JP2000253671A - High frequency wave power converting circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high frequency wave power converting circuit that operates stably even if an input voltage varies to a large extent due to the switching of input power supply. SOLUTION: This converting circuit comprises a piezoelectric transformer 2, a transformer drive circuit 1 to drive the piezoelectric transformer by receiving an input voltage, an output detecting circuit 4 for outputting a detection voltage through conversion to a detection voltage by multiplying an output supplied to the load from the piezoelectric transformer with a conversion ratio, a comparator 5 for comparing the detection voltage with the reference voltage and an oscillator 6 for controlling the frequency depending on the output voltage of the comparator to transfer it to the transformer drive circuit, wherein the conversion ratio takes a value depending on two modes corresponding to high and low input voltages.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power conversion circuit using a high frequency, and more particularly to a high frequency 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 input voltage of the inverter depends on the remaining amount of the rechargeable battery,
It changes by switching between AC adapter and rechargeable battery,
It is necessary to control the output supplied to the discharge tube to be constant regardless of the input voltage. In order to make the output constant, there is a method of controlling the voltage applied to the input terminal of the piezoelectric transformer or controlling the transformation ratio by changing the operating frequency.

[0005]

In the case of a notebook personal computer, the input voltage of the inverter varies in the range of about 7 to 13 V with a rechargeable battery, and in the range of 7 to 21 V when an AC adapter is included, and the maximum input voltage is the minimum. The change width is three times as large as the input voltage. It is difficult to keep the input voltage of the piezoelectric transformer constant for such a wide variation width. Further, when trying to control with the operating frequency, the allowable range of the input voltage change is determined by the ratio between the maximum value and the minimum value of the boosting ratio within the appropriate range of the operating frequency. The ratio between the maximum value and the minimum value of the boosting ratio within an appropriate range of the operating frequency for maintaining high power conversion efficiency is generally about twice regardless of the structure of the piezoelectric transformer. Some triples are unacceptable.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems and to provide a high-frequency power conversion circuit that operates stably even when the input voltage greatly fluctuates due to switching of an input power supply.

[0007]

According to the present invention, there is provided a piezoelectric transformer, a transformer driving circuit for receiving the input voltage and driving the piezoelectric transformer, and a detection voltage obtained by multiplying an output supplied from the piezoelectric transformer to a load by a conversion ratio. An output detection circuit that converts and outputs a detection voltage, a comparator that compares the detection voltage with a reference voltage, and an oscillator that controls a frequency according to the output voltage of the comparator and transmits the frequency to the transformer drive circuit. Prepared,
The conversion ratio is a high-frequency power conversion circuit that takes a value according to two modes corresponding to the level of the input voltage.

According to the present invention, even if the input voltage fluctuates greatly due to switching of a rechargeable battery due to attachment / detachment of an AC adapter which is an input power supply of a device equipped with the high-frequency power conversion circuit, an output from the piezoelectric transformer is obtained. The change step-up ratio can be kept within the allowable range.

Further, the present invention is characterized in that, in the above high-frequency power conversion circuit, each mode of the detection voltage has a region in which the detection voltage changes in accordance with the input voltage.

According to the present invention, in each of the modes of the detection voltage, the detection voltage takes a constant value within a preset set range of each input voltage, and is proportional to the input voltage when deviated from the set range. It is preferable to take a value.

According to the present invention, for example, when the AC adapter is attached and detached, even if the mode of the detection voltage is not switched at the same time, the detection voltage is automatically changed, and the piezoelectric transformer has a high frequency conversion range. Will work with.
That is, when the AC adapter is plugged in in the low current mode, the detection voltage changes so as to approach the value in the high current mode, thereby preventing a decrease in efficiency and a failure of the circuit. Further, when the AC adapter is removed in the high current mode, the detection voltage changes so as to approach the value in the low current mode, thereby preventing unexpected turning off. Since the change in the detection voltage follows the change in the input voltage, even if there is a voltage fluctuation such as chattering, the detection voltage also changes in accordance with the fluctuation, so that it is possible to operate stably.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to specific examples. Of the above two modes, in the low current mode in which the rechargeable battery is used as the input power source, the detection voltage is constant in the voltage fluctuation range of the rechargeable battery, and the detection voltage is proportional to the input voltage in other voltage ranges. To do it. In the high current mode in which an AC adapter is used as an input power supply, the detection voltage is constant in the voltage fluctuation range of the AC adapter, and the detection voltage is proportional to the input voltage in other voltage ranges. In this example, the expected fluctuation range of the voltage of the rechargeable battery or the AC adapter may be set in advance as the respective input voltage ranges. The two modes of the detection voltage are A
It is only necessary to judge whether or not the C adapter is attached or detached by a personal computer, and to switch.

By the way, when the AC adapter is attached and detached, a phenomenon called chattering occurs in which the conduction and the interruption in the connector are repeated in a short time. on the other hand,
Switching between the two modes of the detection voltage is performed by judging the attachment or detachment of the AC adapter with a personal computer.
There is a time difference in the voltage fluctuation due to the attachment / detachment of the C adapter, and according to this, the mode is instantaneously switched.
The PC voltage may fluctuate and the PC may malfunction. Therefore, it is preferable that the mode switching does not occur during the chattering period. For example, it is preferable to switch the mode several seconds after detecting a change in the input voltage.

However, if the AC adapter is operated in the low-current mode, it operates in a state where the power conversion efficiency is extremely reduced, so that the power element of the inverter may be overheated and cause a failure. Conversely, if the AC adapter is stopped in the high current mode, the voltage will drop, exceeding the maximum value of the frequency dependence of the transformer's transformation ratio, and the tube current required for lighting the discharge tube will not flow. Then, the discharge tube goes out.

If the detection voltage is changed in accordance with the input voltage from the power supply, the detection voltage automatically changes when the AC adapter is connected or disconnected, even if the detection voltage mode is not switched at the same time. As a result, the piezoelectric transformer operates in a frequency range where the power conversion efficiency is high. That is, when the AC adapter is plugged in in the low current mode, the detection voltage changes so as to approach the value in the high current mode, thereby preventing a decrease in efficiency and a failure of the circuit. Further, when the AC adapter is removed in the high current mode, the detection voltage changes so as to approach the value in the low current mode, thereby preventing unexpected turning off. The change of the detection voltage is
Since it follows the change in the input voltage, even if there is a voltage fluctuation such as chattering, the detection voltage also changes in accordance with the fluctuation, so that it is possible to operate stably.

[0016]

FIG. 1 illustrates a first embodiment of the present invention. The high-frequency power conversion circuit includes a piezoelectric transformer 2 for supplying power to a load 3 such as a discharge tube, a transformer drive circuit 1 for supplying power to the transformer, a switch drive circuit 7 for driving a switching element of the drive circuit, and a piezoelectric transformer. The load current flowing through the load 3, which is the output, is detected,
An output detection circuit 4 for multiplying the detection voltage by a conversion ratio to convert the detection voltage to a detection voltage; an error amplifier 5 for amplifying an error between the detection voltage and the reference voltage; and a switch driving circuit whose frequency changes in accordance with the output voltage of the error amplifier. And an oscillator 6 for determining an operating frequency of the oscillator 7. Here, the error amplifier 5 is a comparator that compares the detection voltage with the reference voltage. The oscillator 6 amplifies the difference between the detection voltage and the reference voltage, converts the error voltage output from the error amplifier 5 into a frequency, and transmits the frequency to the transformer driving circuit via the switch driving circuit. The switch drive circuit is for driving two switch elements for converting DC into AC in the transformer drive circuit.

Next, an output detection circuit 4 which detects the output of the piezoelectric transformer and converts it into a detection voltage will be described. The load current flowing through the load is converted to an AC voltage by the resistor r1, and the AC voltage is converted to a DC by the rectifying and smoothing device 8 including a diode and a capacitor. This DC voltage is divided by a voltage divider 9 to obtain a detection output. At this time, the detection output is the transistor Tr
By changing the partial pressure ratio by 1, there are two ways. That is, the input signal of the terminal 10 is Lo
When the transistor Tr1 is off at w, the voltage division ratio is high, and when the input signal at the terminal 10 is high and the transistor Tr1 is on, the voltage division ratio is low. For example, a microprocessor (not shown) may determine the switching of the input power supply and switch and supply the Low or High signal to the terminal 10 after a certain period of time from the time until the end of chattering. This partial pressure ratio becomes the conversion ratio.

FIG. 3 shows the relationship between the input voltage at the input terminal 11 and the output voltage to the load of the piezoelectric transformer. In the present embodiment, the output voltage is constant in the voltage fluctuation range of the rechargeable battery in the low current mode as indicated by a straight line a. Also, in the high current mode, the output voltage can be controlled to be constant even if the voltage of the AC adapter fluctuates as indicated by a straight line b. In the conventional circuit, the control is performed in the range from the straight line a to the straight line c. However, since the straight line c enters the anti-resonance region of the piezoelectric transformer and operates at a low efficiency, a problem occurs due to heat generation or the like. By the way, in the upper left of the proper output region in FIG. 3, the piezoelectric transformer is operated at a frequency lower than the resonance frequency of the piezoelectric transformer, which results in an output insufficient region.

[0019]

Second Embodiment FIG. 2 shows a block diagram of a high-frequency power conversion circuit according to a second embodiment. An input voltage is applied to the input terminal 28, and the input voltage is converted into AC by a transformer drive circuit 21 having a switch element driven by the switch drive circuit 27, and the piezoelectric transformer 22 is driven. The alternating current boosted by the piezoelectric transformer is supplied to the load 23. The load current flowing through the load is converted into a detection voltage by the output detection circuit 24, and an error from the reference voltage is compared and amplified by an error amplifier 25 which is a comparator. By being fed back to the transformer drive circuit 21, the output of the piezoelectric transformer is controlled according to the detected voltage. Next, an output detection circuit for detecting the output of the piezoelectric transformer, multiplying the output by a conversion ratio, and converting the output to a detection voltage will be described. In the embodiment of the present invention, the detection voltage is set in two modes, and is changed as follows according to the input voltage of the high-frequency power conversion circuit.

The input voltage applied from the input terminal 28 is divided by a proportional voltage generator 30 composed of a resistance element to obtain two voltages V1 and V2 proportional to the input voltage. Here, the higher voltage is V1 and the lower voltage is V2. Among them, V1 is input to the constant voltage generator 31.
The voltage V4 generated by the constant voltage generator 31 takes the same value as V1 when the input voltage V1 to the constant voltage generator is lower than the threshold Vth of the constant voltage diode, and V1 is higher than the threshold. At this time, the voltage becomes the same as the threshold value Vth. The maximum value selector 29 outputs the larger one of V4 and V2 as the detection voltage V3. At this time, V2 becomes V
When it is larger than 4, the detection voltage V3 becomes the same value as V2 which is proportional to the input voltage.

A constant voltage generator 31 and a terminal 32 connected via a resistor Tr1 and a resistor have a High level voltage in a low current mode and a Low level voltage in a high current mode, for example, an external microprocessor. It is made to take from. Then, by turning on or off the transistor Tr1 according to the voltage of each level, the threshold value of the constant voltage diode is selected from either the value of only D1 or the value of D1 and D2 in series. That is, the threshold value of the constant voltage diode can be set according to the mode of the detection voltage.

The output V3 of the maximum value selector 29 is input to the base terminal of the transistor of the output detection circuit 24 via a resistor. The voltage dividing ratio takes two values when the transistor is in the cut-off region and when the transistor is in the saturation state. The voltage dividing ratio can be set to the active region depending on the value of V3. In that case, the value between the two values is changed. Take. This partial pressure ratio becomes the conversion ratio.

Next, the relationship between the output of the piezoelectric transformer and the input voltage at the input terminal 28 when a discharge tube is used as the load 23 is shown in FIG. In the low current mode of the present embodiment, a region where the tube current is constant by a voltage as indicated by a polygonal line a;
There is an area that is proportional to the input voltage as a transient area. Further, in the high current mode, it becomes like a polygonal line b.

[0024]

According to the present invention, even when the input voltage fluctuates due to switching of the input power supply, stable operation can be performed while maintaining high power conversion efficiency.

[Brief description of the drawings]

FIG. 1 is a block diagram of a first embodiment of the present invention.

FIG. 2 is a block diagram of a second embodiment of the present invention.

FIG. 3 is a diagram illustrating a relationship between an input voltage and a tube current according to the first embodiment.

FIG. 4 is a diagram showing a relationship between an input voltage and a tube current.

[Explanation of symbols]

 1, 21 Transformer drive circuit 2, 22 Piezoelectric transformer 3, 23 Load 4, 24 Output detection circuit 5, 25 Error amplifier 6, 26 Oscillator 7, 27 Switch drive circuit 9 Voltage divider 10.32 Terminal 11, 28 Input terminal 30 Proportional voltage generator 31 Constant voltage generator

Claims (3)

[Claims] 1. A piezoelectric transformer, a transformer driving circuit for receiving the input voltage and driving the piezoelectric transformer, and outputting a detection voltage by converting an output supplied from the piezoelectric transformer to a load to a detection voltage by multiplying the conversion ratio. An output detection circuit, a comparator that compares the detection voltage with a reference voltage, and an oscillator that controls a frequency according to an output voltage of the comparator and transmits the frequency to the transformer drive circuit, wherein the conversion ratio is: A high-frequency power conversion circuit having a value corresponding to two modes corresponding to the level of the input voltage. 2. The high-frequency power conversion circuit according to claim 1, wherein each mode of the detection voltage has a region in which the detection voltage changes according to the input voltage. 3. In each mode of the detection voltage, the detection voltage takes a constant value within a preset set range of each input voltage, and takes a value proportional to the input voltage when deviated from the set range. The high-frequency power conversion circuit according to claim 2, wherein: