JPH10171405A - El element driving circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase luminance and also to prolong the life of an element by clamping the other end of the EL element at a fixed potential. SOLUTION: An EL panel driving circuit 1 is connected with a DC power source 2 and an EL panel 3, and generates a driving signal for driving an EL panel 3 from a DC voltage Vd supplied from the DC power source 2. A clamping circuit 1b is connected across a terminal T3, with which the other end of the EL element 3 is connected, and the ground, and clamps the other end of the EL element 3 at a fixed potential. The clamping circuit 1b comprises a diode D2 and a condenser C2 connected in parallel therewith, and the diode D2 is connected with an anode terminal T3 with which a cathode is connected. The EL element 3 is connected across the terminals T2, T3, and is charged in accordance with a driving signal supplied from the terminal 2 and emits light. The EL element 3 is equivalently composed of a series connection of a resistance ro, an induction element lo, and capacitance element eo, and an inverse direction voltage is generated by a counter electromotive force of the induction element lo.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an EL element driving circuit, and more particularly to an EL element driving circuit for supplying a DC driving voltage to an EL (electroluminescence) element.

[0002]

2. Description of the Related Art EL panels are often used as backlights for display devices such as electronic devices. First, EL
The panel will be described. FIG. 4 is a diagram illustrating the operation of the EL panel. FIG. 4A is a cross-sectional view of an EL panel, and FIG. 4B is a diagram for describing a method for driving the EL panel.

The EL panel 21 has a structure in which a light emitting layer 22 made of a phosphorous material having a light emitting property is sandwiched between a transparent electrode 23 and a back electrode 24. When the EL panel 21 is used as a backlight, it is more efficient to emit light from only one surface, so the back electrode 24 is made of a material that reflects light, such as an aluminum plate.

[0006] A driving voltage is applied between the transparent electrode 22 and the back electrode 24 by an EL panel driving circuit 25 as shown in FIG. The drive voltage supplied from the EL panel drive circuit 25 is applied to the light emitting layer 22. Light emitting layer 2
Reference numeral 2 denotes a configuration in which the phosphor 22b is mixed with the dielectric 2a,
When a driving voltage is applied, free electrons contained in the light emitting layer 22 are moved. When the free electrons of the light emitting layer 22 move,
The phosphor 2 b collides with the phosphor 22 b constituting the light emitting layer 22 and
Light is output from 2b.

At this time, the EL panel drive circuit 25
In order to extend the life of the L panel 21, an AC voltage was applied. FIG. 5 shows a circuit configuration diagram of an example of a conventional EL panel drive circuit. The conventional EL panel driving circuit 25 includes an oscillating circuit 26 for generating an oscillating signal of a predetermined frequency by a DC voltage Vcc, transistors Q11 and Q12, capacitors C11 and C12.
12, a transformer L11 and a resistor R11, and generates a sinusoidal AC voltage from the DC voltage Vcc.
Is applied. The EL panel drive circuit 25 has a DC voltage Vcc
From a so-called DC-AC inverter that generates an AC voltage from the inverter.

FIG. 6 is a diagram for explaining the operation of the conventional example shown in FIG. FIG. 6A is a diagram showing light emission efficiency, and FIG.
(B) shows a diagram illustrating characteristics of the driving voltage and the luminance with respect to time. The EL panel 21 has a property of emitting light in a direction according to the polarity of the applied voltage. For example, when a voltage is applied to the EL panel 21 with the transparent electrode 23 having a positive polarity and the rear electrode 24 having a negative polarity, the light emitting layer 22 emits light in the direction of the transparent electrode 23. When a voltage is applied to the EL panel 21 with the transparent electrode 23 having a negative polarity and the back electrode 24 having a positive polarity, the light emitting layer 22 emits light in the direction of the back electrode 24.

Light emitted in the direction of the transparent electrode 23 is directly emitted from the transparent electrode 23 and can be used with an efficiency of about 100%. Conversely, the light emitted in the direction of the back electrode 24 is once reflected by the back electrode 24 and then emitted from the transparent electrode 23, so that the brightness is attenuated and emitted with an efficiency of about 80%.

In the circuit shown in FIG. 5, a sinusoidal AC voltage as shown by a solid line in FIG. 6B is supplied to the EL panel 21, so that a voltage applied between the transparent electrode 23 and the back electrode 24 is applied. Are alternately inverted. In the light emitting layer 22, light is emitted with a slight delay with respect to the voltage.

Therefore, the brightness of the light emitted from the EL panel 21 is about 100% when the applied voltage is positive.
And the efficiency is about 80 when the applied voltage is negative.
%, And the light is emitted with a slight delay from the applied voltage, so that the characteristics are as shown by the broken line in FIG. 6B. As shown by the broken line in FIG.
The light efficiency is about 100 per cycle of the driving voltage.
% And about 80% alternately, so that the emission of the EL panel 21 appears to flicker or the brightness appears to be dark. In order to improve this, a method of driving in a DC manner by a DC-DC inverter has been proposed.

Next, an EL using a DC-DC converter
A method for driving the panel 21 will be described. FIG. 7 shows a circuit configuration diagram of another conventional example. The EL panel driving circuit 27 shown in FIG. 7 is connected to a DC power supply 28 and the EL panel 21 and converts the DC voltage Vd supplied from the DC power supply 28 into an EL.
A drive signal that changes in DC for driving the panel 21 is generated and supplied to the EL panel 21.

The DC voltage Vd from the DC power supply 28 is supplied to a power supply terminal T21. The power supply terminal T21 is connected to one end of the coil L21 and the power supply terminal of the first oscillation circuit 29,
The DC voltage Vd from the DC power supply 28 is applied to the coil L21 and the first
Is supplied to the oscillation circuit 29 of FIG.

The first oscillating circuit 29 generates a first oscillating signal having a frequency of 10 kHz using the DC voltage Vd from the DC power supply 28. The first oscillation signal generated by the first oscillation circuit 28 is supplied to the base of the first transistor Q21. The first transistor Q21 has the collector L
21 is connected to the other end, the emitter is grounded, and the first
The switching is performed in response to the oscillating signal to supply a current to the coil L21.

The DC voltage Vd from the DC power supply 28
Are supplied to the second oscillation circuit 30 via the first oscillation circuit 29. The second oscillation circuit 30 has a DC voltage Vd
Generates a second oscillation signal having a frequency of 10 Hz.
The second oscillation signal generated by the second oscillation circuit 30 is supplied to the base of the second transistor Q22. The second transistor Q22 has an emitter grounded and a collector connected to the first transistor Q21 and the coil L via a diode D21.
21 and to the output terminal T22. The second transistor Q22 is connected to the second oscillation circuit 3
Switching is performed according to the second oscillation signal generated at 0, and the electric charge accumulated in the EL element 21 is discharged. At this time, the capacitor C21 is connected between the terminal T22 for protecting the EL element 21 and the ground.

The diode D21 has an anode connected to a coil L21.
Is connected to a connection point between the collector of the first transistor Q21 and a collector of the first transistor Q21, and a cathode is connected to a connection point between the collector of the first transistor Q21 and the output terminal T22. To prevent backflow.

FIG. 8 is a diagram for explaining the operation of another example of the prior art shown in FIG. FIG. 8A shows a state of light emission of the EL panel 21 by the EL panel drive circuit 27, and FIG.
FIG. 4 is a diagram showing characteristics of a driving voltage and a luminance with respect to time. In the EL panel drive circuit 27 shown in FIG. 7, the transistor Q21 is switched by the first oscillation signal generated by the first oscillation circuit 29, and a current of a predetermined frequency (20 kHz) is drawn into the coil L21. Coil L21
Accumulates energy according to the fluctuation of the current and supplies the energy to the EL panel 21 via the diode D21. The EL panel 21 holds charges by the energy of the coil L21, and gradually holds the charges.

On the other hand, the second oscillating circuit 30 generates a second oscillating signal (frequency 70) having a sufficiently longer period than the first oscillating signal.
Hz) is supplied to the second transistor Q22.
Therefore, the second transistor Q22 is connected to the EL panel 2
1, the electric charge is held by the coil L21, and the EL panel 21
Is turned on when the potential of the
Discharges the charge stored in the

As described above, a driving voltage having a waveform that changes in a DC manner as shown by a solid line in FIG. 8B is supplied to the EL panel 21. The driving voltage shown by a solid line in FIG. 8B supplied to the EL panel 21 always has a positive polarity on the transparent electrode 23 and a negative polarity on the back electrode 24 of the EL panel 21. Voltage is applied in the direction. Therefore, as shown in FIG. 8A, light is emitted from the light emitting layer 22 toward the transparent electrode 23.

At this time, since the emitted light is emitted with a delay with respect to the drive voltage, as shown by a broken line in FIG. Light is emitted according to the frequency.

[0019]

SUMMARY OF THE INVENTION However, the conventional EL
Among the panel drive circuits, the circuit shown in FIG. 5 is an alternating current (AC)
Since the driving is performed, the light emitted in the opposite direction according to the polarity of the driving voltage as shown in FIG. 6A, and therefore, when the driving voltage becomes positive as shown in FIG. In addition, when the driving voltage becomes negative, the brightness varies, and there is a problem that the light emission is recognized as flickering and the brightness is reduced.

Since the circuit shown in FIG. 7 is driven by direct current (DC), the polarity of the driving voltage is always in one direction as shown in FIG. Is applied, the light-emitting layer 22 is likely to be subjected to electrostatic stress, and the lifetime of the EL panel 21 is shortened.

[0021] The present invention has been made in view of the above-mentioned points, and has a high luminance of light emitted from an EL element,
An object of the present invention is to provide an EL element drive circuit that can extend the life of an element.

[0022]

According to the present invention, there is provided a semiconductor device comprising:
In an EL element driving circuit having a driving signal generating means for supplying a driving signal oscillating in a DC manner to one end of an L element and causing the EL element to emit light, a clamping means for clamping the other end of the EL element to a predetermined potential is provided. It is characterized by having.

According to the first aspect, the voltage applied to the EL element can be shifted to the negative side by a predetermined potential by clamping the other end of the EL element, and the period ineffective by DC driving is shifted to the negative side. Can be driven. Therefore, the EL element can be driven not in one polarity but in both polarities, so that the life of the EL element can be extended. In addition, since the driving is basically performed by DC, the flicker is small. However, the driving can be performed during the ineffective period, so that the flicker can be further reduced.

According to a second aspect of the present invention, the clamping means comprises the EL
The device is characterized in that it has a diode having an anode connected to the other end of the element and a cathode grounded, and a capacitor connected in parallel to the diode. According to a third aspect of the present invention, the other end of the EL element is connected to a DC voltage source for supplying a DC voltage to the drive signal generating means, and the clamp is clamped to a predetermined potential by the DC voltage supplied from the DC power supply. It is characterized by the following.

According to the third aspect, the other end of the EL element is connected to a DC power supply serving as a power supply for a drive signal, and the other end of the EL element is clamped to a predetermined potential, so that it is not necessary to separately provide a clamping means. Therefore, the circuit can be easily configured.

[0026]

FIG. 1 is a circuit diagram of an embodiment of the present invention. This embodiment corresponds to claims 1 and 2 of the claims.
This is equivalent to the embodiment of FIG. EL panel drive circuit 1 of the present embodiment
Is connected to the DC power supply 2 and the EL panel 3, generates a drive signal for driving the EL panel 3 from the DC voltage Vd supplied from the DC power supply 2, and supplies the drive signal to the EL panel 3.

The EL panel driving circuit 1 comprises a driving signal generating circuit 1a corresponding to the driving signal generating means in the claims and a clamp circuit 1b corresponding to the clamping means in the claims. The DC voltage Vd from the DC power supply 2 is supplied to the power supply terminal T1 to the drive signal generation circuit 1a. The power supply terminal T1 is connected to one end of the coil L1 and the power supply terminal of the first oscillation circuit 4, and the DC voltage Vd from the DC power supply 2 is supplied to the coil L1 and the first oscillation circuit 4.

The first oscillating circuit 4 generates a first oscillating signal having a frequency of 10 kHz using the DC voltage Vd from the DC power supply 2. The first oscillation signal generated by the first oscillation circuit 4 is supplied to the base of the first transistor Q1.
The first transistor Q1 has a collector connected to the other end of the coil L1 and an emitter grounded, and switches according to a first oscillation signal to supply a current to the coil L1.

The DC voltage Vd from the DC power supply 2 is
The signal is supplied to the second oscillation circuit 5 via the first oscillation circuit 4. The second oscillating circuit 5 generates a second oscillating signal having a frequency of 10 Hz from the DC voltage Vd. The second oscillation signal generated by the second oscillation circuit 5 is supplied to the base of the second transistor Q2. Second transistor Q
2 has an emitter grounded, a collector connected to a connection point between the first transistor Q1 and the coil L1 via a diode D1, and also connected to an output terminal T2. Second
Transistor Q2 is switched in response to the second oscillation signal generated by the second oscillation circuit 5, and the EL element 3
The electric charge stored in the battery is discharged. At this time, the EL element 3
A capacitor C is connected between the terminal T2 for protecting
1 is connected.

The diode D1 has an anode connected to the coil L1.
Is connected to the connection point between the collector of the first transistor Q1 and the output terminal T2, and the cathode is connected to the connection point between the collector of the first transistor Q1 and the output terminal T2 from the EL element 3 side of the energy stored in the coil L1. To prevent backflow.

The clamp circuit 1b is connected between the terminal T3 to which the other end of the EL element 3 is connected and the ground, and clamps the other end of the EL element 3 to a predetermined potential. Clamp circuit 3
Has a diode D2 and a capacitor C2 connected in parallel. The diode D2 has an anode connected to the terminal T3 and a cathode grounded.

The EL element 3 is connected between the terminal T2 and the terminal T3, and is charged and emits light in accordance with a drive signal supplied from the terminal T2. The EL element 3 is equivalently a resistor r0,
An inductive element 10 and a capacitive element c0 are connected in series, and a reverse voltage is generated by the back electromotive force of the inductive element 10.

FIG. 2 shows an operation waveform diagram of one embodiment of the present invention. 2A shows the operation waveform of the terminal T3, FIG. 2B shows the operation waveform of the terminal T2, and FIG. 2C shows the operation waveform of the voltage across the EL element 3. In the period T1 in FIG. 2, the first transistor Q1 is switched by the first oscillation signal (frequency 20 kHz) from the first oscillation circuit 4. At this time, the second transistor Q2 is switched by the second oscillation signal (frequency 70 Hz) from the second oscillation circuit 5, becomes low level during the period T1, and the second transistor Q2 is turned off.

For this reason, in the period T1, energy is stored in the coil L1 in accordance with the switching of the first transistor Q1, and is supplied to the EL element 3 via the diode D1. In the EL element 3, the energy stored in the coil L1 is supplied each time the first transistor Q1 is switched, and the potential gradually increases as shown in FIG.

When the second transistor Q2 is turned on at time t1, the charge generated by the switching of the first transistor Q1 is discharged to the coil L1 to the ground, and is not supplied to the EL element 3. The electric charge stored in the EL element 3 is also discharged to ground via the second transistor Q2.

Therefore, the potential of the terminal T2 shown in FIG. 2B drops to the ground level. At this time, the clamp circuit 1
The electric charge held in the capacitor C2 is discharged, and the current flows in the opposite direction due to the diffusion capacitance of the diode D2, so that the potential V2 of the terminal T2 decreases. Therefore, in the period T2, a current flows in the EL element 3 in a direction opposite to that in the period T1 and is driven.

At this time, the capacitance of the capacitor C2 constituting the clamp circuit 1b needs to be set sufficiently smaller than the capacitance element c0 which is the capacitance component of the EL element 3.
This is because the electric charge q2 stored in the capacitor C2 constituting the clamp circuit 1b becomes q2 = C2 × V3 when the voltage of the terminal T3 is V3, and the electric charge q0 stored in the EL element 3 is the terminal T2 Assuming that the voltage is V2, q0 = c0.times. (V2-V3).

Here, since the voltage V3 at the terminal T3 is clamped by the forward voltage VF of the diode D2, about 0.6 [V], the voltage V3 is accumulated in the EL element 3 in order to drive the EL element 3 efficiently. It is necessary to make the electric charge q0 sufficiently large, that is, q0> q2. To this end, the capacitance of the capacitor C2 of the clamp circuit 1b may be set sufficiently smaller than the capacitance c0 of the EL element 3.

For example, if the capacitance element c0 of the EL element 3 is 50
In the case of 00 pF, the capacitance of the capacitor C2 of the clamp circuit 1b may be set to about 1000 pF. As described above, according to the present embodiment, light emission can be performed with voltages in directions opposite to each other in both the period T1 during which the EL element emits light by the DC method and the period T2 which is the discharge period.
The life of the L element 3 can be extended. In addition, since light can be emitted even during a period in which the light emitting device is ineffective, the luminance can be increased.

FIG. 3 shows a circuit configuration diagram of another embodiment of the present invention. In the figure, the same components as those of FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. The present embodiment has a configuration in which the other end of the EL element 3 is clamped by the DC power supply 2 for generating a drive signal.

In the EL panel drive circuit 1 of this embodiment, the clamp circuit 1b is omitted, and the terminal T13 for connecting the other end of the EL element 3 is connected to the terminal T1 to which the positive side of the DC power supply 2 is connected.
Connect to. According to the present embodiment, the other end of the EL element 3 is clamped by the DC power supply 2 by the DC voltage Vd supplied from the DC power supply 2, so that the second transistor Q2 is turned on, and one end of the EL element 3 is closed. At the ground GND level, the other end of the EL element 3 is clamped to the DC voltage Vd by the DC power supply 2, so that a voltage is applied to the EL element 3 from the terminal T13 to the terminal T2.

For this reason, a voltage is applied to the EL element 3 in a direction opposite to that when the second transistor Q2 is turned off, and light is emitted. Therefore, as in the embodiment of FIG. 1, light can be emitted at voltages in mutually opposite directions in both the period T1 during which the EL element emits light by the conventional DC method and the period T2 which is the discharge period. The life of the EL element 3 can be extended. In addition, since light can be emitted even during a period in which the light emitting device is ineffective, the luminance can be increased.

[0043]

As described above, according to the first and second aspects of the present invention, the voltage applied to the EL element can be shifted to the negative side by a predetermined potential by clamping the other end of the EL element. The period that was invalid by driving can be driven to the negative side, and E
Since the L element can be driven not in one polarity but in both polarities,
The life of the L element can be extended, and the flicker is small because it is basically driven by a direct current. However, it can be driven even during the ineffective period, so that the flicker can be further reduced.

According to the third aspect, the other end of the EL element is connected to a DC power supply serving as a power supply of a drive signal, and the other end of the EL element is clamped to a predetermined potential, so that it is not necessary to separately provide a clamping means. Therefore, it has features such as a simple circuit configuration.

[Brief description of the drawings]

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

FIG. 2 is an operation explanatory diagram of one embodiment of the present invention.

FIG. 3 is a circuit configuration diagram of another embodiment of the present invention.

FIG. 4 is an explanatory diagram of an EL panel.

FIG. 5 is a circuit configuration diagram of an example of the related art.

FIG. 6 is a diagram illustrating an operation of an example of the related art.

FIG. 7 is a circuit configuration diagram of another example of the related art.

FIG. 8 is an operation explanatory diagram of another example of the related art.

[Explanation of symbols]

 1, 11 EL panel drive circuit 1a Drive signal generation circuit 1b Clamp circuit 2 DC power supply 3 EL panel 4, 5 Oscillation circuit

Claims (3)

[Claims] 1. An EL element driving circuit having a driving signal generating means for supplying a driving signal vibrating in a direct current to one end of an EL element and causing the EL element to emit light, wherein the other end of the EL element is set to a predetermined potential. An EL element driving circuit, comprising: a clamping means for clamping. 2. The device according to claim 1, wherein the clamping means includes a diode having an anode connected to the other end of the EL element and a cathode grounded, and a capacitor connected in parallel to the diode. The EL element driving circuit according to any one of the preceding claims. 3. The clamp means connects the other end of the EL element to a DC voltage source that supplies a DC voltage to the drive signal generation means, and clamps the EL element to a predetermined potential by a DC voltage supplied from the DC power supply. 2. The EL element driving circuit according to claim 1, wherein: