JPS5838997A - Driving circuit for thin film el display - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] The present invention uses EL (Electr) by applying an alternating electric field.

The present invention relates to a drive circuit for a thin film EL display device that emits light (luminescence).

Conventionally, with regard to thin film EL elements used as display bodies of display devices, ZnS, Zn5, etc., in which the light emitting layer is regularly doped with high AC type Mn (or Cu, A, /, Br, etc.)
Three-layer structure ZnS:Mn (or Z
n5e:Mn) EL elements have been developed, and improvements in various light-emitting characteristics have been confirmed. This thin film EL element is several KH
It emits light with high brightness when an alternating current electric field of z is applied, and has a long lifespan.

FIG. 1 shows the basic structure of a ZnS:Mn thin film EL device as an example of a thin film EL device.

The structure of the thin film EL element will be explained in detail based on FIG.
A first dielectric layer 8 made of Al2O3, 5iBN4.5i02, etc. is formed in an overlapping manner by sputtering, electron beam evaporation, or the like. ZnS on the first dielectric layer 3:
A ZnS light emitting layer 4 obtained by electron beam evaporation of Mn sintered pellets is formed. At this time, the ZnS:Mn sintered pellets used for vapor deposition are pellets in which Mn, which is an active substance, is immersed and fixed at a concentration of 4° depending on the purpose. A second dielectric layer 5 made of the same material as the first dielectric layer 3 is laminated on the ZnS light emitting layer 4, and a back electrode 6 made of A/ or the like is further deposited thereon. Transparent electrode 2 and back electrode 6
is connected to an AC power source 7, and the thin film EL element is driven.

When an AC voltage is applied between the electrodes 2 and 6, the ZnS light emitting layer 4
The above AC voltage is induced between the dielectric layers 3 and 5 on both sides of the layer 4, so that Zn≦electrons are excited and accelerated to the conduction band by the electric field generated in the light-emitting layer 4 and have sufficient energy. The electrons become free electrons and are attracted to the interface of the light-emitting layer, where they are accumulated and form internal polarization. At this time, the free electrons moving at high speed directly excite the Mn luminescent center, and when the excited Mn luminescent center returns to the ground state, it emits yellow-orange light. That is, in the process of free electrons accelerated by a high alternating current electric field moving from one interface of the light emitting layer to the other interface, electrons of Mn atoms entering the Zn site, which is a light emitting center in the ZnS light emitting layer 4, are ejected. When a rare earth fluoride other than Mn is used as the active substance, green and other luminescent colors characteristic of this rare earth element can be obtained.

The thin film EL element having the structure described above can be used as a flat thin display device that takes advantage of the space factor to display characters, symbols, still images, etc. in computer output display terminal equipment and various other display devices containing characters and figures. It can be used as a means of displaying moving images, etc.

Thin-film EL panels as flat flat display devices have a lower operating voltage than conventional cathode ray tubes (CRTs), and are superior in terms of weight and strength compared to plasma display panels (FDPs), which are also flat display devices. It has a wider operating temperature range than liquid crystal display (LCD).
It has many advantages such as fast response speed. In addition, since it can be used as a pure solid matrix type panel, it has a long operating life, and its address accuracy makes it very effective as a means of displaying the number of people in computers, etc.

The conventional thin film EL element can be viewed as a capacitive element that operates like a capacitor. By the way, this thin film EL element has a very high driving voltage of about 200 V and a large capacitance of about 6 nF/ai. Therefore, when calculating the power consumption in driving a light emitting display, even if the power involved in light emission is omitted and the power used to simply charge and discharge a capacitor is regarded as the amount of power consumed, there is not much difference from the power actually consumed. Therefore, considering the thin film EL element as a simple capacitor C, the amount of power required to charge and discharge the voltage vO once is determined. First, we will simplify the charging and discharging operation in the conventional driving force and method.
As shown in the figure. Turn off switch S2, turn off switch Sl
When the capacitor C is charged with the voltage vO through the resistor R by setting the capacitance C to N, the following equation holds true.

Rewriting equation (1) using charge q, we get R+1q-E (2)
The general solution to the equation for di is well known. (However, it is assumed that q=O when 1=0.) That is, the electric power amounts WR and WC in the resistance R and the capacitance C are respectively calculated from the following equations.

where -, Equation (6) and Equation (6) indicate the following values.

l 2 ・・・・・・・・・(7) WR=
Wc-TCV.

This shows that the remaining amount was consumed in R and stored in capacitor C. Furthermore, when the energy stored in the capacitor C is discharged by turning off the switch S1 and turning on the switch S2, it is completely consumed in the resistor R. Therefore, it is difficult to charge and discharge the voltage VO to the capacitor C in the conventional method. It is clear that the total power consumption required for is Cvo.

FIG. 3 is a circuit diagram showing the configuration of a drive circuit in a conventional thin film EL display device. 4 is a diagram of voltage waveforms input to each terminal of the drive circuit and the thin film EL element 8 shown in FIG. 3. Each terminal INI, IN2 of the drive circuit is supplied with the power supply voltage vO.

IN8. By applying a pulse voltage to IN4 at the timing shown in FIG. 4, the base potential of the transistor is changed and switching is performed, and an alternating pulse electric field is applied to the thin film EL element 8, causing it to see-saw. El, luminescence is obtained. That is, terminals INI and IN
When a pulse is applied to transistors Trl and Tr
4 becomes conductive, and the thin film EL from the T-th transistor
A current flows in the direction of transistor Tr4 through element 8,
The thin film EL element 8 enters a charged state. Terminal IN in the next period
When a pulse is applied only to transistor TR2, the transistor TR2 becomes conductive, and the charge in the thin film EL element 8 is discharged. Next, when a pulse is applied to the terminals (,p',2. is in a charged state with a polarity opposite to that described above. In the next period, when a pulse is applied only to the terminal IN4, the transistor Tr4 becomes conductive, and the charge in the thin film EL element 8 is discharged.

The thin film EL element 8 is driven with alternating current by applying the above-mentioned pulse voltage, and an EL light emission pattern is obtained.

The object of the present invention is to establish a new and useful method for driving a thin film EL display device that can reduce the power consumption for display driving by making full use of technical means, and to provide a driving circuit for the method. be.

Hereinafter, the present invention will be explained in detail according to embodiments with reference to the drawings.

FIG. 5 is a simplified configuration diagram of a circuit explaining one embodiment of the basic operation of the present invention.

Hereinafter, one embodiment of the present invention will be described based on FIG.

The switches fs, , S2 are turned off, the switch S8 is turned on, and the capacitor C (thin film EL element) is charged via the resistor R with the power source KV, (0<K<1). Next, switch s2. Turn off s8, turn on switch St, and set capacitance C with power source Vo.
to charge. Hereinafter, this charging method will be referred to as a step drive method. At the time of discharging, only the switch S2 is ONL discharged as in the conventional method.

Next, the amount of power required for charging and discharging using this step drive method is determined as follows.

The amount of electric power in the resistor R and capacitor C due to charging from the power source KVQ can be determined from equation (7) as follows.

tt'j 2 ・・・・・・・・・(
8) WR = Wc = T C (KVo) Next, the amount of electricity when charging the capacitor C from the power source Vo is (2
) and q when t=0. Since = CKVo, it becomes... (10).

When -ω, equations (9) and (10) show the following values.

W'R=-LC(]-K)2VO' ---
----・+Il1wlc=C(1-K)VO2-UC(1-K)"Vo' 02) Therefore, the resistance R2 capacity during charging by the step drive method. The total of each electric energy in one amount C WR5+ wcs shows the following value from equation (8) 2〇l) 2〇2).

WR5=-LCK2VO"+-LC(1-K)2VO'
−・−・−・H2 2 Wcs=iCVo・・・・・・・・・04) Furthermore, (1
4) The energy stored in the capacitor C shown by the formula is completely consumed by the resistor R during discharge. Therefore, in the step drive method, the power consumption Ws required to charge and discharge the voltage vo to the capacitor C takes the following value from equation (131 equation 04).

WS "WR8 + w (5 05) The relationship between the power consumption Ws and the parameter is the sixth
As shown in the figure.

The dashed line P1 in the figure is the conventional driving method, and the curve P1
2 corresponds to the step driving method of this embodiment.

As is clear from FIG. 6, when =T, the power consumption Ws takes the minimum value, and compared to the conventional method, the power consumption becomes 7.

Furthermore, when a thin film EL display device is driven by the step driving method, experimental results are obtained that are in good agreement with the above principle.

FIG. 7 is a circuit configuration diagram in which a drive circuit for realizing the step drive method is constructed based on the circuit configuration of FIG. 3. Figure 8 shows each terminal and thin film E of the drive circuit shown in Figure 72.
3 is a voltage waveform diagram of a pulse voltage input to the L element 8. FIG.

Terminals INI', IN2. A pulse voltage is applied to IN3 and IN4 as in FIG. 4. The step drive method uses terminal I
This is done by a pulse applied to the terminal N5, and the rise of the drive pulse applied to the thin film EL element 8 rises in two steps in synchronization with the rise of the pulse applied to the terminal IN5. In addition, the thin film EL element 8 is charged and discharged in the same manner as before by selecting the heavy charge terminals IN2' and IN4' and applying a pulse voltage. By alternately conducting TrB, the thin film EL element 8 is driven with alternating current, and the thin film EL element 8
By making the transistor Tr5 conductive in the middle of the rise of the forward and reverse pulses applied to, a drive method in which a step drive method is superimposed on a seesaw drive method is established.

By the way, the drawback of this step drive method is that the external power supply is
Two power supplies, VQ and V□, are required, which impairs the convenience of the device.

In addition, in FIG. 2 where a single power supply with voltage vo is used as an external power supply, switch S2 is set to 0FFL, switch S
When the voltage across the capacitor C reaches KV, the switch S1 is set to 0FFL, and then the switch Sl is turned on again to charge the capacitor C until the voltage across the capacitor C reaches Vo.The applied waveform of the capacitor C is a step. You can get something like this. However, in this case the resistance R and the capacitance C
It is clear from the following that the total amount of electric power during charging is the same as the value shown in equation (7). For simplicity, the amount of electricity W at the resistance R and capacitance C when vo is reached is
To find R and Wc, from equation (3), (1B1 equation (5)
Substitute into equation (6) and get 3 2...
...... (17) wR-100 CVO -12... (18) Wc-8Cv.

Find out.

' 1 2 ・・・・・・・・・ (
19) WR=HCVO' 32 ・・・・・・・・・ Shi0
) Wc-TCv.

(1?), (+8+, (19), (20j
From the formula, the total Cvo is obtained, and the power consumption is not reduced.

Therefore, in this embodiment, the driving circuit is constructed by improving this point.

FIG. 9 is a configuration diagram of a drive circuit for a thin film EL display device showing one embodiment of the present invention. FIG. 10 is a timing waveform diagram of the voltage waveform input to FIG. 9.

1 corresponding to K −/2 of the voltage value KVQ as an external power supply
A power supply having a voltage value of /2■o is used. terminal INA
A pulse is applied to and IND to make the transistors TrA and TrD, whose paste ends are connected to each other, conductive, and a voltage of /2×0 is applied to the thin film EL element 8. Subsequently, by applying a pulse to the terminal INH, the transistor TrE
is made conductive, a double voltage power source is formed by coupling with the capacitor Co, and a voltage of vO is applied to the thin film EL element 8. As a result, the thin film EL element 8 has 1/2vO9 in two stages.
A voltage of VO is continuously applied to obtain EL light emission.

Next, a pulse is applied to the terminal INB, and the transistor T r
B is made conductive, and the charge in the thin film EL element 8 is discharged. Further, a pulse is applied to the terminal INF to make the transistor Trp, which grounds the capacitor Co, conductive.

Further, a voltage () is applied to the terminals INB and INC to pass the transistors TrB and TrcG-, a voltage '/2Vo of the opposite polarity to the above is applied to the thin film EL element 8, and a pulse is subsequently applied to the terminal INE. The transistor TrE is made conductive, and a double voltage vO superimposed on the charge charged in the capacitor CO is applied. Therefore, the thin film EL element 8 emits light again upon application of this reverse polarity pulse voltage. Next, a pulse is applied to the terminal IND to make the transistor TrD conductive and the thin film EL element 8 discharged, and a pulse is applied to the terminal INF to make the transistor Trp conductive and the capacitor CO is grounded. By repeating the above operations, the thin film EL element 8 is driven for light emitting display by superimposing seesaw drive and step drive using a single power source.

As explained in detail above, the present invention utilizes the fact that the thin film EL element is a capacitive element to reduce the driving power during display, and is a very effective technique as a drive circuit for a thin film EL display device.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the basic structure of a thin film EL element. FIG. 2 is an explanatory diagram illustrating charging and discharging operations in a conventional driving method. FIG. 3 is a circuit diagram showing the configuration of a drive circuit in a conventional thin film EL display device. FIG. 4 is a timing waveform diagram showing voltage waveforms input to the drive circuit shown in FIG. 3. FIG. FIG. 5 is a simplified configuration diagram of a circuit illustrating an embodiment of the basic operation of the present invention. FIG. 6 is an explanatory diagram for comparing and explaining the power consumption in the conventional driving method and the driving method of the present invention. FIG. 7 is a configuration diagram of a drive circuit of a thin film EL display device for realizing step drive. FIG. 8 is a timing waveform diagram showing voltage waveforms input to the drive circuit shown in FIG. 7. FIG. 9 is a configuration diagram of a drive circuit for a thin film EL display device showing one embodiment of the present invention. FIG. 10 is a timing waveform diagram of the voltage waveform input to FIG. 9. 8... Thin film EL element, Co... Capacitor, INA
. INB, INC, IND, INE, INF-... terminal,
Tr A + TrB ITrc, Trp, Tr
E, Trp-transistor. Figure 1 ρ Figure 3 N1 Eight meter K Figure 7 Figure 8 Figure 9

Claims (1)

[Claims] 1. Thin film EL that emits EL light in response to voltage application
In the drive circuit of the display device, a power supply having a voltage value of KVQ, which is twice (0<K<1) as the drive voltage value Vo for obtaining EL light emission, is connected to the power supply voltage to the thin film EL display device. 1. A drive circuit for a thin film EL display device, comprising a switch circuit that continuously applies a superimposed voltage in which charged charges of a power supply voltage are added to the power supply voltage in a step-like manner.