Abstract:
A power supply apparatus includes a variable frequency oscillating circuit, a driver, a soft start circuit, switching elements for receiving a switching signal, a resonating capacitor connected at a connection point of the switching elements via a primary coil of a transformer, a rectifying circuit provided at a secondary coil of the transformer, an amplifier for comparing an output voltage, Vb obtained at the rectifying circuit and a reference voltage, Vref, a photo-coupler for controlling an impedance of an oscillating element of a variable frequency oscillating circuit based on the comparison output, and a charge voltage control circuit for controlling an oscillation frequency when the variable oscillation circuit is initially driven. A frequency control signal when power is ON is made nonlinear relative to time. As a result, a change in oscillation frequency immediately after power is ON is made gentle, and a rapid change in current that flows in a primary coil is eliminated. Therefore, no over-current flows in the switching elements, damage to switching elements can be reduced more significantly than conventionally, and these switching elements can be reliably protected.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a switching power supply apparatus. More particularly, it relates to a switching power supply apparatus in which a rectifying current that flows in a primary side of a transformer connected to a load side is formed to provide nonlinear characteristics when a power supply is started up, thereby preventing an excessive current from flowing in a switching transistor or the like provided at the primary side of the transformer. 
     As a switching power supply apparatus, there is known an apparatus based on a current resonance system. FIG. 1 shows a conventional example of switching power apparatus of this current resonance system, the apparatus having a SEPP (Single Ended Push Pull) arrangement. 
     A switching power supply apparatus  10  shown in FIG. 1 comprises switching signal generating means  12  including a variable frequency oscillating circuit  14  and a drive circuit  16 . The oscillating circuit&#39;s oscillation signal is supplied to the drive circuit, thereby generating a pair of switching signals, for example, having a reverse phase relationship therebetween. In the case where the switching generating means  12  is composed of IC circuits, an oscillating element (capacitor  18  and resistor  20 ) that determines an oscillation frequency is externally provided at any of external terminals  12   a  and  12   b  of this IC circuit. 
     A pair of switching signals Sp, Sp bar are supplied to a pair of switching elements  22  and  24  having SEPP arrangement. A MOS type electric field effect transistor or the like may be utilized as the switching elements  22  and  24 . An resonating capacitor  28  is connected to both a ground and a connection neutral point ‘p’ between a pair of these switching elements  22  and  24  via a primary coil  26   a  of an insulation transformer  26 . 
     Respective diodes  30   a  and  30   b  rectify a secondary current that flows in a pair of secondary coils  26   b  and  26   c  of the insulation transformer  26  as full-wave rectifier. The full-wave rectified current allows a smoothing capacitor  32  to be charged. Therefore, voltage ‘vb’ obtained at both ends  34  of the smoothing capacitor  32  is supplied to a load (not shown) as an output voltage. 
     The output voltage is supplied to an amplifier  36 , as voltage comparison means, wherein the voltage is compared with a reference voltage, Vref Its comparison output is supplied to a photo-coupler  38  that configures inductance control means  37  provided in order to insulate the primary and secondary sides of the transformer  26 . The photo-coupler  38  comprises a photodiode  40  and a phototransistor  42  that functions as a variable inductance element. A current based on the comparison output flows in this phototransistor  42 . 
     The phototransistor  42  is connected to the external terminal  12   b  through a stationary resistor  44 . Therefore, when the phototransistor  42  is ON, this resistor  44  and serial impedance caused by the phototransistor  42  are connected in parallel to a resistor  20 , which is an oscillation element. 
     In this arrangement, it is known that a relationship between a resonation frequency ‘f’ and a resonation impedance Z of the resonating circuit on the primary side of the transformer  26  formed of its primary coil  26   a  and the capacitor  28  is based on upper side operation as indicated by a curve ‘Lo’ in FIG.  2 . 
     In this resonating circuit, when switching frequencies of the switching signals Sp and Sp bar supplied to a pair of switching elements  22  and  24  are increased, the resonance impedance Z increases. The resonance impedance Z is lowered as the switching impedance Z is lowered. Such change in the resonance impedance Z causes a resonance current i 1  that flows in the primary coil  26   a  to be changed. Thus, controlling this resonance current i 1  allows an output voltage Vb induced at the secondary side of the transformer  26  to be controlled. 
     When the output voltage Vb obtained at an output terminal  34  is illustratively higher than the reference voltage Vref, the phototransistor  42  has its impedance according to the comparison output. Thus, the composite resistance of the external terminal  12   b  becomes smaller than a case of a simplex of the resistor  20 , whereby an oscillation frequency ‘fsw’ increases. 
     When the oscillation frequency ‘fsw’ is increased, the resonance impedance Z determined depending on the primary coil  26   a  and the capacitor  28  increases. Thus, a current that flows in this primary coil  26   a  is limited, and its value decreases. With this decrease in current, the currents induced at the secondary coils  26   b  and  26   c  are reduced as well. As a result, a charge voltage with the capacitor  32  decreases. Namely, the output voltage Vb is controlled in the direction of the reference voltage Vref. 
     Conversely, when the output voltage Vb is lower than the reference voltage Vref, the impedance of the phototransistor  42  increases, and the composite resistance value at the external terminal  12   b  increases. Then, the variable frequency oscillating circuit  14  is controlled so that its oscillation frequency ‘fsw’ may be lowered. As a result, the switching frequency is lowered relevant to the switching elements  22  and  24 , and the primary resonance impedance Z of the transformer  26  is lowered accordingly. This causes the resonance current to increase. When the resonance current increases, the secondary current increases as well. Thus, the charge voltage Vb with the capacitor  32  rises, and a closed loop control is performed so as to be close to the reference voltage Vref. 
     In the meantime, in this switching power supply apparatus  10 , a large amount of resonance current flows from a time when a power supply is turned ON to a time when the capacitor  32  rises to a voltage in its constant state. Thus, this current may damage the switching elements  22  and  24 . 
     In order to reduce such damage, there has been conventionally provided a soft start circuit  50 , which functions as frequency control means  60 , for limiting a resonance current during startup. This soft start circuit  50  is provided in the switching signal generating means  12 . An external charging capacitor  52  is connected to an external terminal  12   c  arranged at this soft start circuit  50 , so that charging for this capacitor  52  is started in synchronism with turning ON the power. Then, a change in charge voltage Va at this time causes a charge current of the oscillating capacitor  18 , which is an oscillating element, connected to the external terminal  12   a  to be changed. 
     When a charge current with the oscillating capacitor  18  changes with an elapse of time, the oscillation frequency ‘fsw’ changes accordingly. This fact will be described with reference to FIGS. 3A to  3 E. 
     FIG. 3A shows a change in charge voltage Va when and after the power is turned ON, wherein the charge characteristics are linear as indicated by line La. The variable frequency oscillating circuit  14  is changed in the oscillation frequency ‘fsw’ by the charge voltage Va of the capacitor  52  associated with the soft start circuit  50  connected to the oscillating circuit  14 . The oscillation frequency fsw changes almost linearly as indicated by characteristic line Lb in FIG.  3 B. When a charge voltage Va is zero volt, oscillation occurs at a high frequency, and the oscillation frequency ‘fsw’ is lowered as the charge voltage Va increases. 
     On the other hand, the primary resonance impedance Z is characterized by characteristic curve Lo such that the resonance impedance Z increases as a frequency increases from the resonance frequency ‘fo’ as shown in FIG. 2. A relationship between the impedance Z and a time is illustrated as shown in FIG.  3 C. Namely, there are nonlinear characteristics that the resonance impedance Z is initially high, and then, lowers rapidly; the impedance gently changes as the charge voltage Va is close to a full charge. 
     As a result, there is provided nonlinear characteristics such that, although not so much primary current i 1  flows in this primary resonance circuit system from a time when the power is turned ON to a predetermined time, as indicated by curve Lc in FIG. 3D, the current il increases rapidly after a certain period of time has elapsed. Accordingly, there is established a charge mode in which, although the output voltage (charge voltage) Vb of the capacitor  32  connected to the output terminals  34  is initially charged gently as indicated by the curve Ld in FIG. 3E, rapid charging is then performed. Immediately before a time ‘tb’ when a soft start mode terminates, the result is gentle charging; and at the time and after the time ‘tb’ this mode transits to a voltage control mode caused by a closed loop. In this control mode, voltage control is performed such that the reference voltage Vref is obtained as indicated by the line Le in FIG.  3 E. 
     Thus, a rapid current ii flows in the primary resonance system until a time has come immediately before the soft start mode terminates because of an effect due to a change in the primary impedance Z. This rapid current il causes a pair of switching elements  22  and  24  an excessive stress, and thus, the switching elements  22  and  24  or the like 
     Although a voltage change applied to a load while the power is ON depends on charge characteristics of the soft start circuit, a voltage applying state that is the most suitable to the load can be achieved if the voltage change state matching such load can be freely set. However, conventional art as described above has been such a disadvantage that flexible response cannot be made, since the charge characteristics of the soft start circuit is merely linear. 
     Accordingly, it is an object of the present invention to provide a switching power supply apparatus in which the charge characteristics of a capacitor connected to the soft start circuit are made gentle when the power is turned ON, whereby damage to at least the switching elements can be reduced. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, the object is accomplished in switching power supply apparatus comprising a frequency control device, preferably such as a soft start circuit and a charging capacitor, for controlling an oscillation frequency when switching signal generating device, preferably such as variable frequency oscillating circuit is initially driven. A frequency control signal of the frequency control devices is formed to provide nonlinear characteristics relevant to a time. In carrying out the present invention in one preferred mode, since the charge characteristics of the soft start circuit are made nonlinear, charging for a capacitor connected to the soft start circuit is rapidly performed when the power is ON, and then, the charging is performed gradually. 
     By doing this, the primary impedance Z that originally changes nonlinearly changes almost linearly. This resonance impedance Z determines the primary current i 1 , and thus, the excessive current of this primary current i 1  is inhibited. Therefore, no excessive current flows in switching elements, and damage to these switching elements can be reduced. 
     In addition, an output voltage to be applied to a load, particularly a voltage change when power is ON depends on the charge characteristics of a soft start circuit. The charge characteristics are provided as charge characteristics suitable to the load, whereby more stable circuit operation can be achieved. 
     According to the present invention, the switching power supply apparatus involves a pair of switching elements for receiving the switching signal, a resonating capacitor connected to a connection point of a pair of these switching elements via a primary coil of a transformer, a rectifying circuit provided at a secondary side of the transformer, comparison devices, preferably such as an amplifier, for comparing an output voltage obtained at this rectifying circuit with a reference voltage, and impedance control devices, preferably such as a photo-coupler, for controlling an impedance of an oscillation element of the variable frequency oscillating circuit based on this comparison output. 
     The switching power supply apparatus according to the present invention is very preferable on applying it to a switching converter having a SEPP configuration or the like. 
     The concluding portion of this specification particularly points out and distinctly claims the subject matter of the present invention. However those skilled in the art will best understand both the organization and method of operation of the invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a connection diagram showing a conventional switching power supply apparatus; 
     FIG. 2 is a characteristic view showing a relationship between an oscillation frequency and impedance that show primary resonance impedance characteristics; 
     FIGS. 3A to  3 E are waveform charts provided for a description of operation of the switching power supply apparatus; 
     FIG. 4 is a connection diagram showing essential portions of a switching power supply apparatus embodying the present invention; 
     FIG. 5 is a connection diagram showing an example of a soft start circuit used in a switching power supply apparatus embodying the present invention; 
     FIG. 6 is a characteristic view showing charge characteristic of a charge voltage control circuit; and 
     FIG. 7 is a connection diagram showing essential portion of another example of a soft start circuit used in a switching power supply apparatus embodying the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Now, embodiments of the switching power supply apparatus according to the present invention will be described in detail with reference to the accompanying drawings. 
     According to the present invention, the charge characteristics of the soft start circuit provided as switching signal generating means are changed from linear to nonlinear, whereby a change in characteristics of the primary resonance impedance of an insulation transformer is made gentle, and thus, damage to the switching element connected to the primary side of the insulation transformer can be reduced. 
     FIG. 4 shows a switching power supply apparatus  10  embodying the present invention when a soft start circuit  50  is used. An arrangement of the soft start circuit  50  will now be described here according to FIG.  5 . 
     In the soft start circuit  50 , a current path  74  of a first current mirror circuit  72  for charging a capacitor  52  at a constant current is connected to the capacitor  52 . The first current mirror circuit  72  comprises a constant current portion  77  composed of a pair of transistors  75  and  76 , a third transistor  78  connected to a base of the transistor  75  and a diode (composed of transistor)  79  for preventing back flow. A current identical to the current that flows in the constant current portion  77  flows in the capacitor  52  via the diode  79  with the capacitor  52  being charged at a constant current. Thus, the charge voltage Va with the capacitor  52  indicates linear charge characteristics. 
     A second current mirror circuit  82  is connected to the current path  74  via a pair of transistors  80  and  81  that are Darlington-connected, which may determine the value of a constant current that flows in a second constant current portion  85 . The second current mirror circuit  82  is also arranged similar to the first constant current portion  72 . This circuit comprises a constant current portion  85  composed of a pair of transistors  83  and  84 , a third transistor  86  connected to a base of the transistor  83  and a diode (composed of transistor)  87  for preventing back flow. A current identical to the current that flows in the constant current portion  85  flows in the capacitor  18  via the diode  87 . Thus, the charge characteristics of the capacitor  18  are controlled according to those of the charge voltage Va. As a result, the oscillation frequency ‘fsw’ of the variable frequency oscillating circuit  14  is controlled as desired, and a soft start mode is achieved. 
     According to the present invention, there is also provided frequency control means for the variable frequency oscillating circuit  14 . In the illustrative embodiment, charge characteristics of the charge capacitor  52  provided at the soft start circuit  50  is controlled, thereby obtaining a control signal for controlling the oscillation frequency. 
     An oscillation frequency control circuit comprises the soft start circuit  50 , the charging capacitor  52  particularly connected to the soft start circuit  50  and a charge voltage control circuit  90  connected to the charging capacitor  52  in the illustrative embodiment of FIG.  4 . The circuit  90  has nonlinear charge voltage characteristics relevant to the capacitor  52 . 
     This circuit  90  has a pair of resistors  91  and  92  connected in series as shown in FIG. 4, and its connection neutral point ‘d’ is connected to an external terminal  12   c.  Namely, the resistor  92  is connected in parallel to the capacitor  52 . A switching transistor  94  is further connected between this connection neutral point ‘d’ and a power supply Vcc through resistor  93 . The partial pressure voltage caused by a pair of resistors  95  and  96  is applied to the transistor  94  as its base voltage. 
     According to the thus configured charge voltage control circuit  90 , the capacitor  52  is charged from a time when the power is ON, and thus, a soft start mode starts. When the power is ON, the transistor  94  is turned ON. At this time, the capacitor  52  is charged by the charge current from the current path  74  and the charge current determined depending on the values of the resistors  91 ,  92 , and  93  (indicated by straight line Pa 1  in FIG.  6 ). When a certain degree of power is charged, an emitter voltage of the transistor  94  rises, whereby the transistor  94  is cut off. Therefore, subsequently, the capacitor  52  is charged by the charge current from the current path  74  and the charge current determined depending on the values of the resistors  91  and  92  (indicated by straight line Pa 2  in FIG.  6 ). 
     As a result, a shown in FIG. 6, the charge characteristics Pa relevant to the capacitor  52  differ before and after a transition point ‘y’ at a point ‘ta’ at which the transistor  94  is cut off as shown in FIG. 6 while the transition point is defined as a reference. Namely, a straight line ‘Pa 1 ’ is obtained until the transistor  94  has been cut off, and then, the straight line ‘Pa 2 ’ with its gentler gradient than the line ‘Pa 1 ’ is obtained after the transistor has been cut off. Therefore, comparatively rapid charging is performed up to the point ‘ta’ when the transistor  94  is cut off (provided if a small amount of current is produced). In contrast, after the transistors  94  have been cut off, gentle charging is performed. Namely, there is provided nonlinear charge characteristics having one transition point. 
     A relationship between the primary current ‘i 1 ’ and the output voltage Vb when the nonlinear characteristics are obtained will be described with reference to FIGS. 3A to  3 E. 
     The curve Pa shown in FIG. 3A indicates charge characteristics relevant to the capacitor  52 . In the variable frequency oscillating circuit  14 , its oscillation frequency ‘fsw’ varies depending on the charge voltage Va of the capacitor  52  associated with the soft start circuit  50  connected to the oscillating circuit  14 . There are provided characteristics in which oscillation frequency ‘fsw’ also changes almost nonlinearly. When the charge voltage Va is zero volt, oscillation occurs at a high frequency as indicated by straight line ‘Pb’ shown in FIG.  3 B and the oscillation frequency ‘fsw’ is lowered as the charge voltage Va increases. The frequency change rate, however, differs before and after the transition point ‘y’. Since the frequency change rate after the transition point ‘y’ is smaller than that before the transition point, the oscillation frequency ‘fsw’ changes gently at a timing when a soft start mode terminates. 
     The primary resonance impedance Z of the insulation transformer  26  changes according to this change in oscillation frequency ‘fsw’, as shown in FIG.  3 C. In this resonance impedance Z, there are provided nonlinear characteristics such that the impedance change rate is originally small where the oscillation frequency (switching signal) ‘fsw’ is high as indicated by the curve ‘Lo’ shown in FIG. 2, and the impedance change rate is great where the oscillation frequency ‘fsw’ is comparatively low. However, since the change in oscillation frequency indicates nonlinear characteristics as shown in FIG. 3B, the impedance Z conversely indicates almost linear change as indicated by the curve Po. 
     As a result, the primary current ‘i 1 ’ also changes almost linearly as indicated by the curve Pc shown in FIG.  3 D. Namely, although a change rate of a current that flows is different from another, there are provided almost linear current characteristics before and after the transition point ‘y’. This prevents a current from rapidly flowing in the primary coil  26   a.    
     Due to the current characteristics, it is found that the charge voltage Va relevant to the capacitor  52  be charged linearly even before and after the transition point ‘y’ as indicated by the curve Pd in FIG.  3 E. 
     The charge characteristics relevant to the capacitor  52  is thus made nonlinear, and the frequency of the variable frequency oscillating circuit  14  is controlled so as not to cause the frequency change to be partially rapid, whereby the current that flows in the primary coil  26   a  of the insulation transformer  26  can be limited linearly. In this manner, a current that flows in a pair of switching elements  22  and  24  is made gentle, and the damage to the switching elements  22  and  24  can be significantly reduced. 
     In addition, the aforementioned output voltage Vb can be changed according to the charge characteristics of the capacitor  52 . When design is made in consideration of the position of the transition point ‘y’ or a gradient of the charge characteristics before and after the transition point ‘y’, there can be achieved a voltage change state that is the most suitable to a load to be connected to the output terminal  34  when the power is turned ON. As a result, there can be achieved characteristics on an output voltage rise suitable to the load, and more stable circuit operation can be obtained. 
     The soft start circuit  100  can be also arranged as another example shown in FIG.  7 . In this example, charge voltage control means relevant to the capacitor  52  is not arranged as an external circuit, but the means is arranged as an IC circuit directly incorporated in switch signal generating means  12 . Therefore, in this case, the charge voltage control circuit  90  shown in FIG. 1 is not required. 
     In the soft start circuit  50  shown in FIG. 7, a current path  101  relevant to a DC power source  104  is connected to the capacitor  52 . To this current path  101 , a switching transistor  102  is connected in series via a resistor  103  and a diode (composed of transistor)  105  for preventing back flow. Further, A first current mirror circuit  106  supplies a constant current to a neutral point ‘s’ of connection between the resistor  103  and the diode  105 . 
     The first current mirror  106  comprises a MOS transistor  107  as a constant current source and a MOS transistor  108  connected to a gate of the transistor  107 . A transistor  109  for determining the value of the constant current is connected to the MOS transistor  107  via a resistor  110 . At the transistor  109 , the minimum partial pressure voltage obtained at the neutral point ‘r 3 ’ of connection of a partial pressure circuit  111  made of a plurality of resistors Ra to Rd is supplied to a base of the transistor  109 . The partial pressure circuit  111  applies an intermediate partial pressure voltage obtained at the connection neutral point ‘r 2 ’ to the switching transistor  102  connected to the current path  101 . 
     A pair of transistors  120  and  121  that are Darlington-connected amplifies a current flowing in the current path  101 . The amplified current is used as a current that flows in a constant current source  125  of a second current mirror circuit  122 . Therefore, this current path is connected to the constant current portion  125  that configures the second current mirror circuit  122  via a switching transistor  123  and a resistor  124 . At the transistor  123 , the maximum partial pressure voltage obtained at the neutral point ‘r 1 ’ of connection of the partial pressure circuit  111  is supplied to a base of the transistor  123 . 
     In this embodiment, there is provided an arrangement in which the capacitor  18  that determines the aforementioned oscillation frequency is charged with the current flowing in the other transistor  126  of the second current mirror  122 . 
     With this circuit configuration, the circuit configuration from the switching transistor  102  connected to the current path  101  to the first current mirror circuit  106  functions as charge voltage control means. Therefore, when the power is ON, the capacitor  52  is charged with the current made by composing the constant current that flows in the transistor  108  and a current that flows in the transistor  102 . Due to the charging, the terminal voltage Va of the capacitor  52  rises, and a potential of the connection neutral point ‘s’ rises, then the potential becomes higher than a base potential of the transistor  102 . Thus, this transistor  102  is cut off. As the result, the capacitor  52  is charged with only a constant current from the first current mirror  106 . 
     Therefore, the voltage change rate of the charge voltage before the transistor  102  is cut off differs from that after it is cut off. Namely, the voltage change rate after the transistor  102  is cut off is smaller than that before cut off, and the charge characteristics similar to those shown in FIG. 6 are obtained. 
     Since a change in the charge current identical to this charge characteristics is also transmitted to the second current mirror circuit  122 , the charge characteristics relevant to the capacitor  18  that determines an oscillation frequency is also provided as nonlinear characteristics having a transition point ‘y’ as shown in FIG.  6 . Therefore, nonlinear characteristics similar to the cases of FIGS. 3A to  3 E can be achieved. 
     The nonlinear characteristics relevant to the soft start circuit  50  can be provided in a way other than the aforementioned method. In addition, in the aforementioned embodiments, a voltage change rate as nonlinear characteristics is expressed by a single transition point. However, the nonlinear characteristics can be achieved by a pure curve, and nonlinear characteristics having a plurality of transition points can be provided. 
     In the illustrative embodiments, although the present invention is applied to a switching power supply apparatus having an SEPP configuration, it can be applied to a push-pull type switching power supply apparatus or a half-bridge configured switching power supply apparatus.