JP3375411B2 - Light-emitting diode sorting method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for selecting light emitting diodes.

[0002]

2. Description of the Related Art An active distance measuring device mounted on a camera or the like uses a light emitting diode to project distance measuring light toward an object to be measured. The light emitting diode used for such an application emits near-infrared light from the PN junction surface by supplying an electric current, and the light is applied to the object to be measured through the light projecting lens. When this light is reflected by the object to be measured and is incident on the light receiving part composed of PSD (Position Sensitive Device), etc., the distance to the object to be measured is measured based on the incident position according to the principle of triangulation. You can

In the active distance measuring device, if the light emitting amount of the light emitting diode is insufficient, the distance range in which the distance can be measured becomes narrow. Therefore, it is necessary to check the light emitting amount when a specified current is applied. is there. Also, in a camera or the like, the light emitting diode is driven by the built-in battery, but since the power supply voltage is about 6 V at most, it is difficult to obtain a sufficient drive current if the voltage drop in the light emitting diode is large.
As a result, the amount of light emission becomes insufficient and the distance range in which distance measurement is possible becomes narrow.

Under these circumstances, individual light emitting diodes delivered for distance measuring devices have been inspected. This inspection is performed by measuring the amount of light emission and its voltage drop when a specified measurement current is applied to the light emitting diode. It was selected as a good product.

[0005]

However, in reality, under the specified measurement current, even if the voltage drop is somewhat large, a sufficient amount of light emission can be obtained, and even if the amount of light emission is slightly small, the voltage drop is small. Some of them have a small value, but there are practically no problems even if they are incorporated into products. Therefore, in the conventional sorting method, since such a light emitting diode is also rejected as a defective product, a decrease in yield,
Eventually, it was also a factor in increasing costs.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method of selecting a light emitting diode that can be selected as a non-defective product if it has no practical problem.

[0007]

In order to achieve the above object, the present invention sets a power supply voltage of a driving circuit for lighting a light emitting diode to V _CC and a value of an internal resistance 6 of a driving circuit serially connected to the light emitting diode. [rho, minimum emission amount P ₀ required for the light-emitting diode, when a turn-on voltage of the light-emitting diode was beta, by applying a measuring current I _E to the light emitting diode during sorting emission amount P _E and the light emitting diode across the voltage V _E Is measured and when P _E ≧ P ₀ · (V _E −β + ρ · _IE ) / (V _cc −β) is satisfied, the light emitting diode is selected as a good product.

The minimum amount of light emission required for the light emitting diode is the maximum object distance (infinity distance) that can be measured in the case where the light emitting diode is used by being incorporated in the light projecting portion of the active distance measuring device of the camera. However, the present invention can be applied not only to the light emitting diode for the active distance measuring device, but also to the selection of the general display light emitting diode.

[0009]

FIG. 2 is an equivalent view of a light projecting circuit of an active distance measuring device for a camera. In a near infrared light emitting diode (hereinafter, IRED) 3, a switching transistor 4 for lighting control is turned on. At this time, the power source battery 5 is driven by the voltage V _CC . This light projecting circuit includes the internal resistance of the power supply battery 5, the conduction resistance of the switching transistor 4, etc., and these internal resistances 6 connected in series to the IRED 3 are connected.
Let ρ be the resistance value of and the current flowing in this circuit be I _F , then the voltage V _F across the IRED 3 when the IRED 3 is lit becomes V _F = −ρ · I _F + V _CC .

FIG. 1 shows a general I _F -V of IRED3.
_{Shows F} characteristics. As seen in the characteristic line S _E , I
When the forward voltage V _F is applied to the RED 3, the turn-on region when the current I _F starts flowing is non-linear, but as shown by the DC load line K (V _F = −ρ · I _F + V _CC ). Since the IRED3 is used in the linear region of the inclination α, the I _F -V _F characteristic of the IRED 3 can be approximated as V _F = α · I _F + β. Although there are individual differences in the slope α even if the model or part number of the IRED3 is specified, the turn-on voltage β is a value that can be approximated by the V _F axis intercept of “V _F = α · I _F ”. ing.

Further, the light emission amount P _{F of the} IRED 3 has the correlation shown in FIG. 3 with respect to the current I _F , and the I _F -P _F characteristic shows linearity until the saturation region is reached. Therefore straight portions and the operation region of the IRED 3, when the slope of the linear portion and gamma, generally I _F -P _F characteristic can be represented by _{P F = γ · I F ·····} . However, there is individual difference in the value of the inclination γ, and variations are recognized even if the model or part number of IRED3 is specified.

From the equations and the equations, I _F = (V _CC −β) / (α + ρ) ... Therefore, substituting the equation into the equation, P _F = γ · (V _CC −β) / ( α + ρ) is obtained. Therefore, if the minimum light emission amount required for the IRED 3 is P ₀ from the maximum object distance that can be measured by the active distance measuring device, P ₀ ≦ γ · (V _CC −β) / (α + ρ)・ ・ It is necessary to drive IRED3 under the conditions.

In the formula, the values of α and γ are IRED3
Due to the individual difference of
_{When E} is flown and the light emission amount P _E and the voltage drop V _{E at} that time are respectively measured, V _E = α · _IE + β ∴α = (V _E −β) / _IE P _E = γ · Since I _E ∴γ = P _E / I _E , by substituting them into the equation and transforming, P _E ≧ P ₀ · (V _E −β + ρ · I _E ) / (V _CC −β) ...・ Is obtained.

In the equation, the value of β can be regarded as known because it can be approximated as a constant value with no individual difference if the model of IRED3 is specified. Further, the resistance value ρ is known as a circuit constant of the light projecting circuit of FIG. 2, and P ₀ is I
Since it is a value preset as the minimum light emission amount of RED3, it is possible to determine whether or not the expression is satisfied by measuring the light emission amount P _E and the voltage drop V _E under the specified current I _E. it can.

The determination of the quality of the IRED3 according to the above equation is based on the assumption that the I _F -P _F characteristic of FIG. 3 will be used in the linear portion, and then the specified current I _E shown in FIG. Thus, the inclination α of the characteristic line included in the operation region X of the DC load line K and the γ of the IRED 3 (FIG. 3) are determined. Therefore, the IRED3 of the light emission amount P _E and the voltage drop V _E satisfying the formula is smaller than the light emission amount P _E under the specified current I _E than the light emission amount which is the standard for determining the quality or the voltage drop. Even if V _E is larger than the conventional acceptance / rejection criterion, it can be selected as a non-defective product.

Next, a specific example will be described. As an example, a certain type of IRED is used as a light emitting element of an active range finder for a camera. Taking histogram data for many of the IREDs, the value of the voltage drop V _E when 1.0 A is applied as the specified current I _E is 1.
At 90V, the control limit value by the 3σ method of quality control is -3
σ was 1.70V and + 3σ was 2.10V. Also,
The turn-on voltage β (see FIG. 1) has a value of 1.30V.

Among the above-mentioned IREDs, those whose characteristics are known in advance are equivalently incorporated in a light projecting circuit of an active distance measuring device shown in FIG. 2, and a current I _F is 1.0 A.
The distance measurement experiment was carried out for a maximum subject distance of about 7 m which can be measured by changing the range from 0.1 to 1.5 A by 0.1 A. As a result, it was found that the IRED3 used in this experiment cannot measure the distance to 7 m unless a current of at least 1.4 A is applied. Since this IRED3 has the I _F -P _F characteristic shown in FIG. 4, it can be seen that the value of the minimum light emission amount P ₀ is about 30 mw.

In FIG. 5, in order to confirm the reliability of the distance measuring device used in the experiment, a current of 1.5 A is passed through the IRED 3 to measure the distance while changing the object distance, and the result appears on the light receiving part of the distance measuring device. It shows the result of measuring the output. The horizontal axis is the reciprocal of the subject distance L, and the vertical axis is the subject distance of 1 m.
The increase / decrease in the output at the time is expressed as a percentage. As shown in the figure, since each measurement point is on a substantially straight line, it can be seen that this distance measuring device has satisfactory reliability.

The constant of the light projecting circuit equivalently shown in FIG.
Based on the results of the battery durability test at room temperature, V _CC
Since ≈2.75V, ρ≈0.35Ω, and the turn-on voltage β is 1.3V, assuming that the minimum light emission amount P ₀ obtained in the above-mentioned experiment is 30 mW, the above equation is P _E ≧ It becomes 20.7 * (V _E -1.3 + 0.35 * _IE ). The value of the specified current I _E at the time of inspection is 1.0 A
When set to, the above equation becomes P _E ≧ 20.7 (V _E −0.95).

Therefore, the IRED to be inspected is 1.0
By measuring the light emission amount P _E and the voltage drop V _E when the current of A is passed, it is possible to easily select pass / fail by the formula. According to this sorting method, it is possible to determine a product having no practical problem as a non-defective product, as compared with a conventional sorting method that requires both the light emission amount and the voltage drop to clear the respective reference values. . Therefore, the non-defective rate is improved as compared with the conventional sorting method. In addition, even if the IRED which became a good product by this method was used, there was no problem in the distance measuring performance.

The power supply voltage V _{CC of the} light projecting circuit and the resistance value ρ of the internal resistance 6 are likely to change due to variations in the switching transistor 4 and the power supply battery 5 or the influence of temperature. Therefore, when the sorting method according to the present invention is actually used, the value of the minimum light emission amount P ₀ is set to a large value in consideration of these influences, or correction is performed in consideration of the temperature characteristic of the IRED itself. Is preferred. The method of the present invention can be applied not only to the light emitting diode used in the active distance measuring device but also to the light emitting diode for general lighting display.

[0022]

As described above, in the method of the present invention, the light emission amount and the voltage drop of the light emitting diode to be inspected are not individually evaluated, but both the light emission amount and the voltage drop are simultaneously evaluated. Since the selection is performed in this manner, the non-defective product rate can be increased within a range where there is no practical problem, and it is very effective in improving the yield and reducing the cost.

[Brief description of drawings]

FIG. 1 is a general I _F -V _F characteristic diagram of IRED.

FIG. 2 is an equivalent circuit of a light projecting circuit used in an active distance measuring device.

FIG. 3 is a general I _F -P _E characteristic diagram of IRED.

FIG. 4 is an I _F -P _E characteristic diagram of IRED used in the experiment.

FIG. 5 is a graph showing an output ratio of the light receiving unit with respect to the reciprocal of the object distance of the active distance measuring device used in the experiment.

[Explanation of symbols] 3 IRED 4 switching transistors 5 power battery 6 Internal resistance

Continuation of front page (56) Reference JP-A-5-190636 (JP, A) JP-A-62-276431 (JP, A) JP-A-58-196468 (JP, A) JP-A-2-198178 (JP , A) JP-A-6-310756 (JP, A) Actual development 5-90466 (JP, U) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01L 33/00 G01R 31/26 H01L 21/66

Claims (2)

(57) [Claims] 1. A method of selecting a light-emitting diode, which is incorporated in a drive circuit to which a voltage V _CC is applied and is driven so as to obtain at least a light emission amount P _0, wherein the drive circuit connected in series to the light-emitting diode is used. When the resistance is ρ and the turn-on voltage of the light emitting diode is β, a measurement current I _E is passed through the light emitting diode at the time of selection to measure the voltage V _E across the light emitting diode and the light emission amount P _E, and P _E ≧ P ₀ · A method of selecting a light emitting diode, characterized in that it is judged as a non-defective product when (V _E −β + ρ · _IE ) / (V _cc −β) is satisfied. 2. The light emitting diode is a light emitting diode that is incorporated in a light projecting unit of an active distance measuring device and projects near infrared light for distance measurement toward a subject. 1. The method for selecting a light emitting diode according to 1.