Patent Grant
7176851
1. Field of the Invention
The present invention relates to a gas discharge panel display apparatus and a method for driving a gas discharge panel used for image display for computers, televisions, and the like. The invention particularly relates to an AC plasma display panel which writes an image by accumulating a charge in a dielectric layer and illuminates discharge cells by performing a sustain discharge.
2. Related Art
In recent years, gas discharge panels including plasma display panels (hereafter referred to as PDPs) have become the focus of attention for their ability to realize a large, slim and lightweight display apparatus for use in computers, televisions, and similar. In these gas discharge panels, a PDP produces an image display by selectively illuminating discharge cells arranged in the form of matrix.
PDPs can be broadly divided into two types: direct current (DC) and alternating current (AC). AC PDPs are suitable for large-screen use and so are at present the dominant type.
Discharge cells in an AC PDP are fundamentally only capable of two display states, ON and OFF. Here, a field timesharing gradation display method in which one frame (one field) is divided into a plurality of sub-frames (sub-fields) and the ON and OFF states in each sub-frame are combined to express a gray scale is used.
For image display in each sub-frame, an ADS (Address Display-period Separation) method is employed. In this method, each sub-frame is composed of the following sequence: a set-up period, a write period, a discharge sustain period, and an erase period, as shown in
This illumination principle is basically the same as that of a fluorescent lamp. When a sustain pulse is applied to cause a normal glow discharge, ultraviolet light (Xe resonance lines with a wavelength of 147 nm) is generated from Xe and excites a phosphor to emit light. However, since the efficiency of the conversion from discharge energy to ultraviolet light and the efficiency of the conversion from ultraviolet to visible light in a phosphor are not high, a PDP cannot produce as high brightness as a fluorescent lamp.
Also, there is the demand for high-definition PDPs, just as other types of display (high-definition television with high resolutions of up to 1920×1080 pixels at full specification is currently being introduced). However, such a high-definition PDP is likely to suffer further decreases in luminous efficiency. In view of these points, an important issue in the PDP technology is to increase luminous efficiency (i.e. the amount of brightness with respect to the amount of power). To achieve this, techniques of improving structures of PDPs and techniques of recovering currents (reactive currents) which do not contribute to ultraviolet light emission are being developed. Also, techniques for suppressing the occurrence of reactive currents are being sought.
Furthermore, a rectangular wave is generally used for sustain pulses, as shown in
However, when applying a sustain pulse, there is a certain probability that so-called “discharge delay” occurs. The discharge delay refers to a substantial time delay from the leading edge of the pulse to the start of the discharge. In particular, the discharge delay tends to occur for a sustain pulse which is first applied in a discharge sustain period.
This discharge delay causes a drop in image quality. Which is to may, if there is a certain probability of occurrence of discharge delay in a PDP in which a large number of discharge cells are aligned, discharge delays may occur in part of the discharge cells which are to be illuminated. When this happens, illumination failures will result, and the quality of the displayed image will decrease. Therefore, techniques for preventing discharge delays are desired, too.
The first object of the present invention is to improve luminous efficiency by suppressing reactive currents, when driving a gas discharge panel such as a PDP.
The second object of the invention is to improve image quality by suppressing discharge delays in a discharge sustain period.
To achieve the first object, a waveform of a sustain pulse is determined so that a current waveform which completes a fall by the time triple a rise time to a peak elapses from when the peak is reached is formed when the sustain pulse is applied.
This particular current waveform can be formed by providing any of the following first to third features to the sustain pulse.
(1) First Feature: Applying a pulse of the opposite polarity briefly before the leading edge of the sustain pulse.
(2) Second Feature: Set the absolute voltage of the sustain pulse higher during a fixed period after the leading edge of the sustain pulse, than during a period following the fixed period.
(3) Third Feature: Applying a pulse of the opposite polarity immediately after the trailing edge of the sustain pulse.
By forming the above particular current waveform, reactive currents are suppressed when compared with the case where a sustain pulse of a conventional waveform is applied, with it being possible to improve luminous efficiency.
In addition, the provision of each of the first to third features to the sustain pulse produces the following effects.
The effects produced by the provision of the first feature are as follows.
Electrons move from one electrode toward the other in a discharge space when the opposite polarity pulse is applied before the leading edge of the sustain pulse, but are pulled back toward the electrode without reaching the other electrode when the sustain pulse is applied.
As a result of such an initial reciprocating motion of the electrons in the discharge space, a lot of charged particles (electrons and ions) that contribute to light emission are generated, which further improves luminous efficiency.
Also, with the reciprocating motion of the charged particles between the two electrodes, a source of discharge is formed, which enables the discharge to start with reliability. Hence the suppression of discharge delays which is the second object of the invention is achieved.
To ensure these effects, the absolute voltage of the opposite polarity pulse is preferably no smaller than the absolute voltage of the sustain pulse, and more preferably no smaller than 1.5 times the absolute voltage of the sustain pulse.
Here, the time for applying the opposite polarity pulse is preferably 100 ns or below.
Also, the time during which the absolute voltage of the opposite polarity pulse is no smaller than the absolute voltage of the sustain pulse is preferably 100 ns or below, and more preferably 50 ns or below.
The effects produced by the provision of the second feature are as follows.
When a high voltage is applied for a fixed period from the leading edge of the sustain pulse, the discharge is started with reliability, and the discharge delay is suppressed.
This effect can be enhanced by applying a voltage no smaller than a discharge firing voltage of the discharge cell, in the fixed period.
Here, it is preferable to apply a voltage which is higher in absolute value than a voltage applied thereafter by 50V or more, in the fixed period.
In general, applying a high voltage tends to cause a dielectric breakdown of a dielectric layer or an increase of power consumption. However, by setting the time for applying the high voltage (which is no smaller than the discharge firing voltage) to a short time of no greater than 100 ns or even no greater than 10 ns, the dielectric breakdown and the power consumption increase can be avoided.
The effects produced by the provision of the third feature are as follows.
When the opposite polarity pulse is applied after the trailing edge of the sustain pulse, reactive currents caused by ions remaining in the discharge cell can be suppressed.
Which is to say, the ions remaining in the discharge cell after the trailing edge of the sustain pulse show low activities and do not contribute to light emission. When such ions reach an electrode, reactive currents are generated and cause a decrease in luminous efficiency. With the provision of the third feature, however, such reactive currents are suppressed, thereby significantly improving luminous efficiency.
Here, the highest absolute voltage of the opposite polarity pulse is preferably 50V or more.
Also, the time for applying the opposite polarity pulse is preferably 100 ns or below, and more preferably ions or below.
It should be noted that usually a plurality of sustain pulses of alternating polarity are successively applied to each discharge cell during one discharge sustain period. Although, it is desirable to add the aforementioned waveform features to all sustain pulses which are applied in the discharge sustain period in order to maximize the effects of the invention, the waveform features may instead be added to only part of the sustain pulses. In such a case, the features should be added at least to a sustain pulse which is first applied to each discharge cell in the discharge sustain period.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the invention. In the drawings:
Overall Construction of a Display Apparatus
First, an overall construction of a PDP display apparatus to which the embodiments of the invention relate is explained The PDP display apparatus includes a surface discharge AC PDP and a driving apparatus for the PDP.
In this PDP, a front substrate 11 and a back substrate 12 are placed in parallel so as to face each other with a space in between. The edges of the substrates 11 and 12 are then sealed.
A scan electrode group 19a and a sustain electrode group 19b are formed in parallel strips on the inward-facing surface of the front substrate 11. The electrode groups 19a and 19b are covered by a dielectric layer 17 composed of lead glass or similar. The surface of the dielectric layer 17 is then covered with a protective layer 18 of magnesium oxide (MgO). A data electrode group 14 is formed in parallel strips on the inward-facing surface of the back substrate 12, and covered by a dielectric layer 13 composed of lead glass or similar. Barrier ribs 15 are placed on top of the dielectric layer 13, in parallel with the data electrode group 14. The space between the front substrate 11 and the back substrate 12 is divided into spaces of about 100 μm to 200 μm by the barrier ribs 15. Discharge gas is sealed in these spaces. The pressure at which the discharge gas is enclosed is usually set below external (atmospheric) pressure, typically in a range of around 1×104 Pa to 7×104 Pa. However, setting the pressure at 8×104 Pa or higher is preferable for higher luminous efficiency.
In monochrome PDPS, a gas mixture composed mainly of neon is used as the discharge gas, emitting visible light when a discharge is performed. However, in a color PDP like the one in
This PDP is driven using the field timesharing gradation display method.
In the example division method shown in
The ADS method is applied to each sub-frame to display an image on the PDP. Each sub-frame is composed of the following sequence: a set-up period, a write period, a discharge sustain period, and an erase period.
In the set-up period, all of the discharge cells are set-up by applying set-up pulses to the scan electrodes 19a.
In the write period, data pulses are applied to selected data electrodes 14 while scan pulses are applied sequentially to the scan electrodes 19a. This causes a wall charge to accumulate in the discharge cells which should be illuminated, writing one screen of pixel information.
In the discharge sustain period, sustain pulses are applied across the scan electrodes 19a and the sustain electrodes 19b, causing a discharge to occur in the discharge cells where the wall charge has accumulated, and light to be emitted for a predetermined period.
In
Detailed Explanation of the Driving Apparatus and Driving Method
The driving apparatus 100 includes a preprocessor 101, a frame memory 102, a synchronization pulse generating unit 103, a scan driver 104, a sustain driver 105, and a data driver 106. The preprocessor 101 processes image data inputted from an external image output device. The frame memory 102 stores the processed image data. The synchronization pulse generating unit 103 generates synchronization pulses for each frame and each sub-frame.
The scan driver 104 applies pulses to the scan electrode group 19a, the sustain driver 105 to the sustain electrode group 19b, and the data driver 106 to the data electrode group 14.
The preprocessor 101 extracts image data for each frame from the input image data, produces image data for each sub-frame (sub-frame image data) from the extracted image data, and stores it in the frame memory 102. Also, the preprocessor 101 outputs sub-frame image data stored in the frame memory 102 line by line to the data driver 106, detects synchronization signals such as horizontal synchronization signals and vertical synchronization signals from the input image data, and sends synchronization signals for each frame and sub-frame to the synchronization pulse generating unit 103.
The frame memory 102 is capable of storing the data for each frame split into sub-frame image data for each sub-frame.
Specifically, the frame memory 102 is a two-port frame memory provided with two memory areas each capable of storing one frame (eight sub-frame images). An operation in which image data for one frame is written in one memory area while image data for another frame written in the other memory area is read can be performed alternately on the memory areas.
The synchronization pulse generating unit 103 generates trigger signals indicating the timing at which each of the setup, scan, sustain, and erase pulses should rise. These trigger signals are generated with reference to the synchronization signals received from the preprocessor 101 regarding each frame and each sub-frame, and sent to the drivers 104 to 106.
The scan driver 104 generates and applies the set-up, scan, sustain, and erase pulses in response to the trigger signals received from the synchronization pulse generating unit 103.
The set-up, sustain, and erase pulses are applied to all of the scan electrodes 19a.
As a result, the scan driver 104 has three pulse generators, one for generating each kind of pulse, as shown in
In
Switches SW1 and SW are arranged in the scan driver 104 to selectively apply the output from the above pulse generators 111, 112a, and 113 and the output from the scan pulse generator 114, to the scan electrode group 19a.
The sustain driver 105 includes a sustain pulse generator 112b. The sustain driver 105 generates sustain pulses in response to trigger signals from the synchronization pulse generating unit 103, and applies the sustain pulses to the sustain electrodes 19b.
The data driver 106 outputs data pulses to the data electrodes 141 to 14M in parallel. The output takes place based on sub-field information which is inputted serially into the data driver 106 one line at a time.
The data driver 106 includes a first latch circuit 121 which fetches one scan line of sub-frame image data at a time, a second latch circuit 122 which stores the fetched data, a data pulse generator 123 which generates data pulses, and gates 124, to 124M located at the entrance to each data electrode 141 to 14M.
In the first latch circuit 121, sub-frame image data sent in order from the preprocessor 101 is synchronized with a CLK (clock) signal and fetched sequentially so many bits at a time. Once one scan line of sub-frame image data (information showing whether each of the data electrodes 141 to 14M, is to have a data pulse applied) has been latched, it is transferred to the second latch circuit 122. The second latch circuit 122 opens AND gates, among the AND gates 1241 to 124M, which correspond to the data electrodes that are to have the pulses applied, in response to trigger signals from the synchronization pulse generating unit 103. The data pulse generator 123 generates the data pulses simultaneously with this, as a result of which the data pulses are applied to the data electrodes with their AND gates opened.
In such a driving apparatus 100, the operations for one sub-frame composed of a sequence of the set-up, write, discharge sustain, and erase periods are repeated eight times to display a one-frame image, as explained below it should be noted here that the number of sub-frames may be set at more than eight to suppress false contours.
In the set-up period, switches SW1 and SW2 in the scan driver 104 are ON and OFF respectively. The set-up pulse generator 111 applies a set-up pulse to all of the scan electrodes 19a, causing a set-up discharge to occur in all of the discharge cells, and a wall charge to accumulate in each discharge cell. Here, applying a certain amount of wall voltage to each discharge cell enables a write discharge occurring in the following write period to commence sooner.
In the write period, switches SW1 and SW2 in the scan driver 104 are OFF and ON respectively. Negative voltage scan pulses generated by the scan pulse generator 114 are applied sequentially from the scan electrode 19a1 in the first row to the scan electrode 19aN in the last row. Simultaneously, the data driver 106 performs a write discharge by applying positive voltage data pulses to data electrodes, among the data electrodes 141 to 14M, which correspond to the discharge cells to be illuminated, thereby accumulating a wall charge in these discharge cells. Thus, a one-screen latent image is written by accumulating the wall charge on the surface of the dielectric layer in the discharge cells which are to be illuminated.
Here, the scan pulses and the data pulses (the write pulses in other words) should be set as narrow as possible, to enable driving to be performed at high speed. However, if the write pulses are too narrow, write defects are likely. Besides, limitations in the type of circuitry that may be used mean that the pulse width usually needs to be set at about 1.0 μs or more.
In the discharge sustain period, switches SW1 and SW2 in the scan driver 104 are ON and OFF respectively. The operation in which the sustain pulse generator 112a applies a sustain pulse of a fixed duration (for example 1 μs to 5 μs) to the entire scan electrode group 19a and the sustain pulse generator 112b in the sustain driver 105 applies a discharge pulse of a fixed duration to the entire sustain electrode group 19b are alternated repeatedly.
This operation raises the electric potential of the surface of the dielectric layer above a discharge firing voltage in the discharge cells in which the wall charge had accumulated during the write period, so that a discharge occurs in such discharge cells. This sustain discharge causes ultraviolet light to be emitted within the discharge cells. The ultraviolet light excites the phosphors in the phosphor layers 16 to emit visible light corresponding to the color of the phosphor layer 16 in each of the discharge cells.
In the erase period, switches SW1 and SW2 in the scan driver 104 are ON and OFF respectively. A narrow erase pulse is applied to the entire scan electrode group 19a by the erase pulse generator 113, erasing the wall charge in each discharge cell by generating a partial discharge.
Pulse Waveform in the Discharge Sustain Period
The following is an explanation on the particular waveform of the sustain pulses applied across the scan electrode group 19a and the sustain electrode group 19b in the discharge sustain period, and its effect.
In this invention, a waveform of a sustain pulse is adjusted so that a current waveform which completes the fall by the time triple the rise time to the peak elapses since the peak is reached is formed when the sustain pulse is applied.
In other words, when applying a sustain pulse, its waveform is adjusted so that the current becomes extremely small by the time triple the rise time taken to reach the peak elapses since the peak is reached, in order to suppress reactive currents and improve luminous efficiency.
The current waveform having such a property is found to be obtained by providing one of the following three features to the sustain pulse which is to be applied.
(1) First Feature: Apply a pulse of the opposite polarity briefly before the leading edge of the sustain pulse.
(2) Second Feature: Set the absolute value of the voltage of the sustain pulse higher in a fixed period after the leading edge of the sustain pulse, than in a period following the fixed period.
(3) Third Feature; Apply a pulse of the opposite polarity immediately after the trailing edge of the sustain pulse.
It has been shown by experiment that providing one of the first to third features when applying a sustain pulse generates the current waveform with the above property (the current waveform which completes the fall by the time triple the rise time to the peak elapses since the peak is reached).
The reason why the generation of this particular current waveform has the effect of suppressing reactive currents is given below.
Regarding the mechanism of light emission in the discharge space, consider an example when a positive sustain pulse is applied to a scan electrode 19a.
When the positive sustain pulse (+V) is applied to the scan electrode 19a, an electric field E emerges in a discharge space 20 in the direction from the electrode 19a to an electrode 19b, as shown in
The electrons (e) and the ions (Xe+) in the discharge space 20 are regarded as current carriers. Accordingly, when the electrons (e) or the ions (Xe+) generated in the discharge space 20 reach the electrode 19a or 19b, currents are generated between the electrodes 19a and 19b.
When comparing the moving speeds of an electron and an ion in an electric field, the electron moves much faster than the ion due to their difference in mass (their moving speeds differ by several orders of magnitude).
Therefore, currents carried by the electrons (electron currents) reach their peak soon after the leading edge of the sustain pulse when the electrons reach the electrode 19a, and currents carried by the ions (ion currents) reach their peak relatively later when the ions reach the electrode 19b, as shown in
Here, the earlier currents which are believed to be caused by the electrons that move fast in the discharge space 20 greatly contribute to light emission, but the later currents which are believed to be caused by the ions that move slowly do not much contribute to light emission. Hence luminous efficiency can be improved by suppressing such later currents.
Also, as noted earlier, if the above first to third features are added to a sustain pulse, such a current waveform that completes the fall by the time triple the rise time to the peak elapses since the peak is reached can be obtained when the sustain pulse is applied. Hence it can be said that the electron currents have this type of waveform.
Accordingly, by forming this particular current waveform, the ion currents which do not much contribute to light emission are suppressed, and the luminous efficiency is increased.
This can be confirmed by the experimental results given below.
The current waveform shown in
Which is to say, if a current waveform which agrees well with the peak of the above electron current waveform is formed when a sustain pulse is applied, power is concentrated on the time when the luminous efficiency is high, with it being possible to improve luminous efficiency.
The following first to fourth embodiments explain the first to third features and their effects, in greater detail.
First Embodiment
In the first embodiment, a pulse of the opposite polarity is briefly applied prior to the leading edge of each of the positive sustain pulses which are alternately applied to the scan electrode group 19a and the sustain electrode group 19b in the discharge sustain period, as shown in
The following explanation focuses on the case where sustain pulses are applied to the scan electrode group 19a. Since the same applies to the case where sustain pulses are applied to the sustain electrode group 19b, the explanation for the latter has been omitted here.
When applying a positive sustain pulse to each scan electrode. 19a, first a pulse of the negative polarity is applied briefly before the rise of the positive sustain pulse, and then the positive sustain pulse (the sustain voltage Vs) is applied.
Here, the value of the sustain voltage Vs is set in such a range that causes a discharge to occur in the discharge cells where the wall charge has accumulated during the write period but does not cause a discharge to occur in the discharge cells where the wall charge has not accumulated. The value of the sustain voltage Vs depends on the design of the PDP (such as the size of the discharge cells, the width of the electrodes, and the thickness of the dielectric layer).
In general, the sustain voltage Vs is set below a discharge firing voltage (Vf) of the discharge cells (in a range of Vf-50V to Vf). In this embodiment, however, the sustain voltage Vs can be set lower than that.
A discharge firing voltage in a PDP can be measured in the following way.
With one's eyes kept on a PDP, a voltage applied from a panel driving apparatus to the PDP is increased little by little. When one discharge cell or a specified number (e.g. three) of discharge cells in the PDP starts emitting light, the applied voltage is read and recorded as the discharge firing voltage.
(Effect of the First Embodiment)
When the simple rectangular wave shown in
On the other hand, if a negative pulse (−V) is applied briefly before the leading edge of the positive sustain pulse when applying the sustain pulse to the electrode 19a as shown in
Thus, when the electrons are moving back and forth in the discharge space 20, the frequency with which the electrons collide with gas particles is high, so that many excited atoms that contribute to light emission are generated. Hence the luminous efficiency is improved when compared with the case where a simple rectangular wave such as the one shown in
Also, when a positive sustain pulse of the conventional rectangular wave is applied, a discharge delay may occur due to a voltage drop at the rise of the sustain pulse. The probable cause of the discharge delay is the following. When the sustain pulse rises, currents flow out abruptly, causing a voltage drop. When this happens, it takes time for the voltage to increase again.
However, if the opposite polarity pulse is applied immediately before applying the sustain pulse, the electrons move back and forth and frequently collide with gas particles, which ensures the formation of a source of discharge. Accordingly, a discharge can be started with a high probability while suppressing a discharge delay.
As a result, the discharge can be performed without fail even when the sustain voltage Vs is set comparatively low. In other words, in spite of the fact that the sustain voltage Vs in
Also, setting the sustain voltage Vs low enables ion currents to be reduced, with it being possible to further improve luminous efficiency.
To achieve the above effects, it is preferable to set the absolute value of the voltage (Vmin in
Also, if the time (Tb) during which the negative pulse is applied prior to the rise of the sustain pulse is too long, a problem in which the power consumption increases due to currents flowing during this time period may arise. Especially in the time Tb, if the time Tc during which the absolute value of the voltage Vmin exceeds that of the sustain, voltage Vs (or the discharge firing voltage) is too long, the power consumption increases due to the amount of currents flowing during this time period. Such an increase in power consumption can, however, be significantly suppressed by setting the time Tc short.
In view of these points, the larger the absolute value of the voltage Vmin of the opposite polarity pulse, the shorter the time Tc need be. In general, it is desirable to set the time Tc at 100 ns or below.
Suppose the gap between the scan electrode 19a and the sustain electrode 19b is 60 μm, and the negative pulse with the voltage Vmin of −400V is applied to the scan electrode 19a prior to the leading edge of the positive sustain pulse. Then, if the voltage is changed to positive within 100 ns after the negative voltage no smaller than the discharge firing voltage in absolute value is applied to the scan electrode 19a, the polarity changes before the charged particles generated in the discharge space by the application of the negative pulse reach the scan electrode 19a (or the sustain electrode 19b), so that the charged particles are pulled back toward the sustain electrode 19b. (or the scan electrode 19a). Accordingly, the amount of currents generated during this period is little. Also, since the charged particles move back and forth between the electrodes 19a and 19b, a source of discharge is generated. Therefore, if the sustain voltage Vs of the positive polarity pulse is set at about 200V, a discharge is performed reliably without an increase in discharge delay.
Furthermore, it is more preferable to set the time Tc during which the absolute value of the voltage Vmin is no smaller than the discharge firing voltage at 50 ns or below, as the amount of currents flowing during such a time is almost zero.
(Circuit for Adding the Opposite Polarity Pulse to the Sustain Pulse)
The opposite polarity pulse can be added to the sustain pulse by providing a pulse combining circuit shown in
This pulse combining circuit is roughly made up of a first pulse generator 131 and a second pulse generator 132.
The first pulse generator 131 generates a pulse of negative voltage, and the second pulse generator 132 generates a pulse of positive voltage. A first pulse generated by the first pulse generator 131 is a relatively narrow wave, whereas a second pulse generated by the second pulse generator 132 is a relatively wide rectangular wave.
Also, the timing at which the second pulse rises is set to roughly coincide with the fall of the first pulse.
The first pulse generator 131 and the second pulse generator 132 are connected in series using a floating ground method, so that the output voltages of the first and second pulses are added together.
In this pulse combining circuit, the pulse generators 131 and 132 generate the first and second pulses and combine the generated pulses to an output pulse, in response to trigger signals sent from the synchronization pulse generating unit 103, in the following manner.
First, the first pulse generator 131 receives a trigger signal from the synchronization pulse generating unit 103, and has the first pulse rise. This first pulse falls after a short time. Almost simultaneously with this, the second pulse generator 132 receives a trigger signal from the synchronization pulse generating unit 103, and has the second pulse rise. After the voltage of the second pulse has been outputted for some time, the second pulse falls.
The pulse combining circuit shown in
(Slope of the Rising Portion of the Opposite Polarity Pulse)
When applying the opposite polarity pulse prior to the sustain pulse, it the slope at which the opposite polarity pulse rises is too sharp, in other words if the applied voltage changes widely in a very short time, a large amount of currents tends to flow and cause a decrease in luminous efficiency.
To ensure high luminous efficiency, the slope of the rising portion of the opposite polarity pulse may be made relatively gentle. However, if the slope of part of the rising portion where the absolute value of the voltage Vmin exceeds the sustain voltage Vs is made gentle, the effect of suppressing discharge delays will be lost.
In consideration of these points, it is preferable that the first half of the rising portion of the opposite polarity pulse is sloped gently to restrict currents, while the latter half of the rising portion is sloped sharply.
The slope at which the opposite polarity pulse rises can be adjusted by adjusting the slope of the rising portion of the first pulse. This can be done by adjusting a time constant of an RLC circuit in the first pulse generator 131.
Second Embodiment
In the second embodiment, the features of the pulses which are formed between the scan electrode group 19a and the sustain electrode group 19b are the same as the first embodiment.
However, the second embodiment differs with the first embodiment in the following point. The first embodiment describes the case where a voltage is applied to only one of the electrode groups 19a and 19b at a time, in other words a voltage is not applied to the sustain electrode group 19b while a sustain pulse is being applied to the sustain electrode group 19a, and a voltage is not applied to the scan electrode group 19a while a sustain pulse is being applied to electrode group 19b. In the second embodiment, on the other hand, a sustain pulse is applied to one of the scan electrode group 19a and the sustain electrode group 19b while a pulse of the same polarity as the sustain pulse is applied to the other one of the scan electrode group 19a and the sustain electrode group 19b, and the applied pulses are combined to form the pulses having the above features between the scan electrode group 19a and the sustain electrode group 19b.
In the example in
Meanwhile, immediately before a rectangular wave of the positive voltage V2 is applied to the sustain electrode 19b, a rectangular pulse of the positive voltage V1 is applied briefly to the scan electrode 19a. As soon as the pulse for the scan electrode 19a falls, the rectangular wave of the positive voltage V2 for the sustain electrode 19b rises. As a result, between the scan electrode 19a and the sustain electrode 19b, the positive voltage V1 is applied for a short time immediately before a leading edge of a negative pulse, and after this the sustain pulse of the negative voltage −V2 is applied for some time and then falls.
Thus, the pulses applied to the electrodes 19a and 19b are both rectangular waves in this example, so that there is no need to use such a pulse combining circuit as the one used in the first embodiment.
Third Embodiment
In the third embodiment, positive sustain pulses are alternately applied to the scan electrode group 19a and the sustain electrode group 19b during the discharge sustain period. Here, a voltage of a higher absolute value than normal is applied during a short time immediately after the leading edge of each sustain pulse, and a pulse of the opposite polarity is applied immediately after the trailing edge of each sustain pulse, as shown in
The following explanation focuses on the case where sustain pulses are applied to the scan electrode group 19a. Since the same applies to the case where sustain pulses are applied to the sustain electrode group 19b, the explanation for the latter case has been omitted here.
(Effect of the Sustain Pulse Waveform of the Third Embodiment)
The second and third features may be added singly. These features each deliver the following effects.
(1) Effect of the Second Feature
When a sustain pulse of a simple rectangular wave shown in
Therefore, even when the sustain voltage Vs is set at a relatively low level, the discharge is performed reliably. Which is to say, in spite of the fact that the sustain voltage Vs is fairly lower in the waveform of
In addition, setting the sustain voltage Vs lower has the effect of reducing ion currents and thereby improving luminous efficiency.
To ensure the above effects, it is preferable to set the voltage (maximum voltage Vmax in
Also, if the time (Tb) during which the higher voltage is applied is too long, a problem may arise in which a dielectric breakdown occurs in a discharge cell which should not be illuminated and causes a discharge in the discharge cell, or the power consumption increases due to currents flowing during this time. Therefore, the time Tb has to be set short to avoid the dielectric breakdown.
In consideration of these points, the higher the voltage Vmax which is applied immediately after the rise of the sustain pulse, the shorter the application time Tb of the voltage Vmax need be. In general, it is preferable to set the time Tb at 100 ns or below to limit the amount of currents flowing during this time as little as possible. Also, it is more preferable to set the time Tb at 10 ns or below, as the amount of currents flowing during such a time is almost zero.
A more remarkable effect might be obtained if the voltage Vmax applied after the rise of the sustain pulse is very high of around 400V. In this case, however, it is necessary to set the application time Tb of the voltage Vmax extremely short (10–20 ns or below). To do so, circuit performance that enables a sharp rise to such a high voltage is likely to be required.
(2) Effect of the Third Feature
In the sustain pulse waveform of
As shown in
After the sustain pulse falls, the ions which were moving toward the opposite electrode remain. These ions do not much contribute to light emission, so that the ions will become reactive currents if they reach the electrode 19b, as noted earlier.
However, if the negative pulse is applied soon after the fall time (Tc in
Here, it is preferable to set the voltage (Vmin in
When only the third feature is added to the sustain pulse, the latter part of the discharge is lost, unlike the conventional rectangular sustain pulse. This may result in a reduction in the amount of wall charge accumulated at the end of discharge. If the amount of wall charge at the end of discharge is small, it would be difficult to start a discharge reliably when the next sustain pulse of the opposite polarity is applied.
Therefore, when only the third feature is added to the sustain pulse, it is desirable to set the sustain voltage Vs higher, in order to ensure a reliable discharge.
(Circuit for Adding the Second and Third Features to the Sustain Pulse)
The above sustain pulse having the second and third features can be applied to the scan electrode group 19a and the sustain electrode group 19b by providing a pulse combining circuit shown in
This pulse combining circuit is roughly made up of a first pulse generator 231, a second pulse generator 232, and a third pulse generator 233 which generate pulses in response to trigger signals.
The first pulse generator 231 and the second pulse generator 232 generate positive voltage pulses, with the voltage of the pulse generated by the latter being set as the sustain voltage Vs.
A first pulse generated by the first pulse generator 231 is a relatively narrow waver whereas a second pulse generated by the second pulse generator 232 is a relatively wide rectangular wave.
The third pulse generator 233 generates a third pulse of negative voltage which has a narrow width. The timing at which the third pulse rises is set to coincide with the fall of the second pulse.
The pulse generators 231–233 are connected in series using a floating ground method, so that the output voltages of the first to third pulses are added together.
In this pulse combining circuit, the pulse generators 231–233 generate the first to third pulses and combine the generated pulses to an output pulse in response to trigger signals sent from the synchronization pulse generating unit 103, in the following way.
First, the first pulse generator 231 and the second pulse generator 232 receive trigger signals from the synchronization pulse generating unit 103, and have the first and second pulses rise almost simultaneously. Accordingly, a high voltage obtained as a result of adding the voltages of the first and second pulses is outputted.
The first pulse falls soon after the rise, after which only the second pulse is outputted.
Then, simultaneously with the fall of the second pulse, the third pulse generator 233 receives a trigger signal from the synchronization pulse generating unit 103, and has the third pulse of negative voltage rise. Since the third pulse falls soon after the rise, the negative pulse is briefly outputted immediately after the fall of the second pulse.
As a result, the waveform such as the one shown in
The pulse combining circuit in
In this case, it is necessary to set the voltage of the first pulse generated by the first pulse generator 231 higher than the voltage of the second pulse by about 50V or more. This requires more sophisticated circuitry, as the first pulse generator 231 has to generate a pulse of an extremely high voltage and a very short width.
(Slope of the Rising Portion of the Sustain Pulse)
When a voltage higher than the normal sustain voltage Vs is briefly applied immediately after the rise of the sustain pulse, the voltage changes more widely than the normal sustain voltage Vs for a short time after the rise. This tends to produce a large amount of currents and thereby decrease luminous efficiency.
Accordingly, to obtain high luminous efficiency, the slope of the rising portion of the sustain pulse may be made gentle in some degree. However, if the slope of part of the rising portion where the voltage exceeds the normal sustain voltage Vs is made gentle, the effect of suppressing discharge delays will be lost.
In consideration of these points, it is preferable that the first half of the rising portion is sloped gently to restrict currents, and the latter half of the rising portion is sloped sharply, as shown in
Likewise, it is preferable to set the slope of the falling portion (Td in
The slope during the rise time Ta of the sustain pulse can be adjusted by adjusting the slope of the rising portion of the first pulse or the slopes of the rising portions of both of the first and second pulses. This can be done by adjusting time constants of RLC circuits in the first pulse generator 231 and second pulse generator 232.
The slope during the fall time Td of the opposite polarity pulse can be adjusted by adjusting the slope of the falling portion of the third pulse. This can be done by adjusting a time constant of an RLC circuit in the third pulse generator 233.
(Modifications to the Third Embodiment)
Also, a modification shown in
This modification is the same as the waveform shown in
Such a modification has the same effect of improving the luminous efficiency as the third embodiment.
Note here that this kind of waveform may be spontaneously generated when a small-capacity power source (driving circuit) is used, or accidentally generated by a combination of circuits.
Also, though the second and third features are both added to the sustain pulses in the above embodiment, a sufficient effect can be obtained by applying just one of the second and third features.
Fourth Embodiment
In the fourth embodiment, the features of the sustain pulses which are applied across the scan electrode group 19a and the sustain electrode group 19b in the discharge sustain period are the same as those in the third embodiment.
However, the fourth embodiment differs with the third embodiment in the following point. The third embodiment describes the case where a voltage is not applied to the sustain electrode group 19b while a sustain pulse is being applied to the scan electrode group 19a, and a voltage is not applied to the scan electrode group 19a while a sustain pulse is being applied to the sustain electrode group 19b. In the fourth embodiment, on the other hand, pulses are applied to the scan electrode group 19a and the sustain electrode group 19b at the same time, and the applied pulses are combined to form the pulse waveform with the second and third features between the scan electrode group 19a and the sustain electrode group 19b.
Time charts in
In
Meanwhile, at the same time a rectangular pulse of the positive voltage V1 is applied to the sustain electrode 19b, a short pulse of the negative voltage −V2 whose leading edge almost coincides with the leading edge of the rectangular pulse and a short pulse of the positive voltage V3 whose leading edge almost coincides with the trailing edge of the rectangular pulse are applied to the scan electrode 19a.
As a result, between the scan electrode 19a and the sustain electrode 19b, a high negative voltage −(V1+V2) is applied for a short time after the rise, and then a negative sustain voltage −V1 is applied for some time. Immediately after the negative sustain voltage −V1 falls, the positive voltage V3 is applied briefly.
In this example, the pulses which are applied to the electrode 19a and 19b are both rectangular waves, so that there is no need to use a pulse combining circuit such as the one used in the third embodiment.
In
A pulse of a high voltage V11 (=Vmax) is applied to the scan electrode 19a, while a pulse of a low voltage V12 (=Vmax−Vs) is applied to the sustain electrode 19b shortly after the leading edge of the pulse of the voltage V11. As a result, between the scan electrode 19a and the sustain electrode 19b, the high positive voltage V11 is applied for a short time after the rise, and then a positive sustain voltage V11−V12 is applied for some time. Immediately after the positive sustain voltage V11−V12 falls, a negative pulse −V12 is applied briefly.
Following this, a pulse of the high voltage V11 is applied to the sustain electrode 19b, while a pulse of .the low voltage V12 is applied to the scan electrode 19a shortly after the leading edge of the pulse of the voltage V11 As a result, between the scan electrode 19a and the sustain electrode 19b, a high negative voltage −V11 is applied for a short time after the rise, and then a negative sustain voltage V12−V11 is applied for some time. Immediately after the negative sustain voltage V12−V11 falls, the positive pulse V12 is applied briefly.
In this example, there is no need for the sustain pulse generators 112a and 112b to apply narrow pulses, unlike in
In
In the meantime, a pulse of a positive voltage V23 is applied to the sustain electrode 19b from point t2 which is a little later than point t1, until point t3. Here, V23=V21−V22. Then a narrow pulse of a positive voltage V24 is applied to the sustain electrode 19b from point t4 to point t5.
The resulting potential difference between the electrodes 19a and 19b is as follows. The high positive voltage V21 is applied for a short time (t1 to t2) after the rise, and then the positive sustain voltage V22 (=V21−V23) is applied subsequently (t2 to t4). After the fall of the sustain voltage V22, a negative voltage −V24 is applied briefly (t4 to t5).
From point t6 to point t10, the scan electrode 19a and the sustain electrode 19b change their places, and the pulses are applied in the same way as above. As a consequence, the same waveform of the opposite polarity is formed between the electrodes 19a and 19b.
In this example, the application time of the high voltage V21 to each of the electrodes 19a and l9b is neither short nor long unlike
The above example sets V21=V22+V23, so that there is no change in potential difference between the electrodes 19a and 19b at point t3. However, this is not a limit for the present invention. A similar effect can be accomplished even when the potential difference between the electrodes 19a and 19b changes lightly at point t3.
Modifications to the First to Fourth Embodiments
The first to fourth embodiments describe the case where the features are added to all sustain pulses in the discharge sustain period. However, when the main purpose is to produce a satisfactory image display, the features do not have to be provided to all sustain pulses in the discharge sustain period but may be limited to part of the sustain pulses.
It should be noted here that when successively applying a plurality of sustain pulses to an electrode in the discharge sustain period, a discharge delay is likely to occur when a sustain pulse is first applied to the electrode. If a discharge by the first sustain pulse is performed with no substantial delay, discharges by the sustain pulses that follow can be performed easily. Accordingly, for a satisfactory image display, the features should be added at least to the first sustain pulse.
One example is that the waveform with the above features is used for the first sustain pulse, and a conventional simple rectangular waveform is used for the sustain pulses that follow.
Another example is that the waveform with the features is used when applying positive sustain pulses to the scan electrode group 19a, and the conventional simple rectangular waveform is used when applying positive sustain pulses to the sustain electrode group 19b.
In such a case, the effect of improving luminous efficiency is not as high as the case where the features are added to all sustain pulses but the effect of suppressing discharge delays is similar.
Also, the above embodiments take the surface discharge AC PDP as an example, but the invention is also applicable to an opposing discharge PDP with the same effect. In general, the invention can be applied to any panel display apparatus that writes an image by applying write pulses to discharge cells and performs a sustain discharge by applying sustain pulses to the discharge cells, and produce the same effect.
Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.
| Number | Date | Country | Kind |
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| Number | Date | Country | |
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| 20010030632 A1 | Oct 2001 | US |
