JP5619558B2 - LED drive circuit - Google Patents

Description

  The present invention relates to an LED drive circuit, and more particularly to an LED drive circuit for emitting LED light using a commercial AC power supply.

  2. Description of the Related Art An LED drive circuit that rectifies an AC power supplied from a commercial AC power and emits an LED (light emitting diode) array with the output is known.

  Among these, there is a circuit that directly drives an LED array with a pulsating flow obtained from a diode bridge circuit. For example, FIG. 6 of Patent Document 1 shows a plurality of light emitting diode elements 18 (LEDs), input terminals 12 and 12 for connection to a commercial AC power source, a bridge diode element 14 (diode bridge circuit), and a resistor R1. , R2, R3, and a Zener diode 82, a light emitting diode drive circuit 80 (LED drive circuit) is shown. In the figure, the output terminal of the bridge diode element 14 is connected to an LED array composed of the light emitting diode elements 18 via a resistor R3. Note that Patent Document 1 deals with heat generation of the resistors R1, R2, and R3 and the Zener diode 82 in the light-emitting diode driving circuit 80 shown in FIG. 6, and the light-emitting diode driving circuit 10 including the capacitor 20 shown in FIG. Thus, the voltage of the AC power source for driving the light emitting diode unit 20 (LED array) is to be lowered.

  As shown in FIGS. 1 and 6 of Patent Document 1, when the LED string is driven by a pulsating current output from the diode bridge circuit, the LED string has a non-lighting period in which the LED string is not lit. That is, when the number of LEDs included in the LED array is n and the forward voltage drop of each LED is Vf (V), the LED array is turned off when the voltage of the pulsating current becomes smaller than n × Vf (V). Since this non-lighting period causes a decrease in light emission luminance and flicker, it is desired to shorten the non-lighting period.

  To eliminate the non-lighting period with a simple method, the pulsating flow may be smoothed with a capacitor. For example, in FIG. 1 of Patent Document 2, the output of the half-wave rectifier circuit 11 (diode) is smoothed by a smoothing capacitor 12, and a light emitting element lighting control device (LED) that lights a plurality of LEDs 20 (LED row) with this voltage. Drive circuit). The circuit of FIG. 1 describes that the current flowing through the LED string (the plurality of LEDs 20) is made constant by the shunt regulator 15 and the bipolar transistor 16 even if some ripples remain in the smoothed voltage. (FIG. 2, paragraph 0024).

  As another technique for shortening the non-lighting period, for example, FIG. 1 of Patent Document 3 shows an LED drive circuit that changes the number of stages of LED rows in accordance with the output voltage of the diode bridge circuit. There, the LED string is divided into four groups (group A (2), group B (4), group C (8), group D (16)), and the output voltage of the diode bridge circuit is low. Applies a voltage only to group A, and controls to apply a voltage to groups A and B, groups A to C, and all four groups when the voltage is highest each time the voltage increases. Yes.

JP-A-7-273371 (FIGS. 1 and 6) JP 2006-73637 A (FIG. 1, FIG. 2, paragraph 0024) JP 2007-123562 A (FIG. 1)

  When there is a large non-lighting period as in the circuit of Patent Document 1, in addition to the above-described decrease in brightness and flicker, a motion break occurs in which an object moving at high speed appears to fly away. Further, this circuit has a large total harmonic distortion (hereinafter referred to as THD). The THD represents the ratio of the sum of harmonic components generated when the sine wave signal is input to the device with respect to the fundamental wave component (sine wave signal).

  In order to eliminate the non-lighting period by smoothing the output of the rectifier circuit as in Patent Document 2, an electrolytic capacitor having a large capacity and withstand voltage is required to obtain a high luminance by supplying a sufficient current to the LED array. Become. This electrolytic capacitor is not only large in size, but also has a problem that its life is extremely shortened in an environment where the temperature is high like a lighting device.

  When trying to shorten the non-lighting period by a method of switching the number of LED rows by the rectified voltage value as in Patent Document 3, it is necessary to frequently set the number of switching levels and control. Also, the THD is poor because the current value changes abruptly when switching.

  Accordingly, an object of the present invention has been made in view of the above problems, and in an LED drive circuit that rectifies an AC power source and emits an LED array, shortening or eliminating the non-lighting period with a capacitor having as small a capacity as possible. An LED driving circuit with good THD is provided.

In order to solve the above problems, an LED drive circuit according to the present invention includes a rectifier circuit and a light emitting circuit including an LED array, and rectifies a commercial AC power source to emit light from the LED array.
A current supply circuit for supplying a current to the light emitting circuit when the rectifier circuit is cut off;
The current supply circuit includes a capacitor and a time constant adjusting element, and the capacitor is discharged through the time constant adjusting element, and the discharge time constant is longer than the charging time constant.
The light emitting circuit includes a first LED string, and a constant current circuit connected in series with the first LED string ,
The + side terminal of the first LED row is connected to the output terminal of the rectifier circuit,
The constant current circuit includes a second LED string and an FET connected in parallel with the second LED string ,
The commercial AC power source voltage (absolute value) is the first LED row and the second threshold value by Ri higher the FET LED strings LED strings linked to is not more when they are cut off, the light emitting circuit The current flowing through the first and second LED strings is turned on, and
The commercial AC power source voltage (absolute value) is higher than the threshold value of the first LED string and LED string obtained by connecting the second LED string, the second while maintaining a current flowing through the first LED constant A period of gradually decreasing the current flowing through the LED array and gradually increasing the current flowing through the FET ;
The voltage (absolute value) of the commercial AC power supply is lower than the threshold value of the first LED row, and only the first LED row is turned on.

The constant current circuit includes a second LED string and pressure evaporator in the FET Lee polar transistor and a first resistor and a second resistor,
A positive side terminal and a negative side terminal of the second LED row are respectively connected to a negative side terminal of the first LED row and a positive side terminal of the second resistor;
A negative terminal of the second resistor is connected to a wiring for returning a current to the rectifier circuit;
The drain of the FET is connected to the + side terminal of the second LED string, the gate is connected to the + side terminal via the first resistor, and the source is connected to the + side terminal of the second resistor,
The collector of the bipolar transistor is connected to the gate, the base is connected to the source, and the emitter is connected to the negative terminal of the second resistor.

  The current supply circuit may include a switch element.

  Since a large amount of current flows through this LED string during a period when the voltage of the commercial AC power supply (absolute value, the same applies hereinafter) is higher than the threshold value of the LED string connecting the first and second LED strings, the current supply circuit is substantially The first and second LED rows are not involved in lighting. However, when the voltage of the commercial AC power supply decreases to the vicinity of the threshold voltage of the LED array, the constant current circuit starts to operate, and the current flowing through the second LED array is gradually reduced while keeping the current flowing through the first LED constant. When the voltage of the commercial AC power supply further decreases and becomes close to the threshold value of the first LED row, the rectifier circuit is cut off while the current supply circuit starts to supply current to the light emitting circuit. This current is limited by the time constant adjusting element and is limited to a small value, but the first LED row can be lit for a relatively long time. As described above, while the voltage of the commercial AC power supply is smaller than the threshold value of the first LED string, the first LED string is turned on with a small current, thereby reducing the capacitance of the capacitor included in the current supply circuit. The non-lighting period can be shortened or eliminated.

  When the voltage of the commercial AC power supply is in the vicinity of the threshold value of the LED array connecting the first and second LED arrays, the current flowing through the first LED is constant and the current flowing through the second LED gradually decreases (or increases). That is, by providing a period in which the current is constant between a period in which both the first and second LED strings are lit with high brightness and a period in which only the first LED string is lit, the first or second LED The change in current is smooth over the entire period during which the column is lit. As a result, harmonic distortion (THD) can be reduced.

  As described above, the present invention reduces the capacitance of the capacitor included in the current supply circuit by lighting the LED string with a small current during a period in which the voltage of the commercial AC power supply is smaller than the threshold value of the first LED string. It is possible to provide an LED driving circuit that shortens or eliminates the non-lighting period and has a good THD.

The circuit diagram of the LED drive circuit in 1st Embodiment of this invention. FIG. 2 is an operation explanatory diagram of the LED drive circuit of FIG. 1. The circuit diagram of the LED drive circuit in 2nd Embodiment of this invention. Operation | movement explanatory drawing of the LED drive circuit of FIG. The circuit diagram of the LED drive circuit of a reference example. FIG. 6 is an operation explanatory diagram of the LED drive circuit of FIG. 5.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to FIGS. In the description of the drawings, the same or equivalent elements will be denoted by the same reference numerals, and redundant description will be omitted.
(First embodiment)

FIG. 1 is a circuit diagram of an LED drive circuit 10 according to the first embodiment of the present invention. The diode bridge circuit 1 (rectifier circuit) is composed of four silicon diodes 12 and is composed of a commercial AC power source 1.
1 is connected. The current supply circuit 2 includes a silicon diode 13, a resistor 14 (time constant adjusting element), and a capacitor 15. The anode of the silicon diode 13 and the + side terminal of the resistor 14 are connected to the output terminal of the diode bridge circuit 1, and the cathode of the silicon diode 13 and the − side terminal of the resistor 14 are connected to the + side terminal of the capacitor 15. The time constant adjusting element is not limited to the resistor 14 and may be a constant current diode.

  The light emitting circuit 3 includes an LED array 4a (first LED array) in which a plurality of LED elements 17 are connected in series and a constant current circuit 5. The constant current circuit 5 includes an LED string 4b (second LED string) in which a plurality of LED elements 17 and resistors 22 are connected in series, a resistor 21 (first resistor), a resistor 23 (second resistor), and an FET 24. The bipolar transistor 25 is used. The + side terminal and the − side terminal of the LED array 4b are connected to the − side terminal of the LED array 4a and the + side terminal of the resistor 23, respectively. The negative terminal of the resistor 23 is connected to a wiring for returning a current to the diode bridge circuit 1 (hereinafter referred to as a common wiring). The FET 24 has a drain connected to the + side terminal of the LED row 4 b, a gate connected to the + side terminal of the LED row 4 b via the resistor 21, and a source connected to the + side terminal of the resistor 23. The bipolar transistor 25 has a collector connected to the gate of the FET 24, a base connected to the source of the FET 24, and an emitter connected to the common wiring. The resistor 22 is a protective resistor for the purpose of current limiting and can be omitted.

  FIG. 2 is an operation explanatory diagram of the LED drive circuit 10 of FIG. (A) has shown the drive voltage of the light emission circuit 3 when the LED drive circuit 10 is not provided with the electric current supply circuit 2. FIG. This drive voltage becomes a pulsating flow (full-wave rectification) obtained by rectifying the commercial AC power supply 11.

  The operation of the light emitting circuit 3 will be described with reference to (b). Similarly, (b) shows the current flowing through the light emitting circuit 3 when the LED drive circuit 10 does not include the current supply circuit 2.

  When the drive voltage of the light emitting circuit 3 rises and exceeds the threshold value of the LED array 4a, current starts to flow through the LED array 4a, the FET 24, and the resistor 23 to the common wiring. At this time, the FET 24 is ON. When the drive voltage further rises and exceeds the sum of the threshold values of the LED strings 4a and 4b, current starts to flow through the LED strings 4b. At this time, the sum of the current flowing through the FET 24 and the current flowing through the LED array 4b is kept constant. When the drive voltage further increases, the current flowing through the FET 24 becomes 0 (A) (the FET 24 is OFF), and the current flowing through the LED array 4b increases in accordance with the drive voltage.

  The shoulder portion of the curve indicating the current value is the portion performing constant current operation. The timing when the current starts to flow for the first time as the drive voltage rises (the period when no current flows through the LED array 4b) to the constant current operation period, and the timing when the current starts increasing again from the constant current operation period The current value changes smoothly at. The period during which the drive voltage decreases is substantially opposite to the period during which the drive voltage increases. A period when the drive voltage is smaller than the LED array 4a is a non-lighting period because no current flows. Further, since the current value changes smoothly, THD becomes small.

  (C) shows the drive voltage of the light emitting circuit 3 of the LED drive circuit 10. There is a large ripple in this drive voltage. In a period in which the voltage of the commercial AC power supply 11 is higher than the threshold value of the LED array 4a, the drive voltage becomes a pulsating flow (part) equivalent to (a). In a period in which the voltage of the commercial AC power supply 11 is smaller than the threshold value of the LED array 4a, the drive voltage becomes a discharge curve.

Similarly, (d) shows the current flowing through the light emitting circuit 3 of the LED drive circuit 10. When the voltage of the commercial AC power supply 11 decreases and approaches the vicinity of the threshold value of the LED array 4a, current starts to flow from the current supply circuit 2 to the light emitting circuit 3. That is, while the voltage of the commercial AC power supply 11 and the current of the light emitting circuit 3 are rapidly decreased, the voltage of the current supply circuit 2 (for example, the positive terminal voltage of the capacitor 15) becomes higher than the voltage of the commercial AC power supply 11, so that the diode bridge Since the circuit 1 is cut off, a current flows from the current supply circuit 2 to the light emitting circuit 3. Since this current is limited by the resistor 14, it becomes a small value, and the LED string 4 a continues to be lit with a small current supplied from the current supply circuit 2 during a period when the voltage of the commercial AC power supply 11 is smaller than the threshold value of the LED string 4 a. . During this period, since the current is small, the FET 24 is ON.

  Looking in more detail, the diode bridge circuit 1 supplies current to the current supply circuit 2 and the light emitting circuit 3 during the period in which the voltage of the commercial AC power supply 11 is higher than the threshold value of the LED string 4a, and the voltage The diode bridge circuit 1 and the current supply circuit 2 supply current to the light emitting circuit 3 during the period during which the voltage falls. However, during this period, the diode bridge circuit 1 supplies a large amount of current to the light emitting circuit 3, whereas the current supply circuit 2 can supply a little current compared to the diode bridge circuit 1 because of the resistor 14. That is, the diode bridge circuit 1 substantially supplies current to the light emitting circuit 3 during a period when the voltage of the commercial AC power supply 11 is higher than the threshold value of the LED array 4 a. Further, during the period when the voltage of the commercial AC power supply 11 is lower than the threshold value of the LED string 4a, the output voltage of the current supply circuit 2 becomes higher than the voltage of the commercial AC power supply 11 as described above, so that the diode bridge circuit 1 is cut off, The current supply circuit 2 supplies current to the light emitting circuit 3. That is, during this period, the current supply circuit 2 discharges the charge stored in the capacitor 15 via the resistor 14 at a low speed, and the drive voltage gradually decreases.

  As described above, the LED driving circuit 10 shortens the charging time constant of the capacitor 15 of the current supply circuit 2 and lengthens the discharging time constant, so that the period during which the voltage of the commercial AC power supply 11 is higher than the threshold value of the LED array 4a is mainly used. A current is supplied from the diode bridge circuit 1 to the light emitting circuit 3, and the current supply circuit 2 supplies current to the light emitting circuit 3 during a period when the voltage of the commercial AC power supply 11 is lower than the threshold value of the LED array 4 a.

  In order to eliminate the non-lighting period, the time constant determined by the product of the resistor 14 and the capacitor 15 is set to be approximately equal to the non-lighting period. For example, when the period of the pulsating flow is 10 ms (frequency 100 Hz) and the non-lighting period is about 2 ms, the resistance 14 is 1 kΩ and the capacitor 15 is 4 μF (time constant is 4 ms). By narrowing the current supplied from the current supply circuit 2 with the resistor 14, the capacity of the capacitor 15 can be reduced, and a ceramic capacitor having a long life can be used. (Second Embodiment)

  FIG. 3 is a circuit diagram of the LED drive circuit 30 according to the second embodiment of the present invention. The current supply circuit 2a includes an FET 31 (switch element, N-type MOS-FET) between the negative terminal of the capacitor 15 and the common wiring. The LED drive circuit 30 also includes a control circuit 32. The control circuit 32 monitors the driving voltage (terminal A) of the light emitting circuit 3, and when the voltage of the commercial AC power supply 11 starts to drop, the gate voltage (terminal B) of the FET 31 is set to a low level (voltage of the common wiring). When the voltage of the power supply 11 drops to near the threshold value of the diode array 4a, the gate voltage (terminal D) of the FET 31 is set to the high level (voltage of the terminal A).

  In the LED drive circuit 10 of FIG. 1, when the voltage of the commercial AC power supply 11 starts to decrease past a peak, the capacitor 15 discharges a part of the electric charge through the resistor 14. On the other hand, since the LED drive circuit 30 of FIG. 3 includes the FET 31, even if the voltage of the commercial AC power supply circuit 11 starts to decrease after the peak, the capacitor 15 is discharged until the desired voltage (near the threshold value of the LED array 4a) is reached. do not do. For this reason, the electric charge which the capacitor | condenser 15 stored can be used effectively. The control circuit 32 that controls the gate voltage of the FET 31 in accordance with the drive voltage includes a ladder resistor, a comparator, and a flip-flop that stores the state. The switch element may be connected not only in series with the capacitor 13 (FET 31) as shown in FIG. 3, but also in series with the resistor 14 in the circuit of FIG. In this case, for example, a P-type MOS-FET is used.

FIG. 4 is an operation explanatory diagram of the LED drive circuit 30. (A) has shown the drive voltage of the light emission circuit 3 when the LED drive circuit 30 is not provided with the electric current supply circuit 2a, and becomes the same pulsating flow as Fig.2 (a). Similarly, (b) shows the current flowing through the light emitting circuit 3 when the LED drive circuit 30 does not include the current supply circuit 2a, and has the same pulsating flow as in FIG. 2 (b).

  (C) shows the drive voltage of the light emitting circuit 3 in the LED drive circuit 30. In a period in which the voltage of the commercial AC power supply 11 is higher than the threshold value of the LED array 4a, the drive voltage becomes a pulsating flow (part) equivalent to (a). The FET 31 is turned on before and after the voltage of the commercial AC power supply 11 falls to the vicinity of the threshold value of the LED array 4a and the diode bridge circuit 1 is cut off. At this time, since the voltage across the capacitor 15 is the peak voltage of the voltage of the commercial AC power supply 11, the drive voltage rises once. Thereafter, since the capacitor 15 is discharged through the resistor 14 and the diode array 4a, a discharge curve appears in the drive voltage.

  Similarly, (d) shows the current flowing through the light emitting circuit 3 in the LED drive circuit 30. When the voltage of the commercial AC power supply 11 decreases and approaches the threshold value of the LED string 4a and the FET 31 is turned on, the current increases once and gradually decreases until the voltage of the commercial AC power supply 11 exceeds the threshold value of the LED string 4a again. As a result of providing the FEF 31, the LED drive circuit 30 allows the current supply circuit 2a to pass a larger current than the current supply circuit 2 of the LED drive circuit 10.

The LED drive circuits 10 and 30 emit light weakly in the LED array 4a during the period in which the voltage of the commercial AC power supply 11 is smaller than the threshold value of the LED array 4a, thereby eliminating the non-lighting period. However, the brightness increase, flicker, and motion break can be reduced by shortening the non-lighting period without completely eliminating the non-lighting period.
(Reference example)

  FIG. 5 is a circuit diagram of the LED drive circuit 50 in the reference example. The current supply circuit 2b includes only the capacitor 15. Others are the same as the LED drive circuit 10.

  FIG. 6 is an operation explanatory diagram of the LED drive circuit 50. (A) has shown the drive voltage of the light emission circuit 3 when the LED drive circuit 50 is not provided with the electric current supply circuit 2b, and becomes the same pulsating flow as Fig.2 (a). Similarly, (b) shows the current flowing through the light emitting circuit 3 when the LED drive circuit 50 does not include the current supply circuit 2b, and has the same pulsating flow as in FIG. 2 (b).

  (C) shows the drive voltage of the light emitting circuit 3 in the LED drive circuit 50. In a period in which the voltage of the commercial AC power supply 11 is higher than the sum of the threshold value of the LED string 4a and the threshold value of the LED string 4b, the drive voltage becomes a pulsating flow (partial) equivalent to (a). Thereafter, the voltage decreases toward the threshold value of the LED 4a, and the voltage is maintained.

  (D) shows the current flowing through the light emitting circuit 3 in the LED drive circuit 50. The period of time until the voltage of the commercial AC power supply 11 exceeds the threshold value of the LED array 4a, passes the peak, and is close to the sum of the threshold value of the LED array 4a and the threshold value of the LED array 4b (part) become. Thereafter, when the constant current circuit 5 starts the constant current operation, the amount of current is limited, so that the capacitor 15 contributes to the drive voltage. As a result, the shoulder when the voltage of the commercial AC power supply 11 decreases is wider than the shoulder of the current curve when the voltage of the commercial AC power supply 11 increases. The constant current circuit 5 continues the constant current operation until the voltage of the capacitor 15 drops to the threshold value of the LED string 4a, and then the current disappears.

The characteristics shown in FIGS. 6C and 6D are obtained by setting the capacitance of the capacitor 15 to a small value of about 5 to 10 μF. In this case, the non-lighting period is shortened to obtain a certain effect, and the ceramic capacitor can be used, so that the life can be extended. In addition, when the LED drive circuit 50 increases the capacitance of the capacitor 15, the non-lighting period disappears. For example, in the case of a commercial AC power supply of 100 V / 50 Hz, if the capacitor is 50 to 100 μF, the non-lighting period is eliminated.

  The LED drive circuit 50 limits the current value in a state where the constant current circuit 5 is operated at a constant current, whereas the LED drive circuits 10 and 30 limit the current by the time constant element (at this time, the FET 24 ON and the constant current circuit 5 is almost in the state of a short circuit). Even though the current limiting method is different in this way, the LED drive circuit 50 uses the constant current operation to extend the non-lighting period. The capacity of the smoothing capacitor required by the LED driving circuit 50 can be made much smaller than the capacity of the smoothing capacitor that a simple LED driving circuit must be inserted. Also, as in Patent Document 2, in an LED drive circuit in which a constant current element and an LED array are connected in series and a relatively small smoothing capacitor is provided, the current flowing through the LED array is constant even during periods when the voltage of the commercial AC power supply is high. On the other hand, the LED drive circuit 50 can be brightened because the current flowing through the LED strings 4a and 4b is increased while the voltage of the commercial AC power supply is high.

  As described above, the LED drive circuit 50 has a feature that the THD is good, the non-lighting period can be shortened or eliminated with a relatively small capacity, and it is bright.

1 ... Diode bridge circuit (rectifier circuit),
2, 2a, 2b ... current supply circuit,
3 ... Light emitting circuit,
4a, 4b ... LED row,
5. Constant current circuit 10, 30, 50 ... LED drive circuit,
11 ... Commercial AC power supply,
12, 13 ... silicon diode,
14: Resistance (time constant adjusting element),
15 ... Capacitor,
17 ... LED element,
21... Resistance (first resistance),
22 ... resistance,
23... Resistance (second resistance),
24 ... FET,
25 ... Bipolar transistor,
31 ... FET (switch element),
32: Control circuit.

Claims (3)

In an LED drive circuit that has a rectifier circuit and a light emitting circuit including an LED string, rectifies a commercial AC power source and emits the LED string,
A current supply circuit for supplying a current to the light emitting circuit when the rectifier circuit is cut off;
The current supply circuit includes a capacitor and a time constant adjusting element, and the capacitor is discharged through the time constant adjusting element, and the discharge time constant is longer than the charging time constant.
The light emitting circuit includes a first LED string, and a constant current circuit connected in series with the first LED string ,
The + side terminal of the first LED row is connected to the output terminal of the rectifier circuit,
The constant current circuit includes a second LED string and an FET connected in parallel with the second LED string ,
The commercial AC power source voltage (absolute value) is the first LED row and the second threshold value by Ri higher the FET LED strings LED strings linked to is not more when they are cut off, the light emitting circuit The current flowing through the first and second LED strings is turned on, and
The commercial AC power source voltage (absolute value) is higher than the threshold value of the first LED string and LED string obtained by connecting the second LED string, the second while maintaining a current flowing through the first LED constant A period of gradually decreasing the current flowing through the LED array and gradually increasing the current flowing through the FET ;
The LED drive circuit characterized in that the voltage (absolute value) of the commercial AC power supply is lower than a threshold value of the first LED row, and only the first LED row is lit.
The constant current circuit includes a second LED string and pressure evaporator in the FET Lee polar transistor and a first resistor and a second resistor,
A positive side terminal and a negative side terminal of the second LED row are respectively connected to a negative side terminal of the first LED row and a positive side terminal of the second resistor;
A negative terminal of the second resistor is connected to a wiring for returning a current to the rectifier circuit;
The drain of the FET is connected to the + side terminal of the second LED string, the gate is connected to the + side terminal via the first resistor, and the source is connected to the + side terminal of the second resistor,
2. The LED driving circuit according to claim 1, wherein the bipolar transistor has a collector connected to the gate, a base connected to the source, and an emitter connected to the negative terminal of the second resistor.
The LED drive circuit according to claim 1, wherein the current supply circuit includes a switch element.