Abstract:
The present invention provides a direct current generator and a pulse generator thereof. The pulse generator includes a comparator to replace a central processing unit and a logic integrated circuit to save the costs and space required by the electronic components. The pulse generator generates pulses to control the activation of the direct current generator and then to control the output current of the direct current generator. The direct current generator generates current having pulses based on pulses signals from the pulse generator to drive a load.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a direct current generator and a pulse generator thereof and specifically to a direct current generator used in a light source system and a pulse generator thereof. 
     2. Description of the Prior Art 
     Currently, technologies used to generate pulse current include the switched mode power supply. The device used to generate pulse current includes electronic components such as central processing units and logic integrated circuits. However, the cost of the above-mentioned electronic components is relatively high and thus may easily increases the production cost of the product. Furthermore, the electronic components generally occupy a certain space in switched mode power supply. The above-mentioned reasons may directly increase the price and the volume of the products and thus reduces the competitiveness of the products. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a direct current generator and a pulse generator thereof, wherein the direct current generator includes a comparator and analog circuits to replace central processing unit and the logic integrated circuit to save costs and space of electronic components. 
     The pulse generator of the present invention generates a plurality of pulse signals for switching the driving switch in the direct current generator and for controlling the output power of the direct current generator. The pulse generator includes a control circuit, a comparator, and a pulse output unit. The control circuit is used to output a switching signal to the comparator based on a control signal from a signal processing unit. The control signal includes a high level and a low level. The control circuit also includes a first reference voltage source for outputting a first reference signal to be processed by the comparator. 
     The direct current generator includes a direct current voltage source, a driving circuit, and a pulse generator. The driving circuit of the direct current generator receives a plurality of pulse signals, wherein the switch of the driving circuit is turned on or off according to the pulse signals. The direct current generator further includes an initiator for providing an initial voltage of high level to initiate and drive a load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating the pulse generator of the present invention; 
         FIG. 2  is a circuit diagram of the pulse generator of the present invention; 
         FIG. 3A  is a waveform diagram of error signal, sawtooth signal from the oscillator, and pulse signal; 
         FIG. 3B  is a waveform diagram of control signal, comparison signal, and output current; 
         FIG. 4  is a block diagram of the direct current generator of the present invention; and 
         FIG. 5  is a circuit diagram of the direct current generator of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a direct current generator and a pulse generator thereof to generate a direct current having pulses. In one embodiment of the present invention, the direct current is used to drive lamps in an image system to emit light, wherein the above-mentioned image system includes consumer products such as projectors and flat display devices, but is not limited thereto. In the embodiments disclosed below, the direct current generator and the pulse generator are used to drive lamps, but are not limited thereto and can be used to drive other devices requiring direct current. 
     The pulse generator of the present invention preferably generates a plurality of pulse signals for activating the switch in a switched-mode power supply. In other words, the pulse generator controls the output power of the direct current generator by turning the switch on or off.  FIG. 1  is a block diagram of the pulse generator of the present invention. As  FIG. 1  shows, the pulse generator  100  includes a control circuit  200 , a comparator  400 , and a pulse output unit  600 . The control circuit  200  is coupled with a signal processing unit  10  and outputs a switching signal V coup  based on a control signal V con  from the signal processing unit  10 . The signal processing unit  10  includes a processor of personal computers or a microprocessor, but is not limited thereto. The signal processing unit  10  includes other signal processors for outputting the control signal V con  based on required luminance or other output requirements. The control signal V con  is a digital signal including a high level voltage and a low level voltage. The control circuit  200  includes a first reference voltage source for outputting a first reference signal V ref1  to be processed by the comparator  400 . As  FIG. 1  shows, the comparator  400  compares the amplitude of the switching signal V coup  and that of the first reference signal V ref1  and outputs a comparison signal V comp  at its first output terminal. The amplitude of the comparison signal V comp  alternates between high level and low level. Furthermore, the comparator  400  is coupled to the pulse output unit  600  and outputs the comparison signal V comp  to the pulse output unit  600 . 
     As shown in  FIG. 1 , the pulse signal V imp  from the pulse output unit  600  is configured to control a driving switch of a direct current generator. The pulse output unit  600  is configured to receive the comparison signal V comp  as well as a feedback signal V fb , and an adjustable voltage V adj  from an adjustable power source, wherein the feedback signal V fb  is directly proportional to the output voltage of the direct current generator (not illustrated). The pulse output unit  600  outputs a plurality of pulse signals based on the comparison signal V comp , the feedback signal V fb , and the adjustable voltage V adj . In the present embodiment, the pulse signals V imp  have a substantially same amplitude while the width of the pulse signal V imp  is directly proportional to the sum of the comparison signal V comp  and the adjustable voltage V adj . 
       FIG. 2  is a circuit diagram of the pulse generator of the present invention. In the embodiment illustrated in  FIG. 2 , point A and point B are coupled to the signal processing unit  10  (not illustrated) to receive the control signal V con . The control signal V con  is a digital signal and includes high level and low level. In the present embodiment, the switching signal V coup  and the control signal V con  have a same waveform and thus the switching signal V coup  also alternates between high level and low level. 
     In the embodiment illustrated in  FIG. 2 , the control circuit  200  includes an optical coupler  220 , a first resistor R 1 , and a second resistor R 2  for outputting the switching signal V coup  based on the control signal V con  from the signal processing unit  10 . The switching signal V coup  is then processed by the comparator  400 . The optical coupler  220  turns on or off based on the control signal V con . The optical coupler  220  includes a light emitting diode  221  and an optical transistor  222 . In the present embodiment, anode and cathode of the light emitting diode  221  are respectively coupled with point A and point B. Furthermore, point A and point B are coupled with the signal processing unit  10  to receive the control signal V con . In different embodiments, point A can be coupled with a constant voltage source (such as 2.5V) while point B can be coupled with the signal processing unit  10  to receive the control signal V con . The control signal V con  can alternate between high level (such as 5V) and low level (such as 0V) in order to control the flow of the current between point A and point B. The light emitting diode  221  selectively emits light to the optical transistor  222  according to the voltage difference and the direction of current flow between point A and point B. The optical transistor  222  conducts when receiving light from the light emitting diode  221  and stops conducting when the light emitting diode  221  stops emitting light. In different embodiments, other suitable electronic switches can be used to replace the photoelectric switch  220 . 
     As  FIG. 2  shows, when the optical transistor  222  conducts, the first resistor R 1  and the second resistor R 2  form a voltage divider which outputs the switching signal V coup . The switching signal V coup  and the control signal V con  have substantially the same waveform but may have different amplitudes. The Zener diode Z 1  is configured to limit the voltage at point C to 5V. In this way, the switching signal V coup  alternates between 5V and 0V. On the other hand, the control circuit  200  includes a first reference voltage source  230  [not shown in FIG.  2 .] for outputting the first reference signal V ref1  to the second input terminal  412  of the comparator  400 . The first reference voltage source  230  includes a fifth resistor R 5  and a sixth resistor R 6  and the two resistors form another voltage divider. The voltage divider then processes the voltage across the zener diode Z 1  and outputs the first reference signal V ref1 . The resistance of the fifth resistor R 5  and that of the sixth resistor R 6  are preferably the same, but are not limited thereto. 
     As  FIG. 2  shows, the comparator  400  is used to compare the amplitude of the switching signal V coup  with that of the first reference signal V ref1 . In the present embodiment, when the amplitude of the switching signal V coup  is greater than that of the first reference signal V ref1 , the first output terminal  413  of the comparator  400  will output a corresponding comparison signal V comp . Furthermore, the comparator  400  preferably has a structure of Schmitt Trigger, but is not limited thereto. In the present embodiment, the first input terminal  411  is a non-inverting input while the second input terminal  412  is an inverting input. Thus the comparator  400  of the present embodiment will output a high level signal when the amplitude of the switching signal V coup  is greater than that of the first reference signal V ref1 . However, in different embodiments, the comparator  400  can include other electronic components used to compare amplitudes of two signals (such as a differential amplifier). In different embodiment, a second reference voltage source  420  is coupled with the first output terminal  413  to control the amplitude of the outputted comparison signal V comp . Thus the second reference voltage source  420  can be used to limit the comparison signal V comp  to other voltage levels such as 5V or 12V. The direct current generator of the present invention outputs an output current whose amplitude depends on the amount of load to be driven. Thus in different embodiments, the second reference voltage source  420  can increase or decrease its output in accordance with the output current. The pulse generator of the present invention can also include a sensing resistor (not illustrated) coupled with the load (not illustrated) for sensing the output current and then outputting a sensing voltage corresponding to the output current. The first diode D 1  limits the direction of current flow between the first output terminal and the pulse output unit  600 . 
     In the embodiment illustrated in  FIG. 2 , the pulse output unit  600  further includes an error detector  610  for measuring a difference between two voltages to output an error signal V err  equal to or proportional to the voltage difference. The difference between the error detector  610  and the comparator  400  is that the output of the comparator  400 , i.e. comparison signal V comp  can only be a high level voltage or a low level voltage while the output of the error detector  610  is the voltage difference among received signals, i.e. the error signal V err . Thus the output of the error detector  610  is not limited to only two different voltages. The error detector  610  has a third input terminal  611 , a fourth input terminal  612 , and a second output terminal  613 . The third input terminal  611  corresponds to the non-inverting input terminal of the error detector  610  while the fourth input terminal  612  corresponds to the inverting input terminal of the error detector  610 . 
     The third input terminal  611  illustrated in  FIG. 2  also receives the feedback signal V fb , wherein the feedback signal V fb  is obtained by processing an output voltage of a driving circuit (not illustrated) using voltage divider. The fourth input terminal  612  is coupled with the first output terminal  413  of the comparator  400  to receive the comparison signal V comp . The error detector  610  outputs the error signal V err  based on the amplitude of the feedback signal V fb  and that of the comparison signal V comp , for a pulse width modulator  630  to process. Furthermore, the fourth input terminal  612  can be coupled with the adjustable voltage source  640  to receive the adjustable voltage V adj . The adjustable voltage V adj  is preferably a constant voltage which fixes the voltage input to the pulse width modulator  630 . In this way, the pulse width modulator  630  will output pulse signals V imp  of the same width. The output voltage of the adjustable voltage source  640  can be selectively adjusted. As  FIG. 2  shows, the voltage inputted into the fourth input terminal  612  is a sum of the adjustable voltage V adj  and the comparison signal V comp , but is not limited thereto. 
     As  FIG. 2  shows, an oscillator  620  of the pulse output unit  620  is used to output a sawtooth signal V saw , but is not limited thereto. The oscillator  620  can output a triangle wave to be processed by the pulse width modulator  630 . Furthermore, the pulse width modulator  630  compares the amplitude of the error signal V err  and that of the sawtooth signal V saw  and then outputs a plurality of pulse signals V imp . The pulse signals V imp  have substantially equal amplitude but may have different width. The pulse signals have a high level voltage or a low level voltage and can alternate between the two levels of voltage. 
       FIG. 3A  is a waveform diagram of the error signal, the sawtooth signal from the oscillator, and the pulse signal. In the embodiment illustrated in  FIG. 3A , the width of the pulse signal V imp  is directly proportional to the time during which the amplitude of the sawtooth signal V saw  is greater than that of the error signal V err . In other words, the pulse signal V imp  of the present embodiment will be at high level when the amplitude of the sawtooth signal V saw  is greater than that of the error signal V err . The pulse signal V imp  will be at low level when the amplitude of the sawtooth signal V saw  is smaller than that of the error signal V err . However, alternatively, the pulse signal V imp  can be at low level when the amplitude of the sawtooth signal V saw  is greater than that of the error signal V err , and the pulse signal V imp  can be at high level when the amplitude of the saw tooth signal V saw  is smaller than that of the error signal V err . 
     Please refer to both  FIG. 3A  and  FIG. 3B , the control signal V con  and the comparison signal V comp  change at substantially the same time. In other words, the control signal V con  can alternate between different levels to control the output of the comparator  400  which in turn control the width of the pulse signal V imp  from the pulse output unit  600 . In the embodiment illustrated in  FIG. 3A , the error signal V err  has two different levels. The error signal V err1  with higher amplitude corresponds to the comparison signal V comp  of lower level while the error signal V err2  corresponds to the comparison signal V comp  of higher level, but is not limited thereto. The relationship between signals mentioned above can be changed by modifying connections within the circuit of the pulse output unit  600 . 
     Please refer to  FIG. 2 ,  FIG. 3A , and  FIG. 3B , wherein  FIG. 3B  is a waveform diagram of the control signal V con , the comparison signal V comp , and the output current I sen . In the present embodiment, signals outputted into the fourth input terminal  612  includes the comparison signal V comp  and the adjustable voltage V adj . Thus the error signal V err  outputted by the error detector  610  can be represented by the following equation:
 
 V   err   ≡V   fb −( V   adj   +V   comp )  (1)
 
It can be seen from Equation (1) that the error signal V err  is inversely proportional to the comparison signal V comp . In the present embodiment, the feedback signal V fb  and the adjustable voltage V adj  are substantially constant. When the comparison signal V comp  and the control signal V con  is at a low level, the corresponding error signal V err1  will produce pulse signals V imp  of equal width. When the comparison signal V comp  and the control signal V con  are both at high level, the voltage of the corresponding error signal V err2  will be lower than that of the error signal V err1 . Consequently, the error signal V err2  with lower voltage will produce pulse signals V imp  with higher width, which in turn allows the direct current generator to produce output current I sen  having pulses.
 
       FIG. 4  is a block diagram of the direct current generator  110  of the present invention, wherein the direct current generator  110  preferably includes a switched mode power supply. The function of the direct current generator  110  is to transform a direct current (DC) voltage into another DC voltage of different amplitude. As  FIG. 4  shows, the direct current generator  110  includes a pulse generator  100 , a driving circuit  800 , a DC voltage source  900 , and an initiator  910 . The DC voltage source  900  of the present invention provides DC voltages to the driving circuit  800 . The DC voltage source  900  of the present embodiment includes a rectifier for transforming an alternating current (AC) voltage into a DC voltage. The DC voltage source  900  also includes a power factor correction circuit for improving the power factor, but is not limited thereto. The DC voltage source  900  can also include other suitable elements for outputting DC voltages. The driving circuit  800  is coupled to the DC voltage source  900  to receive DC voltages. Furthermore, the driving circuit  800  is coupled to the pulse generator  100  to receive a plurality of pulse signals V imp . The direct current generator  110  drives a load  920  by transmitting a current having pulses to the load  920  based on the pulse signal from the pulse generator  100 . The initiator  910  is coupled to the driving circuit  800  to receive an output voltage and then provides an initial voltage to the load  920 . In the present embodiment, the initial voltage is a pulse with high amplitude (substantially 600 volts to 5000 volts). The function of the initial voltage is to initiate the load  920  to be driven by the output voltages from the driving circuit  800  and to provide power to the load  920 . That is, the initiator  940  serves to initiate the load  920  and the supply power. In the present embodiment, the load  920  is a high-intensity discharge lamp, but is not limited thereto. The load  920  can includes other devices driven by the DC power source. 
     Furthermore, the direct current generator  110  further includes a sensing resistor R sen  connected in series with the load  920 , wherein the output current flowing through the load  920  produces a sensing voltage V sen  across the sensing resistor R sen . The sensing voltage V sen  is used to measure the amplitude of the output current flowing through the load  920  and the measurement is used as a reference for the output of the second reference voltage source in the pulse generator  100 . 
       FIG. 5  is a circuit diagram of the direct current generator of the present invention. In the present embodiment, the driving circuit  800  includes a buck converter and is used to transform a higher voltage from the DC voltage source  900  into a lower voltage for the initiator  910  to provide the load  920  with appropriate voltage. The pulse signal V imp  is used to activate the bipolar transistor Q 1  and allow the current to flow through the bipolar transistor Q 1 . Furthermore, the output voltage from the driving circuit  800  will be transformed by the voltage divider formed by the resistors R 7  and R 8  into a feedback signal V fb  of smaller voltage. The feedback signal V fb  is then processed by the error detector  610  of the pulse output unit  600 . The initiator  910  and its function have been explained above and thus will not be elaborated again. 
     The above is a detailed description of the particular embodiment of the invention which is not intended to limit the invention to the embodiment described. It is recognized that modifications within the scope of the invention will occur to a person skilled in the art. Such modifications and equivalents of the invention are intended for inclusion within the scope of this invention.