Abstract:
Provided is a DC-DC converter circuit which is stably operated without being affected by a size of an output capacitor ( 111 ) and a cost thereof. When a switching element is turned on so that a charge current that was flowing into an output capacitor flows into the switching element, the current is converted into a voltage to be added to an output voltage, and a resultant voltage is fed back to a control system.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to phase compensation of a DC-DC converter circuit.  
         [0003]     2. Description of the Related Art  
         [0004]     In general, when phase compensation is not performed on a DC-DC converter circuit, the stable operation thereof cannot be ensured. There has been widely known a method of connecting a resistor with an output capacitor in series as one of the methods of performing the phase compensation.  
         [0005]      FIG. 3  is a circuit diagram showing a conventional chopper type boosting DC-DC converter circuit. For the phase compensation to ensure the stable operation, a phase compensation resistor  112  is connected with an output capacitor  111  in series, or an output capacitor provided with an equivalent series resistor is used, thereby providing a zero point. In order to perform sufficient phase compensation, it is necessary to set a frequency fz at the zero point in a frequency band for which the phase compensation is required. Here, assume that a capacitance value of the output capacitor  111  is given by C out , a resister value of the phase compensation resistor  112  is given by R ESR , the frequency fz at the zero point is expressed by the following expression (1). 
   fz= 1/(2Π× C   out   ×R   ESR )  (1)    FIG. 4  is a timing chart of the chopper type boosting DC-DC converter circuit. In the chopper-type boosting DC-DC converter circuit shown in  FIG. 3 , a current flows through a rectifying device  110  when a switching element  104  is turned off, and the current does not flow through the rectifying device  110  when the switching element  104  is turned on. Therefore, a variation in current Ipk is caused in the rectifying device  110 . Therefore, when the resister value of the phase compensation resistor  112  is given by R ESR , a ripple voltage Vpk expressed by the following expression (2) is generated in an output voltage. 
   Vpk≈Ipk×R   ESR   (2)  
 In general, when the ripple voltage Vpk generated in the output voltage is large, normal feedback control is not performed, so the stable operation cannot be ensured. Therefore, in order to suppress generation of the ripple voltage Vpk in the output voltage, it is necessary to set the resister value R ESR  of the phase compensation resistor  112  to a small value. (See JP 07-274495 A ( FIG. 1 )) 
 
         [0006]     However, when the resister value R ESR  of the phase compensation resistor  112  is set to a small value, it is necessary to increase the capacitance value C out  of the output capacitor  111  in order to achieve sufficient phase compensation. Therefore, there is a problem in that a size of the output capacitor  111  is made large and a cost thereof is increased.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention has been made to solve the conventional problem as described above. An object of the present invention is to provide a DC-DC converter circuit which is stably operated.  
         [0008]     According to the present invention, when a switching element is turned on so that a charge current that was flowing into an output capacitor flows into the switching element, the charge current is converted into a voltage. A variation component in the converted voltage is added to an output voltage and a resultant voltage is fed back to a control system.  
         [0009]     As described above, according to the present invention, it is possible to provide a DC-DC converter circuit which can be stably operated even with a small output capacitor by suppressing a ripple voltage Vpk generated in the output voltage. The improvement of stability similar to a current mode can be also desired. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     In the accompanying drawings:  
         [0011]      FIG. 1  is a diagram showing a chopper type boosting DC-DC converter circuit according to a first embodiment of the present invention;  
         [0012]      FIG. 2  is a timing chart of the chopper type boosting DC-DC converter circuit according to the first embodiment of the present invention;  
         [0013]      FIG. 3  is a diagram showing a conventional chopper type boosting DC-DC converter circuit;  
         [0014]      FIG. 4  is a timing chart of the conventional chopper type boosting DC-DC converter circuit;  
         [0015]      FIG. 5  is a diagram showing a chopper type boosting DC-DC converter circuit according to a second embodiment of the present invention; and  
         [0016]      FIG. 6  is a diagram showing a chopper type inverting DC-DC converter circuit according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0017]      FIG. 1  is a diagram showing a chopper type boosting DC-DC converter circuit according to a first embodiment of the present invention.  
         [0018]     In the chopper type boosting DC-DC converter circuit shown in  FIG. 1 , an input voltage from a power source  101  is subjected to energy conversion by an inductor  109 , a switching element  104 , a rectifying device  110 , and an output capacitor  111 , to generate an output voltage. Then, an error amplifying circuit  106  compares a reference voltage  107  with a voltage obtained by dividing the output voltage by a voltage dividing circuit  108  to generate an output signal for controlling a PWM control circuit  105 . The PWM control circuit  105  outputs a pulse control signal to the switching element  104  to maintain the output voltage at a predetermined level. A phase compensation resistor  112  is inserted between the output capacitor  111  and the rectifying device  110 . A feedback resistor  113  and a feedback capacitor  120 , which are connected in parallel with each other, are connected between a connection point (a point), at which the rectifying device  110  and the phase compensation resistor  112  are connected, and the voltage dividing circuit  108 . A ripple voltage correction resistor  114  is connected between the switching element  104  and GND. A ripple voltage correction capacitor  115  is connected between a connection point (b point), at which the switching element  104  and the ripple voltage correction resistor  114  are connected, and a connection point (c point), at which the feedback resistor  113  and the voltage dividing circuit  108  are connected.  
         [0019]      FIG. 2  is a timing chart of the chopper type boosting DC-DC converter circuit according to the first embodiment of the present invention and the operation thereof will be described below.  
         [0020]     First, when the switching element  104  is turned off, a current from the indictor  109  flows into the rectifying device  110 , to thereby rise a voltage at the connection point (a point) between the rectifying device  110  and the phase compensation resistor  112 . When the current flowing into the rectifying device  110  is given by I Loff  and a resistance value of the phase compensation resistor  112  is given by R ESR , a rising voltage Va is expressed by the following expression (3). 
 
 Va≈I   Loff   ×R   ESR   (3) 
 
         [0021]     Next, when the switching element  104  is turned on, the current from the indictor  109  flows into the switching element  104 . Therefore, the voltage at the connection point (a point) between the rectifying device  110  and the phase compensation resistor  112  lowers, so a voltage rises at the connection point (a point) between the switching element  104  and the ripple voltage correction resistor  114 . When the current flowing into the switching element  104  is given by I Lon  and a resistance value of the ripple voltage correction resistor  114  is given by R SENSE , a rising voltage Vb is expressed by the following expression (4). 
 
 Vb≈I   Lon   ×R   SENSE   (4) 
 
         [0022]     A variation in voltage Vc at the input point (c point) of the voltage dividing circuit  108  is expressed by the following expression (5). 
 
 Vc≈I   Loff ×( R   ESR   −R   SENSE )  (5) 
 
         [0023]     Thus, by adjusting the resistance value R ESR  of the phase compensation resistor  112  and the resistance value R SENSE  of the ripple voltage correction resistor  114 , it is possible to reduce the variation in voltage Vc at the input point (c point) of the voltage dividing circuit  108 , that is, the ripple voltage to be inputted to the voltage dividing circuit  108 .  
         [0024]     According to such a method, even when a capacitor whose size is small, cost is low, and capacitance value is small, is used as the output capacitor  111 , feedback control can be performed based on a signal having a small ripple voltage, so stable operation is possible.  
         [0025]     Further, a capacitance value of the feedback capacitor  120  is set to a value smaller than that of the ripple voltage correction capacitor  115  or the feedback capacitor  120  is removed, or a resistance value of the phase compensation resistor  112  is set to a value smaller than that of the ripple voltage correction resistor  114  or the phase compensation resistor  112  is removed, so that the ripple-voltage Vb which is expressed by the expression (4) and generated by the ripple voltage correction resistor  114  as a main cause becomes dominant as compared with the ripple voltage Va which is expressed by the expression (3) and generated by the phase compensation resistor  112  as a main cause. Therefore, the current flowing into the inductor  109  which is increased or decreased according to a time for which the switching element is being turned on can be fed back during the switching element  104  is being turned on. Thus, it is possible to further expand a stable operational region.  
       Second Embodiment  
       [0026]      FIG. 5  is a diagram showing a chopper type boosting DC-DC converter circuit according to a second embodiment of the present invention. In the chopper type boosting DC-DC converter circuit shown in  FIG. 5 , the ripple voltage correction resistor  114  is connected between the switching element  104  and GND. The ripple voltage correction capacitor  115  is connected between the connection point, at which the switching element  104  and the ripple voltage correction resistor  114  are connected, and an input point of the error amplifying circuit  106 . The feedback capacitor  120  is connected between the input point of the error amplifying circuit  106  and the rectifying device  110 . According to the circuit structure as shown in  FIG. 5 , the output voltage can be fed back to the error amplifying circuit  106  with a state in which the ripple voltage is reduced, so stable operation is possible.  
       Third Embodiment  
       [0027]      FIG. 6  is a diagram showing a chopper type inverting DC-DC converter circuit according to a third embodiment of the present invention. In the chopper type inverting DC-DC converter circuit shown in  FIG. 6 , the input voltage from the power source  101  is inputted to the switching element  104 . The switching element  104  is connected with GND through the inductor  109  and the ripple voltage correction resistor  114 . In addition, the switching element  104  is connected with GND through the rectifying device  110 , the phase compensation resistor  112 , and the output capacitor  111 . The connection point between the rectifying device  110  and the phase compensation resistor  112  is connected with the voltage dividing circuit  108 . A second reference voltage circuit  117  is connected between the voltage dividing circuit  108  and GND. An output from the voltage dividing circuit  108  is inputted to an inverting input terminal of the error amplifying circuit  106 .  
         [0028]     A non-inverting input terminal of the error amplifying circuit  106  is connected with a first reference voltage circuit  116  through the feedback resistor  113 . The ripple voltage correction capacitor  115  is connected with the connection point between the inductor  109  and the ripple voltage correction resistor  114 . An output terminal of the error amplifying circuit  106  is connected with the PWM control circuit  105 . An output terminal of the PWM control circuit  105  is connected with the switching element  104 .  
         [0029]     According to the circuit structure as shown in  FIG. 6 , when the reference voltage inputted to the error amplifying circuit  106  is controlled, the output voltage can be fed back to the error amplifying circuit  106  with a state in which the influence of the ripple voltage is reduced, so stable operation is possible.