Abstract:
A ripple converter includes a transistor for switching an input direct-current voltage, a choke coil and a smoothing capacitor for smoothing the switched direct-current voltage, a flywheel diode for causing a current to flow through the choke coil when the transistor is turned off, and a comparing unit for controlling the ON/OFF of the transistor according to ripple in an output voltage. In the ripple converter, a waveform converter is provided on a connecting path between an output terminal and a non-inverting input terminal of a comparator in the comparing unit. A result of converting the waveform of the output voltage is compared with a reference voltage, and a result of the comparison is fed back to the transistor.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to ripple converters. More specifically, the present invention relates to a ripple converter that maintains stable oscillation for switching regardless of the type or capacitance of an externally attached capacitor for smoothing output.  
         [0003]     2. Description of the Related Art  
         [0004]     A ripple converter generally refers to a DC-DC converter including a switching element for switching an input DC voltage, a choke coil and a smoothing capacitor for smoothing the switched DC voltage, a flywheel diode for causing a current to flow through the choke coil when the switching element is turned off, and a control circuit for controlling the ON/OFF of the switching element according to the magnitude of ripple in an output voltage. Circuits of such a ripple converter are publicly known, as described, for example, in the document, “Transistor Gijutsu Special No. 28,” Tokushuu, Saishin Dengenkairo Sekkeigijutsu no Subete, CQ Publishing Co., Ltd., issued on Jul. 1, 1991.  
         [0005]      FIG. 14A  shows a circuit diagram of a ripple converter that is similar to the circuits disclosed in the document mentioned above.  FIG. 14B  shows the voltage at an output terminal of a switching element and the waveform of an output voltage (voltage waveform at an output terminal Vout).  
         [0006]     A ripple converter  1  according to the circuit diagram shown in  FIG. 14A  includes a PNP transistor Q 1  that functions as a switching element, a flywheel diode D 1 , a choke coil L 1 , a smoothing capacitor C 1 , and a comparing unit  2 . The emitter of the transistor Q 1  is connected to an input terminal Vin, and the collector thereof is connected to an output terminal Vout via the choke coil L 1 . The collector of the transistor Q 1  is connected to the ground via the flywheel diode D 1 . The output terminal Vout is connected to the ground via the smoothing capacitor C 1 . The comparing unit  2  includes a comparator  3  and a reference voltage source Vref having one end connected to the ground. The non-inverting input terminal of the comparator  3  is connected to the output terminal Vout, and the inverting input terminal thereof is connected to the reference voltage source Vref. The output of the comparator  3  is connected to the base of the transistor Q 1 . Of these elements, the comparing unit  2  functions as a control circuit for performing feedback control of the ON/OFF of the switching element according to ripple in an output voltage.  
         [0007]     In the ripple converter  1  constructed as described above, the voltage (output voltage) vo at the output terminal Vout increases when the output of the comparator  3  is at low level (hereinafter abbreviated as L) and the transistor Q 1  is ON. When the output voltage vo exceeds a voltage (reference voltage) vref of the reference voltage source Vref, the output of the comparator  3  changes to a high level (hereinafter abbreviated as H) and the transistor Q 1  is turned off. Between when the output voltage vo exceeds the reference voltage vref and when the transistor Q 1  is turned off, a delay time t 1  such as a delay caused by the comparator  3  and a delay due to a switching time of the transistor Q 1  exists. Thus, the output voltage vo keeps increasing during the delay time t 1 .  
         [0008]     The output voltage vo starts decreasing when the transistor Q 1  is turned off. When the output voltage vo drops below the reference voltage vref, the output of the comparator  3  changes to L and the transistor Q 1  is turned on. Between when the output voltage vo drops below the reference voltage vref and when the transistor Q 1  is turned on, a delay time such as a delay caused by the comparator  3  and a delay due to a switching time of the transistor Q 1  exists. Thus, the output voltage vo keeps decreasing during the delay time t 2 . When the delay time t 2  elapses and the transistor Q 1  is turned on, the status returns to initial status. This is continuously repeated. It is described that, as a result, the output voltage vo substantially forms a triangular wave and the average value thereof is maintained substantially at the reference voltage vref.  
         [0009]     The document mentioned earlier describes that the voltage at the output terminal Vout forms a substantially triangular wave as described above. However, the description is not necessarily accurate. Presumably, it is assumed that a capacitor having a large equivalent series resistance (ESR), such as an aluminum electrolytic capacitor, is used as the smoothing capacitor C 1 . Actually, in some cases, the voltage forms a substantially triangular wave as described above depending on the characteristics of the smoothing capacitor C 1 . However, in other cases, the voltage forms a waveform having sudden voltage changes in the vicinities of the peaks of a triangular wave (the timing of the ON/OFF switching of the switching element), or a waveform obtained by alternately folding back quadratic curves, as will be described later.  
         [0010]     More specifically, for example, when a capacitor having a relatively large equivalent series inductance (ESL), such as a leaded low-impedance electrolytic capacitor, is used as a smoothing capacitor, the waveform has sudden voltage changes at the timing of the ON/OFF switching of the switching element, as shown in  FIG. 15 .  
         [0011]     As another example, a case where the smoothing capacitor is an ideal capacitor and has sufficiently small ESR or ESL will be considered. In this case, the current that flows through the choke coil increases linearly when the switching element is ON, and decreases linearly when the switching element is OFF. That is, the waveform of the current that flows through the choke coil is triangular. When the smoothing capacitor is an ideal capacitor, the voltage across the smoothing capacitor is a value obtained by integrating the capacitor current. Thus, the voltage across the smoothing capacitor (i.e., the voltage waveform at the output terminal) obtained by smoothing the choke coil current having a triangular waveform has a waveform in which two quadratic curves are alternately connected with each other, as shown in  FIG. 16 . A peak point in this case is located in the vicinity of the middle of an ON period or OFF period of the switching element.  
         [0012]     As described above, the waveform of the output voltage of the ripple converter changes depending on response delays (delay times t 1  and t 2 ) of the system and characteristics of the smoothing capacitor.  
         [0013]     Generally, when a DC-DC converter made as a module is used, an additional capacitor for output (another smoothing capacitor) is often externally attached to an output terminal of the module. At the stage of designing the module, it is difficult to predict the characteristics of the capacitor attached to the module. Therefore, in some cases, the driving frequency cannot be set to a desired value when the ripple converter is used. This problem becomes particularly severe when a ceramic capacitor having a small ESR or ESL is used as the output capacitor. More specifically, as will be understood from a comparison between  FIG. 16  and  FIG. 14 (B), when a ceramic capacitor is used, the time when the output voltage vo crosses the reference voltage vref is delayed, which causes a decrease in the driving frequency. Thus, a large value must be chosen for the inductance of the choke coil L 1 , and ripple increases. Therefore, it has been difficult to use a ceramic capacitor as an output capacitor.  
         [0014]     As for the response delays of the system, the main cause is a delay caused by the comparator. The delay caused by the comparator is affected by the amount of overdriving (i.e., the voltage difference between the input terminals, or the voltage difference between a maximum value v 1  of the output voltage vo and the reference voltage vref in  FIG. 14B ), and the delay tends to be greater as the amount of overdriving decreases. That is, the delay decreases as the output voltage changes more rapidly and the delay increases as the output voltage changes more slowly.  
         [0015]     For example, in the ripple converter  1  shown in  FIG. 14A , when the capacitance of the output capacitor is increased in order to reduce ripple in the output voltage, the amount of temporal change in the output voltage decreases. Thus, the amount of overdriving at the time when the time t 1  has elapsed since the output voltage crossed the reference voltage vref is less than v 1 . Thus, the actual delay time is larger than t 1 , which caused the driving frequency to decrease as compared to a case where the capacitance of the output capacitor is not increased. When the driving frequency decreases, ripple in the output voltage increases. Thus, even when the capacitance of the output capacitor is increased, ripple in the output voltage does not decrease substantially. That is, the driving frequency decreases without reducing ripple.  
       SUMMARY OF THE INVENTION  
       [0016]     To overcome the problems described above, preferred embodiments of the present invention provide a ripple converter that maintains stable oscillation regardless of the type or capacitance of an output capacitor that is externally attached when the ripple converter is used.  
         [0017]     A ripple converter according to a preferred embodiment of the present invention includes a switching element for switching an input direct-current voltage, a choke coil and a smoothing capacitor for smoothing the switched direct-current voltage, a flywheel diode for causing a current to flow through the choke coil when the switching element is turned off, and a control circuit for performing feedback control as to whether the switching element is on or off according to ripple in an output voltage. The control circuit includes a waveform converter for converting a waveform of a signal that is proportional to the output voltage and outputting a resulting signal, and a comparing unit for comparing the output of the waveform converter with a reference voltage and outputting a result of the comparison.  
         [0018]     The waveform converter preferably includes a phase converter for converting a phase of the signal that is proportional to the output voltage and outputting a resulting signal. Furthermore, the phase converter preferably includes a differentiator for differentiating an input signal and outputting a resulting signal or an integrator for integrating an input signal and outputting a resulting signal.  
         [0019]     Also, the phase converter preferably includes a current detector for detecting a waveform of the current that flows through the choke coil and outputting the waveform of the current, and a signal processor for processing the signal that is proportional to the output voltage according to an output signal of the current detector. Furthermore, the current detector preferably includes a current detecting resistor arranged in series with the choke coil. Alternatively, the current detector may use a resistive component of the choke coil.  
         [0020]     A ripple converter according to preferred embodiments of the present invention maintains stable oscillation regardless of the type or capacitance of an output capacitor that is externally attached when the ripple converter is used.  
         [0021]     These and various other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is a circuit diagram showing a ripple converter according to a preferred embodiment of the present invention;  
         [0023]      FIG. 2  is a circuit diagram showing a ripple converter according to another preferred embodiment of the present invention;  
         [0024]      FIG. 3  is a waveform chart showing voltage waveforms under a certain condition in parts of the ripple converter shown in  FIG. 2 ;  
         [0025]      FIG. 4  is a waveform chart showing voltage waveforms under another condition in parts of the ripple converter shown in  FIG. 2 ;  
         [0026]      FIG. 5  is a circuit diagram showing a ripple converter according to yet another preferred embodiment of the present invention;  
         [0027]      FIG. 6  is a waveform chart showing voltage waveforms in parts of the ripple converter shown in  FIG. 5 ;  
         [0028]      FIG. 7  is a circuit diagram showing a ripple converter according to yet another preferred embodiment of the present invention;  
         [0029]      FIG. 8  is a circuit diagram showing a ripple converter according to yet another preferred embodiment of the present invention;  
         [0030]      FIG. 9  is a waveform chart showing voltage waveforms in parts of the ripple converter shown in  FIG. 8 ;  
         [0031]      FIG. 10  is a circuit diagram showing a ripple converter according to yet another preferred embodiment of the present invention;  
         [0032]      FIG. 11  is a circuit diagram showing a ripple converter according to yet another preferred embodiment of the present invention;  
         [0033]      FIG. 12  is a circuit diagram showing a ripple converter according to yet another preferred embodiment of the present invention;  
         [0034]      FIG. 13A  is a perspective view and  FIG. 13B  is a sectional view showing an example of a current transformer in the ripple converter shown in  FIG. 12 ;  
         [0035]      FIG. 14A  is a circuit diagram and  FIG. 14B  is a waveform chart showing voltage waveforms under a certain condition in parts of an example of a ripple converter according to the related art;  
         [0036]      FIG. 15  is a waveform chart showing voltage waveforms under another condition in parts of the ripple converter shown in  FIG. 14A ; and  
         [0037]      FIG. 16  is a waveform chart showing voltage waveforms under yet another condition in parts of the ripple converter shown in  FIG. 14A . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     First Preferred Embodiment  
       [0038]      FIG. 1  shows a circuit diagram of a ripple converter according to a preferred embodiment of the present invention. In  FIG. 1 , elements corresponding or equivalent to those in  FIG. 14B  are designated by the same numerals, and descriptions thereof will be omitted.  
         [0039]     In a ripple converter  10  shown in  FIG. 1 , a waveform converter  11  is disposed on a connecting path between the output terminal Vout and the non-inverting input terminal of the comparator  3 . The ripple converter  10  is otherwise the same as the ripple converter  1  according to the related art shown in  FIG. 14 . In the ripple converter  10 , the waveform converter  11  and the comparing unit  2  define a control circuit for exercising feedback control of the ON/OFF of a switching element according to ripple in an output voltage.  
         [0040]     In the ripple converter  10 , the waveform of the output voltage vo is converted by the waveform converter  11 , and a result of the waveform conversion is compared with the reference voltage vref. As will be described later, the waveform converter  11  converts the waveform of the output voltage vo into a different waveform. Thus, regardless of the output capacitor that is attached to the ripple converter, it is possible to change the characteristics of the waveform converter in accordance with the output capacitor to increase an allowable setting range of the driving frequency, thereby maintaining desired oscillation status. Now, the waveform converter will be described more specifically with reference to preferred embodiments.  
       Second Preferred Embodiment  
       [0041]      FIG. 2  shows a circuit diagram of a ripple converter according to another preferred embodiment of the present invention. In  FIG. 2 , elements corresponding to those in  FIG. 1  are designated by the same numerals, and descriptions thereof will be omitted.  
         [0042]     In a ripple converter  15  shown in  FIG. 2 , the output terminal Vout is connected to the ground via resistors R 1  and R 2  in series. Furthermore, a capacitor C 2  and a resistor R 3 , connected in series with each other, are connected in parallel to the resistor R 1 . A node between the resistors R 1  and R 2  is connected to the non-inverting input terminal of the comparator  3 . That is, the resistors R 1 , R 2 , and R 3 , and the capacitor C 2  define a waveform converter  16 .  
         [0043]     Of the elements of the waveform converter  16 , the resistors R 1  and R 2  provide a circuit for inputting a voltage ver 1  that is proportional to the output voltage vo to the non-inverting input terminal of the comparator  3 . The capacitor C 2  and the resistors R 3  and R 2  provide a circuit (differentiator) for inputting a value ver 2  obtained by differentiating the output voltage vo from the non-inverting input terminal of the comparator  3 . Thus, a voltage ver that is actually input to the non-inverting input terminal of the comparator  3  is a sum of these values. The resistor R 3  is provided in order to adjust the amount of feedback of ripple voltage, and may be omitted (short-circuited) when it is unnecessary.  
         [0044]     Now, it is assumed that a capacitor having a small ESR or ESL, such as a ceramic capacitor, is used as the smoothing capacitor C 1 .  FIG. 3  shows the voltage ver 1 , the voltage ver 2 , and the voltage ver in this case. The phase of the voltage ver that is input to the non-inverting input terminal of the comparator  3  is somewhat advanced as compared to the phase of the output voltage vo. Thus, delay times t 1 ′ and t 2 ′ between when the voltage ver crosses the reference voltage vref and when the ON/OFF of the transistor Q 1  is switched are increased as compared to a case where the waveform converter  16  is not used (i.e., when the output voltage vo itself is input to the non-inverting input terminal of the comparator  3 ).  
         [0045]     However, since the delay times are actually constant values determined by the characteristics of the comparator, the ON/OFF of the transistor Q 1  is actually switched after predetermined delay times t 1  and t 2  since the voltage ver crosses the reference voltage vref. The predetermined delay times t 1  and t 2  are shorter than the delay times t 1 ′ and t 2 ′ described above. Thus, the ON/OFF of the transistor Q 1  switches faster than in the waveform shown in  FIG. 3 . This indicates that the driving frequency increases. Therefore, a small choke coil is used as the choke coil L 1 , which enables compact design of the ripple converter  15 . Furthermore, the ripple converter  15  overcomes the problem of the related art that, assuming the same delay times (t 1 , t 2 ), the driving frequency is decreased when a ceramic capacitor is used as the smoothing capacitor C 1  as compared to a case where an electrolytic capacitor is used.  
         [0046]     On the contrary, it is assumed that a capacitor having a large ESL, such as a low-impedance electrolytic capacitor, is used as the smoothing capacitor C 1 .  FIG. 4  shows the voltage ver 1 , the voltage ver 2 , and the voltage ver in this case. That is, the phase of the voltage ver that is input to the non-inverting input terminal of the comparator  3  is somewhat advanced as compared to the phase of the output voltage vo. Thus, similarly to a case where a capacitor having a small ESR or ESL, such as a ceramic capacitor, is used, the driving frequency is increased. This enables compact design of the ripple converter.  
         [0047]     As will be understood from the fact that the phase of the voltage ver that is input to the non-inverting input terminal of the comparator  3  is advanced as compared to the phase of the output voltage vo, the waveform converter  16  essentially includes a phase converter.  
       Third Preferred Embodiment  
       [0048]      FIG. 5  shows a circuit diagram of a ripple converter according to yet another preferred embodiment of the present invention. In  FIG. 5 , elements corresponding to those in  FIG. 2  are designated by the same numerals, and descriptions thereof will be omitted.  
         [0049]     In a ripple converter  18  shown in  FIG. 5 , instead of the capacitor C 2  and the resistor R 3  in  FIG. 2 , a resistor R 4  and a capacitor C 3 , connected in series with each other, are connected in parallel to the resistor R 2 . Thus, the resistors R 1 , R 2 , and R 4 , and the capacitor C 3  define a waveform converter  19 .  
         [0050]     Of the elements of the waveform converter  19 , the resistors R 1  and R 2  provide a circuit for inputting a value ver 1  that is proportional to the output voltage vo to the non-inverting input terminal of the comparator  3 . The resistors R 1  and R 4  and the capacitor C 3  provide a circuit (integrator) for inputting a value ver 2  obtained by integrating the output voltage vo to the non-inverting input terminal of the comparator  3 . Thus, a voltage ver that is actually input to the non-inverting input terminal of the comparator  3  is a sum of these values.  
         [0051]     Now, it is assumed that a capacitor having a large ESR, such as an ordinary electrolytic capacitor, is used.  FIG. 6  shows the voltage ver 1 , the voltage ver 2 , and the voltage ver in this case. The phase of the voltage ver that is input to the non-inverting input terminal of the comparator  3  is somewhat delayed as compared to the phase of the output voltage vo. Thus, delay times t 1 ′ and t 2 ′ between when the voltage ver crosses the reference voltage vref and when the ON/OFF of the transistor Q 1  is switched are decreased as compared to a case where the waveform converter  16  is not used (i.e., when substantially the output voltage vo itself is input to the non-inverting input terminal of the comparator  3 ).  
         [0052]     However, since the delay times are actually constant values determined by the characteristics of the comparator, the ON/OFF of the transistor Q 1  is actually switched after predetermined delay times t 1  and t 2  since the voltage ver crosses the reference voltage vref. The predetermined delay times t 1  and t 2  are longer than the delay times t 1 ′ and t 2 ′ described above. Thus, the ON/OFF of the transistor Q 1  is switched later than in the waveform shown in  FIG. 6 . This indicates that the driving frequency decreases. When the delay time of a system is small and the driving frequency is high without using a waveform converter, resulting in large switching loss, switching loss is reduced by using an integrated value of an output voltage as such a waveform converter to decrease the driving frequency.  
         [0053]     As will be understood from the fact that the phase of the voltage ver that is input to the non-inverting input terminal of the comparator  3  is delayed as compared to the phase of the output voltage vo, the waveform converter  16  essentially includes a phase converter.  
       Fourth Preferred Embodiment  
       [0054]      FIG. 7  is a circuit diagram of a ripple converter according to a preferred embodiment of the present invention. In  FIG. 7 , elements corresponding or equivalent to those in  FIG. 1  are designated by the same numerals, and descriptions thereof will be omitted.  
         [0055]     A ripple converter  20  shown in  FIG. 7  differs from the ripple converter  10  shown in  FIG. 1  only with regard to a waveform converter  21 . The waveform converter  21  includes a voltage detector  22  for detecting an output voltage vo and outputting a signal that is proportional to the output voltage vo, a current detector  23  for detecting a current that flows through the choke coil L 1  and outputting a corresponding signal, and a signal processor  24 . The output terminal Vout is connected to an input terminal of the signal processor  24  via the voltage detector  22 . The current detector  23  is disposed so as to detect a current that flows through a wire connecting the choke coil L 1  with the output terminal Vout, and is connected to another input terminal of the signal processor  24 . An output terminal of the signal processor  24  is connected to the non-inverting input terminal of the comparator  3 . Now, the current converter and functions thereof will be described specifically.  
       Fifth Preferred Embodiment  
       [0056]      FIG. 8  shows a circuit diagram of a ripple converter according to yet another preferred embodiment of the present invention. In  FIG. 8 , elements corresponding to those in  FIG. 14A  are designated by the same numerals, and descriptions thereof will be omitted.  
         [0057]     In a ripple converter  30  shown in  FIG. 8 , a current detecting resistor R 5  having a small resistance is disposed between the choke coil L 1  and the output terminal Vout, two resistors R 1  and R 2  are connected in series between the ground and a node between the choke coil L 1  and the resistor R 5 , and a node between the resistors R 1  and R 2  is connected to the non-inverting input terminal of the comparator  3 . In this case, the resistor R 1  and R 2  and the current detecting resistor R 5  define a waveform converter  31 , among which the current detecting resistor R 5  functions as a current detector.  
         [0058]     In the ripple converter  30  constructed as described above, the voltage across the current detecting resistor R 5  is proportional to the current that flows through the choke coil L 1 . Thus, the voltage at the node between the choke coil L 1  and the current detecting resistor R 5  (denoted as a voltage vr) has a waveform obtained by converting the waveform of the output voltage vo according to the current that flows through the choke coil L 1 . A voltage obtained by dividing the voltage at the node by the resistors R 1  and R 2  is input to the non-inverting input terminal of the comparator  3 , such that a sum of a signal detected by the current detector and a signal detected by the voltage detector is input to the non-inverting input terminal of the comparator  3 . Thus, the waveform converter  31  essentially includes a signal processor for processing a signal that is proportional to the output voltage according to an output signal of the current detector.  
         [0059]     Now, it is assumed that a capacitor having a small ESR or ESL, such as a ceramic capacitor, is used as the smoothing capacitor C 1  in the ripple converter  30 .  FIG. 9  shows the voltage vo, the voltage vr, and the voltage ver in this case. The voltage vo is the same as in the case of the ripple converter  15  shown in  FIG. 2 . The voltage vr has a waveform that increases linearly during an ON period of the transistor Q 1  and decreases linearly during an OFF period of the transistor Q 1 , proportionally to the current that flows through the choke coil L  1 . The voltage ver is a sum of these voltages.  
         [0060]     As will be understood from  FIG. 9 , delay times t 1 ′ and t 2 ′ between when the voltage ver crosses the reference voltage vref and when the ON/OFF of the transistor Q 1  is switched increase as compared to a case where the waveform converter  31  is not used (i.e., when substantially the output voltage vo itself is input to the non-inverting input terminal of the comparator  3 ). Thus, the driving frequency increases similarly to the case of the ripple converter  15  shown in  FIG. 2 .  
         [0061]     Furthermore, in the case of the ripple converter  30 , the voltages across the resistors R 1  and R 2  connected in series with each other can be changed by changing the resistance of the current detecting resistor R 5 , irrespective of the magnitude of ripple voltage. Thus, design flexibility of the waveform converter is increased as compared to the case of the ripple converter  15 . Furthermore, stable operation is achieved. In addition, in the ripple converter  30 , even when an output capacitor is additionally provided externally to the module, the voltage ver only becomes closer to the voltage vr when the amplitude of the output voltage vo decreases. Thus, advantageously, the driving frequency remains substantially the same.  
       Sixth Preferred Embodiment  
       [0062]      FIG. 10  shows a circuit diagram of a ripple converter according to yet another preferred embodiment of the present invention. In  FIG. 10 , elements corresponding to those in  FIG. 2  are designated by the same numerals, and descriptions thereof will be omitted.  
         [0063]     In the ripple converter  15  shown in  FIG. 2 , the capacitor C 2  and the resistor R 3 , connected in series with each other, are connected in parallel to the resistor R 1  in the waveform converter  16 . Thus, one end of the capacitor C 2  is connected to the output terminal Vout. On the other hand, in a ripple converter  40  shown in  FIG. 10 , in waveform converter  41 , a current detecting resistor R 6  is disposed between the choke coil L 1  and the output terminal Vout, and one end of the capacitor C 2  is connected to a node between the choke coil L 1  and the current detecting resistor R 6 . The resistor R 6  has a small resistance and functions as a current detector, similar to the current detecting resistor R 5  in the ripple converter  30  shown in  FIG. 8 . Similar to the case of the ripple converter  15 , the resistor R 3  may be omitted (short-circuited) when it is unnecessary.  
         [0064]     In the ripple converter  40  constructed as described above, the waveform of a signal obtained by the resistors R 1  and R 2  at a node between the resistors R 1  and R 2  is proportional to the output voltage vo, similar to the case of the ripple converter  15 . On the other hand, the waveform of a signal obtained by the capacitor C 2  and the resistors R 3  and R 2  at a node between the resistors R 3  and R 2  is a value obtained by differentiating a sum voltage of the output voltage vo and the component of a current that flows through the choke coil L 1  in relation to the resistor R 6 . Thus, the waveform converter  41  essentially includes a signal processor for processing a signal that is proportional to the output voltage according to an output signal of the current detector.  
         [0065]     In the ripple converter  30 , as opposed to the ripple converter  40 , a voltage detected by the resistors R 1  and R 2  is not the output voltage vo. In this case, the voltage at a node between the choke coil L 1  and the resistor R 5  is controlled so as to be constant. Thus, the voltage drop across the resistor R 5  increases when, for example, the output current increases, which possibly deteriorates load regulation (i.e., the output voltage of the ripple converter changes as the load current is increased).  
         [0066]     On the other hand, in the ripple converter  40 , the output voltage vo itself is used as a DC feedback by the resistors R 1  and R 2 , such that load regulation is not deteriorated. Furthermore, since AC components of the ripple voltage are input to the comparator  3  via the capacitor C 2  and the resistor R 3 , the waveform at the non-inverting input of the comparator  3  is maintained so as to be substantially triangular, similar to the case of the ripple converter  30 . Thus, the accuracy of output voltage is improved while maintaining the advantages of the ripple converter  30 .  
         [0067]     Furthermore, similar to the ripple converter  30 , the magnitude of the voltage that is input to the comparator  3  via the capacitor C 2  and the resistor R 3  can be changed by changing the resistance of the resistor R 6 , irrespective of the magnitude of ripple voltage. This increases design flexibility of the waveform converter, and achieves more stable operation.  
       Seventh Preferred Embodiment  
       [0068]      FIG. 11  shows a circuit diagram of a ripple converter according to yet another preferred embodiment of the present invention. In  FIG. 11 , elements corresponding to those in  FIG. 10  are designated by the same numerals, and descriptions thereof will be omitted.  
         [0069]     In the ripple converter  40  shown in  FIG. 10 , the resistor R 6  for current detection is disposed in series with a current path between the choke coil L 1  and the output terminal Vout in the waveform converter  41 . Thus, power loss caused by the current detecting resistor R 6  cannot be neglected. In view of this, in a ripple converter  50 , a resistive component that the choke coil L 1  includes (hereinafter referred to as a resistance Ri) is used such that a discrete resistor such as the resistor R 6  disposed in series is omitted.  
         [0070]     In the ripple converter  50 , a series circuit including a resistor R 7  and a capacitor C 4  is connected in parallel with the choke coil L 1 . Furthermore, one end of the capacitor C 2  is connected to a node between the resistor R 7  and the capacitor C 4 , defining a waveform converter  51 . That is, the waveform converter  51  includes the resistors R 1 , R 2 , R 3 , and R 7 , and the capacitors C 2  and C 4 .  
         [0071]     Now, the relationship among the choke coil L 1  (inductance l 1  and resistive component ri), the resistor R 7  (resistance r 7 ), and the capacitor C 4  (capacitance c 4 ) in the ripple converter  50  will be considered. It is generally known that the voltage across the capacitor C 4  is proportional to the value of the current that flows through the choke coil L 1  when the values are chosen such that c 4 =l 1 /(ri·r 7 ). Thus, the voltage at the node between the resistor R 7  and the capacitor C 4  is substantially the same as the voltage at the node between the choke coil L 1  and the current detecting resistor R 6  in the ripple converter  40 . Thus, the ripple converter  50  achieves the same advantages as the ripple converter  40  while eliminating unnecessary power loss due to the addition of a discrete current detecting resistor.  
         [0072]     When the values are chosen such that c 4 &lt;l 1 /(ri·r 7 ), the ripple voltage across the capacitor C 4  increases. Thus, the overdrive voltage of the comparator  3  increases, such that the driving frequency increases. On the contrary, when the values are chosen such that c 4 &gt;l 1 /(ri˜r 7 ), the ripple voltage across the capacitor C 4  decreases, such that the driving frequency decreases. That is, an effect equivalent to the effect of increasing or decreasing the resistance of the resistor R 6  in the ripple converter  40  is achieved.  
       Eighth Preferred Embodiment  
       [0073]      FIG. 12  shows a circuit diagram of a ripple converter according to yet another preferred embodiment of the present invention. In  FIG. 12 , elements corresponding to those in  FIG. 2  are designated by the same numerals, and descriptions thereof will be omitted.  
         [0074]     In a ripple converter  60  shown in  FIG. 12 , as a current detector in a waveform converter  61 , a current transformer CT is disposed on a wire connecting the choke coil L 1  with the output terminal Vout. One of the terminals of the current transformer CT is connected to a node between the resistors R 1  and R 2 , and the other terminal is connected to the non-inverting input terminal of the comparator  3 . By the connections described above, a signal adder, i.e., a signal processor, is provided.  
         [0075]     In the ripple converter  60  constructed as described above, a voltage that is proportional to a current that flows through the choke coil L 1  is generated on the current transformer CT. Then, the voltage is added to a voltage ver 1  that is proportional to the output voltage vo appearing at the node between the two resistors R 1  and R 2 , and the result is input to the non-inverting input terminal of the comparator  3 .  
         [0076]     As described above, in the ripple converter  60 , it is possible to convert the waveform by adding a voltage that is proportional to a current that flows through the choke coil L 1  to a voltage that is proportional to the output voltage vo. The current that flows through the choke coil L 1  depends on the difference between input and output voltages and the inductance of the choke coil L 1 , regardless of the type of an output capacitor. Thus, stable control operations are provided regardless of the type or capacitance of an output capacitor.  
         [0077]     Also in this case, similar to the case of the ripple converter  50 , the output voltage is accurately controlled even when the load current is large.  
         [0078]     Instead of the current transformer CT, a wiring electrode  61  disposed in proximity to the choke coil L 1  may be used, as shown in  FIGS. 13A and 13B .  FIG. 13A  is a perspective view and  FIG. 13B  is a sectional view showing positional relationship between the choke coil L 1  and the wiring electrode  61 .  
         [0079]     According to the arrangement described above, a flux (leakage flux) generated by a current that flows through the choke coil L 1  crosses the wiring electrode  61 . Accordingly, although such a large value as in the case where a current transformer is used cannot be expected, a voltage that is proportional to the current that flows through the choke coil L 1  is generated on the wiring electrode  61 . Thus, advantageously, a current transformer need not be separately provided.  
         [0080]     It should be understood that the foregoing description is only illustrative of the present invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the present invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variations that fall within the scope of the appended claims.