JP4642406B2 - Constant current drive circuit - Google Patents

Description

  The present invention relates to a constant current driving circuit for causing a light emitting element such as an LED (light emitting diode) to emit light stably at a target luminance.

  In order to display various colors using the RGB LEDs, it is necessary to cause the LEDs of each color to emit light stably with the same luminance. LED color display is used in various fields such as backlights used in electronic devices such as mobile phones, lighting for automobile speedometers, and display devices such as displays. There are various techniques for driving at a constant current for the purpose of emitting light. For example, Patent Document 1 is a technique for stably driving RGB LEDs by PWM (Pulse Width Modulation) control.

JP 2000-214825 A

  In Patent Document 1, a constant voltage is applied to each LED of each RGB color, and a difference in forward voltage drop in each LED is adjusted by an adjustment resistor provided for each LED, thereby stabilizing light emission at a constant current. This is a technology that enables When used in a relatively small circuit such as a cellular phone, such a technique can be adequately used. For example, a large number of LEDs such as a large electronic device such as a display are used as a backlight. In large-scale circuits that need to achieve high brightness, it is difficult to individually adjust the variation in forward voltage with an adjusting resistor. As a result, the LED brightness can be kept uniform by constant current drive. There is a problem that the image quality of the display such as luminance unevenness is deteriorated.

  In Patent Document 1, the cathode side of the LED to which the voltage generated by the D / D converter is applied on the anode side is connected to the drain of the switching FET, and the control unit applies PWM control (Pulse Width Modulation) to the gate of the switching FET. The LED is driven by controlling the switching operation by applying a pulse voltage. That is, the turning on and off of the LED is controlled by turning on and off the pulse voltage by PWM. At this time, the control unit also controls the D / D converter.

  However, this method may cause a problem when the LED is turned on from an unlit state. Specifically, when the LED is turned off and no light is applied, even if the switching FET is turned on at the same time as the controller starts applying a constant voltage to the LED, at that moment Since the generation and application of the voltage by the D / D converter are started, a sufficient voltage cannot be obtained, or the D / D converter is affected by the electric charge remaining between the control unit and the D / D converter. The generated voltage may not be accurately controlled. Therefore, in terms of the current waveform that flows through the LED, in spite of the fact that it is controlled to rise sharply with a pulse waveform from off to on, a time lag occurs until the intended current value is reached, During that time, the current waveform gradually increased, and the current waveform was sometimes rounded. In this case, the LED cannot emit light with originally intended luminance until the predetermined current value is reached. There is no problem even if such a phenomenon occurs if it is used in a small-scale circuit such as a mobile phone that uses one LED for each color of RGB or a device that does not require high-speed operation. However, in an electronic device such as a display realized by connecting a large number of LEDs in series or using a high-intensity LED driven at high current, a symptom in which luminance changes as the current rises becomes significant. Therefore, there is a problem that it cannot be used for a display that requires a high-speed operation, for example, when displaying a moving image or the like.

  Such a phenomenon can be avoided if a constant voltage is continuously applied to the LED even when the LED is turned off by the switching FET. However, in this case, there is a problem that an excessive inrush current flows at the moment when the switching FET is turned on, and there is a possibility that a component of a circuit such as an LED may be destroyed.

  Accordingly, an object of the present invention is to provide a constant current driving circuit that can stably drive all LEDs with a constant current while utilizing PWM control.

In order to solve the above-mentioned problems, the invention of claim 1 is a constant current driving circuit for driving an element with a predetermined current value, and is supplied with power supplied by repeatedly turning on and off the first switching element. A voltage generating means for generating a high frequency pulsed voltage to be applied to the element from the voltage, and a voltage generating means for applying the generated high frequency pulsed voltage to the element; and the current flowing to the element when the high frequency pulsed voltage is applied Current detection means for detecting a current value, voltage control means for controlling the start and stop of intermittent high-frequency voltage application to the element of the voltage generation means according to the current value detected by the current detection means, and the generation the operation of the device the high frequency pulse voltage is applied that is, the low-frequency PWM periodically off control pulsed by the second switching element Comprising a control means, wherein the voltage generating means, the timing for generating the high frequency pulse voltage, said controlled by the voltage control means, intermittent high frequency voltage application start and stop timing to the element The timing at which the low-frequency PWM control means periodically performs on / off control of the operation of the element in a pulsed manner is determined independently .

According to a second aspect of the present invention, there is provided a constant current driving circuit according to the first aspect of the invention, wherein the element is a light emitting element.

  A voltage generated from the supplied power supply voltage is periodically applied to the element in a pulsed manner, and the current value flowing through the element is monitored while controlling the start or stop of voltage application. As a result, the current flowing through the element can be maintained at a predetermined value.

<Configuration of constant current drive circuit>
FIG. 1 is a configuration diagram showing a constant current drive circuit 1 according to one embodiment of the present invention. As described above, the constant current driving circuit 1 includes the booster circuit unit 7 connected to the power supply 13, the LED group 6 connected to the booster circuit unit 7, the switching FET 5 in which the LED group 6 is connected to the drain, The PWM unit 3 connected to the gate of the FET 5, the resistor 4 connected to the source, the control unit 2 connected to the PWM unit 3, the source of the PWM unit 3, the switching FET 5, the booster circuit unit 7 and the power supply 13. And a constant voltage control unit 8 connected thereto. The booster circuit unit 7 includes a coil 11 connected to the power source 13, a rectifier diode 9 connected to the coil, a capacitor 10 connected to the rectifier diode 9, a drain and a coil 11 and a constant voltage control unit 8. And a switching FET 12 to which a gate is connected.

  The power supply 13 has a function of generating a power supply voltage from which a DC voltage used for driving the LED group 6 is obtained. The power supply voltage has a voltage value that does not destroy these elements even if they are directly applied to the elements such as the LED group 6.

  The booster circuit unit (voltage generating means) 7 has a function of boosting the power supply voltage supplied from the power supply 13 and generating a DC voltage necessary for driving the LED group 6. The booster circuit unit 7 includes a coil 7 and a switching FET 12 inside, and obtains a boosted pulsed DC voltage by using an induced voltage generated in the coil 7 by the switching operation of the switching FET 12. As will be described later, the switching FET 12 is also used to control application of the generated DC voltage to the LED group 6. The other rectifier diode 9 and capacitor 10 are elements for generating a stable predetermined DC voltage. Note that the booster circuit unit 7 used here has the function of boosting the supplied voltage to generate a predetermined DC voltage and the function of controlling the application of the DC voltage, as shown in FIG. It is not limited to the mode shown in the above, and other modes may be used. For example, the booster circuit unit 15 shown in FIG. 2 may be used.

  In this way, by using the power supply 13 having a voltage value that does not break even when directly applied to each element such as the LED group 6, the booster circuit unit 7 can operate the constant current drive circuit 1. Even if there is a failure inside, it is possible to avoid a problem that an excessive current flows and each part is damaged. For example, even if the booster circuit unit 7 is damaged, each element is not damaged, and even if the switching FET 5 is damaged and the drain-source is short-circuited, the current continues to flow through the LED group 6, and the current value is As will be described later, since the resistor 4 is used for control, the LED group 6 is not damaged. As will be described later, the resistor 4 is used to control the voltage applied to the LED group 6, and this can also be easily confirmed by using an element such as a fuse. In other words, no damage to other elements or malfunction of the constant current drive circuit 1 will occur.

  If the constant current drive circuit 1 is an element intended for driving at a constant current, the LED group 6 is, for example, a mode in which a plurality of LEDs of the same color are connected in series and mixed at a predetermined ratio. A mode in which a plurality of LEDs of RGB colors or the like are connected in series may be used. Moreover, although shown as the LED group 6, it does not restrict | limit the number of LEDs and the aspect using only one LED may be sufficient. Moreover, you may be the aspect using other elements instead of light emitting elements, such as LED.

  The switching FET 5 has a function of periodically turning on and off the current between the drain and the source in a pulsed manner in accordance with the pulse voltage applied from the PWM unit 3 to the gate. Specifically, when the pulse voltage is on, the drain-source switching operation is turned on and a current is supplied to light the LED group 6, and when the pulse voltage is off, the switching operation is turned off and the current is cut off. Thus, the LED group 6 operates to be extinguished.

  The PWM unit 3 has a function of generating a pulse voltage according to control by the control unit 2 and applying the pulse voltage to the switching FET 5 as a gate voltage.

  The control unit 2 has a function of controlling the pulse voltage generated by the PWM unit 3. Specifically, the duty ratio of the pulse waveform generated by the PWM unit 3 is controlled. The control unit 2 and the PWM unit 3 constitute PWM control means.

  The operation of the control unit 2, the PWM unit 3, and the switching FET 5 controls the operation of turning on and off the LED group 6 and the luminance during light emission. Specifically, a pulse voltage having a duty ratio controlled by the control unit 2 is generated by the PWM unit 3 and applied as a gate voltage of the switching FET 5, thereby grounding the cathode side of the LED group 6 on and off. The LED group 6 is turned on or off according to this control. If the duty ratio is 100%, the LED group 6 continues to be lit, and if the duty ratio is 0, the LED group 6 continues to be turned off. Further, by changing the duty ratio, the ratio and the number of turn-on times and turn-off times of the LED group 6 can be changed, and as a result, the luminance of the LED group 6 can be controlled.

  In addition, in the constant current drive circuit 1, in addition to this, when the LED group 6 is lit by the control unit 2 and the PWM control unit 3, the current value flowing through the LED group 6 is made constant. In order to control, the resistor 4 and the constant voltage control part 8 are provided.

  The resistor 4 is provided for detecting the current value flowing through the LED group 6, and a current value can be obtained from the potential difference between both ends of the resistor 4 and the resistance value of the resistor 4. However, in actuality, it is not used by converting into a current value, but the potential difference between both ends of the resistor 4 is used as it is, and the magnitude of the current value flowing through the LED group 6 is determined based on the magnitude of the voltage value.

  The constant voltage control unit (voltage control means) 8 has a function of controlling the switching operation of the switching FET 12 in order to obtain a pulsed DC voltage obtained by boosting the power supply voltage by the boosting circuit unit 7 and a potential difference between both ends of the resistor 4. The voltage value applied to the LED group 6 from the booster circuit unit 7 is controlled so that the current value flowing from the LED group 6 to the LED group 6 is detected and the value becomes a target predetermined current value. Although details of the operation will be described later, a configuration example for realizing the constant voltage control unit 8 will be described with reference to FIG.

  For example, as shown in FIG. 3, the constant voltage control unit 8 includes a current detection unit 17 connected to the resistor 4, a reference voltage generation unit 16 connected to the current detection unit 17, a current detection unit 17, and a booster circuit. 7 includes an FET driving unit 18 connected to the internal switching FET 12 and the PWM unit 3, and a synchronization signal generating unit 19 connected to the FET driving unit 18.

  The reference voltage generator 16 is used to compare with the potential difference actually generated at both ends of the resistor 4, and the constant current driving circuit 1 drives the LED group 6 with a predetermined constant current intended. It has a function of generating a potential difference generated at both ends as a reference voltage. The voltage value serving as the reference voltage may be fixed in advance to a predetermined value, or may be changed and may be changed according to an element such as the LED group 6 to be used.

  The current detection unit 17 is a functional unit for detecting the value of the current flowing through the LED group 6 from the potential difference between both ends of the resistor 4 as described above. Actually, the current value is not directly detected, but the potential difference between both ends of the resistor 4 is detected, compared with the reference voltage generated by the reference voltage generator 16, and the current flowing through the LED group 6 is determined based on the comparison result. It is determined whether the value is larger or smaller than a predetermined current value intended for the constant current drive circuit 1. The determination result is notified to the FET drive unit 18.

  The FET drive unit 18 uses the determination result notified from the current detection unit 17 and the information on the pulse voltage for controlling the switching operation of the switching FET 5 acquired from the PWM unit 3, to determine the switching FET 12 in the boost circuit unit 7. It has a function of controlling the switching operation. Specifically, the start and stop of application of the voltage generated by the booster circuit unit 7 to the LED group 6 is controlled according to the pulse voltage generated by the PWM unit 3. Further, the voltage value applied to the LED group 6 is controlled by obtaining information related to the current value actually flowing in the LED group 6 and feeding back the information to the application control of the voltage to the LED group 6. As a result, the current value flowing through the LED group 6 is controlled.

  The synchronization signal generation unit 19 has a function of generating a pulse waveform having a sufficiently high frequency as a synchronization signal as compared with the pulse voltage for controlling the switching operation of the switching FET 5 generated by the PWM unit 3. Using this synchronization signal, the FET drive unit 18 controls the switching operation of the switching FET 12 at a high speed, thereby generating a pulsed DC voltage boosted by the booster circuit unit 7 and a group of LEDs having this DC voltage. 6 can be controlled at a very high speed as compared with the operation in which the control unit 2 controls the lighting and luminance of the LED group 6.

  In the description relating to the operation of the constant current drive circuit 1 described later, the constant voltage control unit 8 will be described as one control unit. However, in practice, the operation is realized by the function and operation of each unit described above. Yes.

  In FIG. 1, the switching FETs 5 and 12 are used for the switching operation of the LED group 6 and the booster circuit unit 7, but the present invention is not limited thereto, and the current is turned on by a control signal such as a gate voltage. Other modes may be used as long as they have a function capable of controlling off. For example, as shown in FIG. 4, a mode in which transistors 20 and 21 are used instead of the switching FETs 5 and 12 may be used.

  FIG. 1 shows one constant current drive circuit 1, but actually, in many cases, a plurality of constant current drive circuits 1 are operated in order to drive, for example, each color LED. In such a case, in addition to a mode in which the plurality of constant current driving circuits 1 are independently operated, for example, the control unit 2 is shared, and the other element group 14 shown in FIG. As shown in FIG. 5, it may be connected as shown in FIG.

  Next, the operation of the constant current drive circuit 1 will be described with reference to FIG.

<Operation of constant current drive circuit>
In order to light the LED group 6 with a predetermined luminance, the control unit 2 generates a pulse voltage as shown in FIG. 6A from the PWM unit 3 and controls the switching FET 5. As shown in the upper part of FIG. 6A, the horizontal direction of FIG. 6 represents time, and FIG. 6A shows that the pulse voltage generated by the PWM unit 3 is on from time t ₀ to t ₁ . a state, indicates that from t ₁ to t ₂ is in an off state.

Ideally, the LED group 6 may be lit while the value of the current flowing through the LED group 6 shows the same waveform as in FIG. However, in the conventional technique, as described above, due to the influence of the electric charge remaining in the circuit, the current waveform that gradually increases from 0 at time t ₀ is gradually rounded, or at the time t ₀ In some cases, an inrush current that greatly exceeds a predetermined current value flows. Therefore, in the constant current drive circuit 1, the LEDs from time t ₀ to time t ₁ are controlled while the lighting operation of the LED group 6 is controlled by the control unit 2, the PWM unit 3, and the switching FET 5 as shown in FIG. The current value flowing through the LED group 6 while the group 6 is lit is further controlled.

  In the constant voltage control unit 8, as shown in FIG. 6B, a sufficiently high-frequency pulse wave is generated as compared with the pulse waveform shown in FIG. 6A generated in the PWM unit 3. Then, the constant voltage control unit 8 controls the switching operation of the switching FET 12 in the booster circuit unit 7 based on this pulse waveform. Thereby, the constant voltage control unit 8 can generate a pulsed DC voltage having a waveform similar to that shown in FIG.

At time t ₀ , when the pulse waveform of the voltage generated by the PWM unit 3 is turned on, the constant voltage control unit 8 that has received this controls the switching FET 12 in the booster circuit unit 7 to direct the LED to the LED group 6. Start applying voltage. The DC voltage applied at this time is obtained by boosting the voltage supplied from the power supply unit 13 by the boosting circuit unit 7. The voltage value after boosting at this time can be controlled as will be described later by applying the pulses periodically in the form of pulses, so that the voltage value at the target is a predetermined constant current. The voltage value may be slightly higher than the voltage value required for the LED group 6 when operating.

  FIG. 6C shows the DC voltage applied to the LED group 6 by the booster circuit unit 7. As described above, the constant voltage control unit 8 controls the switching FET 12 to apply a pulsed boosted DC voltage having the same waveform as in FIG. 6B to the LED group 6.

The constant voltage control unit 8 measures the reference voltage V ₀ generated at both ends of the resistor 4 and the both ends of the resistor 4 when the current value flowing through the LED group 6 matches the target current value of the constant current driving circuit 1. The actual potential difference is compared and monitored. A waveform of a voltage value which is a potential difference between both ends of the resistor 4 at that time is shown in FIG. As shown in FIG. 6D, when the voltage value is V _L at time t ₀ and gradually rises as time passes and reaches the reference voltage V ₀ , as shown in FIG. 6C. The constant voltage control unit 8 controls the switching FET 12 to stop the application of voltage from the booster circuit unit 7 to the LED group 6. However, in actuality, the voltage value exceeds V ₀ not at the moment when the voltage value reaches V ₀ but at time ta when time _a has elapsed from t ₀ due to the influence of the temporal operation limit of each element. Then, the voltage value rises when V _U is reached. When the voltage value gradually decreases from this point and reaches the reference voltage V ₀ , as shown in FIG. 6C, the constant voltage control unit 8 controls the switching FET 12 again to boost the boost circuit unit. Application of a voltage from 7 to the LED group 6 is started. Similarly this time, as shown in FIG. 6 (d), after reaching the lower V _L than the voltage value V ₀ to the time t _b from the time t _a has passed by time b, again the voltage value starts to increase. However, since the actual operation of each element is very fast, the time difference between the time when the voltage application is started or stopped and the time when the voltage value reaches V _L or V _U is very small. The potential difference between the voltage V ₀ and the voltage value V _U or V _L is also very small and does not cause a problem in practice.

  As described above, the constant voltage control unit 8 periodically applies the DC voltage boosted by the boosting circuit unit 7 in a pulse shape at a very high frequency shown in FIG. Further, by controlling the start and stop of the application as shown in FIG. 6C, the voltage value applied to the LED group 6 is controlled, and as a result, the current value flowing to the LED group 6 is controlled. It is.

When the LED group 6 is turned off according to the pulse voltage at time t ₁ in FIG. 6A, only the potential difference information generated at both ends of the resistor 4 described above causes the constant voltage control unit 8 to An attempt is made to increase the voltage applied to the LED group 6. However, since information related to the pulse waveform shown in FIG. 6A generated by the PWM unit 3 is also input to the constant voltage control unit 8, such a phenomenon is avoided and shown in FIG. 6C. As described above, during the time period c until the pulse waveform from the PWM unit 3 is turned on next time t ₂ , the voltage application from the booster circuit unit 7 to the LED group 6 is stopped. Then, the voltage control similar to that described above is started from the moment when time t ₂ is reached.

As described above, the voltage value applied to the LED group 6 is controlled in detail while monitoring the potential difference generated at both ends of the resistor 4, so that the current value flowing through the LED group 6 has a waveform as shown in FIG. Indicates. As shown in FIG. 6D, since the potential difference between both ends of the resistor 4 varies between the upper limit voltage value V _U and the lower limit voltage value V _L , the current value flowing through the LED group 6 is also Accordingly, the current value corresponding to V _U and the current value corresponding to V _L vary. However, as described above, the fluctuation range is actually small and not so large that the luminance of the LED group 6 changes, and thus the operation is not affected. Therefore, the constant current drive circuit 1 can light the LED group 6 with the brightness when a predetermined current flows by the above-described operation.

  According to the pulse waveform generated by the PWM unit 3, when the pulse waveform is off, the constant voltage control unit 8 and the booster circuit unit 7 stop operating, and the current value flowing through the LED group 6 and the resistor 4 becomes completely zero. At this time, even if the boosting circuit unit 7 continues to generate a boosted pulsed DC voltage, at the moment when the lighting of the LED group 6 is started, both ends of the resistor 4 corresponding to the current value flowing through the LED group 6 are detected. Since the DC voltage to be applied is controlled as described above while monitoring the potential difference, it is possible to avoid a problem such as generation of an inrush current which has been a problem in the past.

  Since the voltage applied to the LED group 6 is controlled while being applied in pulses at a high frequency as described above, it is necessary when a predetermined current value is obtained in the LED group 6 in the booster circuit unit 7. A DC voltage having a voltage value slightly higher than a predetermined voltage value can be generated and used. By applying a DC voltage having a voltage value higher than the predetermined voltage value to the LED group 6 while controlling as described above, the LED group 6 is turned on at a target luminance immediately after the LED group 6 is turned on. It is possible to obtain a sufficient current.

  Alternatively, if a boosted DC voltage is generated in the booster circuit section 7 while the LED group 6 is turned off, the LED group 6 is turned on at a desired luminance immediately after the LED group 6 is turned on. Sufficient current can be obtained, and generation of inrush current can be avoided as described above.

  Further, the constant voltage control unit 8 detects and uses the potential difference between both ends of the resistance value 4 provided to detect the current value of the LED group 6, but this resistance 4 is grounded. Therefore, the potential difference detected by the remaining charge is not affected. Therefore, when the LED group 6 is turned on by the control unit 2 and the PWM unit 3, the constant voltage control unit 8 instantaneously and accurately determines the voltage value generated at both ends of the resistor 4 corresponding to the current value flowing through the LED group 6. Can get to.

  As a result, even when the LED group 6 is turned on from the extinguished state, the current value of the LED group 6 is also in accordance with the rise of the pulse waveform generated by the PWM unit 3 shown in FIG. ), A steep rise is exhibited, and a phenomenon such as a curling of a current waveform that gradually increases and reaches a target current value as in the prior art can be avoided.

  Further, since information corresponding to the value of the current flowing through the LED group 6 is obtained from the potential difference between both ends of the resistor 4, the information is based on values unique to the LED group 6 such as the forward voltage drop value of the LED group 6. There is no need to provide adjustment resistors. In addition, it is possible to cope with changes in ambient temperature of each element including the LED group 6 and changes with time without special adjustment.

  Furthermore, according to the above-described aspect, the number of LEDs that can be connected in series and used as the LED group 6 is not limited, and the constant current drive circuit 1 is connected in parallel and used as shown in FIG. Or by adjusting the value of the reference voltage generator 16 in the constant voltage controller 8 shown in FIG. 3, it is possible to change the brightness of the LED for each constant current drive circuit 1. It is possible to realize an apparatus used in various modes.

It is a figure which shows the structure of the constant current drive circuit which concerns on one embodiment of this invention. It is a figure which shows another structure of the booster circuit part which concerns on one embodiment of this invention. It is a figure which shows the structural example of the constant voltage control part which concerns on one embodiment of this invention. It is a figure which shows a structure when the constant current drive circuit which concerns on one embodiment of this invention is implement | achieved using the transistor. It is a figure which shows a structure when using the constant current drive circuit which concerns on one embodiment of this invention connected in parallel. It is a figure explaining operation | movement of the constant current drive circuit which concerns on one embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Constant current drive circuit, 2 Control part, 3 PWM part, 4 resistance, 5,12 switching FET, 6 LED group, 7,15 Boosting circuit part, 8 Constant voltage control part, 9 Rectifier diode, 10 Capacitor, 11 Coil, 13 Power supply.

Claims (2)

A constant current drive circuit for driving an element with a predetermined current value,
The first switching element is repeatedly turned on and off to generate a high-frequency pulsed voltage to be applied to the element from a supplied power supply voltage , and the generated high-frequency pulsed voltage is applied to the element. When,
Current detecting means for detecting a current value flowing through the element when the high-frequency pulsed voltage is applied;
Voltage control means for controlling the start and stop of intermittent high-frequency voltage application to the element of the voltage generation means according to the current value detected by the current detection means;
Low-frequency PWM control means for periodically on / off controlling the operation of the element to which the generated high-frequency pulsed voltage is applied, in a pulsed manner by a second switching element ;
Equipped with a,
The voltage generating means generates the high-frequency pulsed voltage; and
Timing of start and stop of intermittent high frequency voltage application to the element controlled by the voltage control means;
The constant current drive circuit, wherein the low frequency PWM control means determines the timing of periodically turning on and off the operation of the element in a pulsed manner independently of each other . The constant current drive circuit according to claim 1,
The constant current driving circuit , wherein the element is a light emitting element .

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