JP2010278177A - Led driving circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve improvement in safety and expansion of uses by enabling an LED to be turned on and dimmed from a commercial power source in an insulated state. <P>SOLUTION: An LED driving circuit includes a rectification circuit 20 which rectifies a PWM-controlled AC input, a transformer T having primary winding N1 where the rectified output flows, secondary winding N2 to which a series LED group 30 is connected, and detection winding N3, an FET Q3 which is connected to the primary winding N1 in series and turns on and off the current flowing thereto, a control IC which generates a first control signal determining an off time of the FET Q3 and also generates a second control signal determining an on time of the FET Q3, and an additional circuit 70 which applies a signal for making the control IC generate the first control signal using a detected voltage of the detection winding N3 to a first control terminal. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an LED driving circuit for energizing an LED to emit light, and more particularly to an LED driving circuit capable of lighting an LED using a commercial AC power source and capable of dimming.

  In recent years, LED lighting has been attracting attention as highly efficient lighting with low power consumption. Further, as a dimming means for LED lighting using a commercial AC power supply, a triac dimmer having a built-in triac is used, and a control IC for an LED driving circuit is also put into practical use (commercially available). . As this type of control IC, for example, there is an LM3445 “offline LED tribar with triac dimming function” manufactured by National Semiconductor, and the following non-patent document provides a recommended circuit using the LM3445.

National Semiconductor Japan Co., Ltd., LM3445 “Off-Line LED Triba with Triac Dimming Function” catalog issued in February 2009, available from URL “WWW.national.com/jpn/”.

  FIG. 3 shows a conventional LED driving circuit (recommended circuit of Non-Patent Document 1) using LM3445 as a control IC. This LED drive circuit includes a triac dimmer 10 that performs PWM control (phase control) on a commercial power supply (AC 100 V), a full-wave rectifier circuit (bridge rectifier circuit) 20 that rectifies PWM-controlled AC input, and a control IC. LM3445, series LED group 30 in which a plurality of LEDs are connected in series, switching elements Q1 to Q3 (Q1, Q3: FET, Q2: bipolar transistor), diodes D1 to D3, constant voltage diode ZD1, resistors R1 to R5, capacitors C1 to C1 C6 and a coil L1.

  The schematic internal configuration of the LM3445 is as shown in FIG. The function of each terminal of the LM3445 will be described. The “ASNS” terminal outputs a PWM signal of 0 to 4 V having a duty cycle proportional to the on-time of the triac dimmer 10. The PWM signal at the “ASNS” terminal is created based on an input signal to the “BLDR” terminal (a signal whose conduction angle is controlled by the triac dimmer 10), which will be described later.

  The “FLTR1” terminal is a first filter input, and a DC signal obtained by filtering the PWM signal output from the “ASNS” terminal with a resistor R5 and a capacitor C5 is applied. This DC signal is compared with a signal that ramps up to 1 to 3 V at 5.85 kHz inside the LM3445, and generates a higher frequency PWM dimming signal having a duty cycle proportional to the conduction angle of the triac dimmer 10. To do. This PWM dimming signal is drawn out to a “FLTR2” terminal described later.

  The “DIM” terminal is a dimming terminal having a dual function of input and output, and can be driven by an external PWM signal for dimming the LED. Further, it can also be used as an output signal for dimming a plurality of LED drive circuits simultaneously by connecting to the “DIM” terminal of another LM3445 or LED driver.

  The “COFF” terminal is an off time setting terminal, and the off time of the internal switching controller is set by the current set by the user and the capacitor C4 connected to this terminal.

  The “FLTR2” terminal is the second filter input, and the capacitor C6 connected to this terminal filters the PWM dimming signal inside the LM3445 and supplies a DC reference voltage Vref for controlling the LED current.

  The “GND” terminal is a terminal for ground connection of the internal circuit of the LM3445.

  The “ISNS” terminal is an LED current detection terminal, is connected to the source of a FET (power MOSFET) Q3 that is a main switching element, and sets a maximum LED current by a resistor R4 between the source and the ground.

  The “GATE” terminal generates a gate signal for driving the FET Q3.

  The “Vcc” terminal is an input voltage terminal and supplies power to the internal circuit of the LM3445.

  The “BLDR” terminal is a signal input terminal to the angle detection circuit inside the LM3445, and also serves as a current path for properly operating the triac dimmer 10.

  Next, the operation of the conventional LED driving circuit shown in FIG. 3 will be described. The AC input from the commercial power supply (AC 100 V) is PWM controlled (phase control) by the triac dimmer 10 and rectified by the full-wave rectifier circuit (bridge rectifier circuit) 20. This rectified output is supplied to a slice circuit 40 including an FET (MOSFET) Q1, resistors R1 and R2, and a constant voltage diode ZD1, and a conduction angle signal including conduction angle information from the triac dimmer 10 is applied to the “BLDR” terminal. Is done. Further, a voltage obtained by rectifying and smoothing the voltage at the connection point between the source of the FET Q1 and the resistor R2 by the diode D2 and the capacitor C1 is supplied to the “Vcc” terminal as the power supply voltage of the LM3445.

  The rectified output of the full-wave rectifier circuit 20 includes a diode D1 and a capacitor C2 for smoothing, and further includes a series circuit of a series LED group 30, a coil L1, a main switching element FET Q3, a resistor R4, and the like. This is supplied to the circuit 50.

  While the FET Q3 is turned on by the gate signal (high level) of the “GATE” terminal, a current flows through the series circuit of the series LED group 30, the coil L1, the FET Q3 of the main switching element, and the resistor R4, and the current of the coil L1 Will increase. Eventually, when the maximum LED current determined by the voltage of the “ISNS” terminal to which the resistor R4 is connected is reached at a certain point, the gate signal becomes low level and the FET Q3 is turned off. That is, the LM 3445 monitors the voltage of the “ISNS” terminal, and the DC reference voltage Vref obtained from the PWM dimming signal inside the LM 3445 (Vref is high if the conduction angle by the triac dimmer 10 is large, and the conduction angle is small. When the voltage at the “ISNS” terminal reaches (when Vref becomes low), the FET Q3 is turned off. During the off time, the current in the coil L1 flows to the series LED group 30 through the diode D3. Capacitor C3 is for removing ripple current flowing in coil L1.

  When the FET Q3 is turned off, the induced voltage of the coil L1 connected to the base side of the transistor Q2 has a polarity for forward-biasing the transistor Q2, and the transistor Q2 is turned on to start charging the capacitor C4 through the resistor R3. The resistor R3 and the capacitor C4 constitute a time constant circuit. When the charging voltage of the capacitor C4, that is, the terminal voltage of the “COFF” terminal reaches a predetermined voltage value (for example, 1.25 V), the gate of the “GATE” terminal When the signal becomes high level, the FET Q3 is turned on, and the charge of the capacitor C4 is discharged to return the terminal voltage of the “COFF” terminal to OV.

  Thus, in the conventional LED drive circuit, the off-time of the FET Q3 of the main switching element is determined by charging the capacitor C4 with a predetermined time constant by the supply voltage to the series LED group 30, and the off-time Is constant. On the other hand, as the conduction angle increases or decreases according to the PWM control of the TRIAC dimmer 10, the DC reference voltage Vref is increased or decreased (that is, the maximum LED current is increased or decreased), thereby increasing or decreasing the on-time of the FET Q3. Plays.

  Incidentally, in the conventional LED drive circuit of FIG. 3, the series LED group 30 is non-insulated with respect to the commercial power supply AC 100V, and the distance between the series LED group 30 and the other LED drive circuit portions is short. There is no problem, but if the distance between the series LED group 30 and the other LED drive circuit portion is long, a high-voltage line of 100V or more is drawn long, causing a safety problem To do.

  The present invention has been made in view of such a situation, and the purpose thereof is to make it possible to light and dimm an LED or series LED in an insulated state from a commercial power source (for example, AC 100V), thereby improving safety and expanding applications. It is to provide an LED drive circuit that achieves the above.

In order to achieve the above object, an LED drive circuit according to an aspect of the present invention is connected to a rectifier that rectifies a PWM-controlled AC input, a primary winding to which a rectified output of the rectifier is supplied, and an LED. A transformer having a secondary winding and a detection winding, a first switching element connected in series to the primary winding, for turning on and off a current flowing through the primary winding, and the first A control IC that generates a first control signal that determines an OFF time of the switching element and a second control signal that determines an ON time of the first switching element, and a detection voltage of the detection winding are used. And an additional circuit for applying a signal for causing the control IC to generate the first control signal to the first control terminal of the control IC,
A voltage proportional to the current of the first switching element is applied to the second control terminal of the control IC to cause the control IC to generate the second control signal;
The additional circuit includes a diode that connects both ends of the detection winding to the high-voltage side of the primary winding such that the cathode side is the primary winding side,
A series connection of a first capacitor and a coil connected between both ends of the detection winding via one diode;
A second switching element connected between a connection point between the one diode and the first capacitor and the first control terminal;
A time constant circuit including a resistor inserted in series with the second switching element and a second capacitor connected between the first control terminal and the ground;
A connection point between the coil and the first capacitor is connected to the control electrode of the second switching element, and the second switching element is switched on and off according to the polarity of the induced voltage of the coil. It is characterized by.

  In the above aspect, the second switching element may be turned on by turning off the first switching element, and charging of the second capacitor may be started through the resistor.

  In the above aspect, the PWM-controlled AC input may be created by a triac dimmer.

  The said aspect WHEREIN: It is good to be a structure which supplies electric power to said LED by the forward converter containing the said transformer and the 1st switching element which turns on and off the electric current which flows into the said primary winding.

  It should be noted that any combination of the above-described constituent elements, and those obtained by converting the expression of the present invention between methods and systems are also effective as aspects of the present invention.

  According to the LED driving circuit of the present invention, the LED can be lit and dimmed in an insulated state from a commercial power source (for example, AC 100 V). For this reason, safety can be improved, the degree of freedom of LED placement is increased, and outdoor placement and the like are also possible, which can contribute to the expansion of the application of LED lighting.

It is a circuit diagram which shows embodiment of the LED drive circuit which concerns on this invention. It is an operation | movement waveform diagram of the said LED drive circuit. It is a circuit diagram of the conventional LED drive circuit. It is a schematic internal block diagram of LM3445 as control IC used with an LED drive circuit.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In addition, the same code | symbol is attached | subjected to the same or equivalent component, member, process, etc. which are shown by each drawing, and the overlapping description is abbreviate | omitted suitably. In addition, the embodiments do not limit the invention but are exemplifications, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

  FIG. 1 shows an LED driving circuit according to an embodiment of the present invention using an LM3445 as a control IC. This LED drive circuit includes a triac dimmer 10 that performs PWM control (phase control) on a commercial power supply (AC 100 V), a full-wave rectifier circuit (bridge rectifier circuit) 20 that rectifies PWM-controlled AC input, and a control IC. LM3445, series LED group 30 in which a plurality of LEDs are connected in series, switching elements Q1 to Q3 (Q1, Q3: FET, Q2: bipolar transistor), diodes D1 to D3, D10 to D13, constant voltage diode ZD1, resistors R1 and R2 , R4, R5, R10 to R16, capacitors C1, C4 to C6, C10 to C15, coils L2 to L4, and a transformer T having a primary winding N1, a secondary winding N2, and a detection winding N3. Yes. In order to drive the series LED group 30 in an insulated state from a commercial power supply (AC 100 V), the current of the primary winding N1 of the transformer T is turned on and off by an FET (power MOSFET) Q3 as a main switching element. A forward converter 60 in which the series LED group 30 is connected to the secondary winding N2 side is configured.

  The AC input from the commercial power supply (AC 100V) is PWM controlled (phase controlled) by the triac dimmer 10 and rectified by the full-wave rectifier circuit (bridge rectifier circuit) 20, and this rectified output is converted into an FET (MOSFET) Q1, The point that the conduction angle signal supplied to the slicing circuit 40 including the resistors R1 and R2 and the constant voltage diode ZD1 and including the conduction angle information by the triac dimmer 10 is applied to the “BLDR” terminal is the same as the conventional circuit of FIG. It is the same. Further, a voltage obtained by rectifying and smoothing the voltage at the connection point between the source of the FET Q1 and the resistor R2 by the diode D2, the resistor R10, and the capacitor C1 is supplied to the “Vcc” terminal as the power supply voltage of the LM3445.

  The rectified output of the full-wave rectifier circuit 20 is supplied to the primary side of the forward converter 60 through a diode D1 and a π-type filter (for noise removal and smoothing) including a coil L2 and capacitors C10 and C11. That is, the rectified output of the full-wave rectifier circuit 20 is supplied to the series connection of the primary winding N1 of the transformer T, the FET Q3 of the main switching element, and the resistor R4 via the diode D1 and the π-type filter. A series connection of a diode D3 and a resistor R13 that form a current path when the FET Q3 is OFF is connected between both ends of the primary winding N1, and a capacitor C12 is connected in parallel to the resistor R13.

  A series LED group 30 is connected to the secondary winding N2 of the transformer T via a rectifying and smoothing circuit including diodes D12 and D13, a coil L4, and a capacitor C15.

  The detection winding N3 of the transformer T is a winding for indirectly detecting the voltage applied to the series LED group 30, and a detection voltage that is directly proportional to the induced voltage of the secondary winding N2 is obtained. An additional circuit 70 is connected to the detection winding N3. The additional circuit 70 uses a detection voltage of the detection winding N3 to thereby determine a first control signal ("" from the turn-off to the turn-on) of the FET Q3 that is the main switching element of the forward converter 60. It can also be defined as a signal that determines the low level period of the gate signal output from the “GATE” terminal).

  The additional circuit 70 has both ends of the detection winding N3 connected to the high voltage side of the primary winding N1, and the cathode side is the diode D10 and D11 with the primary winding side, and the detection winding via one diode D10. A capacitor C14 and a coil L3 connected between both ends of the line N3, and a resistor R15 connected in parallel to the capacitor C14 are provided. A voltage value obtained by rectifying the induced voltage of the detection winding N3 is obtained at both ends of the parallel connection of the capacitor C14 and the resistor R15. The charging voltage of the capacitor C14 is such that the cathode side of the diodes D10 and D11 is positive.

  Further, the additional circuit 70 includes a transistor Q2 (emitter−) connected between a connection point between the diode D10 and the capacitor C14 (the high voltage side of the primary winding N1) and the “COFF” terminal as the first control terminal of the LM3445. And a time constant circuit including a resistor R16 inserted in series with the transistor Q2 and a capacitor C4 connected between the “COFF” terminal and the ground. A base that is a control electrode of the transistor Q2 is connected to a connection point between the coil L3 and the capacitor C14.

  The operation of the additional circuit 70 will be described. While the FET Q3 which is the main switching element is on, the voltage polarity of the coil L3 is positive on the base side of the transistor Q2, and the transistor Q2 maintains the off state. When the FET Q3 is turned off, the voltage polarity of the coil L3 is inverted, the base side of the transistor Q2 becomes negative, and the transistor Q2 is turned on. For this reason, the charging voltage of the capacitor C4 rises with a time constant substantially determined by the resistor R16 and the capacitor C4 through the transistor Q2. When the charging voltage of the capacitor C4, that is, the terminal voltage of the first “COFF” terminal reaches a predetermined voltage value (for example, 1.25 V), the gate signal of the “GATE” terminal becomes high level to turn on the FET Q3. At the same time, the electric charge of the capacitor C4 is discharged to return the terminal voltage of the “COFF” terminal to OV. In this way, the first control signal (the signal for determining the low level period of the gate signal output from the “GATE” terminal) that determines the OFF time of the FET Q3, that is, the period from the turn-off to the turn-on, is created.

  Next, the overall operation of the LED drive circuit of FIG. 1 will be described.

  The AC input from the commercial power supply is PWM-controlled by the triac dimmer 10 and rectified by the full-wave rectifier circuit 20, and this rectified output is supplied to a slice circuit composed of FETQ1, resistors R1 and R2, and a constant voltage diode ZD1, A conduction angle signal including conduction angle information from the triac dimmer 10 is applied to the “BLDR” terminal of the LM3445. As a result, the LM 3445 outputs a 0 to 4 V PWM signal having a duty cycle proportional to the conduction angle (ON time) of the triac dimmer 10 to the “ASNS” terminal. Then, a DC signal obtained by filtering the PWM signal output from the “ASNS” terminal by the resistor R5 and the capacitor C5 is applied to the “FLTR1” terminal. This DC signal is compared with a signal that ramps up to 1 to 3 V at 5.85 kHz inside the LM3445, and generates a higher frequency PWM dimming signal having a duty cycle proportional to the conduction angle of the triac dimmer 10. To do. This high-frequency PWM dimming signal is drawn out to the “FLTR2” terminal, and a capacitor C6 connected to this terminal filters the PWM dimming signal inside the LM3445 and generates a DC reference voltage Vref for controlling the LED current. Supply. This DC reference voltage Vref changes corresponding to (substantially proportional to) the PWM control of the TRIAC dimmer 10 and becomes a reference level that determines the maximum current flowing through the LED. However, in this embodiment, since the current flowing through the series LED group 30 is not directly turned on or off by the switching element, the maximum value of the current flowing through the FET Q3 which is the main switching element of the forward converter 60 is monitored. And the DC reference voltage Vref.

  The FET Q3 which is the main switching element of the forward converter 60 is turned on when the gate signal of the “GATE” terminal of the LM3445 is at a high level and turned off at a low level. As described above, the additional circuit 70 is added to the LM3445. As a result, the low level period (turn-off to turn-on period) of the gate signal is controlled to be constant by the first control signal (determined by the CR time constant circuit).

  The timing at which the gate signal of the “GATE” terminal in FIG. 2A becomes high level and the FET Q3 is turned on is the charging voltage of the capacitor C4 charged through the transistor Q2 and the resistor R16 in the on state, that is, FIG. ) When the terminal voltage of the “COFF” terminal as the first control terminal of the LM3445 reaches a predetermined voltage value (for example, 1.25 V), and then the charge of the capacitor C4 is discharged by the internal circuit of the LM3445. The terminal voltage of the “COFF” terminal is returned to OV.

  During the period when the gate signal of the “GATE” terminal is at the high level, the FET Q3 continues to be in the on state, and the primary winding N1 of the transformer T, the primary side of the forward converter 60, the FET Q3 of the main switching element, and the resistance R4 A current flows through the series circuit, and a DC voltage obtained by rectifying and smoothing the induced voltage of the secondary winding N2 is supplied to the series LED group 30. While the FET Q3 is on, the current of the series circuit of the primary winding N1, the main switching element FET Q3, and the resistor R4 increases. In the LM3445, the DC reference voltage Vref that defines the maximum LED current, which is the terminal voltage of the “FLTR2” terminal (Vref is high if the conduction angle by the triac dimmer 10 is large, and Vref is low if the conduction angle is small). And the voltage of the “ISNS” terminal as the second control terminal are compared to generate a second control signal that defines the high level period (turn-on to turn-off period) of the gate signal. In other words, the voltage at the connection point between the resistor R4 and the source of the FET Q3 (directly proportional to the current of the FET Q3) is applied through the resistor R14, and the voltage at the “ISNS” terminal as the second control terminal (shown in FIG. 2C). ) Reaches the DC reference voltage Vref at a certain point of time, the gate signal is set to a low level as shown in FIG. 2A by the second control signal to turn off the FET Q3. Here, the FET Q3 of the main switching element does not directly turn on / off the current of the series LED group 30, but the current of the FET Q3 and the current of the series LED group 30 are directly proportional to each other, so the current of the FET Q3 is monitored. Thus, the FET Q3 can be turned off indirectly when the maximum LED current is reached.

  The current induced in the OFF time of the FET Q3 and the primary winding N1 of the transformer T flows through the path of the diode D3 and the resistor R13. On the secondary side of the transformer T, the current in the coil L4 flows to the series LED group 30 through the diode D13.

  According to the present embodiment, the following effects can be achieved.

(1) The series LED group 30 can be lit and dimmed in an insulated state from a commercial power source (for example, AC 100 V). For this reason, safety can be improved, the degree of freedom of LED placement is increased, and outdoor placement and the like are also possible, which can contribute to the expansion of the application of LED lighting.

(2) The triac dimmer 10 can be used, and the brightness of the series LED group 30 can be adjusted according to the dimming operation.

(3) The series LED group 30 can be insulated from the primary side of the transformer T by connecting the series LED group 30 to the secondary side of the transformer T and supplying power using the forward converter 60. In addition, a first control for defining the OFF time of the FET Q3 which is the main switching element of the forward converter 60 is added to the transformer T by the detection winding N3 and the additional circuit 70 using the induced voltage and the internal circuit of the LM3445. A signal can be created.

  At that time, if the voltage obtained by simply rectifying and smoothing the induced voltage of the detection winding N3 is used, when the number of series LEDs in the series LED group 30 is small, the induced voltage becomes small, and the “COFF” terminal of the LM3445 Therefore, there is a possibility that the LM3445 cannot operate satisfactorily. In the present embodiment, by connecting both ends of the detection winding N3 to the high voltage side of the primary winding N1 with the diodes D10 and D11, the voltage on the emitter side of the transistor Q2 as a switching element can be made sufficiently high. Even when the number of series LEDs in the series LED group 30 is small (further, in the case of one LED), good operation is possible.

  The present invention has been described above by taking the embodiment as an example. However, it is understood by those skilled in the art that various modifications can be made to each component and each processing process of the embodiment within the scope of the claims. By the way. Hereinafter, modifications will be described.

  In the embodiment, the case where the series LED group 30 in which a plurality of LEDs are connected in series is turned on has been described. However, on the principle of operation, one LED can be turned on.

DESCRIPTION OF SYMBOLS 10 Triac dimmer 20 Full wave rectifier circuit 30 Series LED group 40 Slice circuit 50 Switching circuit 60 Forward converter 70 Additional circuit C1-C6, C10-C15 Capacitor D1-D3, D10-D13 Diode L1-L4 Coil Q1-Q3 Switching Elements R1 to R5, R10 to R16 Resistance T Transformer ZD1 Constant voltage diode

Claims (4)

A rectifier circuit for rectifying a PWM controlled AC input;
A transformer having a primary winding to which a rectified output of the rectifier circuit is supplied, a secondary winding to which an LED is connected, and a detection winding;
A first switching element connected in series to the primary winding for turning on and off a current flowing through the primary winding;
A control IC for generating a first control signal for determining an off time of the first switching element and for generating a second control signal for determining an on time of the first switching element;
An additional circuit for applying a signal for causing the control IC to generate the first control signal using a detection voltage of the detection winding to the first control terminal of the control IC;
A voltage proportional to the current of the first switching element is applied to the second control terminal of the control IC to cause the control IC to generate the second control signal;
The additional circuit includes a diode that connects both ends of the detection winding to the high-voltage side of the primary winding such that the cathode side is the primary winding side,
A series connection of a first capacitor and a coil connected between both ends of the detection winding via one diode;
A second switching element connected between a connection point between the one diode and the first capacitor and the first control terminal;
A time constant circuit including a resistor inserted in series with the second switching element and a second capacitor connected between the first control terminal and the ground;
A connection point between the coil and the first capacitor is connected to the control electrode of the second switching element, and the second switching element is switched on and off according to the polarity of the induced voltage of the coil. LED drive circuit characterized by the above.   2. The LED driving circuit according to claim 1, wherein the second switching element is turned on by the turn-off of the first switching element, and charging of the second capacitor is started through the resistor.   3. The LED drive circuit according to claim 1, wherein the PWM-controlled AC input is created by a TRIAC dimmer.   4. The LED driving circuit according to claim 1, wherein power is supplied to the LED by a forward converter including the transformer and a first switching element that turns on and off a current flowing through the primary winding. LED drive circuit.

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