JP3366103B2 - Switching power supply - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a closed circuit comprising a DC power supply, a primary winding of a transformer, and a switching element, and rectifies a secondary output of the transformer. The present invention relates to a switching power supply that controls the switching element so that its DC output voltage is stabilized, and is equivalently connected between both ends of a primary winding because of, for example, a configuration of a transformer or a boost type having a high secondary output voltage. The present invention relates to a configuration for improving the efficiency by reducing the charge / discharge current with respect to the line capacitance when the line capacitance appearing in (1) is large. 2. Description of the Related Art A conventional switching power supply forms a closed circuit with a DC power supply, a primary winding of a transformer, and a switching element, and rectifies an output of a secondary winding of the transformer to obtain a DC output. However, a spike voltage having a very high peak value is generated at the moment when the switching element is turned off. Therefore, in order to absorb this spike voltage, it is necessary to provide a circuit in which a series connection circuit of a capacitor and a diode is connected in parallel between the primary windings of the transformer, and a discharging resistor of the capacitor is connected in parallel with the capacitor. It was done. However, in this conventional configuration, there is a problem that a spike voltage is absorbed by a capacitor and consumed by a resistor, so that a loss always occurs and efficiency is reduced. The present applicant has disclosed a circuit capable of solving this problem in Japanese Patent Application Laid-Open No. 61-157264. FIG. 4 shows an example of such a circuit. As described above, the DC power supply 1, the primary winding 2a of the transformer 2, and the switching element 3 composed of a power transistor or FET constitute a closed circuit, and the spike voltage absorbing capacitor 4 Is connected in series with the diode 5, and this series connection circuit is connected to the primary winding 2
This is different from the conventional circuit in that a resistor is not used as a discharging circuit for the capacitor 4, a winding 2 c for discharging the capacitor is provided in the transformer 2, and one end of the winding 2 c is connected to the switching element 3. It is connected to a connection point with one end of the DC power supply 1, and the other end of the winding 2 c is connected via a diode 6 to a connection point between the capacitor 4 and the diode 5. A rectifying / smoothing circuit 7 is connected to the secondary winding 2b of the transformer 2. When the switching power supply is configured as a forward converter, the rectifying and smoothing circuit 7 rectifies from the secondary winding 2b when the switching element 3 is turned on. When a current flows through the smoothing circuit 7 and is configured as a flyback converter, when the switching element 3 is turned off, the secondary winding 2b
Rectifying and smoothing circuit 7 through the rectifier / smoothing circuit 7. Reference numeral 8 denotes a control circuit, which controls the on / off of the switching element 3 by the output voltage so that the output voltage of the rectifying / smoothing circuit 7 is stabilized regardless of the size of the load. In this circuit, the switching element 3
When the switching element 3 is turned off (Toff), the energy generated in the primary winding 2a when the switching element 3 is turned off is stored in the capacitor 4 as shown in FIG. The appearing spike voltage Vp is reduced. When the switching element 3 is turned on (Ton), the electric charge stored in the capacitor 4 is discharged through the switching element 3, the winding 2c, and the diode 6, and therefore, the energy stored in the capacitor 4 is transferred. 2, the energy stored in the capacitor 4 can be used as a transformer output, and high efficiency can be achieved. However, in the circuit of FIG. 4, for example, because of the configuration of the transformer 2 or a step-up transformer having a high output voltage, it is equivalent to the primary winding 2a. The appearing line capacitance Ct increases (when the turns ratio between the primary winding 2a and the secondary winding 2b is n,
Assuming that the line capacitance of the secondary winding 2b or the total line capacitance on the secondary side when a noise removing capacitor (not shown) is added between both ends of the secondary winding 2b is Co, this capacitance becomes On the primary side, the value becomes a large value converted into n × Co, so that as the turns ratio n increases, the line capacitance Ct equivalently appearing in the primary winding 2a increases, and as shown in FIG. There is a problem that the charging current to the line capacitance Ct flows through the switching element 3 at the moment when the element 3 is turned on, and the peak current Ip increases. Further, following the charging current, an oscillating current a flows due to the effect of the leakage inductance of the transformer 2 and the line capacitance Ct
Since the charging current and the oscillating current are not transmitted to the load, the efficiency is deteriorated and the shape of the transformer 2 is increased. [0007] In view of the above problems, the present invention reduces the peak current and the oscillating current that appear when the switching element is turned on even when the line capacitance equivalently appearing at both ends of the primary winding of the transformer is large. It is an object of the present invention to provide a switching power supply capable of achieving improvement in efficiency, downsizing of a transformer, and reduction in price. According to the present invention, in order to achieve the above object, DC power supplied through a primary winding of a transformer is turned on / off by a switching element, and an output of the transformer is taken out through a secondary winding. In a switching power supply, an inductor for reducing the charge / discharge current for the line capacitance of the primary winding of the transformer when the switching element is on is provided in series with the primary winding of the transformer,
A capacitor that stores the energy stored in the inductor when the switching element is on when the switching element is off, and a circuit that discharges the charge stored in the capacitor through the winding of the transformer when the switching element is on , and , the windings for discharging the charge accumulated in the capacitor is used as an auxiliary power supply for the windings
It is characterized by. According to the present invention having the above configuration, the inductor reduces the charging current for the line capacitance between both ends of the primary winding of the transformer when the switching element is turned on, and also reduces the oscillating current. Reduced. The energy stored in the inductor when the switching element is turned on is temporarily stored in the capacitor when the switching element is turned off, and is discharged through the winding of the transformer when the next switching element is turned on, and becomes the output of the switching power supply. FIG. 1 shows the premise of a switching power supply according to the present invention.
FIG. 4 is a circuit diagram showing a circuit to be used . In FIG. 1, reference numeral 9 denotes an inductor provided according to the present invention. The inductor 9 reduces the charging current to the line capacitance Ct equivalently appearing in the primary winding 2a of the transformer 2 and reduces the charging / discharging oscillation current. Are provided in series with the primary winding 2a. The inductor 9 is connected to the primary winding 2 of the transformer 2.
a, the capacitor 4 and the diode 5 are provided so as to form a closed circuit. The inductor 9 stores energy when the switching element 3 is turned on, and transfers the energy to the capacitor 4 when the switching element 3 is turned off, and stores the energy in the capacitor 4. Then, when the switching element 3 is turned on, the energy stored in the capacitor 4 is discharged through the switching element 3, the winding 2c, and the diode 6, and transmitted to the secondary side of the transformer 2. As described above, by providing the inductor 9 in the primary winding 2a of the transformer 2, the switching element 3
Is turned on, the current for charging the line capacitance of the primary winding 2a of the transformer 2 is reduced, thereby reducing the peak current Ip and the oscillating current as shown in FIG. Since the energy stored in the inductor 9 is transmitted to the secondary side, the efficiency can be improved. The capacitor 4 is a switching element 3
Needless to say, since the voltage generated in the primary winding 2a when the switching element 3 is turned off is stored in the capacitor 4, generation of a spike voltage when the switching element 3 is turned off can be prevented as shown in FIG. In order to store the energy stored in the inductor 9 in the capacitor 4, the inductance of the inductor 9 is L and the capacitance of the capacitor 4 is C,
It is preferable that I2 <CV2 (where I is the current flowing through the inductor 9 while the switching element 3 is on, and V is the voltage of the capacitor 4 when the capacitor 4 is charged). In order to effectively reduce the oscillating current, the inductance of the inductor 9 is preferably larger than the leakage inductance of the transformer 2. FIG. 3 shows an embodiment of the present invention. If the current flowing through the winding 2c for discharging the electric charge of the capacitor 4 becomes spike-like, it causes noise, so that the discharge circuit of the capacitor 4 An inductor 10 for limiting a discharge current is provided in series with the diode 6. In this way, by providing an inductor for limiting the discharge current (it may be a resistor, but it is preferable to use an inductor for reducing energy loss), the winding ratio of the winding 2c to the primary winding 2a is reduced, Even if the inductance of the winding 2c is small (the inductance is proportional to the square of the number of turns, so if the number of turns of the winding 2c is small,
Since the inductance of the winding 2c sharply decreases), it is possible to prevent the discharge current from flowing instantaneously. Therefore, the number of turns of the winding 2c can be arbitrarily set, and the degree of freedom in design increases. In the embodiment shown in FIG. 3, the voltage of the DC power supply 1 is 130 V, the output voltage of the rectifying / smoothing circuit 7, that is, the rated voltage of the switching power supply is 500 V, and the rated current is 0.1 V.
1A, number of turns of primary winding 2a of transformer 2: secondary winding 2b
, The exciting inductance of the transformer 2 is 300 μH, the leakage inductance is 3 μH,
When the capacitance of the capacitor 4 is 2 μF, the inductance of the inductors 9 and 10 is 47 μH, and the ratio of the number of turns of the primary winding 2a of the transformer 2 to the number of turns of the winding 2c is 5: 1, comparison is made without the inductors 9 and 10. However, when the inductors 9 and 10 were provided, the peak current Ip was reduced from 12 A to 3 A, and the efficiency was improved by about 2% from 76.4% to 78.3%. In the circuit of FIG. 3 , the winding 2c
Are used as windings of an auxiliary power supply used as a power supply of the control circuit 8 and the like. That is, an auxiliary power supply including the rectifying element 11, the capacitor 12, and the DC output terminal 13 is configured in the winding 2c. Here, as described above, if the degree of freedom of the number of turns of the winding 2c is increased by providing the inductor 10 or the like for limiting the discharge current of the capacitor 4, the design of the auxiliary power supply when the winding 2c is used as the auxiliary power supply is facilitated. Can be Further, since the winding 2c is also used for discharging the auxiliary power supply and the capacitor 4, the number of components can be reduced and the configuration can be simplified. Since the voltage of the auxiliary power supply is relatively low,
Actually, the embodiment of FIG. 3 is a configuration suitable for application to a flyback converter. In the case of a switching power supply configured as a forward converter, the turn ratio between the primary winding 2a and the winding 2c is approximately 1: 1 in order to limit the voltage appearing in the primary winding 2a when the switching element 3 is turned off. It is preferable to set the degree. The present invention is not limited to the above-described embodiment. As disclosed in Japanese Patent Application Laid-Open No. 61-157264, an intermediate tap is provided in the primary winding 2a, and a capacitor 4 is connected to the intermediate tap. It goes without saying that various modifications can be made without departing from the spirit of the present invention, such as a configuration in which the capacitor 4 is provided between the intermediate tap of the primary winding 2a and the intermediate tap of the winding 2c. According to the present invention , even if the line capacitance equivalently appearing at both ends of the primary winding of the transformer is large, the effect of the inductor connected in series with the primary winding of the transformer is large. The peak current due to the charging current of the line capacitance is reduced, and the oscillation current is also reduced. Further, the charging current and the oscillating current of the line capacitance that do not contribute to the transmission of energy between the primary and the secondary in the transformer are reduced, so that the efficiency can be improved. Further, by improving the efficiency, the size and cost of the transformer can be reduced. Further, since the winding used for discharging the capacitor is also used as an auxiliary power supply, the number of parts is reduced, the configuration is simplified, and the cost can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS underlying the switching power supply according to the invention; FIG times
It is a circuit diagram showing a road . FIG. 2 is a voltage and current waveform diagram of the present embodiment. FIG. 3 is a circuit diagram showing one embodiment of a switching power supply according to the present invention. FIG. 4 is a circuit diagram showing a known switching power supply. FIG. 5 is a voltage and current waveform diagram when the line capacitance of the primary winding of the transformer in the circuit of FIG. 4 is large. [Description of Signs] 1 ... DC power supply 2 ... Transformer 2a ... Primary winding 2b ...
Secondary winding 2c: Discharge winding of capacitor 4, 4, 12 ...
Capacitors 5, 6, 11 ... Diodes 7 ... Rectification smoothing circuit 8 ... Control circuit

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 3/28

Claims (1)

(57) [Claim 1] In a switching power supply for turning on / off a DC power supplied through a primary winding of a transformer by a switching element and extracting an output of the transformer through a secondary winding, when the switching element is turned on. In addition to providing an inductor in series with the primary winding of the transformer to reduce the charge / discharge current for the line capacitance of the primary winding of the transformer,
A capacitor that stores the energy stored in the inductor when the switching element is on when the switching element is off, and a circuit that discharges the charge stored in the capacitor through the winding of the transformer when the switching element is on , and Stored in the capacitor
A switching power supply, wherein a winding for discharging electric charges is used as a winding for an auxiliary power supply.