JP6900830B2 - LED lighting circuit and LED lighting device - Google Patents

Description

The present invention relates to an LED lighting circuit and an LED lighting device using the same.

The LED lighting device of Patent Document 1 includes a step-up chopper circuit and a step-down chopper circuit that steps down the output of the step-up chopper circuit to supply a current to the LED. The step-down chopper circuit is a so-called low-side control type, and is provided with a current detection resistor for detecting the current flowing through the switching element. The control circuit PWM drives the switching element so that the integrated value of the voltage generated in the current detection resistor becomes constant at the target value.

Japanese Unexamined Patent Publication No. 2013-69482

By the way, in the above configuration, even if the integrated value of the current flowing through the switching element becomes constant at the target value, the LED current may also fluctuate due to the fluctuation of the forward voltage of the LED. In order to deal with this, as shown in the comparative example of FIG. 3, a voltage detection circuit 140 composed of voltage dividing resistors 41 and 42 is provided between the cathode of LED 2 and the ground G, and the detection voltage which is the voltage dividing value thereof is provided. A configuration has been proposed in which the target value is corrected according to Vd. The control unit 155 outputs a correction signal Sc according to the forward voltage Vf of the LED 2 specified based on the detection voltage Vd, and the DC / DC drive unit 52 corrects the target value based on the correction signal Sc. The DC / DC drive unit 52 PWM drives the switching element 32 so that the integrated value of the current flowing through the current detection resistor 35 matches the correction target value. Here, a voltage obtained by subtracting the forward voltage Vf of the LED 2 from the DC output voltage Vdc of the power factor improving circuit 20 is always applied to the voltage detection circuit 140. Therefore, in such a configuration, the power consumption in the voltage detection circuit 140 becomes a problem, as will be described in detail later.

Therefore, an object of the present invention is to reduce power consumption in a voltage detection circuit to save power in an LED lighting circuit that employs a low-side control type step-down chopper circuit and an LED lighting device that uses the LED lighting circuit.

The LED lighting circuit of the present disclosure includes a DC output circuit having a high potential output end and a low potential output end, an inductor, a switching element, and a diode, and the LED, the inductor, and the switching element from the high potential output end to the low potential output end. A series circuit is formed, and a step-down chopper circuit in which a diode is connected from the connection point of the inductor and the switching element with the direction of the high potential output end as the forward direction, and between the connection point of the LED and the inductor and the low potential output end. A voltage detection circuit that has a series circuit of the connected first resistor, switch element, and second resistor, and outputs the voltage at the connection point (detection point) between the switch element and the second resistor as the detection voltage, and the detection It includes a control circuit that controls the output operation of the step-down chopper circuit based on the voltage, and a switching circuit that turns on / off the switch element at a predetermined timing.

As a result, the output operation of the step-down chopper circuit is optimized by the detection voltage when the switch element is turned on, and the switch element is turned off at a predetermined timing, so that the current of the voltage detection circuit is cut off and the voltage detection circuit. Power consumption is reduced. Therefore, in the LED lighting circuit that employs the low-side control type step-down chopper circuit, power saving is realized by reducing the power consumption in the voltage detection circuit.

In the first embodiment, the control circuit outputs an on signal to the switching circuit when enabling the output operation of the step-down chopper circuit, and outputs an off signal to the switching circuit when stopping the output operation of the step-down chopper circuit. It is configured to output, and the switching circuit is configured to turn on / off the switch elements in response to the on / off signal. As a result, in the standby mode for turning off the light, the current path by the LED and the voltage detection circuit is cut off, the standby power can be reduced, and the low light lighting of the LED can be prevented.

In the first modification of the first embodiment, the control circuit outputs an on signal to the switching circuit within a predetermined period from the start of the output operation of the step-down chopper circuit, and outputs an off signal to the switching circuit after the elapse of the predetermined period. The switching circuit is configured to turn on / off the switch elements in response to the on / off signal. As a result, the effect of the first embodiment can be obtained, and the power consumption of the voltage detection circuit while the LED is lit is minimized.

In the second modification of the first embodiment, the control circuit outputs an on signal and an off signal to the switching circuit at a predetermined cycle during the period in which the output operation of the step-down chopper circuit is enabled, and the output operation of the step-down chopper circuit. Is configured to output an off signal to the switching circuit when stopping, and the switching circuit is configured to turn on / off the switch elements in response to the on / off signal. As a result, the effect of the first embodiment can be obtained, and the power consumption of the voltage detection circuit can be reduced while the operation of the switching element during lighting corresponds to the temporal change of the forward voltage Vf of the LED.

In the third modification of the first embodiment, the control circuit controls the output operation of the step-down chopper circuit according to the dimming rate indicated by the dimming signal from the outside, and is determined after the dimming rate changes. The on signal is output to the switching circuit within the period, and the off signal is output to the switching circuit after a predetermined period of time elapses. The switching circuit turns on / off the switch elements according to the on / off signal. It is configured as follows. As a result, the detection voltage is detected only when the dimming rate changes and the forward voltage Vf of the LED changes, and the current of the voltage detection circuit is cut off in other cases. Therefore, the effect of the first embodiment can be obtained, and the power consumption of the voltage detection circuit while the LED is lit is reduced.

Here, the switch element is a PNP transistor, and the switching circuit is a third resistor between the connection point between the inductor and the LED and the input terminal of the PNP transistor, and between the input terminal of the PNP transistor and the low potential output end. It is provided with an NPN transistor connected to the NPN transistor, and is configured so that the NPN transistor is turned on / off by an on signal / off signal, respectively. As a result, a simple voltage detection circuit and switching circuit are realized in a configuration in which the current path is cut off by the switch element connected to the higher potential side of the detection point and a high voltage is not applied to the control circuit.

In the second embodiment, the control circuit can enable or stop the output operation of the step-down chopper circuit, the switch element is composed of a PNP transistor, and the switching circuit is the connection point between the inductor and the LED and the PNP transistor. The voltage generated in the third resistor between the input terminal of the PNP transistor, the NPN transistor connected between the input terminal of the PNP transistor and the low potential output terminal, the auxiliary winding provided in the inductor, and the auxiliary winding is rectified. The rectifying and smoothing circuit is provided, and the smoothing voltage by the rectifying and smoothing circuit is input to the input terminal of the NPN transistor. As a result, the same effect as that of the first embodiment can be obtained, and the control circuit is simplified.

In the first or second embodiment, the control circuit is preferably configured to stop the operation of the DC output circuit at the same time as stopping the output operation of the step-down chopper circuit. As a result, when the PNP transistor is turned off, the voltage applied to the PNP transistor is reduced, and the withstand voltage of the PNP transistor can be reduced.

The LED lighting device of the present invention includes any of the above LED lighting circuits and an LED. As a result, an LED lighting device that achieves each of the above effects is realized.

It is a circuit diagram of the LED lighting circuit and the LED lighting device according to the first embodiment. It is a circuit diagram of the LED lighting circuit and the LED lighting device according to the second embodiment. It is a circuit diagram of the LED lighting circuit and the LED lighting device of the comparative example.

<First Embodiment>
FIG. 1 shows a circuit diagram of an LED lighting circuit 1 according to the first embodiment of the present invention and an LED lighting device 3 including the LED lighting circuit 1. The LED lighting device 3 includes an LED lighting circuit 1 and an LED 2. An AC power supply AC such as a commercial power supply is input to the LED lighting circuit 1, and a DC output from the LED lighting circuit 1 is supplied to the LED 2. The LED lighting circuit 1 includes a rectifier circuit 10, a DC output circuit 20 (hereinafter referred to as “PFC20”), an auxiliary power supply circuit 70, a step-down chopper circuit 30, a voltage detection circuit 40, a control circuit 50, and a switching circuit 60.

The rectifier circuit 10 is a full-wave rectifier such as a diode bridge, and full-wave rectifies the AC voltage input from the AC power supply AC. If necessary, a current fuse, a noise filter, or the like is connected to the front stage of the rectifier circuit 10. Further, when a DC voltage is input, the rectifier circuit 10 may not be provided.

The PFC 20 includes an inductor 21, a switching element 22, a diode 23, and a smoothing capacitor 24, and constitutes a power factor improving circuit including a step-up chopper circuit. The switching element 22 is, for example, a MOSFET (hereinafter, also referred to as “FET 22”), and is PWM-driven by a control circuit 50 (PFC drive unit 51). During the period when the FET 22 is on, energy is stored in the inductor 21 by flowing the output voltage of the rectifier circuit 10 through the inductor 21 and the FET 22. During the period when the FET 22 is off, the energy stored in the inductor 21 together with the output voltage of the rectifier circuit 10 is charged to the smoothing capacitor 24 via the diode 23. In this way, the voltage charged in the smoothing capacitor 24 becomes the DC output voltage Vdc between the high potential output end and the low potential output end of the PFC 20. The node having the same potential as the low potential output end is referred to as ground G.

The auxiliary power supply circuit 70 generates a control power supply Vcc for each part of the control circuit 50 from the DC output voltage Vdc. The auxiliary power supply circuit 70 includes, for example, a voltage regulator circuit such as a three-terminal regulator or a series regulator, and generates a constant voltage of about 5 to 20 V with the ground G as a reference potential as a control power supply. In the present embodiment, the same control power supply Vcc is supplied to each part of the control circuit 50, but different control power supplies may be supplied from the auxiliary power supply circuit 70 to each part.

The step-down chopper circuit 30 includes an inductor 31, a switching element 32, a diode 33, and a capacitor 34, and constitutes a low-side control type back-type DC / DC converter in which the operation reference point of the switching element 32 is substantially ground G. The switching element 32 is, for example, a MOSFET (hereinafter, also referred to as “FET 32”), and is PWM-driven by a control circuit 50 (DC / DC drive unit 52). Further, the current detection resistor 35 is composed of a low resistance element and is connected between the source of the FET 32 and the ground G. A voltage corresponding to the switching current of the step-down chopper circuit 30 is generated in the current detection resistor 35, and this voltage corresponds to the detection current. During the period when the FET 32 is on, a current flows through the LED 2 → the inductor 31 → the FET 32 → the current detection resistor 35 → the ground G using the DC output voltage Vdc as a power source, and energy is stored in the inductor 31. During the period when the FET 32 is off, a current flows through the inductor 31 → the diode 33 → the LED 2 → the inductor 31 using the energy stored in the inductor 31 as a power source. The capacitor 34 filters the current flowing through the LED 2.

The voltage detection circuit 40 includes resistors 41 and 42, a switch element 43, and a capacitor 44. Although the resistor 41 is shown as one resistor, it may actually be composed of a plurality of resistors connected in series. The switch element 43 is a PNP type MOSFET (hereinafter referred to as “FET 43”). One end of the resistor 41 is connected to the connection point between the LED 2 and the inductor 31, the other end of the resistor 41 is connected to the source of the FET 43, the drain of the FET 43 is connected to one end of the resistor 42, and the other end of the resistor 42 is connected to the ground G. Be connected. Further, the voltage at the connection point between the drain of the FET 43 and the resistor 42 (hereinafter referred to as “detection point”) is input to the control circuit 50 as the detection voltage Vd. The detection voltage Vd is a value obtained by subtracting the forward voltage Vf of the LED 2 from the DC output voltage Vdc.

Here, with reference to FIG. 3, the operation of the LED lighting circuit 4 of the comparative example in which the FET 43 and the circuit portion related thereto are not provided, particularly in the standby mode for turning off the light, will be described. In the comparative example, substantially the same configurations as those of the first embodiment of FIG. 1 are designated by the same reference numerals, and the duplicated description thereof will be omitted. In the comparative example, the voltage detection circuit 140 and the control circuit 150 (control unit 155) are provided instead of the voltage detection circuit 40 and the control circuit 50 (control unit 55) of the first embodiment, and the switching circuit 60 is not provided. .. The difference between the control unit 55 and the control unit 155 is due to the presence or absence of the switching circuit 60. The voltage detection circuit 140 is composed of a series circuit of resistors 41 and 42, and the voltage dividing point thereof serves as a detection point. According to this configuration, even in the standby mode, when the DC output voltage Vdc is applied to the LED2 and the voltage detection circuit 140 and the voltage applied to the LED2 exceeds the lit forward voltage Vfa of the LED2, A minute current flows through the LED 2, the resistor 41, and the resistor 42. The situation where the DC output voltage Vd exceeds the forward voltage Vfa can occur even when the PFC 20 stops the boosting operation (that is, the peak voltage of the AC power supply AC may exceed the forward voltage Vfa). When a current path is generated in the LED 2 and the voltage detection circuit 140 in this way, wasteful standby power is consumed in the voltage detection circuit 140. Further, since a current flows through the LED 2, there is also a problem that the LED 2 is slightly lit even though the standby mode for turning off the light is being executed.

Therefore, returning to FIG. 1, in the present embodiment, the FET 43 is located between the resistor 41 and the detection point in the voltage detection circuit 40 in order to reduce the standby power in the standby mode for turning off the light and prevent the LED 2 from turning on the faint light. A switching circuit 60 for controlling the FET 43 and a control configuration thereof are provided. The FET 43 needs to be connected to a higher potential side than the detection point so that a high voltage is not applied to the detection point when the FET 43 is off.

The control circuit 50 includes a PFC drive unit 51, a DC / DC drive unit 52, and a control unit 55.
The PFC drive unit 51 PWM drives the FET 22 so that the DC output voltage Vdc becomes a predetermined value. The PFC drive unit 51 includes a PFC control IC and its peripheral circuits (input voltage detection circuit, output voltage detection circuit, auxiliary winding of inductor 21 and the like (not shown)), but in the present disclosure, for convenience of explanation, the PFC drive unit 51 51 is described as one block. Further, the boosting operation of the PFC 20 by the PFC drive unit 51 is enabled by the input of the enable signal EN1 (for example, a high level signal) from the control unit 55, and is enabled by the release of the enable signal EN1 (for example, a low level signal). Stop.

The DC / DC drive unit 52 PWM drives the FET 32 of the step-down chopper circuit 30. The DC / DC drive unit 52 includes a DC / DC converter control IC and its peripheral circuits (not shown), but in the present disclosure, the DC / DC drive unit 52 is described as one block for convenience of explanation. The DC / DC drive unit 52 determines the on-time (or on-duty) in the PWM drive so that the integrated value of the voltage of the current detection resistor 35 becomes constant at the target value (current target value). The current target value can be corrected by the correction signal Sc from the control unit 55. The correction signal Sc will be described later. Further, the output operation of the step-down chopper circuit 30 by the DC / DC drive unit 52 is enabled by the input of the enable signal EN2 (for example, a high level signal) from the control unit 55, and the enable signal EN2 is released (for example, low level). Stop by signal).

The control unit 55 includes a microcomputer and its peripheral circuits, and comprehensively controls the operations of the PFC drive unit 51, the DC / DC drive unit 52, and the switching circuit 60. The control unit 55 includes a CPU 56, a memory 57, and a signal input unit 58 if necessary, and the CPU 56 includes an enable / stop unit 561, a correction signal generation unit 562, and a switching determination unit 563, each of which is via a bus. It is connected in such a way that signals and data can be exchanged. The signal input unit 58 functions as an input interface that receives inputs of lighting signals and extinguishing signals from the outside.

The enable / stop unit 561 controls the activation / stop of the boosting operation of the PFC 20 via the PFC drive unit 51 by the enable signal EN1, and the step-down chopper circuit 30 via the DC / DC drive unit 52 by the enable signal EN2. Controls whether output operation is enabled / disabled. In the present disclosure, enabling / stopping the operation of the PFC 20 is synonymous with enabling / stopping the PWM drive of the FET 22 (maintaining the off state), and enabling / stopping the output operation of the step-down chopper circuit 30. Is synonymous with enabling / stopping (maintaining the off state) the PWM drive of the FET 32.

When the AC power supply AC is turned on to the LED lighting circuit 1, the enable / stop unit 561 outputs the enable signal EN1 to the PFC drive unit 51 and the enable signal EN2 to the DC / DC drive unit 52, and outputs the PFC 20 and the step-down chopper circuit. Enable the operation of 30. When the extinguishing signal is input to the signal input unit 58, the enable / stop unit 561 releases the enable signals EN1 and EN2 (or reverses the logic thereof), and stops the operation of the PFC 20 and the step-down chopper circuit 30. After that, when a lighting signal is input to the signal input unit 58, the enable / stop unit 561 outputs enable signals EN1 and EN2 to enable the operation of the PFC 20 and the step-down chopper circuit 30.

Further, the activation / stop unit 561 may release the enable signals EN1 and EN2 to stop the operation of the PFC 20 and the step-down chopper circuit 30 when, for example, the detection value by the voltage detection circuit 40 is abnormal. .. Further, when the above abnormality is resolved, the enable / stop unit 561 may output enable signals EN1 and EN2 (for example, a high level signal). Therefore, even if there is no signal input unit 58, the enable / stop unit 561 can enable / stop the operation of the PFC 20 and the step-down chopper circuit 30. In other words, the detection voltage Vd detected by the voltage detection circuit 40 may be used not only for correcting the current target value, but also for detecting the overvoltage state of the LED2, detecting the short-circuit state of the LED2, and the like.

The correction signal generation unit 562 outputs a correction signal Sc for correcting the current target value in the DC / DC drive unit 52 according to the detection voltage Vd from the voltage detection circuit 40. The correction signal Sc is generated, for example, so that the current target value decreases with respect to an increase in the detection voltage Vd (= Vdc-Vf). If there is no correction signal Sc and the forward voltage Vf of LED2 decreases, even if the LED current integrated value during the PWM drive on period of the FET 32 becomes constant, the LED current integrated value during the off period will be. As it increases, the total LED current integral value increases. Therefore, the correction signal Sc is generated so as to compensate for such an increase in the LED current integrated value that may occur.

The switching determination unit 563 outputs an on signal S1 / off signal S0 to the switching circuit 60 to control on / off of the FET 43 of the voltage detection circuit 40. In the present embodiment, when the activation / stop unit 561 outputs the enable signals EN1 and EN2, the switching determination unit 563 outputs the on signal S1 to turn on the FET 43 in the switching circuit 60. On the other hand, the switching determination unit 563 outputs an off signal S0 when the activation / stop unit 561 releases the enable signals EN1 and EN2, and causes the switching circuit 60 to turn off the FET 43.

The switching circuit 60 includes resistors 61 and 63 and a transistor 62. The resistor 61 is connected between the connection point between the inductor 31 and the LED 2 and the gate (input terminal) of the FET 43. The resistor 61 may also be composed of a plurality of resistors connected in series, as in the case of the resistor 41. The transistor 62 is an NPN type, is connected between the gate of the FET 43 and the ground G, and is turned on / off by the on signal S1 / off signal S0. That is, the on signal S1 turns on the transistor 62 and the FET 43, and the off signal S0 turns off the transistor 62 and the FET 43.

As described above, the LED lighting circuit 1 of the present invention includes a DC output circuit 20, a low-side control type step-down chopper circuit 30 having an inductor 31, an FET 32, and a diode 33, a connection point between the LED 2 and the inductor 31, and a ground G. A voltage detection circuit 40 having a series circuit of a resistor 41, an FET 43, and a resistor 42 connected between the two, and outputting the voltage at the connection point (detection point) of the transistor 43 and the resistor 42 as a detection voltage Vd, and a detection voltage Vd. A control circuit 50 (activation / stop unit 561 and correction signal generation unit 562) that controls the output operation of the step-down chopper circuit 30 based on the above, and a switching circuit 60 that turns on / off the FET 43 at a predetermined timing are provided.

As a result, the output operation of the step-down chopper circuit 30 is optimized by the detection voltage Vd while the FET 43 is turned on, and the FET 43 is turned off at a predetermined timing, so that the power consumption in the voltage detection circuit 40 is reduced. Therefore, in the LED lighting circuit 1 that employs the low-side control type step-down chopper circuit 30 and the LED lighting device 3 that uses the LED lighting circuit 1, power saving is realized by reducing the power consumption in the voltage detection circuit 40.

Further, the FET 43 is a PNP type, and a switching circuit 60 is provided between the resistance 61 between the connection point between the inductor 31 and the LED 2 and the gate (input terminal) of the FET 43, and between the gate (input terminal) of the FET 43 and the ground G. It is provided with a connected NPN type transistor 62, and is configured so that the transistor 62 is turned on / off by the on signal S1 / off signal S0, respectively. As a result, a simple voltage detection circuit 40 and switching circuit 60 are realized in a configuration in which the current path is cut off by the FET 43 connected to the higher potential side of the detection point and a high voltage is not applied to the control circuit 50.

In particular, in the present embodiment, the control circuit 50 (enable / stop unit 561 and switching determination unit 563) outputs the on signal S1 to the switching circuit 60 when enabling the output operation of the step-down chopper circuit 30, and steps down the voltage. The off signal S0 is output to the switching circuit 60 when the output operation of the chopper circuit 30 is stopped. Then, the switching circuit 60 is configured to turn on / off the FET 43 in response to the on signal S1 / off signal S0. As a result, in the standby mode for turning off the light, the current path by the LED 2, the resistor 41, the FET 43 and the resistor 42 is cut off, the standby power can be reduced, and the low light lighting of the LED 2 can be prevented. In addition, the standby power caused by the resistor 61 is also reduced.

<First modification of the first embodiment>
In the above embodiment, the configuration is shown in which the FET 43 is turned on while the LED is lit and the FET 43 is turned off in the standby mode for turning off the LED. However, as a first modification, not only in the standby mode but also after a predetermined period immediately after the start-up. The FET 43 may be turned off.

Specifically, the switching determination unit 563 of the control circuit 50 outputs an on signal S1 within a predetermined period (for example, about 0.1s to 10s) from the start of the output operation of the step-down chopper circuit 30 to turn on the FET 43. After a lapse of a predetermined period, the off signal S0 is output to turn on the FET 43. The detection voltage Vd detected during the period when the on signal S1 is output is stored in the memory 57, and the correction signal generation unit 562 outputs the correction signal Sc based on the stored detection voltage Vd.

As a result, the effect of reducing the standby power in the voltage detection circuit 40 (and the resistor 61) at the time of turning off as described above can be obtained in the above embodiment, and the consumption in the voltage detection circuit 40 (and the resistor 61) during lighting can be obtained. Power is minimized.

<Second variant of the first embodiment>
In the above embodiment, the configuration is shown in which the FET 43 is turned on while the LED is lit and the FET 43 is turned off in the standby mode for turning off the LED. However, as a second modification, the FET 43 is periodically turned on not only in the standby mode but also during the LED lighting. The FET 43 may be turned on / off.

Specifically, the switching determination unit 563 of the control circuit 50 outputs the on signal S1 and the off signal S0 at a predetermined cycle during the period for enabling the output operation of the step-down chopper circuit 30 to turn on the FET 43, and the step-down chopper circuit When the output operation of 30 is stopped, the off signal S0 is output to turn off the FET 43. The predetermined cycle may be, for example, about 0.1 s to 1 h, and the on-period length may be a time sufficient to acquire the detected voltage. Also in this modification, the detection voltage Vd detected during the period when the on signal S1 is output is stored in the memory 57, and the correction signal generation unit 562 outputs the correction signal Sc based on the stored detection voltage Vd. And.

As a result, the effect of reducing the standby power consumption in the voltage detection circuit 40 (and the resistor 61) when the voltage is turned off as described above in the above embodiment can be obtained, and the current target value can be changed with time of the forward voltage Vf of the LED 2. It is possible to reduce the power consumption of the voltage detection circuit 40 (and the resistor 61) during lighting while making it compatible.

<Third variant of the first embodiment>
In the above embodiment, the configuration is shown in which the FET 43 is turned on while the LED is lit and the FET 43 is turned off in the standby mode for turning off the LED. However, as a third modification, the FET 43 is turned on and detected only when the dimming rate changes. The voltage may be acquired and the FET 43 may be turned off otherwise. In this modification, the signal input unit 58 receives a dimming signal from the outside.

Specifically, the correction signal generation unit 562 and the switching determination unit 563 of the control circuit 50 control the output operation of the step-down chopper circuit 30 according to the dimming rate indicated by the dimming signal from the outside, and also control the dimming rate. Is configured to output the on signal S1 to the switching circuit 60 within a predetermined period (for example, about 0.1s to 10s) after the change in, and output the off signal S0 to the switching circuit 60 after the elapse of the predetermined period. It should be noted that the start of lighting can also be defined as being included in "when the dimming rate changes". Also in this modification, the detection voltage Vd detected during the period when the on signal S1 is output is stored in the memory 57, and the correction signal generation unit 562 outputs the correction signal Sc based on the stored detection voltage Vd. And.

As a result, the detection voltage is detected only when the dimming rate changes and the forward voltage Vf of the LED 2 changes, and the current of the voltage detection circuit 40 is cut off in other cases. Therefore, in the above embodiment, the effect of reducing the standby power in the voltage detection circuit 40 (and the resistor 61) at the time of turning off as described above can be obtained, and the power consumption in the voltage detection circuit 40 (and the resistor 61) during lighting can be obtained. Is reduced.

Although the first to third modifications are shown as individual ones, these can be applied in combination. For example, the timing at which the on signal S1 is output from the control circuit 50 (switching determination unit 563) may be a time according to a predetermined cycle at the time of lighting and a change in the dimming rate.

<Second embodiment>
In the first embodiment, the control circuit 50 outputs an on signal S1 / off signal S0 to turn on / off the FET 43, but in the present embodiment, the FET 43 is turned on by the output action of the step-down chopper circuit 30. / Indicates the configuration to be turned off.

FIG. 2 shows a circuit diagram of the LED lighting circuit 1 of the present embodiment and the LED lighting device 3 including the LED lighting circuit 1. The same components as those of the first embodiment are designated by the same reference numerals, and the duplicated description thereof will be omitted. The configuration of the switching circuit 60 and the control circuit 50 (CPU 56) is different from that of the first embodiment in this embodiment. The CPU 56 has an enable / stop unit 561 and a correction signal generation unit 562, and does not have a switching determination unit 563.

The switching circuit 60 of the present embodiment includes an auxiliary winding 31s of the inductor 31, a resistor 61, a transistor 62, a resistor 63, a diode 64, and a capacitor 65. The auxiliary winding 31s is magnetically coupled to the inductor 31. One end of the auxiliary winding 31s is connected to the ground G, and the other end is connected to the capacitor 65 via the diode 64. The diode 64 is connected to the capacitor 65, which constitutes a rectifying and smoothing circuit for the ground G. A resistor (not shown) may be connected in series to the diode 64 to form an integrator circuit with the capacitor 65. The voltage generated in the capacitor 65 controls the transistor 62 via the resistor 63. Although the present embodiment shows a half-wave rectification configuration in which a diode 64 is connected to one end of the auxiliary winding 31s, a full-wave rectification configuration in which a diode bridge is connected to both ends of the auxiliary winding 31s is adopted. May be good.

During the output operation of the step-down chopper circuit 30 (while the LED is lit), the voltage generated in the auxiliary winding 31s by the PWM drive of the FET 32 is rectified and smoothed by the diode 64 and the capacitor 65, and the transistor 62 is turned on by this rectified smoothing voltage. , FET 43 turns on. On the other hand, when the output operation of the step-down chopper circuit 30 is stopped (when the LED is turned off), the voltage is not generated in the auxiliary winding 31s due to the FET 32 being turned off, the transistor 62 is turned off, and the FET 43 is turned off. Therefore, the current of the voltage detection circuit 40 is cut off in the standby mode when the light is turned off.

In the present embodiment, the operation of the control circuit 50 to stop the output operation of the step-down chopper circuit 30 is not only the operation of the activation / stop unit 561 of the control unit 55 to release the enable signal EN2, but also the operation of the DC / DC drive unit. The operation of 52 to execute the drive stop function is also included. In either case, the FET 43 is turned off by the switching circuit 60. This drive stop function includes, for example, an overcurrent protection function when the peak of the detected current detected by the current detection resistor 35 exceeds the upper limit value, a low voltage protection function when the control power supply Vcc falls below the lower limit value, and the like. ..

Further, as in the first embodiment, the enable / stop unit 561 can release the enable signals EN1 and EN2 when the light is turned off to stop the step-up operation of the PFC 20 and the output operation of the step-down chopper circuit 30. On the other hand, even when the output operation of the step-down chopper circuit 30 is stopped by the drive stop function of the DC / DC drive unit 52 regardless of the enable signal EN2, the detection voltage Vd is set to zero according to the fact that the FET 43 is turned off. Become. The activation / stop unit 561 may release the enable signal EN1 when the detection voltage Vd becomes zero. As a result, in various situations, the step-up operation of the PFC 20 is stopped as the output operation of the step-down chopper circuit 30 is stopped.

As described above, in the LED lighting circuit 1 of the present embodiment, the switching circuit 60 includes a resistor 61 between the connection point between the inductor 31 and the LED 2 and the gate (input terminal) of the FET 43, and the gate (input terminal) of the FET 43. A rectifying and smoothing circuit (diode 64 and capacitor 65) that rectifies and smoothes the voltage generated in the NPN type transistor 62 connected between the grounds G, the auxiliary winding 31s provided in the inductor 31, and the auxiliary winding 31s. The smoothing voltage by the rectifying smoothing circuit is input to the base (input terminal) of the transistor 62.

As a result, as in the first embodiment, the output operation of the step-down chopper circuit 30 is optimized by the detection voltage Vd with the FET 43 turned on, and the FET 43 is turned off at a predetermined timing, so that the voltage detection circuit The power consumption at 40 is reduced. Therefore, in the LED lighting circuit 1 that employs the low-side control type step-down chopper circuit 30 and the LED lighting device 3 that uses the LED lighting circuit 1, power saving is realized by reducing the power consumption in the voltage detection circuit 40. In particular, in the present embodiment, the control circuit 50 (control unit 55) does not require the switching determination unit 563, so that the effect of simplifying the control circuit 50 can be obtained. Furthermore, even when the output operation of the step-down chopper circuit 30 is stopped by the drive stop function of the DC / DC drive unit 52, not by the release of the enable signal EN2 from the enable / stop unit 561 of the control unit 55. , FET 43 is turned off. As a result, the chances of reducing the standby power by turning off the FET 43 in the standby mode and preventing the LED 2 from turning on the faint light are expanded.

<Modification example>
Although the preferred embodiments of the present invention have been shown above, the present invention can be transformed into various aspects as shown below, for example.

(1) Modification of Enable Signal EN1 In the first embodiment (excluding the first to third modified examples) and the second embodiment, the enable signal EN2 is released when the output operation of the step-down chopper circuit 30 is stopped. The configuration in which the boosting operation of the PFC 20 is also stopped is shown. As a result, when the FET 43 and the transistor 62 are turned off, the voltage applied to each of them is reduced, and the withstand voltage of the FET 43 and the transistor 62 can be reduced. However, if the withstand voltage of the FET 43 and the transistor 62 does not matter, or if the PFC drive unit 51 does not have an enable terminal (function), the step-up operation of the PFC 20 is not stopped when the output operation of the step-down chopper circuit 30 is stopped. May be good. For example, in the first embodiment, the enable signal EN1 may not be released when the enable signal EN2 is released.

(2) Modification of Transistor Type In each of the above embodiments, a MOSFET is used as the transistor 43 and a bipolar transistor is used as the transistor 62, but the MOSFET and the bipolar transistor can be replaced depending on the design conditions. Throughout the present disclosure, the gate of the MOSFET and the base of the bipolar transistor are collectively referred to as an "input terminal", the drain of the MOSFET and the collector of the bipolar transistor are collectively referred to as an "output terminal", and the source of the MOSFET and the bipolar transistor are collectively referred to. Emitters are collectively referred to as "ground terminals".

1 LED lighting circuit 2 LED
3 LED lighting device 20 DC output circuit (PFC)
30 Step-down chopper circuit 31 Inductor 31s Auxiliary winding 32 Switching element (FET)
33 Diode 40 Voltage detection circuit 41, 42 Resistance 43 Switch element (FET)
50 Control circuit 60 Switching circuit 61 Resistance 62 Transistor 64 Diode 65 Capacitor

Claims (9)

A DC output circuit with a high potential output end and a low potential output end,
It has an inductor, a switching element and a diode, and a series circuit of an LED, the inductor and the switching element is formed from the high potential output end to the low potential output end, and the diode is from a connection point between the inductor and the switching element. A step-down inductor circuit connected with the direction of the high potential output end as the forward direction,
It has a series circuit of a first resistor, a switch element and a second resistor connected between the connection point of the LED and the inductor and the low potential output end, and the switch element and the second resistor. A voltage detection circuit that outputs the voltage at the connection point of
A control circuit that controls the output operation of the step-down chopper circuit based on the detected voltage, and
An LED lighting circuit including a switching circuit for turning on / off the switch element at a predetermined timing. When the control circuit enables the output operation of the step-down chopper circuit, an on signal is output to the switching circuit, and when the output operation of the step-down chopper circuit is stopped, an off signal is output to the switching circuit. Configured to
The LED lighting circuit according to claim 1, wherein the switching circuit is configured to turn on / off the switch element in response to the on signal / off signal. The control circuit is configured to output an on signal to the switching circuit within a predetermined period from the start of the output operation of the step-down chopper circuit, and output an off signal to the switching circuit after the elapse of the predetermined period.
The LED lighting circuit according to claim 1, wherein the switching circuit is configured to turn on / off the switch element in response to the on signal / off signal. When the control circuit outputs an on signal and an off signal to the switching circuit at a predetermined cycle during the period for enabling the output operation of the step-down chopper circuit, and stops the output operation of the step-down chopper circuit, an off signal is output. It is configured to output to the switching circuit.
The LED lighting circuit according to claim 1, wherein the switching circuit is configured to turn on / off the switch element in response to the on signal / off signal. The control circuit controls the output operation of the step-down chopper circuit according to the dimming rate indicated by the dimming signal from the outside, and switches the on signal within a predetermined period after the dimming rate changes. It is configured to output to the switching circuit and output an off signal to the switching circuit after the lapse of the predetermined period.
The LED lighting circuit according to claim 1, wherein the switching circuit is configured to turn on / off the switch element in response to the on signal / off signal. The switch element is a PNP transistor.
The switching circuit is an NPN transistor connected between the connection point of the inductor and the LED and the input terminal of the PNP transistor, and the input terminal of the PNP transistor and the low potential output terminal. The LED lighting circuit according to any one of claims 2 to 5, wherein the NPN transistor is turned on / off by the on signal / off signal, respectively. The control circuit can enable or stop the output operation of the step-down chopper circuit.
The switch element is composed of a PNP transistor.
The switching circuit is a third resistor between the connection point of the inductor and the LED and the input terminal of the PNP transistor, and an NPN transistor connected between the input terminal of the PNP transistor and the low potential output terminal. An auxiliary winding provided in the inductor and a rectifying and smoothing circuit for rectifying and smoothing the voltage generated in the auxiliary winding are provided so that the smoothing voltage by the rectifying and smoothing circuit is input to the input terminal of the NPN transistor. The LED lighting circuit according to claim 1, which is configured in 1. The LED lighting circuit according to claim 2 or 7, wherein the control circuit is configured to stop the operation of the DC output circuit at the same time as stopping the output operation of the step-down chopper circuit. An LED lighting device comprising the LED lighting circuit according to any one of claims 1 to 8 and the LED.

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