Abstract:
Transformer driving coils  24  and  25  are addled to a DC-DC converter composed of a DC power supply  1,  semiconductor switching devices  91  and  92,  a transformer  2,  rectifiers  81  and  82,  a filter capacitor  3,  and others, in order to perform a self-oscillating operation. On the other hand, an output-voltage detection and control circuit  6;  is used to provide control consisting of both frequency an( pulse-width modulation, thus keeping the output voltage constant without the use of any expensive IC circuits or pulse transformers.

Description:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT  
         [0001]    The present invention relates to a DC-DC converter for converting a DC voltage into a DC output via a transformer and. in particular, to a self-oscillating DC-DC converter that can keep an output voltage constant by providing control as consisting of both frequency and pulse-width modulation in response to variations in an input voltage or a load.  
           [0002]    [0002]FIG. 8 shows a conventional example of a self-commutated resonant converter. As shown in this figure, a DC power supply  1 , a capacitor  4 , a primary coil  21  of a transformer  2 , and a semiconductor switching device  91  are connected together in series a parallel circuit of a semiconductor switching device  92  and a capacitor  5  is connected between the capacitor  4  and the primary coil  21  of the transformer in parallel; diodes  81  and  82  and a filter capacitor  3  are connected to secondary coils  22  and  23  of the transformer  2 ; and a DC output is connected to the gates of the semiconductor switching devices  91  and  92  via an output-voltage detection and control circuit  6 , a frequency control circuit  14 , and a high-voltage-resistant driver IC  15 .  
           [0003]    [0003]FIG. 9 shows an example of the operation of the converter illustrated in FIG. 8. References V 92 , V 91 , V 4 , and V 21  denote voltage waveforms from the semiconductor switching device  92 , the semiconductor switching device  91 , the capacitor  4 . and the primary coil  21  of the transformer; and references i 91 , i 92 . i 81 , and i 82  denote current waveforms from the semiconductor switching device  91 , the semiconductor switching device  92 , the diode  81 , and the diode  82 .  
           [0004]    During a period {circle over ( 1 )}, when the semiconductor switching device  91  is turned on, the resonant current i 91  flows through the DC power supply  1 →the capacitor  4 →the primary coil  21  of the transformer→the semiconductor switching device  91  to charge the capacitor  4 . At this time, the difference in voltage between the DC power supply and the capacitor  4  is applied to the primary coil  21  of the transformer to charge the filter capacitor  3  via the diode  81 , while supplying power to a load.  
           [0005]    During a period {circle over ( 2 )}; when the semiconductor switching device  91  is turned off the resonant current is commuted to the output capacities of tile semiconductor switching devices  91  and  92  and the capacitor  5 , thereby gradually raising or lowering the voltages at the semiconductor switching devices  91  anti  92 . During a period {circle over ( 3 )}, once the voltage at the semiconductor switching device  91  reaches the DC power-supply voltage, the resonant current is commuted to a parasitic diode of the semiconductor switching device  92 . At this time, when the semiconductor switching device  92  is turned on, the resonant current i 92  flows through the capacitor  4 →the semiconductor switching device  92 →the primary coil  21  of the transformer to discharge the capacitor  4 . Further the difference in voltage between the DC power supply and the capacitor  4  is applied to the primary coil  21  of the transformer to charge the filter capacitor  3  via the diode  82 , while supplying power to the load.  
           [0006]    During a period {circle over ( 4 )}, when the semiconductor switching device  92  is turned off, the resonant current is commuted to the output capacities of the capacitor  5  and the semiconductor switching devices  91  and  92 , thereby gradually raising or lowering the voltages at the semiconductor switching devices  91  and  92 . During the period {circle over ( 1 )} once the voltage at the semiconductor switching device  92  reaches the DC power-supply voltage, the resonant current is commuted to a parasitic diode of the semiconductor switching device  91 . At this time. when the semiconductor switching device  91  is turned on, such an operation is repeated to supply DC output power insulated from the DC power supply. The circuit illustrated in FIG. 8 operates as illustrated in FIG. 9, regardless of its load stale (light or heavy load) or input voltage.  
           [0007]    In the conventional example illustrated in FIG. 8, in response to variations in the load, the output-voltage detection and control circuit and the frequency control circuit are used to modulate the operating frequencies of the semiconductor switching devices. in order to keep the output voltage constant. This method is not based on the current commonly used pulse-width modulation method, and requires relatively expensive high-voltage-resistant driver ICs to drive the semiconductor switching device  92 . Further, the frequency control circuit may he replaced by a pulse-width modulation circuit. and the high-voltage-resistant driver ICs may be replaced by pulse transformers, though the use of pulse transformers hinders size reduction.  
           [0008]    It is thus an object or the present invent ion to eliminate the need for high-voltage-resistant driver ICs or pulse transformers in order to reduce costs.  
         SUMMARY OF THE INVENTION  
         [0009]    To attain this object, the invention set forth in claim  1  provides A DC-DC converter for converting DC power from a DC power supply into an arbitrary DC output via a transformer, with the DC-DC converter being characterized in that:  
           [0010]    the DC power source, a first capacitor, a primary coil of the transformer, a first semiconductor switching device, and a current-limiting resistor are connected together in series; a parallel circuit of a second semiconductor switching device and a second capacitor is connected between the first capacitor and the primary coil of the transformer in parallel; first and second transformer driving coils are each connected between a gate and a source of the first or second semiconductor switching device, respectively, via a resistor; an activation circuit and a transistor are connected between the gate and source of the first semiconductor switching device; the base of the transistor is connected to a connection between the first semiconductor switching device and the current-limiting resistor via a base resistor; a diode and a filter capacitor are connected to a secondary coil of the transformer; and a DC output is connected to the base of the transistor via an output-voltage detection and control circuit.  
           [0011]    The invention set forth in claim  2  provides a DC-DC Converter for converting DC power from a DC power supply into an arbitrary DC output via a transformer, with the DC-DC converter being characterized in that:  
           [0012]    the DC power source, a first capacitor, a primary coil of the transformer, a first semiconductor switching device, and a current-limiting resistor are connected together in series; a parallel circuit of a second semiconductor switching device and a second capacitor is connected between the first, capacitor and the primary coil of the transformer in parallel: first and second transformer driving coils are each connected between a gate and a source of the first or second semiconductor switching device, respectively, via a resistor; an activation circuit and a transistor are connected between the gate and source of the first semiconductor switching device; the base of the transistor is connected to a connection between the first semiconductor switching device anti the current-limiting resistor via a base resistor; a first diode is connected to one terminal of a first secondary coil of the transformer so as to supply power when a positive voltage is applied to the primary coil of the transformer; a second diode is connected to one terminal of a second secondary coil of the transformer so as to supply power when a negative voltage is applied to the primary coil of the transformer; cathodes of the first and second diodes are connected to one terminal of a filter capacitor; the other terminals of the first and second secondary coils of the transformer are both connected to the other terminal of the filter capacitor; and a DC output is connected to the base of the transistor via an output-voltage detection and control circuit.  
           [0013]    In the invention set forth in claim  2 , magnetic coupling between the primary coil of the transformer and the first secondary coil of the transformer is closer than that between the primary coil of the transformer and the second secondary coil of the transformer (invention set forth in claim  3 ), or magnetic coupling between the primary coil of the transformer and the second secondary coil of the transformer is closer than that between the primary coil of the transformer and the first secondary coil of the transformer (invention set forth in claim  4 ). 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a circuit diagram showing a first embodiment of the present invention.  
         [0015]    [0015]FIG. 2 is a circuit diagram showing a second embodiment of the present invention.  
         [0016]    [0016]FIG. 3 is a waveform diagram useful in explaining the operations of the circuits illustrated in FIGS. 1 and 2.  
         [0017]    [0017]FIG. 4 is a stricture diagram of a transformer showing a third embodiment of the present invention.  
         [0018]    [0018]FIG. 5 is a structure diagram of a transformer showing a fourth embodiment of the present invention.  
         [0019]    [0019]FIG. 6 is a comparative explanatory representation useful in explaining the efficiency characteristics of converters.  
         [0020]    [0020]FIG. 7 is a comparative explanatory representation useful in explaining the “on time” rate characteristics of switching devices.  
         [0021]    [0021]FIG. 8 is a circuit diagram showing a conventional example of a DC-DC converter.  
         [0022]    [0022]FIG. 9 is a waveform diagram useful in explaining the operation of the converter illustrated in FIG. 8.  
         [0023]    [0023]FIG. 10 is a structure diagram showing a general example of the coils of a transformer. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0024]    [0024]FIG. 1 is a circuit diagram showing an embodiment of the present invention. In this figure, a DC power source  1 , a capacitor  4 , a primary coil  21  of a transformer, a first semiconductor switching device  91 , and a current-limiting resistor  121  are connected together in series; a parallel circuit of a semiconductor switching device  92  and a capacitor  5  is connected between the capacitor  4  and the primary coil  21  of the transformer in parallel: a transformer driving coil  24  is connected between a gate and a source of the semiconductor switching device  91  via a resistor  101 ; a transformer driving coil  25  is connected between a gate and a source of the semiconductor switching device  92  via a resistor  102 : an activation circuit  7  and a transistor  111  are connected between the gate and source of the semiconductor switching device  91 ; the base of the transistor  111  is connected to a connection between the semiconductor switching device  91  and the current-limiting resistor  121  via a base resistor  131 ; diodes  81  and  82  and a filter capacitor  3  are connected to secondary coils  22  and  23  of the transformer: and a DC output is connected to the base of the transistor  111  via an output-voltage detection and control circuit  6 . The activation circuit  7  determines the start of self-oscillation.  
         [0025]    The operation performed by the converter illustrated in FIG. 1 while the load is light will be described with reference to FIG. 3.  
         [0026]    During a period {circle over ( 1 )}, when the semiconductor switching device  91  is turned on, the resonant current i 91  flows through the DC power supply  1 →the capacitor  4 →the primary coil  21  of the transformer→the semiconductor switching device  91  to charge the capacitor  4 . At this time, the difference in voltage between the DC power supply and the capacitor  4  is applied to the primary coil  21  of the transformer to charge the filter capacitor  3  via the diode  81 , while supplying power to a load. The voltage applied to the transformer driving coils  24  and  25  is equal to the inverse of the turn ratio of the primary coil  21  of the transformer Once the voltage across the transformer driving coil  24  reaches the gate threshold voltage of the semiconductor switching device  91 , the semiconductor switching device  91  is turned off.  
         [0027]    During a period {circle over ( 2 )} when the semiconductor switching device  91  is off, the resonant current is commuted to the output capacities of the capacitor  5  and the semiconductor switching devices  91  and  92 , thereby gradually raising or lowering the voltages at the semiconductor switching devices  91  and  92 . During a period {circle over ( 3 )}, once the voltage at the semiconductor switching device  91  reaches the DC power-supply voltage, the resonant current is commuted to a parasitic diode of the semiconductor switching device  92 . At this time, when the voltage across the transformer driving coil  25  reaches the gate threshold voltage of the semiconductor switching device  92 , the semiconductor switching device  92  is turned on. The resonant current i 92  thus flows through the capacitor  4 →the semiconductor switching device  92 →the primary coil  21  of the transformer to discharge the capacitor  4 . Further, the difference in voltage between the DC power supply and the capacitor  4  is applied to the primary coil  21  of the transformer, but since the voltage generated in the primary coil  23  of the transformer is lower than the output voltage, the diode  82  is not electrically conductive. Once the voltage across the transformer driving coil  25  reaches the gate threshold voltage of the semiconductor switching device  92 , the semiconductor switching device  92  is turned off.  
         [0028]    During a period {circle over ( 4 )}, when the semiconductor switching device  92  is turned off, the resonant current is commuted to the output capacities of the capacitor  5  and the semiconductor switching devices  91  and  92 . thereby gradually raising or lowering the voltages at the semiconductor switching devices  91  and  92 . During the period {circle over ( 1 )}, once the voltage at the semiconductor switching device  92  reaches the DC power-supply voltage, the resonant current is commuted to a parasitic diode of the semiconductor switching device  91 . At this time. when the voltage across the transformer driving coil  24  reaches the gate threshold voltage of the semiconductor switching device  91 , the semiconductor switching device  91  is turned on. Such an operation is repeated to supply DC output power isolated from the DC power supply.  
         [0029]    The output-voltage detection and control circuit  6  operates to keep the output voltage constant. If the output voltage is lower than a set value, the output-voltage detection and control circuit  6  lowers its output to reduce the base current flowing through the transistor  111  (increases the length of time for which the semiconductor switching device  91  is on). On the contrary, if the output voltage is higher than the set value, the output-voltage detection and control circuit  6  raises its output to increase the base current flowing through the transistor  111  (reduces the length of time for which the semiconductor switching device  91  is on). As a result, control is provided such that the output voltage is kept constant based on the pulse-width modulation method, by which the output-voltage detection and control circuit  6  limits the length of time for which the semiconductor switching device  91  is on.  
         [0030]    The operation performed by the converter illustrated in FIG. 1 while the load is heavy is the same as that illustrated in FIG. 1, so a description thereof is omitted. The operation may be performed as illustrated in FIG. 3 irrespective of the load state, that is, regardless of whether the load is light or heavy. However, the operation depends on the capacity of the capacitor  4 . the number of turns in the primary and secondary coils of the transformer, and the like.  
         [0031]    [0031]FIG. 2 is a circuit diagram showing a second embodiment of the present invention. The second embodiment differs from the embodiment illustrated in FIG. 1 in that the secondary coil  23  of the transformer and the diode  82  are omitted. Accordingly, while the load is heavy, DC output power is supplied only through the secondary coil  22  of the transformer. As a result operating waveforms such as those illustrated in FIG. 3 are obtained regardless of the load state, so that no power is supplied to the load while the semiconductor switching device  92  is on.  
         [0032]    Next, the structure of the coils of the transformer illustrated in FIG. 1 will be discussed.  
         [0033]    [0033]FIG. 10 is a structure diagram showing a general example of a coil of a transformer. References  21  to  25  denote the same components as those in FIG. 1. Reference  26  denotes a bobbin for coils. That is, the secondary coils  22  and  23  of the transformer are at the same distance from the primary coil  21  of the transformer, though they are located at vertically different locations. Consequently, the degree of coupling between the secondary coil  22  of the transformer and the primary coil  21  of the transformer is substantially the same as that between the secondary coil  23  of the transformer and the primary coil  21  of the transformer. Further, in a general separately excited current resonant converter, the semiconductor switching devices  91  and  92  have the same “on” period (the output voltage is kept constant by means of frequency modulation) and thus operate as illustrated in FIG. 9, regardless of whether the load is heavy or light. Consequently, substantially equivalent power is alternately supplied to the load through the secondary coils  22  and  23  of the transformer.  
         [0034]    As described above, when the coils of the transformer in the circuit illustrated in FIG. 1 are configured so that the magnetic coupling between one of the two secondary coils of the transformer and the primary coil of the transformer is substantially the same as that between the other secondary coil of the transformer and the primary coil of the transformer as illustrated in FIG. 10, the conversion efficiency may decrease while the load is light, or it may not he possible to effectively use the secondary coils of the transformer. The present invention solves this problem as follows.  
         [0035]    [0035]FIG. 4 is a structure diagram showing a third embodiment of the present invention. This example is characterized in that the secondary coil  22  of the transformer is arranged closer to the primary coil  21  of the transformer than the secondary coil  23  of the transformer. Thus, the secondary coil  22  of the transformer can efficiently transmit power from the primary coil  21  of the transformer to the load, thereby improving the efficiency while the load is light.  
         [0036]    [0036]FIG. 5 is a structure diagram showing a fourth embodiment of the present invention. The secondary coil  23  of the transformer is arranged closer to the primary coil  21  of the transformer than the secondary coil  22  of the transformer. Thus, more power from the primary coil  21  of the transformer is supplied to the secondary coil  23  of the transformer, thereby improving the utilization of the secondary coil  23  of the transformer and the second diode  82 .  
         [0037]    [0037]FIG. 6 is a graph useful in explaining efficiency characteristics.  
         [0038]    FIGS.  6 ( a ),  6 ( b ), and  6 ( c ) show the characteristics of the transformers illustrated in FIGS. 10, 4, and  5 , respectively, as exhibited during operation. These figures indicate that while the load is heavy (an area with at high output current Io), all of the transformers achieve a substantially equivalent efficiency, whereas while the load is light, the transformer illustrated in FIG. 6( b ) achieves a higher efficiency.  
         [0039]    [0039]FIG. 7 is a graph useful in explaining “on time” rate characteristics.  
         [0040]    FIGS.  7 ( a ),  7 ( b ), and  7 ( c ) show the characteristics of the transformers illustrated in FIGS. 10, 4, and  5 , respectively, as exhibited during operation. These figures indicate that if the semiconductor switching device  91  has a high “on time” rate, the utilization of the secondary coil  23  of the transformer and the diode  82  is improved while the load is heavy, and that the semiconductor switching device  91  has the highest “on time” rate in the case of FIG. 7( c ), with this tendency clearer while the load is heavy.  
         [0041]    According to the present invention, in response to variations in the input voltage or the load, the pulse width and the frequency are simultaneously modulated so that the frequency varies automatically based on the self oscillating operation. This eliminates the need for relatively expensive high-voltage-resistance driver ICs and pulse transformers which hinder size reduction, as both the high-voltage-resistant driver ICs and the pulse transformers are used to drive the semiconductor switching devices.  
         [0042]    Further, as set forth in claims  3  and  4 , the closer magnetic coupling between the primary coil of the transformer and the first secondary coil of the transformer improves the efficiency while the load is light, and the closer magnetic coupling between the primary coil of the transformer and the second secondary coil of the transformer improves the utilization of the second secondary coil of the transformer and the second diode.