JP2008125339A - Inrush current prevention circuit, load drive circuit, and light-emitting device using them - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent inrush current which flows into a smoothing capacitor. <P>SOLUTION: The smoothing capacitor Cs smoothes a voltage Vout of an output terminal 32 of a power supply 30. An inrush current prevention circuit 10 prevents inrush current with respect to the smoothing capacitor Cs. A protective resistor Rp is provided on a path, from a first terminal of the power supply 30 to a second terminal of the power supply 30 via the smoothing capacitor Cs. A light-emitting diode D1 on the input side of a photo relay 12 is provided on the path of a load current Iload that flows into a load 40 driven by the output voltage Vout. A switch SW1 on the output side is connected in parallel with the protective resistance Rp. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a circuit protection technique, and more particularly to a technique for preventing an inrush current flowing into a smoothing capacitor.

In recent years, instead of fluorescent lamps, a technique has been proposed in which a light emitting diode (hereinafter abbreviated as LED), which is expected to have high efficiency and long life, is used as a lighting fixture. For example, Patent Literature 1 discloses an LED lighting device that rectifies a commercial AC voltage, smoothes the commercial AC voltage using a smoothing circuit including a smoothing capacitor, and supplies the commercial AC voltage to an array of LEDs that are loads. According to the LED lighting device described in the document, it is possible to stably supply a voltage necessary for driving the light emitting diode.
JP 2004-273267 A

  The smoothing capacitor is used to stabilize a potential appearing at a predetermined node or terminal. The smoothing capacitor is provided between a node to be stabilized and another terminal such as a ground potential terminal or a reference voltage terminal. However, in the device of the same document, when the rectified voltage is suddenly applied to the smoothing capacitor when the charge stored in the smoothing capacitor is zero or very small at the time of starting the circuit, the rated current of the circuit element is Inrush current may occur, which may affect the reliability of the smoothing capacitor and other circuit elements.

  The present invention has been made in view of such circumstances, and one of its purposes is to provide a circuit for preventing an inrush current flowing in a smoothing capacitor.

  In order to solve the above-described problem, according to an aspect of the present invention, an inrush current prevention circuit for preventing an inrush current with respect to a smoothing capacitor for smoothing a voltage of a predetermined node is provided. This inrush current prevention circuit responds to a protective resistor provided on a path from the power source through the smoothing capacitor to another terminal, and a load current driven by a voltage obtained by smoothing the input side diode by the smoothing capacitor. A photorelay provided on a current path and having an output-side switch element connected in parallel with a protective resistor.

  According to this aspect, in a state where the voltage appearing in the smoothing capacitor is low, in other words, in a state where an inrush current can occur, the current flowing through the diode on the input side of the photorelay does not exceed the threshold value, so the output of the photorelay The switch element on the side is turned off. As a result, the charging current flowing into the smoothing capacitor is limited by the protective resistance, and the occurrence of inrush current is suppressed. When charging of the smoothing capacitor progresses, the voltage appearing at the smoothing capacitor increases to such an extent that no inrush current occurs, and when the drive of the load is started, the current flowing through the diode on the input side of the photorelay exceeds the threshold, and the photorelay The output side switch element is turned on. As a result, the protective resistor provided in the same path as the smoothing capacitor is bypassed by the switch element, and unnecessary power consumption due to the protective resistor can be reduced. The “current corresponding to the load current” includes not only the current flowing through the load itself but also a current obtained by copying the current.

  The light emitting diode on the input side of the photorelay may be provided on the current path of the load. In this case, the switch element can be turned on and off by the current itself flowing through the load.

  Furthermore, the inrush current prevention circuit may further include a second light emitting diode provided in parallel with the light emitting diode on the input side of the photorelay. In this case, since the current flowing through the load branches and flows into the second light emitting diode and the light emitting diode on the input side of the photorelay, it can be suitably used when the load current is larger than the input rated current of the photorelay.

In another aspect of the present invention, an inrush current preventing circuit for preventing an inrush current to a smoothing capacitor for smoothing a voltage of a predetermined node is provided. This inrush current prevention circuit has a variable resistor provided on the path from the power source through the smoothing capacitor to another terminal, and a photo diode in which the diode on the input side is provided on the current path according to the charge state of the smoothing capacitor. And a relay. The resistance value of the variable resistor is changed in conjunction with the ON / OFF state of the switch element on the output side of the photo relay, and the resistance value of the variable resistor in the ON state of the switch element is set lower than the resistance value of the variable resistor in the OFF state. To do.
According to this aspect, since the photorelay switch is turned on and off according to the state of charge, the impedance of the path from the power source to another terminal can be changed according to the state of charge, and the inrush current is suitably Can be prevented.

In a certain aspect, the current according to the state of charge of the smoothing capacitor may be a current according to the current of the load driven by the voltage smoothed by the smoothing capacitor.
The current of the load driven by the smoothing capacitor changes depending on the state of charge, that is, the voltage appearing on the smoothing capacitor. Therefore, according to this aspect, the state of charge can be suitably monitored to prevent inrush current. In addition to this, a current corresponding to the state of charge of the smoothing capacitor may be provided with a voltage-current conversion circuit for converting a voltage appearing in the smoothing capacitor into a current, and this output current may be used.

  Another aspect of the present invention relates to a load driving circuit. The load driving circuit includes a rectifying bridge circuit that rectifies an AC voltage, a smoothing capacitor that smoothes an output voltage of the rectifying bridge circuit and supplies the output voltage to a load to be driven, and a smoothing from a first terminal on the DC side of the rectifying bridge circuit. A protective resistor provided on the path leading to the second terminal on the DC side of the rectifier bridge circuit through the capacitor, a diode on the input side are provided on the drive path of the load, and a switch element on the output side is in parallel with the protective resistor And a connected photorelay.

  According to this aspect, before the load current reaches the threshold current (trigger current) of the photorelay, the protective resistance acts to prevent the inrush current, and the load current becomes the threshold of the photorelay. When the current is exceeded, the switch is turned on and the protective resistor is bypassed, so that wasteful power consumption can be reduced.

  The load drive circuit according to an aspect may further include a constant current circuit provided on the same path as the load and the switch on the output side of the photorelay. In this case, when the output voltage exceeds a voltage necessary for driving the load, the load can be driven with a constant current. Furthermore, by setting the value of the constant current generated by the constant current circuit to be larger than the threshold current of the photorelay, it is possible to reliably turn on the switch element on the output side of the photorelay as the load starts. it can.

  One end of the smoothing capacitor may be connected to the first terminal on the DC side of the rectifier bridge circuit, and one end of the protective resistor is connected to the second terminal on the DC side of the rectifier bridge circuit, and the other end of the smoothing capacitor. It may be connected to the other end. A diode on the input side of the load and the photo relay may be connected in parallel with the smoothing capacitor.

In certain aspects, the rectifier bridge circuit may include a diode bridge. The load driving circuit may further include a discharge control switch connected between positive and negative terminals on the DC side of the diode bridge.
A high voltage exceeding 100 V may appear at one end of the smoothing capacitor. In this case, by turning on the discharge control switch when the load is not driven, a discharge path for the electric charge stored in the smoothing capacitor can be formed, and the terminal voltage of the smoothing capacitor can be lowered. As a result, it is possible to prevent an electric shock to the human body and a short circuit failure due to other factors.

  The load drive circuit may further include a discharge resistor provided in series with the discharge control switch between the positive and negative terminals on the DC side of the diode bridge. In this case, since the time constant can be adjusted, the discharge rate can be arbitrarily set.

The discharge control switch may be configured by a photorelay including a light emitting diode on the input side and a switch element on the output side. The switch element may be disposed as a discharge control switch between the positive and negative terminals on the DC side of the diode bridge, and the light emitting diode may be disposed on the drive path of the load.
By providing a photorelay that turns off the switch element on the output side when current flows through the light-emitting diode on the input side, the discharge control switch is automatically switched off and on according to the driving state of the load. be able to.

  The load driving circuit may further include a discharging resistor connected in series with the switch element of the photorelay. In this case, since the time constant can be adjusted, the discharge rate can be arbitrarily set.

The load to be driven may be a device that becomes high impedance in a non-driven state.
When the load becomes high impedance in the non-driving state, the discharge path of the smoothing capacitor does not exist, so that the advantage of providing the discharge control switch can be further enjoyed.

  The load to be driven may be a plurality of light emitting diodes connected in series. Since the light emitting diode has a high impedance in which almost no current flows in the non-light emitting state, the advantage of providing the discharge control switch can be further enjoyed.

Yet another embodiment of the present invention is a light emitting device. This light emitting device includes a plurality of diodes connected in series and a load driving circuit that drives the plurality of light emitting diodes as a load to be driven.
According to this aspect, in a state where the output voltage of the load driving circuit is low, that is, in a state where an inrush current can be generated, the output voltage is lower than the light emitting threshold voltage of the light emitting diode. Since no current flows, the protective resistor can act on the smoothing capacitor, and the occurrence of inrush current can be suppressed. After that, when the output voltage rises and exceeds the light emission threshold voltage of the light emitting diode, current flows through the light emitting diode and exceeds the threshold current of the photo relay, so the output side switch is turned on and the protective resistor is bypassed. Therefore, most of the output voltage can be effectively utilized for the light emitting diode.

  It should be noted that any combination of the above-described constituent elements and the expression of the present invention converted between methods, apparatuses, etc. are also effective as an aspect of the present invention.

  According to the present invention, it is possible to prevent inrush current flowing through the smoothing capacitor and reduce power consumption by the inrush current prevention circuit.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes shown in the drawings are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate. 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.

(First embodiment)
FIG. 1 is a circuit diagram showing a configuration of a load driving device 100 according to the first embodiment. FIG. 1 shows a load 40 to be driven along with the load driving device 100. The load driving device 100 includes a power supply 30, a smoothing capacitor Cs, and an inrush current prevention circuit 10.

  The power supply 30 generates a drive voltage (hereinafter also referred to as an output voltage) Vout necessary for driving the load 40 and outputs it from the output terminal 32. The power source 30 is an AC / DC converter that converts an AC voltage into a DC voltage, a DC / DC converter that boosts or steps down an input DC voltage, and stabilizes and outputs the DC voltage. Hereinafter, the two terminals of the power supply 30 are referred to as a first terminal P5 and a second terminal P6. The second terminal P6 provides a potential reference.

  The smoothing capacitor Cs is provided for smoothing the output voltage Vout of a predetermined node, in this embodiment, the output terminal 32 of the power supply 30. The smoothing capacitor Cs is a path connecting a node to be smoothed and a fixed voltage terminal, and is arranged on a path parallel to the load 40. In the circuit of FIG. 1, the smoothing capacitor Cs has one end connected to the output terminal 32 to be smoothed and the other end grounded via a protective resistor Rp described later. The capacitance value of the smoothing capacitor Cs may be appropriately selected according to the application. For example, an electrolytic capacitor of several μF to several hundred μF can be used. In addition, a ceramic capacitor or the like may be used.

  In an embodiment, the smoothing capacitor Cs may constitute a part of the power supply 30. For example, when the power supply 30 is a switching regulator or a charge pump circuit, the output capacitor corresponds to the smoothing capacitor Cs.

  The inrush current prevention circuit 10 is provided to prevent an inrush current with respect to the smoothing capacitor Cs, and includes a photorelay 12 and a protection resistor Rp. The protective resistor Rp is provided on a path from the first terminal P5 of the power supply 30 to the second terminal P6 of the power supply 30 through the smoothing capacitor Cs. In the present embodiment, the protective resistor Rp is provided on the low voltage side of the smoothing capacitor Cs. However, the protective resistor Rp may be provided on the high voltage side of the smoothing capacitor Cs.

The protective resistor Rp is connected in series with the smoothing capacitor Cs and functions to prevent the charging current of the smoothing capacitor Cs from increasing rapidly. The charging voltage Vs applied to the smoothing capacitor Cs is given by Vs = (Vout−Vd) using the output voltage Vout of the power supply 30 and the voltage drop Vd of the protection resistor Rp. Therefore, when the charging current Ichg is increased, the voltage drop Vd at the protection resistor Rp is increased, the charging voltage Vs is decreased, and feedback is applied so that the charging current Ichg is reduced, and the rapid increase of the charging current Ichg is suppressed. . From another point of view, the smoothing capacitor Cs and the protective resistor Rp form an RC filter, and the high frequency component of the output voltage Vout is removed by the action of the filter, and the rapid increase in the charging current Ichg is suppressed. It is also possible to see.
Therefore, the resistance value of the protective resistor Rp may be set in consideration of the output voltage Vout, the withstand voltage of the smoothing capacitor Cs, the rated current, and the like. For example, the resistance value of the protective resistor Rp is set in the range of several tens Ω to several kΩ.

  The photorelay 12 includes input terminals P1 and P2 and output terminals P3 and P4. Between the input terminals P1 and P2, the light emitting diode D1, which is a light emitting element, has an anode on the input terminal P1 side and a cathode on the input terminal P2 side. It is provided in the direction. The light emitting diode D1 emits light when a voltage exceeding the forward threshold voltage is applied and a current flows. A switch SW1 (phototransistor) is provided between the output terminals P3 and P4. The switch SW1 and the light emitting diode D1 are optically coupled, and the switch SW1 is turned on when receiving light output from the light emitting diode D1, and turned off when receiving no light. The photorelay 12 may be a packaged light emitting diode D1 and a switch SW1 that are commercially available.

  The light emitting diode D1 on the input side of the photorelay 12 is provided on the path of the monitor current Im corresponding to the current Iload flowing in the load 40 driven by the voltage Vout smoothed by the smoothing capacitor Cs. In the present embodiment, the monitor current Im is the load current Iload.

  The protective resistor Rp functions as a protective element for preventing an inrush current as described above, but also functions as a loss element because it is disposed on the path of the load current Iload. Therefore, it is desirable that the protective resistance Rp acts on the smoothing capacitor Cs only during a period in which an inrush current can occur, and has a resistance value as low as possible in a state where no inrush current occurs. Therefore, the switch SW1 on the output side of the photo relay 12 is connected in parallel with the protective resistor Rp. When the switch SW1 is turned on, the protection resistor Rp is bypassed, so that the combined resistance of the protection resistor Rp and the switch SW1 is lowered, and the substantial resistance value is lowered. From another viewpoint, it is considered that the protective resistance Rp and the switch SW1 constitute a variable resistance.

  The operation of the load driving device 100 configured as described above will be described. FIG. 2 is a time chart showing an operation state of the load driving device 100. FIG. 2 shows, in order from the top, the output voltage Vout, the load current Iload, the charging current Ichg, and the on / off state of the switch SW1. The waveform of the switch SW1 corresponds to a state where the high level is on. Note that the vertical axis and the horizontal axis in FIG. 2 and FIG. 4 are enlarged or reduced as appropriate for easy understanding, and each waveform shown is also simplified for easy understanding. ing.

  During the period from time t0 to t1, the output voltage Vout is not generated and is maintained at 0V. At time t1, the power supply 30 is activated, and the output voltage Vout starts to increase toward the target value. Since the smoothing capacitor Cs is charged as the output voltage Vout increases, the charging current Ichg also starts increasing at time t1.

  When the load current Iload, that is, the monitor current Im rises and exceeds the threshold current Ith of the photorelay 12 at time t2, the switch SW1 is turned on and the protection resistor Rp is bypassed. When the protection resistor Rp is bypassed, the voltage drop Vd at the protection resistor Rp is reduced, and the output voltage Vout of the power supply 30 can be efficiently supplied to the load 40. Thereafter, the load 40 is driven stably based on the output voltage Vout.

  If the protective resistance Rp is not provided, the charging current Ichg rapidly increases and becomes an inrush current as indicated by a broken line in FIG. In the load driving device 100 according to the present embodiment, the switch SW1 is in an OFF state immediately after the start of startup, so that the protection resistor Rp is connected in series with the smoothing capacitor Cs, and an inrush current is generated. It is suitably suppressed.

The conditions for suitably suppressing the inrush current by the inrush current preventing circuit 10 of FIG. 1 are summarized as follows.
The inrush current to the smoothing capacitor Cs can occur when the output voltage Vout is low and the load current Iload does not flow or is small even when it flows. Therefore, when the maximum value of the load current flowing through the load 40 is Imax in a state where the output voltage Vout in a range where an inrush current can be generated is supplied to the smoothing capacitor Cs and the load 40,
Imax <Ith (1)
Should just hold. In this case, since the switch SW1 of the photo relay 12 is not turned on in a state where an inrush current can be generated, the protection resistor Rp can be functioned to prevent the inrush current.

Further, when the output voltage Vout increases beyond the range where the inrush current can occur and is supplied to the load 40, that is, when the minimum value of the load current flowing through the load 40 in the normal driving state is Imin,
Imin> Ith (2)
Should just hold. If the formula (2) is established, the protective resistor Rp is always bypassed in the normal driving state, so that high-efficiency driving can be realized. In other words, the load current Iload may vary within the range as long as it is larger than the threshold current Ith in the normal state.

Thus, according to the inrush current prevention circuit 10 according to the present embodiment, when the smoothing capacitor Cs is charged while suitably preventing the inrush current to the smoothing capacitor Cs, the current of the light emitting diode D1 increases. As the current to the light emitting diode D1 increases, the switch SW1 is turned on and the protection resistor Rp is bypassed. As a result, unnecessary power consumption due to the protective resistance Rp can be suppressed, and the voltage applied to the load can be kept high.
Moreover, when applied to an LED lamp, the number of LEDs can be increased in series, and a brighter lamp can be realized. That is, the voltage drop of the protective resistor Rp can be effectively used for the LED lighting voltage.

  In the present embodiment, the photo relay 12 is used to bypass the protective resistance Rp. Since the photorelay 12 is composed of the light emitting diode D1 and the switch SW1, the elements necessary for protecting the smoothing capacitor Cs are the protection resistor Rp, the light emitting diode D1, and the switch SW1, and a very simple circuit. It can be configured. Since a very small photorelay of several millimeters square can be used, an increase in circuit area due to the provision of the inrush current prevention circuit 10 can be suppressed slightly.

  Further, the conditions to be considered in order to realize suitable circuit protection are the above formulas (1) and (2), but it is safer that the threshold current Ith is closer to the minimum value Imin of the load current. There is a need to select and connect the photorelay 12 that satisfies this upper limit, so that the design is very easy.

Hereinafter, the superiority of the inrush current prevention circuit 10 according to the present embodiment over the circuit to be compared will be described.
(First Comparative Example) When a switch composed of a MOSFET is provided in parallel with the protective resistor Rp without using the photorelay 12 In this case, in order to turn the switch on and off, a load current or a load voltage is detected. It is necessary to control the on / off of the switch by comparing this with a threshold using a comparator or the like. In this case, a power supply circuit and a control circuit for turning on and off the switch are required, which causes an increase in the number of components and a corresponding increase in circuit area and cost. The same applies when a thyristor is used as a switch.

  In contrast to the first comparative example, the inrush current prevention circuit 10 according to the present embodiment uses a photorelay 12, so that the switch and its control circuit are composed of an output side switch SW1 and an input side light emitting diode D1. There is an advantage of being configured.

(Second Comparative Example) When a Thermistor is Provided at the Position of the Protection Resistance Rp In this case, the resistance value automatically provided for preventing inrush current decreases as the temperature rises, so that it is not necessary to provide a switch. However, if the charge of the smoothing capacitor Cs is discharged in a state where the temperature of the thermistor is rising, charging is performed in a state where the resistance value of the thermistor is low, so that an inrush current may occur. Further, the amount of change in the resistance value of the thermistor is about several times at most. Therefore, even if the temperature rises to some extent, the resistance value only decreases to a fraction, and there is a limit in reducing loss.

  In contrast to the second comparative example, the inrush current prevention circuit 10 according to the embodiment directly monitors the operating state of the load, not the indirect state quantity such as the temperature, and thus the charging state of the smoothing capacitor Cs. Since the resistance value is switched by indirectly monitoring, it is possible to more reliably prevent the occurrence of an inrush current. The voltage drop Vd when the switch SW1 is on is proportional to the on-resistance of the switch SW1 of the photorelay 12, but this on-resistance is in the range of several tens of mΩ to several Ω. Then, the power loss due to the voltage drop Vd can be reduced compared to the second comparative example.

It should be noted that the following inrush current prevention method is realized in the inrush current prevention circuit 10 according to the present embodiment. What is that method,
(1) providing a variable resistor on a path from the power source 30 through the smoothing capacitor Cs to the fixed voltage terminal;
(2) the step of providing the photorelay 12 on the path of the current Im corresponding to the charging state of the smoothing capacitor Cs by the diode D1 on the input side;
(3) changing the resistance value of the variable resistor in conjunction with the ON / OFF state of the switch SW1 on the output side of the photo relay 12.
Here, the variable resistor (resistance value switching unit) includes a protection resistor Rp and a switch SW1, and the resistance value is switched in conjunction with the on / off of the switch SW1. Further, as the charged state of the smoothing capacitor Cs, the current Iload of the load 40 driven by the voltage Vout smoothed by the smoothing capacitor Cs is monitored by the monitor current Im. The resistance value of the variable resistor in the on state of the switch SW is set lower than the resistance value of the variable resistor in the off state. Therefore, a circuit capable of realizing this method is included in the scope of the present invention.

(Second Embodiment)
In the first embodiment, a general load driving circuit has been described. In the second embodiment, a light emitting device in which an LED is provided in a load will be described as a circuit using the inrush current prevention circuit 10.

  FIG. 3 is a circuit diagram showing a configuration of the light emitting device 200 according to the second embodiment. The light emitting device 200 includes a load 40 including a plurality of LEDs 42 and an LED drive circuit 100a for driving the plurality of LEDs 42.

  The LED drive circuit 100a includes a power supply 30, a constant current circuit 20, an inrush current prevention circuit 10, and a smoothing capacitor Cs. The power supply 30 includes an AC power supply 34 and a rectifier circuit 36. The AC power supply 34 generates an AC voltage Vac, and the rectifier circuit 36 rectifies the AC voltage Vac. When a commercial AC voltage is used as the AC voltage Vac, the AC power supply 34 is provided outside the LED drive circuit 100a.

The output terminal of the rectifier circuit 36 is the output terminal 32 of the power supply 30. Therefore, the output voltage of the rectifier circuit 36 is smoothed by the smoothing capacitor Cs and supplied to the load 40 as the output voltage Vout.
In order to keep the brightness of the LED 42 constant, it is necessary to drive at a constant current. Therefore, a constant current circuit 20 that generates a predetermined constant current Ic is provided on the drive path of the LED 42. This constant current Ic matches the load current Iload. The configuration of the constant current circuit 20 is not particularly limited, and a known circuit may be used. Alternatively, a constant current diode may be used. In order to set the load current Iload to a predetermined value, a switching element may be provided on the current path, and a mechanism for adjusting the current by switching driving based on a PWM (pulse width modulation) signal may be provided.

  As described in the first embodiment, the inrush current prevention circuit 10 includes the photo relay 12 and the protection resistor Rp. The protective resistor Rp is provided on a path from the first terminal of the power supply 30 to the second terminal of the power supply 30 via the smoothing capacitor Cs. Also in the present embodiment, the protective resistor Rp is provided on the low voltage side of the smoothing capacitor Cs, but it may be provided on the high voltage side of the smoothing capacitor Cs.

  The light emitting diode D1 on the input side of the photorelay 12 is provided in series with the load 40 and the constant current circuit 20 on the drive path of the load 40. The switch SW1 is connected in parallel with the protection resistor Rp. Also in the present embodiment, the monitor current Im is the load current Iload. Also in the present embodiment, the load 40 and the light emitting diode D1 on the input side of the photorelay 12 are connected in parallel with the smoothing capacitor Cs. The threshold current Ith of the photorelay 12 is set so that Ith <Ic.

When n (n is a natural number) LEDs 42 are connected to the load 40, the voltage drop of the constant current circuit 20 in a state where the constant current Ic flows through the LEDs 42 (hereinafter referred to as a stable drive state) is expressed as Vcs, When the forward threshold voltage of the light emitting diode D1 of the photorelay 12 is Vf ′,
Vout = Vd + Vcs + Vf ′ + Vload
Holds. The load voltage Vload is
Vload = n × Vf
Given in.

As described above, since Ic> Ith is satisfied, the switch SW1 of the photorelay 12 is turned on in a state where the predetermined constant current Ic flows and the LED 42 emits light. When the on-resistance of the switch SW1 is Ron, the voltage drop Vd across the protection resistor Rp in the stable drive state is
Vd = (Rp // Ron) × Ic
It is a constant value given by. Note that (Rp // Ron) represents a parallel combined resistance of Rp and Ron. Further, the forward threshold voltage Vf ′ of the light emitting diode D1 when a predetermined current flows is also a constant value.

Therefore, when a certain output voltage Vout is given, the number n of LEDs 42 is
n ≦ (Vout−Vd−Vcs−Vf ′) / Vf
It becomes. When the commercial AC voltage is rectified, the output voltage Vout is 140V. For example, when Vout = 140V, Ic = 20mA, Rp // Ron = 10Ω, Vcs = 4V, Vf ′ = 1V, Vf = 3V, n ≦ 44.9. The number of LEDs 42 is preferably as large as possible from the viewpoint of effective use of power, and therefore n = 44.

  The operation of the LED drive circuit 100a configured as described above will be described. FIG. 4 is a time chart showing an operation state of the LED drive circuit 100a of FIG. FIG. 4 shows, in order from the top, the AC voltage Vac, the output voltage Vout, the charging current Ichg, the load current Iload, and the on / off state of the switch SW1. A broken line Ichg ′ indicates an inrush current when the protective resistance Rp is not provided.

  At time t1, the AC voltage Vac starts to increase from 0V, and accordingly, the output of the rectifier circuit 36 also increases according to the waveform indicated by the broken line Vout ′. As a result, the charging current Ichg begins to flow, and charging of the smoothing capacitor Cs is started. As the smoothing capacitor Cs is charged, the output voltage Vout increases.

  When the load current Iload exceeds the threshold current of the photorelay 12 at time t2, the switch SW1 is turned on.

  According to the LED drive circuit 100a according to the present embodiment, in a state where the output voltage Vout is low, that is, in a state where an inrush current can occur, the current does not flow through the LED 42, so the switch SW1 is turned off, and the protection resistor Rp Inrush current is suppressed. Thereafter, when the output voltage Vout rises and the load current Iload exceeds the threshold value, the switch SW1 is turned on, and the voltage drop Vd across the protective resistor Rp is reduced.

  Thus, the inrush current prevention circuit 10 according to the present embodiment can be suitably used for an LED drive circuit.

  It will be understood by those skilled in the art that the above-described embodiments are exemplifications, and various modifications can be made to combinations of the respective constituent elements and processing processes, and such modifications are also within the scope of the present invention. By the way.

  In the embodiment, the case where the light emitting diode D1 of the photorelay 12 is arranged on the path of the load current Iload and the load current Iload is monitored has been described, but the present invention is not limited to this. For example, the monitor current Im only needs to be able to monitor the state of the load 40, and thus may be a current proportional to the load current Iload. FIG. 5 is a circuit diagram showing a part of the configuration of the inrush current prevention circuit 10a according to the modification. In the inrush current prevention circuit 10a shown in FIG. 5, a current mirror circuit 44 for multiplying the load current Iload flowing through the load 40 by k is provided. The light emitting diode D1 of the photorelay 12 is connected on the current path on the output side of the current mirror circuit 44, and monitors the current Im = k × Iload.

  For example, when the load current Iload flowing through the load 40 exceeds the rated current of the photorelay 12, the load current Iload may be multiplied by k (k <1) using a current mirror circuit. Alternatively, since the threshold current of the photorelay 12 is constant, the apparent threshold current with respect to the load current Iload can be adjusted by adjusting the mirror ratio k.

FIG. 11 is a circuit diagram showing a part of the configuration of an inrush current prevention circuit 10e according to another modification. The inrush current prevention circuit 10e is preferably used when the load current Iload is larger than the rated input current of the photorelay 12, like the inrush current prevention circuit 10a of FIG.
Photorelay 12 includes a resistance component R1 connected in series with light emitting diode D1. The inrush current prevention circuit 10e in FIG. 11 includes a light emitting diode D3 provided in parallel with the light emitting diode D1 (and the resistance component R1) of the photorelay 12. That is, the load current Iload flowing through the load 40 branches to the light emitting diode D1 and the light emitting diode D3. As a result, a current Im = k × Iload corresponding to the current division ratio k determined by the light emitting diode D1, the resistance component R1, and the light emitting diode D3 flows through the light emitting diode D1. Specifically, when the resistance values of the light emitting diodes D1 and D3 are written as r1 and r3, respectively, k = r3 / (r1 + r3 + R1). According to the inrush current prevention circuit 10e of FIG. 11, by inserting a light emitting diode D3 as a load in parallel to the input side of the photorelay 12, it is possible to adapt to the rated current on the input side of the photorelay 12.

  Further, in the embodiment, the case where the photorelay 12 is controlled based on the current corresponding to the load current Iload has been described, but the present invention is not limited to this. The load current Iload is an example of a state quantity indicating the state of charge of the smoothing capacitor Cs, that is, the voltage of the smoothing capacitor Cs. The load current Iload is a configuration that controls the photorelay 12 based on the current according to the voltage of the smoothing capacitor Cs. Good. For example, a voltage-current conversion circuit that converts the voltage of the smoothing capacitor Cs into a current may be provided, and the light-emitting diode D1 of the photorelay 12 may be provided on the output current path of the voltage-current conversion circuit.

  Although the case where only one photorelay 12 is provided has been described in the embodiment, the present invention is not limited to this. FIG. 6 is a circuit diagram of an inrush current prevention circuit 10b according to a modification including a plurality of photorelays 12. In the circuit of FIG. 6, the loads 40a and 40b branched to a plurality of paths are driven by the output voltage Vout. The light emitting diodes D1a and D1b on the input side of the plurality of photorelays 12a and 12b are arranged on the drive paths of the loads 40a and 40b. The switches SW1a and SW1b on the output side of the photorelays 12a and 12b are connected in parallel to the protection resistor Rp.

  In the circuit of FIG. 6, the protection resistor Rp can be bypassed if the output voltage Vout increases to some extent and at least one of the loads 40a and 40b is driven and any one of the load currents Iloada and Iloadb flows. This circuit is effective when inrush current does not occur in a state where at least one of the loads 40a and 40b is driven.

  FIG. 7 is a circuit diagram showing another configuration of the inrush current prevention circuit 10 including a plurality of photorelays 12. The inrush current prevention circuit 10c of FIG. 7 includes two protection resistors Rpa and Rpb. The switches SW1a and SW1b of the two photorelays 12a and 12b are connected in parallel with the protection resistors Rpa and Rpb, respectively.

  In the inrush current prevention circuit 10c, the protection resistors Rpa and Rpb connected in series contribute to prevention of inrush current to the smoothing capacitor Cs. Therefore, for example, when only the load 40a is driven, the protective resistance is only Rpb. When only the load 40b is driven, the protective resistance is only Rpa. When neither is driven, the protective resistance is Rpa + Rpb. . Therefore, the value of the protective resistance can be changed according to the driving state of the load. Further, when both the loads 40a and 40b are driven and the total load current is large, the protection resistors Rpa and Rpb are both bypassed, so that the power loss can be reduced suitably.

  FIG. 8 is a circuit diagram showing still another configuration of an inrush current prevention circuit including a plurality of photorelays 12. In the circuit of FIG. 8, light-emitting diodes D1a and D1b on the input side of the plurality of photorelays 12a and 12b are connected in series on the same path as the load 40. Further, protective resistors Rpa and Rpb are provided in series on the path of the smoothing capacitor Cs, and the switches SW1a and SW1b on the output side of the photorelays 12a and 12b are connected in parallel to the protective resistors Rpa and Rpb. Has been. The photorelay 12a and the photorelay 12b have different threshold currents Ith1 and Ith2.

  According to the circuit of FIG. 8, when the load current Iload flowing through the load 40 changes, the resistance value of the protection resistor Rp can be switched stepwise in accordance with the load current Iload. In general, the load current Iload changes according to the magnitude of the output voltage Vout. Therefore, when the output voltage Vout is low, that is, when there is a high possibility that an inrush current is generated, the load current Iload is low, Iload <Ith1 <Ith2, and the value of the protective resistance is Rpa + Rpb. Increases prevention ability.

  When the load current Iload increases as the output voltage Vout increases and Ith1 <Iload <Ith2, the switch SW1a is turned on, and the value of the protective resistance is Rpb. In this state, the inrush current is suppressed by the protection resistor Rpb, and the power loss due to the protection resistor Rpa is reduced.

  When the output voltage Vout further rises and Ith1 <Ith2 <Iload, both the switches SW1a and SW1b are turned on, both the protection resistors Rpa and Rpb are bypassed, and the power loss is further reduced.

  As described above, according to the inrush current prevention circuit 10d of FIG. 8, the functions of suppressing the inrush current and reducing the power loss can be exhibited with respect to the load in which the load current Iload changes.

(Third embodiment)
In the first and second embodiments, the inrush current prevention technique for the smoothing capacitor has been described. That is, it is a technique that focuses on the operation at the time of startup of the device and circuit. On the other hand, in the third embodiment, a technique that focuses on the operation at the time of using or stopping the device or circuit will be described.

  Consider the light emitting device 200 of FIG. As described above, the smoothing capacitor Cs is applied with the voltage (Vout) obtained by smoothing the commercial AC voltage, and charges proportional to the applied voltage are stored. Here, when driving of the load 40 such as an LED is stopped, the power supply 30 is stopped.

  When the output voltage Vout is somewhat higher than Vf × n after the power supply 30 is stopped, a voltage exceeding the forward threshold voltage Vf is applied to each LED 42, so that a forward current flows through the LED 42 and the smoothing capacitor Charge is discharged from Cs. However, when the output voltage Vout drops to a certain extent due to the discharge, the voltage applied to each LED 42 becomes smaller than the forward threshold voltage Vf, so that the LED 42 is cut off or enters a high impedance state. That is, there is a problem that the charge stored in the smoothing capacitor Cs loses the discharge path, and the output voltage Vout does not decrease from around Vf × n, and a high voltage remains.

  For example, in a situation where the light emitting device 200 of FIG. 3 is used as illumination, it is assumed that the user replaces the LED. In this case, if electric charge is stored in the smoothing capacitor Cs, the user may be electrocuted. Such a problem may also occur when the load 40 is other than the LED 42. The third embodiment provides a technique for solving such a problem.

  FIG. 9 is a circuit diagram showing a configuration of a light emitting device 200b according to the third embodiment. Hereinafter, the light emitting device 200b of FIG. 9 will be described focusing on differences from the light emitting device 200 of FIG. The light emitting device 200b further includes a discharge control switch SWdis, a discharging resistor Rdis, and a power switch SWp, as compared with the light emitting device 200 of FIG.

  The rectifier circuit 36 of the load driving device 100b is configured by a diode bridge. The discharge control switch SWdis is connected between the positive and negative terminals P5 and P6 on the DC side of the diode bridge. From another viewpoint, the discharge control switch SWdis is provided between both ends of the smoothing capacitor Cs. The discharge control switch SWdis is turned off while the load 40 is being driven, and is turned on after the driving is stopped. The power switch SWp is a switch that controls the supply of the AC voltage Vac by the power supply 30b. When the power switch SWp is turned on, the output voltage Vout is supplied to the load 40, and the LED 42 is lit. When the power switch SWp is turned off, the AC voltage Vac is interrupted, so that power supply to the load 40 is stopped and the LED 42 is turned off. Therefore, on / off of the discharge control switch SWdis may be linked with on / off of the power switch SWp so that when one is on, the other is off. The power switch SWp and the discharge control switch SWdis may be mechanical switches or electronic switches. In the case where a mechanical switch is used, the device can be configured at low cost.

  The discharge resistor Rdis is provided in series with the discharge control switch Swdis between the positive and negative terminals P5 and P6 on the DC side of the diode bridge.

The operation of the light emitting device 200b in FIG. 9 will be described. When the power switch SWp is turned on, the discharge control switch SWdis is turned off, and the light emitting device 200b operates in the same manner as the light emitting device 200 of FIG.
When the power switch SWp is turned off, the power supply to the load 40 is stopped and the LED 42 is turned off. At this time, since the discharge control switch SWdis is turned on, a discharge path for the charge stored in the smoothing capacitor Cs is formed by the discharge control switch SWdis and the discharge resistor Rdis. Since the charge stored in the smoothing capacitor Cs is discharged through the discharge control switch SWdis and the discharge resistor Rdis, the output voltage Vout can be lowered to around 0V.

As described above, according to the light emitting device 200b of FIG. 9, the problem that the high voltage remains in the smoothing capacitor Cs can be solved by providing the discharge control switch SWdis and turning on the load 40 in the non-driven state. .
Furthermore, since the CR time constant is adjusted by providing the discharge resistor Rdis on the discharge path, it is possible to suitably adjust the time until the output voltage Vout decreases to 0V. Note that the discharge resistor Rdis may be omitted when the on-resistance of the discharge control switch SWdis is sufficiently large or when the output voltage Vout is to be rapidly decreased.

  FIG. 10 is a circuit diagram showing a part of another configuration of the light emitting device according to the third embodiment. In the circuit diagram of FIG. 10, the constant current circuit 20 and the photorelay 12 on the drive path of the power source 30 and the load 40 are omitted. The light emitting device 200c of FIG. 10 includes a discharge photorelay 50. The discharging photorelay 50 includes an input side light emitting diode D2 and an output side switch SW2. In the discharge photorelay 50, when a current flows through the light emitting diode D2 on the input side and emits light, the switch SW2 on the output side is turned off, and when no current flows through the light emitting diode D2, the switch SW2 is turned on. That is, the operation reverse to that of the photorelay 12 of FIGS. Such a photorelay is commercially available.

  In the light emitting device 200c of FIG. 10, the discharge control switch SWdis of FIG. That is, the switch SW2 of the discharge photorelay 50 is connected in series with the discharge resistor Rdis at both ends of the smoothing capacitor Cs as the discharge control switch SWdis. In FIG. 10, the discharge resistor Rdis plays a role of adjusting the discharge rate in the same manner as in FIG. The light emitting diode D2 on the input side is disposed on the drive path of the load 40 (LED 42).

  The operation of the light emitting device 200c of FIG. 10 will be described. The power source 30 is stopped to turn off the LED 42, and after a while, no current flows through the light emitting diode D2. As a result, the switch SW2 is turned on, the electric charge of the smoothing capacitor Cs is discharged through the discharging resistor Rdis and the switch SW2, and the output voltage Vout decreases to 0V. Therefore, the light emitting device 200c of FIG. 10 can also solve the problem that a high voltage remains in the smoothing capacitor Cs, similarly to the light emitting device 200b of FIG.

  Furthermore, in the light emitting device 200c of FIG. 10, since the discharge path is provided on the load 40 side, there is an advantage that it is easy to install the switch SW2 and the discharge resistor Rdis. That is, for example, when the light emitting device 200c of FIG. 10 is used as fixed illumination on a ceiling or the like, if the discharge resistor Rdis, the discharge photorelay 50, and the constant current circuit (not shown) are modularized, a discharge control switch is provided on the wall. The work of installing SWdis is unnecessary and convenient.

  In the circuit of FIG. 10, when the discharge resistance Rdis is too small, no current flows through the LED 42 and the light emitting diode D2 of the discharge photorelay 50. Therefore, immediately after startup, there is a problem that the switch SW2 of the discharging photorelay 50 is kept on, and the LED 42 cannot be turned on. Therefore, the resistance value of the discharge resistor Rdis needs to be larger than the combined operation resistance of the load 40.

  In the third embodiment, the discharge mechanism of the residual voltage of the smoothing capacitor Cs has been described with reference to FIGS. 9 and 10. 9 and 10, the LED 42 has been described as an example of the load 40. However, the present invention is not limited to this, and load driving is performed by driving various devices having high impedance in the non-driving state as loads. It can be used for the apparatus 100.

  9 and FIG. 10, a discharge mechanism for the smoothing capacitor Cs using the discharge control switch SWdis is provided in addition to the inrush current prevention mechanism using the photorelay 12 described in the first and second embodiments. Explained the case. However, in one aspect of the present invention, the present invention is not limited to this. Even in a circuit that does not include an inrush current prevention mechanism, the effect of eliminating the residual voltage can be obtained by providing the discharge control switch SWdis.

It is a circuit diagram which shows the structure of the load drive device which concerns on 1st Embodiment. It is a time chart which shows the operation state of a load drive device. It is a circuit diagram which shows the structure of the light-emitting device which concerns on 2nd Embodiment. It is a time chart which shows the operation state of the LED drive circuit of FIG. It is a circuit diagram which shows a part of structure of the inrush current prevention circuit which concerns on a modification. It is a figure which shows the modification of an inrush current prevention circuit provided with a some photorelay. It is a circuit diagram which shows another structure of the inrush current prevention circuit provided with a some photorelay. It is a circuit diagram which shows another structure of the inrush current prevention circuit provided with a some photorelay. It is a circuit diagram which shows the structure of the light-emitting device which concerns on 3rd Embodiment. It is a circuit diagram which shows a part of another structure of the light-emitting device which concerns on 3rd Embodiment. It is a circuit diagram which shows a part of structure of the inrush current prevention circuit which concerns on another modification.

Explanation of symbols

  10 inrush current prevention circuit, 12 photorelay, 20 constant current circuit, 30 power supply, 32 output terminal, 34 AC power supply, 36 rectifier circuit, 40 load, 42 LED, 44 current mirror circuit, D1 light emitting diode, SW1 switch, 100 load Drive device, 100a LED drive circuit, 200 light emitting device, Rp protection resistor, Cs smoothing capacitor, SWdis discharge control switch, Rdis discharge resistor, 50 discharge photorelay.

Claims (15)

An inrush current prevention circuit for preventing an inrush current for a smoothing capacitor for smoothing a voltage of a predetermined node,
A protective resistor provided on a path from the power source to the other terminal via the smoothing capacitor;
A photo diode in which an input side light emitting diode is provided on a current path corresponding to a current of a load driven by a voltage smoothed by the smoothing capacitor, and an output side switch element is connected in parallel with the protection resistor. Relay,
An inrush current prevention circuit comprising:   The inrush current prevention circuit according to claim 1, wherein the light emitting diode on the input side of the photorelay is provided on a current path of the load.   The inrush current prevention circuit according to claim 2, further comprising a second light emitting diode provided in parallel with the light emitting diode on the input side of the photorelay. An inrush current prevention circuit for preventing an inrush current for a smoothing capacitor for smoothing a voltage of a predetermined node,
A variable resistor provided on a path from the power source to the other terminal through the smoothing capacitor;
A photorelay in which a light-emitting diode on the input side is provided on a current path according to a charge state of the smoothing capacitor;
With
The resistance value of the variable resistor is changed in conjunction with the on / off state of the switch element on the output side of the photorelay, and the resistance value of the variable resistor in the on state of the switch element is changed to that of the variable resistor in the off state. An inrush current prevention circuit characterized by being set lower than the resistance value.   5. The inrush current prevention according to claim 4, wherein the current according to the state of charge of the smoothing capacitor is a current according to a current of a load driven by a voltage smoothed by the smoothing capacitor. circuit. A rectifier bridge circuit for rectifying an AC voltage;
A smoothing capacitor for smoothing the output voltage of the rectifying bridge circuit and supplying the output voltage to a load to be driven;
A protective resistor provided on a path from the first terminal on the DC side of the rectifier bridge circuit to the second terminal on the DC side of the rectifier bridge circuit through the smoothing capacitor;
A photorelay in which a diode on the input side is provided on the drive path of the load, and a switch element on the output side is connected in parallel with the protection resistor;
A load driving circuit comprising:   The load drive circuit according to claim 6, further comprising a constant current circuit provided on the same path as the load and the switch element on the output side of the photorelay. The smoothing capacitor has one end connected to the first terminal on the DC side of the rectifying bridge circuit,
The protective resistor has one end connected to the second terminal on the DC side of the rectifying bridge circuit and the other end connected to the other end of the smoothing capacitor.
The load driving circuit according to claim 6, wherein the load and a diode on the input side of the photorelay are connected in parallel with the smoothing capacitor. The rectifier bridge circuit includes a diode bridge;
9. The load drive circuit according to claim 8, further comprising a discharge control switch connected between positive and negative terminals on the DC side of the diode bridge.   The load driving circuit according to claim 9, further comprising a discharging resistor provided in series with the discharge control switch between positive and negative terminals on the DC side of the diode bridge. The discharge control switch is
It consists of a photorelay that includes a light emitting diode on the input side and a switch element on the output side,
The light emitting diode is arranged on the drive path of the load between the positive and negative terminals on the DC side of the diode bridge, wherein the switch element is the discharge control switch. Load drive circuit.   The load driving circuit according to claim 11, further comprising a discharging resistor connected in series with the switch element of the photorelay.   The load driving circuit according to claim 6, wherein the load to be driven is a device having a high impedance in a non-driving state.   The load driving circuit according to claim 6, wherein the load to be driven is a plurality of light emitting diodes connected in series. A plurality of light emitting diodes connected in series;
The load driving circuit according to any one of claims 6 to 13, wherein the plurality of light emitting diodes are driven as the load to be driven.
A light emitting device comprising:

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