Light emitting diode array

An LED array composed of a plurality of LEDs is disposed on either side of a signal interconnection board, while metal plates are disposed on the other side of the signal interconnection board as positive and negative power source lines for driving the LEDs, respectively, and a capacitor is provided between the positive metal plate for power supply and the negative metal plate for power supply. As a result, the LED array in which an output light is not lowered in power, no variations in light output arises in every LEDs, and all the LEDs can be turned on simultaneously is provided.

FIELD OF THE INVENTION
 The present invention relates to a light emitting diode (hereinafter
 referred to simply as "LED") array, and more particularly to an LED array
 in which an output light is not lowered in power, the uniformity of output
 light of each LED is improved, and all of the LEDs are turned on
 simultaneously.
 BACKGROUND OF THE INVENTION
 A conventional LED array is shown in FIGS. 1 and 2 wherein an LED chip 1 is
 mounted on either side (the top surface in this case) of a signal
 interconnection board 2, while (positive-) and (negative-) N-patterned
 power supply lines 11 and 12 for driving the LED chip (LED chips) are
 provided on the other side (the bottom surface in this case) of the signal
 interconnection board 2 to constitute the LED array. As the LED chip 1,
 either the one which has been prepared from a plurality of LEDs into a
 single chip, or a plurality of chips each of which has been prepared in
 the form of a single LED may be applied. In either case, an LED array is
 constituted on the top surface of the signal interconnection board 2. The
 patterned power supply lines 11 and 12 are prepared by a printed wiring
 operation wherein the P-patterned power supply line 11 and the N-patterned
 power supply line 12 are provided by a printed wiring manner,
 respectively. These patterned power supply lines 11 and 12 are connected
 to an external power supply through an interconnecting means.
 It is to be noted that there has heretofore been no such LED array wherein
 a capacitor is provided between positive and negative power supply lines.
 Meanwhile, power supply voltage for driving LED is applied between positive
 and negative power supply lines. In this case, voltage drop occurs due to
 rush current at a moment when the LED is turned on. For this reason when
 all the LEDs are turned on at the same time, lowering of light output and
 variations in light output among the respective LEDs arise. In order to
 prevent occurrences of such lowering of light output and such variations
 in light output, LEDs have been divided into a plurality of groups, and
 they have been sequentially turned on while accomplishing time shifts in
 every groups. Because of such sequential turn-on for LEDs, it was
 difficult to establish prompt operation for selecting turn-on or turn-off
 action for LED arrays.
 Furthermore, since power supply lines are ones which have been patterned on
 a signal interconnection board, large amounts of current cannot be
 applied. Hence, current to be applied to respective LEDs are also
 restricted, so that high light output cannot be obtained.
 SUMMARY OF THE INVENTION
 Accordingly, an object of the present invention is to provide an LED array
 wherein an output light is not lowered in power, no variations in output
 light appear in every LEDs, and all the LEDs can be turned on
 simultaneously. According to the invention, an LED array, comprises: a
 signal interconnection board having signal interconnections; a plurality
 of LEDs each connected to corresponding terminals of the signal
 interconnections of the signal interconnection board, the plurality of
 LEDs being arranged in array on a first plane of the signal
 interconnection boards; positive and negative power supply metal plates
 connected to a power supply at first ends thereof and to the signal
 interconnections of the signal interconnection board at a second ends
 thereof, the positive and negative power supply metal plates being
 arranged on a second plane of the signal interconnection board; and at
 least one capacitor provided between the positive and negative power
 supply metal plates.
 The above described capacitor may have a capacitance C meeting the
 equation:
EQU C.gtoreq.(K+I) (VO.times.t)
 where VO is a power supply voltage for driving the plurality of LEDs at an
 ordinary state, I is a total current at the time of turning the plurality
 of LEDs on, t is a period of time in which a voltage lowered instantly at
 a time of turning the plurality of LED on is restored to be the voltage
 VO, and K is a constant.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 A side of the LED array according to the invention is shown in FIG. 3, and
 the bottom of which is shown in FIG. 4. Further, a circuit diagram for the
 LED array is shown in FIG. 5. In these figures, components common to those
 in the figures are designated by the same reference numerals,
 respectively.
 As shown in the figures, the LED array of the invention is constituted in
 such that an LED chip 1 is mounted on the top of a signal interconnection
 board 2, metal plates 3 and 4 are disposed along the bottom of the signal
 interconnection board 2 as positive and negative power source lines for
 driving the LED separately from a pattern of a substrate, and a capacitor
 6 is provided between the metal plate 3 being a power source line on the
 positive (+) side and the metal plate 4 being a power source line on the
 negative (-) side. Each of the metal plates 3 and 4 covers substantially a
 half section of the signal interconnection board 2 in its breadth
 direction, and each of the metal plates projects from both ends of the
 signal interconnection board 2 in the longitudinal direction thereof, and
 each terminal for interconnection is provided on the projected extra
 portion.
 The metal plates 3 and 4 are connected to an external DC power supply 5
 through metal interconnections, respectively. As shown in FIGS. 3 and 4,
 the capacitor 6 is positioned in the vicinity of the metal plates 3 and 4
 on the way from these metal plates to the DC power supply. Moreover, as
 shown in FIG. 5, the capacitor 6 is positioned in the vicinity of terminal
 connecting apertures 3a and 4a for connecting the interconnections
 extending from the metal plates 3 and 4 to the DC power supply 5. As
 described herein, it is preferable to position the capacitor 6 in the
 vicinity of a place where the metal plates 3 and 4 are connected to the
 interconnections toward the DC power supply as much as possible.
 The LED chip 1 is the one wherein a MOS transistor 8 for turning on and off
 an LED is provided to each LED 1. An anode of each LED a is connected to
 the positive metal plate 3 for power supply, while a cathode of each LED 1
 is connected to a terminal on the load side of each MOS transistor 8, and
 a terminal on the ground side of each MOS transistor 8 is connected to the
 negative metal plate 4 for power supply.
 In operation, a procedure of turning simultaneously all of LEDs 1 on for a
 predetermined period of time, and then, turning simultaneously all the
 LEDs 1 off for a predetermined period of time is repeated under the
 condition where a power supply voltage VO for driving the LEDs at an
 ordinary state is 3.5 V. In this case, the power supply voltage for
 driving the LEDs exhibits behavior of lowering itself at the moment when
 all the LEDs 1 are simultaneously turned on as shown in FIG. 7. However,
 since the electric charge stored in the capacitor is discharged, the
 voltage is restored to its initial state. As a result, the LEDs 1 can
 obtain a normal voltage during substantially the whole turn-on period of
 time. In order to compare such action with that of a prior art, when the
 above described turn-on and turn-off operations are repeated in a
 conventional LED array, a power supply voltage for driving the LEDs lowers
 drastically at the moment when all the LEDs are turned simultaneously on,
 thereafter, the voltage is hardly restored to its initial state, and at
 last, a normal power supply voltage for driving the LEDs cannot be
 obtained until a turn-on period of time is finished as shown in FIG. 6.
 Thus, it is understood that the remarkable voltage fluctuations as shown
 in FIG. 6 are significantly improved by the present invention as shown in
 FIG. 7.
 When specific numerical values as indicated hereinafter substitute for the
 following equation:
EQU C.gtoreq.(K.times.I) (VO.times.t)
 a capacitance C of the capacitor 6 was determined as C.sub.-- 3000 .mu. F
 wherein a power supply voltage for driving LEDs at an ordinary state is
 3.5 V, a total current I at the time of turning all the LEDs on is 25 A, a
 period of time t in which a voltage lowered instantly at a time of turning
 the LEDs on is restored to be the voltage VD is 2.5 .mu.s, and a constant
 x is 1.05.times.10-8. As a result, C=3000 .mu.F in the present embodiment.
 When a capacitor having a capacitance of 3000 .mu.F is used as the
 capacitor 6, the results observed of a power supply voltage for driving
 the LEDs are as shown in FIG. 7. As described above, a capacitance of the
 capacitor can be calculated dependent upon a desired period of time for
 restoring voltage drop by employing a rated voltage and current as well as
 a fixed constant, so that a capacitance of the capacitor in the case where
 a high-speed operation is required can easily be also determined.
 In the following, another embodiment will be described.
 While one capacitor having a high capacitance has been used in the above
 described embodiment, a plurality of capacitors each having a low
 capacitance may be employed. For instance, the same effects as in the
 above described embodiment can be obtained in the case where a number of
 capacitors 6 are provided between metal plates 3 and 4 for power supply
 disposed on the bottom of a signal interconnection board 2 as shown in
 FIGS. 8 and 9.
 Although the invention has been described in only the case where all of
 LEDs are turned on simultaneously, the present invention is also effective
 for the case where LEDs are partly turned on as a matter of course.
 As described above, since metal plates have been provided separately from
 patterned interconnections as power supply lines for driving LEDs in the
 present invention, voltage drop due to a large current is suppressed as a
 result of lowering impedance of the power supply lines. Besides, since a
 capacitor is provided between a positive metal plate for power supply and
 a negative metal plate for power supply, a rapid voltage drop is
 moderated, and the voltage can be promptly restored. Accordingly, even if
 all of LEDs are simultaneously turned on by means of a high output, there
 are no lowering of light output; and no occurrence of variations therein
 so that an operation for switching turn-on to turn-off and vice versa at
 high speed becomes also possible. Thus, when the LED array of the
 invention is applied to an optical printer head and the like, high-speed
 printing becomes possible while maintaining even and high print quality.
 According to the present invention, the following excellent advantages can
 be obtained.
 (1) Since fluctuations in a power supply voltage for driving LEDs are
 suppressed, variations in light output among the LEDs can be canceled.
 (2) It becomes possible to turn simultaneously all the LEDs on, whereby
 high-speed operation comes to be possible.
 It will be appreciated by those of ordinary skill in the art that the
 present invention can be embodied in other specific forms without
 departing from the spirit or essential characteristics thereof.
 The presently disclosed embodiments are therefore considered in all
 respects to be illustrative and not restrictive. The scope of the
 invention is indicated by the appended claims rather than the foregoing
 description, and all changes that come within the meaning and range of
 equivalents thereof are intended to be embraced therein.