Abstract:
An object of the present invention is to create the optimum resonating condition to constantly decrease switching losses. A converter transformer capable of varying the leakage inductance is employed as a converter transformer, and a control circuit is arranged to detect an input voltage applied to a switching circuit and a voltage drop brought about in a current detecting resistor which allows a load current to flow. Thus, the leakage inductance of the converter transformer can be controlled.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
         [0001]    The present document is based on Japanese Priority Document JP 2000-391186, filed in the Japanese Patent Office on Dec. 22,  2000 , the entire contents of which being incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a resonance type switching power supply unit having a switching circuit for carrying out switching operation on an inputted direct current, a converter transformer supplied with a switched output from the switching circuit, a resonator including a coil of the converter transformer as a resonator element, a rectifier for rectifying an output of the converter transformer and supplying the resultant output to a load connected to the resonance type switching power supply unit, a switching controller for controlling the switching frequency of the switching circuit depending on the rectified output deriving from the rectifier.  
           [0004]    2. Description of Related Arts  
           [0005]    A switching power supply unit has been utilized in a situation in which a commercially available alternative current is rectified and smoothed to create a direct current, and this direct current is subjected to a switching operation at a high frequency such as  100 kHz and converted into a current having a desired voltage by a transformer at a high efficiency.  
           [0006]    A system for controlling the output voltage in the above-described switching power supply unit may be a pulse width modulation control system in which a duty ratio of the switching pulse is controlled depending on the fluctuation of the output voltage. The system for controlling the output voltage in the above-described switching power supply unit may be a frequency control system or a phase control system of a resonance type in which the frequency or the phase of the switching pulse is controlled. Other variations may be possible for the system for controlling the output voltage in the above-described switching power supply unit.  
           [0007]    [0007]FIG. 8 is a diagram showing a fundamental circuit configuration of a conventional current resonance type switching power supply unit  200 .  
           [0008]    As shown in FIG. 8, the current resonance type switching power supply unit  200  is arranged to include an AC rectifying unit  3  connected to a commercially available power supply source  1  through a noise filter  2 , a smoothing condenser  4  for smoothing the rectified output generated from the AC rectifying unit  3 , a switching circuit  5  for carrying out switching operation on the direct current which has undergone smoothing by the smoothing condenser  4 , and so on. A current resonant circuit  6  composed of a primary coil  10 A of a converter transformer  10  and a resonant condenser  6 C connected in series is connected to the AC rectifying unit  3  through the switching circuit  5 . A secondary coil  10 B of the converter transformer  10  is connected to a rectifying/smoothing circuit  20  which is composed of diodes  21 A and  21 B, condensers  22 A and  22 B, and a choke coil  23 . Further, the rectifying/smoothing circuit  20  is connected with a switching control circuit  25  for controlling the switching operation of the switching circuit through an error detecting circuit  24 . The rectifying/smoothing circuit  20  also is connected with output terminals  26 A and  26 B.  
           [0009]    According to the above arrangement of the current resonance type switching power supply unit  200 , the secondary side voltage is outputted from the rectifying/smoothing circuit  20  at the output terminals  26 A and  26 B, and the secondary side voltage is compared with a reference voltage V ref  by a voltage comparator  24 A in the error detecting circuit  24  to create an error voltage. Then, the resultant error voltage is fed back to the switching control circuit  25  through a photocoupler  24 B, whereby switching elements  5 A and  5 B provided in the switching circuit  5  are switched therebetween at a frequency which is varied depending on the error voltage. Thus, even if the input voltage or the load is fluctuated, a stable voltage can always be obtained.  
           [0010]    According to the above-described arrangement of the current resonance type switching power supply unit  200 , owing to the resonant circuit formed of the leakage inductance le of the converter transformer  10  and the capacity of the resonant condenser  6 C, energy loss can be decreased.  
           [0011]    According to the conventional current resonance type switching power supply unit  200 , the leakage inductance le of the converter transformer  10  and the capacity of the resonant condenser  6 C are fixedly determined. The switching circuit  5  creates the minimum switching loss from the switching elements  5 A and  5 B at a range near a self-resonance frequency fr, and consequently the maximum output voltage can be obtained and the loss ratio becomes the minimum due to the operation characteristic. The operation at this time can be illustrated as shown in FIG. 9. That is, when a condition that the input voltage becomes the minimum and the load current becomes the maximum is satisfied, the switching loss becomes the lowest, with the result that the conversion efficiency becomes the highest.  
           [0012]    If the input voltage is increased or the load is increased, then the switching frequency is increased so that the output voltage becomes constant. The operation at this time can be illustrated as shown in FIG. 10. That is, since the switching element  5 A is forcibly turned off to cut the current which is going to flow at the self-resonance point denoted as IQ 1 , the turning-off operation at a timing when the current value is large results in an increased switching loss.  
           [0013]    When the above power supply unit is utilized in a practical situation, however, the input voltage will vary in a range from 100V to 240V depending on the region where the unit is driven, with the result that the load current will also vary depending on the operation of an apparatus connected to the power supply unit. Further, in an ordinary case, as the input voltage is increased, and also as the load becomes smaller, the switching frequency is increased so that the power converted into one on the secondary side can be saved and the output becomes stable. Accordingly, when the conventional current resonance type switching power supply unit  200  is operated under an ordinary condition, it is not operated at a region where the loss ratio becomes low.  
         SUMMARY OF THE INVENTION  
         [0014]    The present invention is made in view of the above problem concerning the above-described conventional current resonance type switching power supply unit. That is, according to the present invention, there is provided a novel current resonance type switching power supply unit which can always convert an inputted power into one having a desired voltage at a high converting efficiency.  
           [0015]    According to the present invention, the leakage inductance of the converter transformer is varied in accordance with the fluctuation of the inputted voltage and a load imposed on the power supply unit, whereby an optimum resonating condition can always be created and the switching loss can be constantly suppressed.  
           [0016]    According to the present invention, in order to attain the above purpose, there is provided a resonance type switching power supply unit having a switching circuit for carrying out switching operation on an inputted direct current, a converter transformer supplied with a switched output from the switching circuit, a resonator including a coil of the converter transformer as a resonator element, a rectifier for rectifying an output of the converter transformer and supplying the resultant output to a load connected to the resonance type switching power supply unit, a switching controller for controlling the switching frequency of the switching circuit depending on the rectified output deriving from the rectifier, wherein the converter transformer is arranged as one capable of varying the leakage inductance thereof, and the resonance type switching power supply unit includes a detector for detecting an input voltage applied to the switching circuit and an output current supplied from the rectifier to the load and a leakage inductance controller for variably controlling the leakage inductance of the converter transformer.  
           [0017]    According to the present invention, it becomes possible to vary the leakage inductance of the converter transformer depending on fluctuation of the inputted voltage and the load imposed on the power supply unit. Therefore, an optimum resonating condition can always be created and the switching loss can be constantly suppressed.  
           [0018]    Accordingly, with the above invention, it becomes possible to provide a resonance type switching power supply unit which can always convert an inputted power into one having a desired voltage at a high converting efficiency.  
           [0019]    The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The above and other objects, features and advantages of the present invention will become more apparent from the following description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which:  
         [0021]    [0021]FIG. 1 is a circuit diagram showing an arrangement of a current resonance type switching power supply unit according to the present invention;  
         [0022]    [0022]FIGS. 2A and 2B are diagrams each schematically showing a structure of a converter transformer which is employed in the current resonance type switching power supply unit and operated in a manner allowable of varying leakage inductance;  
         [0023]    [0023]FIG. 3 is a diagram schematically showing another structure of the converter transformer which is employed in the current resonance type switching power supply unit and operated in a manner allowable of varying the leakage inductance;  
         [0024]    [0024]FIG. 4 is a diagram illustrative of a relationship between a control current flowed through a control coil and the leakage inductance of the converter transformer;  
         [0025]    [0025]FIG. 5 is a set of waveform diagrams illustrative of the current resonance type switching power supply unit;  
         [0026]    [0026]FIGS. 6A, 6B,  6 C and  6 D are diagrams each schematically showing a structure of the converter transformer which is employed in the current resonance type switching power supply unit and operated in a manner allowable of varying the leakage inductance;  
         [0027]    [0027]FIG. 7 is a circuit diagram showing an arrangement of a modification of the current resonance type switching power supply unit according to the present invention;  
         [0028]    [0028]FIG. 8 is a circuit diagram showing a fundamental circuit configuration of a conventional current resonance type switching power supply unit;  
         [0029]    [0029]FIG. 9 is a set of waveform diagrams illustrative of an ideal operation condition of the conventional current resonance type switching power supply unit; and  
         [0030]    [0030]FIG. 10 is a set of waveform diagrams illustrative of an actual operation condition of the conventional current resonance type switching power supply unit. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0031]    Embodiments of the present invention will be hereinafter described in detail with reference to attached drawings.  
         [0032]    The resonance type switching power supply unit according to the present invention is arranged as shown in FIG. 1, for example.  
         [0033]    A current resonance type switching power supply unit  100  shown in FIG. 1 is a unit in which the present invention is applied to the current resonance type switching power supply unit  200  shown in FIG. 8. That is, the converter transformer  10  in which the leakage inductance is fixedly settled is replaced with a converter transformer  50  in which a leakage inductance is variably settled. Also, the current resonance type switching power supply unit  100  is provided with a control circuit  60  for controlling the leakage inductance of the converter transformer  50 .  
         [0034]    In the current resonance type switching power supply unit  100  shown in FIG. 1, like components corresponding to those constituting the current resonance type switching power supply unit  200  shown in FIG. 8 are identified by the same reference numerals, and they will not be described in detail.  
         [0035]    As for example shown in FIGS. 2A and 2B, the converter transformer  50  allowable of varying the leakage inductance thereof is configured to have a magnetic core  51  composed of a couple of core members made of a ferrite material having four magnetic legs brought into opposition to each other so that each of the magnetic legs abuts on opposing one of the legs of the opposing core member. A primary coil  50 A and a secondary coil  50 B are wound around the magnetic core  51  so that both the coils extend over common two of the four legs. A control coil  50 C is wound around the magnetic core  51  so that the control coil  50 C forms a perpendicular posture, that is, being orthogonal, with respect to the primary coil  50 A and the secondary coil  50 B.  
         [0036]    As for example shown in FIG. 3, the converter transformer  50  may be configured to have a magnetic core  52  composed of a couple of core members made of ferrite material having three magnetic legs forming an E-letter shape brought into opposition to each other so that each of the magnetic legs abuts on opposing one of the legs of the opposing core member. The control coil  50 C may be wound around the center magnetic leg, and the primary coil  50 A and the secondary coil  50 B may be wound around the legs of both sides, respectively.  
         [0037]    In the converter transformer  50  having the above-described structure, when a control current Ic is flowed through the control coil  50 C, a part of the core utilized for winding the primary coil  50 A and the secondary coil  50 B becomes saturated, with the result that, as shown in FIG. 4, the leakage inductance L 1  can be changed together with the inductance value L formed in the coil.  
         [0038]    In the current resonance type switching power supply unit  100 , the secondary coil  50 B of the converter transformer  50  is connected at its one end to an output terminal  26 B through a current detecting resistor  70 .  
         [0039]    The control circuit  60  is arranged to include a first voltage comparator  61  for detecting the fluctuation of an input voltage applied to the switching circuit  5 , a second voltage comparator  62  for detecting the fluctuation of the rectified output voltage outputted from a rectifying/smoothing circuit  20 , a third voltage comparator  63  for detecting a voltage which is proportional to the load current flowing through the current detecting resistor  70  and dropped due to the current detecting resistor  70 , and a transistor  64  connected to the control coil  50 C coupled to the converter transformer  50 .  
         [0040]    The first voltage comparator  61  compares the input voltage applied to the switching circuit  5  with a first reference voltage Vref 1  to detect a fluctuation of the input voltage, and then supplies the detected fluctuation to a resistor adding circuit  66  through a first photocoupler as a first error voltage.  
         [0041]    The resistor adding circuit  66  adds the first error voltage to the rectified output voltage outputted from the rectifying/smoothing circuit  20 . The second voltage comparator  62  compares the rectified output voltage added with the first error voltage with a second reference voltage Vref 2  so as to create voltage fluctuation information corresponding to the fluctuation of the input voltage and the rectified output voltage as the compared output thereof. The compared output of the second voltage comparator  62  is applied to a base of the transistor  64  through a diode  67 , whereby the transistor  64  is controlled in such a manner that the control current corresponding to the fluctuation of the input voltage and the rectified output voltage is flowed through the control coil  50 C.  
         [0042]    The third voltage comparator  63  detects the dropped voltage which is proportional to the load current flowing through the current detecting resistor  70  and dropped due to the current detecting resistor  70 . The compared output of the third voltage comparator  63  is applied to the base of the transistor  64 , whereby the transistor  64  is controlled in such a manner that the control current in proportion to the load current is flowed through the control coil  50 C.  
         [0043]    The control circuit  60  detects the input voltage applied to the switching circuit  5  and the dropped voltage which is proportional to the load current flowing through the current detecting resistor  70  and dropped due to the current detecting resistor  70 . In accordance with the result of detection, the control circuit  60  controls the converter transformer  50  in the leakage inductance L 1  in the following manner.  
         [0044]    That is, when the input voltage stays in a low level and the load current is relatively large, the control circuit  60  prohibits the control current Ic from being flowed through the control coil  50 C so that the output power can be obtained at the maximum level and the loss ratio stays in the minimum level.  
         [0045]    If the input voltage is increased and/or the load current is decreased, as shown in FIG. 5, the switching control circuit  25  operates so that a switching frequency fsw is increased to decrease the output voltage. At this time, the control current Ic is flowed through the control coil  50 C depending on the degree of increase in the input voltage and/or decrease in the load current. Thus, the leakage inductance L 1  of the converter transformer  50  is decreased. Consequently, the control circuit  60  controls the leakage inductance Ll of the converter transformer  50  so that a resonance frequency fr of the current resonant circuit  6  comes to a vicinity of the switching frequency fsw.  
         [0046]    As described above, according to the arrangement of the current resonance type switching power supply unit  100 , the control current Ic flowed through the control coil  50 C is controlled depending on the degree of increase in the input voltage and/or decrease in the load current. Therefore, it becomes possible to suppress the switching loss deriving from the switching element  5 A which forcibly turns off to cut the flow of the current IQ 1  due to the self-resonance, and consequently a high converting efficiency can be maintained.  
         [0047]    While in the above-described embodiment the control circuit  60  is supplied with information indicative of the input voltage applied to the switching circuit  5  and the dropped voltage brought about on the current detecting resistor  70 , the oscillating frequency of the switching control circuit  25  may be utilized as information to be supplied to the control circuit  60  to obtain the similar effect.  
         [0048]    Although in the above-described embodiment, as the converter transformer  50  capable of varying the leakage inductance L 1 , there are shown one having an iron core formed to have four magnetic legs or an E-letter shape with three magnetic legs. However, the converter transformer may be one having a structure in which the control coil is employed for variably changing the inductance of the transformer. Alternatively, the transformer may be one having an arrangement in which the resistor of the magnetic circuit of the transformer is varied by a control signal (e.g., by changing the size of the magnetic gap) so that the leakage inductance is correspondingly varied.  
         [0049]    [0049]FIG. 6A is a circuit diagram of a fundamental structure of a converter transformer T 1 . FIGS. 6B, 6C and  6 D are diagrams each showing a variation of the structure of the converter transformer shown in FIG. 6A. As shown in FIG. 6B, 6C and  6 D, each of transformers T 2 , T 3 , and T 4  may be coupled to coils L 2 , L 3 , and L 4  and L 5 , respectively. Specifically, a coil corresponding to the leakage inductance and the first coil are connected in series on a primary side as shown in FIG. 6B. Alternatively, the coil and the first coil are connected in parallel on the primary side as shown in FIG. 6C. Furthermore, the coil is composed of two portions on the primary side, a first portion being connected in parallel with the first coil and a second portion being connected in series with the parallel connection of the first portion and the first coil as shown in FIG. 6D. In the above arrangements, if the inductance (corresponding to the leakage inductance) of the coils L 2 , L 3 , and L 4  and L 5  coupled to the transformers T 2 , T 3 , and T 4  are made variable by a control signal, then the converter transformer  50  may be utilized as one capable of varying the leakage inductance. Meanwhile, although in the above example the respective coils L 2 , L 3 , and L 4  and L 5  are provided on the primary side, these coils may be provided on a secondary side.  
         [0050]    Furthermore, as shown in FIG. 7, the converter transformer may be arranged to include a coil  10 D provided independently of the primary coil and the secondary coil of the converter transformer T 1 , and the current resonant circuit  6  may be made up of the coil  10 D and the resonant condenser  6 C. In this arrangement, if the inductance of the coil  10 D of the current resonant circuit  6  is controlled by the control circuit  60  so that the resonance frequency fr thereof is made coincident with the switching frequency fsw, a high converting efficiency can be also maintained.