JP2000270543A - Inverter circuit using a piezoelectric transformer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the circuit that can realize stable voltage boosting ratio for variation of input voltage and output side load and can maintain higher operation efficiency. SOLUTION: An output voltage of a voltage boosting coil 13 in the preceding stage of a piezoelectric transformer 14 can be varied or switched for varying the voltage boosting ratio of a voltage boosting coil 13. Intermediate taps T1, T2 is provided to a winding for connection with switch means SW1, SW2, and the winding ratio of the primary side and secondary side is varied. As a result, variation in the drive frequency of the piezoelectric transformer can be set to a small value, and high efficiency can be maintained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter circuit for lighting a fluorescent tube or the like of a backlight for a liquid crystal, and more particularly, to a piezoelectric transformer for performing boosting necessary for lighting using a piezoelectric transformer and a boosting coil. It relates to the used inverter circuit.

[0002]

2. Description of the Related Art A transmissive liquid crystal display device used for a display of a personal computer or the like uses a fluorescent tube as a backlight, and a high voltage is required for lighting the fluorescent tube. For this purpose, means for boosting the power supply voltage is required, and a high-voltage output has conventionally been obtained using a large winding transformer.

[0003] The use of a large-output transformer is not preferable for small-sized portable equipment and the like because the apparatus becomes large and heavy, and the use of a piezoelectric transformer has been studied and put into practical use. However, when a piezoelectric transformer is used, it is necessary to change the frequency depending on a change in input voltage of a battery or the like or a load state. In addition, there is a problem that an output maintaining sufficient luminance cannot be obtained when used alone.

Therefore, it is common practice to boost the voltage in two stages by using a booster coil in front of the piezoelectric transformer to obtain a stable output. However, even in this case, it may be necessary to drive the piezoelectric transformer at a frequency greatly deviated from the resonance frequency due to a change in the input voltage or the load on the output side, and the efficiency is significantly reduced.

FIG. 4 shows that the input voltage range is set to 5 V to 10 V so that a predetermined output voltage can be obtained with the minimum input voltage of 5 V, an insufficient voltage is obtained by the winding transformer, and the output of the piezoelectric transformer is obtained. FIG. 9 shows characteristics of input voltage versus efficiency of an inverter circuit that obtains a sum as an entire output. FIG. This shows that as the input voltage increases, the driving frequency of the piezoelectric transformer deviates from the resonance frequency, and the efficiency decreases.

[0006]

SUMMARY OF THE INVENTION The present invention reduces the decrease in the efficiency of the piezoelectric transformer caused by the frequency control for controlling the step-up ratio required when the input voltage fluctuates and the load on the output side fluctuates. Thus, efficiency is improved.

[0007]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems by making the output variable by using a step-up coil to suppress the deviation of the driving frequency of the piezoelectric transformer.

That is, in an inverter circuit using a piezoelectric transformer, a booster coil is provided before the piezoelectric transformer, and the output of the booster coil is made variable.

Specifically, in an inverter circuit using a piezoelectric transformer, a step-up coil is provided in a stage preceding the piezoelectric transformer, a plurality of taps are provided in the step-up coil, and each tap is connected to switch means. The switch circuit connected to a predetermined tap is operated by the drive circuit of the above, and a switching means for changing a turn ratio of the input side and the output side of the tap is provided, and the output of the booster coil is made variable. is there

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The step-up voltage that is applied to a step-up coil (transformer) is changed by changing the turn ratio between a primary winding and a secondary winding by switching taps.
When the input voltage becomes equal to or higher than a predetermined value, the switch means is operated to reduce the turns ratio, thereby lowering the output voltage of the boosting coil.

[0011]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the present invention. A coil 13 and a piezoelectric transformer 14 are connected in series between the input terminal 11 and the output terminal 12. Two taps on coil 13
T1 and T2 are provided, and switches SW1 and SW2 formed by transistors are connected to these taps, respectively. One of the switches SW1 and SW2 receives a driving signal from a terminal 17 via a switching circuit 16 that is switched by a switching signal input from a terminal 15.

The coil 13 operates as a transformer in which the number of turns on the primary side and the number of turns on the secondary side are variable by tapping. That is, when the switch SW1 connected to the tap T1 is turned on, the number of turns on the secondary side increases, and a relatively high voltage is obtained. On the other hand, when the switch SW2 is turned on, the number of turns on the input side of the tap T2 increases, the number of turns on the secondary side decreases, and the output voltage decreases.

The case where the input voltage is changed in the range of 5V to 10V will be described with reference to FIG. The dotted line in the figure shows the characteristics of the conventional circuit, and the solid line shows the characteristics of the circuit according to the present invention. This example shows an example in which a switch is switched near an intermediate input voltage of 7.5 V to change the step-up ratio of the coil.

As the input voltage increases from 5 V, the efficiency gradually decreases as described above. Switching the transistor switch around 7.5V, which is less than 80%, so that the boost ratio became smaller, the efficiency once increased and recovered to 84%. When the input voltage further increases, the efficiency starts to decrease again, but the efficiency is maintained at about 78% even at 10 V, and it has been confirmed that the efficiency is improved by about 10% compared with the conventional circuit.

This indicates that the efficiency of the piezoelectric transformer can be maintained to some extent by changing the output of the boosting coil, whereas the frequency was conventionally adjusted by adjusting the frequency of the piezoelectric transformer. This shows that the piezoelectric transformer is driven at a position that does not significantly deviate from the resonance frequency. That is, the change in the step-up ratio of the piezoelectric transformer is absorbed by the coil.

FIG. 2 is a circuit diagram showing another embodiment of the present invention. Winding transformer 23 between input terminal 21 and output terminal 22
And the piezoelectric transformer 24 are connected in series.
A tap T3 is provided on the primary winding 23a, and a switch SW3 is connected to this tap, and a switch SW4 is connected to an end of the primary winding. One of the switches SW3 and SW4 is turned on by a switching signal supplied from a terminal 25 via a switching circuit.

Also in this example, the turns ratio of the primary side and the secondary side of the transformer can be changed by the operation of the switch of the transistor, and the output of the transformer can be switched.

The present invention is not limited to the above-described example, and it is also possible to provide more taps and switch finely. Further, a circuit using a MOS-FET or the like as a switch may have a similar circuit configuration.

[0020]

According to the present invention, the efficiency of the piezoelectric transformer can be maintained in a good state over a wide input voltage range.

Further, even when there are a plurality of load modes, taps can be formed so as to have a turns ratio suitable for each mode, so that optimum conditions can be set in all modes.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the present invention.

FIG. 3 is an explanatory diagram of characteristics of an inverter circuit according to the present invention.

FIG. 4 is an explanatory diagram of characteristics of a conventional inverter circuit.

[Explanation of symbols]

 13, 23: Coil (transformer) 14, 24: Piezoelectric transformer T1, T2, T3: Tap SW1, SW2, SW3, SW4: Switch (transistor) 16, 26: Switching circuit

Claims (5)

[Claims] 1. An inverter circuit using a piezoelectric transformer, wherein a booster coil is provided before the piezoelectric transformer, and an output of the booster coil is variable. 2. An inverter circuit using a piezoelectric transformer, comprising: a booster coil in a preceding stage of the piezoelectric transformer; switching means for changing a turns ratio between a primary side and a secondary side of the booster coil; An inverter circuit using a piezoelectric transformer. 3. An inverter circuit using a piezoelectric transformer, comprising a step-up coil in front of the piezoelectric transformer, providing a plurality of taps in the step-up coil and selecting the tap, thereby reducing the turns ratio between the primary side and the secondary side. An inverter circuit using a piezoelectric transformer, comprising a switching means for changing the output of the step-up coil. 4. An inverter circuit using a piezoelectric transformer, comprising: a booster coil in a preceding stage of the piezoelectric transformer, a plurality of taps provided in the booster coil, each tap being connected to switch means, and a drive circuit for the switch means. A switching means for operating a switch means connected to a predetermined tap to change a turn ratio between an input side and an output side of the tap, and using a piezoelectric transformer characterized in that the output of the step-up coil is made variable. Inverter circuit. 5. An inverter circuit using a piezoelectric transformer, comprising a step-up transformer in front of the piezoelectric transformer, providing a tap on the primary side of the step-up transformer, connecting the tap to switch means, and using a drive circuit for the switch means. An inverter circuit using a piezoelectric transformer, comprising switching means for changing the number of turns on the primary side of the transformer by operating switch means connected to the tap, and making the output of the step-up transformer variable.