Power supply circuit

Abstract

To provide a power supply circuit capable of appropriately suppressing inrush current.SOLUTION: A power supply circuit (1) includes an inrush resistance element (3) and a switching element (4) parallel with each other and connected to a negative-side second output terminal (T4), and a control circuit (10). The control circuit turns on the switching element when an input voltage (V1) is below a first input threshold and an output voltage (V2) is at or above a first output threshold and turns off the switching element when the input voltage is at or above the first input threshold and the output voltage is below a second output threshold, which is greater than the first output threshold.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a power supply circuit. [Background technology]

[0002] Patent Document 1 discloses an inrush current suppression circuit, which switches a changeover switching element connected in parallel to a first resistor in accordance with a terminal voltage detected by a voltage detection circuit. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-122158 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when the input voltage fluctuates, switching the circuit based solely on the output voltage can result in large inrush currents.

[0005] An object of one embodiment of the present invention is to provide a power supply circuit that can appropriately suppress inrush current. [Means for solving the problem]

[0006] A power supply circuit according to a first aspect of the present invention comprises a first positive output terminal and a second negative output terminal, a resistive element connected to the second output terminal, a switching element connected in parallel to the resistive element, and a control circuit that detects an input voltage and an output voltage, wherein the control circuit is configured to turn on the switching element if the input voltage is less than a first input threshold and the output voltage is greater than or equal to a first output threshold, turn off the switching element if the input voltage is greater than or equal to the first input threshold and the output voltage is less than a second output threshold that is greater than the first output threshold, and turn on the switching element if the input voltage is greater than or equal to the first input threshold and the output voltage is greater than or equal to the second output threshold.

[0007] According to the above configuration, when the difference between the input voltage and the output voltage is large, the switching element is turned off, and when the difference becomes small, the switching element is turned on, thereby making it possible to appropriately suppress inrush current.

[0008] A power supply circuit according to a second aspect of the present invention may be configured in the above-mentioned first aspect such that the control circuit includes a first diode having a cathode connected to the first output terminal, a first transistor having a control terminal connected to an anode of the first diode, a second diode having a cathode connected to the first output terminal, a second transistor having a control terminal connected to an anode of the second diode, and a control transistor having a control terminal connected to one end of the first transistor and one end of the second transistor, one end of the control transistor being connected to the first output terminal and the other end of the control transistor being connected to a control terminal of the switching element, and the control circuit turns off the first transistor when the input voltage is equal to or greater than the first input threshold.

[0009] According to the above configuration, the inrush current can be suppressed by an analog circuit that operates in response to the input voltage and the output voltage, and therefore there is no need to increase the circuit size.

[0010] A power supply circuit according to a third aspect of the present invention may be configured in the second aspect above such that, in the control circuit, if the input voltage is less than the first input threshold and the output voltage is greater than or equal to a first output threshold, the first transistor is turned on, the control transistor is turned on, and the switching element is turned on; if the input voltage is greater than or equal to the first input threshold and the output voltage is less than the second output threshold, the first transistor is turned off, the second transistor is turned off, the control transistor is turned off, and the switching element is turned off; and if the input voltage is greater than or equal to the first input threshold and the output voltage is greater than or equal to the second output threshold, the first transistor is turned off, the second transistor is turned on, the control transistor is turned on, and the switching element is turned on.

[0011] A power supply circuit according to a fourth aspect of the present invention may be configured in accordance with the second aspect above, wherein the control circuit turns off the second transistor when the input voltage is equal to or greater than a second input threshold that is greater than the first input threshold.

[0012] According to the above configuration, for example, when an input voltage greater than the maximum allowable voltage is input, the first transistor and the second transistor are turned off. This allows the switching element to be kept off, thereby suppressing inrush current.

[0013] A power supply circuit according to aspect 5 of the present invention may be configured in the above-mentioned aspect 2 such that, when the input voltage is equal to or greater than the first input threshold, the control circuit turns off the first transistor by reducing the potential difference between the control terminal of the first transistor and the other end of the first transistor.

[0014] A power supply circuit according to a sixth aspect of the present invention may be configured such that, in the fifth aspect described above, the control circuit includes a third diode through which a reverse current flows when the input voltage is equal to or greater than the first input threshold, and a photocoupler connected to the third diode, and when the photocoupler is on, the photocoupler provides electrical conduction between the control terminal of the first transistor and the other end of the first transistor.

[0015] According to the above configuration, the control circuit can operate in accordance with the input voltage and the output voltage via the photocoupler, thereby switching the switching element on and off in accordance with the input voltage and the output voltage.

[0016] A power supply circuit according to a seventh aspect of the present invention may be configured in the second aspect above, wherein the control circuit includes a capacitor connected between the control terminal of the first transistor and the other end of the first transistor.

[0017] According to the above configuration, even if the input voltage is an AC or pulsating current, the first transistor can be stably maintained in an off state according to the input voltage.

[0018] A power supply circuit according to an eighth aspect of the present invention may be configured in the above-mentioned second aspect such that the first transistor and the second transistor are NPN bipolar transistors, and the control transistor is a PNP bipolar transistor.

[0019] A power supply circuit according to a ninth aspect of the present invention may be configured in the above-mentioned first aspect, wherein the switching element is an n-type field effect transistor.

[0020] According to the above configuration, the n-type field effect transistor with a small on-resistance is connected to the second negative output terminal, which reduces loss during normal operation.

[0021] A power supply circuit according to a tenth aspect of the present invention is the power supply circuit of any one of the first to ninth aspects above, wherein the control circuit detects the input voltage as a smoothed DC voltage.

[0022] According to the above configuration, there is no need to provide a capacitor for the first transistor, and the first transistor can stably operate in accordance with the input voltage.

[0023] The power supply circuit according to an eleventh aspect of the present invention is the power supply circuit of any one of the first to ninth aspects, further comprising a rectifier circuit that rectifies the AC input voltage, and the control circuit may be configured to detect the AC input voltage.

[0024] A power supply circuit according to aspect 12 of the present invention comprises a first positive output terminal and a second negative output terminal, a resistive element connected to the second output terminal, a switching element connected in parallel to the resistive element, and a control circuit that detects an input voltage and an output voltage, wherein the control circuit is configured to turn on the switching element if the input voltage is less than a first input threshold and the output voltage is equal to or greater than a first output threshold, and to turn off the switching element regardless of the output voltage if the input voltage is equal to or greater than the first input threshold.

[0025] According to the above configuration, for example, when an input voltage higher than the maximum allowable voltage is input, the switching element can be kept off, thereby appropriately suppressing inrush current. [Effects of the Invention]

[0026] According to one aspect of the present invention, it is possible to appropriately suppress inrush current. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a circuit diagram showing a configuration of a power supply circuit according to an embodiment of the present invention; [Figure 2] FIG. 2 is a circuit diagram of the power supply circuit, showing a specific example of a control circuit. [Figure 3]1 is a circuit diagram showing a configuration of a power supply circuit according to an embodiment of the present invention; [Figure 4] 1 is a circuit diagram showing a configuration of a power supply circuit according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0028] [Embodiment 1] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment according to one aspect of the present invention (hereinafter also referred to as "the present embodiment") will be described below with reference to the drawings.

[0029] §1 Application Examples 1 is a circuit diagram showing the configuration of a power supply circuit 1 of this embodiment. The power supply circuit 1 receives an AC voltage from its input side, converts the AC to DC, and outputs the DC voltage from its output side to a load 90. The power supply circuit 1 includes a first input terminal T1, a second input terminal T2, a first positive output terminal T3, a second negative output terminal T4, a rectifier circuit 2, an inrush resistance element 3, and a switching element 4. The inrush resistance element 3 is connected to the second negative output terminal T4. The switching element 4 is connected in parallel to the inrush resistance element 3. The inrush resistance element 3 and the switching element 4 are used to suppress inrush current.

[0030] Immediately after the input voltage is applied, the switching element 4 is off. The current flows through the inrush resistance element 3, so the inrush current is suppressed. The output voltage V2 between the first output terminal T3 and the second output terminal T4 gradually increases and approaches the input voltage V1. When V1 - V2 becomes sufficiently small, the switching element 4 is switched from off to on, suppressing the inrush current and stabilizing the power supply.

[0031] However, because the second negative output terminal T4 and the rectifier circuit 2 are connected via the inrush resistor 3, a potential difference occurs across the inrush resistor 3. In a situation where the input voltage is unknown, this potential difference cannot be determined from the output voltage V2 alone. Therefore, if the switching element 4 is switched from off to on based only on the output voltage V2, V1 - V2 may still be large, and a large inrush current may flow.

[0032] Therefore, the power supply circuit 1 of this embodiment detects the input voltage V1 and the output voltage V2, and switches the switching element 4 based on the input voltage V1 and the output voltage V2. In this way, the power supply circuit 1 appropriately suppresses the inrush current.

[0033] §2 Configuration example The configuration of the power supply circuit 1 will be described in more detail with reference to Figure 1. The power supply circuit 1 includes a first input terminal T1, a second input terminal T2, a first positive output terminal T3, a second negative output terminal T4, a rectifier circuit 2, an inrush resistance element 3, a switching element 4, a smoothing capacitor 5, a coil 6, a first diode 7, a second diode 8, a PFC transistor 9, and a control circuit 10.

[0034] The first input terminal T1 and the second input terminal T2 are terminals to which an AC input voltage is input. The rectifier circuit 2 rectifies the AC to DC. Here, the rectifier circuit 2 is a bridge circuit including a plurality of diodes, but is not limited to this and may be a circuit including a plurality of switching elements. The rectifier circuit 2 has a first AC terminal 2a connected to the first input terminal T1, a second AC terminal 2b connected to the second input terminal T2, a positive terminal 2c, and a negative terminal 2d. The rectifier circuit 2 outputs a DC (pulsating) input voltage V1 from the positive terminal 2c and the negative terminal 2d.

[0035] One end of the inrush resistance element 3 is connected to the negative terminal 2d of the rectifier circuit 2, and the other end of the inrush resistance element 3 is connected to the second output terminal T4. The inrush resistance element 3 may be, for example, a thermistor whose resistance decreases as the temperature increases.

[0036] The switching element 4 is connected in parallel to the inrush resistance element 3. The switching element 4 is, for example, a transistor. The switching element 4 is preferably an n-type field effect transistor. The n-type field effect transistor has a small on-resistance, which makes it possible to reduce loss. Because the n-type field effect transistor is controlled by a positive gate voltage, it is more convenient to provide the switching element 4 on the low side (between the negative side terminal 2d and the second output terminal T4) rather than on the high side (between the positive side terminal 2c and the first output terminal T3).

[0037] The smoothing capacitor 5 is connected between the first output terminal T3 and the second output terminal T4.

[0038] The coil 6 is connected to the positive terminal 2c. The anode of the first diode 7 is connected to the coil 6, and the cathode is connected to the first output terminal T3. The second diode 8 is connected in parallel to the coil 6 and the first diode 7. The cathode of the second diode 8 is connected to the first output terminal T3.

[0039] One end of the PFC transistor 9 is connected to the node between the coil 6 and the first diode 7, and the other end of the PFC transistor 9 is connected to the second output terminal T4. The coil 6, the first diode 7, the second diode 8, and the PFC transistor 9 form a boost circuit. The PFC transistor 9 is driven by a PFC control circuit (not shown) and improves the power factor of the power supply circuit 1.

[0040] The first output terminal T3 is a positive output terminal. The second output terminal T4 is a negative output terminal. The first output terminal T3 and the second output terminal T4 output an output voltage V2 between the first output terminal T3 and the second output terminal T4 to the load 90.

[0041] The control circuit 10 detects an input voltage V1 and an output voltage V2. The control circuit 10 controls the switching element 4 based on the input voltage V1 and the output voltage V2. For example, the control circuit 10 turns on the switching element 4 if the input voltage V1 is less than a first input threshold and the output voltage V2 is equal to or greater than a first output threshold. The control circuit 10 turns off the switching element 4 if the input voltage V1 is equal to or greater than the first input threshold and the output voltage V2 is less than a second output threshold that is greater than the first output threshold. The control circuit 10 turns on the switching element 4 if the input voltage V1 is equal to or greater than the first input threshold and the output voltage V2 is equal to or greater than a second output threshold.

[0042] As a result, when the input voltage V1 is larger, the control circuit 10 can switch on the switching element 4 when the output voltage V2 becomes larger. Therefore, even if the input voltage V1 fluctuates, the switching element 4 can be switched on when V1 - V2 becomes smaller. Therefore, the power supply circuit 1 can appropriately suppress inrush current.

[0043] (Configuration example of control circuit 10) 2 is a circuit diagram of the power supply circuit 1, showing a specific example of the control circuit 10. The control circuit 10 includes a first input diode Da1, a second input diode Da2, a third input diode Da3, a first photocoupler P1, a second photocoupler P2, a third photocoupler P3, a third diode 21, and an input capacitor 22. The control circuit 10 includes a first output diode Db1, a second output diode Db2, a third output diode Db3, a first transistor Q1, a second transistor Q2, a third transistor Q3, and a control transistor T. The control circuit 10 also includes a plurality of resistor elements for adjusting the current, as appropriate.

[0044] The cathode of the first output diode Db1 is connected to the first output terminal T3. The anode of the first output diode Db1 is connected to the control terminal of the first transistor Q1 via the resistor R1. The first output diode Db1 is a Zener diode in this example, but may be another type of diode.

[0045] One end of the first transistor Q1 is connected to the control terminal of the control transistor T via a resistor R7. The other end of the first transistor Q1 is connected to the second output terminal T4. The other end of the first transistor Q1 is connected to the control terminal of the first transistor Q1 via a resistor R4. Here, the first transistor Q1 is an NPN bipolar transistor. One end of the first transistor Q1 is the collector, the other end is the emitter, and the control terminal is the base.

[0046] The cathode of the second output diode Db2 is connected to the first output terminal T3. The anode of the second output diode Db2 is connected to the control terminal of the second transistor Q2 via a resistor R2. The second output diode Db2 is a Zener diode in this example, but may be another type of diode.

[0047] One end of the second transistor Q2 is connected to the control terminal of the control transistor T via a resistor R7. The other end of the second transistor Q2 is connected to the second output terminal T4. The other end of the second transistor Q2 is connected to the control terminal of the second transistor Q2 via a resistor R5. Here, the second transistor Q2 is an NPN bipolar transistor. One end of the second transistor Q2 is the collector, the other end is the emitter, and the control terminal is the base.

[0048] The cathode of the third output diode Db3 is connected to the first output terminal T3. The anode of the third output diode Db3 is connected to the control terminal of the third transistor Q3 via a resistor R3. The third output diode Db3 is a Zener diode in this example, but may be another type of diode.

[0049] One end of the third transistor Q3 is connected to the control terminal of the control transistor T via a resistor R7. The other end of the third transistor Q3 is connected to the second output terminal T4. The other end of the third transistor Q3 is connected to the control terminal of the third transistor Q3 via a resistor R6. Here, the third transistor Q3 is an NPN bipolar transistor. One end of the third transistor Q3 is the collector, the other end is the emitter, and the control terminal is the base.

[0050] One end of the control transistor T is connected to the first output terminal T3. The other end of the control transistor T is connected to the control terminal (gate) of the switching element 4. Here, the control transistor T is a PNP bipolar transistor. One end of the control transistor T is the emitter, the other end is the collector, and the control terminal is the base. Since the control terminal of the control transistor T is on the low potential side, it is advantageous for the control transistor T to be a PNP bipolar transistor.

[0051] The anode of the third diode 21 is connected to the positive terminal 2c of the rectifier circuit 2. The input capacitor 22 is connected between the cathode of the third diode 21 and the negative terminal 2d of the rectifier circuit 2.

[0052] The cathode of the first input diode Da1 is connected to the cathode of the third diode 21. The anode of the first input diode Da1 is connected to one end of the first photocoupler P1 on the light-emitting element side. Here, the first input diode Da1 is a Zener diode, but it may be another diode. The other end of the first photocoupler P1 on the light-emitting element side is connected to the negative terminal 2d of the rectifier circuit 2 via a resistor R11. One end of the first photocoupler P1 on the light-receiving element side is connected to the control terminal of the first transistor Q1. The other end of the first photocoupler P1 on the light-receiving element side is connected to the other end of the first transistor Q1.

[0053] The cathode of the second input diode Da2 is connected to the cathode of the third diode 21. The anode of the second input diode Da2 is connected to one end of the second photocoupler P2 on the light-emitting element side. Here, the second input diode Da2 is a Zener diode, but it may be another diode. The other end of the second photocoupler P2 on the light-emitting element side is connected to the negative terminal 2d of the rectifier circuit 2 via a resistor R12. One end of the second photocoupler P2 on the light-receiving element side is connected to the control terminal of the second transistor Q2. The other end of the second photocoupler P2 on the light-receiving element side is connected to the other end of the second transistor Q2.

[0054] The cathode of the third input diode Da3 is connected to the cathode of the third diode 21. The anode of the third input diode Da3 is connected to one end of the third photocoupler P3 on the light-emitting element side. Here, the third input diode Da3 is a Zener diode, but it may be another diode. The other end of the third photocoupler P3 on the light-emitting element side is connected to the negative terminal 2d of the rectifier circuit 2 via a resistor R13. One end of the third photocoupler P3 on the light-receiving element side is connected to the control terminal of the third transistor Q3. The other end of the third photocoupler P3 on the light-receiving element side is connected to the other end of the third transistor Q3.

[0055] (Operation of control circuit 10) The control circuit 10 detects the input voltage V1 smoothed by the third diode 21 and the input capacitor 22. The smoothed input voltage V1 is applied to the first input-side diode Da1, the second input-side diode Da2, and the third input-side diode Da3. Let the breakdown voltages of the first input-side diode Da1, the second input-side diode Da2, and the third input-side diode Da3 be Va1, Va2, and Va3, respectively. Va1, Va2, and Va3 are the thresholds at which reverse current flows. However, Va1 < Va2 < Va3. Therefore, as the input voltage V1 increases, the first input-side diode Da1, the second input-side diode Da2, and the third input-side diode Da3 will have reverse current flowing through them in sequence. Therefore, as the input voltage V1 increases, the first photocoupler P1, the second photocoupler P2, and the third photocoupler P3 will switch on in sequence.

[0056] For example, if the input voltage V1 is less than the first input threshold Va1, the first photocoupler P1, the second photocoupler P2, and the third photocoupler P3 are off. If the input voltage V1 is greater than or equal to the first input threshold Va1 and less than the second input threshold Va2, the first photocoupler P1 is on, and the second photocoupler P2 and the third photocoupler P3 are off. If the input voltage V1 is greater than or equal to the second input threshold Va2 and less than the third input threshold Va3, the first photocoupler P1 and the second photocoupler P2 are on, and the third photocoupler P3 is off. If the input voltage V1 is greater than or equal to the third input threshold Va3, the first photocoupler P1, the second photocoupler P2, and the third photocoupler P3 are on.

[0057] The control circuit 10 detects the output voltage V2. The output voltage V2 is applied to the first output-side diode Db1, the second output-side diode Db2, and the third output-side diode Db3. Let the yield voltages of the first output-side diode Db1, the second output-side diode Db2, and the third output-side diode Db3 be Vb1, Vb2, and Vb3, respectively. Vb1, Vb2, and Vb3 are the thresholds at which reverse current flows. However, Vb1 < Vb2 < Vb3. Therefore, as the output voltage V2 increases, the first output-side diode Db1, the second output-side diode Db2, and the third output-side diode Db3 sequentially have reverse current flowing through them.

[0058] Suppose the first photocoupler P1, the second photocoupler P2, and the third photocoupler P3 are off. In this case, if the output voltage V2 is less than the first output threshold Vb1, the first transistor Q1, the second transistor Q2, and the third transistor Q3 are off. If the output voltage V2 is greater than or equal to the first output threshold Vb1 and less than the second output threshold Vb2, the first transistor Q1 is on and the second transistor Q2 and the third transistor Q3 are off. If the output voltage V2 is greater than or equal to the second output threshold Vb2 and less than the third output threshold Vb3, the first transistor Q1 and the second transistor Q2 are on and the third transistor Q3 is off. If the output voltage V2 is greater than or equal to the third output threshold Vb3, the first transistor Q1, the second transistor Q2, and the third transistor Q3 are on.

[0059] When the first transistor Q1, the second transistor Q2, and the third transistor Q3 are off, the control transistor T turns off and the switching element 4 turns off. When at least one of the first transistor Q1, the second transistor Q2, and the third transistor Q3 is on, the control transistor T turns on, thereby switching the switching element 4 to on.

[0060] However, when the first photocoupler P1 is on, the first photocoupler P1 reduces the potential difference between the control terminal of the first transistor Q1 and the other end of the first transistor Q1. More specifically, when the first photocoupler P1 is on, the first photocoupler P1 establishes conduction (short-circuit) between the control terminal of the first transistor Q1 and the other end of the first transistor Q1. As a result, when the first photocoupler P1 is on, the first transistor Q1 is maintained off regardless of the magnitude of the output voltage V2.

[0061] Similarly, when the second photocoupler P2 is on, the second transistor Q2 is kept off regardless of the magnitude of the output voltage V2, and when the third photocoupler P3 is on, the third transistor Q3 is kept off regardless of the magnitude of the output voltage V2.

[0062] From this, the relationship between the input voltage V1, the output voltage V2, and the state of the switching element 4 can be summarized as follows: When the input voltage V1 is less than the first input threshold Va1 and the output voltage V2 is less than the first output threshold Vb1, the switching element 4 is turned off. If the input voltage V1 is less than the first input threshold Va1 and the output voltage V2 is equal to or greater than the first output threshold Vb1, the switching element 4 is turned on. When the input voltage V1 is equal to or greater than the first input threshold Va1 and less than the second input threshold Va2, and the output voltage V2 is less than the second output threshold Vb2, the switching element 4 is turned off. When the input voltage V1 is equal to or greater than the first input threshold Va1 and less than the second input threshold Va2, and the output voltage V2 is equal to or greater than the second output threshold Vb2, the switching element 4 is turned on. When the input voltage V1 is equal to or greater than the second input threshold Va2 and less than the third input threshold Va3, and the output voltage V2 is less than the third output threshold Vb3, the switching element 4 is turned off. When the input voltage V1 is equal to or greater than the second input threshold Va2 and less than the third input threshold Va3, and the output voltage V2 is equal to or greater than the third output threshold Vb3, the switching element 4 is turned on. If the input voltage V1 is equal to or greater than the third input threshold Va3, the switching element 4 is turned off.

[0063] In this way, the control circuit 10 switches the switching element 4 on and off based on the input voltage V1 and the output voltage V2. If the input voltage V1 is relatively small, the control circuit 10 switches the switching element 4 on based on a threshold value for the relatively small output voltage V2. If the input voltage V1 is relatively large, the control circuit 10 switches the switching element 4 on based on a threshold value for the relatively large output voltage V2. In this way, the control circuit 10 increases the threshold value for the output voltage V2 for switching the switching element 4 as the input voltage V1 increases. This allows the control circuit 10 to switch the switching element 4 on when V1-V2 becomes smaller to a certain extent in response to the input voltage V1, even if the input voltage V1 fluctuates. Furthermore, if the input voltage V1 is small, the control circuit 10 switches the switching element 4 on at a stage when the output voltage V2 is smaller, i.e., at an earlier stage. Therefore, the power supply circuit 1 can appropriately suppress inrush current in response to the input voltage V1 and quickly stabilize the power supply.

[0064] Furthermore, when an input voltage V1 higher than the maximum allowable voltage (V1≧Va3) is input, the power supply circuit 1 keeps the switching element 4 turned off to protect the load 90.

[0065] (Variation) Here, the configuration in which the output threshold value is switched in three stages has been exemplified, but it may be switched in two stages or in four or more stages.

[0066] The third input diode Da3 and the third photocoupler P3 may be omitted. In this case, when the output voltage V2 is equal to or greater than the third output threshold Vb3, the switching element 4 is turned on.

[0067] Alternatively, the circuits including the second input diode Da2, the second photocoupler P2, the third input diode Da3, the third photocoupler P3, the second output diode Db2, the second transistor Q2, the third output diode Db3, and the third transistor Q3 may be omitted. In this case, the output threshold is the first output threshold Vb1 only. On the other hand, if the input voltage V1 is equal to or greater than the first input threshold Va1, the switching element 4 is turned off regardless of the output voltage V2. In other words, if an input voltage V1 higher than the maximum allowable voltage (V1≧Va1) is input, the switching element 4 remains off to protect the load 90.

[0068] The control transistor T is not limited to a bipolar transistor but may be a field-effect transistor. The first transistor Q1, the second transistor Q2, and the third transistor Q3 are not limited to bipolar transistors but may be field-effect transistors. The switching element 4 is not limited to an n-type field-effect transistor but may be a p-type field-effect transistor or a bipolar transistor.

[0069] The control circuit 10 may be implemented as a logic circuit that controls the switching element 4 based on the input voltage V1 and the output voltage V2.

[0070] [Embodiment 2] Other embodiments of the present invention will be described below. For ease of explanation, the same reference numerals will be used to designate components having the same functions as those described in the above embodiment, and the description thereof will not be repeated.

[0071] 3 is a circuit diagram showing the configuration of a power supply circuit 1a according to this embodiment. The power supply circuit 1a includes a control circuit 10a instead of the control circuit 10. The control circuit 10a does not include the third diode 21 and the input capacitor 22. The control circuit 10a includes a first capacitor C1, a second capacitor C2, and a third capacitor C3.

[0072] The cathode of the first input side diode Da1, the cathode of the second input side diode Da2, and the cathode of the third input side diode Da3 are connected to the positive terminal 2c of the rectifier circuit 2.

[0073] The first capacitor C1 is connected between the control terminal of the first transistor Q1 and the other end of the first transistor Q1. The second capacitor C2 is connected between the control terminal of the second transistor Q2 and the other end of the second transistor Q2. The third capacitor C3 is connected between the control terminal of the third transistor Q3 and the other end of the third transistor Q3.

[0074] The input voltage V1 applied to the first input diode Da1, the second input diode Da2, and the third input diode Da3 is a pulsating current. Therefore, in the power supply circuit 1a, the first photocoupler P1 repeatedly turns on and off even when the peak of the input voltage V1 is equal to or greater than the first input threshold Va1. While the first photocoupler P1 is off, the reverse current flowing through the first output diode Db1 flows into the first capacitor C1. When the first photocoupler P1 repeatedly turns on and off with the pulsating cycle of the input voltage V1, no current flows between the control terminal of the first transistor Q1 and the other end of the first transistor Q1, and the first transistor Q1 remains off.

[0075] When the peak of the input voltage V1 is below the first input threshold Va1, the first photocoupler P1 remains off. In this case, when the first capacitor C1 is charged to a certain extent by the reverse current flowing through the first output diode Db1, current begins to flow between the control terminal of the first transistor Q1 and the other end of the first transistor Q1. This turns on the first transistor Q1. The other second and third capacitors C2 and C3 function in a similar manner.

[0076] Therefore, the power supply circuit 1a operates in the same manner as the power supply circuit 1 of embodiment 1. In the power supply circuit 1a, the third diode 21 and the input capacitor 22 of the power supply circuit 1 of embodiment 1 are omitted, and instead a first capacitor C1, a second capacitor C2, and a third capacitor C3 are provided. Note that when the number of output threshold stages is large, the power supply circuit 1 of embodiment 1 can reduce costs.

[0077] [Embodiment 3] Other embodiments of the present invention will be described below. For ease of explanation, the same reference numerals will be used to designate components having the same functions as those described in the above embodiment, and the description thereof will not be repeated.

[0078] 4 is a circuit diagram showing the configuration of a power supply circuit 1b of this embodiment. The power supply circuit 1b includes a control circuit 10b instead of the control circuit 10. The control circuit 10b does not include the third diode 21 and the input capacitor 22. The control circuit 10b includes a first capacitor C1, a second capacitor C2, a third capacitor C3, a fourth input diode Da4, a fifth input diode Da5, and a sixth input diode Da6.

[0079] The cathode of the first input diode Da1 is connected to the first input terminal T1. The anode of the first input diode Da1 is connected to one end of the first photocoupler P1 on the light-emitting element side. The other end of the first photocoupler P1 on the light-emitting element side is connected to the anode of the fourth input diode Da4. The cathode of the fourth input diode Da4 is connected to the second input terminal T2 via a resistor R11.

[0080] The cathode of the second input diode Da2 is connected to the first input terminal T1. The anode of the second input diode Da2 is connected to one end of the second photocoupler P2 on the light-emitting element side. The other end of the second photocoupler P2 on the light-emitting element side is connected to the anode of the fifth input diode Da5. The cathode of the fifth input diode Da5 is connected to the second input terminal T2 via the resistor R12.

[0081] The cathode of the third input diode Da3 is connected to the first input terminal T1. The anode of the third input diode Da3 is connected to one end of the third photocoupler P3 on the light-emitting element side. The other end of the third photocoupler P3 on the light-emitting element side is connected to the anode of the sixth input diode Da6. The cathode of the sixth input diode Da6 is connected to the second input terminal T2 via a resistor R13.

[0082] The breakdown voltage of the fourth input diode Da4 is the same as the breakdown voltage Va1 of the first input diode Da1. The breakdown voltage of the fifth input diode Da5 is the same as the breakdown voltage Va2 of the second input diode Da2. The breakdown voltage of the sixth input diode Da6 is the same as the breakdown voltage Va3 of the third input diode Da3.

[0083] The control circuit 10b detects an AC input voltage V1. The AC input voltage V1 is input between the first input terminal T1 and the second input terminal T2. The AC input voltage V1 is applied to the first input diode Da1 and the fourth input diode Da4. If the absolute value of the input voltage V1 is equal to or greater than the first input threshold Va1, the first photocoupler P1 is turned on. Similarly, if the absolute value of the input voltage V1 is equal to or greater than the second input threshold Va2, the second photocoupler P2 is turned on. If the absolute value of the input voltage V1 is equal to or greater than the third input threshold Va3, the third photocoupler P3 is turned on. In the control circuit 10b, the first capacitor C1, the second capacitor C2, and the third capacitor C3 function similarly to the control circuit 10a of the second embodiment. Therefore, the power supply circuit 1b operates similarly to the power supply circuit 1 of the first embodiment.

[0084] 〔summary〕 A power supply circuit according to a first aspect of the present invention comprises a first positive output terminal and a second negative output terminal, a resistive element connected to the second output terminal, a switching element connected in parallel to the resistive element, and a control circuit that detects an input voltage and an output voltage, wherein the control circuit is configured to turn on the switching element if the input voltage is less than a first input threshold and the output voltage is greater than or equal to a first output threshold, turn off the switching element if the input voltage is greater than or equal to the first input threshold and the output voltage is less than a second output threshold that is greater than the first output threshold, and turn on the switching element if the input voltage is greater than or equal to the first input threshold and the output voltage is greater than or equal to the second output threshold.

[0085] A power supply circuit according to a second aspect of the present invention may be configured in the above-mentioned first aspect such that the control circuit includes a first diode having a cathode connected to the first output terminal, a first transistor having a control terminal connected to an anode of the first diode, a second diode having a cathode connected to the first output terminal, a second transistor having a control terminal connected to an anode of the second diode, and a control transistor having a control terminal connected to one end of the first transistor and one end of the second transistor, one end of the control transistor being connected to the first output terminal and the other end of the control transistor being connected to a control terminal of the switching element, and the control circuit turns off the first transistor when the input voltage is equal to or greater than the first input threshold.

[0086] A power supply circuit according to a third aspect of the present invention may be configured in the second aspect above such that, in the control circuit, if the input voltage is less than the first input threshold and the output voltage is greater than or equal to a first output threshold, the first transistor is turned on, the control transistor is turned on, and the switching element is turned on; if the input voltage is greater than or equal to the first input threshold and the output voltage is less than the second output threshold, the first transistor is turned off, the second transistor is turned off, the control transistor is turned off, and the switching element is turned off; and if the input voltage is greater than or equal to the first input threshold and the output voltage is greater than or equal to the second output threshold, the first transistor is turned off, the second transistor is turned on, the control transistor is turned on, and the switching element is turned on.

[0087] A power supply circuit according to a fourth aspect of the present invention may be configured in accordance with the second or third aspect above, wherein the control circuit turns off the second transistor when the input voltage is equal to or greater than a second input threshold that is greater than the first input threshold.

[0088] A power supply circuit according to aspect 5 of the present invention may be configured such that, in any of aspects 2 to 4 above, the control circuit turns off the first transistor by reducing the potential difference between the control terminal of the first transistor and the other end of the first transistor when the input voltage is equal to or greater than the first input threshold.

[0089] A power supply circuit according to a sixth aspect of the present invention may be configured such that, in the fifth aspect described above, the control circuit includes a third diode through which a reverse current flows when the input voltage is equal to or greater than the first input threshold, and a photocoupler connected to the third diode, and when the photocoupler is on, the photocoupler provides electrical conduction between the control terminal of the first transistor and the other end of the first transistor.

[0090] A power supply circuit according to a seventh aspect of the present invention may be configured in any one of the second to sixth aspects above, wherein the control circuit includes a capacitor connected between the control terminal of the first transistor and the other end of the first transistor.

[0091] A power supply circuit according to aspect 8 of the present invention may be configured in any one of aspects 2 to 7 above, such that the first transistor and the second transistor are NPN bipolar transistors, and the control transistor is a PNP bipolar transistor.

[0092] A power supply circuit according to a ninth aspect of the present invention may be configured in any one of the first to eighth aspects, wherein the switching element is an n-type field effect transistor.

[0093] A power supply circuit according to a tenth aspect of the present invention is the power supply circuit of any one of the first to ninth aspects above, wherein the control circuit detects the input voltage as a direct current.

[0094] The power supply circuit according to an eleventh aspect of the present invention is the power supply circuit of any one of the first to ninth aspects, further comprising a rectifier circuit that rectifies the AC input voltage, and the control circuit may be configured to detect the AC input voltage.

[0095] A power supply circuit according to aspect 12 of the present invention comprises a first positive output terminal and a second negative output terminal, a resistive element connected to the second output terminal, a switching element connected in parallel to the resistive element, and a control circuit that detects an input voltage and an output voltage, wherein the control circuit is configured to turn on the switching element if the input voltage is less than a first input threshold and the output voltage is equal to or greater than a first output threshold, and to turn off the switching element regardless of the output voltage if the input voltage is equal to or greater than the first input threshold.

[0096] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Explanation of symbols]

[0097] 1, 1a, 1b power circuit 2 Rectifier circuit 3 Inrush resistance element 4 Switching elements 5 smoothing capacitors 9 PFC transistor 10, 10a, 10b Control circuit 21 Third diode 22 Input capacitor 90 load C1 First capacitor C2 Second capacitor C3 Third capacitor Da1 First input diode Da2 Second input diode Da3 Third input diode Da4 4th input diode Da5 Fifth input diode Da6 6th input diode Db1 First output diode Db2 Second output diode Db3 Third output diode P1 1st photocoupler P2 Second photocoupler P3 3rd photocoupler Q1 First transistor Q2 Second transistor Q3 Third transistor T control transistor T1 First input terminal T2 Second input terminal T3 1st output terminal T4 Second output terminal V1 input voltage V2 Output voltage

Claims

1. a first positive output terminal and a second negative output terminal; a resistive element connected to the second output terminal; a switching element connected in parallel to the resistance element; a control circuit for detecting an input voltage and an output voltage; The control circuit If the input voltage is less than a first input threshold and the output voltage is greater than or equal to a first output threshold, turning on the switching element; turning off the switching element when the input voltage is equal to or greater than the first input threshold and the output voltage is less than a second output threshold that is greater than the first output threshold; The power supply circuit turns on the switching element if the input voltage is equal to or greater than the first input threshold and the output voltage is equal to or greater than the second output threshold.

2. The control circuit a first diode having a cathode connected to the first output terminal; a first transistor having a control terminal connected to the anode of the first diode; a second diode having a cathode connected to the first output terminal; a second transistor having a control terminal connected to the anode of the second diode; a control transistor having a control terminal connected to one end of the first transistor and one end of the second transistor, one end of the control transistor is connected to the first output terminal, and the other end of the control transistor is connected to a control terminal of the switching element; 2. The power supply circuit according to claim 1, wherein the control circuit turns off the first transistor when the input voltage is equal to or greater than the first input threshold.

3. In the control circuit, If the input voltage is less than the first input threshold and the output voltage is greater than or equal to a first output threshold, the first transistor is turned on, the control transistor is turned on, and the switching element is turned on; When the input voltage is greater than or equal to the first input threshold and the output voltage is less than the second output threshold, the first transistor is turned off, the second transistor is turned off, the control transistor is turned off, and the switching element is turned off; 3. The power supply circuit of claim 2, wherein when the input voltage is greater than or equal to the first input threshold and the output voltage is greater than or equal to the second output threshold, the first transistor is turned off, the second transistor is turned on, the control transistor is turned on, and the switching element is turned on.

4. 3. The power supply circuit according to claim 2, wherein the control circuit turns off the second transistor when the input voltage is equal to or greater than a second input threshold that is greater than the first input threshold.

5. 3. The power supply circuit according to claim 2, wherein the control circuit turns off the first transistor by reducing a potential difference between a control terminal of the first transistor and the other end of the first transistor when the input voltage is equal to or greater than the first input threshold.

6. The control circuit a third diode through which a reverse current flows when the input voltage is equal to or higher than the first input threshold; a photocoupler connected to the third diode, 6. The power supply circuit according to claim 5, wherein when said photocoupler is on, said photocoupler establishes conduction between said control terminal of said first transistor and the other end of said first transistor.

7. 3. The power supply circuit according to claim 2, wherein the control circuit comprises a capacitor connected between a control terminal of the first transistor and the other end of the first transistor.

8. the first transistor and the second transistor are NPN bipolar transistors, 3. The power supply circuit according to claim 2, wherein the control transistor is a PNP bipolar transistor.

9. 2. The power supply circuit according to claim 1, wherein the switching element is an n-type field effect transistor.

10. 10. The power supply circuit according to claim 1, wherein the control circuit detects the input voltage as a smoothed DC voltage.

11. a rectifier circuit that rectifies the AC input voltage; The power supply circuit according to claim 1 , wherein the control circuit detects the input voltage as an AC voltage.

12. a first positive output terminal and a second negative output terminal; a resistive element connected to the second output terminal; a switching element connected in parallel to the resistance element; a control circuit for detecting an input voltage and an output voltage; The control circuit If the input voltage is less than a first input threshold and the output voltage is greater than or equal to a first output threshold, turning on the switching element; If the input voltage is equal to or higher than the first input threshold, the power supply circuit turns off the switching element regardless of the output voltage.

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