JPH0648432B2 - EL drive method - Google Patents

Description

Detailed Description of the Invention a. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving method for an EL display panel, and more particularly to a pulse driving method for a matrix type thin film EL element.

B. Problems to be Solved by the Related Art and Invention In pulse driving of a general thin film type EL element, one pulse of about 200 V is applied to the EL element to charge the EL element to cause it to emit light, and then the charge is discharged. The power consumption is the sum of the power (charging energy) for charging the EL element to a predetermined potential and the power (light emitting energy) for emitting light with the required brightness. Here, the ratio (%) of the charging energy and the emission energy which the power consumption occupies varies depending on the EL driving frequency and the brightness, but as a general tendency, the charging energy is about 80%.

By the way, since the driving voltage of the thin film type EL element is as high as about 200 (V), the power consumption is considerably higher than that of the dispersion type EL element which is widely used as a backlight of a liquid crystal display. However, the thin film type EL device is excellent in terms of life and brightness. Therefore, recently, it has been desired to apply a long-life and high-brightness thin film type EL element to a backlight of a liquid crystal display, and part of it is being put to practical use. Electrification was a big issue.

C. Means for Solving the Problems An object of the present invention is to provide a novel method for driving an EL element with low power consumption. According to the present invention, a first switching element capable of switching the electrode to a required voltage for light emission, a ground potential, and a floating state is provided on each electrode at both ends of the EL element, one or more capacitors, and the capacitor. Via a second switching element capable of switching one side terminal of the capacitor to a ground potential and a floating state via the capacitor, and when discharging the charge charged in the EL element, the one side electrode of the EL element is The first switching element is set to the ground potential, and one side terminal of the capacitor connected to the other electrode is sequentially grounded one by one by the second switching element, thereby recovering electric charges in stages to each capacitor,
After the collection to the capacitor is completed, both electrodes of the EL are set to the ground potential by the first switching element to discharge all the electric charges of the EL element, and when the EL element is charged, one side of the EL element is charged. The EL element is gradually charged from each capacitor by grounding one terminal of the capacitor connected to the electrode one by one in reverse to the order in which the second switching element recovered the discharge, and Disclosed is an EL driving method characterized in that after the end of charging, the first switching element is charged to a voltage required for light emission. According to this method, the power consumption of the EL element can be recovered by the capacitor and can be reduced by the amount of charging energy supplied to the EL element at the next EL driving.

E. Embodiment FIG. 1 shows a basic implementation circuit of the present invention.
L element, (2a) and (2b) are electrodes of EL element (1), (3a) and (3
b) is a capacitor connected to each electrode (2a), (2b).
(4a) and (4b) are EL elements on both electrodes (2a) and (2b) of the EL element (1).
Driving voltage required to make the device (1) emit light with required brightness
The first switching element for selectively applying VEL, (5
a) and (5b) are drive circuits for the switching elements (4a) and (4b), which set the electrodes (2a) and (2b) to the drive voltage VEL application state, the open floating state, and the ground potential.
Switch to (V) three ground states. (6a) and (6b) are second capacitors that selectively ground the open ends of the capacitors (3a) and (3b).
Switching elements, (7a), (7b) are switching elements (6
This is a drive circuit that switches between a) and (6b).

Now, assuming that the EL element (1) and the capacitors (3a) and (3b) have the same capacitance and VEL = 200 (V), the basic operation of the circuit of FIG. 1 will be described from (a) to (d) of FIG. To do. First, as shown in FIG. 2B, both capacitors (3a) and (3b) are in an open (floating) state, and one electrode of the EL element (1)
(2b) is grounded to 0 (V) and a pulse of drive voltage 200 (V) is applied to the other electrode (2a) to cause the EL element (1) to emit light. Next, as shown in (b) of FIG. 2, the electrode (2a) is opened (floating) and the open end of one capacitor (3a) is brought to a 0 (V) ground state. Then, the capacitor (3a) is charged to 100 (V) by the charge of the EL element (1), and the charge is partially recovered. Next, in Figure 2 (C)
Return the capacitor (3a) to the open state, ground the electrode (2a), and set the voltage between the terminals of the EL element (1) to 0
Make to (V). In this state, immediately before the next application of one pulse of the same polarity, the electrode (2a) is opened as shown in (d) of FIG.
When the capacitor (3a) is grounded, the charged electric charge of the capacitor (3a) is supplied to the EL element (1) and becomes 50 (V). After this charge is supplied, VEL is applied as shown in FIG. 2B, but since the EL element (1) has already been charged to 50 (V) at this second time, the replenishment charge necessary for light emission is 150 remaining
(V) is enough, and it can be seen that the charging energy of 50 (V) can be saved here. After that, the above operation is repeated.

Here, the capacitance of the EL element (1) is Co, and both capacitors (3
A) and (3b) are ACo times A times ACo, and the voltage across the capacitor terminals due to the collected charges of the capacitors (3a) and (3b) based on the above operation is Vd, and capacitors (3a) and (3b) are Assuming that the voltage between the EL element terminals at the nth time when electric charge is supplied to the EL element (1) is Vpn, the operation after the nth time of the circuit in FIG. 1 is repeated from (a) to (h) in FIG. The light emission pulse waveform at this time is expressed as shown in FIG. That is,
3A shows the n-th state of the state of FIG. 2D, and the inter-terminal voltage of the EL element (1) is Vpn due to the charge supply from the capacitor (3a). In this state, as shown in (b) of FIG. 3, when the capacitor (3a) is opened and VEL is applied to the electrode (2a), the EL element (1) becomes VEL-Vpn
It emits light only with the charging energy of, and it is saved by the charging energy of Vpn. Next, as shown in (c) of Fig. 3, open the electrode (2a) and ground the capacitor (3a) to EL.
Part of the charge stored in the device (1) is recovered by the capacitor (3a), and then the capacitor (3a) is used as shown in (d) of FIG.
Open a) and ground the electrode (2a). The light emission pulse waveform during this period is shown by P1 in FIG.

Next, the operation by the pulse of the opposite polarity from (d) of FIG. 3 is performed in the process shown in (e) to (h) of FIG. This reverse polarity pulse drive is performed on the other capacitor (3b) side in the same manner as described above, and the light emission pulse waveform at this time is shown in P2 of FIG. Then, returning from FIG. 3C to FIG. 3A again, the above operation is repeated.

In the above pulse driving, the voltage Vpn between terminals of the EL element (1) at the n-th time is obtained from the following equation.

Bn + 1 = Vp (n + 1) −Vpn (2), here Therefore From equation (2) As n → ∞, from equation (3) Therefore, since the normal EL drive frequency is 50Hz or higher,
Vpn instantly becomes as shown in equation (4). Further, Vpn increases as the capacitance of the capacitors (3a) and (3b) increases (A increases), and the amount of power consumption (charging energy) that can be saved also increases. Specifically, the upper limit of Vpn is 1/2 VEL,
The power consumption W at this upper limit is Therefore, it can be seen that the power consumption can be reduced by about 25%.

Next, another embodiment circuit example of the present invention will be described with reference to FIG. This FIG. 5 shows that the electrodes (2a) and (2b) of the EL element (1) each have a first switching element (not shown) and a plurality of m capacitors (3a1) to (3am), 3b1) to (3bm) and a second switching element (not shown) connected to these capacitors, respectively. When supplying electric charge to the EL element (1), the capacitor (3a1) (3am), or (3
From b1) to (3bm), one terminal on one side of the capacitor is sequentially grounded one by one by the second switching element, and the charge recovery of the EL element (1) is conversely performed by the capacitor (3am).
To (3a1), or from (3bm) to (3b1), one side terminal of the capacitor is sequentially connected by the second switching element.
Do this by grounding each one. By doing so, further reduction in power consumption is realized. For example, each capacitor (2a1) ~ (2am), (2
Assuming that the capacity of b1) to (2bm) is 100Co (A = 100) and the saving rate (%) of the charging energy with respect to m when the above-mentioned sequential driving is performed, the results shown in the graph of Fig. 6 are obtained. Was given. In other words, about 25% when m = 1 and about 40% when m = 2
Strong, m = 20 can save about 90%. Theoretically, if m = ∞, the charging energy can be saved by 100%, but the actual problem is about m = 20. Even if the total EL power consumption at this level cannot reduce the emission energy, the overall
It was found that the savings can be achieved up to about 50-70%.

In the above embodiments, the case where capacitors are connected to the respective electrodes on both ends of the EL element has been described, but the present invention is not limited to the above embodiments, and capacitors may be provided on only one side of the electrodes. It may be connected. In this case, although the charge recovery rate is reduced, the number of parts used is reduced and the cost is reduced.

E. EFFECTS OF THE INVENTION As described above, according to the present invention, it is possible to reduce power consumption, and especially in a thin film EL display panel, 50 to 70%.
Thus, it is possible to significantly reduce power consumption and to expand the applications of thin film EL elements, for example, to provide an effective liquid crystal backlight.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a basic implementation of the effect method of the present invention, FIGS. 2A to 2D are circuit diagrams showing respective basic operations of the circuit shown in FIG. (A) to (h) are circuit diagrams at each operation showing the operation of the circuit of FIG. 1 from the n-th time onward, FIG. 4 is a light emission pulse waveform diagram in the operation of FIG. 3, and FIG. Another embodiment circuit diagram, FIG. 6 is a graph showing the relationship between the number of capacitors (m) and the charging energy saving rate (%) in the circuit of FIG. (1) ... EL element, (2a), (2b) ... electrode, (3a), (3
b), (3a1) ~ (3am), (3b1) ~ (3bm) ... capacitors.

Claims (1)

[Claims] 1. A first switching element capable of switching the electrode to a floating state between a voltage required for light emission, a ground potential, and one or more capacitors on each electrode at both ends of the EL element. A second switching element capable of switching one terminal of the capacitor to a ground potential and a floating state is connected, and when discharging the electric charge charged in the EL element, one electrode of the EL element is switched to the first switching element. The element is set to the ground potential, and one side terminal of the capacitor connected to the other electrode is sequentially grounded one by one by the second switching element, thereby gradually collecting the electric charge to each capacitor,
After the collection to the capacitor is completed, both electrodes of the EL are set to the ground potential by the first switching element to discharge all the electric charges of the EL element, and when the EL element is charged, one side of the EL element is charged. The EL element is gradually charged from each capacitor by grounding one terminal of the capacitor connected to the electrode one by one in reverse to the order in which the second switching element recovered the discharge, and After the completion of the charging, the EL driving method is characterized in that the first switching element is charged to a voltage required for light emission.