Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
1999-07-07
2001-01-16
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S2090SC, C315S283000, C315S360000, C315S291000
Reexamination Certificate
active
06175191
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a drive circuit for an EL (Electroluminescent) element.
2. Description of the Related Art
There are examples at the present time of an EL element which is used as a backlight for a liquid crystal display provided on a small electronic appliance, such as a wristwatch. A drive circuit is required to generate an alternating current to drive an EL element of this kind. Although it has been a general practice to use a transformer as a voltage increasing element, this is not advantageous in encouraging size reduction.
FIG. 6
shows the type of drive circuit previously proposed. This circuit includes a voltage increasing circuit X
1
to increase the voltage between a power terminal VDD and a power terminal VSS, and an H bridge circuit X
3
to apply an output voltage of the voltage increasing circuit X
1
to an EL element X
2
. The voltage increasing circuit X
1
has a coil X
4
and switch element X
5
connected in series between the power terminal VDD and the power terminal VSS, and a diode X
6
connected to a connection point between the coil X
4
and the switch element X
5
. By intermittently turning on and off the switch element X
5
, an induced voltage is caused on the coil X
4
and outputted through the diode X
6
. The H bridge circuit X
3
has switch elements X
7
and X
8
connected in series between the diode X
6
and the power terminal VSS, and switch elements X
9
and X
10
connected in series similarly between the diode X
6
and the power terminal VSS, and an EL element X
2
provided between a connection point between the switch elements X
7
and X
8
and a connection point between the switch elements X
9
and X
10
, thereby charging and discharging the EL element in respective directions.
More explicitly, with reference to the timing graph of
FIG. 7
, if at time t0 the switch elements X
7
and X
10
are first turned on and then the switch element X
5
begins intermittent on and off operation, the EL element X
2
beings to be charged to a gradually increasing terminal-to-terminal voltage of the EL element X
2
of approximately 100 V. Next, if at time t1 the switch element X
5
is stopped from its intermittent on and off operation and the switch element X
7
is turned off and the switch element X
8
is turned on, then the voltage on the EL element X
2
is discharged. Next, at time t2 the switch element X
10
is turned off and the switch element X
9
is turned on, and then the switch element X
5
begins intermittent on and off operation. This causes the EL element X
2
to be charged from the opposite terminal side thereof. Next, if at time t3 the switch element X
5
is stopped from its intermittent on and off operation and the switch element X
9
is turned off and the switch element X
10
is turned on, then the voltage on the EL element X
2
is discharged. By repeating the above operation, the EL element X
2
is discharged in respective directions. Each switch element may be, for example, a bipolar transistor or a MOS transistor.
SUMMARY OF THE INVENTION
In the structure shown in
FIG. 6
, discharge is abruptly carried out in a short circuit state, thereby posing the following problems.
(1) Where the EL element uses a piezoelectric material, the abruptly changed charging voltage causes deformation in the EL element, producing sound.
(2) Instantaneous flow of electric current might cause burning at or around an electrode of the EL element where electric current concentrates.
(3) A large current at discharge causes radio wave noise, exerting noise on a peripheral circuit.
(4) Because the EL element has terminal-to-terminal voltage waveforms containing a high frequency component at discharge, the luminance half value period (life) becomes short.
(5) Because of the large flowing current, the transistor used as a switch element requires increased size.
Therefore, in the present invention, a coil and a switch element are provided on a discharge path for an EL element and the switch element is kept in the on position for a proper duration to cause intermittent discharge through the coil, thereby suppressing the EL element from abruptly discharging. Due to this, it is possible to suppress noise, electrode burning and reduce the size of the switch element.
In a drive circuit for an EL element in which the EL element is repeatedly gradually charged to increase the terminal-to-terminal voltage of the EL element to a required voltage and thereafter the voltage is discharged, a coil and a switch element are provided on a discharge path from the EL element such that the switch element is turned on for an appropriate duration so as to cause intermittent discharge through the coil.
Preferably, a drive circuit for an EL element comprises: a first switch element for intermittently connecting for a first time period a coil between a first power source and a second power source having a potential lower than the first power source, causing an induced voltage on the coil; an H bridge circuit for connecting the EL element in alternating directions between a terminal allowing output of the induced voltage and a terminal of the second power source; and a second switch element for intermittently connecting the terminal on the higher potential side of the EL element, charged by the induced voltage, to the coil for a second time period, after the first time period, to cause the EL element to discharge through the coil.
More preferably, a drive circuit for an EL element, comprises: a first switch element for intermittently connecting for a first time period a first coil between a first power source and a second power source having a potential lower than the first power source, causing an induced voltage on the coil; an H bridge circuit for connecting the EL element in alternating directions between a terminal allowing output of the induced voltage and a terminal of the second power source; and a second switch element for intermittently connecting the terminal on a higher potential side of the EL element, charged by the induced voltage, to a second coil for a second time period, after the first time period, to cause the EL element to discharge through the coil.
More preferably, the induced voltage is supplied to the EL element after a capacitor is charged, and the charge voltage of the capacitor being discharged together with discharge of the EL element through the coil.
More preferably, the intermittent discharge time starts out relatively short and is gradually increased.
REFERENCES:
patent: 4070663 (1978-01-01), Kanatani et al.
patent: 4540899 (1985-09-01), Pella
patent: 5313141 (1994-05-01), Kimball
patent: 5336978 (1994-08-01), Alessio
patent: 5349269 (1994-09-01), Kimball
patent: 5789870 (1998-08-01), Remson
patent: 5886475 (1999-03-01), Horiuchi et al.
patent: 6005350 (1999-12-01), Hachiya
patent: 2 317 722 (1977-02-01), None
patent: 07-240289 (1995-09-01), None
Amster Rothstein & Ebenstein
Philogene Haissa
Seiko Precision Inc.
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