JP3553493B2 - Control circuit for piezoelectric transformer - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a control circuit of a piezoelectric transformer used in a piezoelectric inverter for turning on a cold cathode tube. The present invention relates to a change in characteristics of the cold cathode tube, an open load disconnection of the cold cathode tube, and a load short circuit of the piezoelectric transformer due to an external factor. On the other hand, the present invention relates to a control circuit for protecting components and circuits in a piezoelectric inverter and peripheral circuit elements.
[0002]
[Prior art]
Liquid crystal display devices are mainly used as displays for information devices because of their thin and lightweight characteristics. A backlight is necessary for a transmissive liquid crystal display, and most of the backlight is a cold cathode tube. Lighting of a cold cathode tube includes a process of starting discharge before lighting and a process of continuing lighting. Piezoelectric transformers are not only compact and lightweight, but also have characteristics such as a high voltage at the start of discharge and a ballast effect at the time of continuous lighting, and are advantageous as inverters for cold-cathode tubes, and their use range is expanding. .
[0003]
The method shown in FIG. 3 is used for starting the discharge of the cold cathode tube by the piezoelectric inverter and continuing the lighting with the lighting current being constant. As shown in FIG. 3, the no-load characteristic curve 31 (dashed line) when the secondary electrode load is released and the load characteristic curve 32 (solid line) when the secondary electrode load is connected are higher in the frequency axis as shown in FIG. The load characteristic curve 32 exists on the lower side of the characteristic curve 31. Further, the boost ratio of the primary electrode driving AC voltage to the secondary electrode AC output voltage of the piezoelectric transformer is high in the no-load characteristic curve 31 and low in the load characteristic curve 32.
[0004]
Utilizing this characteristic, the discharge start of the cold-cathode tube gradually lowers the driving frequency from the driving upper limit frequency f1 that is sufficiently higher than the bottom of the no-load characteristic curve when the piezoelectric inverter is started. This is performed when the voltage reaches the discharge start voltage a of the cold cathode tube at the secondary electrode output due to the high boosting ratio. After the start of the discharge, a transition is made to the load characteristic curve 32 due to the ballast effect of the piezoelectric transformer.
[0005]
When the current is large, that is, when the secondary electrode voltage is high, the driving frequency is increased to decrease the voltage, and when the lighting current is small, the driving frequency is decreased so that the lighting current becomes constant after the cold cathode tube is lit. By the circuit control for increasing the current by increasing the next electrode voltage, the drive frequency moves to the slope portion slightly higher than the peak of the load characteristic curve 32, and the control for keeping the lighting current constant is continued at the point c. The transition locus 33 (thin solid line) of the secondary electrode voltage of the piezoelectric transformer is shown along the no-load characteristic curve and the load characteristic curve.
[0006]
In the above-described conventional piezoelectric inverter, the cold-cathode tube must be discharged, the load does not come off from the secondary electrode of the piezoelectric transformer, and a short-circuit load is connected to the secondary electrode of the piezoelectric transformer. The control is performed on the assumption that there is no control. In actual use, the secondary electrode of the piezoelectric transformer is substantially opened or short-circuited due to a characteristic change due to the environment of the cold cathode tube or a load fluctuation due to an external factor. The problem of the inability to start discharge of the cold-cathode tube due to the change in the characteristics of the cold-cathode tube. Is corrected to the set value, a phenomenon such as heat generation of the drive switch caused by lowering the piezoelectric transformer to the capacitive lower drive frequency f2 occurs.
[0007]
[Problems to be solved by the invention]
The present invention relates to the inability to start discharge between cold cathodes due to changes in the characteristics of the cold cathode tubes, opening or short-circuiting of the secondary electrode of the piezoelectric transformer, deterioration of components in the inverter, accidental discharge to the surroundings, and heat generation of the drive switches caused by frequency reduction. It is intended to provide a control circuit for a piezoelectric transformer of a piezoelectric inverter, which can prevent the occurrence of an electric current and can operate stably.
[0008]
[Means for Solving the Problems]
The present invention solves the above problem by detecting the output of the secondary side of a piezoelectric transformer and controlling the operation of the driving circuit of the piezoelectric transformer when the output is not the value at the time of continuing lighting. Is what you do.
[0009]
That is, a drive circuit that drives the primary electrode of the piezoelectric transformer with an AC voltage starts the discharge of the cold-cathode tube by the output voltage from the secondary electrode, and continuously turns on the piezoelectric transformer. A circuit that performs negative feedback control so that the lighting current of the cathode tube is increased when it is high and reduced when it is low, and the drive frequency is gradually reduced from the upper limit frequency that is sufficiently higher than the maximum output of the no-load characteristic curve of the piezoelectric transformer at startup. In the control circuit of the piezoelectric transformer with the circuit,
When a voltage equal to or higher than a preset voltage is detected at the secondary electrode of the piezoelectric transformer, the drive frequency is returned to the upper limit frequency at the switch speed, and the drive frequency is gradually reduced again from the upper limit frequency to drive the piezoelectric transformer. And / or a circuit for stopping the operation of the driving circuit of the piezoelectric transformer when a preset lighting current cannot be obtained in the secondary electrode of the piezoelectric transformer. .
[0010]
Hereinafter, embodiments of the present invention will be described with reference to the drawings. First, the operation will be described. FIG. 4 is an explanatory diagram of the operation of the piezoelectric transformer according to the present invention. The no-load characteristic curve and the load characteristic curve of the piezoelectric transformer, and the locus of the secondary electrode voltage of the piezoelectric transformer for normal discharge start and lighting current control are the same as those in FIG.
[0011]
The trajectory 34 is a trajectory of the secondary electrode voltage when the secondary electrode load of the piezoelectric transformer is substantially opened due to an increase in the discharge voltage due to the characteristics of the cold-cathode tube in a cool and dark environment or an external factor. A locus of the secondary electrode voltage when the secondary electrode is substantially short-circuited is shown as a voltage locus 35, and a load curve obtained by substantially short-circuiting the secondary electrode of the piezoelectric transformer is shown as a short-circuit load curve 36 (dashed line).
[0012]
When the discharge starting voltage rises due to the characteristics of the cold cathode fluorescent lamp under a dark environment or when the secondary electrode load of the piezoelectric transformer is substantially opened due to an external factor, the driving frequency of the secondary electrode voltage of the piezoelectric transformer gradually decreases from f1. Thus, even if the secondary electrode voltage rises, it passes the normal discharge starting voltage a and continues to rise toward the peak of the no-load characteristic curve. Since the material of the piezoelectric transformer and the material of its peripheral connection parts have a withstand voltage limit, it is not preferable in terms of life to apply the peak voltage of the no-load characteristic curve.
[0013]
In order to cope with this, a limit voltage b is set with a margin for the withstand voltage limit of the material, and when the voltage is detected at the secondary electrode, the drive frequency is returned to f1 at the circuit element switch speed, and the inverter is again turned on. It is gradually lowered from f1 in the same manner as when starting. This cycle is the trajectory 34. By this operation, even if the discharge starting voltage rises due to the characteristics of the cold cathode fluorescent lamp in a cold and dark environment or the secondary electrode load of the piezoelectric transformer is substantially open due to an external factor, an excessively high voltage is applied to the piezoelectric transformer and its peripheral connection parts. Is not applied.
[0014]
At the same time, when the cycle of the trajectory 34 is caused only by the increase in the discharge starting voltage due to the characteristics of the cold cathode tube under the cold and dark environment, there is a possibility that the cold and dark environment of the cold cathode tube is improved and the discharge can be started after several cycles. In this case, the locus 34 changes from the discharge start voltage a to the locus 33. For this reason, the locus 4 is also useful as an inverter characteristic in which a failure in starting discharge is small in a cool and dark environment.
[0015]
When the secondary electrode of the piezoelectric transformer is short-circuited, the driving frequency is reduced to f2 in locus 35, and when the cold cathode tube does not return to the set lighting current within a predetermined time, the driving current is stopped. The drive frequency is lowered and moves to f2 in order to correct the insufficient lighting current value by negative feedback control. When the piezoelectric transformer is driven at a frequency lower than the peak of the load characteristic or the short-circuit load characteristic as in f2, the piezoelectric transformer is in a capacitive region, and the drive switch generates heat. In order to prevent heat generation, it is necessary to stop driving when staying at the frequency f2 for a predetermined time or more.
[0016]
FIG. 1 is a circuit diagram showing an embodiment of the present invention. , 14 and 15 are input portions of a piezoelectric inverter, 1 is a control IC, 2 is a matching inductor, 3 is a drive switch, and 4 is a piezoelectric transformer. 4 'is a primary electrode of the piezoelectric transformer, 4 "is a secondary electrode of the piezoelectric transformer. 5 is a cold cathode tube, R6 is a lighting current detecting resistor, R4 and R5 are resistors for a limit voltage detecting circuit, and 6 is a limit voltage. Reference numerals 10 and 11 denote switches driven by the limit voltage detector, 9 a differential amplifier for comparing a voltage corresponding to the lighting current with a reference voltage, and 8 a VCO based on the output voltage of the differential amplifier. R1, R2, R3, and C1 are resistors and capacitors for applying a bias to the VCO, 12 is a power supply that can turn on / off a power supply at a SW terminal that applies a reference voltage to the inside and around the IC, and 13 detects an abnormal output of the error amplifier. Fig. 2 shows the VF characteristics of the VCO of the circuit of Fig. 1.
[0017]
The operation of the circuit of FIG. 1 will be described with reference to FIG. When the inverter is started, a bias obtained by dividing the reference voltage by R1, R2, R3, and C1 is applied to the VCO, and driving of the piezoelectric transformer by the oscillation frequency of the VCO is started. Immediately after startup, the bias is low because there is no charge in C1, and the driving frequency is high from the characteristics of FIG. 2, which corresponds to f1 described above. As time elapses, electric charges are accumulated in C1, so that the bias gradually increases and the driving frequency gradually decreases. In FIG. 4, the locus of the secondary electrode voltage of the piezoelectric transformer up to this point is such that the no-load characteristic curve of the locus 33 or the locus 34 rises from the f1 side.
[0018]
If the normal cold-cathode tube is a load, it discharges at the discharge starting voltage a and starts lighting. In a cold and dark environment, when the cold cathode tube whose discharge starting voltage has risen is a load or the secondary electrode is unloaded due to an external factor, the secondary electrode voltage rises on the load characteristic curve 31 to the limit voltage b. This voltage is detected by the limit voltage detector 6, and the switches 10 and 11 at the output of this detector are turned on. Since the switch 10 removes the charge of C1 of the VCO bias circuit at the switch speed, the bias of the VCO decreases rapidly, and the drive frequency becomes f1 at the switch speed.
[0019]
This time corresponds to the straight line portion of the locus 34 in FIG. Thereafter, as in the case of the start-up, electric charges are accumulated in C1 with the passage of time, the secondary electrode voltage increases, reaches the limit voltage b, and the cycle of the locus 34 is repeated. During the repetition, when the discharge start voltage becomes equal to or lower than the limit voltage b due to a discharge start voltage increase due to a cold / dark environment of the cold cathode tube or a change in environment during the repetition of the cold cathode tube, a locus close to the locus 33 moves to a point c and continuous lighting is performed. In FIG. 4, a circuit is added to prevent the reference power supply from being turned off by resetting the C2 of the SW terminal of the reference power supply by the switch 11 every cycle by the switch 11 so as not to enter a latch stop operation described later during the operation of the repetition locus 34.
[0020]
When the load of the piezoelectric transformer is short-circuited or short-circuited, the lighting current detection voltage generated in R6 is lower than the reference voltage corresponding to the set target current, the output of the error amplifier 9 rises to the upper limit, and the frequency of the VCO is lowered to light up. The current is controlled to be increased, but the lighting current does not increase, so that the driving frequency is lowered to the lower limit f2 from FIG. This is the negative feedback control itself of a series of lighting current control. In FIG. 4, it is represented by a locus 35. If the output of the error amplifier continues to be at the upper limit, the output of the error amplifier output abnormality detector 13 will continue to be at a high level, charge C2, and then turn off the reference power through the reference power SW. The IC 1 whose reference power supply is cut off stops its operation, and stops the operation of the drive circuit. As a result, the drive switch 3 is also stopped, and the switch does not generate heat.
[0021]
【The invention's effect】
According to the present invention, it is possible to obtain a piezoelectric inverter with less discharge failure with respect to a change in the discharge starting voltage characteristic of the cold cathode tube, and to substantially open or substantially short-circuit the secondary electrode load of the piezoelectric transformer in the inverter. Deterioration of parts and accidents of electric discharge to the surroundings can be prevented, and heat generation of the drive switch caused by lowering of the drive frequency can be prevented, so that stable operation can be obtained.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing an embodiment of the present invention. FIG. 2 is an explanatory diagram of VF characteristics of a VCO. FIG. 3 is an explanatory diagram of an operation of a conventional piezoelectric transformer. FIG. 4 is an operation of a piezoelectric transformer according to the present invention. [Description of the symbols]
4: Piezoelectric transformer 5: Cold cathode tube 8: VCO
31: no-load characteristic curve 32: load characteristic curve

Claims (3)

A driving circuit for driving the primary electrode of the piezoelectric transformer with an AC voltage, a discharge of the cold cathode tube by the output voltage from the secondary electrode, and a piezoelectric transformer for starting and continuously lighting the cold cathode tube; A circuit that performs negative feedback control so as to increase when the lighting current is high and decrease when it is low, and a circuit that gradually lowers the drive frequency from the upper limit frequency that is sufficiently higher than the frequency that is the maximum output of the no-load characteristic curve of the piezoelectric transformer at startup. In the control circuit of the equipped piezoelectric transformer,
When a voltage equal to or higher than a preset voltage is detected at the secondary electrode of the piezoelectric transformer, the drive frequency is returned to the upper limit frequency at the switch speed, and the drive frequency is gradually reduced again from the upper limit frequency to drive the piezoelectric transformer. A control circuit for stopping operation of a driving circuit of the piezoelectric transformer when a preset lighting current cannot be obtained in a secondary electrode of the piezoelectric transformer. A driving circuit for driving the primary electrode of the piezoelectric transformer with an AC voltage, a discharge of the cold cathode tube by the output voltage from the secondary electrode, and a piezoelectric transformer for starting and continuously lighting the cold cathode tube; A circuit that performs negative feedback control so as to increase when the lighting current is high and decrease when it is low, and a circuit that gradually lowers the drive frequency from the upper limit frequency that is sufficiently higher than the frequency that is the maximum output of the no-load characteristic curve of the piezoelectric transformer at startup. In the control circuit of the equipped piezoelectric transformer,
When a voltage equal to or higher than a preset voltage is detected at the secondary electrode of the piezoelectric transformer, the drive frequency is returned to the upper limit frequency at the switch speed, and the drive frequency is gradually reduced again from the upper limit frequency to drive the piezoelectric transformer. A control circuit for a piezoelectric transformer, comprising: A driving circuit for driving the primary electrode of the piezoelectric transformer with an AC voltage, a discharge of the cold cathode tube by the output voltage from the secondary electrode, and a piezoelectric transformer for starting and continuously lighting the cold cathode tube; A circuit that performs negative feedback control so as to increase when the lighting current is high and decrease when it is low, and a circuit that gradually lowers the drive frequency from the upper limit frequency that is sufficiently higher than the frequency that is the maximum output of the no-load characteristic curve of the piezoelectric transformer at startup. In the control circuit of the equipped piezoelectric transformer,
A control circuit for a piezoelectric transformer, comprising: a circuit for stopping an operation of a driving circuit of the piezoelectric transformer when a preset lighting current cannot be obtained in a secondary electrode of the piezoelectric transformer.

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