Abstract:
A DC-AC converter is applicable for transforming direct current (DC) to alternating current (AC). The DC-AC converter includes a voltage boost module and a DC-AC converter module. Herein the voltage boost module includes a voltage bypass circuit and a voltage boost circuit, both of which receive input voltage from DC input power. Meantime, the voltage bypass circuit sends out the received input voltage, and the voltage boost circuit will operate to increase DC output voltage from the DC input as the DC output voltage from the voltage bypass circuit is not high enough to meet requirement by AC output power. The DC-AC converter module receives the output voltage from the voltage boost module and converts the received voltage to the required AC output power.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the priority benefit of Taiwan application serial no. 93133756, filed on Nov. 5, 2004. All disclosure of the Taiwan application is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of Invention  
         [0003]     The present invention relates to an electric converter. More particularly, the present invention relates to a DC-AC converter.  
         [0004]     2. Description of the Prior Art  
         [0005]     A structure of DC-AC converter of the conventional art applicable to the situation with input voltage in large range is illustrated in  FIG. 1 , wherein the DC-AC converter  100  includes stage-1 voltage boost circuit  102 , stage-2 DC-DC converter  104  and stage-3 DC-AC converter  106 .  
         [0006]     Therein, stage-1 voltage boost circuit  102  is comprised by a DC input power  108 , an inductor  110 , a transistor switch  112 , a diode  114  and a capacitor  116 . The stage-1 voltage boost circuit  102  boosts its voltage to a level required by output end by means of pulse width modulation (PWM) operation of the transistor switch  112 , and has effect to keep voltage stabilized.  
         [0007]     Stage-2 DC-DC converter  104  is connected to the output end of the stage-1 voltage boost circuit  102  and comprises a primary control circuit  152 , a voltage transformer  126  and a secondary control circuit  154 .  
         [0008]     And, the stage-2 DC-DC converter  104  receives the DC power voltage boosted by the stage-1 voltage boost circuit  102 , and then delivers the output voltage from the stage-1 voltage boost circuit  102  to the primary winding of voltage transformer  126  by means of ON/OFF operations of the transistor switches  118 ,  120 ,  122  and  124  in primary control circuit  152 . Further, the voltage transformer  126  delivers the received voltage to the rectifier formed by diodes  128 ,  130 ,  132  and  134 . The received voltage is also transmitted to the filter formed by inductor  136  and capacitor  138 , wherefrom the output is sent to the stage-3 DC-AC converter  106 .  
         [0009]     The following stage-3 DC-AC converter  106  is coupled with the output end of the stage-2 DC-DC converter  104 , comprising transistor switches  140 ,  142 ,  144 ,  146 , inductor  148  and capacitor  150 , and converts the received DC power voltage to AC power voltage forwarding to a load terminal by means of switching operations of transistor switch  140 ,  142 ,  144  and  146 , and also by the filter. The filter includes inductor  148  and capacitor  150 . In the application of a conventionally known DC-AC converter with input voltage in large range, the input DC power voltage must be boosted to its maximum extent by the stage-1 voltage boost circuit and kept stabilized. Thereafter it is transmitted by the voltage transformer to the stage-3 DC-AC converter, wherein the DC power voltage is converted to an AC power voltage needed by the load terminal.  
         [0010]     In summary from above-described, there are the following shortages for the conventional DC-AC converter in application for the situation with input voltage in large range:  
         [0011]     1. No matter how the AC power voltage is high or low required by output end, the DC input power voltage in stage-1 voltage boost circuit must be boosted to its highest voltage. Consequently, the components thereof must be operated or switched in the condition with high voltage, resulting in energy consumption.  
         [0012]     2. The converting structures of three stages are required in it, which causes an increasing energy loss and reduces the converting efficiency.  
         [0013]     3. And two sets of heavy reservoir capacitors are employed for its configuration, leading to an increase of cost which doesn&#39;t meet economically saving consideration.  
       SUMMARY OF THE INVENTION  
       [0014]     It is an object of the invention to provide a DC-AC converter applicable for converting a DC input power to an AC output power and regulating the DC input voltage to a just level required by output to reduce effectively energy loss during the DC-AC converter&#39;s operation. Also, the heavy reservoir capacitors are saved to reduce the cost and increase the converting efficiency.  
         [0015]     The present invention provides a DC-AC converter for applications, which comprises a voltage boost module, a DC-AC converting module and a feedback module, wherein the voltage boost module comprises a voltage bypass circuit and a voltage boost circuit. These two circuits are used for receiving DC input voltage from the DC input power, and the voltage bypass circuit conveys the received DC input voltage directly to the DC-AC converting module.  
         [0016]     The voltage boost circuit is on duty if the DC output voltage from the voltage bypass circuit is insufficiently high to meet the need of the AC output voltage level. Hereupon, the voltage boost circuit receives the input DC voltage, and boosts this voltage sent to DC-AC converting module wherein the received DC is converted to the required AC output power.  
         [0017]     In accordance with the preferred embodiment&#39;s description, the foregoing voltage bypass circuit includes a diode, which is electrically coupled with a DC input power at an end and with a capacitor at another end of the diode.  
         [0018]     In accordance with the description of another preferred embodiment of DC-AC converter, the above-mentioned feedback module determines a required value of voltage provided by the voltage boost circuit based on the difference between the output voltage level from DC-AC converting module and the preset value.  
         [0019]     In the present invention, due to utilizing the voltage bypass circuit, if the DC input voltage is high enough to provide AC output power, the voltage boost circuit is not in action and the DC input is transferred to the DC-AC converting module via the voltage bypass circuit. On the contrary, when the DC input voltage is not high enough to provide AC output power, the voltage boost circuit starts to work and regulates the DC input voltage to a level required by output to reduce effectively energy loss during the DC-AC converter&#39;s operation. Also and, the high-capacity capacitors can be saved, resulting in saving cost and improvement of the converting efficiency. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0021]      FIG. 1  is a drawing, schematically showing a conventional DC-AC converter.  
         [0022]      FIG. 2A  is a drawing, schematically showing a DC-AC converter, according to an embodiment of the invention.  
         [0023]      FIG. 2B  is a drawing, schematically showing the waveform of output voltage measured at an end of reservoir capacitors after the DC input power voltage is boosted by the voltage boost circuit in a DC-AC converter, according to an embodiment of the invention.  
         [0024]      FIG. 3  is a drawing, schematically showing the output voltage in a DC-AC converter and the switching waveforms of the first transistor switch and the second transistor switch in a secondary control circuit respectively, according to an embodiment of the invention.  
         [0025]      FIG. 4  is a block diagram, schematically showing a feedback module in a DC-AC converter, according to an embodiment of the invention.  
         [0026]      FIG. 5  is a drawing, schematically showing an alternative of the secondary control circuit in a DC-AC converter, according to an embodiment of the invention.  
         [0027]      FIG. 6  is a drawing, schematically showing an alternative of the primary control circuit in a DC-AC converter, according to an embodiment of the invention. 
     
    
     DESCRIPTION OF EMBODIMENTS  
       [0028]      FIG. 2A  is an electric diagram of a DC-AC converter, according to an embodiment of the invention, wherein the DC-AC converter includes voltage boost module  202 , DC-AC converting module  204  and feedback module  216 .  
         [0029]     The voltage boost module  202  of the embodiment includes a voltage bypass circuit  208  and a voltage boost circuit  206 . Here the voltage boost circuit  206 , in fact, can be seen as a dual voltage boost circuit, which includes inductors  224  and  226 , diodes  218  and  220 , capacitor  232 , the first transistor switch  228  and the second transistor switch  230 .  
         [0030]     Wherein, one end of the inductors  224  and  226  is coupled with the positive end of the DC input power  258 , and the another end thereof is coupled with an end of diodes  218  and  220 . One end of the first transistor switch  228  is coupled with the common end of the inductor  224  and the diode  218 , and the other end thereof is coupled with the common end of the capacitor  232  and the DC input power  258 .  
         [0031]     The capacitor  232  is coupled, at its one end, with the common end of diode  218  and  220 , but at its another end is coupled with the negative end of DC input power  258 . Fourthly, one end of the second transistor switch  230  is coupled with the common end of inductor  226  and diode  220 , and the another end thereof is coupled with the common end of capacitor  232  and DC input power  258 .  
         [0032]     In the voltage boost circuit  206  of the embodiment, the inductor  224 , the diode  218  and the first transistor switch  228  can be removed and only the inductor  226 , the diode  220  and the second transistor switch  230  are used. In this manner, it becomes a single voltage boost circuit which functions same as the dual voltage boost circuit for boosting the DC output voltage to the required amount and keeping voltage stabilized.  
         [0033]     Voltage bypass circuit  208  includes diode  222 , one end of which is connected with the positive end of DC input power  258 , and the another end is connected with one end of the capacitor  232 .  
         [0034]     In the embodiment, when the DC voltage, after delivering via voltage bypass circuit  208 , is high enough to supply AC output end, the transistor switches  234 ,  236 ,  238  and  240  in the primary control circuit  212  start to make PWM switching in the DC-AC converting module  204  of the embodiment, and voltage boost circuit  206  is not on duty. By means of diode  222  in the voltage bypass circuit  208 , the voltage boost module  202  carries DC voltage directly from the DC input power and turns over it as DC output power to DC-AC converting module  204 , whereby an AC with the voltage required by output is converted.  
         [0035]     As the voltage transferred by voltage bypass circuit  208  from DC input power  258  is high enough to supply AC output end, the transistor switches  234 ,  236 ,  238  and  240  in primary control circuit  212  of DC-AC converting module  204  is by PWM control to generate a proper voltage level for AC output.  
         [0036]     Therefore, voltage boost circuit  206  is not on duty at this stage. Also and, the voltage boost module  202  continues to carries DC voltage, by the diode  222  of voltage bypass circuit  208 , directly from the DC input power and turns over it as DC output power to the DC-AC converting module  204 , whereby an AC voltage with the voltage required by output is converted.  
         [0037]     As voltage bypass circuit  208  output sourced from DC input power is not high enough to supply the preset output voltage level, it indicates that the transferred DC output voltage is not high enough to meet the need by the output end. Meanwhile, the transistor switches  234 ,  236 ,  238  and  240  in primary control circuit  212  of DC-AC converting module  204  keep their duty cycles to the maximum extent. Thus, the first transistor switch  228  and the second transistor switch  230  starts to make PWM switching. Then, the DC voltage from DC input power  258  is, by means of the voltage boost circuit  206 , boosted to an amount required by output.  FIG. 2B  is a drawing, schematically showing the waveform of output voltage measured at the output end of capacitor  232  after the DC input power voltage is boosted by the voltage boost circuit.  
         [0038]     DC-AC converting module  204  in the embodiment is comprised by voltage transformer  210 , primary control circuit  212  and secondary control circuit  214 , wherein the voltage transformer  210  includes a primary winding and a secondary winding. The output voltage at the secondary winding is determined by the provided voltage at the primary winding.  
         [0039]     In this embodiment, the primary control circuit  212  is electrically coupled between the voltage boost module  202  and the primary winding of voltage transformer  210 . The primary control circuit  212  includes the first transistor switch  234 , the second transistor switch  236 , the third transistor switch  238  and the fourth transistor switch  240 .  
         [0040]     The one end of the first transistor switch  234  is coupled with one end of the second transistor switch  236  and one end of the primary winding of voltage transformer  210 . The another ends of the first transistor switch  234  and the second transistor switch  236  are coupled with two output ends of voltage boost module  202 . And, the one end of the third transistor switch  238  is coupled with one end of the fourth transistor switch  240 , also another end of the primary winding of voltage transformer  210 . Further, the another ends of the third transistor switch  238  and the fourth transistor switch  240  are coupled with two output ends of voltage boost module  202 . Alternatively, the above-mentioned primary control circuit  212  can be substituted by a DC-DC converter with a similarly function, referring to  FIG. 6 .  
         [0041]     Secondary control circuit  214  in the embodiment is connected with the secondary winding of voltage transformer  210 , wherefrom the output voltage is received and converted to the AC output power voltage to meet the need by the output. Alternatively, the above-mentioned secondary control circuit  214  can be substituted by the circuit shown in  FIG. 5  with a same function.  
         [0042]     In the above-described DC-AC converting module  204 , the PWM operation with the transistor switches of the primary control circuit  212  allows the voltage to be transferred from the voltage boost module  202  to the primary winding of the voltage transformer  210 , the transferred voltage goes further to the secondary control circuit  214  via windings of the voltage transformer  210 , then across a rectifier formed by diodes  242 ,  244 ,  246  and  248 , and starts DC-AC converting by means of the first transistor switch  250  and the second transistor switch  252 .  
         [0043]     As the AC output voltage takes positive half-cycle, the first transistor switch  250  is conducted. Instead, as the AC output voltage takes negative half-cycle, the second transistor switch  252  is conducted. And, the switching frequencies of the first transistor switch  250  and the second transistor switch  252  are the same as the one of output voltage. As a result, the AC voltage goes through a filter, formed by inductor  254  and capacitor  256 , exports the AC output power needed by load.  
         [0044]     The feedback module  216  in the embodiment includes the first controller  302  and the second controller  304 . The first controller  302  has the first error compensator  310 , the first comparator  316  and the first PWM signal generator  320 . Wherein, the first error compensator  310  is to compare the feedback signal  306  from the DC-AC converting module  204  with the reference signal  308 . Besides, it gives out the first error compensating signal  312  to an input end of the first comparator  316  which makes a comparison between the first sawtooth waveform signal  314  and the first error compensating signal  312 , and gives out the first comparison signal  318  into the first PWM signal generator  320 .  
         [0045]     The second controller  304  includes a peak-holding circuit  322 , a first subtracter  324 , a second comparator  330  and a second PWM signal generator  334 . Wherein, the peak-holding circuit  322  is to receive the first sawtooth waveform signal  314  and to hold the peak voltage thereof. The first subtracter  324  is to receive the first error compensating signal  312  from the first error compensator  310  and the output signal value from the peak-holding circuit  322 . The resulting signal by subtraction operation is then sent to an input end of the second comparator  330 , where the resulting signal of the subtraction  328  and the second sawtooth waveform signal are compared with each other, and the second comparison signal  332  is given out into the second PWM signal generator  334 .  
         [0046]     In the embodiment, as DC voltage is high enough to supply a voltage needed by at the output end, the first transistor switch  228  and the second transistor switch  230  in the voltage boost circuit keep at off status according to the feedback signal from the second controller  304  of the feedback module  216 . At this moment, the inputting DC voltage is transferred to the DC-AC converting module  204  via the voltage bypass circuit  208 . The feedback signal  306  (current signal or voltage signal) from DC-AC converting module  204  is compared with the reference signal  308  in the first error compensator  310  located in the first controller  302  of feedback module  216 , then the first error compensating signal  312  is sent out to an input end of the first comparator  316 .  
         [0047]     And, the first comparator  316  serves to compare the first sawtooth waveform signal  314  with the first error compensating signal  312  for outputting the first comparison signal  318  to the first PWM signal generator  320 . The first PWM signal generator  320  is based on the first comparison signal  318  to generate PWM signals to control the transistor switches  234 ,  236 ,  238  and  240  in the primary control circuit  212  for PWM.  
         [0048]     When the inputting DC voltage is not high enough to supply the AC voltage needed at the output end, the feedback current signal or the voltage signal  306  from DC-AC converting module  204  is compared with reference signal  308  via the first error compensator  310  of feedback module  216 , then the first error compensating signal  312  is exported to an input end of the first comparator  316 .  
         [0049]     The first comparator  316  serves to compare the first sawtooth waveform signal  314  with the first error compensating signal  312  for outputting the first comparison signal  318  to the first PWM signal generator  320 . The first PWM signal generator  320  is based on the first comparison signal  318  to generate the outputting signals to control the transistor switches  234 ,  236 ,  238  and  240  in primary control circuit  212  for keeping their duty cycles to maxim extent.  
         [0050]     In this instance, the voltage boost circuit  206 , according to peak-holding circuit  322  of the controller  304 , is to receive the first sawtooth waveform signal  314  and to hold the peak value of the sawtooth waveform signal  314 . The first subtracter  324  is to subtract the first error compensating signal  312  by the output signal value from the peak-holding circuit  322 , and send the resulting signal of subtraction  328  to an input end of the second comparator  330 .  
         [0051]     The second comparator  330  serves to compare the resulting signal of subtraction  328  with the second sawtooth waveform signal  326  for giving out the second comparison signal  332  to the second PWM signal generator  334 . The second PWM signal generator  334  is based on the second comparison signal  332  to generate the outputting wave signal to control the operation of voltage boost circuit  206 . As a result, the first transistor switch  228  and the second transistor switch  230  start the switching operation for PWM and convert the DC input voltage to an AC output power required by the output end.  
         [0052]     In summary from above-described, the present invention can effectively reduce energy loss during operation of the DC-AC converter. Also and, the high-capacity capacitors can be saved, resulting in saving cost and improvement of the converting efficiency.  
         [0053]     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.