  and R 8  and inductor L are chosen to allow rapid start-up of the DC-to-AC inverter circuit without undesired current spikes through the transformer T. As would be understood, actual values for the above mentioned electrical components would differ for various applications and would be known by those skilled in the art. 
     In another embodiment, the preferred Royer inverter circuit  60  described above is altered to provide transistors Q 3  and Q 4  that are mismatched so that they have sufficiently different saturation characteristics. The values for Q 3  and Q 4  are chosen to allow rapid start-up of the DC-to-AC inverter circuit without the undesired current spikes flowing to the back light. Thus, resistors R 7  and R 8  are no longer needed because they would not be required to achieve a mismatched transistor pair. 
     In operation, before the DC-to-AC inverter circuit  46  receives an input DC pulse width modulated (PWM) signal from the CPU  36 , the Royer inverter circuit  60  is off and does not generate the AC PWM signal to the back light  22 . As the DC PWM signal goes high, the switch circuit  58  is activated, as described below, and sends the base current signal to the Royer inverter circuit  60 . When the DC PWM signal remains high, the Royer inverter circuit  60  begins operation, as will be subsequently discussed, by oscillating and outputting an AC PWM signal via the secondary winding  66  of transformer T to turn on the back light  22 . 
     When the DC PWM signal switches from low to high at input  52 , both transistors Q 1  and Q 2  in the switch circuit  58  turn on. This allows the voltage source  56  to provide the necessary base current to turn on transistors Q 3  and Q 4 . When the PWM signal goes low, transistors Q 1  and Q 2  turn off resulting in shutting off the base current driving transistors Q 3  and Q 4 . 
     As the switch circuit  58  sends current to the Royer inverter circuit  60 , turn on of the Royer inverter circuit  60  occurs very rapidly. The switch circuit  58  sends base current that drives transistors Q 3  and Q 4 . The base current driving Q 3  and Q 4  in conjunction with inductor L allows current to flow through the primary winding  64 . Because of the resistor combination R 7  and R 8  coupled to transistor Q 3 , transistor Q 4  has a greater gain characteristic and is turned on before transistor Q 3 . As current begins flowing through transistor Q 4 , most of the current in the inductor L flows in primary winding  64   a . The current in winding  64   a  causes the flux density within the core  62  to increase so that the base winding  68  develops a voltage and by this time, Q 4  is saturated. The size of inductor L determines the ramp voltage across winding  64   a.    
     The full amplitude of voltage source  56  is rapidly applied across primary winding  64   a . As the flux increases, the flux gets large enough to reverse the voltage on the base winding  68  that drives transistor Q 3  into saturation and allows current to flow through the collector of transistor Q 3  and primary winding  64   b . The Royer inverter circuit  60  has thus begun oscillating and will continue to do so until the DC PWM signal received at the switch circuit  58  goes low. Oscillation frequency is dependant upon a combination of primary and secondary inductances and capacitances. In the preferred embodiment, this oscillation frequency ranges from 20 to 70 kHz. As the Royer inverter circuit  60  oscillates, transformer T steps up the resulting AC PWM voltage signal and outputs it via the secondary winding  66  of transformer T to turn on the back light  22 . Preferably, the AC PWM signal has a peak value sufficient to ignite the CCF tubes that make up the display. 
     When transistors Q 3  and Q 4  are perfectly matched, the current flowing through the primary winding  64  and their collectors will spike through the transformer T at rapid start-up. Even though transistors Q 3  and Q 4  are matched, oscillation will still occur because of small mismatches within the transistors and circuits driven. Resistors R 7  and R 8  are added to the emitter of transistor Q 3  to create a current and flux imbalance through the transformer T that thereby negates the current spike. 
     From the foregoing, it will be seen that this invention is one well adapted to attain all the ends and objects hereinabove set forth together with other advantages that are inherent to the structure. 
     It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.