JP2005311090A - Led drive circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an LED drive circuit which can drive to a high luminance by suppressing heating of the LED with a small size. <P>SOLUTION: In the LED drive circuit (1), the LED (5) is connected between the positive output side of an AC power supply rectifying circuit (3) and an FET (7), and a drive current detecting resistor (R2) is connected between the FET and the negative output side. An FET control circuit (11) applies a control voltage to a gate terminal (7g) to turn on the FET, receives the drive current detection of the detecting resistor, lowers the control voltage to less than the threshold voltage of the FET, and expedites discharging of the charge charged in the FET. The on-state of the FET is maintained by the discharge of the charge, and the drive current continuously flow during its period. This realizes the increase of the luminance of the LED. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a drive circuit for a light emitting diode for driving a high output large current light emitting diode of 1 W or more with a commercial AC power supply, for example.

Light-emitting diodes suitable for lighting devices such as street lighting, spotlights, and guide lights (in this specification, simply referred to as “LEDs”) include the following three methods as main methods for driving them with a commercial AC power supply. is there. The first method is a method in which alternating current is converted into direct current, and the LED is driven by the direct current obtained by the conversion. However, according to the first method, a power transformer or a large-capacity capacitor is required to convert alternating current into direct current, so that the power supply becomes large. Therefore, it is difficult to reduce the size of a lighting device using LEDs. Furthermore, since loss due to conversion occurs, the power consumption of the entire lighting device also increases. The second method is a method in which the LED 105 is driven by the rectified current rectified by the rectifier circuit 103 as shown in FIG. A current limiting resistor 107 for preventing damage is connected in series to the LED. According to the second method, there is a problem in that the amount of heat generated by the current limiting resistor increases because it is necessary to use the current flowing in a constantly flowing state. Therefore, as a third method, the applicant of the present application uses a rectifier circuit for rectifying alternating current and a transistor functioning as a switching element, and turns on and off the drive current in one half wave in the output of the rectifier circuit. An LED drive circuit has been proposed (see Patent Document 1). According to the third method, the base voltage of the transistor is lowered in response to the detection of the LED drive current that flows when the transistor is turned on, and the transistor is turned off by this drop. By turning it off, it is not necessary to constantly drive the drive current, and the amount of heat generation is reduced.
Japanese Patent No. 312870 Publication

  According to the third method described above, it is possible to provide an LED drive circuit that has an advantage of being relatively small and generating relatively little heat. However, in recent years, LEDs with a high output and high current of 1 W or more have been supplied in order to satisfy the demand for higher brightness. Along with this, for example, a current of 300 mA may flow through the LED. However, if a current of 300 mA or more is passed in the third system, the heat generation of the transistor cannot be ignored due to the on-resistance. As the amount of current is increased in order to increase the brightness, the amount of generated heat increases. For this reason, the third method has been desired to be improved so that a large current flows while suppressing heat generation. The problem to be solved by the present invention is, for example, a high-output, high-current LED of 1 W or more with a commercial power supply while being small by adding improvements to the third system without impairing the advantages of the above-described third method It is an object of the present invention to provide an LED drive circuit that can be driven with high brightness while suppressing heat generation.

  In order to solve the above-described problems, the present invention has the following configuration. It should be noted that the definition of terms used in the description of the invention according to any claim is applicable to the invention according to other claims as long as it is possible in nature.

(Characteristics of the invention described in claim 1)
The LED drive circuit according to the invention described in claim 1 (hereinafter, appropriately referred to as “drive circuit of claim 1”) is a circuit for driving one LED. A circuit for driving the plurality of LEDs will be described later. Specifically, an AC power supply rectifier circuit having a power supply connection terminal on the input side, one LED having an anode terminal connected to the positive output side of the AC power supply rectifier circuit, and the LED connected to the cathode terminal of the LED An FET for turning on and off the drive current; a current detection resistor disposed between the FET and the negative output side of the AC power supply rectifier circuit for detecting the drive current that has passed through the FET; and An FET control circuit for controlling the FET connected via a gate resistor is included in the gate terminal of the FET. Then, the FET control circuit applies a control voltage to the gate resistor for applying a gate voltage exceeding the threshold voltage to the gate terminal as the rectified voltage applied from the positive output side of the AC power supply rectifier circuit increases. Applied to turn on the FET and detect that the current detection resistor has detected a drive current exceeding a predetermined value, and the control voltage is lowered to a voltage lower than the threshold voltage to reduce the charge from the gate terminal. The FET is configured to delay-off by prompting discharge. That is, both the anode terminal of the LED and the FET control circuit are connected to the positive output side of the AC power supply rectifier circuit, and the cathode terminal of the LED is connected to the FET.

  According to the drive circuit of the first aspect, the FET control circuit applies the control voltage to the gate resistance due to the rise of the rectified voltage, and thereby the FET is turned on when the gate voltage exceeds the threshold voltage. By turning on, a drive current flows through the LED to which the rectified voltage is applied, and thereby the LED is lit. The drive current is detected by a current detection resistor, and the FET control circuit that receives this detection drops the control voltage. This drop causes the potential of the gate resistance on the FET control circuit side to be lower than the potential on the gate terminal side. As a result, the charged electric charge is discharged from the gate terminal via the gate resistance, and the gate voltage decreases with the discharge. When the gate voltage drops and falls below the threshold voltage, the FET is turned off, thereby turning off the LED. That is, since the drive current flows until the gate voltage reaches the threshold voltage due to charge discharge (until the FET is turned off), the LED can be lit with high brightness. In this way, the detection of the drive current by the current detection resistor is used as a trigger to turn off the FET, so that the latter operates with a delay from the former. This delay operation realizes high luminance even when the low voltage and large current, that is, the applied voltage is low. Further, since the on-resistance value of the FET is generally smaller than the on-resistance value of the transistor, the former generates less heat than the latter even when the same current is passed. Further, in order to light the LED with a direct current, a power transformer, a large-capacitance capacitor, and the like are required, but the drive circuit according to claim 1 does not need such a circuit. The drive circuit can be miniaturized as much as it is not necessary.

(Characteristics of the invention described in claim 2)
An LED drive circuit according to the invention described in claim 2 (hereinafter referred to as “drive circuit of claim 2” as appropriate) is a circuit for driving an LED group formed by connecting a plurality of LEDs in series. Specifically, an AC power supply rectifier circuit having a power supply connection terminal on the input side, an LED group formed by serially connecting a plurality of LEDs having an anode terminal connected to the positive output side of the AC power supply rectifier circuit, and the LED group Between the FET and the negative output side of the AC power supply rectifier circuit for detecting the drive current that has passed through the FET. The distributed current detection resistor and the FET control circuit for controlling the FET connected to the gate terminal of the FET via the gate resistor are included. Then, the FET control circuit applies a control voltage to the gate resistor for applying a gate voltage exceeding the threshold voltage to the gate terminal as the rectified voltage applied from the positive output side of the AC power supply rectifier circuit increases. Applied to turn on the FET and detect that the current detection resistor has detected a drive current exceeding a predetermined value, and the control voltage is lowered to a voltage lower than the threshold voltage to reduce the charge from the gate terminal. By delaying off the FET by encouraging discharge, the drive current can be increased almost in proportion to the increase in the number of LEDs constituting the LED group. That is, both the anode terminal of the LED group and the FET control circuit are connected to the positive output side of the AC power supply rectifier circuit, and the cathode terminal of the LED group is connected to the FET.

  According to the drive circuit of the second aspect, the FET control circuit applies the control voltage to the gate resistance due to the rise of the rectified voltage, whereby the FET is turned on when the gate voltage exceeds the threshold voltage. By turning on, a drive current flows through the LED group to which the rectified voltage is applied, and thereby the LED group is lit. The drive current is detected by a current detection resistor, and the FET control circuit that receives this detection drops the control voltage. This drop causes the potential of the gate resistance on the FET control circuit side to be lower than the potential on the gate terminal side. As a result, the charged electric charge is discharged from the gate terminal via the gate resistance, and the gate voltage decreases with the discharge. When the gate voltage drops and falls below the threshold voltage, the FET is turned off, and the LED group is turned off. That is, the drive current flows until the gate voltage reaches the threshold voltage due to charge discharge (until the FET is turned off), and this drive current increases almost in proportion to the increase in the number of LEDs. It can be lit with brightness. That is, for example, when comparing the drive current when two LEDs are connected in series with the drive current when three LEDs are connected in series, the drive current when three LEDs are connected is more than when two are connected. Will increase. As described above, the detection of the drive current by the current detection resistor is used as a trigger to turn off the FET, so that the latter operates with a delay from the former. This delay operation realizes high luminance even when the low voltage and large current, that is, the applied voltage is low. Further, since the on-resistance value of the FET is generally smaller than the on-resistance value of the transistor, the former generates less heat than the latter even when the same current is passed. Further, in order to light the LED with a direct current, a power transformer, a large-capacitance capacitor, and the like are required, but the drive circuit according to claim 2 does not need such a circuit. The drive circuit can be miniaturized as much as it is not necessary.

(Characteristics of the invention described in claim 3)
The LED drive circuit according to the invention described in claim 3 (hereinafter, appropriately referred to as “drive circuit of claim 3”) is for driving one LED or a group of LEDs connected in series or series-parallel. Circuit. Specifically, an AC power supply rectifier circuit having a power supply connection terminal on the input side, one LED, or a group of LEDs connected in series or series-parallel, the anode terminal of the LED or the LED group, and the AC FET for turning on / off the drive current of the LED or LED group connected between the positive output side of the power supply rectifier circuit and the drive current that has passed through the LED or LED group (drive current that has passed through the FET) ) To detect a current detection resistor disposed between the cathode terminal of the LED or the LED group and the negative output side of the AC power supply rectifier circuit, and the gate terminal of the FET via a gate resistor. And an FET control circuit for controlling the FET. Then, the FET control circuit applies a control voltage to the gate resistor for applying a gate voltage exceeding the threshold voltage to the gate terminal as the rectified voltage applied from the positive output side of the AC power supply rectifier circuit increases. Applied to turn on the FET and detect that the current detection resistor has detected a drive current exceeding a predetermined value, and the control voltage is lowered to a voltage lower than the threshold voltage to reduce the charge from the gate terminal. The FET is configured to delay-off by prompting discharge. That is, the anode terminal of the LED or LED group is connected to the positive output side of the AC power supply rectifier circuit via the FET, and the cathode terminal of the LED or LED group is connected to the negative output side of the AC power supply rectifier circuit via the current detection resistor Is connected to. On the positive output side of the AC power supply rectifier circuit, an FET control circuit is connected together with the FET.

  According to the drive circuit of the third aspect, the FET control circuit applies the control voltage to the gate resistance due to the rise of the rectified voltage, and thereby the FET is turned on when the gate voltage exceeds the threshold voltage. When turned on, a drive current flows through the LED by the rectified voltage applied via the FET, and thereby the LED or the LED group is turned on. The drive current is detected by a current detection resistor, and the FET control circuit that receives this detection drops the control voltage. This drop causes the potential of the gate resistance on the FET control circuit side to be lower than the potential on the gate terminal side. As a result, the charged electric charge is discharged from the gate terminal via the gate resistance, and the gate voltage decreases with the discharge. When the gate voltage drops and falls below the threshold voltage, the FET is turned off, thereby turning off the LED or LED group. That is, since the drive current flows until the gate voltage reaches the threshold voltage due to charge discharge (until the FET is turned off), the LED or LED group can be lit with high brightness. In this way, the detection of the drive current by the current detection resistor is used as a trigger to turn off the FET, so that the latter operates with a delay from the former. This delay operation realizes high luminance even when the low voltage and large current, that is, the applied voltage is low. Further, since the on-resistance value of the FET is generally smaller than the on-resistance value of the transistor, the former generates less heat than the latter even when the same current is passed. Further, in order to light the LED or LED group with a direct current, a power transformer, a large-capacitance capacitor, and the like are required. The drive circuit can be miniaturized as much as it is not necessary.

(Feature of the invention described in claim 4)
The LED drive circuit according to the invention described in claim 4 (hereinafter, appropriately referred to as “drive circuit of claim 4”) is for driving one LED or a group of LEDs connected in series or series-parallel. Circuit. Specifically, an AC power supply rectifier circuit having a power supply connection terminal on the input side and a single LED having an anode terminal connected to the positive output side of the AC power supply rectifier circuit, or a series or series-parallel connection. An LED group, an FET connected to the LED or a cathode terminal of the LED group, an FET for turning on / off the drive current of the LED group, and the FET for detecting the drive current that has passed through the FET Including a current detection resistor disposed between the negative output side of the AC power supply rectifier circuit and a FET control circuit for controlling the FET connected to the gate terminal of the FET via a gate resistor. It is configured. The FET control circuit applies a gate voltage exceeding the threshold voltage to the gate terminal as the rectified voltage applied from the positive output side of the AC power supply rectifier circuit via the LED or the LED group increases. A control voltage is applied to the gate resistor to turn on the FET, and the control voltage is lowered to a voltage lower than the threshold voltage in response to detection of a drive current that the current detection resistor exceeds a predetermined value. The FET is delayed off by encouraging charge discharge from the gate terminal. That is, the anode terminal of the LED or LED group is connected to the positive output side of the AC power supply rectifier circuit, and the cathode terminal of the LED or LED group is connected to the FET control circuit.

  According to the drive circuit of claim 4, the FET control circuit applied the control voltage to the gate resistance due to the rise of the rectified voltage applied through the LED or the LED group, and thereby the gate voltage exceeded the threshold voltage. By the way, FET turns on. By turning on, a drive current flows through the LED or LED group to which the rectified voltage is applied, and thereby the LED or LED group is turned on. The drive current is detected by a current detection resistor, and the FET control circuit that receives this detection drops the control voltage. This drop causes the potential of the gate resistance on the FET control circuit side to be lower than the potential on the gate terminal side. As a result, the charged electric charge is discharged from the gate terminal via the gate resistance, and the gate voltage decreases with the discharge. When the gate voltage decreases and falls below the threshold voltage, the FET is turned off, and thus the LED or LED group is turned off. (In practice, however, the FET is controlled through the LED or LED group even when the FET is off.) A slight drive current flows in the circuit). That is, since the drive current flows until the gate voltage reaches the threshold voltage due to charge discharge (until the FET is turned off), the LED or the LED group can be lit with high brightness. In this way, the detection of the drive current by the current detection resistor is used as a trigger to turn off the FET, so that the latter operates with a delay from the former. This delay operation realizes high luminance even when the low voltage and large current, that is, the applied voltage is low. Further, since the on-resistance value of the FET is generally smaller than the on-resistance value of the transistor, the former generates less heat than the latter even when the same current is passed. In order to light the LED or LED group with a direct current, a power transformer, a large-capacitance capacitor, and the like are required, but the drive circuit according to claim 4 does not need such a circuit. The drive circuit can be miniaturized as much as it is not necessary.

(Feature of the invention of claim 5)
The LED driving circuit according to the invention described in claim 5 (hereinafter referred to as “the driving circuit of claim 5” as appropriate) is for driving one LED or a group of LEDs connected in series or series-parallel. Circuit. Specifically, an AC power supply rectifier circuit having a power supply connection terminal on the input side and a single LED having a cathode terminal connected to the negative output side of the AC power supply rectifier circuit, or a series or series-parallel connection. The LED group, the FET connected to the positive output side of the AC power supply rectifier circuit or the FET for turning on / off the drive current of the LED group, and the FET for detecting the drive current passing through the FET And a current detection resistor disposed between the LED and the anode terminal of the LED group, and a FET control circuit for controlling the FET connected to the gate terminal of the FET via a gate resistor. Configured. The FET control circuit applies a control voltage to the gate resistor for applying a gate voltage exceeding the threshold voltage to the gate terminal as the rectified voltage applied from the positive output side of the AC power supply rectifier circuit increases. The FET is turned on, and the control voltage is lowered to a voltage lower than the threshold voltage in response to detecting that the current detection resistor has detected a drive current exceeding a predetermined value, and charge discharge from the gate terminal is performed. The FET is configured to delay off by prompting. The drive circuit of claim 5 is different from the drive circuit of claim 4 in that the latter LED or LED group is arranged on the positive output side of the AC power supply rectifier circuit, whereas the former LED or LED group is AC. This is the point arranged on the negative side of the power rectifier circuit.

  According to the drive circuit of the fifth aspect, the FET control circuit applies the control voltage to the gate resistance due to the rise of the rectified voltage applied from the positive output side of the AC power supply rectifier circuit, whereby the gate voltage reduces the threshold voltage. When it exceeds, FET turns on. By turning on, a drive current flows through the LED or LED group to which the rectified voltage is applied, and thereby the LED or LED group is turned on. The drive current is detected by a current detection resistor, and the FET control circuit that receives this detection drops the control voltage. This drop causes the potential of the gate resistance on the FET control circuit side to be lower than the potential on the gate terminal side. As a result, the charged electric charge is discharged from the gate terminal via the gate resistance, and the gate voltage decreases with the discharge. When the gate voltage drops below the threshold voltage, the FET is turned off, thereby turning off the LED or LED group (in practice, however, the positive output of the AC power supply rectifier circuit even when the FET is off) A slight drive current flows from the side to the FET control circuit). That is, since the drive current flows until the gate voltage reaches the threshold voltage due to charge discharge (until the FET is turned off), the LED or the LED group can be lit with high brightness. In this way, the detection of the drive current by the current detection resistor is used as a trigger to turn off the FET, so that the latter operates with a delay from the former. This delay operation realizes high luminance even when the low voltage and large current, that is, the applied voltage is low. Further, since the on-resistance value of the FET is generally smaller than the on-resistance value of the transistor, the former generates less heat than the latter even when the same current is passed. Further, in order to light the LED or LED group with a direct current, a power transformer, a large-capacitance capacitor, and the like are required, but the drive circuit according to claim 5 does not need such a circuit. The drive circuit can be miniaturized as much as it is not necessary.

(Characteristics of the invention described in claim 6)
In the LED drive circuit according to the invention described in claim 6 (hereinafter, referred to as “drive circuit of claim 6” as appropriate), in addition to the basic configuration of the drive circuit according to any one of claims 1 to 5, The bypass path is connected in parallel, and the bypass path includes a forward diode toward the FET and a bypass resistor connected in series to the diode, and the value of the bypass resistance is equal to the gate resistance. It is set smaller than the value.

  According to the drive circuit of claim 6, in addition to the basic function and effect of the drive circuit of any of claims 1 to 5, the value of the bypass resistor is smaller than the value of the gate resistance, although depending on the difference between the two resistance values. Therefore, at least half of the gate current of the FET, or if the difference between the two resistance values is large, most of the gate current during charge charging flows through the bypass resistor. Since the discharge direction is the reverse direction of the diode, the charge discharge is performed only through the gate resistance, and there is no discharge through the bypass resistance. Since the value of the bypass resistance is smaller than the value of the gate resistance, the charge charge to the FET performed via the former is performed in a shorter time than the charge discharge performed via the latter. Since the FET can be turned on at a low output voltage by shortening the charging time, the heat generation of the FET can be reduced accordingly.

(Feature of the invention of claim 7)
In the LED drive circuit according to the invention described in claim 7 (hereinafter, appropriately referred to as “drive circuit of claim 7”), in addition to the basic configuration of the drive circuit of claim 6, the gate resistance is directed to the FET. It is preferable that diodes in the reverse direction are connected in series.

  According to the drive circuit of claim 7, in addition to the basic operation and effect of the drive circuit of claim 6, the gate current at the time of charge charging flows only through the bypass resistance, and the charge discharge is performed only through the gate resistance. Done. Therefore, the charge charge to the FET and the charge discharge from the FET are independent from each other. As a result, the gate resistance and the bypass resistance can be easily designed.

According to the LED drive circuit of the present invention, it is possible to drive the LED with high luminance while suppressing heat generation while being small.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of an LED drive circuit (hereinafter referred to as “drive circuit” as appropriate) according to the first embodiment. FIG. 2 is a diagram illustrating waveforms of a rectified voltage and a drive current of the drive circuit according to the first embodiment. FIG. 3 is a circuit diagram of a drive circuit according to the second embodiment. FIG. 4 is a diagram illustrating a waveform of the drive current of the drive circuit according to the second embodiment. FIG. 5 is a circuit diagram of a drive circuit according to the third embodiment. 6 and 7 are circuit diagrams of the drive circuit according to the fourth embodiment. FIG. 8 is a circuit diagram of a drive circuit according to the fifth embodiment. FIG. 9 is a circuit diagram of another drive circuit according to the fifth embodiment.

(First embodiment)
This will be described with reference to FIGS. The drive circuit 1 includes a rectifier circuit (AC power supply rectifier circuit) 3, an LED (light emitting element) 5, an FET (field effect transistor) 7, a current detection resistor R 2, and an FET control circuit 11. The rectifier circuit 3 has AC input terminals (power supply connection terminals) 3a and 3a for connecting an AC power supply E on the input side, and a rectification output terminal 3b (+ for extracting a rectified voltage (rectified current) on the output side. ), 3b (−) respectively. A commercial power supply (for example, 100 V to 220 V, 50 Hz to 60 Hz) is generally used as the AC power supply E because of its high versatility, but an AC power supply other than the commercial power supply can also be used. In the present embodiment, a single-phase AC of 100 V 50 Hz is used as a power source, but a three-phase AC or the like may be used as a power source. The rectifier circuit 3 is configured as a full-wave bridge type, but can also be configured as a rectifier circuit such as a half-wave rectifier type.

  As the LED 5 in the first embodiment, a LED with a large output current of 3.42 V forward voltage and 350 mA forward current is employed. LEDs with ratings other than the above ratings can also be used. The color of the LED 5 is white because the drive circuit 1 is mainly used for illumination. It is not necessary to limit to white, and it does not prevent the adoption of colors other than white according to preference and application. The LED driven by the drive circuit 1 is only one LED 5. Driving the plurality of LEDs will be described later. The LED 5 includes an anode terminal 5a and a cathode terminal 5k. The former is connected to the rectified output terminal 3b (+), and the latter is connected to the drain terminal 7d of the FET 7. Although the heat generation amount of the drive circuit 1 is relatively small, it goes without saying that a heat radiating plate or the like for coping with the use environment such as the ambient temperature of the drive circuit 1 can be provided in the LED 5 as needed.

  As the FET 7, a P-channel FET can also be adopted, but here, an N-channel MOS type FET is adopted. The FET 7 includes a source terminal 7s and a gate terminal 7g which is a control electrode, in addition to the drain terminal 7d described above. The source terminal 7s is connected to one end of the current detection resistor R2 via the connection point S1, and the other end of the current detection resistor R2 is connected to the rectified output terminal 3b (−). The gate terminal 7g is connected to the FET control circuit 11 (emitter terminal 13e of the transistor 13) via the gate resistor R4, and the FET 7 turns on the driving current of the LED 5 in response to a signal applied to the gate terminal 7g. -It is configured to work as a switching element that turns off. An example of a field effect transistor suitable for the FET 7 is 2SK2914. This 2SK2914 has a standard on-resistance of 0.42Ω as standard and 0.5Ω at maximum.

  The FET control circuit 11 includes a PNP transistor 13, an NPN transistor 15, a resistor R3, and a resistor R5. The resistor R3 is connected between the rectified output terminal 3b (+) of the rectifier circuit 3 and the emitter terminal 13e of the transistor 13, and the resistor R5 is connected between the emitter terminal 13e of the transistor 13 and the base terminal 13b of the transistor 13. Connected in between. The base terminal 13 b of the transistor 13 is connected to the collector terminal 15 c of the transistor 15. The collector terminal 13 c of the transistor 13 is connected to the base terminal 15 b of the transistor 15. The emitter terminal 15e of the transistor 15 is connected to one rectified output terminal 3b (-). The base terminal 15b of the transistor 15 is connected to the connection point S1 through the resistor R1. As described above, the emitter terminal 13e of the transistor 13 is connected to the gate terminal 7g of the FET 7 via the gate resistor R4. Hereinafter, a connection point between the gate resistor R4 and the resistor R3 is referred to as a connection point S2. The connection point S2 coincides with the emitter terminal 13e.

(Operation of the drive circuit)
This will be described with reference to FIGS. The alternating current supplied from the alternating current power source E to the rectifier circuit 3 via the power supply connection terminals 3 a and 3 a is full-wave rectified by the rectifier circuit 3. The waveform of the output voltage e between the rectified output terminals 3b (+) and 3b (−) of the rectifier circuit 3 is a waveform in which half cycles of sine waves are arranged as shown in FIG. In FIG. 2, the output voltage e is applied to both the anode terminal 5 a of the LED 5 and the FET control circuit 11. The FET control circuit 11 to which the output voltage e is applied applies the control voltage to the gate resistor R4. That is, the output voltage e is applied to the gate terminal 7g of the FET 7 via the resistor R3 included in the FET control circuit 11 and the gate resistor R4 that applies the control voltage. Here, when the output voltage e is zero, the gate current does not flow, but as the output voltage e rises, the gate current flows and the gate region of the FET 7 is charged, and the control voltage becomes the threshold value of the FET 7 due to the further rise. When the voltage Vth is exceeded, the FET 7 is turned on. The potential of the gate terminal 7g in the on state is held at a potential higher than the threshold voltage Vth by charge charging. When the FET 7 is turned on, a circuit that returns to the rectifier circuit 3 through the rectifier circuit 3, the positive rectifier output terminal 3b (+), the LED 5, the FET 7, the current detection resistor R2, and the rectifier output terminal 3b (-) is formed. Thus, the drive current i flows through the LED 5 to which the output voltage e is applied, and the LED 5 is lit.

  The drive current i that has passed through the LED 5 flows through the current detection resistor R2, and at this time, between the potential at the connection point S1 of the FET 7 and the current detection resistor R2 and the potential on the rectification output terminal 3b (−) side of the current detection resistor R2. In addition, since a potential difference due to a voltage drop occurs, the potential at the connection point S1 of the current detection resistor R2 is higher than the potential on the rectification output terminal 3b (−) side of the current detection resistor R2. Generation of a potential difference at the connection point S1 becomes a trigger, and a base current flows to the base terminal 15b of the transistor 15 via the resistor R1 to turn on the transistor 15. When the transistor 15 is turned on, a base current flows through the resistor R3 and the resistor R5 to the base terminal 13b of the transistor 13 to turn on the transistor 13. When the transistor 13 is turned on, the potential at the connection point S2 is lowered. Here, since the resistor R3 is set so that the lowered potential is lower than the threshold voltage Vth, the potential at the connection point S2 becomes lower than the threshold voltage Vth due to the potential drop. That is, the potential of the gate terminal 7g is higher than the potential of the connection point S2 across the gate resistor R4. Due to the potential difference between both ends of the gate resistor R4, charge discharge occurs and the charge in the gate region gradually flows into the connection point S2 via the gate resistor R4. With this charge discharge, the gate voltage drops. The FET 7 is kept on until the gate voltage drops and reaches a voltage equal to the threshold voltage Vth, but the FET 7 is turned off when the gate voltage becomes lower than the threshold voltage Vth.

  That is, the FET control circuit 11 in the first embodiment causes the drive current i to flow by turning on the FET 7 to light the LED 5, while receiving the fact that the drive current i is detected by the current detection resistor R 2 and turning off the FET 7 Is turned off, but the FET 7 is not turned off almost simultaneously with the detection of the drive current i, but delayed by charge discharge. If a transistor is used instead of an FET, the transistor has less chargeable charge than an FET, so it turns off when the gate voltage drops below the threshold voltage and interrupts the flow of drive current. (Indicated by i 'in FIG. 2). On the other hand, in the case of an FET, the turning-off of the FET is delayed by discharging the charge charged in the gate region. Due to this delay, the FET causes an extra driving current to flow than the transistor. That is, the brightness of the LED is increased. Further, the amount of heat generation can be greatly reduced as compared with the case where the drive current constantly flows by turning on / off the FET 7. In addition, since the on-resistance of the FET is smaller than that of the transistor, the amount of heat generated by the FET can be suppressed by the small on-resistance even if the same drive current is passed.

(Second Embodiment)
This will be described with reference to FIGS. The drive circuit 21 according to the second embodiment differs from the drive circuit 1 according to the first embodiment in that the former has only one LED 5 and the former is connected in series with a plurality of LEDs 5. It is the point which has the LED group 5 which becomes. Therefore, in the following, the description will focus on the points where the drive circuit 21 is different from the drive circuit 1, and the same reference numerals as those used in FIG. 1 are used as much as possible in FIG. The description of is omitted. The waveform of the drive current shown in FIG. 4A is a waveform when there is one LED, and is almost the same as the waveform of the drive current shown in FIG. Fig.4 (a) is a figure for comparing with the case where several LED is connected. The waveform of the drive current shown in FIG. 4B is a waveform when there are two LEDs, and the waveform of the drive current shown in FIG. 4C is a waveform when there are three LEDs. The waveform of the drive current shown in FIG. 4 is obtained when the current detection resistor R2 is 2Ω, and one scale on the vertical axis indicates 250 mA, and one scale on the horizontal axis indicates 2 mS.

  The drive circuit 21 shown in FIG. 3 has an LED group 5 composed of three LEDs connected in series. The number of LEDs 5 does not prevent the number of LEDs 5 from being two or four or more depending on the application of the drive circuit 21 or the like. In order to light up n LEDs connected in series, a voltage equal to n times the forward voltage of each LED is required. Therefore, when three LEDs 5,... Are connected in series, the forward voltage of each LED 5 (LED A voltage equal to three times the voltage is required. If one LED 5 is used, one forward voltage is required, and if two LEDs 5 are used, a voltage equal to two forward voltages is required. As described above, since the forward voltage of LED 5 is 3.42V, if it is one, it is 3.42V, if it is 2, 6.84 (3.42 × 2) V, if it is 3, it is 10.26 ( A voltage of 3.42 × 3) V is required at least.

  In FIG. 4, the waveform of two LEDs 5 (FIG. 4 (b)) is the waveform of three LEDs 5 rather than the waveform of two LEDs 5 than the waveform of one LED 5 (FIG. 4 (a)). In FIG. 4C, the height of the single peak waveform is higher. That is, it can be seen that as the number of LEDs 5 increases, the amount of current increases almost in proportion to the rate of increase. This indicates that according to the drive circuit 21, the luminance of each LED 5 when a plurality of LEDs 5 are connected in series increases from the luminance when a single LED 5 is connected. If the brightness of each LED 5 is significantly lower than the brightness when only one LED 5 is turned on by connecting a plurality of LEDs 5, a device using a large number of LEDs is unsuitable. If it is the drive circuit 21, it can be used suitably also for the apparatus which has many LED for the reason mentioned above, for example, an illumination lamp. The LED group 5 in the second embodiment is formed by connecting a plurality of LEDs 5 in series, but connecting one or a plurality of LEDs in parallel with a part or all of the LEDs 5 connected in series, That is, it can also be composed of a plurality of series-parallel LEDs.

(Third embodiment)
This will be described with reference to FIG. The drive circuit 31 according to the third embodiment is different from the drive circuit 1 according to the first embodiment in the connection position of the LED 5. For this reason, in the following description, the driving circuit 31 will be described with a focus on differences from the driving circuit 1, and portions common to both are given the same reference numerals as those used in FIG. 5 in FIG. 5 as much as possible. Those descriptions are omitted.

  The LED 5 of the drive circuit 1 according to the first embodiment is connected between the rectified output terminal 3b (+) and the drain terminal 7d of the FET 7, but the LED 5 in the drive circuit 31 is connected to the source terminal 7s of the FET 7 and current detection. It is connected between the resistor R2. Specifically, the anode terminal 5a of the LED 5 is connected to the source terminal 7s and the cathode terminal 5k is connected to the terminal of the current detection resistor R2 on the FET 7 side, and the rectified output terminal 3b (+) is controlled by the FET. The circuit 11 is connected to one end of a resistor R3. The operation and effect in the drive circuit 31 is basically not different from the operation and effect of the drive circuit 1, and the case where the LED 5 is upstream of the FET 7 and the case where the LED 5 is also downstream is seen from the direction in which the drive current i flows. There is only a difference. It goes without saying that a plurality of LEDs connected in series or series-parallel can also be connected in the drive circuit 31. Even when a plurality of LEDs are connected in series or series-parallel in the drive circuit 31, the brightness of each LED 5 increases from the brightness of the single LED 5 as in the drive circuit 21 according to the second embodiment described above. It is almost constant.

(Fourth embodiment)
This will be described with reference to FIG. The drive circuit 41 according to the fourth embodiment differs from the drive circuit 1 according to the first embodiment in the connection position of the LED 5. For this reason, the following description will focus on the differences between the drive circuit 41 and the drive circuit 1, and the same reference numerals as those used in FIG. Those descriptions are omitted.

  The LED 5 in the drive circuit 41 is connected to the rectification output terminal 3b (+) and one end of the resistor R3 included in the FET control circuit 11. Specifically, the anode terminal 5a of the LED 5 is connected to the rectified output terminal 3b (+), and the cathode terminal 5k is connected to one end of the resistor R3. The cathode terminal 5k is connected to the drain terminal 7d of the FET 7 together, so that the output voltage e is applied to the drain terminal 7d and the FET control circuit 11 via the LED 5. The operation and effect in the drive circuit 41 are basically not different from the operation and effect of the drive circuit 1, except that the voltage applied to the FET control circuit 11 is lowered by the LED 5. Also in the drive circuit 41, a plurality of LEDs connected in series or in series-parallel can be connected. Even when a plurality of LEDs are connected in series or series-parallel in the drive circuit 41, the brightness of each LED 5 is increased from the brightness of the single LED 5 as in the drive circuit 21 according to the second embodiment described above. It is almost constant.

  The LED 5 of the drive circuit 41 shown in FIG. 6 is connected between the rectified output terminal 3b (+) and one end of the resistor R3 of the FET control circuit 11 as described above, and this LED 5 is connected to the rectified output terminal 3b ( It can also be placed on the-) side. That is, as shown in FIG. 7, the cathode terminal 5k of the LED 5 is connected to the rectified output terminal 3b (−), and the anode terminal 5a of the LED 5 is connected to the source terminal 7s of the FET 7 via the current detection resistor R2. The rectified output terminal 3b (+) is directly connected to the drain terminal 7d of the FET 7 and one end of the resistor R3. The configuration other than the arrangement of the LEDs 5 is not different between the drive circuit 41 and the drive circuit 41 ′. This is the same in terms of the operational effect of the drive circuit and in that a plurality of LEDs can be connected in series or series-parallel.

(Fifth embodiment)
This will be described with reference to FIGS. The drive circuit 51 according to the fifth embodiment is structurally different from the drive circuit 1 according to the first embodiment in the connection method between the FET 7 and the FET control circuit 11. For this reason, in the following, the description will focus on the differences between the drive circuit 51 and the drive circuit 1, and the parts common to both can be given the same reference numerals as those used in FIG. The description thereof is omitted as much as possible.

  That is, a gate resistor R4 ′ is connected between the gate terminal 7g of the FET 7 and the FET control circuit 11 (ie, the connection point S2), and a bypass path 23 is connected in parallel with the gate resistor R4 ′. It is. The bypass path 23 is composed of a diode D1 in the forward direction toward the FET 7 and a bypass resistor R6 connected in series to the diode D1. The value of the bypass resistor R6 is set to a value smaller than that of the gate resistor R4 ′. For example, when the bypass resistor R6 is 300 KΩ, the gate resistor R4 ′ may be about 1 MΩ. The value of the bypass resistor R6 is set to be smaller than the value of the gate resistor R4 ′ so that the gate current flows mainly through the bypass resistor R6 and the charge discharge from the gate terminal 7g flows only through the gate resistor R4 ′. It is to make it. That is, since the diode D1 is in the forward direction toward the FET 7, the gate current flows through both the bypass resistor R6 and the gate resistor R4 ′, but the former resistance value is smaller than the latter resistance value (the internal resistance of the diode D1). Is negligible), the gate current mainly flows through the bypass resistor R6. On the other hand, since the direction of charge discharge is opposite to that of the diode D1, the charge discharge does not flow through the bypass path 23 but flows exclusively through the gate resistor R4 ′. Since the value of the bypass resistor R6 is smaller than the value of the gate resistor R4 ′, the charge of the FET 7 can be charged in a shorter time than the charge discharge by appropriately setting the value of the resistor R3. If the charging time is shortened, the FET is turned on at a lower output voltage, so that the heat generation of the FET can be reduced accordingly. The same effect can be obtained even if the relative position of the diode D1 and the bypass resistor R6 is reversed from that shown in FIG. 8 and the bypass resistor R6 is set closer to the FET 7 than the diode D1.

  Further, as shown in FIG. 9, a diode D2 in the reverse direction (forward direction of charge discharge) can be connected in series with the gate resistor R4 ′ toward the FET 7. Providing the diode D2 prevents the gate current from flowing through the gate resistor R4 ′. Therefore, the charge charge to the FET 7 and the charge discharge from the FET 7 are independent from each other. As a result, the gate resistor R4 ′ and the bypass resistor R6 can be easily designed. Furthermore, the above-described bypass path 23 and further D2 may be employed in the drive circuits 1, 21, 31, 41, and 41 'according to the above-described first to fourth embodiments.

FIG. 3 is a circuit diagram of a drive circuit according to the first embodiment. It is a figure which shows the waveform of the rectification voltage and drive current of the drive circuit which concern on 1st Embodiment. FIG. 6 is a circuit diagram of a drive circuit according to a second embodiment. It is a figure which shows the waveform of the drive current of the drive circuit which concerns on 2nd Embodiment. FIG. 6 is a circuit diagram of a drive circuit according to a third embodiment. It is a circuit diagram of the drive circuit concerning a 4th embodiment. It is a circuit diagram of the drive circuit concerning a 4th embodiment. FIG. 10 is a circuit diagram of a drive circuit according to a fifth embodiment. FIG. 10 is a circuit diagram of another drive circuit according to the fifth embodiment. It is a circuit diagram of the conventional drive circuit.

Explanation of symbols

1, 21, 31, 41, 41 ', 51 LED drive circuit (drive circuit)
3 Rectifier circuit 3a Power connection terminal 3b Rectification output terminal 5 LED (LED group)
7 FET
11 FET control circuit 13, 15 Transistor 23 Bypass path D1, D2 Diode R1, R3, R5 Resistance R2 Current detection resistance R4 Gate resistance R6 Bypass resistance S1, S2 Connection point

Claims (7)

An AC power rectifier circuit having a power connection terminal on the input side;
One LED having an anode terminal connected to the positive output side of the AC power supply rectifier circuit;
FET for turning on / off the drive current of the LED connected to the cathode terminal of the LED;
A current detection resistor disposed between the FET and the negative output side of the AC power supply rectifier circuit to detect the drive current that has passed through the FET;
An FET control circuit for controlling the FET connected to the gate terminal of the FET via a gate resistor,
The FET control circuit applies a control voltage to the gate resistor for applying a gate voltage exceeding the threshold voltage to the gate terminal as the rectified voltage applied from the positive output side of the AC power supply rectifier circuit increases. The FET is turned on, and the control voltage is lowered to a voltage lower than the threshold voltage in response to detecting that the current detection resistor has detected a drive current exceeding a predetermined value, and charge discharge from the gate terminal is performed. An LED driving circuit configured to delay-off the FET by prompting. An AC power rectifier circuit having a power connection terminal on the input side;
A group of LEDs in which a plurality of LEDs having anode terminals connected to the positive output side of the AC power supply rectifier circuit are connected in series;
FET for turning on / off the drive current of the LED group connected to the cathode terminal of the LED group;
A current detection resistor disposed between the FET and the negative output side of the AC power supply rectifier circuit to detect the drive current that has passed through the FET;
An FET control circuit for controlling the FET connected to the gate terminal of the FET via a gate resistor,
The FET control circuit applies a control voltage to the gate resistor for applying a gate voltage exceeding the threshold voltage to the gate terminal as the rectified voltage applied from the positive output side of the AC power supply rectifier circuit increases. The FET is turned on, and the control voltage is lowered to a voltage lower than the threshold voltage in response to detecting that the current detection resistor has detected a drive current exceeding a predetermined value, and charge discharge from the gate terminal is performed. By delaying off the FET by prompting,
An LED drive circuit characterized in that the drive current can be increased substantially in proportion to the increase in the number of LEDs constituting the LED group. An AC power rectifier circuit having a power connection terminal on the input side;
One LED or a group of LEDs connected in series or in series and parallel;
FET for turning on / off the drive current of the LED or the LED group connected between the anode terminal of the LED or the LED group and the positive output side of the AC power supply rectifier circuit;
A current detection resistor disposed between the cathode terminal of the LED or the LED group and the negative output side of the AC power supply rectifier circuit in order to detect the drive current that has passed through the LED or the LED group;
An FET control circuit for controlling the FET connected to the gate terminal of the FET via a gate resistor,
The FET control circuit applies a control voltage to the gate resistor for applying a gate voltage exceeding the threshold voltage to the gate terminal as the rectified voltage applied from the positive output side of the AC power supply rectifier circuit increases. The FET is turned on, and the control voltage is lowered to a voltage lower than the threshold voltage in response to detecting that the current detection resistor has detected a drive current exceeding a predetermined value, and charge discharge from the gate terminal is performed. An LED driving circuit configured to delay-off the FET by prompting. An AC power rectifier circuit having a power connection terminal on the input side;
One LED having an anode terminal connected to the positive output side of the AC power supply rectifier circuit, or a group of LEDs formed in series or series-parallel connection,
FET for turning on / off the drive current of the LED or the LED group connected to the cathode terminal of the LED or the LED group;
A current detection resistor disposed between the FET and the negative output side of the AC power supply rectifier circuit to detect the drive current that has passed through the FET;
An FET control circuit for controlling the FET connected to the gate terminal of the FET via a gate resistor,
The FET control circuit applies a gate voltage exceeding the threshold voltage to the gate terminal as the rectified voltage applied from the positive output side of the AC power supply rectifier circuit via the LED or the LED group increases. A control voltage is applied to the gate resistor to turn on the FET, and the control voltage is lowered to a voltage lower than the threshold voltage in response to detection of a drive current that the current detection resistor exceeds a predetermined value. The LED driving circuit is configured to delay-off the FET by prompting charge discharge from the gate terminal. An AC power rectifier circuit having a power connection terminal on the input side;
One LED having a cathode terminal connected to the negative output side of the AC power supply rectifier circuit, or a group of LEDs connected in series or in series and parallel,
FET for turning on / off the drive current of the LED or LED group connected to the positive output side of the AC power supply rectifier circuit;
In order to detect the drive current that has passed through the FET, a current detection resistor disposed between the FET and the anode terminal of the LED or the LED group;
An FET control circuit for controlling the FET connected to the gate terminal of the FET via a gate resistor,
The FET control circuit applies a control voltage to the gate resistor for applying a gate voltage exceeding the threshold voltage to the gate terminal as the rectified voltage applied from the positive output side of the AC power supply rectifier circuit increases. The FET is turned on, and the control voltage is lowered to a voltage lower than the threshold voltage in response to detecting that the current detection resistor has detected a drive current exceeding a predetermined value, and charge discharge from the gate terminal is performed. An LED driving circuit configured to delay-off the FET by prompting. A bypass path is connected in parallel to the gate resistor,
The bypass path includes a forward diode toward the FET and a bypass resistor connected in series to the diode;
The LED drive circuit according to claim 1, wherein a value of the bypass resistor is set smaller than a value of the gate resistor. The LED drive circuit according to claim 6, wherein a diode in a reverse direction toward the FET is connected in series to the gate resistor.

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