Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2001-11-30
2004-03-09
Vo, Tuyet T. (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C347S237000, C347S238000
Reexamination Certificate
active
06703790
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for driving a self-scanning light-emitting element array, particularly to a method for driving a self-scanning light-emitting element array using three-terminal light-emitting thyristors.
BACKGROUND ART
A light-emitting element array in which a plurality of light-emitting elements are arrayed on the same substrate is utilized as a light source of a printer, in combination with a driver circuit. The inventors of the present invention have interested in a three-terminal light-emitting thyristor having a PNPN-structure as a component of the light-emitting element array, and have already filed several patent applications (for example, Japanese Patent Publication Nos. 1-238962, 2-14584, 2-92650, and 2-92651). These publications have disclosed that a self-scanning function for light-emitting elements may be implemented, and further have disclosed that such self-scanning light-emitting element array has a simple and compact structure for a light source of a printer, and has smaller arranging pitch of light-emitting elements.
The inventors have further provided a self-scanning light-emitting device having such structure that a transfer portion including a transfer element array is separated from a light-emitting portion including a light-emitting element array (see Japanese Patent Publication No. 2-263668).
Referring to
FIG. 1
, there is shown a self-scanning light-emitting element array chip
10
in which a transfer portion
10
-
1
is separated from a light-emitting portion
10
-
2
, and a driver circuit
40
for driving the transfer portion and light-emitting portion, the self-scanning light-emitting element array chip being a type of two-phase (clock pulses Ø1 and Ø2) driving and diode coupling. The transfer portion
10
-
1
comprises transfer elements T
1
, T
2
, T
3
, . . . , diodes D, and load resistors R
1
, R
2
, R
3
, . . . . The light-emitting portion
10
-
2
comprises light-emitting elements L
1
, L
2
, L
3
, . . . The transfer element and light-emitting element are composed of a three-terminal light-emitting thyristor, respectively.
The transfer portion
10
-
1
further comprises a Ø1 line
11
, a Ø2 line
12
, and a power supply (V
GK
) line
14
. The Ø1 line
11
is connected to a Ø1 terminal
21
through a current limiting resistor
31
provided within the chip
10
, the Ø2 line
12
is connected to a Ø2 terminal
22
through a current limiting resistor
32
provided within the chip
10
, and the V
GK
line
14
is connected to a V
GK
terminal
24
. The gate of a transfer element T
1
is connected to a start pulse (Ø
s
) terminal
23
through a current limiting resistor
33
.
The light-transmitting portion
10
-
2
comprises a write signal (Ø
I
) line
15
which is connected to a Ø
I
terminal
25
.
The driver circuit
40
comprises four CMOS inverters
50
-
1
,
50
-
2
,
50
-
3
and
50
-
5
each consisting of a PMOS transistor (normally on)
51
and an NMOS transistor (normally off)
52
. Each high level terminal of the CMOS converters is connected to a common power supply (+5V) line (or +5V power supply)
48
.
The driver circuit
40
further comprises an input terminal
41
for Ø1, an input terminal
42
for Ø2, an input terminal
43
for Ø
S
, and an input terminal
45
for Ø
I
.
A current limiting resistor
35
is provided between the CMOS inverter
50
-
5
of the driver circuit
40
and the Ø
I
terminal
25
of the light-emitting element array chip
10
, and outside the chip
10
.
The operating voltage of the transfer portion
10
-
1
of the self-scanning light-emitting array shown in
FIG. 1
is needed to be at least 2V
D
(V
D
is a forward voltage of PN-junction in a light-emitting thyristor). V
D
is about 1.5V when the material for PNPN-structure is GaAs, so that the minimum operating voltage for the transfer portion becomes 3 volts. In practice, the self-scanning light-emitting element array is operated by a single power supply of about 5 volts in order not to be unstable in operation due to a parasitic resistance and a noise.
In the conventional self-scanning light-emitting array shown in
FIG. 1
, when a power supply voltage of 5 volts is used, the turn-on voltage of a light-emitting thyristor in the light-emitting portion
10
-
2
is substantially equal to the forward voltage V
D
(1.5 volts) of PN-junction. Therefore, a voltage drop of 3.5 (=5-1.5) volts is caused across the resistor
35
provided outside the chip. Assuming that a current through the light-emitting portion is 10 mA (an average value in time), a power consumed in the resistor
35
is 35 mW. On the other hand, a power consumed in the light-emitting portion is 15 mW. Therefore, when a plurality of self-scanning light-emitting element array chips, for example 60 chips are arrayed to form an optical writing head, the total power consumed in the head when one light-emitting element per chip is lighted up becomes 3W (=50 mW×60 chips). The heat generated by consumed power causes the temperature rising of the self-scanning light-emitting element array chip, resulting in the problem of the decrease of the luminous efficiency of light-emitting elements. In addition, the optical writing head is positioned in a narrow and bad exhaust head environment, so that the temperature in the printer rises to have an effect on the image formed by an electrophotographic printer.
As to the effect on the image due to the temperature rising of self-scanning light-emitting element array chips, the following causes are conceivable.
(1) When a pattern being light in color such as half-tone is printed just after printing a table including horizontal rules, a part of the pattern corresponding to the horizontal rules is missed in color, resulting in the degradation of an image quality. This is because the particular distribution in temperature is caused on the chip due to the printing of horizontal rules, and the luminous efficiency of the light-emitting elements contributing to the printing of horizontal rules is decreased.
(2) Whereas the temperature of the head at the start of printing is low, the temperature within the printer is gradually increased, so that the light output of the head is varied. This variation is large at the beginning of the printing, resulting in the problem.
(3) In the case of a self-scanning light-emitting element array having a structure such that adjacent light-emitting elements may be possible to be lighted up at the same time, the temperature rising is varied based on whether one light-emitting element is lighted up together with the other light-emitting element. As a result, there is a problem in that the photographic density is varied depending upon a pattern to be printed.
(4) The volume of a body through which the heat is dispersed at the light-emitting elements at the both ends of the chip is one-half of that at the center of the chip, resulting in a high heat resistance of said body. Therefore, the temperature rising of the light-emitting elements at the both ends of the chip becomes two times that at the center of the chip. As a result, there is a problem in that the light output at the both ends of the chip is decreased.
In order to resolve these problems, the technique has been proposed in which a uniform temperature distribution may be realized through a chip by causing the power consumption at the transfer portion when the light-emitting elements are not lighted up (see Japanese Patent Publication Nos. 8-264838 and 11-170596). According to this technique, the problem of an image degradation in the case (1) described above may be addressed, but the temperature rising of the head becomes larger because the same power as that when all the light-emitting elements are lighted up is consumed. The percentage of light-emitting elements lighted up is less than 20% in the case of conventional color printing, so that it is not effective to design an optical writing head under the assumption that all the light-emitting elements are always lighted up. Also, this technique may not address the problem of
Nippon Sheet Glass Co. Ltd.
RatnerPrestia
Vo Tuyet T.
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