JP2000270557A - Power conversion circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To drive a piezoelectric transducer with high efficiency and stability near a desired resonance frequency in a power inverter circuit, using the piezoelectric transducer by providing an output detecting circuit and a phase waveform converting circuit and feeds back to a transducer drive circuit which supplies AC power to the piezoelectric transducer. SOLUTION: DC input is converted into an AC on a transducer drive circuit 4 to drive a piezoelectric transducer 1, and an AC power transformed by the piezoelectric transducer 1 is outputted to a load 2. The current flowing through the load 2 is converted 5 into an AC voltage V3 by a load current detector, and its phase is displaced by a phase device 6 to be converted into respective rectangular waves V1, V2, whose phases are different from each other by a waveform converter 7, thereby forming a self-excited circuit which drives the respective switch elements SW1, 2 of a drive circuit 4. A circuit constant of the phase device 6 is set, so that the displacement of the phase generated by the piezoelectric transducer 1 is compensated, and the piezoelectric transducer 1 is operated with a desired frequency at all times.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a power conversion circuit using a piezoelectric transformer.

[0002]

2. Description of the Related Art Liquid crystal display panels are widely used as display devices for portable electronic devices such as notebook personal computers. A discharge tube such as a cold cathode tube is used as a backlight light source for displaying a liquid crystal display panel. In many cases, a notebook computer uses a rechargeable battery and an AC adapter together. In this case, the power supply voltage is a direct current of about 20 V at the maximum, and a high voltage of several hundred kilovolts and a high frequency of several tens of kilohertz is required to light the discharge tube. Therefore, in order to convert a DC voltage into a high-frequency high voltage, a power conversion circuit including a high-frequency transformer, that is, a so-called inverter is used.

[0003] Recently, notebook computers have been reduced in size and weight. In particular, the thinning of liquid crystal display panels is remarkable, and inverters are also required to be small and thin, and as high-frequency transformers that generate high voltage at high frequencies,
Attention has been paid to a method using a piezoelectric transformer using a piezoelectric effect instead of an electromagnetic transformer. The piezoelectric transformer has a structure in which input and output electrodes are printed on a rectangular piezoelectric ceramic plate, and is an element that performs power conversion using resonance generated by vibration due to a piezoelectric effect. In order to increase power conversion, it is necessary to operate the piezoelectric ceramic plate at a frequency at which the length of the piezoelectric ceramic plate is an integral multiple of half the vibration wavelength, that is, near the resonance frequency.

The transformation ratio of a piezoelectric transformer has a frequency characteristic as shown in FIG. That is, the resonance frequency that gives the maximum value of the transformation ratio is 80 kHz, 120 kHz, 16 kHz.
There are a plurality depending on the order of resonance, such as 0 kHz and 200 kHz. Among them, the operation is most efficient when operating near a specific resonance frequency determined by the electrode pattern. In addition, in the vicinity of the resonance frequency, driving at a frequency higher than the frequency showing the maximum value generally results in smaller loss. In this example, the operation at 120 kHz to 130 kHz shows good characteristics. Therefore, when the output decreases, the driving frequency is lowered, and
When the output increases, it is possible to control the output to be constant by increasing the drive frequency. Since the bandwidth of the resonance frequency of the piezoelectric transformer is very narrow, about several percent, it is necessary to precisely control the frequency. A voltage controlled oscillator such as a VCO is used to drive the piezoelectric transformer.
Generally, a separate excitation method for driving a switch element of a transformer driving circuit is used.

FIG. 4 shows a conventional discharge tube lighting circuit using a piezoelectric transformer. An output current supplied from a piezoelectric transformer 51 to a discharge tube 52 is detected by a load current detector 53 and a reference voltage is used as an error amplifier. The comparison is made at 54, and the error signal is converted to a frequency by the VCO 55 and fed back to the transformer drive circuit 57 through the switch drive circuit 56. By feeding back the error from the reference value, the circuit is configured so that the output from the piezoelectric transformer follows the characteristic change and becomes constant. On the other hand, at the time of startup, since it is not determined which frequency is the steady operation frequency, the operation starts at a frequency different from the steady operation frequency. For example, when the operation is started at a frequency slightly higher than the steady operation frequency, the output is insufficient, so that the operation frequency is controlled to be lower, and the operation is performed at the steady operation frequency. By the way, considering the cost and the like, the tolerance of the initial oscillation frequency is generally 20% or more due to the tolerance and the temperature characteristics of the components of the oscillator. Even when the initial frequency becomes a resonance frequency band different from a desired resonance frequency band due to this tolerance, if the transformation ratio is sufficient, the device may operate at that frequency. In this case, the efficiency decreases and the loss increases.

In order to solve this problem, a self-excited system in which the output of a piezoelectric transformer is fed back to drive a switch element of a transformer drive circuit without using an oscillator has been studied. Since the self-excited system operates at a preferable frequency according to the characteristics of the piezoelectric transformer, the problem of operating at a different frequency does not occur. FIG. 5 shows a conventional self-excited circuit, in which an output voltage output from a piezoelectric transformer 61 to a load 62 is fed back to an oscillation circuit to provide a transistor T.
The operation frequency is adjusted by driving r10.

[0007]

In such a conventional circuit, since the transistor Tr10 is operated in the active region, the loss in the transistor is large, and it cannot be applied to a power conversion circuit. When the switching operation is performed by using the transistor Tr10 in the cutoff region and the saturation region in order to reduce the loss of the transistor, the input waveform of the piezoelectric transformer does not become a sine wave or the operation at a desired frequency becomes impossible. The transformer cannot be efficiently driven, and the efficiency of the entire circuit is reduced. As described above, the conventional self-excited system has no power conversion circuit that operates with high efficiency. SUMMARY OF THE INVENTION It is an object of the present invention to provide a power conversion circuit for operating a piezoelectric transformer stably with high efficiency near a desired resonance frequency.

[0008]

The present inventors have found that the phase of the alternating current output from the piezoelectric transformer and supplied to the load is different from the phase of the alternating current for driving the piezoelectric transformer, and it is not desirable that the phase be fed back with a different phase. Oscillation at a frequency was found. Therefore, the present inventors have reached the idea that the phase of the output to be fed back may be shifted so as to cancel the phase difference, and the present invention has been made.

That is, the present invention provides a piezoelectric transformer, a transformer drive circuit for supplying AC power to the piezoelectric transformer, an output detection circuit for detecting an output of the piezoelectric transformer and outputting an AC voltage, and a phase of the AC voltage. And a phase waveform conversion circuit that changes the phase, and feeds back the AC voltage whose phase has changed to the transformer drive circuit. According to the present invention, the piezoelectric transformer is driven by a signal having a certain phase shifted from the output of the piezoelectric transformer. Since the phase shift is different from the resonance frequency different from the desired resonance frequency for operating the piezoelectric transformer, the piezoelectric transformer always operates at the desired frequency. Therefore, it is possible to prevent the efficiency of the piezoelectric transformer from decreasing and the loss from increasing as described above.

The power conversion circuit according to the present invention comprises a first coil and a second coil connected to one end of DC power supplied from the outside, the other ends of the two coils, and the other end of the DC power. And a first switch element and a second switch element connected therebetween, and the input electrode of the piezoelectric transformer is preferably connected to a connection point between the switch element and the coil. By doing so, the driving waveform of the piezoelectric transformer becomes a sine wave, and the piezoelectric transformer operates efficiently.

Further, the phase waveform conversion circuit of the present invention further comprises:
It is preferable to include an oscillator that oscillates at an arbitrary frequency.
By doing so, the self-excited vibration occurs reliably regardless of the frequency of the excitation, so that the problem that the oscillation stops at the time of startup can be prevented. Further, by driving the first and second switch elements alternately at an on-duty of 50%, the drive waveform of the piezoelectric transformer becomes a sine wave without distortion, so that the piezoelectric transformer operates efficiently.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to a specific circuit diagram.

[0013]

Embodiment 1 FIG. 1 shows a circuit of this embodiment. In this circuit, DC power input from an input terminal 3 is converted into AC by a transformer drive circuit 4 to drive the piezoelectric transformer 1. Then, the AC power transformed by the piezoelectric transformer is supplied to the load 2. Then, the current flowing through the load is converted into an AC voltage by the load current detector 5 including the resistor r1. The phase of this AC V3 is shifted by a phase shifter 6 composed of a resistor r2 and a capacitor C1 to obtain an AC V4. Thereafter, by passing through a waveform converter 7 composed of an inverter, rectangular waves V1 and V2 out of phase with the load current can be obtained. By driving the switch elements SW1 and SW2 with this rectangular wave, oscillation continues, and self-excitation starts. These AC V
The waveforms of 1, V2, V3, and V4 are shown in FIG. Since the phase shift differs depending on the resonance frequency, the operation at the different resonance frequency is prevented by setting so as to cancel the phase shift generated in the piezoelectric transformer, and the operation always occurs at the desired frequency. Further, even in a desired frequency band, the phase shift differs depending on the frequency. Therefore, the operating frequency can be made constant by making the phase shift constant.

The phase waveform conversion circuit is a phase shifter 6 for shifting the phase of a detection signal in a load current detector as an output detection circuit.
And a waveform converter 7 for converting a sine wave detection signal into a rectangular wave having a duty of approximately 50%. The same effect can be obtained by changing the order of these phase shifters and waveform converters, or connecting a plurality of phase shifters with reduced phase shift in series. The amount of phase shift may be determined as follows. The two switching elements SW1 and SW2 of the transformer driving circuit are respectively operated by two rectangular waves having phases shifted by 180 °, and a sine wave power having a duty of 50% is supplied to the piezoelectric transformer to drive it. The output waveform supplied to the load from the piezoelectric transformer is observed with the signal of the load current detector. At the same time, the phase difference between the two may be determined by observing the waveform of the rectangular wave supplied to the switching element and determining the circuit constant of the phase shifter so as to cancel the phase difference. This phase difference occurs when AC power is converted by the piezoelectric transformer.

Here, in the transformer driving circuit 4, two coils L1 and L2 have one ends connected to a terminal 3 for inputting DC power supplied from the outside, and the other ends connected to two switch elements SW1. And SW2 are connected respectively. In the piezoelectric transformer 1, two input electrodes are formed with a piezoelectric substrate interposed therebetween, and each input electrode is connected to a connection point between the coil and the switch element.
The other ends of the two switch elements are connected to a ground terminal that is the other end of a DC power supply that supplies DC power. By doing so, the driving waveform of the piezoelectric transformer becomes a sine wave, and the piezoelectric transformer operates efficiently.

[0016]

Embodiment 2 FIG. 3 shows a second embodiment. In this power conversion circuit, switch elements SW3 and
The piezoelectric transformer 11 is driven with a sine wave by a voltage by alternately operating the SW4 with an on-duty of 50%.
The output of the piezoelectric transformer is supplied to the load current detector 15 through the load.
Flow through the resistance. The load current detector outputs an AC voltage having the same phase as the load current of the sine wave. This AC voltage has a sinusoidal shape oscillating around the ground level, but is passed through the waveform converter 16 to be converted into a rectangular wave having a ground level as a lower limit voltage and an on-duty of 50%. At the time of steady operation, since the oscillator 17 is driven by this signal, the operating frequency of the oscillator matches the operating frequency of the piezoelectric transformer. The phase of this rectangular wave signal is shifted by the phase shifter 18 so that the phase of the square wave signal coincides with that of the original drive signal, whereby the self-excitation operation is continued.

In the present embodiment, the phase waveform conversion circuit includes an oscillator 17 composed of two inverters U4 and U5. By doing so, there is no signal to be fed back when the load current is 0 at the time of starting, for example, but if the oscillation frequency of the oscillator is set to an arbitrary frequency at this time,
The piezoelectric transformer can oscillate at this frequency.
Also, once the load current is detected, this oscillator is pulled to the self-excited frequency, so there is no problem even if the operating frequency of the oscillator is in a frequency band different from the desired resonance frequency. It operates at the frequency of

[0018]

According to the present invention, the piezoelectric transformer is driven by changing the phase of the output of the piezoelectric transformer to be fed back so as to match the phase of the alternating current for driving the piezoelectric transformer.
Oscillation can be performed near a desired resonance frequency, and stable operation with high efficiency can be achieved.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a circuit according to a first example of the present invention.

FIG. 2 is a diagram illustrating voltage waveforms at various parts in the circuit according to the first embodiment.

FIG. 3 is a diagram illustrating a circuit according to a second embodiment.

FIG. 4 is a diagram showing a conventional another excitation circuit.

FIG. 5 is a diagram showing a conventional self-excited oscillation circuit.

FIG. 6 is a diagram illustrating a relationship between a transformation ratio and a frequency of a piezoelectric transformer.

[Explanation of symbols]

 1, 11, 51 ··· Piezoelectric transformer 2, 12 · · Load 3, 13, 58 · · Terminal 4, 14 · · Transformer drive circuit 5, 15 · · Load current detector 6, 18 · · Phase shifter 7, 16 ··· Waveform converter 17 ··· Oscillator

Claims (3)

[Claims] A piezoelectric transformer, a transformer driving circuit for supplying AC power to the piezoelectric transformer, an output detecting circuit for detecting an output of the piezoelectric transformer and outputting an AC voltage, and a phase for changing a phase of the AC voltage. A power conversion circuit, comprising: a waveform conversion circuit; and feeding back an AC voltage having a changed phase to the transformer driving circuit. 2. A transformer driving circuit comprising: a first coil and a second coil connected to one end of DC power supplied from the outside; and the other ends of the two coils and the other end of the DC power. And a first switch element and a second switch element connected between the first and second coils, and an input electrode of the piezoelectric transformer is connected to a connection point between the switch element and the coil. Item 2. The power conversion circuit according to item 1. 3. The power conversion circuit according to claim 1, wherein the phase waveform conversion circuit includes an oscillator that oscillates at an arbitrary frequency.