Constant current circuit
The constant current circuit design with a voltage conversion unit and comparator achieves stable operation and a simple configuration by controlling the voltage conversion unit to maintain a constant current without operational amplifiers, addressing the complexity and cost issues of existing circuits.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- MINEBEAMITSUMI INC
- Filing Date
- 2024-12-26
- Publication Date
- 2026-07-08
Smart Images
Figure 2026114087000001_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to a constant current circuit.
Background Art
[0002] Patent Document 1 discloses a constant current circuit used for charging a battery or the like. The constant current circuit includes a transistor connected to a DC power supply, a filter circuit connected to the transistor, a resistance element through which a current passing through the filter circuit flows, an operational amplifier that amplifies a voltage drop generated in the resistance element and outputs a voltage, and a voltage control circuit having a comparator that controls the switching of the transistor according to a comparison result between the voltage output from the operational amplifier and a reference voltage. Since the constant current circuit uses an operational amplifier, it has a complex configuration. In particular, an operational amplifier that operates at several hundred kHz requires a high-cost one. Also, since an operational amplifier is likely to oscillate in a high-frequency region, the design difficulty is high.
Prior Art Documents
Patent Documents
[0003]
Patent Document 1
Summary of the Invention
Problems to be Solved by the Invention
[0004] Since the constant current circuit of Patent Document 1 uses an operational amplifier, it has a complex configuration. However, if the operational amplifier is simply omitted to configure the constant current circuit, the levels of the positive input voltage and the negative input voltage in the comparator are reversed immediately when a current flows, so that stable operation cannot be guaranteed. Therefore, there is room for improvement from the viewpoints of stable operation and simple configuration.
[0005] An object of the present invention is to realize stable operation and a simple configuration in a constant current circuit.
Means for Solving the Problems
[0006] The present invention A voltage conversion unit having an input terminal, an output terminal, a control terminal, and a switching unit that turns on / off in accordance with a signal input to the control terminal, and which steps down the voltage input from the power supply to the input terminal using the switching unit and outputs it from the output terminal, A first resistor element having one end connected to the output terminal of the voltage conversion unit and the other end connected to the load, A comparator having a positive input terminal connected to one end of the first resistive element, to which a voltage from the voltage conversion unit is input; a negative input terminal connected to the other end of the first resistive element, to which a voltage from the load is input; and an output terminal connected to the control terminal of the voltage conversion unit, which outputs an on / off signal in accordance with a comparison between the voltage input to the positive input terminal and the voltage input to the negative input terminal, A voltage offset section is provided between the negative input terminal of the comparator and the other end of the first resistive element, which offsets the voltage from the load. A constant current circuit is provided that includes the following features.
[0007] In this configuration, the current output from the voltage conversion unit is detected by a first resistor element, and a constant current is generated from the power supply to the load by controlling the on / off operation of the voltage conversion unit according to this current. Specifically, a comparator compares the voltage across the first resistor element, which is generated by the current flowing through the first resistor element, and controls the operation of the voltage conversion unit according to the comparison result so that the current output from the voltage conversion unit remains constant. When the voltage conversion unit is operating, a predetermined voltage is applied to the negative input terminal of the comparator by a voltage offset unit so that a predetermined voltage difference is created between the positive and negative input terminals of the comparator. Due to this predetermined voltage difference, even when current starts to flow, the high and low positions of the positive and negative input voltages at the comparator do not immediately reverse, so that the comparator can stably turn the voltage conversion unit on and off, and a constant current flows from the voltage conversion unit. Therefore, in a constant current circuit, stable operation and a simple configuration can be achieved without using complex and expensive operational amplifiers.
[0008] The voltage offset section may include a second resistive element, a third resistive element, a fourth resistive element, a first Zener diode, and a first PNP transistor. One end of the second resistive element may be connected to the power supply and the cathode of the first Zener diode. The other end of the second resistive element may be connected to the emitter terminal of the first PNP transistor. One end of the third resistive element may be connected to the anode of the first Zener diode. The other end of the third resistive element may be connected to ground. One end of the fourth resistive element may be connected to the negative input terminal of the comparator. The other end of the fourth resistive element may be connected to the other end of the first resistive element. The base terminal of the first PNP transistor may be connected to one end of the third resistive element. The collector terminal of the first PNP transistor may be connected to the one end of the fourth resistive element.
[0009] This configuration allows for a simpler construction of the voltage offset section. Specifically, a constant current is generated, and this constant current is passed through a fourth resistive element connected to the negative input terminal of the comparator to generate a predetermined voltage, which can then be applied to the negative input terminal of the comparator. Thus, a predetermined voltage difference can be created between the positive and negative input terminals of the comparator.
[0010] The voltage offset section may include a fifth resistive element, a sixth resistive element, a second Zener diode, and a second PNP transistor. One end of the fifth resistive element may be connected to the one end of the second resistive element and the cathode of the second Zener diode. The other end of the fifth resistive element may be connected to the emitter terminal of the second PNP transistor. One end of the sixth resistor element may be connected to the anode of the second Zener diode. The other end of the sixth resistor element may be connected to the output terminal of the comparator. The base terminal of the second PNP transistor may be connected to the one end of the sixth resistor element. The collector terminal of the second PNP transistor may be connected to the one end of the fourth resistor element.
[0011] According to this configuration, the voltage offset section is provided with a hysteresis function, and when the difference between the positive input voltage and the negative input voltage of the comparator does not exceed a predetermined value, the on / off signal output from the output terminal of the comparator cannot be switched. As a result, the on / off switching cycle of the switching section of the voltage conversion section can be slowed down, and the heat generation amount of the switching section can be reduced.
Effect of the Invention
[0012] According to the present invention, in a constant current circuit, stable operation and a simple configuration can be realized.
Brief Description of the Drawings
[0013] [Figure 1] Circuit diagram showing the constant current circuit of the first reference example. [Figure 2] Circuit diagram showing the constant current circuit of the second reference example. [Figure 3] Graph showing the input / output voltage of the comparator and the output current of the first resistor element in the constant current circuit of the second reference example. [Figure 4] Circuit diagram showing the constant current circuit according to the first embodiment of the present invention. [Figure 5] Graph showing the input / output voltage of the comparator and the output current of the first resistor element in the constant current circuit of the first embodiment. [Figure 6] Operation flowchart in the constant current circuit of the first embodiment. [Figure 7] Circuit diagram showing the constant current circuit according to the second embodiment of the present invention. [Figure 8] Graph showing the input / output voltage of the comparator and the output current of the first resistor element in the constant current circuit of the second embodiment. [Figure 9] Circuit diagram showing the voltage offset section in the second embodiment.
Embodiments for Carrying Out the Invention
[0014] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. For convenience of explanation, first, first and second reference examples different from the embodiments of the present invention will be described.
[0015] (First Reference Example) FIG. 1 is a circuit diagram showing a constant current circuit 1 of the first reference example.
[0016] The constant current circuit 1 performs switching control of the DCDC converter 12 so as to keep the current supplied from the power supply 10 to the load 11 at a predetermined value. For example, the power supply 10 is a battery serving as a power supply source, and the load 11 is an electric double layer capacitor serving as a charging destination.
[0017] The DCDC converter 12 includes a switching transistor 13, a diode 14 for smoothing the output of the switching transistor 13, an inductor 15, and a capacitor 16. The DCDC converter 12 is connected to the power supply 10 at one end and to the resistor element 17 at the other end. The resistor element 17 is connected to the load 11. The current from the power supply 10 is supplied to the load 11 as an output current (charging current) controlled to a constant value through the DCDC converter 12 and the resistor element 17.
[0018] The constant current circuit 1 includes an amplifier circuit 18 that amplifies the voltage drop of the resistor element 17 for the above current control, and a comparator 19 that compares the voltage Vopo output from the amplifier circuit 18 with the reference voltage Vref and controls the signal to the DCDC converter 12 according to the comparison result.
[0019] The amplification circuit 18 includes an operational amplifier 20. The voltage Vopo output from the operational amplifier 20 is set to be equal to the reference voltage Vref when the desired output current Iout is at a gain determined by the resistors 21-24.
[0020] The comparator 19 outputs an on / off signal (Vcompo) according to the result of comparing the magnitude of the voltage Vopo output from the operational amplifier 20 with a reference voltage Vref. The output signal Vcompo of the comparator 19 repeatedly and continuously switches the on / off operation of the switching transistor 13 of the DC-DC converter 12, generating a constant output current Iout.
[0021] As shown in this example, it is possible to construct a constant current circuit 1 using an amplification circuit 18 (operational amplifier 20), but this is complex and costly. Furthermore, an operational amplifier 20 that can operate at several hundred kHz requires a high-cost model. In addition, operational amplifier 20 is prone to oscillation in the high-frequency range, making the design difficult.
[0022] (2nd reference example) Figure 2 is a circuit diagram showing the constant current circuit 2 of the second reference example.
[0023] In the second reference example, the amplifier circuit 18 (see Figure 1) and related parts are omitted from the first reference example. The same reference numerals are used for the components shown in the first reference example.
[0024] Figure 3 is a graph showing the input and output voltages Vcomp+, Vcomp-, and Vcompo of the comparator 19 and the output current Iout of the resistor 17 in the constant current circuit 2 of this reference example. In Figure 3, the area indicated by the dashed line frame in the upper figure is shown enlarged in the lower figure. In both the upper and lower figures, the positive input voltage Vcomp+ and the negative input voltage Vcomp- of the comparator 19 are shown in the upper section, the output signal Vcompo of the comparator 19 is shown in the middle section, and the output current Iout is shown in the lower section.
[0025] As shown in this example, simply omitting the amplifier circuit 18 (operation amplifier 20) in Figure 1 will cause the high and low positions of the positive input voltage Vcomp+ and the negative input voltage Vcomp- at the comparator 19 to reverse as soon as current flows. This causes the output signal Vcompo to switch on and off, and the desired output current Iout is not achieved.
[0026] (First Embodiment) Figure 4 is a circuit diagram showing a constant current circuit 100 according to the first embodiment of the present invention.
[0027] The constant current circuit 100 of this embodiment includes a voltage conversion unit 110, a first resistive element 120, a comparator 130, and a voltage offset unit 140. Despite its simple configuration without an operational amplifier, the constant current circuit 100 can operate stably thanks to the voltage offset unit 140. That is, the constant current circuit 100 of this embodiment can control the switching of the voltage conversion unit 110 to keep the current supplied from the power supply 150 to the load 160 constant without using an operational amplifier. For example, the power supply 150 is a battery that provides power, and the load 160 is an electric double-layer capacitor that is charged.
[0028] The voltage conversion unit 110 is a DC-DC converter and has an input terminal 111, an output terminal 112, a control terminal 113, and a switching unit (switching transistor) 114. The voltage conversion unit 110 also has a diode 115, an inductor 116, and a capacitor 117 to smooth the output of the switching unit 114. The switching unit 114 turns on / off in response to a signal input to the control terminal 113. The voltage input from the power supply 150 to the input terminal 111 is stepped down by the switching unit 114 and output from the output terminal 112.
[0029] The first resistive element 120 is connected at one end to the output terminal 112 of the voltage conversion unit 110 and at the other end to the load 160.
[0030] The comparator 130 has a positive input terminal 131, a negative input terminal 132, and an output terminal 133. The positive input terminal 131 is connected to one end of the first resistive element 120 and receives the voltage from the voltage conversion unit 110 (Vcomp+). The negative input terminal 132 is connected to the other end of the first resistive element 120 and receives the voltage from the load 160 (Vcomp-). The output terminal 133 is connected to the control terminal 113 of the voltage conversion unit 110 and outputs an on / off signal (Vcompo) according to the comparison result between the voltage input to the positive input terminal 131 and the voltage input to the negative input terminal 132.
[0031] The voltage offset section 140 is provided between the negative input terminal 132 of the comparator 130 and the other end of the first resistive element 120, and offsets the voltage from the load 160. The voltage offset section 140 includes a second resistive element 141, a third resistive element 142, a fourth resistive element 143, a first Zener diode 144, and a first PNP transistor 145.
[0032] One end of the second resistor 141 is connected to the power supply 150 and the cathode of the first Zener diode 144, and the other end of the second resistor 141 is connected to the emitter terminal of the first PNP transistor 145. One end of the third resistor 142 is connected to the anode of the first Zener diode 144, and the other end of the third resistor 142 is connected to ground. One end of the fourth resistor 143 is connected to the negative input terminal 132 of the comparator 130, and the other end of the fourth resistor is connected to the other end of the first resistor 120. The base terminal of the first PNP transistor 145 is connected to one end of the third resistor 142, and the collector terminal of the first PNP transistor 145 is connected to one end of the fourth resistor 143.
[0033] Figure 5 is a graph showing the input and output voltages Vcomp+, Vcomp-, and Vcompo of the comparator and the output current Iout of the first resistor element 120 in the constant current circuit 100 of this embodiment. In Figure 5, the portion indicated by the dashed line frame in the upper figure is shown enlarged in the lower figure. In both the upper and lower figures, the positive input voltage Vcomp+ and the negative input voltage Vcomp- of the comparator 130 are shown in the upper section, the output signal Vcompo of the comparator 130 is shown in the middle section, and the output current Iout is shown in the lower section.
[0034] In this embodiment, time Δt is required from the start of current flow until the positive input voltage Vcomp+ at the comparator 130 exceeds the negative input voltage Vcomp-. Therefore, unlike the second reference example (Figure 3), the output signal Vcompo does not switch on / off immediately when current flows, and the desired output current Iout is achieved.
[0035] Figure 6 shows an operation flowchart of the constant current circuit 100 in this embodiment.
[0036] When current begins to flow from the power supply 150, it is determined whether the positive input voltage Vcomp+ at the comparator 130 is smaller than the negative input voltage Vcomp- (step S1). If the positive input voltage Vcomp+ at the comparator 130 is not smaller than the negative input voltage Vcomp- (N: step S1), the output signal Vcompo of the comparator 130 turns on, which turns off the switching unit 114, and it remains in standby mode without charging. If the positive input voltage Vcomp+ at the comparator 130 is smaller than the negative input voltage Vcomp- (Y: step S1), the output signal Vcompo of the comparator 130 turns off, which turns on the switching unit 114, and it charges the load (electric double-layer capacitor) 160 (step S2). Next, it is determined whether the positive input voltage Vcomp+ at the comparator 130 is larger than the negative input voltage Vcomp- (step S3). If the positive input voltage Vcomp+ at comparator 130 is not greater than the negative input voltage Vcomp- (N: step S3), charging continues without changing the state (step S2). If the positive input voltage Vcomp+ at comparator 130 is greater than the negative input voltage Vcomp- (Y: step S3), the output signal Vcompo of comparator 130 turns on, which turns off the switching unit 114 and stops charging (step S4). Then, it is determined again whether the positive input voltage Vcomp+ at comparator 130 is greater than the negative input voltage Vcomp- (step S3). In this way, as shown in Figure 5, a constant output current Iout is generated by repeatedly switching the charging on and off in a repetitive pulse manner.
[0037] The constant current circuit 100 of this embodiment provides the following effects.
[0038] The current output from the voltage conversion unit 110 is detected by the first resistor element 120, and a constant current is generated from the power supply 150 to the load 160 by controlling the on / off operation of the voltage conversion unit 110 according to the current. Specifically, the comparator 130 compares the voltage across the first resistor element 120, which is generated by the current flowing through the first resistor element 120, and controls the operation of the voltage conversion unit 110 so that the current output from the voltage conversion unit 110 remains constant according to the comparison result. When the voltage conversion unit 110 is operating, a predetermined voltage is applied to the negative input terminal 132 of the comparator 130 by the voltage offset unit 140 so that a predetermined voltage difference is generated between the positive input terminal 131 and the negative input terminal 132 of the comparator 130. Due to the predetermined voltage difference, even when current begins to flow, the relative positions of the positive and negative input voltages in the comparator 130 do not immediately reverse. As a result, the comparator 130 can stably switch the voltage conversion unit 110 on and off, and a constant current flows from the voltage conversion unit 110. Therefore, the constant current circuit 100 can achieve stable operation and a simple configuration without using a complex and costly operational amplifier.
[0039] Furthermore, the voltage offset section 140 can be easily constructed. Specifically, a constant current is generated, and this constant current is passed through a fourth resistive element 143 connected to the negative input terminal 132 of the comparator 130 to generate a predetermined voltage, which can then be applied to the negative input terminal 132 of the comparator 130. Thus, a predetermined voltage difference can be created between the positive input terminal 131 and the negative input terminal 132 of the comparator 130.
[0040] (Second Embodiment) The constant current circuit 200 of the second embodiment shown in Figure 7 differs from the first embodiment in its voltage offset section 140. Aside from this difference, it is substantially the same as the first embodiment. Therefore, explanations of the parts shown in the first embodiment may be omitted.
[0041] In this embodiment, the voltage offset unit 140 further includes a fifth resistive element 146, a sixth resistive element 147, a second Zener diode 148, and a second PNP transistor 149, in addition to the components of the first embodiment.
[0042] One end of the fifth resistor 146 is connected to one end of the second resistor 141 and the cathode of the second Zener diode 148, and the other end of the fifth resistor 146 is connected to the emitter terminal of the second PNP transistor 149. One end of the sixth resistor 147 is connected to the anode of the second Zener diode 148, and the other end of the sixth resistor 147 is connected to the output terminal 133 of the comparator 130. The base terminal of the second PNP transistor 149 is connected to one end of the sixth resistor 147, and the collector terminal of the second PNP transistor 149 is connected to one end of the fourth resistor 143.
[0043] Figure 8 is a graph showing the input and output voltages Vcomp+, Vcomp-, and Vcompo of the comparator 130 and the output current Iout of the first resistor element 120 in the constant current circuit 200 of this embodiment. In Figure 8, the portion indicated by the dashed line frame in the upper figure is shown enlarged in the lower figure. In both the upper and lower figures, the positive input voltage Vcomp+ and the negative input voltage Vcomp- of the comparator 130 are shown in the upper section, the output signal Vcompo of the comparator 130 is shown in the middle section, and the output current Iout is shown in the lower section.
[0044] In this embodiment, as in the first embodiment, time Δt is required from the start of current flow until the positive input voltage Vcomp+ at the comparator 130 exceeds the negative input voltage Vcomp-. Therefore, as in the first embodiment, the output signal Vcompo does not switch on / off immediately after current flows, and the desired output current Iout is achieved. In addition, in this embodiment, a predetermined amount of offset (Vhys) is provided as hysteresis to the negative input voltage Vcomp- when the relative magnitudes of the positive input voltage Vcomp+ and the negative input voltage Vcomp- are reversed. That is, when the output signal Vcompo of the comparator 130 switches on, the negative input voltage Vcomp- decreases by a predetermined amount, and when the output signal Vcompo of the comparator 130 switches off, the negative input voltage Vcomp- increases by a predetermined amount. This prevents abnormally high-speed switching from occurring.
[0045] Figure 9 is a circuit diagram showing the voltage offset unit 140 in this embodiment.
[0046] The current Ioffset of the fourth resistor element 143 can be determined by the following equation (1). TIFF2026114087000002.tif6150 TIFF2026114087000003.tif10150 TIFF2026114087000004.tif11150 TIFF2026114087000005.tif11150
[0047] In equation (1) above, Iec represents the current through the second resistor 141. Ihys represents the current through the fifth resistor 146. Ihys is 0 [A] when Vcompo is on, and when Vcompo is off, it follows equation (1) above. Vbat represents the voltage of the power supply 150. Vzd1 represents the voltage through the first Zener diode 144. Vzd2 represents the voltage through the second Zener diode 148. Vbe1 represents the voltage between the base and emitter terminals of the first PNP transistor 145. Vbe2 represents the voltage between the base and emitter terminals of the second PNP transistor 149. Rhys represents the resistance of the fifth resistor 146. Re represents the resistance of the second resistor 141.
[0048] Furthermore, the output current Iout can be determined as shown in equation (2) below. TIFF2026114087000006.tif6150 TIFF2026114087000007.tif6150 TIFF2026114087000008.tif6150 TIFF2026114087000009.tif6150 TIFF2026114087000010.tif10150
[0049] In equation (2) above, Rout represents the resistance of the first resistive element 120. RESR represents the resistance of the load 160. Vsc represents the voltage across the load 160. Therefore, the output current Iout is determined by the current Rout of the first resistive element 120 and the constant current flowing through the voltage offset section 140, and takes a constant value regardless of the voltage of the power supply 150, etc.
[0050] According to this embodiment, the voltage offset unit 140 is given a hysteresis function, so that the on / off signal output from the output terminal 133 of the comparator 130 does not switch unless the difference between the positive input voltage and the negative input voltage of the comparator exceeds a predetermined value. This makes it possible to slow down the on / off switching cycle of the switching unit 114 of the voltage conversion unit 110, and reduces the amount of heat generated by the switching unit 114.
[0051] Although specific embodiments and variations of the present invention have been described above, the present invention is not limited to the above embodiments and can be implemented with various modifications within the scope of this invention.
[0052] For example, instead of the circuit configuration for providing hysteresis described in the second embodiment, the comparator 130 in the first embodiment may be configured to have a hysteresis function. [Explanation of Symbols]
[0053] 1,2 Constant current circuit 10 Power supply 11 Load 12 DC-DC converters 13 Switching Transistors 14 diodes 15 Inductors 16 Capacitors 17 Resistors 18 Amplifier Circuit 19 Comparator 20 operational amplifiers 21-24 Resistor elements 100,200 constant current circuit 110 Voltage conversion section 111 Input Terminals 112 Output terminals 113 Control terminal 114 Switching section 115 diode 116 Inductors 117 Capacitors 120 First Resistor Element 130 Comparator 131 Positive input terminal 132 Negative input terminal 133 Output terminals 140 Voltage offset section 141 Second Resistor Element 142 Third Resistor Element 143 Fourth Resistor Element 144 First Zener Diode 145 First PNP Transistor 146 Fifth Resistor Element 147 Sixth Resistor Element 148 Second Zener Diode 149. Second PNP Transistor 150 Power supply 160 load
Claims
1. A voltage conversion unit having an input terminal, an output terminal, a control terminal, and a switching unit that turns on / off in accordance with a signal input to the control terminal, and which steps down the voltage input from the power supply to the input terminal using the switching unit and outputs it from the output terminal, A first resistor element having one end connected to the output terminal of the voltage conversion unit and the other end connected to the load, A comparator having a positive input terminal connected to one end of the first resistive element, to which a voltage from the voltage conversion unit is input; a negative input terminal connected to the other end of the first resistive element, to which a voltage from the load is input; and an output terminal connected to the control terminal of the voltage conversion unit, which outputs an on / off signal according to the comparison result between the voltage input to the positive input terminal and the voltage input to the negative input terminal, A voltage offset section is provided between the negative input terminal of the comparator and the other end of the first resistive element, which offsets the voltage from the load. A constant current circuit equipped with the following features.
2. The voltage offset section comprises a second resistive element, a third resistive element, a fourth resistive element, a first Zener diode, and a first PNP transistor. One end of the second resistive element is connected to the power supply and the cathode of the first Zener diode. The other end of the second resistive element is connected to the emitter terminal of the first PNP transistor. One end of the third resistive element is connected to the anode of the first Zener diode. The other end of the third resistive element is connected to ground. One end of the fourth resistive element is connected to the negative input terminal of the comparator. The other end of the fourth resistive element is connected to the other end of the first resistive element. The base terminal of the first PNP transistor is connected to the one end of the third resistive element, The constant current circuit according to claim 1, wherein the collector terminal of the first PNP transistor is connected to one end of the fourth resistive element.
3. The voltage offset section comprises a fifth resistive element, a sixth resistive element, a second Zener diode, and a second PNP transistor. One end of the fifth resistor is connected to the one end of the second resistor and the cathode of the second Zener diode. The other end of the fifth resistor is connected to the emitter terminal of the second PNP transistor. One end of the sixth resistive element is connected to the anode of the second Zener diode, The other end of the sixth resistive element is connected to the output terminal of the comparator. The base terminal of the second PNP transistor is connected to one end of the sixth resistor element, The constant current circuit according to claim 2, wherein the collector terminal of the second PNP transistor is connected to one end of the fourth resistive element.