JPH0245197B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention utilizes EL (Electro
This invention relates to a method for driving a thin film EL display device that emits light (luminescence).

Thin films conventionally used as display bodies in display devices
Regarding EL elements, a high alternating current electric field (about 10 ⁶ V/cm) is regularly applied to the light emitting layer, and in order to improve dielectric strength, luminous efficiency, and operation stability, etc.
A semiconductor light-emitting layer made of ZnS, ZnSe, etc. doped with 2.0 Wt% Mn (or Cu, Al, Br, etc.) is made of Y ₂ O ₃ ,
A three-layer structure ZnS:Mn (or ZnSe:Mn) EL device sandwiched with a dielectric thin film such as TiO ₂ has been developed, and improvements in various light-emitting properties have been confirmed. This thin film EL element emits high-intensity light when an alternating current electric field of several KHz is applied, and has a long lifespan.

As an example of a thin film EL device, the basic structure of a ZnS:Mn thin film EL device is shown in FIG.

The structure of a thin film EL element will be explained in detail based on FIG. 1. A transparent electrode 2 made of 1n ₂ O ₃ , SnO ₂ , etc. is placed on a glass substrate 1, and Y ₂ O ₃ , etc.
A first dielectric layer 3 made of TiO ₂ , Al ₂ O ₃ , Si ₃ N ₄ , SiO ₂ or the like is formed in an overlapping manner by sputtering or electron beam evaporation. A ZnS light emitting layer 4 is formed on the first dielectric layer 3 by electron beam evaporation of ZnS:Mn sintered pellets. At this time, the ZnS:Mn sintered pellets used for deposition are pellets in which the concentration of Mn, which is an active substance, is set to suit 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 Al or the like is further deposited thereon. The transparent electrode 2 and the back electrode 6 are 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 above AC voltage is induced between the dielectric layers 3 and 5 on both sides of the ZnS luminescent layer 4, and therefore the electric field generated within the ZnS luminescent layer 4 Electrons that are excited and accelerated into the conduction band and have acquired sufficient energy 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 Nm emission center,
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 luminescent layer to the other interface, they excite the electrons of the Mn atoms that have entered the Zn site, which is the luminescence center, in the ZnS luminescent layer 4, returning them to the ground state. When it falls, it emits strong EL light in a wide wavelength range with a peak of approximately 5850 Å. When a rare earth fluoride other than Mn is used as an 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, moving images, etc. It can be used as a means of displaying 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 (PDPs), which are flat display devices. It has many advantages over liquid crystals (LCDs), such as a wider operating temperature range and faster response speed. Furthermore, 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 an input/output display means for computers and the like.

The conventional thin film EL element described above 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 200V, and its capacity is about 200V.
It exhibits a large value of ^about 6nF/cm2. 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 consumed by simply charging and discharging a capacitor is regarded as the power consumption, there is not much difference from the power actually consumed. Therefore, the above thin film EL
Considering the element as a simple capacitor C, the voltage V ₀ is 1
Find the amount of electricity required to charge and discharge twice. First, FIG. 2 shows a simplified charging/discharging operation in a conventional driving method. Switch S ₂
OFF, capacity C by turning on switch _S1
When charging with voltage V ₀ through resistor R, the following equation holds true.

Ri+1/C∫idt=E...(1) If we rewrite equation (1) using charge q, Rdq/dt+1/Cq=E...(2) becomes.

The general solution to this equation is well known. (however,
Consider q=o at t=o. ) That is, q=CV ₀ (1-e ^-1/CRt ) ...(3) i=dq/dt=V/Re ^-t/CR ...(4) Electric energy in resistance R and capacitance C W _R , W _C is calculated from the following formulas.

W _R =∫ ^t _p i ² Rdt=1/2CV ₀ ² (1-e ^-2/CRt ) ...(5) W _C =∫ ^t _p Vidt=1/2CV ₀ ² (1-e ^-1/CRt ) ² ...(6) At t→∞, equations (5) and (6) show the following values.

W _R = W _C = 1/2CV ₀ ² ...(7) Equation (7) means that 1/2 of the energy supplied from the power supply is consumed in the resistor R, and the remaining 1/2 is stored in the capacitor C. It shows. In addition, the energy accumulated in capacitor C can be released by turning off switch _S1 and turning off switch _S2.
When it is discharged by turning it on, it is all consumed by the resistor R. Therefore, in the conventional method, the capacity C
It is clear that the power consumption required to charge and discharge the voltage V ₀ is the total voltage V ₀ ² .

FIG. 3 is a circuit diagram showing the configuration of a drive circuit in a conventional thin film EL display device. In addition, FIG. 4 shows each terminal of the drive circuit shown in FIG. 3 and the thin film EL element 8.
FIG.

Each terminal IN of the circuit supplied with the power supply voltage V ₀
By applying a pulse voltage to 1, IN2, IN3, and IN4 at the timing shown in FIG. It will be driven,
EL emission can be obtained. That is, when a pulse is applied to terminals IN1 and IN4, transistors Tr ₁ and Tr ₄
becomes conductive, and the _thin film EL
A current flows in the direction of the transistor Tr ₄ through the element 8, and the thin film EL element 8 enters a charged state. In the next period, when a pulse is applied only to the terminal IN2, 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 IN2 and IN3, the transistors Tr ₂ and Tr ₃ become conductive, and the thin film EL element 8 is connected to the transistor Tr ₃ .
A current flows in the direction of the transistor Tr ₂ through the transistor Tr 2 , and the thin film EL element 8 enters 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.

An object of the present invention is to provide a new and useful method for driving a thin film EL display device that can reduce the power consumption for display driving by driving technical means.

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.

Switches S ₁ and S ₂ are turned OFF, switch S ₃ is turned ON, and a capacitor C (thin film EL element) is charged via a resistor R with a power source WV ₀ (O<K<1). Next, switch S ₂ ,
Turn S ₃ OFF, turn S ₁ ON, and capacitance C with power supply V ₀ .
to charge. Hereinafter, this charging method will be referred to as the step drive method. During discharging, only switch _S2 is turned on to discharge, as in the conventional method. Next, the amount of power required for charging and discharging using this step drive method is calculated as follows.

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

W _R ′ = W _C ′ = 1/2C (KV ₀ ) ² ...(8) Next, the amount of electricity when charging the capacitor C from the power source V ₀ is calculated using equation (2) and when t = o, q ₀ = Since CKV is ₀ , W _R = V ₀ ² (1-K) ² /R∫ ^t _p e ^-2/RCt dt ...(9) W _C ″=V ₀ ² (1-K)/R∫ ^t _p e ^-t/CR {1 - (1 - K) e ^-t/CR } dt ...(10).

At t→∞, equations (9) and (10) show the following values.

W _R ″=1/2C(1-K) ² V ₀ ² …(11) W _C ″=C(1-K)V ₀ ² −1/2C(1-K) ² V ₀ ² …( 12) Therefore, the resistance R during charging by the step drive method,
The total amount of electric power in capacity C, W _RS and W _CS , is
From equations (8), (11), and (12), the following values are shown.

W _RS = 1/2CK ² V ₀ ² + 1/2C (1-K) ² V ₀ ² ... (13) W _CS = 1/2CV ₀ ² ... (14) Note that the capacitance C shown in equation (14) The energy stored in is completely consumed by the resistor R during discharge. Therefore, in the step drive method, the power consumption W _S required to charge and discharge the voltage V ₀ to the capacitor C is expressed by Equations (13) and (14).
Take the following value from the formula.

W _S = W _RS + W _CS = {(K-1/2) ² + 3/4} CV ₀ ² ... (15) The relationship between the power consumption W _S in equation (15) and the parameter K is shown in Figure 6. .

The dashed line _P1 in the figure corresponds to the conventional driving method, and the curve _P2 corresponds to the step driving method of this embodiment.
As is clear from FIG. 6, when K=1/2, the power consumption W _S takes a minimum value, and the power consumption becomes 4/3 compared to the conventional method.

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

FIG. 7 is a circuit diagram showing one embodiment of a drive circuit for realizing the step drive method described above. Figure 8 shows each terminal and thin film of the drive circuit shown in Figure 7.
3 is a voltage waveform diagram of a pulse voltage input to the EL element 8. FIG.

Pulse voltages are applied to the terminals IN1', IN2', IN3' and IN4' in the same manner as in FIG. The step drive method is performed by a pulse applied to the terminal IN5, 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. Also thin film
The charge charged in the EL element 8 is transferred to terminals IN2' and IN
By selecting 4' and applying a pulse voltage, discharge is performed in the same manner as in the conventional case. Transistor Tr ₁ and
The thin film EL element 8 is AC driven by alternately conducting Tr ₄ , Tr ₂ and Tr ₃ , and the seesaw driving method is performed by making the transistor Tr ₅ conductive in the middle of the rise of the forward and reverse pulses applied to the thin film EL element 8. A driving system in which a step driving method is superimposed on the driving method is established.

As detailed above, the present invention takes advantage of the fact that the thin-film EL element is a capacitive element, and in particular, when charging by applying a driving voltage of V ₀ , the voltage near 1/2 of V ₀ and the voltage of V ₀ are sequentially applied. By adding this, the V ₀ voltage application charging process of the thin film EL element has a step-like charging state of a voltage near 1/2 V ₀ and a voltage of V ₀ applied from the same direction, and is different from the conventional 3 This technology reduces driving power by as much as /4, and is a very effective technology as a method for driving thin-film EL display devices.

[Brief explanation of drawings]

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 explaining one 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. Figure 7 shows part 1 of the present invention.
FIG. 2 is a configuration diagram of a drive circuit of a thin film EL display device showing an example. FIG. 8 is a timing waveform diagram showing voltage waveforms input to the drive circuit shown in FIG. 7. 8... Thin film EL element, Tr ₁ , Tr ₂ , Tr ₃ , Tr ₄ , Tr ₅
...transistor.

Claims (1)

[Claims] 1. In a method for driving a capacitive thin film EL display device that emits EL light in response to the application of a voltage,
When applying and charging a capacitive thin film EL display element with a driving voltage having a value of V ₀ , after applying and charging the thin film EL display element with a voltage having a value close to 1/2 of the V ₀ , By continuously applying and charging the same thin film EL display element with a voltage having a value of V ₀ from the same direction as the voltage application in the vicinity of 2,
The thin-film EL display element is characterized by having a step-like charging state of a voltage near 1/2 V ₀ and a voltage of V ₀ applied from the same direction in the charging step of applying a voltage of V ₀ to the thin film EL display element. A method for driving a thin film EL display device.