JP5645470B2 - LED drive device - Google Patents

Description

  The present invention relates to an LED driving device that turns on a light emitting diode (LED) serving as a light source, and is particularly suitable for an LED illumination device that requires high-speed response such as an image processing device for product inspection.

In recent years, production lines for various industrial products have built-in product identification devices and product inspection devices that use images, and as the speed of image processing increases, shape determination is instantaneously performed for all products that flow at high speed. It is also possible to do.
In such a case, it is necessary to capture images continuously in a short time, and for this purpose, an illumination device using an LED (light emitting diode) as a light source is used.
When the LEDs are continuously lit, there is a problem that not only energy loss occurs when the LEDs are lit while the image is not taken, but also the LEDs generate heat due to continuous lighting.
For this reason, normally, for example, the LED is pulsed in sync with a digital camera whose shutter is released at the timing when the product arrives at the imaging position, thereby suppressing energy loss and heat generation and capturing images at high speed. ing.

FIG. 4 shows a general driving device of such an LED lighting device. In the case of LED lighting used in image processing, control is performed using a constant current circuit in order to ensure the uniformity of the luminance value of the LED. Often done.
In the driving device 42 for lighting the LED array 41 having one or more LEDs arranged, a constant current circuit 45 is interposed in a power supply circuit 44 that supplies power from a main power supply 43.
The constant current circuit 45 uses a current detection resistor 46 that outputs a detection voltage corresponding to the current flowing through the power supply circuit 44, and a lighting current that lights the LED array 41 as a target current, and a reference that generates a reference voltage corresponding to the target current. A voltage generator 47 and a comparison circuit 49 that compares the detected voltage with a reference voltage and outputs a control signal to a current control element 48 that is a field effect transistor so that the detected voltage matches the reference voltage are provided.
Then, for blinking the LED array 41 at a predetermined timing, it is provided with a switch S ₁ or S ₂ in one of the power supply circuit 44 and the constant current circuit 45.
According to this, the LED array 41 can be blinked by providing any _{one of} the switches S ₁ and S ₂ and turning it on / off in accordance with the shutter timing of the digital camera (not shown).

However, when the switch S ₁ provided in the power supply circuit 44 is turned on and off to blink the LED, the comparison circuit 49 of the constant current circuit 45 does not flow even when the switch S ₁ is cut off and no current flows through the power supply circuit 44. Since the detection voltage is maintained at 0 and does not rise to the reference voltage, the maximum control voltage is applied from the comparison circuit 49 to the current control element 48.
In this state, when the LED light by conducting the switch S _1, since the overcurrent (rush current) flows through the current control device 48, unfavorably may shorten the LED lifetime. In addition, since it takes time to stabilize, there is a limit to high-speed response.

Further, by turning on and off the switches S ₂ provided in the constant current circuit 45 performs switching of the current control element 48 by shorting between gate / source of the current control element 48, but the LED may be flashing, Also in this case, since the LED blinks in a state where the comparison circuit 49 is always driven, the excessive current cannot be solved.

  Therefore, as shown in FIG. 5, a constant voltage circuit 53 that applies a constant voltage lower than the threshold voltage of the LED 52 in the forward direction is connected between both terminals of the LED 52 interposed in the constant current power supply circuit 51. There has been proposed a circuit in which a switch 54 is interposed in the constant voltage circuit 53 and the circuit resistance of the constant voltage circuit 53 when the switch 54 is turned on is smaller than the circuit resistance of the LED 52 (see Patent Document 1).

  According to this, when the switch 54 is cut off, a constant current power supply circuit 51 supplies a lighting current for turning on the LED 52. Therefore, if the LED 52 is turned on and the switch 54 is turned on when the switch 54 is turned off, the constant current power supply circuit Since the current supplied from 51 flows into the constant voltage circuit 53 having a small circuit resistance, the LED 52 is extinguished. At the same time, a standby voltage lower than the threshold voltage is applied to both ends of the LED 52 from the constant voltage circuit 53 and floats on the LED 52. Since the capacity is accumulated, the rise time can be shortened when the LED 51 is turned on.

However, since the power supply circuit 51 and the constant voltage circuit 53 form a closed circuit, and the reverse bias voltage is applied to the constant current power supply circuit 51 by the constant voltage circuit 53, the voltage of the constant current power supply circuit 51 at that time point And a voltage difference between the voltage of the constant voltage circuit 53 is applied to the constant current power supply circuit 51 and causes heat generation.
This Patent Document 1 is a driver circuit for an LED 52 as a light source for optical communication. Since it is sufficient to perform blinking control of one LED, the differential pressure is small, and the current flowing through the LED 52 is as small as about 100 mA. The amount of heat generated in the power supply circuit 51 can also be ignored.
However, when it is used as an illumination circuit for lighting a large number of LEDs simultaneously, it is expected that both current and voltage will be several to several tens of times higher.
In particular, when a large number of LEDs are connected and used as a lighting device, individual differences occur in the forward voltage as the characteristics of the LED, or the forward voltage changes due to the heat of the LED. If the difference between the LED and the threshold voltage of the LED is small, malfunction may occur.
For this reason, when used for illumination, the difference between the voltage of the constant voltage circuit 53 and the threshold voltage of the LED must be large, and the reverse bias applied to the constant current power supply circuit 51 when the LED is turned off. The voltage increases, causing heat generation.

  For this reason, a device has also been proposed in which a standby current lower than the lighting start current (operating current) is supplied to the LED array, and the LED lighting rise time is shortened when the lighting current is supplied (see Patent Document 2). ).

In this case, a method of constantly supplying the standby current and two methods of starting the supply immediately before the lighting of the LED can be considered.
However, in the case of constant supply, there is a problem that the life of the LED is shortened because a current flows through the LED even when it is turned off, and the LED is always in a heat generating state.
In addition, when supplying the LED immediately before the lighting of the LED, in addition to the timing control for lighting the LED, the timing control for starting the supply of the standby current in a predetermined time slightly earlier than that must be performed. There is a problem of being troublesome.
Furthermore, in order to switch between the operating current and the standby current, the operating current indication value and the standby current indication value must be switched and supplied in accordance with the lighting timing of the LED, so that there is still a problem that the control is complicated and troublesome. .

JP-A-63-250873 JP 2008-27874 A

  Therefore, according to the present invention, even when the LED is blinked by pulse control, the start-up time can be shortened when the LED is turned on, and at the same time, the LED generates heat without flowing any current when the LED is turned off. The technical challenge is to reduce the amount.

In order to solve this problem, according to the present invention, a current control element, an LED array, and a current detection resistor are arranged in series in a main circuit to which a DC power supply voltage is applied, and are detected by the current detection resistor during lighting . In the LED lighting device provided with a constant current circuit for controlling the current control element so that the current is maintained constant,
The bypass circuit connected in parallel to the LED array is connected in series with a switching element that conducts / cuts off the circuit and a current limiting resistor that restricts a current flowing through the circuit when conducting ,
The constant current circuit is connected to the positive side of the current detection resistor positive input terminal for detecting the current through the current limiting resistor, minus-side input terminal connected to the ground side of the current detection resistor Rutotomoni ,
The resistance value of the current limiting resistor is set such that a voltage applied to a branch point between the bypass circuit and the main circuit when a switching element is turned on is lower than a threshold voltage of the LED array . .

  According to the LED lighting device of the present invention, the potential of the voltage drop generated between the positive side input terminal and the negative side input terminal of the constant current circuit is detected, and this is set to a preset voltage (for example, the rating of the LED array). The current flowing through the main circuit is controlled by the current control element so that the reference voltage is maintained at that reference voltage.

At the time of lighting, when a power supply voltage equal to or higher than the threshold voltage of the LED array is applied to the main circuit, the LED array is turned on, and at the same time, a voltage drop occurs in the current detection resistor due to the current flowing through the main circuit.
The positive side input terminal of the constant current circuit that detects the voltage drop is connected to the positive side of the current detection resistor through the current limiting resistor of the bypass circuit. Since no current flows through the current limiting resistor, only the potential of the voltage drop caused by the current detection resistor is detected by the constant current circuit, and this is compared with the reference voltage.
As a result, the current flowing through the main circuit is controlled so that the detection potential is maintained constant, and the current flowing through the LED array is maintained at the rated current.

On the other hand, when the light is turned off, a part of the current flowing through the main circuit flows through the bypass circuit when the switching element is turned on, so that a voltage drop occurs in both the current limiting resistor and the current detection resistor. Is detected and compared with a reference voltage.
Since the detection potential becomes higher than that during lighting, the current control element works to lower this, and as a result, a turn-off current lower than the turn-on current flows to the bypass circuit. At this time, the main circuit responds to the turn-off current. A turn-off voltage is applied.
Therefore, if the resistance value of the current limiting resistor is set so that the extinguishing voltage is lower than the threshold voltage of the LED array, the LED array can be extinguished.

At this time, it is not necessary to switch the reference voltage to be compared between when it is turned on and when it is turned off, and it is only necessary to keep it constant, so that the control can be simplified.
In addition, since the extinguishing voltage lower than the threshold voltage is applied to both sides of the LED array, there is a merit that the rise time of the LED array when the switching element is cut off to light the LED array is shortened .
Furthermore, since the current flows through the bypass circuit when the light is turned off, it does not flow to the LED array. As a result, there is an advantage that heat generation of the LED array can be suppressed.

The circuit diagram which shows an example of the LED lighting apparatus which concerns on this invention. Explanatory drawing which shows the operation | movement at the time of lighting. Explanatory drawing which shows the operation | movement at the time of light extinction. Explanatory drawing which shows the circuit of a conventional apparatus. Explanatory drawing which shows the circuit of a conventional apparatus.

In the present invention, even when the LED is blinked by pulse control, the startup time can be shortened when the LED is turned on, and at the same time, no current is supplied to the LED when the LED is turned off. In order to achieve the purpose of suppressing the amount of heat generated in the current circuit, a current control element, an LED array, and a current detection resistor are arranged in series in the main circuit to which a DC power supply voltage is applied, and are detected by the current detection resistor. In the LED lighting device provided with a constant current circuit for controlling the current control element so that the current is kept constant,
The bypass circuit connected in parallel to the LED array is connected in series with a switching element that conducts when the light is turned off and a current limiting resistor that restricts the current that flows when the light is turned off.
In the constant current circuit, the positive input terminal for detecting current is connected to the positive side of the current detection resistor via the current limiting resistor, and the negative input terminal is connected to the ground side of the current detection resistor.

FIG. 1 is a circuit diagram showing an LED lighting device 1 according to the present invention, in which a main circuit 3 having one end connected to a DC power source 2 and the other end connected to a ground G is connected to a current control element 4 and an LED. array 5 and the current detection resistor R _S is arranged in series, a constant current circuit 6 to control the current control element 4 so that the current detected by the current detection resistor R _S is maintained constant are arranged .
The power supply voltage VC of the DC power supply 2 is set to a predetermined value higher than the threshold voltage of the LED array 5.

A bypass circuit 7 is formed in the main circuit 3 in parallel with the LED array 5. The bypass circuit 7 includes a switching element 8 such as a FET that is turned on when the light is turned off, a diode 9, and a bypass circuit 7 that is turned off. A current limiting resistor _RL for limiting the current flowing through the capacitor is connected in series.
A pulse generator 10 that blinks the LED array 5 by pulse control is connected to the gate of the switching element 8.
As a result, when the pulse signal from the pulse generator 10 is at a low level, the switching element 8 is cut off and only the main circuit 3 is turned on to turn on the LED array 5, and when it is at a high level, the switching element 8 is cut off. Since the current flows through the bypass circuit 7, the LED array 5 is turned off as will be described later.

The constant current circuit 6 includes positive and negative input terminals 6P and 6M that detect current, and the positive input terminal 6P is connected to the positive side of the current detection resistor _RS via a current limiting resistor _RL. The negative input terminal 6M is connected to the ground side of the current detection resistor _RS .
That is, when no current flows in the bypass circuit 7, the potential of the voltage drop generated in the current detection resistor _RS is detected by the current flowing in the main circuit 3, and when the current flows in the bypass circuit 7, the current limiting resistor The sum of the potentials of the voltage drop generated in both R _L and the current detection resistor R _S is detected.

The constant current circuit 6, with a differential amplifier 11 for amplifying the plus-side and minus-side input terminal 6P, the detection signal input from 6M to a predetermined amplification factor is provided, and its output voltage V _1, the reference voltage A comparator 13 that compares the reference voltage V ₀ output from the generator 12 is provided.
In the comparator 13, the difference between the output voltage V ₁ and the reference voltage V ₀ which differential amplifier 11 is output to the gate of the current control element 4 as a control signal, the current flowing in the main circuit 3 in response to the control signal is adjusted It has become so.
As a result, the output voltage V ₁ of the differential amplifier 11 becomes equal to the reference voltage V ₀ , that is, the potential of the voltage drop generated in the current detection resistor _RS due to the current flowing through the main circuit 3 or the bypass circuit 7. Thus, the sum of the potentials of the voltage drops generated in both the current limiting resistor _RL and the current detecting resistor _RS is controlled so as to become a predetermined voltage.

Here, for example, if the current limiting resistor _RL is set so that the following equation is established, the LED array 5 is turned on / off by the operation of the constant current circuit 6 when the switching element 8 is turned on / off.
R _S / (R _S + R _L ) ≈Ioff / Ion
R _S : current detection resistor
R _L : current limiting resistor
Ioff: Light-off current << Lighting start current of the LED array 5
Ion: LED array 5 lighting current (rated current)
In this example, R _S = 0.1Ω
R _L = 22Ω
Ioff = 11mA
Ion = 2.5A (rated current)
The amplification factor of the differential amplifier 11 is set to 10 times, and the reference voltage V ₀ output from the reference voltage generator 12 is set to 2.5V.

In this example, the lighting head H in which the LED array 5 and the current detection resistor _RS are arranged supplies a cable or a connector (see FIG. 5) to the power supply device S that supplies power to the LED array 5 through the main circuit 3. (Not shown) is detachably connected.
The power supply device S is provided with the constant current circuit 6, the switching element 8 constituting the bypass circuit 7, the current limiting resistor 4, and the like.

The above is one configuration example of the LED lighting device 1 according to the present invention, and the operation thereof will be described next.
When the pulse signal input from the pulse generator 10 to the gate of the switching element 8 is at a low level, the bypass circuit 7 is shut off by the switching element 8, so that only the main circuit 3 is conducted, and when it is at a high level, switching is performed. The bypass circuit 7 is turned on by the element 8, and this is repeated alternately.

When the bypass circuit 7 is interrupted, a current flows through the main circuit 3 as shown in FIG. 2, and when the current is Ion, the detection voltage Von input to the differential amplifier 11 of the constant current circuit 6 It is equal to the voltage drop caused by the detection resistor Rs, and Von = Ion × Rs.
In addition, since the potential V ₁ amplified 10 times by the differential amplifier 11 becomes stable when it becomes equal to the reference potential V ₀ ,
V ₀ = V ₁ = 10 × Von = 10 × Ion × Rs
Thus, the lighting current Ion is maintained at 2.5A, which is equal to the rated current of the LED array 5, so that the LED array 5 is lit with a constant luminance.

Next, as shown in FIG. 3, when the current that flows when the bypass circuit 7 is turned on is Ioff, the detection voltage Voff that is input to the differential amplifier 11 of the constant current circuit 6 is the current detection resistor Rs and the current limiter. Equal to the sum of the potentials of the voltage drop caused by resistor _RL ;
Voff = Ioff × (R _S + R _L ).
In addition, since the potential V ₁ amplified 10 times by the differential amplifier 11 becomes stable when it becomes equal to the reference potential V ₀ ,
V ₀ = V ₁ = 10 × Voff = 10 × Ioff × (R _S + R _L )
Thus, Ioff≈11 mA.
At this time, Voff≈0.25 V, and this voltage is equal to the voltage applied to the plus side branch point P of the bypass circuit 7 and the ground G, and is much smaller than the threshold voltage of the LED array 5. No current flows, and therefore the LED array 5 is turned off.

In this way, the resistance that causes a voltage drop is changed by pulse-controlling the switching element 8 to turn on / off the bypass circuit. By comparing this with the constant reference voltage V ₀ , the main circuit 3 and The current flowing through the bypass circuit 7 can be controlled.
At this time, the turn-off current Ioff can be suppressed sufficiently low by the current limiting resistor _RL, so that the power consumption during turn-off in the constant current circuit 6 can be reduced from the power consumption during turn-on. Therefore, the amount of heat generated when the lights are turned off can be suppressed, and unnecessary energy consumption can be suppressed.

Further, since the extinguishing voltage Voff is applied to both ends of the LED array 5 at the time of extinguishing, the time for boosting from the extinguishing voltage Voff to the threshold voltage when the switching element 8 is turned off to light the LED array 5 Lights on.
Accordingly, since the time required for boosting from the voltage 0 to the extinguishing voltage Voff is shortened as compared with the case of boosting from the voltage 0, the rise time can be shortened and the LED is blinked by pulse control. Excellent high-speed response.

  The present invention can be applied to the use of LED lighting that requires high-speed response, such as a lighting device used in an image processing apparatus for product inspection.

DESCRIPTION OF SYMBOLS 1 LED lighting apparatus 2 DC power supply G Ground 3 Main circuit 4 Current control element 5 LED array R _S Current detection resistance R _L Current limiting resistance 6 Constant current circuit 6P Positive side input terminal 6M Negative side input terminal 7 Bypass circuit 8 Switching element 10 Pulse generator 11 Differential amplifier 13 Comparator

Claims (3)

A current control element, an LED array, and a current detection resistor are arranged in series in a main circuit to which a DC power supply voltage is applied, and the current control is performed so that the current detected by the current detection resistor is kept constant during lighting. In the LED lighting device provided with a constant current circuit for controlling the element,
The bypass circuit connected in parallel to the LED array is connected in series with a switching element that conducts / cuts off the circuit and a current limiting resistor that restricts a current flowing through the circuit when conducting ,
The constant current circuit is connected to the positive side of the current detection resistor positive input terminal for detecting the current through the current limiting resistor, minus-side input terminal connected to the ground side of the current detection resistor Rutotomoni ,
The resistance value of the current limiting resistor is set such that a voltage applied to a branch point between the bypass circuit and the main circuit when a switching element is turned on is lower than a threshold voltage of the LED array. LED lighting device. The constant current circuit includes a differential amplifier that amplifies the detection potential input from the positive and negative input terminals at a predetermined amplification factor, and compares the output of the differential amplifier with a preset reference voltage. 2. The LED lighting device according to claim 1, further comprising: a comparator that maintains a constant value, wherein an output signal of the comparator is input to the current control element to adjust a current flowing through the main circuit.
An illumination head provided with the LED array and a current detection resistor, and a power supply device that supplies power to the LED array via the main circuit, the illumination head being detachably connected to the power supply device, and The LED lighting device according to claim 1, wherein a switching element and a current limiting resistor constituting the bypass circuit and the constant current circuit are arranged in the power supply device.

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