Patent Publication Number: US-2007114952-A1

Title: Light source driver circuit

Description:
FIELD OF THE INVENTION  
      The present invention relates to a light source driver circuit, and more particularly to a light source driver circuit that provides a high efficiency of power conversion and avoids a high cost incurred by the requirements of the safety regulations.  
     BACKGROUND OF THE INVENTION  
      Referring to  FIG. 1  for the schematic diagram of a prior art light source driver circuit applied to a liquid crystal display (LCD) backlight module, an alternate current  1  (AC 50/60 Hz) is converted into a low voltage DC output power DCOP by an AC-to-DC converter  2 , and that voltage is usually between 12 volts to 36 volts, and the power of the DC output power DCOP is converted into a high-frequency AC power by a DC input inverter  3 , and that power usually has a voltage of 500 volts to 1500 volts and a frequency of 100 KHz for driving a light source device  4  such as a cold cathode fluorescent lamp.  
      Referring to  FIG. 2  for the schematic view of a switch power supply, an example of the aforementioned prior art AC-to-DC converter includes an input AC power supply  5  (AC 50/60 Hz) comprising a rectifier circuit of a rectifier filter  6  and a capacitor  7 , a pulse width control circuit  8 , a transformer T 1 , a secondary rectifier filter circuit  9  and a feedback control circuit  10 . The switch power supply converts city electricity into a direct current required by the prior art DC input inverter.  
      Referring to  FIG. 3  for an example of the prior art DC input inverter, the DC input inverter comprises a DC input power, a pulse width control circuit  11  of the DCIP, a resonant circuit  12 , a transformer T 2  and a feedback control circuit  13 . The function of the resonant circuit  12  is to produce an AC power (AC voltage or alternate current) and step up the voltage by the transformer T 2  or not step up the voltage by the transformer to drive the light source device  14  such as a cold cathode fluorescent lamp. The pulse width control circuit  11  provides power to drive the resonant circuit  12  to produce an AC voltage or an alternate current, and such power could be in form of a voltage or a current. The signal of the feedback control circuit  13  is fed back for controlling the power supplied by the pulse width control circuit  11  and further controlling the AC power supply produced by the resonant circuit  12  to achieve the effect of modulating light.  
      The foregoing prior art light source driver circuit has the following drawbacks: 
          1. The total efficiency is low. Since the prior art light source driver circuit comprises an AC-to-DC converter and an inverter, and the efficiency of the present AC-to-DC converter is approximately 85% (0.85), therefore the efficiency of the prior art light source driver circuit is approximately 0.85×0.85=0.7225(72.25%).     2. Referring to  FIG. 3  for the DC input inverter, the input voltage usually ranges from 5 volts to 30 bolts. If a rectifier filter circuit (not shown in the figure) is installed to the DCIP of the DC input power, the same effect of the prior art DC input inverter can be achieved theoretically and its efficiency is even higher than the foregoing prior art circuit. However, the safety distance between the primary and seconding windings of the transformer T 2  should be at least 16˜18 mm according to the requirements of safety regulations and should be able to pass a HI-POT Test of 25K volts. Thus, it will be difficult to make such DC input inverter and its cost will be high.        

     SUMMARY OF THE INVENTION  
      In view of the shortcomings of the prior art, the present invention provides a light source driver circuit that provides a high power conversion efficiency and avoids a high cost incurred by the requirements of safety regulations.  
      Therefore, it is a primary objective of the present invention to provide a light source driver circuit comprising: a pulse width control circuit that uses an AC power supply to produce a square wave; a first transformer having its primary winding for inputting the square wave and its secondary winding for outputting a transformation square wave; a resonant circuit that inputs the transformation square wave to produce a resonance sine wave; and a second transformer having its primary winding for inputting the resonant sine wave and its secondary winding for outputting a transformation sine wave to drive a light source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram of a prior art light source driver circuit;  
       FIG. 2  is a schematic diagram of a prior art switch power supply;  
       FIG. 3  is a schematic diagram of a prior art DC input inverter;  
       FIG. 4  is a schematic diagram of a light source driver circuit according to a first preferred embodiment of the present invention;  
       FIG. 5  is a schematic diagram of a light source driver circuit according to a first preferred embodiment of the present invention;  
       FIG. 6A  is a schematic diagram of a light source driver circuit according to a second preferred embodiment of the present invention;  
       FIG. 6B  is a schematic diagram of waveforms according to a second preferred embodiment of the present invention; and  
       FIG. 7  is a schematic diagram of waveforms according to a third preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Referring to  FIG. 4  for the schematic diagram of a light source driver circuit according to a first preferred embodiment of the present invention, the light source driver circuit  15  comprises a pulse width control circuit  16  that uses an AC power supply to produce a square wave, and the AC power supply could be city electricity passing through a rectifier filter circuit having a rectifier filter and a rectifier capacitor CC; a first transformer T 3  having its primary winding for inputting the square wave and its secondary winding for outputting a transformation square wave V 2 ; a resonant circuit  17  for inputting the transformation square wave V 2  to produce a resonant sine wave; a second transformer T 4  having its primary winding for inputting the resonant sine wave and its secondary winding for outputting a transformation sine wave to drive the light source device  18 , and the light source device  18  could be a light source of an LCD backlight module; and a feedback control circuit  19  that uses the transformation sine wave to produce a feedback to a feedback signal of the pulse width control circuit  16 .  
      Compared with the prior art light source driver, the light source driver circuit of the present invention simplifies and combines the two sets of pulse width control circuits and the two sets of feedback control circuits (respectively used in the switch power supply and the DC input inverter), and only uses one set of pulse width control circuit and one set of feedback control circuit. In the meantime, the secondary rectifier filter circuit of the switch power supply is simplified or omitted, and the first transformer T 3  converts the high voltage of a general rectified city electricity into a low voltage (defined as DC 42 volts or AC 30 volts by the safety regulations) and then converts the low voltage square wave into a sine wave voltage or current directly by the resonant circuit, and finally outputs the current to drive a light source device  18  such as a cold cathode fluorescent lamp after the voltage is boosted by the second transformer T 4 . Therefore, the light source driver circuit of the present invention has a higher efficiency and a lower manufacturing cost than the prior art light source driver circuit. Furthermore, the present invention can avoid the aforementioned safety regulation issue.  
      Referring to  FIG. 5  for the schematic diagram of a light source driver circuit according to a first preferred embodiment of the present invention, the resonant circuit is connected in parallel. In  FIGS. 4 and 5 , the light source driver circuit of the invention comprises a pulse width control circuit  16  for inputting city electricity through a rectifier filter circuit including a rectifier filter and a rectifier capacitor CC, a pulse width modulator  20 , two current switches Q 1 , Q 2 , and a capacitor C 0 ; a first transformer T 3  having its primary winding for inputting a square wave produced by the pulse width control circuit  16  and its secondary input transformation square wave V 2  complies with the low-voltage requirement of the safety regulations; a resonant circuit  17  for inputting the transformation square wave V 2  through the rectifier diode D 1 , D 2  and includes a resonant inductor L 1 , L 2 , a resonant capacitor C 1 , and a rectifier transistor Q 3 , Q 4  controlled by the signals produced respectively by the voltage divider resistor R 1 , R 2 , so as to produce a resonant sine wave; a second transformer T 4  having its primary winding for inputting the resonant sine wave and its secondary winding for outputting a transformation sine wave to drive the light source device  18 , and the light source device  18  could be a light source used for an LCD backlight module and includes a cold cathode fluorescent lamp  21 ,  22  connected to the capacitor C 2 , C 3  in series respectively; and a feedback control circuit  19  that uses the transformation sine wave to produce a feedback to a feedback signal of the pulse width control circuit  16 .  
      Referring to  FIG. 6A  for the schematic diagram of a light source driver circuit according to a second preferred embodiment of the present invention, the resonant circuit is connected in parallel. In  FIGS. 4 and 6 A, a light source driver circuit of the present invention comprises a pulse width control circuit  16  for inputting city electricity that passes through a rectifier filter circuit including a rectifier filter and a rectifier capacitor CC, and a pulse width modulator  20 , two current switches Q 1 , Q 2 , and a capacitor C 0 ; a first transformer T 3  having its primary winding for inputting a square wave V 1  produced by the pulse width control circuit  16  and its secondary input transformation square wave V 2  complies with a low-voltage requirement of the safety regulations; a resonant circuit  17  including a resonant inductor L and a resonant capacitor C 4 , C 5  to produce a resonant sine wave; a second transformer T 4  having its primary winding for inputting the resonant sine wave and its secondary winding for outputting a transformation sine wave to drive the light source device  18 , and the light source device  18  could be a light source used for an LCD backlight module and includes a cold cathode fluorescent lamp  21 ,  22  connected with the capacitor C 2 , C 3  in parallel respectively; and a feedback control circuit  19  that uses the transformation sine wave to produce a feedback to a feedback signal of the pulse width control circuit  16 .  
      The second preferred embodiment of the present invention as shown in  FIG. 6A  is used for illustrating the actions of the light source driver circuit of the present invention.  
      Refer to  6 B for the schematic diagram of waveforms according to a second preferred embodiment of the present invention. In  FIGS. 6A and 6B , the city electricity is inputted from a rectifier filter circuit including a rectifier filter and a rectifier capacitor CC and the pulse width control circuit  16  outputs a square wave V 1  to the primary winding of the first transformer T 3  by the actions of the current switches Q 1 , Q 2  and produces a low-voltage transformation square wave V 2  of the first transformer T 3  to meet the low voltage requirements of safety regulations, and the alternate current produced by the resonant circuit  17  is boosted by the second transformer T 4  to drive the light source device  18 . Referring to  FIGS. 6A and 6B , the pulse width control circuit  16  controls the high potential of the switch signal Q 2 VGS between the gate and the source of the current switch within the time from t 0  to t 1 , such that the current switch Q 2  is electrically connected to input the current from the first transformer T 3 , the capacitor C 0  and pass from the drain to the source of the current switch Q 2 , and finally to the ground. A voltage V 1  (not shown in the figure) induced at the primary winding of the first transformer T 3  goes through the first transformer T 3  to induce a voltage V 2  having a high potential lower than the voltage V 1  at the secondary winding and has the same polarity, and the voltage V 2  is applied to the resonant circuit  17  to produce a resonant sine wave through the resonant inductor L and the resonant capacitor C 5 . Its frequency is ½π× (the capacitance of the resonant inductor L×the capacitance of the resonant capacitor C 5 ) ½, where the time between t0 and t1 must be smaller than or equal to a half cycle of the sine wave. With the time off between t1 and t2, the pulse width control circuit 16 controls the low potential of the switch signal Q2VGS of the current switch Q2 between its gate and source, such that the current switch Q2 is not electrically connected so as to disconnect the current that passes through the first transformer T3, the capacitor C0 and from the drain to the source of the current switch Q2. Now, the resonant inductor L produces a counter electromotive force to maintain the original current direction, so as to reverse the polarity of the voltage V2 of the secondary winding of the first transformer T3 and also reverse the voltage polarity of the primary inductance of the first transformer T3. The current passes from the primary winding of the first transformer T3 through the capacitor C0 and the internal diode D11 of the current switch Q1, and then the current returns to the primary winding of the first transformer T3. In the meantime, the pulse width control circuit 16 at the time t1a between t1 and t2 controls and coverts the switch signal Q1VGS outputted between the gate and source of the current switch Q1 into a high potential, such that the current switch Q1 is electrically connected. Within the time from t2 to t3, the phase of the current IL of the resonant inductor L is in a negative half cycle due to the resonance, and the current switch is electrically connected, therefore the polarity of the current is reversed. The current passes from the primary winding of the first transformer T3 through the drain to the source of the current switch Q1 and then passes the capacitor C4 and returns to the primary winding of the first transformer T3, and the action is very similar to that occurred at the time from t0 to t1. At the time off toff from t3 to t4, the pulse width control circuit 16 controls and outputs a low potential for the switch signal Q1VGS from the gate and source of the current switch Q1, such that the current switch Q1 is not electrically connected, so as to disconnect the current originally passing through the primary winding of the first transformer T3 from the drain to the source of the current switch Q1 and then to the resonant capacitor C4. Now, the resonant inductor L produces a counter electromotive force to maintain the original current direction, so that the polarity of the voltage V2 induced at the primary winding of the first transformer T3 is reversed, and the polarity of the voltage V1 induced at the primary winding of the first transformer T3 is reversed as well. The current passes from the positive (+) end of the capacitor C3 of the primary winding of the first transformer T3 to the ground and then through the internal diode D21 and the capacitor C4 of the current switch Q2 back to the primary winding of the first transformer T3. In the meantime, the pulse width control circuit 16 at the time t3   a  between the t 3  and t 4  controls and converts the switch signal Q 2 VGS between the gate and the source of the current switch Q 2  into a high potential, so that the current switch Q 2  is electrically connected. The foregoing process is repeated to form a resonance and the second transformer T 4  is boosted to drive the light source device  18 .  
      In the light source driver circuit of the present invention, it preferably includes a current feedback control circuit  19  for producing a feedback to a feedback signal of the pulse width control circuit  16 , so that the pulse width control circuit  16  can appropriately control the operating frequency of the light source driver circuit to control the change of the current outputted to the light source device  18 , and can further achieve the effect of producing the required brightness of the light source device  18 , and the foregoing light modulation could be in a continuous mode or a burst mode.  
      Referring to  FIG. 7  for the schematic diagram of waveforms according to a third preferred embodiment of the present invention, the resonant circuit is connected in series. In  FIGS. 4 and 7 , a light source driver circuit of the present invention comprises: a pulse width control circuit  16  that inputs city electricity through a rectifier filter circuit including a rectifier filter and a rectifier capacitor CC and a pulse width modulator  20 , two current switches Q 1 , Q 2 , and a capacitor C 0 ; a first transformer T 3  having its primary winding for inputting a square wave V 1  produced by the pulse width control circuit  16  and its secondary winding for inputting a transformation square wave V 2  to meet the low-voltage requirements of safety regulations; a resonant circuit  17  that uses the stray inductance (not shown in the figure) of the resonant capacitor C 6 , C 7  and the second transformer T 4  to produce a resonant sine wave; a second transformer T 4  having its primary winding for inputting the resonant sine wave and its secondary winding for outputting a transformation sine wave to drive the light source device  18 , and the light source device  18  could be a light source used in an LCD backlight module and includes a cold cathode fluorescent lamp  21 ,  22  respectively connected with the capacitor C 2 , C 3  in series; and a feedback control circuit  19  that uses the transformation sine wave to produce a feedback to a feedback signal of the pulse width control circuit  16 .  
      While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.