JP6411261B2 - LED drive circuit - Google Patents

Description

  The present invention relates to an LED drive circuit whose emission color changes when brightness is adjusted.

  An LED lighting device has been proposed that imitates an incandescent bulb and has a high color temperature when emitting light brightly and a low color temperature when emitting light darkly. In order to easily realize this, two LEDs having different color temperatures may be prepared, and the balance of light emission intensity may be adjusted for one LED and the other LED according to the luminance (for example, Patent Document 1). Then, FIG. 2 of patent document 1 is re-displayed in FIG. 5, and the illuminating device from which a luminescent color changes according to a brightness | luminance is demonstrated. In addition, the code | symbol is changed in the figure. The terms used in Patent Document 1 are shown in parentheses.

  FIG. 5 is a circuit diagram of the illumination device 102 shown as a conventional example. The lighting device 102 includes a dimmer 160 and an LED lamp 101, and an AC power source 150 is connected to the lighting device 102. The LED lamp 101 includes a rectifying / smoothing circuit 170, a variable current source 180, and the LED module 100. The LED module 100 includes an LED array 111A (LED array) that emits light at a high color temperature, an LED array 121A that emits light at a low color temperature, a resistor 123, and a transistor 122. In the LED row 111A, the LEDs 111 are connected in series, and in the LED row 121A, the LEDs 121 are connected in series.

  The rectifying / smoothing circuit 170 rectifies and smoothes the output voltage of the dimmer 160, and the variable current source 180 outputs the current It based on the smoothed voltage. The current It is divided into a current I1 flowing through the LED array 111A and a current I2 flowing through the LED array 121A. The current I2 is divided into the base current Ib and the collector current Ic of the transistor 122. When the current It is increased or decreased by the dimmer 160 in the lighting device 102, the transistor 122 operates at a constant current (the current Ic is constant), so that the ratio between the current I1 and the current I2 changes and the emission color is adjusted.

JP2014-157744 (FIG. 2)

  However, in the LED lamp 101 shown in FIG. 5, the current I2 continues to flow through the LED array 121A even when light is emitted with high luminance. In other words, the LED lamp 101 that adjusts the color temperature to be low during low-luminance light emission and to increase the color temperature during high-luminance light emission, while the LED 111 included in the LED array 111A emits light at a high color temperature, Since the included LED 121 emits light at a low color temperature, the color temperature is lowered during high luminance light emission. Therefore, the LED lamp 101 has a color temperature of the LED array 111A that is higher than necessary, and accurately maintains the balance of the light emission amounts of the LED array 111A and the LED array 121A during high-luminance light emission. The emission color must be managed. That is, the LED lamp 101 has a problem that it is difficult to manage the emission color.

  In general, a phosphor having a low emission color temperature also has low emission efficiency. Therefore, if the LED 121A is caused to emit light during high luminance emission, the emission efficiency of the LED lamp 101 cannot be increased. Further, the loss of the transistor 122 during high-luminance emission cannot be ignored. As a result, the LED lamp 101 shown in FIG. 5 has a problem that the light emission efficiency at the time of high luminance light emission is poor.

  In addition, it is not preferred to perform wide dimming in a state where the color temperature is high, and it may be preferred that the emitted color has a low color temperature when emitting light at a moderate brightness or lower.

  Therefore, the present invention has been made in view of the above problems, and emits light with a single emission color up to a predetermined luminance at the time of dimming while having high luminous efficiency and easy management of the emission color. Provided is an LED driving circuit that emits light in other emission colors.

In order to solve the above problems, the LED drive circuit of the present invention is:
A variable constant current source;
A first LED array that emits light in a first emission color;
A second LED array that emits light in a second emission color;
A switching circuit that switches between a state in which the first LED row emits light and a state in which the second LED row emits light,
The first LED row and the second LED row are connected in parallel to the variable constant current source,
The switching circuit has a first switch having one end connected in series with the first LED array, and a second switch having one end connected in series with the second LED array, and the current output from the variable constant current source is a predetermined current. When it is smaller, the first switch is turned on and the second switch is turned off. When the current output from the variable constant current source is larger than the predetermined current, the first switch is turned off and the second switch Is turned on.

  In the LED drive circuit of the present invention, while the value of the current output from the variable constant current source is small, the switching circuit turns on the first switch and turns off the second switch, so that the current flows only in the first LED row, The first LED row emits light with the first emission color and low luminance. When the output current of the variable constant current source is increased and the output current exceeds a predetermined value, the switching circuit turns off the first switch and turns on the second switch, and the output current of the variable constant current source is only in the second LED row. Control to flow. As a result, when the value of the output current of the variable constant current source is large, only the second LED array emits light with high brightness in the second emission color, and therefore the LED drive circuit of the present invention can easily manage the emission color. As described above, the LED drive circuit of the present invention increases the light emission efficiency at the time of high luminance light emission by turning off the first LED row at the time of high luminance light emission. More preferably, the light emission efficiency of the second LED array is made larger than the light emission efficiency of the first LED array.

  The switching circuit includes a current detection element and a comparator that switches an output with a voltage at one end of the current detection element, the one end of the current detection element, the other end of the first switch, and the other end of the second switch. And the first switch and the second switch may be controlled based on the output of the comparator.

  The first switch may be controlled by the output of the comparator, and the second switch may be controlled by the output of the one end of the first switch.

  The current detection element is a resistor, the comparator includes a bipolar transistor and another resistor, the output of the comparator is a collector voltage of the bipolar transistor, and the first switch and the second switch are enhancement-type FETs It may be.

  The current detection element is a resistor, the comparator includes a bipolar transistor and another resistor, the output of the comparator is a current shunted from the collector of the bipolar transistor, and the first switch and the second switch are It may be a bipolar transistor.

  The one end of the second switch may be connected to a control terminal of the first switch via a resistor.

  The threshold voltage of the first LED array may be higher than the threshold voltage of the second LED array.

  As described above, the LED drive circuit of the present invention emits light with a single emission color up to a predetermined luminance at the time of dimming while having high luminous efficiency and easy management of the emission color. Can emit light.

The circuit diagram of the LED drive circuit shown as 1st Embodiment of this invention. The graph which shows the voltage-current characteristic of the LED drive circuit shown in FIG. The circuit diagram of the LED drive circuit shown as 2nd Embodiment of this invention. The circuit diagram of the LED drive circuit shown as 3rd Embodiment of this invention. The circuit diagram of the LED drive circuit shown as a prior art example.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to FIGS. In the description of the drawings, the same or corresponding elements are denoted by the same reference numerals, and redundant description is omitted. The relationship with the invention specific matters described in the claims is described in parentheses.

(First embodiment)
An LED driving circuit 10 shown as a first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a circuit diagram of the LED drive circuit 10, and FIG. 2 is a graph showing the voltage-current characteristics of the LED drive circuit 10.

  As shown in FIG. 1, the LED drive circuit 10 includes a variable constant current source 13, a first LED row 11, a second LED row 21, and a switching circuit 30. The first LED array 11 has a plurality of LEDs 12 connected in series, and emits light in a reddish color (first emission color) with a low color temperature. The second LED array 21 includes a plurality of LEDs 22 connected in series, and emits light with a high color temperature and a bluish color (second emission color). The anodes of the first LED row 11 and the second LED row 21 are connected to the current output terminal of the variable constant current source 13. The number of series stages of LEDs 12 in the first LED string 11 is larger than the number of series stages of LEDs 22 in the second LED string 21. In addition, LED12 and LED22 coat | cover with the different fluorescent resin with respect to the same LED die. That is, the threshold voltage of the first LED string 11 (the product of the forward voltage drop of the LED 12 and the number of series stages) is larger than the threshold voltage of the second LED string 21 (the product of the forward voltage drop of the LED 22 and the number of series stages). The series connection of the LEDs 12 and 22 in the first and second LED rows 11 and 21 includes a series-parallel connection or a parallel-series connection of the LEDs 12 and 22 as is well known (the same applies hereinafter).

  The switching circuit 30 includes resistors 31, 32, 33, 34, Zener diodes 35, 36, bipolar transistors (hereinafter referred to as transistors) 37, and enhancement type field effect transistors (hereinafter referred to as FETs) 38, 39. The resistor 31 is a pull-up resistor, and the current output terminal of the variable constant current source 13 and the collector of the transistor 37 are connected via the resistor 31. The resistor 32 is a current detection resistor, and the base and emitter of the transistor 37 are connected via the resistor 32. The emitter of the transistor 37 is connected to a terminal (ground) to which the current of the variable constant current source 13 returns.

The FET 38 operating as the first switch has a gate (control terminal) connected to the collector of the transistor 37 and is connected to its own source via the Zener diode 35 and the resistor 33, and a drain (one end) is connected to the first LED row 11. The source (the other end) is connected to one end of the current detection resistor 32. FET acting as a second switch
39, the gate (control terminal) is connected to the drain of the FET 38, is connected to its own source via the Zener diode 36 and the resistor 34, and the drain (one end) is connected to the cathode of the second LED row 21; The other end is connected to one end of the current detection resistor 32.

  Next, the operation of the LED drive circuit 10 will be described with reference to FIG. In this description, each member and block in FIG. 1 are referred to without any special instruction. 2 is the current It output from the variable constant current source 13, and the vertical axis is the voltage V at the current output terminal of the variable constant current source 13 (terminal (ground) to which the current of the variable constant current source 13 returns. Is set to 0 (V)).

  In FIG. 2, it is assumed that the current It output from the variable constant current source 13 is 0 (A) and the voltage at the output terminal of the variable constant current source 13 is V0 (V). The gate voltage of the FET 38 becomes the cathode voltage of the Zener diode when the voltage V0 is divided by the resistor 31, the Zener diode 35, and the resistor 33. Since this voltage exceeds the threshold voltage (also referred to as Vth) of the FET 38 (hereinafter, the voltage exceeding the threshold voltage of the FET is referred to as a high level), the FET 38 is in a conductive state (hereinafter, the conductive state is referred to as ON). To do. Since the FET 38 is on, the gate voltage of the FET 39 is equal to or lower than the threshold voltage of the FET 39 (hereinafter, the voltage equal to or lower than the threshold voltage of the FET is referred to as a low level), and the FET 39 is in a cutoff state (hereinafter, the cutoff state is referred to as off). Become.

  When the output current It of the variable constant current source 13 is in the current range I3 from 0 (A) to I4 (A), the voltage drop of the resistor 32 becomes 0.6 V or less, and the collector-emitter of the transistor 37 is turned off. . For this reason, the gate voltage of the FET 38 is maintained at a high level, and the gate voltage of the FET 39 is maintained at a low level. As a result, the current I1 flows only in the first LED row 11 (I1 = It, I2 = 0).

  That is, as the current It increases, the voltage V of the output terminal of the variable constant current source 13 increases, and the light emission luminance of the first LED row 11 increases. The Zener diode 35 is a protective element that prevents an excessive voltage from being applied to the gate of the FET 38 even when the voltage V at the output terminal of the variable constant current source 13 increases, and has a breakdown voltage slightly higher than the threshold voltage of the FET 38. Is used.

  In the current range I5 where the output current It of the variable constant current source 13 exceeds I4 (A), the voltage drop of the resistor 32 exceeds 0.6V, so that the collector-emitter of the transistor 37 is turned on. As a result, the gate voltage of the FET 38 becomes low level and the FET 38 is turned off. When the FET 38 is turned off, no current flows through the first LED string 11 and the gate voltage of the FET 39 becomes high level (the voltage drop of the first LED string 11 is reduced, and the gate voltage of the FET 39 becomes the breakdown voltage of the Zener diode 36). As a result, the current I2 flows only through the second LED row 21 (I1 = 0, I2 = It).

  Since the second LED string 21 has a lower threshold voltage than the first LED string 11, when the output current It of the variable constant current source 13 exceeds I4 (A), the voltage V of the output terminal of the variable constant current source 13 is The voltage decreases from V1 (V) to a voltage V2 (V) near the threshold voltage of the second LED array 21 once. As the current It increases, the voltage V at the output terminal of the variable constant current source 13 increases, and the light emission luminance of the second LED array 21 increases.

The Zener diode 36 is a protective element that prevents an excessive voltage from being applied to the gate of the FET 39 even when the voltage V at the output terminal of the variable constant current source 13 rises, and has a breakdown voltage slightly larger than the threshold value of the FET 39. Use. In the LED driving circuit 10, the threshold voltage of the first LED array 11 is increased by several forward voltage drops of the LED 12 from the threshold voltage of the second LED array 21, so that the variable constant current when the current It is greater than I4 (A) The voltage V at the output terminal of the source 13
No current flows through the first LED array 11 even if the voltage rises. This corresponds to the fact that the voltage value at the right end of the graph in FIG. 2 does not reach the voltage V0.

  If the threshold voltage of the first LED string 11 is made smaller than the threshold voltage of the second LED string 21, the LED drive circuit 10 starts from the first LED string 11 to the zener when the current It is in the current range I5 larger than I4 (A). There is a possibility that the current I1 flows through the diode 36 and the resistor 34. However, if the resistance 34 is set to a large value, this current I1 (and lighting of the first LED string 11) can be ignored. Even in this case, the states of the FETs 38 and 39 are not changed.

  As described above, in the LED drive circuit 10, in the current period I3 in which the value of the output current It of the variable constant current source 13 is small, the switching circuit 30 turns on the FET 38 and turns off the FET 39, and the current I1 is supplied only to the first LED row 11. The first LED row 11 emits light with a low luminance in the first emission color. When the output current It of the variable constant current source 13 is increased and the output current exceeds a predetermined value (I4 (A)), the switching circuit 30 turns off the FET 38 and turns on the FET 39 to output the variable constant current source 13. Control is performed so that the current It flows only through the second LED array 21. As a result, in the LED drive circuit 10, the second LED array 21 emits light with high brightness in the second emission color in the current range I5 where the value of the output current It of the variable constant current source 13 is large.

  That is, the LED drive circuit 10 emits light with the light emission color of the second LED row 21 without being mixed with the light emission color of the first LED row 11 at the time of high luminance light emission, so that the light emission color can be managed in the LED drive circuit 10 and products using the LED drive circuit 10. It becomes easy. The LED driving circuit 10 emits light because the second LED array 21 emits light at a color temperature with high light emission efficiency, and the first LED array 11 that emits light at a color temperature with low light emission efficiency is turned off while emitting light with high luminance. Efficiency is high.

  As shown in FIG. 1, the switching circuit 30 in the LED drive circuit 10 includes a resistor 32 as a current detection element and a transistor 37 as a comparator. In other words, the transistor 37 as a comparator compares the base-emitter threshold voltage of 0.6 V with the voltage at one end of the resistor 32 (base voltage) and inverts the collector voltage output. Also, one end of the resistor 32 that is a current detection element is connected to the source (the other end) of the FET 38 that is the first switch and the source (the other end) of the FET 39 that is the second switch, and the first LED row 11 and the second LED row 21 are connected. Currents I1 and I2 flowing in the resistor 32 flow into the resistor 32. The FETs 38 and 39 are controlled based on the collector voltage of the transistor 37. The FET 38 is controlled by the collector voltage of the transistor 37 based on the collector voltage of the transistor 37, and the FET 39 is further controlled by the drain voltage of the FET 38.

  The current detection element may be not only a resistor but also a coil or an integrated sensor. As a comparator, not only a transistor but also a comparator or an A / D converter may be used. The current detection element may be provided separately for the first switch and the second switch. The circuit that controls the first and second switches may be a digital control circuit. However, the LED drive circuit 10 enables switching between the first LED array 11 and the second LED array 21 based on the current It with a small number of elements.

  As described above, in the LED drive circuit 10, the color temperature of the first emission color is set low and the color temperature of the second emission color is set high, so that the first and second LED arrays 11 for the currents I1 and I2 are used. , 21 has a higher luminous efficiency in the second LED array 21. Under this condition, the LED drive circuit 10 increases the number of LEDs 12 included in the first LED array 11 from the number of LEDs 22 included in the second LED array 21, and the first LED array 11 and the first LED array before and after the current I4 (A). The brightness of the two LED rows 21 was made equal. That is, since the brightness does not change when the emission color is switched, natural light control is possible.

  In the LED driving circuit 10, the number of LEDs 12 and 22 is adjusted in order to make the threshold voltage of the first LED row 11 higher than the threshold voltage of the second LED row 21. The reason why the threshold voltages of the first and second LED rows 11 and 12 can be adjusted by the number of the LEDs 12 and 22 is that the LED dies of the LED 12 and the LED 22 are common in the LED driving circuit 10 as described above. This is because the forward voltage drop was equal. On the other hand, there may be a case where different types and types of LED dies are used for the LEDs included in the first LED row and the second LED row. In this case, it is necessary to adjust the threshold voltages of the first LED row and the second LED row based on the forward voltage drop of each LED die.

(Second Embodiment)
As described above, in the switching circuit 30 of the LED driving circuit 10 shown in FIG. 1, the current detection element is the resistor 32, the comparator is the transistor 37 (more precisely, the pull-up resistor 31), and the output of the comparator. Is the collector voltage of the transistor 37, and the first and second switches are enhancement type FETs 38 and 39. On the other hand, the first switch and the second switch may be other switching elements. FIG. 3 shows a circuit diagram of an LED driving circuit 40 in which the first switch and the second switch are bipolar transistors as a second embodiment of the present invention.

  As shown in FIG. 3, in the LED drive circuit 40, the variable constant current source 13 and the first and second LED rows 11 and 21 included in the LED drive circuit 10 shown in FIG. 1 are common, and the switching circuit 40a is LED driven. Different from the switching circuit 30 of the circuit 10.

  In the LED drive circuit 40, the switching circuit 40a includes resistors 41, 42, 43, 44, and 45, and bipolar transistors (hereinafter referred to as transistors) 47, 48, and 49. The resistor 41 is a pull-up resistor, and the current output terminal of the variable constant current source 13 and the collector of the transistor 47 are connected via the resistor 31. The resistor 42 is a current detection resistor, and the base and emitter of the transistor 47 are connected via the resistor 42 and a resistor 43 that limits the base current. The emitter of the transistor 47 is connected to a terminal (ground) from which the current of the variable constant current source 13 returns.

  The transistor 48 operating as the first switch has a collector (one end) connected to the cathode of the first LED row 11 and a base (control terminal) connected to the collector of the transistor 47 via the resistor 44. The transistor 49 operating as the second switch has a collector (one end) connected to the cathode of the second LED row 21 and a base (control terminal) connected to the collector of the transistor 48 via the resistor 45. The emitters (other ends) of the transistors 48 and 49 are both connected to one end of the resistor 42.

  In the switching circuit 40a, the comparator is a transistor 47 (more precisely, the resistor 41 is included) as in the switching circuit 30 (see FIG. 1). However, since the first switch and the second switch are the transistors 48 and 49, ON and OFF are controlled by the base current. That is, the transistor 48 is controlled by the current shunted from the collector of the transistor 47 as the output of the comparator, and the transistor 49 is controlled by the current shunted from the collector of the transistor 48.

  If the base current is ignored, the light emission operation of the LED drive circuit 40 is equal to the light emission operation of the LED drive circuit 10 shown in FIG. 1 and FIG. Further, the LED drive circuit 40 is slightly simpler than the LED drive circuit 10 shown in FIG. 1 by using bipolar transistors (transistors 48 and 49) as the first and second switches.

(Third embodiment)
In the LED drive circuits 10, 40, the first switch (FET 38, transistor 48) is controlled by the output of the comparator (transistors 37, 47), and the second switch (FET 39, transistor 49) is the output (drain) of one end of the first switch. Voltage, current shunted from the collector). Such a circuit configuration may cause oscillation when the current It is in the vicinity of I4 (A) in FIG. 2, for example. FIG. 4 shows a circuit diagram of an LED drive circuit 50 having a hysteresis characteristic when the first switch and the second switch are switched to prevent oscillation as a third embodiment of the present invention.

  As shown in FIG. 4, the LED drive circuit 50 is obtained by adding a resistor 51 to the LED drive circuit 10 shown in FIG. 1. That is, in the LED drive circuit 50, the gate (control terminal) of the FET 38 serving as the first switch and the drain (one end) of the FET 39 serving as the second switch are connected via the resistor 51 in the switching circuit 50a.

  The circuit composed of the FET 38 and the first LED string 11 can be regarded as an inverting amplifier having the gate of the FET 38 as an input and the drain as an output. Similarly, the circuit composed of the FET 39 and the second LED array 21 can be regarded as an inverting amplifier in which the gate of the FET 39 is an input and the drain is an output. That is, the resistor 51 can be regarded as a feedback resistor that connects the input and output of an amplifier composed of a two-stage inverting amplifier. This feedback resistor generates hysteresis as is well known. As a result, the LED drive circuit 50 is suppressed from oscillating near the current I4 (A) in FIG.

10, 40, 50 ... LED drive circuit,
11 ... 1st LED row,
12, 22 ... LED,
13 ... Variable constant current source,
21 ... 2nd LED row,
30, 40a, 50a ... switching circuit,
31, 32, 33, 34, 41, 42, 43, 44, 45, 51 ... resistance,
35, 36 ... Zener diode,
37, 47 ... transistor (bipolar transistor, comparator),
38 ... FET (enhancement type field effect transistor, first switch),
39 ... FET (enhancement type field effect transistor, second switch),
48 ... transistor (bipolar transistor, first switch),
49: Transistor (bipolar transistor, second switch).

Claims (7)

A variable constant current source;
A first LED array that emits light in a first emission color;
A second LED array that emits light in a second emission color;
A switching circuit that switches between a state in which the first LED row emits light and a state in which the second LED row emits light,
The first LED row and the second LED row are connected in parallel to the variable constant current source,
The switching circuit has a first switch having one end connected in series with the first LED array, and a second switch having one end connected in series with the second LED array, and the current output from the variable constant current source is a predetermined current. When it is smaller, the first switch is turned on and the second switch is turned off. When the current output from the variable constant current source is larger than the predetermined current, the first switch is turned off and the second switch LED driving circuit characterized by turning on.   The switching circuit includes a current detection element and a comparator that switches an output with a voltage at one end of the current detection element, the one end of the current detection element, the other end of the first switch, and the other end of the second switch. The LED driving circuit according to claim 1, wherein the first switch and the second switch are controlled based on an output of the comparator.   The LED driving circuit according to claim 2, wherein the first switch is controlled by an output of the comparator, and the second switch is controlled by an output of the one end of the first switch.   The current detection element is a resistor, the comparator includes a bipolar transistor and another resistor, the output of the comparator is a collector voltage of the bipolar transistor, and the first switch and the second switch are enhancement-type FETs The LED driving circuit according to claim 2 or 3, wherein   The current detection element is a resistor, the comparator includes a bipolar transistor and another resistor, the output of the comparator is a current shunted from the collector of the bipolar transistor, and the first switch and the second switch are 4. The LED driving circuit according to claim 2, wherein the LED driving circuit is a bipolar transistor.   6. The LED drive circuit according to claim 1, wherein the one end of the second switch is connected to a control terminal of the first switch via a resistor.   The LED drive circuit according to any one of claims 1 to 6, wherein a threshold voltage of the first LED array is higher than a threshold voltage of the second LED array.

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