JP2003111402A - Variable output dc-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a variable output DC-DC converter with a simple structure and high efficiency capable of promptly responding to output fluctuations and obtaining high-voltage output on a secondary side, and to provide a variable output DC-DC converter of a simple structure and high efficiency capable of promptly responding to output fluctuations and obtaining high-voltage output on a secondary side. SOLUTION: A winding with a center tap is used for primary windings 8a of a transformer 8, two switching devices 4, 5 are connected with each of the primary windings 8a of the transformer 8 so that current alternately flows, a choke coil 2 with a transformer function is inserted between two switching devices 4, 5, and the primary windings 8a of the transformer 8. Energy stored in the choke coil 2 is returned to a DC power 1 through the primary windings 8a during an off period of the switching devices 4, 5, thereby attaining high efficiency. The output voltage can be easily changed by changing the on-off rate of the switching devices 4, 5, which provides a prompt response to the secondary side output fluctuations. Choke coils and the like are not required for the secondary side, thereby increasing the voltage on the secondary side.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit, and more particularly to a switching type variable output DC / DC converter. 2. Description of the Related Art Conventional variable output power supplies include a method in which a primary voltage supplied to a DC / DC converter is varied by another method and a secondary voltage is changed, or FIG.
The on / off control of four switching elements of a DC / DC converter which performs a resonant push-pull type switching operation having a resonance choke coil on the primary side as disclosed in Japanese Patent Application Laid-Open No. 5-176535 And FIG. 4 shows an example of a method of varying the secondary voltage.
As disclosed in Japanese Patent No. 242579, a resonance push-pull type in which a choke coil is inserted on the secondary side to control two switching elements to change the voltage, or a full bridge method can be used. For example, in the method shown in FIG. 3, a choke coil 2 to which a DC power supply 1 is inputted, a commutation circuit 3, pulse-width modulated (PWM) push-pull signals a and b, and signals of opposite phases synchronized with the signals. a control circuit 12 for outputting c and d,
Switch elements 4, 5, 6, 7 operated by this control signal
And a transformer 8 for voltage transformation, rectifier circuits 9 and 10, and a smoothing circuit 11. The primary side of the transformer 8 is provided with a primary winding 8a and a 50% duty primary winding 8b. In particular, the 50% duty primary winding 8b is connected to the secondary side of the choke coil 2. Have been. The switching elements 4 and 5 are connected to the primary winding 8a, and the switching elements 6 and 7 are connected to the 50% duty primary winding 8b. In this power supply circuit, the on-time of the switch elements 4 and 5 is controlled by appropriately setting the duty of the push-pull signals a and b output from the control circuit 12 to control the secondary voltage of the transformer 8. Control. The 50% duty signals c and d are set to have a duty of 50% having opposite phases to each other, and the rises of these signals a and c and the signals b and d are the same, whereby the switching elements 4 and 6 are switched. The turn-on is synchronized, and the turn-on of the switch elements 5 and 7 is also synchronized. According to this configuration, when the switch element 4 is turned on, the energy supplied from the DC voltage 1 passes through the choke coil 2 and flows through the primary winding 8a of the transformer 8 and the switch element 4. It is transmitted to the next side. When the switch element 4 is turned off, the energy stored in the choke coil 2 due to the current that has flowed so far passes from the commutation circuit 3 to the secondary side through the switch element 6 via the 50% duty primary winding 8b. Is transmitted. At the same time when the switch element 6 is turned off, the switch elements 5 and 7 are turned on, and the same energy transmission is performed. The method shown in FIG. 4 uses resonance as a high-efficiency switching power supply. In the block diagram of FIG. 4, reference numeral 1 denotes a DC power supply for supplying energy to the switch elements 4 and 5, and the other end of the output is a primary winding of a transformer 8 for outputting an AC signal output from the switch element to the secondary side. 8a is connected to the middle point. Reference numerals 4 and 5 denote switch elements having stray capacitances that are alternately turned on and off, 16 denotes a capacitor for current resonance, 9 and 10 denote rectifier circuits that convert a signal output from the transformer 8 into direct current, 13 denotes a secondary choke coil, 11 is a smoothing capacitor for smoothing the output from the secondary choke coil 13, 20 is a load, and 14 and 15 are resonance inductors. In this power supply circuit, switching elements 4, 5
Are alternately turned on and off, the voltage applied to the switching elements 4 and 5 is about twice the voltage of the DC power supply 1, and the output of the secondary side of the transformer 8 is turned on while any of the switching elements is on. The current resonates by the series resonance of the combined inductance of the resonance inductors 14 and 15 and the leakage inductance of the transformer and the current resonance capacitor 16, and the current generated from the primary side of the transformer 8 is
Are sequentially input to the rectifier circuits 9 and 10 via the
And 10 are rectified and the resulting DC current is passed through the secondary choke coil 13 to the smoothing capacitor 1.
Charge to 1. In this case, the output voltage on the secondary side is controlled by the on / off control of the two switch elements 4 and 5. When the two switch elements 4 and 5 are off, voltage resonance is performed by parallel resonance of the self-inductance of the transformer 8, the combined inductance of the resonance inductors 14 and 15, and the stray capacitance of the switch elements 4 and 5. That is, when the switch element 4 is turned off, the voltage applied to the switch element 4 gradually rises due to the parallel resonance of the combined inductance of the self-inductance of the transformer 8 and the resonant inductor 14 and the stray capacitance of the switch element 4, and rises with a gentle slope. When the voltage reaches twice, the resonance stops. On the other hand, when the switching element 4 is turned off, the self-inductance of the transformer 8 and the combined inductance of the resonance inductor 14 and the switching element 4
The primary current is1 flows in the positive direction due to the parallel resonance with the stray capacitance, and the charge charged in the switch element 5 is discharged, the voltage value of the switch element 5 decreases, and the resonance ends when the discharge ends. Next, when the switch element 5 is turned on, the primary current is2 starts flowing in the forward direction, and the current il2 output from the secondary side of the transformer 8 passes through the rectifier circuit 10 and then flows through the secondary side choke coil 13. Via smoothing capacitor 11
And the current resonance is performed by the series resonance of the combined inductance of the resonance inductor 15 and the leakage inductance of the transformer and the current resonance capacitor 16. By repeating the above operation, the switching element 4,
5 drives the transformer 8 by performing a push-pull operation. Therefore, the secondary-side current output through the transformer 8 is rectified by the rectifier circuits 9 and 10, further charged into the smoothing capacitor 11 through the choke coil 13, and a resonance current is output from the secondary side of the transformer 8. . As described above, voltage resonance is performed by the stray capacitance of the two switch elements of the push-pull circuit, the combined inductance of the self-inductance of the transformer and the resonance inductor, and the current resonance is performed to resonate the secondary current of the transformer. This is a method in which series resonance is performed by a combined inductance of a capacitor, a resonance inductor, and a leakage inductance of a transformer to perform zero voltage switching and zero current switching. As described above, various methods have been put to practical use for a conventional variable DC power supply.
However, in the method of changing the primary voltage supplied to the DC / DC converter by another method and changing the secondary voltage, a new control circuit is used to change the primary DC voltage by another method. Is necessary, and the response when changing the output is poor. In the example of the switching method shown in FIG. 3 or the full bridge method, four switching elements are required, and four types of signals for turning on and off each switching element are required, which complicates the control. In the method using the resonance circuit shown in FIG. 3, it is necessary to insert a choke coil on the secondary side of the transformer, and when the secondary side has a high voltage output, the size is large because the withstand voltage of the choke coil needs to be increased. No. At present, use at 100 V or less is limited. The present invention solves the above-mentioned problems, and can simplify the circuit configuration, facilitate high-efficiency and easy-to-change voltage, have a fast response to secondary-side output fluctuations, and further choke the secondary-side output voltage. It is an object of the present invention to provide a variable output type DC / DC converter that can be increased without limiting the withstand voltage of the coil. The variable output type DC / DC converter provided by the present invention uses a winding having a center tap as a primary winding of a transformer in order to solve the above problems. Two switching elements are connected to each of the primary windings of the transformer, current is alternately passed, and a choke coil having a transformer function is inserted between the two switching elements and the primary winding of the transformer. The output voltage is changed by changing the on / off ratio of the switching element. As a result, the response to the output fluctuation on the secondary side is fast, the noise is reduced by the choke coil inserted on the primary side, and the magnetic energy stored in the choke coil when the switching element is off is reduced to 1
Since the power is returned to the DC power supply through the next winding, the conversion efficiency can be increased as in the resonance type. Further, since no choke coil or the like is required on the secondary side, it is possible to increase the voltage, and a variable output type DC / DC having a wide range of power supply applications.
A converter can be put to practical use. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to an embodiment shown in the drawings. Variable output type D provided by the present invention
The C / DC converter includes a DC power supply 1, a choke coil 2 having a transformer function, switching elements 4, 5, a transformer 8, a primary winding 8a having a center tap,
The secondary winding 8c includes commutation diodes 17 and 18, a diode bridge group 19 on the secondary side, a load 20, and a control circuit 12. FIG. 1 is a circuit diagram showing a main part of a variable output type DC / DC converter according to the present invention. Components indicated by the same reference numerals as in FIGS. 3 and 4 are the same components. The primary winding 8a of the transformer 8 of the present invention has a center tap, and the DC power supply 1 is connected to the center point. Both ends of the primary winding 8a are connected to the switching elements 4 and 5 through respective windings of the choke coil 2 having a transformer function. Transformer 8-2
On the secondary side, the energy on the primary side is boosted to a predetermined voltage by a booster circuit in which a voltage doubler rectifier circuit by a diode bridge group 19 is connected in multiple stages through a transformer 8 through a transformer 8. The circuit configuration of the variable output type DC / DC converter according to the present invention is as described above, and FIG. 2 shows a time chart of the operation. (A) shows the timing of the signal a for controlling the switching element 4 from the control circuit 12, and the switching element 4 is controlled by this signal. When the switching element 4 is turned on, the current is1
Flows as shown in FIG. When the switching element 4 is turned off, the energy stored in the choke coil 2 having a transformer function is returned to the DC power supply 1 through the commutation diode 18 and the winding on the side opposite to the primary winding 8a of the transformer. The current is2 at this time is shown in FIG. Next, the timing of the control signal b applied to the switching element 5 is shown in FIG. (E) shows the current is3 flowing when the switching element 5 is on, and (f) shows the current is4 flowing due to the energy stored in the choke coil 2 when the switching element 5 is off. As described in detail above, the variable output type DC / DC converter provided by the present invention is configured such that is2 and is4 are obtained by storing the magnetic energy stored in the transformer type choke coil 2 through the primary winding 8a of the transformer 8. Power supply 1
Is the current sent back to Therefore, these currents are also used to excite the transformer 8 in the opposite phase, so that the efficiency does not deteriorate. As a result, a current proportional to the ON time of the switching elements 4 and 5 appears in the secondary winding 8c of the transformer 8 by the action of the inductance of the transformer 8. The secondary side voltage is adjusted by changing the on / off ratio of the switching elements 4 and 5 between 0% and 100%. In the circuit example of FIG. Are connected in multiple stages to obtain a high voltage output. The secondary side rectification method may be any type, and the present invention covers all these modified embodiments. The variable output type DC / DC provided by the present invention
The converter is as described in detail above, is compact and simple in structure, and has a fast response to output fluctuations. Transformer 8
Since the choke coil 2 is connected to the primary side, noise is small. Although the efficiency as a DC converter is good as described above, only two switching elements are required. In addition, the signals for controlling the switching elements 4 and 5 have the same reference potential, and the control circuit 12 can use a simple circuit. Further, a variable output type DC / DC converter having various features such as various types of rectification methods for the secondary circuit of the transformer 8 and no limitation on the secondary circuit voltage can be realized. .

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a main part of a circuit of a variable output type DC / DC converter according to the present invention. FIG. 2 is a time chart of the operation of the variable output DC / DC converter according to the present invention. FIG. 3 is an example of a diagram showing a main part of a circuit of a conventional variable output DC / DC converter. FIG. 4 is an example of a diagram showing a main part of a circuit of a conventional variable output type DC / DC converter. [Description of Signs] 1 DC power supply 2 Choke coils 4 and 5 Switching element 8 Transformer 8a Primary winding 8c Secondary winding 12 Control circuits 17, 18 Commutating diode 19 Diode bridge group 20 ... Load

Claims (1)

Claims 1. A transformer having a center tap on a primary side of a transformer for transformer, a DC power supply is connected to the center tap, and a switching element is provided at each of the other ends of the winding. In a DC / DC converter of a type in which current flows alternately through connection, a choke coil having a transformer function is inserted between each switching element and a primary winding of the transformer, and the output voltage of the DC / DC converter is reduced. A variable output type DC / DC converter characterized by controlling by changing an on / off time ratio of each switching element.