JP2012155893A - Power supply circuit and lighting system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: an inexpensive power supply circuit capable of detecting abnormality generated in a diode such as an LED; and a lighting system.SOLUTION: An LED lighting circuit 100 includes: a constant current circuit 110 supplying current to an LED 200; a current restricting resistor 122 provided on a current path between a cathode of the LED 200 and the ground of the constant current circuit 110; and a short circuit protection circuit 120 blocking the supply of the current to the LED 200 when a potential difference between the cathode of the LED 200 and the ground of the constant current circuit 110 exceeds a predetermined voltage. Thereby, abnormality, such as a short circuit, generated in the LED 200 can be inexpensively detected without using an expensive circuit, such as an operational amplifier and a photo coupler.

Description

  The present invention relates to a power supply circuit and a lighting device.

  An illuminating device that uses an LED (Light Emitting Diode) as a light source usually has a configuration in which a plurality of light emitting diodes are connected in series or in parallel, and emits light even if an abnormality such as a short circuit occurs in one of the light emitting diodes. Continue. However, if the lighting device is continuously used while an abnormality has occurred, there is a risk of causing problems such as abnormal heat generation. Patent Document 1 discloses an LED lighting circuit that detects a short circuit generated in an LED by comparing the voltage across the LED with a reference voltage.

JP 2008-130377 A

  However, since the LED lighting circuit described in Patent Document 1 detects an abnormality by comparing the voltage across the LED and the reference voltage, it is necessary to use an integrated circuit such as a comparator or an operational amplifier. Since an integrated circuit is more expensive than a single element such as a diode or a transistor, the circuit of Patent Document 1 has a problem that the circuit cannot be configured at low cost.

  The present invention has been made in view of such problems, and an object thereof is to provide an inexpensive power supply circuit and lighting device that can detect an abnormality occurring in a diode such as an LED.

In order to achieve the above object, a power supply circuit according to a first aspect of the present invention includes:
A current supply circuit for supplying current to the diode;
A current limiting resistor installed on a current path between the cathode of the diode and the ground of the current supply circuit;
A protection circuit that cuts off the supply of current to the diode when the potential difference between the cathode of the diode and the ground of the current supply circuit exceeds a predetermined voltage;
It is characterized by that.

In order to achieve the above object, a lighting device according to a second aspect of the present invention includes:
The power supply circuit;
A light emitting diode to which a current is supplied by the power supply circuit,
It is characterized by that.

  It is possible to provide an inexpensive power supply circuit and lighting device that can detect an abnormality occurring in a diode such as an LED.

It is a block diagram explaining the LED lighting circuit of this embodiment. It is a figure for demonstrating the structure of LED connected to the LED lighting circuit shown in FIG. It is a figure which shows a mode that the short circuit generate | occur | produced in one of the several light emitting diode which comprises LED. It is a figure for demonstrating operation | movement of the LED lighting circuit of this embodiment, (A) is the voltage of the both ends of LED, (B) is the anode voltage of a Zener diode, (C) is FB terminal voltage of control IC, respectively. FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The LED lighting circuit 100 of the present embodiment is a power supply circuit having a function of detecting an abnormality that has occurred in the LED 200, for example, a short circuit that has occurred in one of the plurality of light emitting diodes constituting the LED 200, as shown in FIG. In addition, the constant current circuit 110, the short circuit protection circuit 120, and the LED 200 are included.

  The constant current circuit 110 is a circuit having a function of keeping a current flowing through the LED 200 (hereinafter referred to as “LED current”) constant. For example, the constant current circuit 110 controls the LED current by turning on and off a switching element such as an FET at high speed. It is a chopper circuit. The two input terminals (Vin terminal and input side GND terminal) of the constant current circuit 110 are connected to the Vdd terminal (positive electrode terminal) and the GND (ground) terminal (negative electrode terminal) of the DC power supply device 300, respectively. In addition, two output terminals (Vout terminal and output-side GND terminal) of the constant current circuit 110 are connected to the short-circuit protection circuit 120.

  The short circuit protection circuit 120 is a circuit for protecting the LED lighting circuit 100 from an accident such as abnormal heat generation. As shown in FIG. 1, the switching element 121, the current limiting resistor 122, the control IC 123, the diode 124, , A capacitor 125, a coil 126, a capacitor 127, and a voltage detection circuit 130.

  The switching element 121 includes an N-channel type MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The switching element 121 has a drain connected to one end of the coil 126, a source connected to one end of the current limiting resistor 122, and a gate connected to a GATE terminal that is one of output terminals of the control IC 123. When a predetermined voltage (hereinafter referred to as “set voltage”) set to the GATE terminal of the control IC 123 is applied from the control IC 123 to the gate, the switching element 121 connects (drains) between the drain and the source to generate a current. Let it pass. When the GND voltage (installation voltage) is applied from the control IC 123 to the gate, the switching element 121 cuts off (switches off) between the drain and the source and blocks the current flowing through the LED 200.

  The current limiting resistor 122 is a resistor for limiting the current flowing through the LED 200. The current limiting resistor 122 is installed on a current path between the VL terminal of the LED 200 (on the cathode side of the LED 200) and the GND terminal of the constant current circuit 110, and between the VL terminal of the LED 200 and the GND terminal of the constant current circuit 110. A potential difference is generated between them. One end of the current limiting resistor 122 is connected to the source of the switching element 121, and the other end is connected to the GND terminal of the constant current circuit 110. One end of the current limiting resistor 122 (the side connected to the source of the switching element 121) is connected to the CS terminal of the control IC 123 as shown in FIG.

  The control IC 123 includes a processor and the like. The Vin terminal of the control IC 123 is connected to the Vout terminal of the constant current circuit 110, and the GND terminal is connected to the GND terminal of the constant current circuit 110. The CS terminal of the control IC 123 is connected to one end of the current limiting resistor 122, and the control IC 123 preliminarily stores the voltage at one end of the current limiting resistor 122 input from the CS terminal and an internal memory (not shown). The LED current is calculated based on the stored resistance value of the current limiting resistor 122.

  In addition, the control IC 123 includes a GATE terminal that is a port output terminal for controlling the switching element 121. When the LED current calculated based on the voltage input to the CS terminal is equal to or less than a predetermined current value, the control IC 123 applies a set voltage to the gate of the switching element 121 and causes the drain-source between the switching element 121 to Connecting. When the calculated LED current exceeds a predetermined current value, the GND voltage is applied to the switching element 121 to cut off the drain-source of the switching element 121 in order to protect the LED 200 from overcurrent.

  The control IC 123 includes an FB terminal for detecting a short circuit that has occurred in the LED 200. The FB terminal is connected to a pull-up resistor (not shown) inside the control IC 123, and is kept at the Vin terminal voltage (operating voltage V1) of the control IC 123 in a steady state. The FB terminal is connected to a voltage detection circuit 130 (that is, one end of the resistor 133), which will be described later, and when the short circuit of the LED 200 is detected by the voltage detection circuit 130 (that is, the voltage detection circuit 130 sets the FB terminal to the GND voltage). The control IC 123 stops applying the set voltage to the switching element 121 (that is, applies the GND voltage to the gate of the switching element 121), assuming that the LED 200 is short-circuited.

The diode 124, the capacitor 125, and the coil 126 function as a surge absorbing element for protecting the LED 200 and the like from steep voltage / current changes when the switching element 121 is switched.
The cathode of the diode 124 is connected to the Vout terminal of the constant current circuit 110, the VH terminal of the LED 200, and one end of the capacitor 125. The anode is connected to the drain of the switching element 121 and one end of the coil 126.
One end of the capacitor 125 is connected to the Vout terminal of the constant current circuit 110, the VH terminal of the LED 200, and the cathode of the diode 124, and the other end is connected to the VL terminal of the LED 200, one end of the coil 126, and the voltage detection circuit 130 (that is, The cathode of the Zener diode 131).
One end of the coil 126 is connected to the Vout terminal of the constant current circuit 110, the VH terminal of the LED 200, and the cathode of the diode 124, and the other end is connected to the VL terminal of the LED 200, one end of the coil 126, and the cathode of the Zener diode 131. Has been.

  The capacitor 127 has a capacity for storing the current leaked from the FB terminal. One end is connected to the FB terminal of the control IC 123 and one end of the resistor 133, and the other end is connected to the GND terminal of the constant current circuit 110. It is connected.

  The voltage detection circuit 130 is a circuit for detecting a short circuit generated in the LED 200 and notifying the control IC 123 of the short circuit of the LED 200 by dropping the FB terminal of the control IC 123 to the GND voltage. It is arranged between the circuit 110 GND terminal. As shown in FIG. 1, the voltage detection circuit 130 includes a Zener diode 131, a transistor 132, and a resistor 133.

  The Zener diode 131 is composed of a constant voltage diode having a Zener voltage Vz. The Zener diode 131 has a cathode connected to the base of the transistor 132 and an anode connected to the VL terminal of the LED 200. When a short circuit occurs in one of the plurality of light emitting diodes constituting the LED 200, the Zener diode 131 applies a voltage to the base of the transistor 132 by causing a current to flow from the anode to the cathode.

  The transistor 132 is composed of an NPN transistor. The base of the transistor 132 is connected to the cathode of the Zener diode 131, the emitter is connected to the GND terminal of the constant current circuit 110, and the collector is connected to one end of the resistor 133. When a short circuit occurs in the LED 200 and a voltage is applied to the base via the Zener diode 131, the transistor 132 connects the emitter and the collector.

  The resistor 133 functions as a surge absorbing element for protecting the FB terminal of the control IC 123 from a steep voltage change when the transistor 132 is switched. One end of the resistor 133 is connected to the collector of the transistor 132, and the other end is connected to the FB terminal of the control IC 123.

  The LED 200 functions as a light source for the LED lighting circuit 100. The LED 200 is composed of inorganic crystal light-emitting diodes, and as shown in FIG. 2, the three light-emitting diodes 201 to 203 each having a forward voltage drop Vf are arranged in series. The VH terminal of the LED 200 is connected to the Vout terminal of the constant current circuit 110, the cathode of the diode 124, and one end of the capacitor 125. The VL terminal of the LED 200 is connected to one end of the capacitor 125, one end of the coil 126, and the cathode of the Zener diode 131.

Next, the operation of the LED lighting circuit 100 having such a configuration will be described.
When the DC power supply 300 is turned on by a user or the like, the DC power supply 300 starts applying the power supply voltage Vdd to the constant current circuit 110. The constant current circuit 110 starts applying the operating voltage V1 to the control IC 123 while keeping the output current to the LED 200 constant.

  When application of the operating voltage V1 to the control IC 123 is started, the control IC 123 causes a current to flow through the pull-up resistor to the FB terminal. Then, the capacitor 127 connected to the FB terminal accumulates electric charge with the current leaked from the FB input terminal. As the electric charge is stored in the capacitor 127, the FB terminal gradually increases in pressure, and when the FB terminal exceeds a predetermined charge and reaches a predetermined potential, the control IC 123 applies a set voltage to the gate of the switching element 121. To start.

  When application of the set voltage to the gate of the switching element 121 is started, the switching element 121 connects the drain and the source. As a result, the VL terminal of the LED 200 is connected to the GND terminal of the constant current circuit 110 via the coil 126, the switching element 121, and the current limiting resistor 122, and the LED 200 starts to light.

  The control IC 123 constantly monitors the CS terminal, and the voltage at one end of the current limiting resistor 122 input to the CS terminal is stored in advance in an internal memory (not shown) every predetermined time. The current value flowing through the LED 200 is calculated by dividing by the resistance value of the limiting resistor 122. Then, when the calculated current exceeds a certain value, the control IC 123 applies a GND voltage to the gate of the switching element 121 on the assumption that an excessive current flows in the LED 200. Then, the switching element 121 disconnects between the drain and the source, and no current flows through the LED 200.

Next, the operation of the short circuit protection circuit 120 when a short circuit occurs in one of the light emitting diodes constituting the LED 200 will be described.
As shown in FIG. 3, when a short circuit occurs in one of the light emitting diodes constituting the LED 200 (the light emitting diode 201 in FIG. 3), the voltage drop of the LED 200 is reduced by the voltage drop Vf of the shorted light emitting diode. The VL terminal voltage of the LED 200 increases by Vf (that is, a large voltage is applied to the current limiting resistor 122 by Vf). When the VL terminal voltage rises and exceeds the Zener voltage Vz as shown in FIG. 4A, the Zener diode 131 causes a current to flow from the anode to the cathode. Then, since a voltage is applied to the base V2 of the transistor 132 as shown in FIG. 4B, the emitter and collector of the transistor 132 are thereby connected. Then, the FB terminal of the control IC 123 is connected to the GND terminal of the constant current circuit 110 through the resistor 133 and the transistor 132, and the FB terminal of the control IC 123 is connected to the GND as shown in FIG. It becomes voltage (0V). When the control IC 123 detects that the FB terminal has become the GND voltage, the control IC 123 stops applying the set voltage to the gate of the switching element 121 (that is, applies the GND voltage to the gate of the switching element 121). When the GND voltage is applied to the gate, the switching element 121 cuts off the drain-source connection. As a result, the connection between the VL terminal of the LED 200 and the GND terminal of the constant current circuit 110 is disconnected, and a current flows through the LED 200. Disappear.

  According to the present embodiment, the short circuit of the LED 200 is detected by detecting whether or not the voltage between the VL terminal of the LED 200 and the GND terminal of the constant current circuit 110 exceeds a predetermined voltage. Unlike the method, it is not necessary to use an expensive operational amplifier to compare the voltage across the LED 200 with the reference voltage.

  Further, as in the conventional method, if it is intended to detect a short circuit by comparing the voltage across the LED 200 with the reference voltage, the voltage across the LED 200 (voltage between the VH terminal and VL terminal) needs to be fed back to the control IC 123. However, since there is a potential difference between the VL terminal voltage of the LED 200 and the GND terminal voltage of the control IC 123, it is necessary to feed back the voltage across the LED 200 to the control IC 123 using an expensive photocoupler. However, in this embodiment, since the potential of the Zener diode 131 that is GND and the voltage of the control IC 123 are the same potential, both ends of the LED 200 are fed back to the control IC 123 using an expensive photocoupler. There is no need.

  Note that the LED 200 is not limited to a light emitting diode made of an inorganic crystal. It may be an organic EL composed of organic molecules. Moreover, other diodes other than LED, organic EL, etc. may be sufficient.

  Further, the switching element 121 is not limited to an N-channel MOSFET. Unipolar transistors (field effect transistors) other than N-channel MOSFETs, such as P-channel MOSFETs, P-channel JFETs (Junction FETs), and N-channel JFETs, may be used. Further, it may be a PNP type or NPN type bipolar transistor.

  The constant current circuit 110 may be a switching control type DC / DC converter such as a step-down chopper circuit, a step-up chopper circuit, a flyback converter, or a forward converter. At this time, the control IC used for switching control of the DC / DC converter and the control IC 123 may be integrated.

  A part or all of the above-described embodiment can be described as in the following supplementary notes, but is not limited thereto.

(Appendix 1) a current supply circuit for supplying current to the diode;
A current limiting resistor installed on a current path between the cathode of the diode and the ground of the current supply circuit;
A protection circuit that cuts off the supply of current to the diode when the potential difference between the cathode of the diode and the ground of the current supply circuit exceeds a predetermined voltage;
A power supply circuit characterized by that.

(Appendix 2) The protection circuit is
A switching element that starts output of current to the diode by switching on, and shuts off the output of current by switching off;
It is installed between the cathode of the diode and the ground of the current supply circuit, and allows a current to pass when a potential difference between the cathode of the diode and the ground of the current supply circuit exceeds a predetermined potential, and a predetermined voltage A Zener diode that cuts off current in the following cases:
Switching control means for switching off the switching element to cut off the supply of current to the diode when the Zener diode passes current;
The power supply circuit according to appendix 1, wherein:

(Appendix 3) The switching control means includes:
A current detecting means for detecting a current flowing through the diode;
When the current detected by the current detection exceeds a predetermined threshold, the switching element is switched off to cut off the supply of current to the diode;
The power supply circuit according to appendix 2, wherein

(Supplementary Note 4) The power supply circuit according to any one of Supplementary notes 1 to 3,
A light emitting diode to which a current is supplied by the power supply circuit,
A lighting device characterized by that.

DESCRIPTION OF SYMBOLS 100 LED lighting circuit 110 Constant current circuit 120 Short circuit protection circuit 121 Switching element 122 Current limiting resistor 123 Control IC
124 diode 125 capacitor 126 coil 127 capacitor 130 voltage detection circuit 131 Zener diode 132 transistor 133 resistance 200 LED
201-203 Light emitting diode 300 DC power supply device

Claims (4)

A current supply circuit for supplying current to the diode;
A current limiting resistor installed on a current path between the cathode of the diode and the ground of the current supply circuit;
A protection circuit that cuts off the supply of current to the diode when the potential difference between the cathode of the diode and the ground of the current supply circuit exceeds a predetermined voltage;
A power supply circuit characterized by that. The protection circuit is
A switching element that starts output of current to the diode by switching on, and shuts off the output of current by switching off;
It is installed between the cathode of the diode and the ground of the current supply circuit. A Zener diode that cuts off current in the following cases:
Switching control means for switching off the switching element to cut off the supply of current to the diode when the Zener diode passes current;
The power supply circuit according to claim 1. The switching control means includes
A current detecting means for detecting a current flowing through the diode;
When the current detected by the current detection exceeds a predetermined threshold, the switching element is switched off to cut off the supply of current to the diode;
The power supply circuit according to claim 2. The power supply circuit according to any one of claims 1 to 3,
A light emitting diode to which a current is supplied by the power supply circuit,
A lighting device characterized by that.

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