JPS5842472B2 - Daiyouryyouseihiyoujisoshino Kudo Cairo - Google Patents

Description

Detailed Description of the Invention The present invention is applicable to devices with extremely large interelectrode capacitance, such as large-area thin-film EL display devices, liquid crystal display devices, electro-optic effect display devices such as ferroelectric transparent porcelain plates, and plasma display devices. This relates to a drive circuit.

As shown in FIG. 1, a thin film EL display device has a glass substrate 1.
Transparent electrodes 2 are arranged in a striped manner on top of
A dielectric material 3 such as O3 is further deposited on top of the fluorescent layer 4 such as ZnS doped with Mn, and the same dielectric material 3' as described above is deposited on top of this by vapor deposition, sputtering, etc. to form a three-layer structure. structure, and an electrode 5 perpendicular to the transparent electrode 2 is placed thereon.
It is constructed by arranging them in a striped pattern.

With such a structure, one of the first electrode group 2 and
When a suitable alternating current voltage is applied to one of the second electrode group 5, only a small area sandwiched between the two electrodes will emit light, which corresponds to one pixel on the screen. .

In EL with this kind of structure, brightness and lifespan are important.

In terms of stability, it has superior characteristics compared to conventional dispersion type EL elements, and each picture element newly exhibits a hysteresis phenomenon between brightness and applied voltage as shown in Figure 2.

To explain this characteristic according to Fig. 2, first, as shown in Fig. 2 a (when a pulse of voltage amplitude v1 is applied, the brightness is as shown in Fig. 2, c
As shown, it is at the B1 level.

Here, vl is the emission threshold voltage, and if th, then V1≧vt
It is h.

When the write voltage V2 is applied to this, the brightness increases to 83 at once.
After that, even if the voltage value is returned to the maintenance voltage V1 again, the brightness settles to B2, which is higher than B1.

When the erasing voltage v3 is applied to this, the brightness level decreases rapidly, and when it is returned to the sustain voltage 1 again, the brightness settles to B1.

These temporal relationships are indicated by the symbol t1 in FIG. 2a. t
3...t21 is shown by corresponding to the positions of the same symbols in C of the figure.

As shown by the thin line in FIG. 2, this hysteresis phenomenon can take any small loop depending on the amplitude and pulse width (not shown) of the write voltage.

That is, it is also possible to display halftones. A major feature of the EL display device, which is not found in other display elements of the EL display device, is that once a write voltage is applied, each picture element continues to emit light without losing the gradation given by the sustain pulse.

The above voltages vary depending on the composition, film thickness, and applied waveform, but in a prototype example, Vth=200V, V1=
210V, ■2-210~280■, V3=190■.

When the above EL display device is configured to have a large area, the inter-electrode capacitance becomes large, and in particular, the sustain pulse must be commonly applied to all electrodes after writing is completed, and the actual capacitance at this time is extremely large. .

When directly charging and discharging such an element, it is necessary to select a switching element that has a large transient current, has a small on-resistance, and can flow a large current.

When current limiting is applied to the switching element to avoid damage to the switching element due to large transient currents during switching, or when a current limiting resistor R1 is inserted between the thin film EL element and the switching element, the switching element
Ineffective power loss for light emission in the limiting resistor portion becomes large.

On the other hand, when viewed from the thin film EL element side, flowing a large transient current causes heat loss in the transparent electrode and terminal portion.

The present invention provides a single power supply type circuit that effectively limits current and greatly reduces power loss that is ineffective for light emission.

Therefore, the present invention is characterized in that the current is effectively limited by an inductance that resonates with the capacitive component of the EL display device, and that a switching element is inserted to maintain the potential of the capacitive component.

Another object of the present invention is to obtain a drive circuit using one power source.

FIG. 3 shows the basic drive circuit of the invention.

A diode D1, a coil L1, a resistor R1, a switching element S1. B3 series circuit and diode D2,
Coil L2, resistor R2, switching element S2. Insert the B4 series circuit.

The diodes D1 and D2 are connected to currents if and ir in the forward direction, respectively.

Here, R1 and R2 represent the total resistance such as the electrode resistance of the element and the equivalent resistance of the circuit.

In this circuit, first, switching element S1°B3 is turned on, switching element S2. Period a when B4 is off
Then, as shown in period a in FIG. 4a, a charging current if from the power source E flows.

When a half period of the LC resonance frequency has elapsed, the potential of the capacitive component C becomes a voltage V exceeding the power supply voltage as shown in FIG. 4.

Therefore, diode D1 is reverse biased and turns off.
This voltage ■ is held.

During this hold period, switching element S1. B3 is on or off, and switching element S2.B3 is on or off. B4 is off.

Next, switching element S1. B3 is off, switching element S2. During period C when B4 is on, current ir from power source E flows as shown in FIG. 4a.

Then, when half a period has elapsed, the diode D2 is turned off, and the voltage -■ is held.

During this hold period, switching element S1. B3 is off and switching element S2.B3 is off. B4 is on or off.

The above operation is repeated, and the sustain pulse is continuously applied to the EL display element.

As described above, according to the drive circuit of the present invention, the current to the capacitive component C is limited by the resonant frequency of the LC resonant circuit,
This potential is then held by a diode.

Therefore, by selecting the inductance values of the coils L1 and L2, the state of current limitation can be changed and the switching element S1. B3 and B2. B4 on.

The repetition frequency of the sustain pulse can be changed by the off repetition frequency.

Furthermore, since the present invention uses four switching elements to supply currents if and ir in opposite directions to the capacitance component C from one power supply E, the number of components in the power supply circuit that requires large components is small. Can be configured to be small.

Furthermore, according to the present invention, power losses are significantly reduced.

The reason for this will be explained below. First, consider the case in which a current if flows from the positive DC power source E to the capacitive component C in the circuit of the present invention shown in FIG.

If there is no diode D1, it is a normal LCR series resonant circuit, and when the switches S1 and S3 are short-circuited to the series power supply E, the current if flowing in the circuit and the voltage between the terminals of capacitor 〇 +eO are calculated by the formula (1), ( 2).

FIG. 5 shows the current if and voltage e.

This shows the changes in

In the case of series resonance without the diode D1, damped oscillation occurs under the above vibration conditions as shown by the broken lines in FIGS. 5a and 5b.

Next, when the diode D1 is connected in series, a forward current flows through the diode D1 during the first half cycle of the natural resonance after the switch S1 is turned on, and resonance can be performed. ) and thereafter, diode D1 is reverse biased*.

For example, if we consider an ideal case where R''=i0, the forward voltage drop of the diode and the forward resistance can be ignored, and there is no loss in the capacitance component C and the coil L, we can obtain e from equation (3) from α.

b==2E. That is, the amplitude is maintained at twice the amplitude of the power supply voltage E.

The energy loss of the power supply during the second half of the cycle when the switch S is turned on in this ideal resonance is expressed by equations (1) and (2).

The energy supplied from the power supply during this half cycle is only the energy required to charge the capacitance component C to 2E.

Next, switch S1. A period C (shown in FIG. 4a) in which S2 is off and switches S2 and S4 are on.

) is the discharge period of the capacitive component C, and the energy returned to the power supply during this half cycle of resonance is calculated from equation (4) as −C(2E)2
It is.

Furthermore, during the other periods t and d, the voltage is maintained at 2E volts from O volts, and no energy is transferred.

It can be seen that there is no power loss in such ideal resonant drive.

However, although a thin film EL element is a capacitive element, it cannot be considered a lossless capacitor.

In small-signal driving that does not involve light emission, the loss is negligibly small, but in large-amplitude driving that accompanies light emission, non-linear light emission loss occurs.

Furthermore, there are electrode resistance losses (mainly on the transparent electrode side), on-resistance of switching elements, coil losses, etc., and ideal resonance that does not require energy supply from the power supply is impossible, so these losses can be eliminated from the power supply. It is necessary to replenish from

Calculating non-linear losses with respect to drive amplitude such as light emission loss becomes complicated, so electrode resistance, switching element on-resistance, coil resistance, etc. are combined into constant resistances R1 and R.
Figure 3 shows the LCR series resonant circuit incorporated in 2.

In this circuit, the power supply voltage required when driving from +V to -V in a steady state is given by equation (2).

In equation (5), when ideal resonance R'=i00, Eξ0 becomes Eξ0, which means that even if the capacitor 〇 (thin film EL element) is first charged by V, even if the power source E (-E) is set to zero, +V
This shows that it is possible to drive from -■.

On the other hand, when the resistance 1 and R2 are R=2jf, E=V.

This depends on critical braking conditions. In the actual circuit O<E
<V, in other words, E<V<2E'C-A6of
It can be said that the larger fl"J=■"-"2=ef, the less loss there is in the circuit.

The basic drive circuit of the present invention shown in FIG. 5 was configured as shown in FIG. 6 in a more specific embodiment of the invention.

Here, C is the capacitance component of the thin film EL element, and Dl.

D2 is a holding diode, T is a transformer for resonance coils L1 and L2, Trl, Tr2, Tr3, and Tr4 are switching transistors, T1. T2 is an interstage coupling transformer, D is a protection diode, Ul and U2 are open collector TTL inverters, and Pl and P2 are switching pulses.

Note that the ratio between the primary and secondary windings of the transformer T is 1:1.

Pulse P1. The pulse width of P2 is one or more natural vibration periods.

In the circuit of FIG. 6, when timing pulses P1 and P2 are applied as shown in FIG. 7a, the transistors Tr1. Tr2゜Tr3. The on/off operation of Tr4 is controlled, and the voltage shown in FIG. 7 is applied to the thin film EL element.

This voltage is applied to all the picture element points as a sustaining pulse for the thin film EL element explained in FIGS. 1 and 2 above.

A basic circuit diagram of another embodiment of the invention is shown in FIG.

In this circuit, the capacitive component C is charged by the power source E through the switch S1, the coil L1, and the diode D1, and is discharged from the capacitive component C through the diode D2, the coil L2, and the switch S2.

[Brief explanation of drawings]

FIG. 1 is an exploded perspective view of a thin film EL device, FIG. 2 is a diagram of applied voltage and luminance characteristics to explain the operation of the device, and FIG. 3 is a basic drive circuit diagram of the present invention. Figure 4 is a current waveform diagram and voltage waveform diagram of the element in the circuit of Figure 3, Figure 5 is a current waveform diagram and voltage waveform diagram applied to the element to explain the operation of the circuit of the present invention, and Figure 6 is A specific circuit diagram of one embodiment of the present invention, FIG. 1 shows a time chart of the circuit of FIG. 6, and FIG. 8 shows a basic circuit diagram of another embodiment. C is a thin film EL element, Dl and D2 are holding diodes, Ll
, L2 is a coil, Ro and R2 are resistors, Trl, Tr2°Tr3. Tr4 is a switching element, and E is a DC power supply.

Claims (1)

[Claims] 1. In a drive circuit that writes, erases, or maintains a large-capacitance display element in a written state and an erased state using an alternating current voltage, two series A series of coils constituting a resonant circuit and diodes connected in opposite directions in the above two series to stop the resonant operation after half a cycle of the resonant operation of each resonant circuit and maintain the potential of the display element at the time of stop. 1. A driving circuit for a large-capacitance display element, characterized in that the circuits are connected to each other, and a switching means is provided for alternately supplying a voltage in the opposite direction to one DC power source to each of the series circuits.