JPH1126181A - Cold-cathode tube lighting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To decrease loss and increase efficiency of a field effect transistor by detecting voltage in the input part of a piezoelectric transformer constituting a DC/AC inverter, and supplying a control signal by which the field effect transistor is set on when the voltage in the input part is equal to reference voltage or lower than it to a driving circuit. SOLUTION: When input voltage Vdc is increased, tube current I flowing through a cold-cathode tube FL is increased, and in order to keep the tube current I constant, a frequency signal (f) from a V/F conversion circuit 7 is made high, and the period T of a field effect transistor Q is made short. As a result, on-duty D is varied to make on time short. When the input voltage Vdc is decreased, the tube current I flowing through the cold-cathode tube FL is decreased, and in order to keep the tube current I constant, the frequency signal (f) from the V/F conversion circuit 7 is made low, and the period T of the field effect transistor Q is made long. As a result, on-duty D is varied to make on time long.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-cathode tube lighting device used for a backlight of a liquid crystal display and the like, and more particularly to a cold-cathode tube lighting device using a DC / AC inverter comprising a resonance coil, a field effect transistor and a piezoelectric transformer. The present invention relates to a cathode-ray tube lighting device.

[0002]

2. Description of the Related Art A conventional cold cathode tube lighting device using a DC / AC inverter including a piezoelectric transformer will be described with reference to FIG. As shown in FIG. 5, a conventional cold-cathode tube lighting device 50 is used.
Is composed of an input power supply 1, a resonance coil L, a field effect transistor Q, and a piezoelectric transformer T, and converts a low DC voltage Vdc from the input power supply 1 into a high AC voltage V2.
A C / AC inverter 2, a drive circuit 3 for controlling on / off of the field effect transistor Q by an on-duty D, that is, on-time (ton) / on-time (ton) + off-time (toff) [%]; A cold-cathode tube FL lit by an AC voltage V2 generated from an output portion Tb of the transformer T, a tube current detection circuit 4 for detecting a tube current I flowing through the cold-cathode tube FL, and an AC output from the tube current detection circuit 4. A rectifier circuit 5 for rectifying the voltage V3, an error amplifier circuit 6 for amplifying a difference voltage between the DC voltage V4 of the rectifier circuit 5 and the dimming voltage Vo, and a converter for converting the DC voltage V5 of the error amplifier circuit 6 into a frequency signal f. And a V / F conversion circuit 7 for outputting the frequency signal f to the drive circuit 3, and constitutes a feedback control circuit as a whole.

Next, the operation of each block of the cold-cathode tube lighting device 50 will be described. The input power supply 1 has a DC voltage Vd
This is a power supply for generating c.

The DC / AC inverter 2 stores power in the resonance coil L when the field effect transistor Q is on, and discharges power from the resonance coil L to the input portion Ta of the piezoelectric transformer T when it is off. And a function of converting a low-voltage DC voltage Vdc into an AC voltage V1 that is a single-wave sine wave. At this time, the time tV1 during which the AC voltage V1 is generated depends on the inductance value of the resonance coil L and the capacitance value of the input section Ta of the piezoelectric transformer T.

[0005] The piezoelectric transformer T constituting the DC / AC inverter 2 transmits energy due to mechanical vibration generated by an AC voltage V1 applied from the resonance coil L to an input portion Ta of the piezoelectric transformer T to an output portion Tb, thereby transmitting the AC voltage V2. Raise,
The AC voltage V2 is supplied to the cold cathode fluorescent lamp FL.

The drive circuit 3 has a predetermined on-duty D, ie, on-time (ton) / (on-time (ton) + off-time (toff)) [%], and V / F
The frequency signal f from the conversion circuit 7, that is, the on-time (t
on) + off time (toff), DC / AC
The on-time ton and the off-time toff of the field-effect transistor Q forming the inverter 2 are determined. And
During the on-time ton, a gate voltage is applied to the gate of the field-effect transistor Q to turn on the field-effect transistor Q. During the off-time toff, no gate voltage is applied to the gate of the field-effect transistor Q. Turn off Q.

[0007] The cold cathode fluorescent lamp FL is lit when an AC voltage V2 of the piezoelectric transformer T is input. At this time, a tube current I flowing through the cold-cathode tube FL is supplied to the tube current detection circuit 4.

[0008] The tube current detection circuit 4 detects the supplied tube current I
Is supplied to the rectifier circuit 5.

The rectifier circuit 5 rectifies and smoothes the supplied AC voltage V3 to generate a DC voltage V4, and supplies the DC voltage V4 to one input terminal of the error amplifier circuit 7.

The error amplifier circuit 6 compares the DC voltage V4 supplied from the rectifier circuit 5 supplied to one input terminal with the dimming voltage Vo supplied to the other input terminal, amplifies the difference voltage, and amplifies the DC voltage. It is supplied to the V / F conversion circuit 7 as V5.

The V / F conversion circuit 7 converts the DC voltage V5 supplied from the error amplification circuit 6 into a frequency signal f, and converts the frequency signal f into an ON state of the field effect transistor Q constituting the DC / AC inverter 2. Time ton and off time to
ff, that is, the denominator of the on-duty D that controls the on-time ton of the field-effect transistor Q, is input to the drive circuit 3.

A series of operations as described above are performed in each block of the cold-cathode tube lighting device 50, and feedback control is performed.

The period T of the field effect transistor Q constituting the DC / AC inverter 2, that is, the sum of the on-time ton and the off-time toff is determined by the V / F conversion circuit 7.
, The on-time ton of the field-effect transistor Q is determined by the on-duty D of the drive circuit 3. The time tV1 at which the voltage V1 of the input portion of the piezoelectric transformer is generated is determined by the inductance value of the resonance coil and the capacitance value of the input portion of the piezoelectric transformer.

[0014]

However, in the above-described conventional cold-cathode tube lighting device, the inductance value of the resonance coil and the piezoelectric transformer are affected by the product variation and temperature characteristics of the resonance coil and the piezoelectric transformer, or the fluctuation of the input voltage. Of the input portion of the piezoelectric transformer, which is determined by the capacitance value of the input portion of the piezoelectric transformer. As a result, there is a difference between the time tV1 at which the AC voltage V1 is generated and the cycle of the on-time ton of the field-effect transistor determined by the frequency signal f from the V / F conversion circuit and the on-duty D of the drive circuit. As a result, as shown in FIG.
Is increased (region I), that is, the field-effect transistor is turned on before the AC voltage V1 becomes zero, or the time tV1 when the AC voltage V1 is generated as shown in FIG. (Region I
I) That is, when the AC voltage V1 is zero, the time during which the field-effect transistor is turned off becomes longer.

Similarly, by using different input voltages, for example, using a battery (about 6 V) and an AC adapter (about 12 V), the period T of the field-effect transistor constituting the DC / AC inverter, that is, the on-time t
The sum of on and off time toff varies. as a result,
A difference occurs between the time tV1 at which the AC voltage V1 is generated and the cycle of the on-time ton of the field-effect transistor determined by the frequency signal f from the V / F conversion circuit and the on-duty D of the drive circuit. As shown in FIG. 7A, as the input voltage increases, the period T of the field effect transistor is shortened (region I), that is, the field effect transistor is turned on before the AC voltage V1 becomes zero,
As shown in FIG. 7B, as the input voltage decreases, the period T of the field effect transistor increases (region I).
I) That is, when the AC voltage V1 is zero, the time during which the field effect transistor is turned off becomes longer.

As a result, as shown in FIGS. 6A and 7A, when the field effect transistor is turned on before the AC voltage V1 becomes zero, the surge current is supplied to the field effect transistor. Flow, the loss of the field effect transistor increases, and the efficiency decreases. Further, when a surge current flows through the field-effect transistor, noise is generated and the cold-cathode tube flickers, which causes a problem that the screen of a liquid crystal display using the cold-cathode tube lighting device as a backlight is disturbed.

Further, as shown in FIGS. 6 (b) and 7 (b), when the time during which the field-effect transistor is turned off becomes longer when the AC voltage V1 is zero, the electric field is increased during that time. There is also a problem that a forward current flows through the parasitic diode of the effect transistor, the loss of the field effect transistor increases, and the efficiency decreases.

The present invention has been made to solve such a problem, and provides a cold-cathode tube lighting device which reduces the loss of a field effect transistor constituting a DC / AC inverter and improves the efficiency. The purpose is to do.

[0019]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention comprises an input power supply, a resonance coil, a field effect transistor, and a piezoelectric transformer, and controls the on / off of the field effect transistor. A DC / AC inverter for converting a DC voltage from an input power supply into an AC voltage, a drive circuit for controlling on / off of the field effect transistor, a cold cathode tube which is turned on by an output voltage of the piezoelectric transformer, and the cold cathode A tube current detection circuit for detecting a tube current flowing in a tube, a rectification circuit for rectifying an AC voltage output from the tube current detection circuit, and an error for amplifying a difference voltage between an output voltage of the rectification circuit and a dimming voltage. A cold-cathode tube lighting device including an amplifier circuit and a V / F converter circuit that converts an output voltage of the error amplifier circuit into a frequency signal and supplies the frequency signal to the drive circuit. Te, the driving circuit, the DC /
A control circuit having a function of detecting a voltage of an input section of a piezoelectric transformer constituting an AC inverter and supplying a control signal for turning on the field effect transistor to the drive circuit when the voltage of the input section is equal to or lower than a reference voltage. It is characterized by having.

Further, the invention is characterized in that the control circuit has a function of supplying a control signal for controlling a sum of an on time and an off time of the field effect transistor to the drive circuit.

According to the cold-cathode tube lighting device of the present invention,
When the input voltage of the piezoelectric transformer constituting the C / AC inverter is equal to or lower than the reference voltage, the control circuit supplies a control signal for turning on the field effect transistor to the drive circuit. In this case, only the current flowing from the resonance coil to the field effect transistor is provided, and it is possible to eliminate the surge current flowing from the piezoelectric transformer to the field effect transistor.

[0022]

Embodiments of the present invention will be described below with reference to the drawings. Note that the same or equivalent parts as those in the conventional example are denoted by the same reference numerals, description thereof will be omitted, and only different parts will be described.

FIG. 1 is a block diagram showing an embodiment of a cold cathode tube lighting device according to the present invention. The cold cathode tube lighting device 10 of FIG.
However, the difference from the conventional cold-cathode tube lighting device 50 of FIG. 5 is that the voltage V1 of the input section Ta of the piezoelectric transformer T constituting the DC / AC inverter 2 is detected and the voltage is equal to or lower than the reference voltage Vref. Sometimes, a control circuit 11 for supplying a control signal for turning on the field effect transistor Q to the drive circuit 3 is provided.

FIG. 2 is a block diagram showing the configuration of the control circuit 11 of the cold cathode tube lighting device 10 of FIG. Control circuit 1
1 is a comparator 12 composed of, for example, a comparator, and OR
A circuit 13 and a pulse generator 1 comprising for example a PWMIC
4, one input terminal of the comparator 12 is connected to a connection point between the piezoelectric transformer T and the field effect transistor Q, and the other input terminal is connected to the reference voltage Vref.

The output of the comparator 12 is connected to one input terminal of an OR circuit 13, and the other input terminal of the OR circuit 13 is connected to a pulse generator 14. Further, the output terminal of the OR circuit 13 is connected to the drive circuit 3, and the pulse generator 14 is connected to the V / F conversion circuit 7.

Note that the pulse generator 14 generates a cycle T (on time ton and off time toff) of the field effect transistor Q constituting the DC / AC inverter 2 according to the frequency signal f from the V / F conversion circuit 7. Of the field effect transistor Q based on the period T and the on-duty D set by the CR constant of the pulse generator 14 or the like.

Here, the operation of the control circuit 11 will be described.
For example, the detected voltage V1 of the input portion Ta of the piezoelectric transformer T is compared with the reference voltage Vref. When the voltage V1 of the input portion Ta of the piezoelectric transformer T is equal to or lower than the reference voltage Vref, the comparator 12 The output is low.

The output of the comparator 12 is low, and the output of the pulse generator 14 is low (the field-effect transistor Q
When the off time (toff) is reached, the output of the OR circuit 13 becomes low, and when the low signal is input to the drive circuit 3, the drive circuit 3
The gate voltage for turning on the field effect transistor Q
Is applied to the gates.

That is, the output of the pulse generator 14 is low
Despite w, the output of the comparator 12 is high (the voltage V1 of the input portion Ta of the piezoelectric transformer T is equal to the reference voltage Vref).
In the above case, no gate voltage is applied to the gate of the field effect transistor Q, and the field effect transistor Q is turned off.

When the input voltage Vdc generated due to the difference in the input power source 1 changes, the cold cathode fluorescent lamp FL
The tube current I flowing through the V / F converter 7 also fluctuates. In order to keep the tube current I constant, the frequency signal f from the V / F conversion circuit 7 moves depending on the tube current I, and the pulse signal depends on the frequency signal f. The period T of the field effect transistor Q determined by the generator 14 moves.

That is, when the input voltage Vdc increases, the tube current I flowing through the cold-cathode tube FL also increases. In order to keep the tube current I constant, the frequency signal from the V / F conversion circuit 7 f increases, and the period T of the field effect transistor Q decreases. As a result, the on-duty D
To shorten the on-time.

Conversely, when the input voltage Vdc decreases, the tube current I flowing through the cold-cathode tube FL also decreases, and the frequency from the V / F conversion circuit 7 is maintained to keep the tube current I constant. The signal f becomes lower, and the period T of the field effect transistor Q becomes longer. As a result, the on-duty D is changed to increase the on-time.

According to the above-described embodiment, the control circuit operates as follows:
It has a function of detecting the voltage of the input section of the piezoelectric transformer constituting the C / AC inverter and turning on the field effect transistor when the voltage of the input section of the piezoelectric transformer is equal to or lower than the reference voltage. FIG. 3 (A) shows that even when the time of generation of the voltage at the input section of the piezoelectric transformer determined by the inductance value of the resonance coil and the capacitance value of the input section of the piezoelectric transformer varies due to the product variation and the temperature characteristics or the fluctuation of the input voltage. As shown in FIG. 3A, when the generation time tV1 of the AC voltage V1 becomes longer, the off time toff of the field effect transistor becomes longer, and FIG.
As shown in (b), when the generation time tV1 of the AC voltage V1 is shortened, the off-time toff of the field-effect transistor is shortened.

Similarly, since the control circuit has a function of supplying a control signal for controlling the sum of the on-time and the off-time, which is the cycle of the field-effect transistor, to the drive circuit, the DC / Even if the sum of the on-time and the off-time, which is the cycle of the field-effect transistor constituting the AC inverter, fluctuates, as shown in FIG. 4B, the on-time ton of the field-effect transistor becomes short, and as shown in FIG. 4B, when the period of the field-effect transistor becomes longer due to an increase in the input voltage, Becomes longer.

That is, even if the generation time of the voltage at the input section of the piezoelectric transformer varies due to the product variation and temperature characteristics of the resonance coil and the piezoelectric transformer, or the fluctuation of the input voltage, and the input voltage varies, the DC
Even when the sum of the on-time and the off-time, which is the cycle of the field effect transistor constituting the / AC inverter, fluctuates, the timing at which the voltage of the input portion of the piezoelectric transformer becomes zero and the timing at which the field effect transistor turns on are substantially the same. Since the surge current can flow from the input portion of the piezoelectric transformer to the field effect transistor or when the AC voltage V1 is zero, the field effect transistor is turned off, and the parasitic diode of the field effect transistor is forward-directed. The current does not flow.

Therefore, the loss of the field effect transistor can be reduced, and the efficiency can be improved.

Since the maximum rated current of the field effect transistor can be reduced, the size of the field effect transistor can be reduced, and accordingly, the size of the cold-cathode tube lighting device can be reduced.

Further, since a high-precision resonance coil and a piezoelectric transformer or a step of selecting them are not required,
The manufacturing cost can be reduced, and the degree of freedom in design can be improved.

A battery (about 6 V) is used as an input power supply.
And an AC adapter (approximately 12 V), so that even if the input voltage is different, it is possible to sufficiently cope without reducing the efficiency of the field effect transistor.

In the above embodiment, the case where the DC / AC inverter has a function of converting a low-voltage DC voltage generated from the input power supply into a single-wave sine-wave AC voltage has been described. A similar effect can be obtained even when a function of converting the DC voltage of the above to a sine wave AC voltage is provided.

[0041]

According to the cold cathode tube lighting device of the first aspect,
The control circuit has a function of detecting the voltage of the input portion of the piezoelectric transformer constituting the DC / AC inverter and turning on the field effect transistor when the voltage of the input portion of the piezoelectric transformer is equal to or lower than the reference voltage. The timing at which the voltage at the input section of the piezoelectric transformer becomes zero and the field effect transistor is turned on even if the time at which the voltage at the input section of the piezoelectric transformer varies varies due to product variations and temperature characteristics of the piezoelectric transformer and fluctuations in the input voltage. Timing can be substantially matched.

As a result, when a surge current flows from the input portion of the piezoelectric transformer to the field effect transistor or when the voltage of the input portion of the piezoelectric transformer is zero, the field effect transistor is turned off, and the parasitic diode of the field effect transistor is turned off. This eliminates the case where a forward current flows through the circuit.

Therefore, the loss of the field effect transistor can be reduced, and the efficiency can be improved. Further, since the maximum rated current of the field-effect transistor can be reduced, the size of the field-effect transistor can be reduced, and the size of the cold-cathode tube lighting device can be reduced. Furthermore, since a high-precision resonance coil or piezoelectric transformer or a step of selecting them is not required, the manufacturing cost can be reduced and the degree of freedom in design can be improved.

According to the cold-cathode tube lighting device of the second aspect, the control circuit supplies the drive circuit with a control signal for controlling the sum of the on-time and off-time of the field-effect transistor constituting the DC / AC inverter. Therefore, a battery (approximately 6 V) and an AC adapter (approximately 12 V) are used in combination as an input power source, so that even when the input voltage is different, it is possible to sufficiently cope without reducing the efficiency of the field effect transistor. .

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment according to a cold cathode tube lighting device of the present invention.

FIG. 2 is a block diagram of a control circuit constituting the cold cathode tube lighting device of FIG.

3 shows operations when the period of the single-wave sine-wave AC voltage generated by the DC / AC inverter of the cold-cathode tube lighting device of FIG. 1 is (a) longer and (b) shorter. FIG.

4A and 4B are diagrams illustrating an operation when an input voltage generated from an input power supply of the cold cathode tube lighting device of FIG. 1 is (a) increased and (b) is decreased, respectively.

FIG. 5 is a block diagram showing a conventional cold-cathode tube lighting device.

FIG. 6 shows a conventional cold-cathode tube lighting device of FIG.
FIG. 9 is a diagram illustrating an operation when the cycle of the single-wave sine wave AC voltage generated by the / AC inverter is (a) longer and (b) shorter.

7A and 7B are diagrams illustrating an operation when an input voltage generated from an input power source of the conventional cold-cathode tube lighting device of FIG. 5 is (a) increased and (b) is decreased, respectively.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Cold-cathode tube lighting device 11 Control circuit 1 Input power supply 2 DC / AC inverter 3 Drive circuit 4 Tube current detection circuit 5 Rectifier circuit 6 Error amplifier circuit 7 V / F conversion circuit FL Cold-cathode tube L Resonant coil Q Field effect transistor T Piezoelectric transformer Ta input section Vdc DC voltage V1 AC voltage Vref Reference voltage

Claims (2)

[Claims] 1. A DC / AC inverter comprising an input power supply, a resonance coil, a field effect transistor, and a piezoelectric transformer, controlling on / off of the field effect transistor to convert a DC voltage from the input power supply into an AC voltage. When,
A drive circuit for controlling on / off of the field-effect transistor, a cold-cathode tube lit by an output voltage of the piezoelectric transformer, a tube-current detection circuit for detecting a tube current flowing through the cold-cathode tube, A rectifier circuit for rectifying an AC voltage output from the circuit, an error amplifier circuit for amplifying a difference voltage between the output voltage of the rectifier circuit and the dimming voltage, and converting the output voltage of the error amplifier circuit into a frequency signal; A cold cathode tube lighting device comprising a V / F conversion circuit for supplying the frequency signal to the drive circuit, wherein the drive circuit detects a voltage of an input part of a piezoelectric transformer constituting the DC / AC inverter; A cold-cathode tube lighting device comprising: a control circuit having a function of supplying a control signal for turning on the field-effect transistor when a voltage of an input unit is equal to or lower than a reference voltage. 2. The cold cathode according to claim 1, wherein the control circuit has a function of supplying a control signal for controlling a sum of an on time and an off time of the field effect transistor to the drive circuit. Tube lighting device.