JP5357033B2 - DC low voltage power supply - Google Patents

Description

  The present invention relates to a DC low voltage power supply device using a piezoelectric transformer.

  2. Description of the Related Art Conventionally, a DC low voltage power supply device that generates a low voltage DC voltage is known. For example, in such a DC low-voltage power supply device, a drive circuit that converts a DC voltage into an AC voltage by a switching operation, a piezoelectric transformer that converts the AC voltage into a low AC voltage, and an AC voltage that is the output of the piezoelectric transformer. Some are constituted by a rectifier circuit for converting to a DC voltage. For such a DC power supply device drive circuit, a half-bridge drive circuit that steps down the voltage during DC-AC conversion is often used. Moreover, a current doubler type synchronous rectifier circuit is often used as a rectifier circuit having a step-down action in a rectifier circuit that converts an AC voltage output from a piezoelectric transformer into a DC voltage (see Patent Document 1).

The DC / DC converter described in Patent Document 1 uses a current doubler rectification / smoothing circuit instead of the bridge rectification circuit, and halves the forward voltage drop due to the diode. For cost reduction, synchronous rectification and a special gate waveform shaping circuit are used instead of the diode. In this way, the DC / DC converter described in Patent Document 1 reduces the loss of the rectification / smoothing circuit of the DC / DC converter using the piezoelectric transformer and increases the efficiency of the converter.
JP 11-55941 A

  As described above, by obtaining a low voltage using a current doubler type synchronous rectifier circuit or the like, the DC low voltage power supply device can efficiently generate a DC low voltage. However, simply configuring a circuit for obtaining a low voltage does not necessarily provide sufficient power supply efficiency. Piezoelectric transformers have characteristics that the step-down efficiency (conversion efficiency) of the piezoelectric transformer differs depending on the load connected to the output of the piezoelectric transformer. Therefore, the efficiency of the DC low-voltage power supply is reduced unless a piezoelectric transformer that meets the load specifications is used. The present invention has been made in view of such circumstances, and an object of the present invention is to provide a DC low-voltage power supply device with high step-down efficiency by a piezoelectric transformer.

  (1) In order to achieve the above object, a DC low-voltage power supply device according to the present invention includes a switching element and a choke coil, and a drive circuit that converts a DC input voltage into an AC voltage by the operation of the switching element; Synchronous rectification which has a piezoelectric transformer driven by inputting the converted AC voltage and outputs a stepped-down AC voltage, a switching element and a choke coil, and converts the output AC voltage into a DC voltage And the piezoelectric transformer is based on the resistance of the load, the output voltage of the piezoelectric transformer required by the synchronous rectification action of the synchronous rectifier circuit, and the output of the piezoelectric transformer derived from the power conservation law It has an output impedance within a predetermined range determined by the current.

  Thus, the DC low-voltage power supply device of the present invention includes a piezoelectric transformer having an output impedance corresponding to the output voltage and output current. Thereby, since the specification of the piezoelectric transformer conforms to the load specification of the DC low-voltage power supply device, the step-down efficiency of the piezoelectric transformer can be increased and the power supply efficiency of the DC low-voltage power supply device can be increased.

  (2) Moreover, the DC low-voltage power supply device according to the present invention includes a switching element and a choke coil, a drive circuit that converts a DC input voltage into an AC voltage by the operation of the switching element, and the converted AC voltage. Including a piezoelectric transformer that outputs a stepped-down AC voltage, a switching element and a choke coil, and converts the output AC voltage into a DC voltage. The piezoelectric transformer is an output of the piezoelectric transformer required according to the synchronous rectification action of the synchronous rectifier circuit based on the input voltage of the piezoelectric transformer converted by the drive circuit based on the input voltage and the resistance of the load. It has a maximum boost ratio within a predetermined range determined by the voltage.

  In the DC low-voltage power supply device of the present invention, the piezoelectric transformer is designed so that the maximum step-up ratio falls within a predetermined range according to the voltage input to the piezoelectric transformer and the voltage output from the piezoelectric transformer. Thereby, the efficiency of the step-down by the piezoelectric transformer can be increased, and the power efficiency of the DC low-voltage power supply device can be increased.

(3) Further, a DC low-voltage power supply device according to the present invention includes a switching element and a choke coil, a drive circuit that converts a DC input voltage into an AC voltage by the operation of the switching element, and the converted AC voltage A piezoelectric transformer that outputs a stepped-down AC voltage, a switching element and a choke coil, and a current doubler type synchronous rectifier circuit that converts the output AC voltage into a DC voltage; , Where the output voltage is V _OUT and the output current is I _OUT , the output impedance Z _OPT of the piezoelectric transformer satisfies the following equation.

  Thus, in the DC low-voltage power supply device of the present invention, the output impedance of the piezoelectric transformer is designed within the range of the above formula determined according to the output voltage and output current of the DC low-voltage power supply device. Since the specification of the piezoelectric transformer is adapted to the load specification of the DC low-voltage power supply device, the efficiency of the piezoelectric transformer is increased, and the power supply efficiency of the DC low-voltage power supply device can be increased.

(4) A DC low-voltage power supply device according to the present invention has two switching elements and one choke coil, and converts the DC input voltage into an AC voltage by the operation of the switching elements. And a piezoelectric transformer that is driven by input of the converted AC voltage and outputs a stepped-down AC voltage, a switching element, and a choke coil, and converts the output AC voltage to a DC voltage. and a synchronous rectification circuit current doubler type, the input voltage _{V iN,} the output voltage _{V OUT,} when the output current and _{I OUT,} the piezoelectric transformer (π _^2/2) of the _{(V OUT / I} OUT) The maximum step-up ratio Av when connecting the load resistance satisfies the following expression.

  The step-up ratio of the piezoelectric transformer is determined by the shape and structure of the piezoelectric transformer. Although it is not impossible to change the step-up ratio of the piezoelectric transformer depending on the drive frequency, the efficiency of the piezoelectric transformer decreases if it is too far from the resonance frequency of the piezoelectric transformer. Therefore, it is necessary to use a piezoelectric transformer having an appropriate step-up ratio according to the voltage input to the piezoelectric transformer or the voltage output from the piezoelectric transformer. In the DC low-voltage power supply apparatus using the half-bridge type drive circuit of the present invention, the piezoelectric transformer is designed so that the maximum step-up ratio falls within the above range according to the input voltage and output voltage. Thereby, the efficiency of a piezoelectric transformer can be made high and the power supply efficiency of a DC low voltage power supply device can be made high.

(5) A DC low-voltage power supply apparatus according to the present invention includes a switching circuit and a choke coil, a drive circuit that converts a DC input voltage into an AC voltage by the operation of the switching element, and the converted AC voltage. A piezoelectric transformer that outputs a stepped-down AC voltage, a switching element and a choke coil, and a current doubler type synchronous rectifier circuit that converts the output AC voltage into a DC voltage; , Wherein the capacitance of the primary side of the piezoelectric transformer is C, and the drive frequency of the piezoelectric transformer is f, the inductance capacity L of the choke coil included in the drive circuit satisfies the following equation: It is a feature.

  Thus, by adopting a choke coil having an inductance capacity matched with a piezoelectric transformer having an optimum specification for the drive circuit, the efficiency of the function of the choke coil can be increased. The choke coil is a single winding and may have a small size enough to realize a filter function for shaping the applied rectangular wave voltage into a sine wave voltage. Accordingly, it is possible to reduce the size and weight of the apparatus while maintaining a predetermined operation at the time of designing without adopting a special magnetic shielding structure in a strong magnetic field environment.

(6) A DC low-voltage power supply device according to the present invention includes a switching circuit and a choke coil, a drive circuit that converts a DC input voltage into an AC voltage by the operation of the switching element, and the converted AC voltage. A piezoelectric transformer that outputs a stepped-down AC voltage, a switching element and a choke coil, and a current doubler type rectifier circuit that converts the output AC voltage into a DC voltage; When the output voltage is V _OUT , the output current is I _OUT , and the drive frequency of the piezoelectric transformer is f, the inductance capacitance L of the choke coil included in the current doubler type synchronous rectifier circuit satisfies the following equation: It is characterized by that.

When the reactance of the choke coil used in the synchronous rectifier circuit is equal to the output impedance of the piezoelectric transformer, the synchronous rectifier circuit and the piezoelectric transformer operate with high efficiency. Depending on the output impedance of the output specifications and the piezoelectric transformer of the DC voltage source, it is desirable reactance of the choke coil is in the vicinity of ^{(π 2/2) (V} OUT / I OUT), the choke coil of the synchronous rectification circuit inductance The capacitance L is preferably in the vicinity of (π / 4f) (V _OUT / I _OUT ). Therefore, high power supply efficiency can be obtained when the inductance value of the choke coil is in the range of the above equation.

  (7) Moreover, the DC low-voltage power supply device according to the present invention is characterized in that the choke coil included in the drive circuit is an air-core coil. As described above, the DC low-voltage power supply device can shape the rectangular wave voltage applied to the sine wave voltage by using the air-core coil. The air core coil may be small enough to realize the filter function.

  According to the present invention, since the specification of the piezoelectric transformer conforms to the load specification of the DC low-voltage power supply device, the step-down efficiency by the piezoelectric transformer can be increased and the power supply efficiency of the DC low-voltage power supply device can be increased. it can.

It is a figure which shows the structure of the direct-current low voltage power supply device which concerns on this invention. It is a schematic diagram which shows the relationship between the output voltage and output current of the DC low-voltage power supply device which concerns on this invention, and the output impedance of a piezoelectric transformer. It is a schematic diagram which shows the relationship between the input voltage and output voltage of the direct-current low voltage power supply device which concern on this invention, and the input voltage and output voltage of a piezoelectric transformer. It is a graph which shows an experimental result. It is a graph which shows an experimental result. It is a graph which shows an experimental result.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 DC low voltage power supply device 110 Drive circuit 111, 112 Switching element 117 Choke coil 120 Piezoelectric transformer 130 Synchronous rectifier circuit 131, 132 Switching element 136, 137 Choke coil 140 Load 150, 155 Detection resistance 160 Error amplifier 170 Voltage control oscillation circuit C piezoelectric transformer primary side of the electrostatic capacitance _{C 2} piezoelectric transformer secondary side of the electrostatic capacitance f driving frequency _{I OPT} input current _{I OUT} output current L inductance capacitance _{R L} resistor _{V iN} input voltage _{V OUT} output voltage _{V IPT} output voltage _{Z OPT} piezoelectric transformer output impedance _{V ref} reference voltage from the input voltage _{V OPT} piezoelectric transformer to the piezoelectric transformer

  Next, embodiments of the present invention will be described with reference to the drawings. In order to facilitate understanding of the description, the same reference numerals are given to the same components in the respective drawings, and duplicate descriptions are omitted.

(Configuration of DC low-voltage power supply)
FIG. 1 is a diagram illustrating a configuration of a DC low-voltage power supply device 100. As shown in FIG. 1, the DC low-voltage power supply device 100 includes a drive circuit 110, a piezoelectric transformer 120, a synchronous rectifier circuit 130, a load 140, detection resistors 150 and 155, an error amplifier 160, and a voltage-controlled oscillation circuit 170. Yes. The DC low-voltage power supply device 100 is used as an AC adapter in combination with, for example, an input stage rectifier circuit.

  The drive circuit 110 generates an AC voltage from the DC voltage, and applies the generated AC voltage to the input unit of the piezoelectric transformer 120. The AC voltage is generated according to the oscillation signal of the voltage controlled oscillation circuit 170. The drive circuit 110 includes two switching elements 111 and 112 and one choke coil 117, and is configured as a half-bridge type drive circuit. The drive circuit 110 receives the oscillation signal from the voltage controlled oscillation circuit 170 and applies a sine wave input voltage to the input terminal of the piezoelectric transformer by alternately turning on and off the switching element 111 and the switching element 112. A DC voltage is converted into an AC voltage on a rectangular wave by such a switching operation. For example, MOSFETs are used for the switching elements 111 and 112. In the present invention, since it is important to obtain a low voltage with high efficiency, it is preferable that the switching elements 111 and 112 have a small parasitic capacitance and a high switching speed. In order to reduce power loss, the switching elements 111 and 112 preferably have low on-resistance.

  Since the piezoelectric transformer 120 uses resonance vibration, it is necessary to drive at a resonance frequency caused by its shape. The resonance frequency is approximately 100 to 200 kHz although it depends on the element shape, and of course cannot be driven by a commercial frequency of 50 Hz or 60 Hz. Therefore, a switching drive circuit that converts the input AC voltage into a DC voltage and converts the DC voltage into an AC voltage of 100 to 200 kHz that matches the resonance frequency of the piezoelectric transformer is required.

In the drive circuit 110, an alternating current voltage is generated by switching the input direct current voltage by switching elements 111 and 112 such as FETs. The choke coil 117 matches the circuit and the piezoelectric transformer 120, and shapes the waveform input to the piezoelectric transformer 120 into a sine wave. As the choke coil 117, a choke coil having a characteristic matched to the frequency of the capacitance on the primary side of the piezoelectric transformer 120 is used. The choke coil 117 can be an air-core coil. The half-bridge type drive circuit 110 has two FETs and one choke coil and has a small number of parts, which is advantageous in terms of mounting area and cost.

  Further, unlike the push-pull type and full-bridge type drive circuits, the half-bridge type has a step-down action and is a drive circuit 110 suitable for the step-down circuit. If a voltage can be stepped down by the drive circuit 110 by using the half-bridge type drive circuit, it is not necessary to step down by the piezoelectric transformer. Among the full-bridge type, push-pull type, and half-bridge type as the drive circuit 110, the half-bridge type drive circuit 110 having a step-down action is preferable. In the above embodiment, a full-bridge type drive circuit or a push-pull type drive circuit is used as the drive circuit 110, but other drive circuits may be used.

The choke coil 117 is connected between the switching elements 111 and 112 and the piezoelectric transformer 120. The rectangular wave voltage is converted into an AC voltage having a substantially sinusoidal waveform by the filter effect of the capacitance on the primary side of the choke coil 117 and the piezoelectric transformer 120 and input to the piezoelectric transformer 120. The characteristic of the half-bridge type driving circuit, when the DC voltage input to the drive circuit 110 to a _{V IN,} the effective value _{V IPT} AC voltage input to the piezoelectric transformer 120 and the (3 / _{√2π) V IN} Become.

A choke coil 117 is connected to the front stage of the piezoelectric transformer 120. The choke coil 117 is an air-core choke coil in which a winding coil is wound around a bobbin made of synthetic resin, for example, and is suitable for use in a strong magnetic field environment. The choke coil 117 is matched with the piezoelectric transformer 120 in order to realize a highly efficient resonance of the piezoelectric transformer 120. The inductance capacitance L of the choke coil 117 is a matched value based on the capacitance C on the primary side of the piezoelectric transformer 120, and satisfies the relationship of the following mathematical formula. Since the choke coil 117 is not a transformer having an auxiliary boosting function, the choke coil 117 may be a single winding and may have a small size enough to realize a filter function for shaping the applied rectangular wave voltage into a sine wave voltage. As a result, the DC low-voltage power supply apparatus 100 can reduce the size and weight of the apparatus while maintaining a predetermined operation at the time of design without adopting a special magnetic shielding structure in a strong magnetic field environment.

  As the piezoelectric transformer 120, for example, a piezoelectric transformer having a planar shape and having two piezoelectric ceramic plates polarized in the thickness direction laminated in the thickness direction can be used. Each piezoelectric ceramic plate has electrodes on the front and back surfaces, and one piezoelectric ceramic plate is an input portion, and the other piezoelectric ceramic plate is an output portion. The drive circuit 110 is connected to input electrodes provided with an input unit interposed therebetween. The piezoelectric transformer 120 steps down the alternating voltage input to the input electrode and outputs the stepped down alternating voltage from the output electrode provided across the output unit. The output impedance of the piezoelectric transformer 120 can be adjusted by the dielectric constant of the material and the electrode shape, and the step-up ratio can be adjusted by the number of stacked layers. The piezoelectric transformer 120 is effective in reducing the circuit size and reducing the size and weight of a liquid crystal panel or the like.

The output terminal of the piezoelectric transformer 120 is connected to the synchronous rectifier circuit 130. The piezoelectric transformer 120 steps down the input voltage by the vibration of the piezoelectric body and outputs it. As the piezoelectric transformer 120, for example, a general Rosen-type piezoelectric transformer can be used. The piezoelectric transformer 120 may be a single layer type or a laminated type. Piezoelectric transformer 120, as described below, the output impedance _{^{(π 2/8) (V}} OUT / I OUT) over ^{_{2π 2 (V OUT / I OUT}} ) below the maximum step-up ratio is (π ^2/3) _(V OUT _{/ V iN)} or _{^{(2π 2/3) (V}} OUT / V iN) which is preferably designed to become less. Note that the input voltage of the DC low-voltage power supply apparatus 100 is expressed as V _IN and the output voltage is expressed as V _OUT .

The synchronous rectifier circuit 130 is a current doubler type synchronous rectifier circuit, and includes two switching elements 131 and 132 and two choke coils 136 and 137. The synchronous rectification circuit 130 uses these switching elements 131 and 132 to switch the current flow and rectify. FETs can be used for the switching elements 131 and 132. When the output voltage from the piezoelectric transformer 120, that is, the effective value of the input voltage to the synchronous rectifier circuit 130 is V _OPT and the output voltage of the synchronous rectifier circuit 130 is V _OUT , the input / output relationship in the synchronous rectifier circuit 130 is theoretically. In general, it is expressed by the following equation.

  As shown in this equation, the current doubler type synchronous rectifier circuit has a step-down action, and it is not necessary to step down by the piezoelectric transformer 120 correspondingly. In this way, the AC voltage output from the piezoelectric transformer 120 is converted to a DC voltage and stepped down.

  A diode is generally used for the rectifier circuit, but in a rectifier circuit using a diode, a voltage drop due to the diode causes a loss of power. If the output voltage of the rectifier circuit is about 10 to 20 V and there is a voltage drop of the diode of about 2 V, a power loss of 10 to 20% occurs due to the voltage drop. Therefore, in the present invention, a synchronous rectification type rectifier circuit is adopted, and a current flow is rectified by switching using a switching element instead of a diode.

The load 140 is connected to the output terminal of the synchronous rectifier circuit 130 and has a resistance _RL . The detection resistors 150 and 155 are resistors for detecting the output voltage of the DC power supply. In this way, a voltage detection circuit can be easily configured by using a resistor. The detection resistors 150 and 155 transmit the obtained voltage to the error amplifier 160 as a detection signal.

The error amplifier 160 generates a differential signal corresponding to the difference between the detected voltage and the reference voltage _Vref . The error amplifier 160 is connected to the voltage controlled oscillation circuit 170 and transmits a differential signal to the voltage controlled oscillation circuit 170. The voltage controlled oscillation circuit 170 receives the differential signal and controls the frequency or duty ratio of the oscillation signal. The voltage controlled oscillation circuit 170 generates an oscillation signal. The oscillation signal is a rectangular wave having a constant frequency and a duty ratio. The voltage controlled oscillation circuit 170 outputs an oscillation signal having a frequency or duty ratio corresponding to the differential signal output from the error amplifier 160.

(Output impedance of piezoelectric transformer)
FIG. 2 is a schematic diagram showing the relationship between the output voltage and output current of the DC low-voltage power supply apparatus 100 and the output impedance of the piezoelectric transformer 120. The piezoelectric transformer 120 operates with high efficiency when the output impedance Z _OPT on the secondary side of the piezoelectric transformer 120 matches the resistance _{RL of the} load 140. _{V OPT} the effective value of the output voltage of the piezoelectric transformer 120, when the output voltage _{V OUT} of the DC low voltage power supply 100, the relationship of _{V OPT = (π / √2)} V OUT is established under the conditions of matching.

Also, assuming that the output current of the piezoelectric transformer 120 is I _OPT and the output current of the DC low-voltage power supply device 100 is I _OUT , when assuming that there is no loss in the synchronous rectifier circuit 130, the values are mutually determined according to the power conservation law. _Satisfies the relationship I _OPT = (√2 / π) I _OUT . When the output voltage of the DC low-voltage power supply device 100 is V _OUT and the output current I _OUT , the input voltage of the current doubler type synchronous rectifier circuit 130, that is, the output voltage V _OPT of the piezoelectric transformer 120 is (π / √2) V. _OUT . When the loss of the synchronous rectifier circuit is assumed to be zero, the relationship between the input current I _OPT of the synchronous rectifier circuit and the output current I _OUT of the DC low voltage power supply device is I _OPT = (√2 / π) according to the power conservation law. I _OUT . With this formula, the specification of the most efficient output impedance Z _OPT of the piezoelectric transformer 120 is obtained.

From the above relation, the equivalent load resistance looking into the synchronous rectifier circuit 130 from the output side of the piezoelectric transformer 120 _V OPT _{/ I OPT,} that is, _{^{(π 2/2) (V}} OUT / I OUT). As a result, it is found that it is preferable to design the piezoelectric transformer 120 so that the output impedance of the piezoelectric transformer 120 to _{^{(π 2/2) (V}} OUT / I OUT).

However, in practice, the output impedance Z _OPT of the highly efficient piezoelectric transformer 120 has a certain width. Considering the experimental results described later, it is preferable that the output impedance Z _OPT of the piezoelectric transformer 120 be in a range satisfying the following mathematical formula. Thereby, the DC low-voltage power supply device 100 operates with high efficiency. Note that the following formula is applicable regardless of the type of the drive circuit 110.

On the other hand, the driving frequency f, and the capacitance of the secondary side of piezoelectric transformer 120 and _{C 2,} the output impedance of the piezoelectric transformer 120 is given by _Z OPT = 1 / _{2πfC 2.} The resonance frequency of the piezoelectric transformer 120 is used as the driving frequency f. For example, when the piezoelectric transformer 120 is a laminate of disk-like piezoelectric ceramic plates, the resonance frequency of the diameter spreading vibration mode is a contour spreading vibration when the resonance frequency of the square piezoelectric ceramic plate is laminated. The mode's resonant frequency is used. In each case, in addition to the λ / 2 mode oscillating at half wavelength, there is a λ mode oscillating at one wavelength. Generally, the λ / 2 mode is used as the resonance mode. Therefore, when designing the piezoelectric transformer 120 having a planar shape and two piezoelectric ceramic plates polarized in the thickness direction laminated in the thickness direction, a plane perpendicular to the polarization axis of each piezoelectric ceramic plate. The above f can be controlled by adjusting the size of the direction. Further, the value of the electrostatic capacitance C ₂ on the secondary side of the piezoelectric transformer 120 is determined by the piezoelectric material used for the piezoelectric transformer 120, the distance between the electrodes of the piezoelectric transformer 120, and the electrode area. Therefore, it is possible to control the capacitance C ₂ in controlling these.

(Pressure ratio of piezoelectric transformer)
In the half-bridge type drive circuit 110, a switching operation is performed by an input differential signal, and thereby a DC voltage is converted into an AC voltage on a rectangular wave. Further, the rectangular wave voltage is converted into an AC voltage having a substantially sinusoidal waveform by the filter effect due to the capacitance on the primary side of the choke coil 117 of the drive circuit 110 and the piezoelectric transformer 120 and is input to the piezoelectric transformer 120. At this time, when the direct-current voltage input to the half-bridge type drive circuit 110 is _VIN , the effective value of the alternating-current voltage _VIPT input to the piezoelectric transformer is (3 / √2π) _VIN .

On the other hand, when the output voltage of the DC low-voltage power supply device 100 is V _OUT , the effective value of the output voltage V _OPT of the piezoelectric transformer 120 necessary for outputting the DC voltage of _VOUT is ( π / √2) V _OUT . Therefore, when the load resistor is connected to the piezoelectric transformer _{^{120 (π 2/2) (}} V OUT / I OUT), the maximum step-up ratio required piezoelectric transformer _{120 Av (= V OPT / V} IPT) is, ([pi ² / 3) (V _OUT / V _IN ). FIG. 3 is a schematic diagram showing the relationship between the input voltage and output voltage of the DC low-voltage power supply apparatus 100 and the input voltage and output voltage of the piezoelectric transformer 120.

In practice, however, the range of the step-up ratio of the piezoelectric transformer 120 that can maintain high efficiency has a certain range. In view of the experimental results described later, it is preferable to design the piezoelectric transformer 120 so that the maximum step-up ratio Av of the piezoelectric transformer 120 satisfies the following mathematical formula.

(Inductance capacity of choke coil)
When the reactance of the choke coils 136 and 137 used in the synchronous rectifier circuit 130 is equal to the output impedance of the piezoelectric transformer 120, the synchronous rectifier circuit 130 and the piezoelectric transformer 120 operate with high efficiency. Here, the DC low voltage power supply output voltage 100 _{V OUT,} when the output current is _{I OUT,} the output impedance of the piezoelectric transformer 120 is _{^{(π 2/2) (V}} OUT / I OUT). Reactance of the choke coil 136 and 137 is preferably _{^{(π 2/2) (V}} OUT / I OUT) around. Therefore, from this relationship, the inductance capacity of the choke coil of the preferred synchronous rectifier circuit is obtained as L = (π / 4f) (V _OUT / I _OUT ). Actually, when the efficiency was measured by changing the inductance value of the choke coil in the vicinity, it showed high power supply efficiency within the range of the following formula. Note that f is a driving frequency.

(Operation of DC low voltage power supply)
Next, the operation of the DC low-voltage power supply apparatus 100 configured as described above will be described below. First, an oscillation signal is transmitted from the voltage control oscillation circuit 170 to the drive circuit 110. The drive circuit 110 outputs a sine wave voltage to the piezoelectric transformer 120 when the switching elements 111 and 112 are turned on / off by the oscillation signal. In general, the driving frequency of the piezoelectric transformer 120 is set to the λ / 2 mode.

The piezoelectric transformer 120 is driven by this sine wave voltage. The synchronous rectification circuit 130 rectifies the AC voltage input from the piezoelectric transformer 120 and outputs it as a DC voltage. A low-voltage DC voltage is output to the load 140. The detection resistors 150 and 155 detect the output voltage (tube voltage) applied to the load 140 as a voltage. The error amplifier 160 compares the detection voltage with the reference voltage _Vref , amplifies the difference between the comparison results, and generates a differential signal corresponding to the difference.

  The voltage controlled oscillation circuit 170 outputs an oscillation signal having a frequency or duty ratio corresponding to the differential signal transmitted from the error amplifier 160. A rectangular wave signal as an oscillation signal is transmitted to the switching elements 111 and 112 of the drive circuit 110. In this way, the DC low voltage power supply apparatus 100 outputs a DC low voltage to the load 140 with high power supply efficiency.

An experiment for obtaining the output impedance of the piezoelectric transformer 120 with high efficiency was performed on the DC low-voltage power supply device 100. Experiments were performed by changing the resistance _{RL of the} load 140 from about 1 ohm to 100 ohm using a piezoelectric transformer designed to set the output voltage _VOUT of the DC low voltage power supply device 10 to 10 V, the output current I _OUT to 1 A, and the output impedance to 50 ohms. Went.

  As a result, in the range where the resistance of the load 140 is 2.5 ohms or more and 40 ohms or less, a power efficiency of 95% or more (relative value with respect to the maximum point) was obtained. FIG. 4 is a graph showing experimental results. By setting the power supply efficiency within a range where 95% or more is obtained, overheating of the piezoelectric transformer 120 can be prevented.

In other words, the efficiency is high in a range from 1/4 to 4 times the resistance 10 ohm of the load 140. _Therefore, V OUT _{/ I} load conditions _OUT to the output impedance (π _^2/8) of the piezoelectric transformer 120 to be used for DC low voltage power supply device 100 _{(V OUT / I} OUT) over 2π ² _{(V OUT} / I _OUT) that it is preferably designed such that the following was demonstrated.

For example, a piezoelectric transformer with an output impedance of 50 ohms is optimal for a DC low-voltage power supply device with an output voltage of 10 V and an output current of 1 A, and about 1/4 to 4 times the DC power supply for high efficiency. It is preferable to use a piezoelectric transformer having an output impedance of 12.5 to 200 ohms.

In addition, an experiment was performed on the DC low-voltage power supply device 100 to obtain the maximum boost ratio of the piezoelectric transformer 120 with high efficiency. In the experiment, a voltage controlled oscillation circuit 170 that varies the frequency by a control signal from the error amplifier 160 was used. As the synchronous rectifier circuit 130, a synchronous rectifier circuit having two switching elements 131 and 132 and two choke coils 136 and 137 was used. As conditions, the input voltage VIN was 48V and the output voltage VOUT was 10V. FIG. 5 is a graph showing experimental results. The piezoelectric transformer 120 having a maximum step-up ratio smaller than about 0.6 times did not operate because the step-up ratio was insufficient. Further, when the piezoelectric transformer 120 having a maximum step-up ratio greater than 1.2 times was used, the power supply efficiency was reduced by 5% or more compared to the case where the piezoelectric transformer 120 having a step-up ratio of 1.0 times was used. This is because the difference between the frequency at which the piezoelectric transformer 120 is driven and its inherent frequency is increased.

To summarize these results, the maximum step-up ratio of the piezoelectric transformer 120 exhibiting the maximum efficiency is about 1.4 to 1.5 times the minimum necessary step-up ratio. When the maximum step-up ratio of the piezoelectric transformer 120 exceeds about twice the minimum required step-up ratio, the piezoelectric transformer 120 is driven on the higher frequency side than the inherent frequency, so that the power supply efficiency is lowered. . Therefore, to achieve an operable low DC voltage power supply 100 at a high efficiency, the maximum step-up ratio _{^{(π 2/3) (V}} OUT / V IN) or _{^{(2π 2/3) (V}} OUT / V IN It has been demonstrated that it is preferable to use the following piezoelectric transformer 120.

In addition, an experiment was performed on the DC low-voltage power supply device 100 to determine the inductance capacity of the choke coils 136 and 137 used in the synchronous rectifier circuit 130. The power supply efficiency (relative value) was measured by changing the inductance of the choke coil used in the synchronous rectifier circuit 130 at the output voltage V _OUT = 10 V and the output current I _OUT = 1A from 22 uH to 270 uH. A piezoelectric transformer 120 having a resonance frequency of 130 kHz was used. FIG. 6 is a diagram showing experimental results. As shown in FIG. 6, the efficiency of 95% or more is shown at 33 uH or more and 220 uH or less, and in order to increase the efficiency of the DC power source from this, (π / 8f) (V _OUT / I _OUT ) or more (π / f) ) (V _OUT / I _OUT ) or less was found to be maintained.

When the reactance of the choke coils 136 and 137 used in the synchronous rectifier circuit 130 is equal to the output impedance of the piezoelectric transformer 120, the synchronous rectifier circuit 130 and the piezoelectric transformer 120 operate with high efficiency. Here, the output specifications _{(V OUT, I} OUT) of the DC voltage source when it is, the output impedance of the piezoelectric transformer 120 is _{^{(π 2/2) (V}} OUT / I OUT). Therefore, the reactance of the choke coil 136, 137 can be said to be desirable in the vicinity of _{^{(π 2/2) (V}} OUT / I OUT). Therefore, from this relationship, the inductance capacity of the choke coil of the synchronous rectifier circuit 130 is obtained as L = (π / 4f) (V _OUT / I _OUT ). Actually, the inductance value of the choke coil was changed in the vicinity and the efficiency was measured. As a result, a high power supply was obtained at (π / 8f) (V _OUT / I _OUT ) or more and (π / f) (V _OUT / I _OUT ) or less. Showed efficiency.

Claims (5)

A drive circuit having a switching element and a choke coil, and converting a DC input voltage into an AC voltage by the operation of the switching element;
A piezoelectric transformer that is driven by input of the converted AC voltage and outputs a reduced AC voltage;
A current doubler type synchronous rectifier circuit that has a switching element and a choke coil, and converts the output AC voltage into a DC voltage;
A DC low-voltage power supply apparatus characterized in that when the output voltage is V _OUT and the output current is I _OUT , the output impedance Z _OPT of the piezoelectric transformer satisfies the following expression.

A half-bridge type drive circuit that has two switching elements and one choke coil, and converts a DC input voltage into an AC voltage by the operation of the switching elements;
A piezoelectric transformer that is driven by input of the converted AC voltage and outputs a reduced AC voltage;
A current doubler type synchronous rectifier circuit that has a switching element and a choke coil, and converts the output AC voltage into a DC voltage;
When the input voltage is V _IN , the output voltage is V _OUT , and the output current is I _OUT ,
Piezoelectric transformer ^{(π 2/2) (V} OUT / I OUT) up to the step-up ratio Av DC low voltage power supply and satisfies the following expression when connecting the load resistor.

A drive circuit having a switching element and a choke coil, and converting a DC input voltage into an AC voltage by the operation of the switching element;
A piezoelectric transformer that is driven by input of the converted AC voltage and outputs a reduced AC voltage;
A current doubler type synchronous rectifier circuit that has a switching element and a choke coil, and converts the output AC voltage into a DC voltage;
When the capacitance on the primary side of the piezoelectric transformer is C and the drive frequency of the piezoelectric transformer is f, the inductance capacity L of the choke coil included in the drive circuit satisfies the following equation: Low voltage power supply.

A drive circuit having a switching element and a choke coil, and converting a DC input voltage into an AC voltage by the operation of the switching element;
A piezoelectric transformer that is driven by input of the converted AC voltage and outputs a reduced AC voltage;
A current doubler type rectifier circuit that has a switching element and a choke coil, and converts the output AC voltage into a DC voltage;
When an output voltage is V _OUT , an output current is I _OUT , and a driving frequency of the piezoelectric transformer is f, an inductance capacity L of a choke coil included in the current doubler type synchronous rectifier circuit satisfies the following equation: DC low voltage power supply.

The DC low-voltage power supply device according to any one of claims 1 to 4 , wherein the choke coil included in the drive circuit is an air-core coil.

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