JPH03215166A - Voltage-resonance type switching power source - Google Patents

Abstract

PURPOSE:To make an OFF time variable and to facilitate a voltage control by using a variable capacitor. CONSTITUTION:A direct-current power supply from a power source 1 is turned into an alternating current by a switching element 7. It is turned into a direct current by an output rectifyingsmoothing circuit 14 through a transformer 5 and supplied to a load 17. A control circuit 8 gives a pulse of a prescribed time width to a gate of the switching element 7 from the moment when a voltage between the two terminals of the switching element 7 and thereby the switching element 7 is turned ON. When the pulse turns OFF, the switching element 7 is biased by an inductance of a primary winding 6 and this bias is discharged by a capacitor 19 and a variable capacitor 20. A bias control circuit 22 controls an output voltage by varying the capacity of the variable capacitor 20 in accordance with an amplitude of the output voltage and by varying an OFF time of the switching element 7.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a switching power supply device such as a resonant DC-DC converter.

[Prior Art] A switching regulator connects a transformer and a switching element in series, controls the switching element to turn on and off, generates alternating current on the secondary side of the transformer, and rectifies the alternating current to obtain a direct current output. In,
A system in which a capacitor that resonates with the inductance of the transformer is connected or coupled to the transformer and the switching element is turned on when the voltage of the switching element becomes zero due to the resonance effect is known as a voltage resonant converter.

[Problems to be Solved by the Invention] By the way, the off period of a voltage resonant converter is inevitably determined based on the inductance of the transformer and the capacitance of the resonant capacitor, and is almost constant. Therefore, in order to control the output voltage, It was necessary to change the on-time width. However, if only the on-time width is changed significantly, it becomes difficult to stably obtain a desired resonance state. Also, the on/off period of the switching element changes. Further, in the case of a resonant converter, the maximum amplitude value of the voltage applied to the switching element is larger than that of a conventional rectangular wave converter.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a voltage resonant switching power supply device that can easily change the off period. [Means for Solving the Problems] The present invention for achieving the above objects comprises: a DC power supply; a transformer connected between one end and the other end of the DC power supply;
It has first and second main terminals and a control terminal, wherein the first main terminal is connected to one end of the DC power supply via the transformer, and the second main terminal is connected to the other end of the DC power supply. a switching element connected to the transformer, an output circuit coupled to the transformer, a variable capacitor connected or coupled to the transformer so as to resonate with the inductance of the transformer, and a capacitance of the variable capacitor. a circuit for applying a bias voltage or a control voltage to the variable capacitor in order to change the voltage between the first main terminal and the second main terminal of the switching element, which becomes zero or near zero during an off period; The present invention relates to a switching power supply device characterized by comprising a control circuit that detects a time point and provides an on control signal to the control terminal of the switching element. Moreover, as shown in claim 2, the on-time width of the switching element can be made constant.

Moreover, as shown in claim 3, the on/off period of the switching element can be made constant.

E-effect] The variable capacitor according to the present invention forms a resonant circuit with the inductance of the transformer. Note that, if necessary, a resonant circuit may be created with a stray capacitance such as a fixed resonant capacitor or a transformer connected separately from the variable capacitor. If you change the control voltage or bias voltage of the variable capacitor, the capacitance value of the variable capacitor will change, and the resonance frequency will also change. Therefore, it becomes possible to control the off period of the switching element. A variable capacitor such as a variable capacitor diode is used in which the capacitance increases as the voltage applied between both terminals of the variable capacitor decreases, and the voltage of the difference between the voltage of the transformer and the voltage of the variable bias voltage source is variable. If the voltage is applied across both terminals of a capacitive capacitor, the capacitance of the variable capacitor increases as the amplitude of the resonant voltage increases during the off period, and the voltage applied across both terminals of the switching element increases. The maximum value of is suppressed.

As shown in claim 2, if the on-time width of the switching element is made constant, voltage control can be performed over a wide range while satisfying the resonance condition.

As shown in claim 3, if the on/off period of the switching element is kept constant and the off period is varied, noise countermeasures etc. can be facilitated. [First Embodiment] Next, a voltage resonant switching power supply device 1 according to a first embodiment of the present invention will be described with reference to FIGS. 1 to 4.

A DC power supply 1 of this switching power supply device includes a full-wave rectifier 3 and a smoothing capacitor 4 connected to an AC power supply terminal 2. A switching element 7 consisting of an insulated gate field effect transistor is connected between one end and the other end (ground terminal) of the DC power supply 1 via the primary winding 6 of the high frequency transformer 5.
but! #Continued. Control electrode of switching element 7 (
The gate electrode is connected to the control circuit 8. The secondary winding 9 of the transformer 5 is connected to an output rectifying and smoothing circuit 14 consisting of diodes 10 and 11, a reactor 12, and a capacitor 13.
are connected to the DC output terminals 15 and 16 via. A load 17 is connected between the pair of output terminals 15 and 16. A diode 18 is connected in parallel to the switching element 7 in order to cause a current to flow in the opposite direction to the switching element 7. In this example, the switching element 7 is an N-channel insulated gate field effect transistor whose source is connected to the substrate, so it has a built-in diode. Therefore, it is not necessary to connect the diode 18 externally, but
Shown separately for ease of understanding.

A resonance capacitor 19 is connected in parallel to the primary winding 6 to form a resonant circuit with the inductance of the primary winding 6 of the transformer 5 . Furthermore, C in the LC resonant circuit
In order to change the value of , a variable capacitor 20 consisting of a variable capacitance diode is connected in parallel to the primary winding 6.
A variable bias voltage source 21 for applying a reverse bias voltage is connected here. This variable bias voltage source 21
is connected in series to the variable capacitor 20, so the voltage value obtained by subtracting the voltage of the primary winding 6 from the voltage of the variable bias voltage source 21 becomes the actual reverse bias voltage value of the variable capacitor 20. The variable bias voltage source 21 is controlled by a bias control circuit 22 and applies various bias values to the variable capacitor 20. A bias control circuit 22 is connected to the output terminal 15 and controls the bias voltage to keep the output voltage constant. In this example,
A bias control circuit 22 controls the variable bias voltage source 21 so that the bias voltage becomes low when the output voltage becomes high.

FIG. 2 shows the switch control circuit 8 of FIG. 1 in principle.

The switch control circuit 8 includes a zero point detection circuit 23 for detecting the point in time when the voltage applied between both terminals of the switching element 7 becomes zero based on resonance during the off period of the switching element 7; Timer 2 generates a pulse with a constant time width in response to the zero point detection signal obtained from
4 and a drive circuit 25 connected to the gate of the switching element 7. Next, the operation of the circuits shown in Figs. 1 and 2 will be explained in Figs. 3 and 4.
This will be explained with reference to the figure. During the ON period of the switching element 7, the power supply 1, the primary winding 6, and the switching element 7
A current flows through the closed circuit consisting of the following, a voltage is generated in the secondary winding 9, and this voltage is rectified and smoothed and applied to the load 17. If the switching element 7 is turned off at time t1 in FIG. 4, a flyback voltage based on the energy stored in the inductance of the primary winding 6 is generated. At this time, a <LC voltage resonant circuit is formed based on the inductance L of the primary winding 6, the capacitance C1 of the capacitor 19, and the capacitance C2 of the variable capacitor 20. As a result, the voltage between both terminals of switching element 7 changes. That is, since the resonant capacitor 19 is connected in parallel to the switching element 7 via the smoothing capacitor 4, the voltage of the switching element 7 also changes in response to a change in the voltage of the resonant capacitor l9. In FIG. 4(A) showing the voltage across the switching element 7 (drain-source voltage), the solid line indicates the variable bias voltage source 2.
The dotted line shows the voltage when the capacitance C2 of the variable capacitor 20 is large because the voltage of the variable bias voltage source 21 is small. During the off period, a voltage having a sinusoidal waveform or a waveform close to this is applied to the switching element 7 due to resonance operation. Zero point detection circuit 2 in Figure 2
3 detects that the voltage of the switching element 7 is zero or near zero at time t2 in FIG. 4, as shown in FIG. 2(B).
A trigger pulse shown in FIG. 4C is generated, the timer 24 is triggered, and a pulse with a constant time width T1 is generated as shown in FIG. 4(C). Then, this is applied as an on control signal to the switching element 7 via the drive circuit 25. When the on period ends at t3, the off period begins again, and a voltage ' based on resonance is applied to the switching element 7. Now, if the output voltage becomes higher than a predetermined value, the voltage of the variable bias voltage source 21 is controlled to be lower. As a result, the reverse bias voltage value applied to the variable capacitor 20 during the off period decreases, and the value of the reverse bias voltage applied to the variable capacitor 20 decreases.
The capacitance C2 increases. In addition, variable capacitor 2
The relationship between 0 bias voltage and capacitance is shown in Figure 3. As the capacitance C2 of the variable capacitor 20 increases, the resonance frequency decreases and the off period of the switching element 7 increases. In this embodiment, since the on-period T1 is constant, the duty ratio becomes small and the output voltage decreases.Since the resonant voltage is a sine wave or a waveform similar to this,
The reverse bias voltage value of the variable capacitor 20 changes during one off period. That is, in the rising and falling regions of the off period, the reverse bias voltage consisting of the value obtained by subtracting the resonance voltage from the value of the variable bias voltage source 21 is large, so the capacitance C2 is small, and in the central region, the reverse bias voltage is small. Therefore, the capacitance C2 increases. Therefore,
By providing the variable capacitor 20, the vicinity of the maximum value of the resonant voltage becomes a waveform that is more compressed than other parts.
The maximum value of the voltage applied to the switching element 7 is lower than in the conventional case.

The voltage resonant switching power supply device of this embodiment has the advantage that since the on period is kept constant, voltage control can be performed over a wide range while satisfying the resonance condition. [Second Embodiment] Next, a voltage resonant switching power supply device according to a second embodiment of the present invention will be described with reference to FIG. are given the same reference numerals and their explanations will be omitted. In this embodiment, a variable capacitor 20 is provided via a tertiary winding 30 of a transformer 5. In FIG. 5, an LC resonant circuit is formed in the same manner as in FIG. 1, and by changing the capacitance of the variable capacitor 20, the resonant frequency changes and the off period of the switching element 7 changes.

[Third Embodiment] FIGS. 6 and 7 show the switch control circuit 8 of the third embodiment.
Indicates a. The configuration of the main circuit of the switching power supply device of the third embodiment is the same as that in FIG. 1, so illustration thereof is omitted. Zero point detection circuit 2 connected to switching element 7
3 has the same configuration as that in Fig. 2, and generates the trigger pulse shown in Fig. 7(C). Flip 7 lop 31 is shown in Figure 7 (
It is set by the trigger pulse shown in C) and reset by the clock pulse of a constant period shown in FIG. The pulse shown in FIG. 7(D) is applied to the switching element 7 via the drive circuit 25, turning it on. 7th
The resonance operation during the period t1 to t2 in FIG. 4B is the same as that in FIG. 4, and a voltage approximately equal to the sine wave is applied to the switching element 7.

This embodiment has the feature that the on/off period of the switching element 7 is constant. [Modifications] The present invention is not limited to the above-described embodiments, and, for example, the following modifications are possible.

(1) As shown by the broken line in Figure 1, the resonance capacitor 1
9 can be connected in parallel to the switching element 7. Further, a resonance capacitor 19 can be connected to the secondary winding 9. In short, the resonance capacitor 19 may be connected anywhere as long as it is possible to form a resonance circuit with the inductance of the primary winding 6.

(2) Stray capacitance of the primary winding 6 etc. is connected to the resonance capacitor 1
9, and the resonance capacitor 19 can be omitted. Furthermore, when the necessary resonance capacitance can be obtained only with the variable capacitor 20, the capacitor 19 can be omitted.

(3) The variable capacitor 20 can be a variable capacitor multilayer ceramic capacitor of a type in which a control electrode 43 is provided between a pair of t-poles 41 and 42, as shown in FIG. In this case, when the DC voltage value of the control electrode 43 is changed, a pair of t[! The capacitance between 41 and 42 changes. <<4>> Diodes other than diode 18 can be connected in series to give directionality to switching element 7. [Effects of the Invention] As described above, according to the present invention, it becomes possible to easily change the off period and perform voltage control.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing a voltage resonant switching power supply device according to the first embodiment of the present invention, Fig. 2 is a block diagram showing the switch control circuit of Fig. 1, and Fig. 3 is a bias diagram of a variable capacitor. FIG. 4 is a waveform diagram showing the state of each part in FIG. 2, FIG. 5 is a circuit diagram showing the voltage resonance type switching power supply device of the second embodiment, and FIG. FIG. 7 is a waveform diagram showing the state of each part in FIG. 6, and FIG. 8 is a principle diagram showing another variable capacitor. 1...Power supply, 5...Transformer, 6...Primary winding,
7... Switching element, 8... Control circuit, 19.
... Resonance capacitor, 20... Variable capacitor, 21... Variable bias voltage source.

Claims (1)

[Scope of Claims] [1] A direct current power source, a transformer connected between one end and the other end of the direct current power source, and first and second main terminals and a control terminal, the first a switching element whose main terminal is connected to one end of the DC power supply via the transformer, and whose second main terminal is connected to the other end of the DC power supply; an output circuit coupled to the transformer; a variable capacitor connected or coupled to the transformer so as to resonate with the inductance of the transformer; a circuit that applies a bias voltage or a control voltage to the variable capacitor in order to change the capacitance of the variable capacitor; a control circuit that provides an on control signal to the control terminal of the switching element at a point in time when a voltage between the first main terminal and the second main terminal of the switching element becomes zero or near zero during an off period; A switching power supply device comprising: [2] The voltage resonance type switching power supply device according to claim 1, wherein the control circuit is a circuit that controls the switching element so as to keep an on period of the switching element constant. [3] The control circuit controls whether the switching element is turned on or off.
The voltage resonance type switching power supply device according to claim 1, characterized in that the circuit controls the switching element so as to keep an off period constant.