Protection circuit for dimmer, and dimmer

A control unit controls a bidirectional switch so as to turn the switch from ON to OFF when an amount of time, varying according to a lighting level, passes since a starting point of a half cycle of an AC voltage. When a voltage between both terminals of a capacitive element, connected to a control terminal of the bidirectional switch, becomes equal to or greater than a threshold voltage, the bidirectional switch turns from OFF to ON A first charging/discharging regulator circuit and a second charging/discharging regulator circuit each make a rate of fall of the voltage between both of the terminals of the capacitive element when the bidirectional switch turns from ON to OFF lower than a rate of rise of the voltage between both of the terminals of the capacitive element when the bidirectional switch turns from OFF to ON.

RELATED APPLICATIONS

This application is the U.S. National Phase under 35 U.S.C. § 371 of International Application No. PCT/JP2017/030824, filed on Aug. 29, 2017, which in turn claims the benefit Japanese Application No. 2016-183349, filed Sep. 20, 2016, the disclosures of which are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a protection circuit for a dimmer configured to control the amount of light emitted by lighting load and also relates to a dimmer itself.

BACKGROUND ART

Dimmers for controlling the amount of light emitted by a lighting load have been known in the art (see, for example, Patent Literature 1).

The dimmer disclosed in Patent Literature 1 includes a pair of terminals, a control circuit unit, a control power supply unit for supplying control power to the control circuit unit, and a lighting control operating unit for setting the lighting level of the lighting load.

Between the pair of terminals, connected in parallel are the control circuit unit and the control power supply unit. In addition, a series circuit of an AC power supply and the lighting load is also connected between the pair of terminals. The lighting load includes a plurality of light-emitting diodes (LEDs) and a power supply circuit for turning these LEDs ON. The power supply circuit includes a smoothing circuit including a diode and an electrolytic capacitor.

The control circuit unit includes a switch unit for performing phase control on an AC voltage supplied to the lighting load, a switch drive unit for driving the switch unit, and a control unit for controlling the switch drive unit and the control power supply unit.

The control power supply unit is connected to the switch unit in parallel. The control power supply unit converts the AC voltage of the AC power supply into control power. The control power supply unit includes an electrolytic capacitor for storing the control power.

The control unit is supplied with the control power by the control power supply unit via the electrolytic capacitor. The control unit includes a microcomputer. The microcomputer performs an inverse phase control for cutting off, according to the lighting level set by the lighting control operating unit, the supply of the power to the lighting load in the middle of a period set for every half cycle of the AC voltage.

The dimmer of Patent Literature 1 performs cutoff control on the switch unit (bidirectional switch) in the middle of the period set for every half cycle of the AC voltage. That is why cutting off the switch unit during a period with a large current value could have counter electromotive force generated by an inductive component (inductance component) included in the impedance of a line, for example.

CITATION LIST

Patent Literature

Patent Literature 1: JP 2013-149498 A

SUMMARY OF INVENTION

It is therefore an object of the present invention to provide a protection circuit, having the ability to reduce the counter electromotive force generated when a bidirectional switch is cut off, for a dimmer and also provide a dimmer itself.

A protection circuit according to an aspect of the present invention is for use in a dimmer including a pair of input terminals, a bidirectional switch, and a control unit. The pair of input terminals is configured to be electrically connected in series between a lighting load and an AC power supply. The bidirectional switch is configured to selectively cut off or pass a bidirectional current between the pair of input terminals. The control unit is configured to control the bidirectional switch so as to turn the bidirectional switch from ON to OFF at a point in time when an amount of time, varying according to a lighting level, passes since a starting point of a half cycle of an AC voltage of the AC power supply. The bidirectional switch has a control terminal, to which a capacitive element is electrically connected. The bidirectional switch turns from OFF to ON when a voltage between both terminals of the capacitive element becomes equal to or greater than a threshold voltage. The protection circuit includes a charging/discharging regulator circuit. The charging/discharging regulator circuit makes a rate of fall of the voltage between both of the terminals of the capacitive element when the control unit turns the bidirectional switch from ON to OFF lower than a rate of rise of the voltage between both of the terminals of the capacitive element when the control unit turns the bidirectional switch from OFF to ON.

A dimmer according to another aspect of the present invention includes a pair of input terminals, a bidirectional switch, a control unit, and the protection circuit for the dimmer. The pair of input terminals is electrically connected in series between a lighting load and an AC power supply. The bidirectional switch is configured to selectively cut off or pass a bidirectional current between the pair of input terminals. The control unit controls the bidirectional switch so as to turn the bidirectional switch from ON to OFF at a point in time when an amount of time, varying according to a lighting level, passes since a starting point of a half cycle of an AC voltage of the AC power supply.

DESCRIPTION OF EMBODIMENTS

First Embodiment

Note that an exemplary configuration to be described below is only one of various embodiments of the present invention and should not be construed as limiting. Rather, those embodiments may be readily modified in various manners depending on a design choice or any other factor without departing from a true spirit and scope of the invention.

As shown inFIG. 1, the dimmer1of this embodiment includes a pair of input terminals11and12, a bidirectional switch2, a phase detection unit3, an input unit4, a power supply unit5, a control unit6, a switch drive unit7, and a protection circuit8. As used herein, the “input terminals” do not have to be parts (terminals) to which electric wires, cables, or any other members are connected but may also be leads of an electronic part or a portion of a conductor included in a circuit board, for example.

The dimmer1is implemented as a two-wire dimmer and is used as being electrically connected in series to a load9with respect to an AC power supply10. The load9is a lighting load and is lit when supplied with electricity. The load9includes LED elements serving as a light source and a lighting circuit for lighting the LED elements. The AC power supply10may be implemented as a single-phase 100 V, 60 Hz commercial power supply, for example. This dimmer1is applicable to a wall switch, for example.

The bidirectional switch2may be made up of two elements, namely, a switching element Q1and a second switching element Q2, which are electrically connected in series between the input terminals11and12, for example. Each of these switching elements Q1and Q2may be implemented as, for example, a semiconductor switching element such as an enhancement n-channel metal-oxide-semiconductor field-effect transistor (n-MOSFET).

These switching elements Q1and Q2are connected together in a so-called “anti-series” fashion between the input terminals11and12. That is to say, the switching elements Q1and Q2have their respective sources connected together. The drain of the switching element Q1is connected to the input terminal11, and the drain of the switching element Q2is connected to the input terminal12. The respective sources of the switching elements Q1and Q2are connected to the ground of the power supply unit5. The ground of the power supply unit5defines a reference potential for the internal circuit of the dimmer1.

The bidirectional switch2may have its state switched from one of four different states to another according to the combination of ON and OFF states of the switching elements Q1and Q2. Specifically, the four states consist of: a bidirectional OFF state in which the switching elements Q1and Q2are both OFF; a bidirectional ON state in which the switching elements Q1and Q2are both ON; and two unidirectional ON states in each of which only one of the switching elements Q1and Q2is ON. In each of the unidirectional ON states, the elements between the pair of input terminals11and12become electrically conductive unidirectionally via one ON-state switching element and a parasitic diode of the other OFF-state switching element, out of the two switching elements Q1and Q2. For example, in a situation where the switching element Q1is ON and the switching element Q2is OFF, a first unidirectional ON state is established where a current is allowed to flow from the input terminal11toward the input terminal12. On the other hand, in a situation where the switching element Q2is ON and the switching element Q1is OFF, a second unidirectional ON state is established where a current is allowed to flow from the input terminal12toward the input terminal11. Thus, in a situation where an AC voltage Vac is applied from the AC power supply10to between the input terminals11and12, in a half cycle in which the AC voltage Vac is positive (i.e., the input terminal11is positive), the first unidirectional ON state becomes a “forward ON state” and the second unidirectional ON state becomes a “reverse ON state.” On the other hand, in a half cycle in which the AC voltage Vac is negative (i.e., the input terminal12is positive), the second unidirectional ON state becomes a “forward ON state” and the first unidirectional ON state becomes a “reverse ON state.”

In this case, the bidirectional switch2is ON in both of the “bidirectional ON state” and the “forward ON state,” and is OFF in both of the “bidirectional OFF state” and the “reverse ON state.”

The phase detection unit3detects the phase of the AC voltage Vac applied between the input terminals11and12. As used herein, the “phase” includes a zero-crossing point of the AC voltage Vac and the polarity (which is positive or negative) of the AC voltage Vac. The phase detection unit3is configured to output, on detecting a zero-crossing point of the AC voltage Vac, a detection signal to the control unit6. The phase detection unit3includes a diode D31, a first detection unit31, a diode D32, and a second detection unit32. The first detection unit31is electrically connected to the input terminal11via the diode D31. The second detection unit32is electrically connected to the input terminal12via the diode D32. The first detection unit31detects a zero-crossing point when the AC voltage Vac makes a transition from a half cycle with negative polarity to a half cycle with positive polarity. The second detection unit32detects a zero-crossing point when the AC voltage Vac makes a transition from a half cycle with positive polarity to a half cycle with negative polarity.

That is to say, on detecting that the voltage regarding the input terminal11as a positive electrode has made a transition from a state where the voltage is less than a predetermined value to a state where the voltage is equal to or greater than the predetermined value, the first detection unit31determines that a zero-crossing point should have been detected and outputs a first detection signal ZC1to the control unit6. Likewise, on detecting that the voltage regarding the input terminal12as a positive electrode has made a transition from a state where the voltage is less than a predetermined value to a state where the voltage is equal to or greater than the predetermined value, the second detection unit32determines that a zero-crossing point should have been detected and outputs a second detection signal ZC2to the control unit6. The predetermined value is a value (absolute value) set at around 0 V. For example, the first detection unit31may have a predetermined value on the order of a few V, and the second detection unit32may also have a predetermined value on the order of a few V. Therefore, the zero-crossing point detected by each of the first detection unit31and the second detection unit32is slightly delayed with respect to a zero-crossing point (of 0 V) in a strict sense.

The input unit4receives a signal representing a lighting level from an operating unit to be operated by the user and outputs it as a lighting control signal to the control unit6. In outputting the lighting control signal, the input unit4may or may not process the received signal. The lighting control signal may be, for example, a numerical value specifying the magnitude of the optical output of the load9, and may sometimes include an “OFF level” that turns the load9into an extinct state. The operating unit only needs to be configured to output a signal representing the lighting level to the input unit4in response to the user's operation, and may be implemented, for example, as a variable resistor, a rotary switch, a touchscreen panel, a remote controller, or a telecommunications device such as a smartphone.

The control unit6includes a microcomputer, for example. The microcomputer performs the function of the control unit6by having a program stored in a memory of the microcomputer executed by a central processing unit (CPU). The program executed by the CPU may be stored in advance in the memory of the microcomputer, or may be provided by being stored in a storage medium such as a memory card or by being downloaded through a telecommunications line. In other words, the program is designed to make a computer (e.g., the microcomputer in this example) serve as the control unit6.

The control unit6controls the bidirectional switch2in accordance with the detection signals from the phase detection unit3and the lighting control signal from the input unit4. The control unit6controls the switching elements Q1and Q2separately. Specifically, the control unit6controls the switching elements Q1and Q2with a first control signal Sb1and a second control signal Sb2, respectively.

Optionally, the control unit6may include a level shifter. The level shifter transforms a voltage value of the control signal output from the microcomputer into voltage values with which the switching elements Q1and Q2are able to be driven, and outputs the latter voltage values to the switch drive unit7. For example, the microcomputer outputs a control signal with H (High) level (with a voltage of 5 V, for example) to turn the switching elements Q1and Q2ON, and outputs a control signal with L (Low) level (with a voltage of 0 V, for example) to turn the switching elements Q1and Q2OFF. When the microcomputer outputs an H-level control signal, the level shifter transforms a voltage value of this control signal into 12 V, for example, and outputs a first control signal Sb1and a second control signal Sb2, both having an H level, to the switch drive unit7. On the other hand, when the microcomputer outputs an L-level control signal, the level shifter outputs a first control signal Sb1and a second control signal Sb2, both having an L level (with a voltage of 0 V, for example), to the switch drive unit7. In the following description, the first control signal Sb1and second control signal Sb2, of which the signal level is H level, will be hereinafter referred to as “ON” signals, and the first control signal Sb1and second control signal Sb2, of which the signal level is L level, will be hereinafter referred to as “OFF” signals.

The switch drive unit7includes a first drive unit71for driving (i.e., controlling the ON/OFF states of) the switching element Q1and a second drive unit72for driving (i.e., controlling the ON/OFF states of) the switching element Q2.

The first drive unit71includes Zener diodes ZD11and ZD12, resistors R11and R12, a capacitive element (capacitor) C11, and a diode D11. The capacitive element C11and the resistor R11are connected in series between the gate (control terminal) and source of the switching element Q1. In addition, the Zener diode ZD11is also connected between the gate and source of the switching element Q1for the purpose of overvoltage protection. The gate of the switching element Q1is connected to the output terminal P1of the control unit6via the resistor R12. The anode of the diode D11is further connected to the gate of the switching element Q1. The cathode of the Zener diode ZD12is connected to the cathode of the diode D11, and the anode of the Zener diode ZD12is connected to the output terminal P1of the control unit6. The control unit's6outputting a first control signal Sb1at H level from the output terminal P1to the first drive unit71allows a current to flow through the capacitive element C11and charges the capacitive element C11, thus applying a voltage between both terminals of the capacitive element C11to the gate of the switching element Q1. When the voltage between both terminals of the capacitive element C11becomes equal to or greater than the threshold voltage of the switching element Q1, the switching element Q1turns ON. On the other hand, the control unit's6outputting a first control signal Sb1at L level from the output terminal P1to the first drive unit71discharges the capacitive element C11. When the voltage between both terminals of the capacitive element C11becomes less than the threshold voltage of the switching element Q1, the switching element Q1turns OFF. In this manner, the first drive unit71controls the ON/OFF states of the switching element Q1in accordance with the first control signal Sb1from the control unit6. In this case, the first drive unit71generates a gate voltage by reference to the source potential of the switching element Q1.

The second drive unit72includes Zener diodes ZD21and ZD22, resistors R21and R22, a capacitive element (capacitor) C21, and a diode D21. The capacitive element C21and the resistor R21are connected in series between the gate (control terminal) and source of the switching element Q2. In addition, the Zener diode ZD21is also connected between the gate and source of the switching element Q2for the purpose of overvoltage protection. The gate of the switching element Q2is connected to the output terminal P2of the control unit6via the resistor R22. The anode of the diode D21is further connected to the gate of the switching element Q2. The cathode of the Zener diode ZD22is connected to the cathode of the diode D21, and the anode of the Zener diode ZD22is connected to the output terminal P2of the control unit6. The control unit's6outputting a second control signal Sb2at H level from the output terminal P2to the second drive unit72allows a current to flow through the capacitive element C21and charges the capacitive element C21, thus applying a voltage between both terminals of the capacitive element C21to the gate of the switching element Q2. When the voltage between both terminals of the capacitive element C21becomes equal to or greater than the threshold voltage of the switching element Q2, the switching element Q2turns ON. On the other hand, the control unit's6outputting a second control signal Sb2at L level from the output terminal P2to the second drive unit72discharges the capacitive element C21. When the voltage between both terminals of the capacitive element C21becomes less than the threshold voltage of the switching element Q2, the switching element Q2turns OFF. In this manner, the second drive unit72controls the ON/OFF states of the switching element Q2in accordance with the second control signal Sb2from the control unit6. In this case, the second drive unit72generates a gate voltage by reference to the source potential of the switching element Q2.

The protection circuit8is provided to reduce the counter electromotive force generated in the AC voltage Vac when the bidirectional switch2turns from ON to OFF. The protection circuit8of this embodiment includes a first charging/discharging regulator circuit81for reducing the counter electromotive force generated when the switching element Q1is cut off and a second charging/discharging regulator circuit82for reducing the counter electromotive force generated when the switching element Q2is cut off.

The first charging/discharging regulator circuit81includes the resistor R12, diode D11, and Zener diode ZD12of the first drive unit71and an auxiliary charging circuit83. The auxiliary charging circuit83includes a series circuit of a capacitive element (capacitor) C83and a resistor R83, which are connected in series between the input terminal11and the gate of the switching element Q1. Note that the diode D11and the Zener diode ZD12are not essential constituent elements for the first charging/discharging regulator circuit81. In other words, the first charging/discharging regulator circuit81does not have to include diode D11and the Zener diode ZD12.

The second charging/discharging regulator circuit82includes the resistor R22, diode D21, and Zener diode ZD22of the second drive unit72and an auxiliary charging circuit84. The auxiliary charging circuit84includes a series circuit of a capacitive element (capacitor) C84and a resistor R84, which are connected in series between the input terminal12and the gate of the switching element Q2. Note that the diode D21and the Zener diode ZD22are not essential constituent elements for the second charging/discharging regulator circuit82. In other words, the second charging/discharging regulator circuit82does not have to include diode D21and the Zener diode ZD22.

The power supply unit5may include an electrolytic capacitor, for example. The power supply unit5is electrically connected to the input terminal11via a diode D1and also electrically connected to the input terminal12via a diode D2. Also, the ground of the power supply unit5is electrically connected to a connection node where the respective parasitic diodes of the switching elements Q1and Q2are connected together. This allows the AC voltage Vac applied between the input terminals11and12to be full-wave rectified, and supplied to the power supply unit5, by a diode bridge formed by the diodes D1and D2and the respective parasitic diodes of the switching elements Q1and Q2. Therefore, while the bidirectional switch2is OFF, the full-wave rectified AC voltage Vac (i.e., a pulsating voltage output from the diode bridge) is applied to the power supply unit5. The power supply unit5smooths out the full-wave rectified AC voltage Vac and supplies an operating voltage to the phase detection unit3, the control unit6, and the switch drive unit7.

A lighting circuit for the load9detects the lighting level thereof based on the waveform of the AC voltage Vac, of which the phase is controlled by the dimmer1, thus changing the magnitude of the optical output of the LED elements. In this embodiment, the lighting circuit may include, for example, a current securing circuit such as a bleeder circuit. This allows a current to flow through the load9even in a period during which the bidirectional switch2of the dimmer1is electrically unconductive.

(1.2.1) Activation Operation

First of all, it will be described how the dimmer1of this embodiment performs an activation operation when electricity starts to be supplied thereto.

The dimmer1with the configuration described above allows, when the AC power supply10is connected between the input terminals11and12via the load9, the AC voltage Vac applied from the AC power supply10to between the input terminals11and12to be rectified and supplied to the power supply unit5. In the meantime, the power supply unit5supplies an operating voltage to the control unit6and other units, thus activating the control unit6.

When activated, the control unit6determines the frequency of the AC power supply10in accordance with a detection signal supplied from the phase detection unit3. Then, the control unit6refers to a numerical value table, stored in advance in a memory, according to the frequency determined, thus setting time and various other parameters. In this case, if the lighting level input to the input unit4is OFF level, then the control unit6maintains the bidirectional switch2in bidirectionally OFF state, thus keeping the impedance between the pair of input terminals11and12high. This allows the load9to maintain an extinct state.

(1.2.2) Lighting Control Operation

Next, it will be described how the dimmer1of this embodiment performs lighting control operation. Note that in the following description, the phrase “from a point in time A” refers herein to a period that starts from, and does include, the point in time A. For example, the phrase “from a starting point of a half cycle” refers herein to a period including the starting point of the half cycle. Meanwhile, the phrase “to a point in time A” refers herein to a period that terminates immediately before, and does not include, the point in time A. For example, the phrase “to an end point of a half cycle” refers herein to a period that terminates immediately before, and does not include, the end point of the half cycle.

First, it will be described how the dimmer1operates in a half cycle during which the AC voltage Vac is positive.

The dimmer1makes the phase detection unit3detect a zero-crossing point of the AC voltage Vac, which is used as a reference for phase control. When the AC voltage Vac, which is making a transition from a half cycle with negative polarity to a half cycle with positive polarity, reaches a prescribed value with positive polarity, the first detection unit31outputs a first detection signal ZC1to the control unit6.

On receiving the first detection signal ZC1from the first detection unit31in the half cycle during which the AC voltage Vac is positive, the control unit6turns the first control signal Sb1and the second control signal Sb2into “ON” signals.

This allows, in the first drive unit71, a current to flow through the capacitive element C11via the resistor R12to increase the voltage between both terminals of the capacitive element C11. When the voltage between both terminals of the capacitive element C11becomes equal to or greater than a threshold voltage, the switching element Q1turns ON. Likewise, in the second drive unit72, a current is allowed to flow through the capacitive element C21via the resistor R22to increase the voltage between both terminals of the capacitive element C21. When the voltage between both terminals of the capacitive element C21becomes equal to or greater than a threshold voltage, the switching element Q2turns ON. At this time, the switching elements Q1and Q2both turn ON to let the bidirectional switch2enter the bidirectional ON state. Thus, power is supplied from the AC power supply10to the load9via the bidirectional switch2, thus lighting the load9.

Meanwhile, at the end point of the half cycle during which the AC voltage Vac is negative, the bidirectional switch2is controlled to maintain the bidirectional OFF state. Thus, in the interval from the starting point of the half cycle with positive polarity to a point in time when the first detection signal ZC1is input, the control unit6maintains the bidirectional switch2in the bidirectional OFF state.

At a point in time when an ON-state period, of which the duration is determined by the lighting control signal, passes since a point in time when the first detection signal ZC1is input, the control unit6turns the first control signal Sb1into an “OFF” signal while keeping the second control signal Sb2the “ON” signal.

At this time, in the first drive unit71, if the voltage between both terminals of the capacitive element C11is greater than the Zener voltage of the Zener diode ZD12, then the Zener diode ZD12turns ON (i.e., becomes electrically conductive). Since a discharging current flows from the capacitive element C11through the diode D11and the Zener diode ZD12, the voltage between both terminals of the capacitive element C11falls down to the Zener voltage of the Zener diode ZD12in a short time. Once the voltage between both terminals of the capacitive element C11has become lower than the Zener voltage of the Zener diode ZD12, the Zener diode ZD12turns OFF (i.e., becomes electrically unconductive, or no longer allows a current to flow therethrough), and a discharging current flows from the capacitive element C11via the resistor R12. Thereafter, when the voltage between both terminals of the capacitive element C11becomes lower than the threshold voltage, a load current flowing through the switching element Q1decreases.

In this case, the impedance of the electrical path and other components connecting the dimmer1, the AC power supply10, and the load9together includes an inductance component. Thus, when the load current flowing through the switching element Q1decreases, counter electromotive force is generated by the inductance component included in the impedance of the electrical path and other components.FIG. 2Aillustrates the waveforms of the AC voltage Vac of the AC power supply10and a load current I1for a dimmer with no protection circuits8. In this case, the counter electromotive force, generated at a timing when the bidirectional switch2turns from ON to OFF, is superposed on the AC voltage Vac. Also, as the amount of the load current flowing when the switching element Q1turns from ON to OFF increases, the counter electromotive force generated by the inductance component of the electrical path increases.

The dimmer1of this embodiment includes the first charging/discharging regulator circuit81for reducing the counter electromotive force generated when the switching element Q1is cut off. The first charging/discharging regulator circuit81includes the auxiliary charging circuit83between the input terminal11and the gate of the switching element Q1. The auxiliary charging circuit83is configured to allow a charging current to flow from the input terminal11to the capacitive element C11according to the voltage value of the voltage applied to the input terminal11.

Therefore, when counter electromotive force is generated on an electrical path, to which the dimmer1is connected, while the switching element Q2is turning from ON to OFF, a current flows from the input terminal11to the capacitive element C11via the capacitive element C83and resistor R83of the auxiliary charging circuit83. This makes the rate of fall of the voltage between both terminals of the capacitive element C11lower than in a situation where no charging current flows through the auxiliary charging circuit83to the capacitive element C11. This slows down the rate of decrease in the amount of the current flowing through the switching element Q1. Therefore, this slows down the rate of decrease in the amount of the load current I1flowing through the switching element Q1, thus reducing the counter electromotive force to be generated on the electrical path.FIG. 2Billustrates the waveforms of the AC voltage Vac of the AC power supply10and the load current I1for the dimmer1according to this embodiment. As can be seen, the counter electromotive force to be generated on the electrical path when the switching element Q2turns from ON to OFF has decreased. In this case, the time constant of the series circuit of the capacitive element C83and the resistor R83(hereinafter referred to as a “CR series circuit”) that form the auxiliary charging circuit83is set at such a value as to allow the counter electromotive force, generated on the electrical path, to be reduced to the extent that the circuit components are not affected adversely by the counter electromotive force.

Thereafter, the voltage between both terminals of the capacitive element C11falls gradually and the load current flowing through the switching element Q1decreases to turn the switching element Q1OFF eventually. At this time, only the switching element Q1turns OFF, out of the switching elements Q1and Q2, to let the bidirectional switch2enter a reverse ON state. This cuts off the supply of power from the AC power supply10to the load9.

Also, at a point in time when a half cycle minus a certain amount of time passes from the point in time of generation of the first detection signal ZC1, the control unit6turns the first control signal Sb1and the second control signal Sb2into “OFF” signals. The point in time to turn the first control signal Sb1and the second control signal Sb2into “OFF” signals only needs to be before the end point of the half cycle (zero-crossing point), and the time length (corresponding to the certain amount of time) through the end point of the half cycle may be set appropriately.

At this time, in the second drive unit72, if the voltage between both terminals of the capacitive element C21is greater than the Zener voltage of the Zener diode ZD22, then the Zener diode ZD22turns ON. Since a discharging current flows from the capacitive element C21through the diode D21and the Zener diode ZD22, the voltage between both terminals of the capacitive element C21falls down to the Zener voltage of the Zener diode ZD22in a short time. Once the voltage between both terminals of the capacitive element C21has become lower than the Zener voltage of the Zener diode ZD22, the Zener diode ZD22turns OFF, and a discharging current flows from the capacitive element C21via the resistor R22. Thereafter, when the voltage between both terminals of the capacitive element C21becomes lower than the threshold voltage, the switching element Q2turns OFF. That is to say, the switching elements Q1and Q2both turn OFF to let the bidirectional switch2enter the bidirectional OFF state.

In the half cycle during which the AC voltage Vac is negative, the dimmer1operates basically in the same way as in the half cycle during which the AC voltage Vac is positive.

In the half cycle with negative polarity, when the AC voltage Vac reaches a prescribed value with negative polarity, the second detection unit32outputs a second detection signal ZC2to the control unit6.

On receiving the second detection signal ZC2from the second detection unit32in the half cycle during which the AC voltage Vac is negative, the control unit6turns the first control signal Sb1and the second control signal Sb2into “ON” signals.

This allows, in the first drive unit71, a current to flow through the capacitive element C11via the resistor R12to increase the voltage between both terminals of the capacitive element C11. When the voltage between both terminals of the capacitive element C11becomes equal to or greater than a threshold voltage, the switching element Q1turns ON. Likewise, in the second drive unit72, a current is allowed to flow through the capacitive element C21via the resistor R22to increase the voltage between both terminals of the capacitive element C21. When the voltage between both terminals of the capacitive element C21becomes equal to or greater than a threshold voltage, the switching element Q2turns ON. At this time, the switching elements Q1and Q2both turn ON to let the bidirectional switch2enter the bidirectional ON state. Thus, power is supplied from the AC power supply10to the load9via the bidirectional switch2, thus lighting the load9.

Meanwhile, at the end point of the half cycle during which the AC voltage Vac is positive, the bidirectional switch2is controlled to maintain the bidirectional OFF state. Thus, in the interval from the starting point of the half cycle with negative polarity to a point in time when the second detection signal ZC2is input, the control unit6maintains the bidirectional switch2in the bidirectional OFF state.

Thereafter, at a point in time when an ON-state period, of which the duration is determined by the lighting control signal, passes since a point in time when the second detection signal ZC2is input in the half cycle during which the AC voltage Vac is negative, the control unit6turns the second control signal Sb2into an “OFF” signal while keeping the first control signal Sb1the “ON” signal.

Thus, in the second drive unit72, a discharging current flows from the capacitive element C21via the resistor R22. When the voltage between both terminals of the capacitive element C21becomes lower than the threshold voltage, the load current flowing through the switching element Q2decreases.

In this case, when the load current flowing through the switching element Q2decreases, counter electromotive force is generated by, for example, the inductance component included in the impedance of the electrical path and other components that connect the dimmer1, the AC power supply10, and the load9together. As the amount of the load current flowing when the switching element Q2turns from ON to OFF increases, the counter electromotive force generated by the inductance component of the electrical path increases.

The dimmer1of this embodiment includes the second charging/discharging regulator circuit82for reducing the counter electromotive force to be generated when the switching element Q2is cut off. The second charging/discharging regulator circuit82includes the auxiliary charging circuit84between the input terminal12and the gate of the switching element Q2. The auxiliary charging circuit84is configured to allow a charging current to flow from the input terminal12to the capacitive element C21according to the voltage value of the voltage applied to the input terminal12.

Therefore, when counter electromotive force is generated as a surge voltage on an electrical path, to which the dimmer1is connected, while the switching element Q2is turning from ON to OFF, a current flows from the input terminal12to the capacitive element C21via the capacitive element C84and resistor R84of the auxiliary charging circuit84. This slows down the rate of fall of the voltage between both terminals of the capacitive element C21, and therefore, slows down the rate of decrease in the amount of the load current flowing through the switching element Q2. This reduces the counter electromotive force to be generated on the electrical path. In this case, the time constant of the series circuit of the capacitive element C84and the resistor R84(hereinafter referred to as a “CR series circuit”) that form the auxiliary charging circuit84is set at such a value as to allow the counter electromotive force, generated on the electrical path, to be reduced to the point that the circuit components are not affected adversely.

Thereafter, the voltage between both terminals of the capacitive element C21falls gradually and the load current flowing through the switching element Q2decreases to turn the switching element Q2OFF eventually. At this time, only the switching element Q2turns OFF, out of the switching elements Q1and Q2, to let the bidirectional switch2enter a reverse ON state. This cuts off the supply of power from the AC power supply10to the load9.

Also, at a point in time when a half cycle minus a certain amount of time passes since the point in time of generation of the second detection signal ZC2in the half cycle during which the AC voltage Vac is negative, the control unit6turns the first control signal Sb1and the second control signal Sb2into “OFF” signals.

At this time, in the first drive unit71, if the voltage between both terminals of the capacitive element C11is greater than the Zener voltage of the Zener diode ZD12, then the Zener diode ZD12turns ON. Since a discharging current flows from the capacitive element C11through the diode D11and the Zener diode ZD12, the voltage between both terminals of the capacitive element C11falls down to the Zener voltage of the Zener diode ZD12in a short time. Once the voltage between both terminals of the capacitive element C11has become lower than the Zener voltage of the Zener diode ZD12, the Zener diode ZD12turns OFF, and a discharging current flows from the capacitive element C11via the resistor R12. Thereafter, when the voltage between both terminals of the capacitive element C11becomes lower than the threshold voltage, the switching element Q1turns OFF. That is to say, the switching elements Q1and Q2both turn OFF to let the bidirectional switch2enter the bidirectional OFF state.

The dimmer1of this embodiment alternately and repeatedly performs the operation for the half cycle with positive polarity and the operation for the half cycle with negative polarity every half cycle of the AC voltage Vac, thereby controlling the amount of light emitted by the load9.

As used herein, the “bidirectional ON state” refers to the ON state of the bidirectional switch2, and the “reverse ON state” refers to the OFF state of the bidirectional switch2. Also, when a variable amount of time passes since the starting point of a half cycle of the AC voltage Vac (i.e., at a switching point), the bidirectional switch2turns from ON to OFF.

As used herein, the “variable amount of time” corresponds to an interval from the starting point of a half cycle to a point in time when an ON-state period, of which the duration is determined by the lighting control signal, has passed since the generation of the first detection signal ZC1and the second detection signal ZC2. That is to say, the “variable amount of time” is the sum of the interval from the starting point of the half cycle to the point in time of generation of the first detection signal ZC1and the second detection signal ZC2and the ON state period with the duration determined by the lighting control signal. Thus, the variable amount of time has a length varying according to the lighting level, and the phase of the switching point with respect to the AC voltage Vac also varies according to the lighting level. That is to say, to decrease the optical output of the load9, the variable amount of time is defined to be a shorter amount of time. On the other hand, to increase the optical output of the load9, the variable amount of time is defined to be a longer amount of time. This allows the magnitude of the optical output of the load9to be adjusted according to the lighting level provided for the input unit4.

A protection circuit8according to a first aspect is designed for use in a dimmer1. The dimmer1includes a pair of input terminals11and12configured to be electrically connected in series between a lighting load (load9) and an AC power supply10, a bidirectional switch2, and a control unit6. The bidirectional switch2is configured to selectively cut off or pass a bidirectional current between the pair of input terminals11and12. The control unit6controls the bidirectional switch2so as to turn the bidirectional switch2from ON to OFF at a point in time when an amount of time, varying according to a lighting level, passes since a starting point of a half cycle of an AC voltage Vac of the AC power supply10. A capacitive element C11, C21is electrically connected to a control terminal of the bidirectional switch2(the gate of the switching element Q1, Q2). The bidirectional switch2turns from OFF to ON when a voltage between both terminals of the capacitive element C11, C21becomes equal to or greater than a threshold voltage. The protection circuit8includes a charging/discharging regulator circuit (the first charging/discharging regulator circuit81or the second charging/discharging regulator circuit82). The charging/discharging regulator circuit makes a rate of fall of the voltage between both of the terminals of the capacitive element C11, C21when the control unit6turns the bidirectional switch2from ON to OFF lower than a rate of rise of the voltage between both of the terminals of the capacitive element C11, C21when the control unit6turns the bidirectional switch2from OFF to ON.

In the dimmer1, when the bidirectional switch2turns from ON to OFF, counter electromotive force could be generated on an electrical path, to which the dimmer1is connected, by an inductance component such as the electrical path. The protection circuit8of this embodiment slows down the rate of the bidirectional switch2turning ON to OFF, compared to a configuration in which the dimmer1includes no protection circuits8. This reduces the counter electromotive force to be generated on an electrical path connected to the input terminals11and12when the bidirectional switch2turns from ON to OFF.

The dimmer1of this embodiment includes the pair of input terminals11and12, the bidirectional switch2, the control unit6, and the protection circuit8. This reduces the counter electromotive force to be generated on the electrical path connected to the input terminals11and12when the bidirectional switch2turns from ON to OFF.

In a protection circuit8for a dimmer1according to a second aspect, which may be implemented in conjunction with the first aspect, the charging/discharging regulator circuit (the first charging/discharging regulator circuit81or the second charging/discharging regulator circuit82) includes an auxiliary charging circuit83,84configured to allow a charging current to flow from the pair of input terminals11and12to the capacitive element C11, C21. The auxiliary charging circuit83,84changes a current value of the charging current according to a voltage value of a voltage applied between the pair of input terminals11and12. If counter electromotive force is generated on the electrical path when the bidirectional switch2turns from ON to OFF, the amount of the charging current allowed by the auxiliary charging circuit83,84to flow through the capacitive element C11, C12increases. This slows down the rate of fall of the voltage between both terminals of the capacitive element C11, C12, thus reducing the counter electromotive force to be generated on the electrical path connected to the input terminals11and12when the bidirectional switch2turns from ON to OFF. In this case, as the voltage value of the voltage applied between the pair of input terminals11and12increases, the auxiliary charging circuit83,84may increase the current value of the charging current. If necessary, the auxiliary charging circuit83,84may change the current value of the charging current within a range including zero.

In a protection circuit8for a dimmer1according to a third aspect, which may be implemented in conjunction with the second aspect, the auxiliary charging circuit83,84includes a series circuit of another capacitive element C83, C84and a resistor R83, R84. This allows a rate of the bidirectional switch2turning from ON to OFF to be adjusted with a time constant set by the capacitive element C83, C84and the resistor R83, R84.

In a protection circuit8for a dimmer1according to a fourth aspect, which may be implemented in conjunction with the second or third aspect, the auxiliary charging circuit83,84includes a passive component (the capacitive element C83, C84and the resistor R83, R84). Implementing the auxiliary charging circuit83,84as at least one passive component eliminates the need for providing any power supply for operating the auxiliary charging circuit83,84or a circuit for controlling the operation of the auxiliary charging circuit83,84.

Next, variations of the embodiment described above will be enumerated one after another. Note that any of various configurations of the variations to be described below may be adopted as appropriate in combination with any of the configurations described for the embodiment.

The protection circuit8of the embodiment described above includes the auxiliary charging circuits83and84, each of which is implemented as a CR series circuit. However, this is only an example and should not be construed as limiting. The auxiliary charging circuits83and84do not have to be implemented as CR series circuits. Alternatively, the auxiliary charging circuits83and84may each consist of a capacitive element. Still alternatively, the auxiliary charging circuits83and84may each include a resistor connected between the input terminal11,12and the capacitive element C11, C12, and a Zener diode connected to the resistor in series such that a current flows in the opposite direction from the input terminal to the capacitive element. In that case, only when the voltage applied to the input terminal11,12is greater than the voltage between both terminals of the Zener diode, the charging current is allowed to flow through the capacitive element C11, C12. Yet alternatively, the auxiliary charging circuits83and84may each consist of a Zener diode connected between the input terminal11,12and the capacitive element C11, C12such that a current flows in the opposite direction from the input terminal11,12to the capacitive element C11, C12. Yet alternatively, the auxiliary charging circuits83and84may each include a diode connected between the input terminal11,12and the capacitive element C11, C12in the direction in which a current flows from the input terminal11,12to the capacitive element C11, C12, and a Zener diode connected to the diode in series in the opposite direction from the diode. Yet alternatively, the auxiliary charging circuits83,84may each include a resistor and a diode connected to the resistor in series in the direction in which a current flows from the input terminal11,12to the capacitive element C11, C12or may consist of the resistor.

The dimmer1of the embodiment described above does not have to be applied to such a load9using LED elements as a light source but is also applicable to a light source including a capacitor input type circuit, having high impedance, and lighting with a small amount of current supplied. Examples of this type of light sources include an organic electroluminescent (EL) element. The dimmer is also applicable to a discharge lamp and various other types of loads9as well.

Also, the control unit6does not have to control the bidirectional switch2by the exemplary method described above. Alternatively, a method of alternately turning the first control signal or the second control signal into an “ON” signal at the same intervals as the AC voltage Vac may also be adopted. In that case, while one, having the higher potential with respect to the AC voltage Vac, of the switching elements Q1and Q2is ON, the bidirectional switch2becomes electrically conductive. That is to say, according to this variation, so-called “reverse phase control,” in which the components between the pair of input terminals11and12become electrically conductive during an interval from the zero-crossing point of the AC voltage Vac to a middle of a half cycle, is realized. In that case, adjusting the phase difference between the first control signal and the second control signal and the AC voltage Vac allows the electrically conductive period of the bidirectional switch2to be controlled.

Furthermore, the switching elements Q1and Q2that form the bidirectional switch2do not have to be implemented as enhancement n-channel MOSFETs but may also be implemented as insulated gate bipolar transistors (IGBTs) as well. Furthermore, in the bidirectional switch2, the rectifiers (diodes) that realize the unidirectional ON state do not have to be parasitic diodes of the switching elements Q1and Q2but may also be external diodes. Each of the diodes and an associated one of the switching elements Q1and Q2may be built in the same package. Furthermore, the diodes D1and D2of the embodiment described above are not essential constituent elements for the dimmer1but may be omitted as appropriate.

Furthermore, the bidirectional switch2is made up of the two switching elements Q1and Q2, the respective sources of which are connected together. Alternatively, the bidirectional switch may also be implemented as a single switching element configured to selectively cut off or pass bidirectional currents. Examples of this type of switching elements include a switching element with a double gate structure. That is to say, the bidirectional switch may also be implemented as a semiconductor device having a double gate (or dual gate) structure and made of a wide-bandgap semiconductor material such as GaN (gallium nitride).

Also, the protection circuit8of this embodiment is for use in the dimmer1including the pair of input terminals11and12, the bidirectional switch2, and the control unit6. In the embodiment described above, the protection circuit8is provided inside of the dimmer1. However, this is only an example and should not be construed as limiting. Alternatively, the protection circuit8may also be provided outside of the dimmer1. That is to say, the protection circuit8does not always include the pair of input terminals11and12, the bidirectional switch2, and the control unit6as essential constituent elements.

Second Embodiment

Next, a dimmer1A according to a second embodiment will be described with reference toFIG. 3.

The dimmer1A of the second embodiment includes a protection circuit8A with a different configuration from the protection circuit8of the first embodiment, which is a major difference between the first and second embodiments. In the following description, any constituent member of the dimmer1A of the second embodiment, having the same function as a counterpart of the dimmer1of the first embodiment described above, will be designated by the same reference numeral as that counterpart's, and a detailed description thereof will be omitted herein.

The protection circuit8A of this embodiment includes a first charging/discharging regulator circuit81A for reducing counter electromotive force to be generated when the switching element Q1is cut off and a second charging/discharging regulator circuit82A for reducing counter electromotive force to be generated when the switching element Q2is cut off.

The first charging/discharging regulator circuit81A includes the resistor R12, a diode D85, and a resistor R85, which form parts of the first drive unit71. The resistor R12is connected between a connection node, where the gate of the switching element Q1and the capacitive element C11are connected together, and the output terminal P1of the control unit6. The series circuit of the diode D85and the resistor R85is connected between both terminals of the resistor R12. The diode D85is connected in such a direction that a current flows from the output terminal P1of the control unit6to the capacitive element C11. Thus, in the first charging/discharging regulator circuit81A, a charging circuit allowing a charging current to flow from the control unit6toward the capacitive element C11is implemented as a circuit in which the resistor R12and the series circuit of the diode D85and the resistor R85are connected together in parallel. On the other hand, in the first charging/discharging regulator circuit81A, a discharging circuit allowing a discharging current to flow from the capacitive element C11is implemented as a resistor R12. Thus, the discharging circuit allowing the discharging current to flow from the capacitive element C11comes to have a greater impedance than the charging circuit allowing the charging current to flow toward the capacitive element C11. That is to say, the rate of fall of the voltage between both terminals of the capacitive element C11when the control unit6turns the switching element Q1from ON to OFF becomes lower than the rate of rise of the voltage between both terminals of the capacitive element C11when the control unit6turns the switching element Q1from OH to ON. In this embodiment, the resistance value of the resistor R21and the resistance value of the resistor R85are both 22 kΩ, and the impedance of the discharging circuit is approximately twice as high as the impedance of the charging circuit. Note that the resistance value of the resistor R21and resistor R85is only an example and should not be construed as limiting, but may be changed as appropriate.

The second charging/discharging regulator circuit82A includes the resistor R22, a diode D86, and a resistor R86, which form parts of the second drive unit72. The resistor R22is connected between a connection node, where the gate of the switching element Q2and the capacitive element C21are connected together, and the output terminal P2of the control unit6. The series circuit of the diode D86and the resistor R86is connected between both terminals of the resistor R22. The diode D86is connected in such a direction that a current flows from the output terminal P2of the control unit6to the capacitive element C21. Thus, in the second charging/discharging regulator circuit82A, a charging circuit allowing a charging current to flow from the control unit6toward the capacitive element C21is implemented as a circuit in which the resistor R22and the series circuit of the diode D86and the resistor R86are connected together in parallel. On the other hand, in the second charging/discharging regulator circuit82A, a discharging circuit allowing a discharging current to flow from the capacitive element C21is implemented as a resistor R22. Thus, the discharging circuit allowing the discharging current to flow from the capacitive element C21comes to have a greater impedance than the charging circuit allowing the charging current to flow toward the capacitive element C21. That is to say, the rate of fall of the voltage between both terminals of the capacitive element C21when the control unit6turns the switching element Q2from ON to OFF becomes lower than the rate of rise of the voltage between both terminals of the capacitive element C21when the control unit6turns the switching element Q2from OFF to ON. In this embodiment, the resistance value of the resistor R22and the resistance value of the resistor R86are both 22 kΩ2, and the impedance of the discharging circuit is approximately twice as high as the impedance of the charging circuit. Note that the resistance value of the resistor R22and resistor R86is only an example and should not be construed as limiting, but may be changed as appropriate.

Next, it will be described how the dimmer1A of this embodiment performs a lighting control operation.

First, it will be described how the dimmer1operates in a half cycle during which the AC voltage Vac is positive.

At the end point of the half cycle during which the AC voltage Vac is negative, the bidirectional switch2is controlled to maintain the bidirectional OFF state. Thus, in the interval from the starting point of the half cycle with positive polarity to a point in time when the first detection signal ZC1is input, the control unit6maintains the bidirectional switch2in the bidirectional OFF state.

When the AC voltage Vac reaches a positive prescribed value in the half cycle during which the AC voltage Vac is positive, the first detection unit31outputs the first detection signal ZC1to the control unit6.

On receiving the first detection signal ZC1from the first detection unit31in the half cycle during which the AC voltage Vac is positive, the control unit6turns the first control signal Sb1and the second control signal Sb2into “ON” signals.

This allows, in the first drive unit71, a current to flow through the capacitive element C11via the circuit in which the resistor R12and the series circuit of the resistor R85and the diode D85are connected together in parallel to increase the voltage between both terminals of the capacitive element C11. When the voltage between both terminals of the capacitive element C11becomes equal to or greater than a threshold voltage, the switching element Q1turns ON. Likewise, in the second drive unit72, a current is allowed to flow through the capacitive element C21via the circuit in which the resistor R22and the series circuit of the resistor R86and the diode D86are connected together in parallel to increase the voltage between both terminals of the capacitive element C21. When the voltage between both terminals of the capacitive element C21becomes equal to or greater than a threshold voltage, the switching element Q2turns ON. At this time, the switching elements Q1and Q2both turn ON to let the bidirectional switch2enter the bidirectional ON state. Thus, power is supplied from the AC power supply10to the load9via the bidirectional switch2, thus lighting the load9.

At a point in time when an ON-state period, of which the duration is determined by the lighting control signal, passes since a point in time when the first detection signal ZC1is input, the control unit6turns the first control signal Sb1into an “OFF” signal while keeping the second control signal Sb2the “ON” signal.

At this time, in the first drive unit71, if the voltage between both terminals of the capacitive element C11is greater than the Zener voltage of the Zener diode ZD12, then the Zener diode ZD12turns ON. Since a discharging current flows from the capacitive element C11through the diode D11and the Zener diode ZD12, the voltage between both terminals of the capacitive element C11falls down to the Zener voltage of the Zener diode ZD12in a short time. Once the voltage between both terminals of the capacitive element C11has become lower than the Zener voltage of the Zener diode ZD12, the Zener diode ZD12turns OFF, and a discharging current flows from the capacitive element C11via the resistor R12. Thereafter, when the voltage between both terminals of the capacitive element C11becomes lower than the threshold voltage, the load current flowing through the switching element Q1decreases.

In this case, when the load current flowing through the switching element Q1decreases, counter electromotive force is generated by the inductance component included in the impedance of the electrical path connecting the dimmer1, the AC power supply10, and the load9together and other components. As the amount of the load current flowing when the switching element Q1turns from ON to OFF increases, the counter electromotive force generated by the inductance component of the electrical path increases.

In this embodiment, the discharging circuit allowing a discharging current to flow from the capacitive element C11when the switching element Q1is turned from ON to OFF has a greater impedance than the charging circuit allowing a charging current to flow toward the capacitive element C11when the switching element Q1is turned from OFF to ON. Therefore, the rate of fall of the voltage between both terminals of the capacitive element C11becomes lower than in a situation where the impedance of the charging circuit is equal to the impedance of the discharging circuit. This slows down the rate of decrease in the amount of the load current flowing through the switching element Q1, thus reducing the counter electromotive force to be generated on the electrical path.

Thereafter, the voltage between both terminals of the capacitive element C11falls gradually and the load current flowing through the switching element Q1decreases to turn the switching element Q1OFF eventually. At this time, only the switching element Q1turns OFF, out of the switching elements Q1and Q2, to let the bidirectional switch2enter a reverse ON state. This cuts off the supply of power from the AC power supply10to the load9.

Also, at a point in time when a half cycle minus a certain amount of time passes since the point in time of generation of the first detection signal ZC1, the control unit6turns the first control signal Sb1and the second control signal Sb2into “OFF” signals.

At this time, in the second drive unit72, if the voltage between both terminals of the capacitive element C21is greater than the Zener voltage of the Zener diode ZD22, then the Zener diode ZD22turns ON. Since a discharging current flows from the capacitive element C21through the diode D21and the Zener diode ZD22, the voltage between both terminals of the capacitive element C21falls down to the Zener voltage of the Zener diode ZD22in a short time. Once the voltage between both terminals of the capacitive element C21has become lower than the Zener voltage of the Zener diode ZD22, the Zener diode ZD22turns OFF, and a discharging current flows from the capacitive element C21via the resistor R22. Thereafter, when the voltage between both terminals of the capacitive element C21becomes lower than the threshold voltage, the switching element Q2turns OFF. That is to say, the switching elements Q1and Q2both turn OFF to let the bidirectional switch2enter the bidirectional OFF state.

In the half cycle during which the AC voltage Vac is negative, the dimmer1operates basically in the same way as in the half cycle during which the AC voltage Vac is positive.

At the end point of the half cycle during which the AC voltage Vac is positive, the bidirectional switch2is controlled to maintain the bidirectional OFF state. Thus, in the interval from the starting point of the half cycle with negative polarity to a point in time when the second detection signal ZC2is input, the control unit6maintains the bidirectional switch2in the bidirectional OFF state.

When the AC voltage Vac reaches a negative prescribed value in the half cycle during which the AC voltage Vac is negative, the second detection unit32outputs the second detection signal ZC2to the control unit6.

On receiving the second detection signal ZC2from the second detection unit32in the half cycle during which the AC voltage Vac is negative, the control unit6turns the first control signal Sb1and the second control signal Sb2into “ON” signals.

This allows, in the first drive unit71, a current to flow through the capacitive element C11via the circuit in which the resistor R12and the series circuit of the resistor R85and the diode D85are connected together in parallel to increase the voltage between both terminals of the capacitive element C11. When the voltage between both terminals of the capacitive element C11becomes equal to or greater than a threshold voltage, the switching element Q1turns ON. Likewise, in the second drive unit72, a current is allowed to flow through the capacitive element C21via the circuit in which the resistor R22and the series circuit of the resistor R86and the diode D86are connected together in parallel to increase the voltage between both terminals of the capacitive element C21. When the voltage between both terminals of the capacitive element C21becomes equal to or greater than a threshold voltage, the switching element Q2turns ON. At this time, the switching elements Q1and Q2both turn ON to let the bidirectional switch2enter the bidirectional ON state. Thus, power is supplied from the AC power supply10to the load9via the bidirectional switch2, thus lighting the load9.

Thereafter, at a point in time when an ON-state period, of which the duration is determined by the lighting control signal, passes since a point in time when the second detection signal ZC2is input in the half cycle during which the AC voltage Vac is negative, the control unit6turns the second control signal Sb2into an “OFF” signal while keeping the first control signal Sb1the “ON” signal.

At this time, in the second drive unit72, if the voltage between both terminals of the capacitive element C21is greater than the Zener voltage of the Zener diode ZD22, then the Zener diode ZD22turns ON. Since a discharging current flows from the capacitive element C21through the diode D21and the Zener diode ZD22, the voltage between both terminals of the capacitive element C21falls down to the Zener voltage of the Zener diode ZD22in a short time. Once the voltage between both terminals of the capacitive element C21has become lower than the Zener voltage of the Zener diode ZD22, the Zener diode ZD22turns OFF, and a discharging current flows from the capacitive element C21via the resistor R22. Thereafter, when the voltage between both terminals of the capacitive element C21becomes lower than the threshold voltage, the load current flowing through the switching element Q2decreases.

In this case, when the load current flowing through the switching element Q2decreases, counter electromotive force is generated by the inductance component included in the impedance of the electrical path connecting the dimmer1, the AC power supply10, and the load9together and other components. As the amount of the load current flowing when the switching element Q2turns from ON to OFF increases, the counter electromotive force generated by the inductance component of the electrical path increases.

In the second charging/discharging regulator circuit82, the discharging circuit allowing a discharging current to flow from the capacitive element C21when the switching element Q2is turned from ON to OFF has a greater impedance than the charging circuit allowing a charging current to flow toward the capacitive element C21when the switching element Q2is turned from OFF to ON. Therefore, the rate of fall of the voltage between both terminals of the capacitive element C21becomes lower than in a situation where the impedance of the charging circuit is equal to the impedance of the discharging circuit. This slows down the rate of decrease in the amount of the load current flowing through the switching element Q2, thus reducing the counter electromotive force to be generated on the electrical path.

Thereafter, the voltage between both terminals of the capacitive element C21falls gradually and the load current flowing through the switching element Q2decreases to turn the switching element Q2OFF eventually. At this time, only the switching element Q2turns OFF, out of the switching elements Q1and Q2, to let the bidirectional switch2enter a reverse ON state. This cuts off the supply of power from the AC power supply10to the load9.

Also, at a point in time when a half cycle minus a certain amount of time passes since the point in time of generation of the second detection signal ZC2in the half cycle during which the AC voltage Vac is negative, the control unit6turns the first control signal Sb1and the second control signal Sb2into “OFF” signals.

At this time, in the first drive unit71, if the voltage between both terminals of the capacitive element C11is greater than the Zener voltage of the Zener diode ZD12, then the Zener diode ZD12turns ON. Since a discharging current flows from the capacitive element C11through the diode D11and the Zener diode ZD12, the voltage between both terminals of the capacitive element C11falls down to the Zener voltage of the Zener diode ZD12in a short time. Once the voltage between both terminals of the capacitive element C11has become lower than the Zener voltage of the Zener diode ZD12, the Zener diode ZD12turns OFF, and a discharging current flows from the capacitive element C11via the resistor R12. Thereafter, when the voltage between both terminals of the capacitive element C11becomes lower than the threshold voltage, the switching element Q1turns OFF. That is to say, the switching elements Q1and Q2both turn OFF to let the bidirectional switch2enter the bidirectional OFF state.

The dimmer1of this embodiment alternately and repeatedly performs the operation for the half cycle with positive polarity and the operation for the half cycle with negative polarity every half cycle of the AC voltage Vac, thereby controlling the lighting level of the load9.

In a protection circuit8A for a dimmer1according to a fifth aspect, which may be implemented in conjunction with the first aspect, the charging/discharging regulator circuit (the first charging/discharging regulator circuit81A or the second charging/discharging regulator circuit82A) includes: a charging circuit configured to allow a charging current to flow therethrough toward the capacitive element C11, C12; and a discharging circuit configured to allow a discharging current to flow therethrough from the capacitive element C11, C12. The discharging circuit has a greater impedance than the charging circuit. The discharging circuit's having greater impedance than the charging circuit decreases the amount of a discharging current flowing from the capacitive element C11, C12compared to a situation where the impedance of the charging circuit is equal to that of the discharging circuit, thus slowing down the rate of fall of the voltage between both terminals of the capacitive element C11, C12. This reduces the counter electromotive force to be generated when the bidirectional switch2turns from ON to OFF.

In a protection circuit8for a dimmer1according to a sixth aspect, which may be implemented in conjunction with the fifth aspect, the charging/discharging regulator circuit includes a first impedance element (the resistor R12, R22), a diode (D85, D86), and a second impedance element (the resistor R85, R86). The first impedance element is electrically connected between a connection node, where the control terminal of the bidirectional switch2and the capacitive element (C11, C21) are connected together, and an output terminal (P1, P2) of the control unit6. The series circuit of the diode and the second impedance element is electrically connected between both terminals of the first impedance element. The diode is connected in such a direction that a charging current is allowed to flow toward the capacitive element (C11, C12) via the diode itself and the second impedance element. The charging circuit includes a parallel circuit in which the first impedance element and the series circuit of the diode and the second impedance element are connected together in parallel. The discharging circuit includes the first impedance element. This allows the discharging circuit to have a greater impedance than the charging circuit, thus reducing the amount of the discharging current flowing from the capacitive element C11, C21and slowing down the rate of fall of the voltage between both terminals of the capacitive element C11, C21. This reduces the counter electromotive force to be generated when the bidirectional switch2turns from ON to OFF.

Note that the charging/discharging regulator circuit (the first charging/discharging regulator circuit81A or the second charging/discharging regulator circuit82A) does not have to have the circuit configuration described for the exemplary embodiment. Rather, the circuit configuration of the charging/discharging regulator circuit may be modified as appropriate as long as the circuit configuration allows the discharging circuit in which the discharging current flows from the capacitive element C11, C21to have a greater impedance than the charging circuit in which the charging current flows toward the capacitive element C11, C21. In the embodiment described above, the first impedance element and the second impedance element are each implemented as a single resistor. However, this is only an example and should not be construed as limiting. Alternatively, each of the first and second impedance elements may also be implemented as a plurality of resistor connected together in series or in parallel. Also, in the embodiment described above, the diode (D85, D86) is connected between the second impedance element (R85, R86) and the capacitive element (C11, C21). Alternatively, the diode may also be connected between the second impedance element and the output terminal (P1, P2) of the control unit6.

The configuration described for the second embodiment may be employed as appropriate in combination with the configuration described for the first embodiment (including variations thereof).

A dimmer1according to a seventh aspect includes a pair of input terminals11and12, a bidirectional switch2, a control unit6, and a protection circuit8according to any one of the first to sixth aspects. The pair of input terminals11and12is configured to be electrically connected in series between a lighting load9and an AC power supply10. The bidirectional switch2is configured to selectively cut off or pass a bidirectional current between the pair of input terminals11and12. The control unit6is configured to control the bidirectional switch2so as to turn the bidirectional switch2from ON to OFF at a point in time when an amount of time, varying according to a lighting level, passes since a starting point of a half cycle of an AC voltage of the AC power supply10.

In other words, the dimmer1of the embodiment described above includes a switch unit (bidirectional switch2), a synch signal generation unit (phase detection unit3), a control power supply unit (power supply unit5), and a control unit6, and further includes the protection circuit8,8A described above. The switch unit is connected to the load9in series with respect to the AC power supply10to perform phase control on the AC voltage Vac applied to the load9. The synch signal generation unit generates a synch signal (the first detection signal ZC1or the second detection signal ZC2) synchronized with an AC voltage waveform of the AC power supply10. The control power supply unit includes a capacitive element, which is connected to the switch unit in parallel, converts the AC power supply10into a predetermined control power supply, makes the start and stop of the conversion operation controllable, and stores the control power. The control unit6is supplied with the control power by the control power supply unit via the capacitive element, and divides the period of each half cycle of the AC voltage Vac into three intervals in accordance with the synch signal generated by the synch signal generation unit. In the first interval (i.e., an interval from the starting point of each half cycle to a point in time when the first detection signal ZC1or the second detection signal ZC2is input), the control unit6makes the switch unit electrically unconductive to cut off the supply of power to the load9and activate the conversion operation of the control power supply unit. In the second interval (i.e., an interval from the point in time when the first detection signal ZC1or the second detection signal ZC2is input to a point in time when the ON period passes), the control unit6makes the switch unit electrically conductive to supply power to the load9and deactivate the operation of the control power supply unit. In the third interval (i.e., an interval from the point in time when the ON period passes since when the first detection signal ZC1or the second detection signal ZC2was input through the end point of each half cycle), the control unit6makes the switch unit electrically unconductive to cut off the supply of power to the load9and activate the conversion operation of the control power supply unit.

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