Abstract:
An object of the present invention is to lower the voltage applied to the starting resistor of the starting circuit in a switching power supply circuit to reduce the power loss especially when the receiving voltage is high and thereby provide a small and inexpensive switching power supply circuit. 
     According to the present invention, a switching power supply circuit comprises: a DC voltage section having two or more capacitors connected in series; and a PWM control circuit for receiving DC power supply from the DC voltage section and performing switching control on a primary side of a transformer in order for the switching power supply circuit to output a DC voltage of different voltage specifications; wherein the starting resistor for the PWM control circuit is connected to a connection point of the capacitors.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a device whose DC voltage section has two or more capacitors connected in series, and more particularly to a circuit configuration of a switching power supply circuit whose DC voltage section supplies a DC voltage of different voltage specifications. 
   A related prior art technique disclosed in, for example, FIG. 6 of Japanese Laid-Open Patent Publication No. 2000-60118 provides a switching power supply circuit which converts received AC power to a DC voltage and, from this converted DC voltage, produces a DC voltage of different voltage specifications. Specifically, this switching power supply circuit receives DC power from its DC voltage section and performs switching control on the primary side of the transformer by use of a PWM control circuit to output a DC voltage of different voltage specifications. The switching power supply circuit, however, is disadvantageous in that the voltage of its DC voltage section increases with increasing voltage of the received AC power (for example, such high receiving voltages as 200 V and 400 V). This means that the power loss produced in resistors  8  and  9  used in the starting circuit of the switching power supply circuit increases substantially in proportion to the square of the increase in the receiving voltage. To solve the above problems, the invention disclosed in Japanese Laid-Open Patent Publication No. 2000-60118 provides a method in which power is supplied to a power control IC used as the above PWM control circuit through a resistor only when the switching power supply circuit is started. The current flowing through the resistor is cut off after the switching power supply circuit has begun to operate. In this arrangement, however, the circuit for performing the cutoff control on the current must employ an active device which has a withstand voltage matching the voltage of the DC voltage section, resulting in a complicated circuit and increased cost. 
   Other methods have been also used to solve the above problems, as described in FIG. 6 of the above Japanese Laid-Open Patent Publication No. 2000-60118. Specifically, two or more resistors connected in series are installed in the starting circuit to divide the voltage; the dimensions of the resistor itself are increased to dissipate the heat produced in the resistor; the number of resistors connected in series is increased even when the voltage of each resistor does not exceed the maximum permissible voltage in order to spread the heat generation sources (resistors) in a wider area; or a sufficient heat dissipation space is provided around the resistor. 
   In the above switching power supply circuits, however, the voltage applied to the resistor of the starting circuit increases with increasing voltage Vi of the DC voltage section, causing the problems described in paragraphs (1) to (3) below. 
   (1) If the voltage or power to be applied to the resistor of the starting circuit exceeds its maximum permissible value, the resistor must be replaced by two or more resistors connected in series. The higher the voltage, the larger the number of the series-connected resistors, which is a factor in reducing the reliability and increasing the cost. 
   (2) Furthermore, the heat generated in the resistor due to the power loss increases with increasing voltage applied to the resistor, making it necessary to provide a sufficient heat dissipation space. This is a factor in increasing the equipment size and the cost. 
   (3) As for the related prior art technique of the invention disclosed in Japanese Laid-Open Patent Publication No. 2000-60118 which solves the above problems, it is disadvantageous in that the circuit for performing cutoff control on the current must employ an active device which has a withstand voltage matching the voltage of the DC voltage section. This means that an active device of high withstand voltage is required when the receiving voltage is high, which leads to an increase in the circuit complexity and the cost. 
   It is, therefore, an object of the present invention to provide a small and inexpensive switching power supply circuit with a simple circuit configuration which lowers the voltage applied to a resistor of its starting circuit to reduce the power loss, especially when the receiving voltage is high. 
   SUMMARY OF THE INVENTION 
   To solve the above problems, the present invention utilizes the fact that with a high receiving voltage, it is necessary to install two or more series-connected smoothing capacitors in the DC voltage section, instead of just one smoothing capacitor. Specifically, according to the present invention, a switching power supply circuit comprises: a DC power supply section having two or more capacitors connected in series; and a PWM control circuit for receiving DC power supply from the DC voltage section and performing switching control on a primary side of a transformer in order for the switching power supply circuit to output a DC voltage of different voltage specifications; wherein a DC voltage of the DC voltage section is divided, and a power supply to the PWM control circuit is connected through a resistor to a dividing point of the voltage division. This arrangement reduces the power loss produced in the resistor for the above starting circuit as well as the size and cost. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing the configuration of a switching power supply according to an embodiment of the present invention; 
       FIG. 2  is a diagram showing the power loss characteristic of a starting circuit according to the present invention; 
       FIG. 3  is a diagram showing an equivalent circuit for illustrating an embodiment of the present invention; 
       FIG. 4  is a diagram showing the circuit configuration of a switching power supply according to an embodiment of the present invention; 
       FIG. 5  is a diagram showing the circuit configuration of a switching power supply according to an embodiment of the present invention; and 
       FIG. 6  is a diagram showing a power supply circuit for a frequency converter according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Detailed description will be made below of the circuit configurations of switching power supply circuits according to preferred embodiments of the present invention with reference to the accompanying drawings.  FIG. 1  shows a switching power supply circuit according to an embodiment of the present invention. Referring to  FIG. 1 , reference numeral DR denotes a rectifying diode for rectifying AC power V AC  to produce a DC voltage. Reference numerals CB 1  and CB 2  denote capacitors for smoothing the DC voltage produced by the rectifying diode DR. Reference numerals RB 1  and RB 2  denote voltage balancing resistors for equalizing the voltages applied to the capacitors CB 1  and CB 2 . Reference numeral A denotes a connection point at which the capacitors CB 1  and CB 2  as well as the resistors RB 1  and RB 2  are connected in series. Reference numeral B indicates a portion (point) at which the DC voltage Vi smoothed by the capacitors CB 1  and CB 2  appears. Reference numeral CTRL denotes a PWM control circuit for performing PWM control on the primary side of the transformer of the switching power supply circuit. Specifically, the PWM control circuit is made up of a power control IC which outputs a PWM control signal. For example, an IC such as HA16107FP available from Hitachi, Ltd. is used. Reference numeral C indicates a terminal used for supplying necessary power to the PWM control circuit CTRL. Reference numeral M denotes a switching device. Reference numerals CS, DS, and RS denote a snubber capacitor, a snubber diode, and a snubber resistor for surge voltage absorption, respectively. Reference numeral R 1  denotes a resistor of the starting circuit used for supplying power to the PWM control circuit CTRL during and after the startup of the switching power supply circuit. Even though  FIG. 1  shows only one resistor R 1 , a plurality of resistors may be connected in series and the number of the series-connected resistors may be adjusted as necessary. Reference numerals D 1 , D 2 , C 1 , and C 2  denote diodes and smoothing capacitors, respectively. In the rectifier system with the rectifying diode DR shown in  FIG. 1 , a single-phase current is rectified by a bridge circuit. However, the present invention is not limited to a particular type of rectifier system. 
   The present embodiment of the invention connects the resistor R 1  to the point A, whereas the related prior art technique connects the resistor R 1  of the starting circuit to the point (portion) B located at a position after the smoothing capacitors CB 1  and CB 2 . According to the present embodiment, the voltage V A  at the point A is lower than the voltage Vi at the point B, making it possible to reduce the voltage applied to the resistor R 1  and thereby lower the loss produced in the resistor R 1 . Furthermore, it is possible to reduce the number of series-connected resistors and thereby reduce the heat dissipation space. As a result, the reliability can be enhanced.  FIG. 2  shows the relationship between the voltage V A  at the voltage dividing point A and the power loss produced in the resistor R 1 , assuming that the PWM control circuit CTRL requires substantially an equal current for all values of the voltage V A . The figure indicates that the power loss produced in the resistor R 1  can be reduced substantially in proportion to the decrease in the voltage at the dividing point A. Specifically, the resistor R 1  receives the DC voltage V A  at the voltage dividing point A minus the voltage V CTRL  applied to the PWM control circuit CTRL. In most cases, the voltage applied to the PWM control circuit CTRL is substantially constant and ten-odd volts or less. Furthermore, the operational current required for the PWM control circuit CTRL is also substantially constant. Therefore, when power is supplied to the PWM control circuit with the resistor R 1  connected to the voltage dividing point A, the power loss can be reduced substantially in proportion to the decrease in the voltage at the voltage dividing point A. 
   Consider, by way of example, that the voltages applied to the capacitors CB 1  and CB 2  are made equal to each other and thereby the voltage V A  at the voltage dividing point A is set equal to half of the voltage Vi at the point B. In such a case, the voltage applied to the resistor R 1  is substantially half of that for the related prior art technique. At that time, the values of the resistors RB 1 , RB 2 , and R 1  are set such that V A =Vi/2 based on Formula 1 described later. 
   In this case, if a current is drawn to the starting circuit from the voltage dividing point A at which the capacitors CB 1  and CB 2  are connected in series as shown in  FIG. 1 , the voltages of the capacitors CB 1  and CB 2  become unbalanced since the values of the balancing resistances connected in parallel to the capacitors are not equal. Therefore, the combined impedances must be set so as to balance the voltages. 
     FIG. 3  shows an equivalent circuit of the portion of the switching power supply circuit in  FIG. 1  which consists of the resistors RB 1  and RB 2 , the capacitors CB 1  and CB 2 , and the input impedance R CTRL  of the PWM control circuit CTRL. Let V A  denote the voltage at the voltage dividing point A at which the capacitors are connected in series and Vi the voltage at the point B in the DC voltage section. The current I 1  flowing through the resistor RB 1  is the sum of the current I 2  in the resistor RB 2  and the current I R  in the resistor R 1 . The voltage V A  at the voltage dividing point A is determined by the voltage drop across the resistor RB 1 , while the values of I 2  and I R  are determined by the voltage V A  at the voltage dividing point A and the values of the resistors RB 2 , R 1 , and R CTRL . The following formula (Formula 1) is derived from the above relations. 
   
     
       
         
           
             
               
                 
                   V 
                   A 
                 
                 = 
                 
                   
                     1 
                     
                       1 
                       + 
                       
                         RB 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                         ⁢ 
                         
                           
                             
                               RB 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                             
                             + 
                             
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                             + 
                             
                               R 
                               CTRL 
                             
                           
                           
                             RB 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                             ⁢ 
                             
                               ( 
                               
                                 
                                   R 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   1 
                                 
                                 + 
                                 
                                   R 
                                   CTRL 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   ⁢ 
                   Vi 
                 
               
             
             
               
                 ( 
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 ) 
               
             
           
         
       
     
   
   Based on Formula 1, the values of the resistors RB 1 , RB 2 , and R 1  are determined such that the voltage V A  at the voltage dividing point A is set to a desired value. It should be noted that the value of R CTRL  is determined based on the specifications of the PWM control circuit, separately. Furthermore, the relation V C &lt;V A &lt;Vi must hold. 
   As described above, the resistor R 1 , and the resistors RB 1  and RB 2  connected in parallel with the capacitors CB 1  and CB 2  can be set to appropriate values to stably set the voltages of the capacitors CB 1  and CB 2  to arbitrary values. Thus, the voltages applied to the capacitors CB 1  and CB 2  can be set to arbitrary values and balanced. Furthermore, the power supplied to the control circuit CTRL from the voltage dividing point A at which the capacitors CB 1  and CB 2  are connected to each other is set smaller than that for the related prior art technique. 
     FIG. 4  shows a switching power supply circuit according to another embodiment of the present invention, wherein three capacitors are connected in series. Referring to  FIG. 4 , reference numerals CB 1 , CB 2 , and CB 3  denote capacitors, while reference numerals RB 1 , RB 2 , and RB 3  denote resistors connected in parallel to the capacitors. In  FIG. 4 , the resistor R 1  is connected to the connection point at which the capacitors CB 2  and CB 3  are connected in series. For example, the voltages applied to the capacitors CB 1 , CB 2 , and CB 3  can be balanced by setting the values of the resistors RB 1 , RB 2 , RB 3 , and R 1  such that they satisfy the following formula (Formula 2). 
   
     
       
         
           
             
               
                 
                   V 
                   A 
                 
                 = 
                 
                   
                     1 
                     
                       1 
                       + 
                       
                         
                           ( 
                           
                             
                               RB 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                             + 
                             
                               RB 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           
                             
                               RB 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               3 
                             
                             + 
                             
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                             + 
                             
                               R 
                               CTRL 
                             
                           
                           
                             RB 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                             ⁢ 
                             
                               ( 
                               
                                 
                                   R 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   1 
                                 
                                 + 
                                 
                                   R 
                                   CTRL 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
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                     V 
                     i 
                   
                 
               
             
             
               
                 ( 
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
                 ) 
               
             
           
         
       
     
   
   In this case, if the value of the resistor RB 1  is set equal to that of the resistor RB 2 , the voltage V A  at the voltage dividing point A shown in  FIG. 4  is equal to one-third of the voltage Vi at the point B shown in  FIG. 4 . Even though  FIG. 4  shows an example using 3 capacitors, the present embodiment is not limited to the number of series-connected capacitors. The function-of the present embodiment can be also realized by use of 4 or more capacitors. 
     FIGS. 1 and 4  show circuit configurations of the so-called flyback converter. However, the present embodiment is not limited to a particular type of switching power supply circuit system. For example, it may be applied to a forward converter or a half-bridge or full-bridge. 
     FIG. 5  shows a switching power supply circuit according to still another embodiment of the present invention, wherein capacitors CB 1  and CB 2  are connected in series and two resistors RB 1  and RB 2  or RB 3  and RB 4  are connected to each capacitor. As shown in  FIG. 5 , a resistor R 1  is connected to the connection point at which the resistors RB 3  and RB 4  are connected in series. The arrangement shown in  FIG. 5  makes it possible to further reduce the voltage applied to the resistor R 1 . Even though  FIG. 5  shows an example in which two resistors are connected in parallel to each of the capacitors CB 1  and CB 2 , the present embodiment is not limited to the number of such resistors (for example, 3 or more resistors may be connected). Furthermore, an equal number of resistors need not be connected to each of the capacitors CB 1  and CB 2 . 
     FIG. 6  shows an example in which electrical equipment incorporates a switching power supply using a power supply system according to an embodiment of the present invention; specifically the switching power supply is used in a frequency converter. In  FIG. 6 , reference numerals D 1  to D 6  denote rectifying diodes for three-phase current which collectively constitute a rectifier (converter section) for receiving an AC current from an AC power supply V AC  and converting it to a DC voltage. Reference numerals Q 1  to Q 6  denote inverter switching devices. Power transistors such as IGBTs or bipolar transistors are used as the inverter switching devices. Reference numerals FWD 1  to FWD 6  denote inverter flywheeling diodes constituting the inverter section for converting a DC intermediate voltage (P-N DC voltage) to a three-phase AC voltage whose amplitude and frequency can be arbitrarily changed. The inverter (or frequency converter) of the present embodiment shown in  FIG. 6  has at least the above inverter section or converter section. It should be noted that the example shown in  FIG. 6  is configured such that the output (voltage) of the converter section is used as the DC intermediate voltage (P-N DC voltage). However, the present embodiment is not limited to the above configuration in which the inverter has a converter section therein. Therefore, for example, the inverter may be configured such that it has no converter section and a DC intermediate voltage (P-N DC voltage) is supplied from an external power supply. 
   A thyristor Thy and a resistor RD collectively constitute an inrush current blocking circuit to the capacitors C 1  and C 2 . In  FIG. 6 , reference numerals CB 1  and CB 2  denote smoothing capacitors in the DC intermediate voltage section of the frequency converter. In the example of  FIG. 6 , the two capacitors CB 1  and CB 2  are connected in series and voltage balancing resistors RB 1  and RB 2  are connected in parallel to the capacitors CB 1  and CB 2 . The (power supply) PS shown in  FIG. 6  is the same as that shown in  FIG. 1  and is made up of a PWM control circuit CTRL, a switching device M, a transformer T, etc. (constituting a switching power supply circuit). The power supply PS is used as a control source for supplying a control voltage to the inverter control circuit CTRL of this frequency converter. In  FIG. 6 , a resistor R 1  is connected to the connection point A at which the capacitors CB 1  and CB 2  are connected in series, as in  FIG. 1 . Furthermore, the primary winding L 1  of the transformer is connected to the point P on the positive side of the DC voltage in  FIG. 6  (for the power supply PS, the point P corresponds to the point B in  FIG. 1 ). The point N on the negative side in  FIG. 6 , on the other hand, is connected to a common potential for the PWM control circuit CTRL and the switching device M. 
   In the above embodiments, the values of the resistors (impedance values) connected in parallel to the capacitors CB 1  and CB 2  are set such that the voltages applied to the capacitors are equalized. This arrangement is effective in efficiently using the capacities of the capacitors CB 1  and CB 2  since capacitors of the same rated voltage are usually used as the capacitors CB 1  and CB 2 . Further, consider the case where with a high receiving voltage, the voltages applied to the capacitors exceed their withstand voltages or a certain margin must be provided for these voltages. In such a case, it is necessary to install two or more capacitors connected in series. The embodiments of the present invention utilize this series-connection arrangement to reduce the power loss produced in the starting resistor. Therefore, it is not necessary to prepare a special capacitor for startup, resulting in a simple configuration and low cost. 
   The present invention can lower the voltage applied to the resistor for supplying power to a switching power supply and thereby reduce the power loss, resulting in a reduced amount of generated heat. Furthermore, the present invention makes it possible to miniaturize the device and reduce the cost.