Switching power supply, ac-dc circuit, input voltage detection circuit and method

Abstract

This invention provides a switching power supply, an AC-DC circuit, an input voltage detection circuit, and a method. The first end of the secondary coil of a transformer is connected to the anode of a rectifier diode, and the cathode of the rectifier diode is connected to the positive terminal of a filter capacitor. The second end of the secondary coil of the transformer, the negative terminal of the filter capacitor, and the positive terminal of the sampling capacitor are all grounded. The cathode of the diode is connected to the first end of the secondary coil of the transformer, and the anode is connected to the negative terminal of the sampling capacitor. The negative terminal of the sampling capacitor is grounded sequentially through a third sampling resistor and a second sampling resistor. The common terminal of the third and second sampling resistors is connected to a voltage detection terminal and, through a first sampling resistor, to a reference voltage input terminal. A reference voltage is input to the reference voltage input terminal. The control unit detects the voltage at the voltage detection terminal and calculates the magnitude of the input AC voltage based on the relationship between the AC voltage and the voltage at the voltage detection terminal.

Description

Technical Field

[0001] This invention relates to the field of power supplies, specifically to switching power supplies, AC-DC circuits, input voltage detection circuits and methods. Background Technology

[0002] AC-DC circuits are used in switching power supplies in various electronic devices. In an AC-DC circuit, the AC voltage is first rectified into DC voltage by a rectifier circuit and output to the bus. Then, the DC-DC unit converts the voltage on the bus to the required target voltage.

[0003] In some cases, AC-DC circuits need to know the magnitude of the input AC voltage. There are some existing technical solutions for detecting the magnitude of the input AC voltage in AC-DC circuits, but these solutions are either too complex or not accurate enough. Summary of the Invention

[0004] Based on the above situation, the main objective of this invention is to provide a switching power supply, an AC-DC circuit, an input voltage detection circuit, and a method.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] An input voltage detection circuit for an AC-DC circuit, wherein the AC-DC circuit includes a rectifier circuit and a DC-DC unit, the DC-DC unit including a transformer, a filter capacitor, and a rectifier diode; the rectifier circuit rectifies the input AC voltage to obtain a DC voltage and provides the DC voltage to the bus; the DC-DC unit converts the voltage on the bus into a target output voltage; the input voltage detection circuit includes a sampling capacitor, a first sampling resistor, a second sampling resistor, a third sampling resistor, a diode, and a control unit; the control unit also has a reference voltage input terminal and a voltage detection terminal; the first terminal of the secondary coil of the transformer is connected to the anode of the rectifier diode, and the cathode of the rectifier diode is connected to the positive terminal of the filter capacitor. The second terminal of the secondary coil of the transformer, the negative terminal of the filter capacitor, and the positive terminal of the sampling capacitor are all grounded. The end of the primary coil of the transformer connected to the busbar is the same terminal as the second terminal of the secondary coil. The cathode of the diode is connected to the first terminal of the secondary coil of the transformer, and the anode is connected to the negative terminal of the sampling capacitor. The negative terminal of the sampling capacitor is grounded in sequence through a third sampling resistor and a second sampling resistor. The common terminal of the third sampling resistor and the second sampling resistor is connected to the voltage detection terminal, and is connected to the reference voltage input terminal through the first sampling resistor. The reference voltage input terminal inputs a reference voltage. The control unit detects the voltage at the voltage detection terminal and calculates the magnitude of the input AC voltage based on the relationship between the AC voltage and the voltage at the voltage detection terminal.

[0007] Preferably, the input voltage detection circuit further includes a dummy load resistor; during a set period after the AC-DC circuit is powered on, the control unit controls the dummy load resistor to connect to the discharge circuit of the filter capacitor, and the control unit detects the voltage at the voltage detection terminal, and calculates the magnitude of the input AC voltage based on the relationship between the AC voltage and the voltage at the voltage detection terminal; after the set period, the control unit controls the dummy load resistor not to connect to the discharge circuit of the filter capacitor, and the control unit stops detecting the voltage at the voltage detection terminal.

[0008] Preferably, the relationship between the AC voltage and the voltage at the voltage detection terminal is: Vac={-Vsen+[(Vref-Vsen) / R1+Vsen / R2]*R3+VD1}*NP / (NS*k); where VD1 is the voltage drop of diode D1, NS and NP are the number of turns of the secondary and primary coils of the transformer, respectively, Vsen is the voltage detected at the voltage detection terminal, Vref is the reference voltage, R1, R2 and R3 are the resistance values ​​of the first sampling resistor, the second sampling resistor and the third sampling resistor, respectively, and k is the ratio of the voltage output by the rectifier circuit to the input AC voltage.

[0009] Preferably, the input voltage detection circuit further includes a resistor switch, and the dummy load resistor and the resistor switch are connected in series between the positive and negative terminals of the filter capacitor; during a set period after the AC-DC circuit is powered on, the control unit controls the resistor switch to be turned on so that the dummy load resistor is connected to the discharge circuit of the filter capacitor; after the set period, the control unit controls the resistor switch to be turned off so that the dummy load resistor is not connected to the discharge circuit of the filter capacitor.

[0010] Preferably, the AC-DC circuit further includes a first bus capacitor, a second bus capacitor, a capacitor switch, and an optocoupler; one end of the first bus capacitor is connected to the bus and the other end is connected to ground, and the second bus capacitor and the capacitor switch are connected in series between the bus and ground; when the AC voltage is less than a voltage threshold, the control unit controls the capacitor switch to conduct through the optocoupler, thereby connecting the second bus capacitor to the charging and discharging circuit of the bus; when the AC voltage is greater than or equal to the voltage threshold, the control unit controls the capacitor switch to disconnect through the optocoupler, thereby preventing the second bus capacitor from being connected to the charging and discharging circuit of the bus.

[0011] The present invention also provides an AC-DC circuit, including any of the input voltage detection circuits described above.

[0012] The present invention also provides a switching power supply, including the aforementioned input voltage detection circuit.

[0013] The present invention also provides an input voltage detection method for the aforementioned AC-DC circuit, wherein the control unit detects the voltage at the voltage detection terminal and calculates the magnitude of the input AC voltage based on the relationship between the AC voltage and the voltage at the voltage detection terminal.

[0014] Preferably, the input voltage detection method employs the input voltage detection circuit. During a set period after the AC-DC circuit is powered on, the control unit controls the dummy load resistor to connect to the discharge circuit of the filter capacitor, and the control unit detects the voltage at the voltage detection terminal, calculating the magnitude of the input AC voltage based on the relationship between the AC voltage and the voltage at the voltage detection terminal. After the set period, the control unit controls the dummy load resistor not to connect to the discharge circuit of the filter capacitor, and the control unit stops detecting the voltage at the voltage detection terminal.

[0015] Preferably, the input voltage detection method employs the input voltage detection circuit. When the AC voltage is less than the voltage threshold, the control unit controls the capacitor switch to turn on, thereby connecting the second bus capacitor to the charging and discharging circuit of the bus. When the AC voltage is greater than or equal to the voltage threshold, the control unit controls the capacitor switch to turn off, thereby preventing the second bus capacitor from being connected to the charging and discharging circuit of the bus.

[0016] [Beneficial Effects]

[0017] This invention achieves the detection of input AC voltage using a sampling capacitor, a first sampling resistor, a second sampling resistor, a third sampling resistor, a diode, and a control unit, resulting in a simple circuit. In some embodiments, during a set period of the AC-DC circuit power-on process, a dummy load resistor is connected to the discharge circuit of the filter capacitor to complete the input voltage detection. This allows the voltage at the negative terminal of the sampling capacitor to be more stable rather than fluctuating significantly, thus improving the accuracy of the detected input voltage. Furthermore, since the input voltage detection is completed during the AC-DC circuit power-on process, the impact on the actual load of the AC-DC circuit is reduced.

[0018] In some embodiments, when the AC voltage is less than the voltage threshold, the second bus capacitor is connected to the bus charging and discharging circuit by controlling the capacitor switch to turn on. The second bus capacitor can also provide a portion of the discharge current. Overall, the first and second bus capacitors only need to perform small-amplitude charging and discharging, resulting in relatively small ripple on the bus and fewer harmonics in the AC circuit connected to the rectifier circuit. When the AC voltage is greater than or equal to the voltage threshold, the bus voltage is larger. To meet the output power requirements at the output terminal and maintain the target voltage, only a small discharge current needs to be provided by the bus capacitor. Providing this discharge current solely through the first bus capacitor will not cause large ripple on the bus. Therefore, by controlling the capacitor switch to turn off, the second bus capacitor is not connected to the bus charging and discharging circuit, avoiding the connection of more components such as the second bus capacitor and capacitor switch to the bus, which would lead to more power consumption. At the same time, the smaller bus ripple results in smaller ripple in the AC circuit. This embodiment, with a simple circuit structure, can adaptively balance the reduction of harmonics and power consumption in various regions with different AC voltage levels.

[0019] Other beneficial effects of the present invention will be explained in detail through the introduction of specific technical features and technical solutions in specific embodiments. Those skilled in the art should be able to understand the beneficial technical effects brought about by these technical features and technical solutions through the introduction of these technical features and technical solutions. Attached Figure Description

[0020] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. In the drawings:

[0021] Figure 1 This is a schematic diagram of an AC-DC circuit according to an embodiment of the present invention;

[0022] Figure 2 This is a schematic diagram of an AC-DC circuit according to another embodiment of the present invention;

[0023] Figure 3 This is a timing diagram of multiple voltages during the power-on process of an AC-DC circuit in one embodiment of the present invention. Detailed Implementation

[0024] The present invention is described below based on embodiments, but the present invention is not limited to these embodiments. In the following detailed description of the present invention, some specific details are described in detail, but well-known methods, processes, procedures, and elements are not described in detail in order to avoid obscuring the essence of the present invention.

[0025] Furthermore, those skilled in the art should understand that the accompanying drawings provided herein are for illustrative purposes only and are not necessarily drawn to scale.

[0026] Unless the context explicitly requires it, the words "comprising," "including," and similar terms throughout the specification and claims should be interpreted as encompassing rather than being exclusive or exhaustive; that is, meaning "including but not limited to."

[0027] In the description of this invention, it should be understood that the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0028] Figure 1 This is an embodiment of the AC-DC circuit (i.e., AC-DC converter circuit) of the present invention, including a rectifier circuit, a first bus capacitor EC1, a DC-DC unit (i.e., DC-DC converter unit) and an input voltage detection circuit.

[0029] The input terminal of the rectifier circuit is used to input an AC voltage Vac. The rectifier circuit rectifies the input AC voltage Vac to obtain a DC voltage and supplies the DC voltage to the bus. The rectifier circuit can be a full-bridge rectifier or a half-bridge rectifier, etc.

[0030] The input terminal of the DC-DC unit receives the voltage from the bus. The DC-DC unit converts this bus voltage into the target output voltage. Specifically, by controlling the DC-DC unit, the bus voltage is converted into the target output voltage. This target voltage can be set according to appropriate needs. For common consumer electronics products, 5V, 12V, and 24V are relatively common target voltages. Figure 2 As shown, the DC-DC unit includes a switching transistor Q1, a transformer T, a rectifier diode D2, and a filter capacitor EC3. The first end of the primary coil of the transformer T is connected to the bus, and the second end is grounded through the switching transistor Q1. The first end of the secondary coil of the transformer T is connected to the anode of the rectifier diode D2, and the cathode of the rectifier diode D2 is connected to the positive terminal of the filter capacitor EC3. The negative terminal of the filter capacitor EC3 is grounded, and the second end of the secondary coil of the transformer T is grounded. The cathode of the rectifier diode D2 serves as the output terminal Vout of the DC-DC unit, which is also the output terminal Vout of the AC-DC circuit. This output terminal Vout outputs the target voltage. By adjusting the duty cycle of the control signal PWM of the switching transistor Q1, the voltage at the output terminal Vout can be stabilized at the set target voltage.

[0031] The input voltage detection circuit includes a sampling capacitor C1, a first sampling resistor R1, a second sampling resistor R2, a third sampling resistor R3, a diode D1, and a control unit IC. The control unit IC also has a reference voltage input terminal VREF and a voltage detection terminal Vsen. The control unit IC can be a control chip, and its power supply terminal Vin is connected to the positive terminal of the filter capacitor, meaning the power supply voltage of the control chip IC is the output voltage of the AC-DC circuit. The first terminal of the secondary coil of transformer T is connected to the anode of rectifier diode D2, and the cathode of rectifier diode D2 is connected to the positive terminal of filter capacitor EC3. The second terminal of the secondary coil of transformer T, the negative terminal of filter capacitor EC3, and the positive terminal of sampling capacitor C1 are all grounded. The primary coil of transformer T... One end of the coil connected to the busbar is the same terminal as the second end of the secondary coil; the cathode of diode D1 is connected to the first end of the secondary coil of transformer T, and the anode is connected to the negative terminal of sampling capacitor C1. The negative terminal of sampling capacitor C1 is grounded through the third sampling resistor R3 and the second sampling resistor R2 in sequence. The common terminal of the third sampling resistor R3 and the second sampling resistor R2 is connected to the voltage detection terminal Vsen, and is connected to the reference voltage input terminal VREF through the first sampling resistor R1. The reference voltage input terminal VREF inputs the reference voltage Vref; the control unit IC calculates the magnitude of the input AC voltage Vac based on the relationship between the AC voltage Vac and the voltage of the voltage detection terminal Vsen by detecting the voltage of the voltage detection terminal Vsen.

[0032] Since the positive terminal of sampling capacitor C1 is grounded and the negative terminal is connected to the first end of the secondary coil through diode D1, the voltage VC1 at the negative terminal of sampling capacitor C1 satisfies: VC1=0-(VDC*NS / NP-VD1), where VDC is the voltage on the bus, VD1 is the voltage drop of diode D1, and NS and NP are the number of turns of the secondary coil and primary coil of transformer T, respectively. Since the voltage VC1 at the negative terminal of sampling capacitor C1 is negative, the current flowing through the third sampling resistor R3 is equal to the sum of the current flowing through the first sampling resistor R1 and the current flowing through the second sampling resistor R2, i.e.: (Vsen-VC1) / R3=(Vref-Vsen) / R1+Vsen / R2, where Vsen is the voltage detected at the voltage detection terminal, Vref is the reference voltage, and R1, R2, and R3 are the resistance values ​​of the first sampling resistor R1, the second sampling resistor R2, and the third sampling resistor R3, respectively. By solving the above equation, the voltage VC1 can be calculated. Combining this with the previous formula, the voltage VDC on the bus can be calculated. If the ratio of the output voltage of the rectifier circuit (i.e., the voltage VDC on the bus) to the input AC voltage Vac is k (for example, in a certain type of rectifier circuit, k=√2 (i.e., the square root of 2)), then the relationship between the AC voltage Vac and the voltage at the voltage detection terminal is:

[0033] Vac={-Vsen+[(Vref-Vsen) / R1+Vsen / R2]*R3+VD1}*NP / (NS*k);

[0034] In this embodiment, since the positive terminal of the sampling capacitor C1 is grounded and the negative terminal is connected to the first end of the secondary coil through the diode D1, and the end of the primary coil of the transformer T connected to the bus is the same terminal as the second end of the secondary coil, the voltage VC1 of the negative terminal of the sampling capacitor C1 can follow the change of the bus voltage.

[0035] To minimize the impact on the actual load of the AC-DC circuit, this input voltage detection circuit detects the input voltage within a set time period after the AC-DC circuit starts powering on (e.g., between 10ms and 1s after power-on). However, in actual use, the AC-DC circuit may be in an unloaded state during this power-on process. Research has shown that when the AC-DC circuit is unloaded, the accuracy of the input voltage detected by the above-mentioned input voltage detection circuit is low because the voltage VC1 at the negative terminal of the sampling capacitor C1 is unstable at this time. To minimize the impact on the actual load of the AC-DC circuit while maintaining the accuracy of the calculated AC voltage Vac, some embodiments include a dummy load resistor R6 in the input voltage detection circuit. During a set period after the AC-DC circuit powers on (e.g., between 10ms and 1s after power-on), the control unit IC controls the dummy load resistor R6 to connect to the discharge circuit of the filter capacitor EC3, causing the DC-DC unit to exit the no-load mode (Burst mode). At this time, the dummy load resistor R6 acts as a load, drawing current from the filter capacitor EC3. This makes the voltage VC1 at the negative terminal of the sampling capacitor C1 more stable rather than fluctuating significantly, thus improving the accuracy of the detected input voltage. Furthermore, since the input voltage detection is completed during the AC-DC circuit power-on process, the impact on the actual load of the AC-DC circuit is reduced. After the set period, the control unit IC controls the dummy load resistor R6 to disconnect from the discharge circuit of the filter capacitor EC3, and the control unit IC stops detecting the voltage at the voltage detection terminal Vsen.

[0036] In some embodiments, the input voltage detection circuit further includes a resistor switch Q2. The dummy load resistor R6 and the resistor switch Q2 are connected in series between the positive and negative terminals of the filter capacitor EC3. The control unit IC also includes a control terminal IO1. During a set period after the AC-DC circuit is powered on, the control unit IC controls the resistor switch Q2 to be turned on through the control terminal IO1 so that the dummy load resistor R6 is connected to the discharge circuit of the filter capacitor EC3. After the set period, the control unit IC controls the resistor switch Q2 to be turned off through the control terminal IO1 so that the dummy load resistor R6 is not connected to the discharge circuit of the filter capacitor EC3. Figure 3This is a timing diagram of multiple voltages during the power-on process of an AC-DC circuit in one embodiment of the present invention, such as... Figure 3 As shown, the AC-DC circuit starts powering on (i.e., time 0) and continues until a certain time (at... Figure 3 The reference voltage Vref only stabilizes after 10ms. At this time, the control unit IC outputs a high level at the control terminal IO1, thereby connecting the dummy load resistor R6 into the discharge circuit of the filter capacitor EC3; when a certain moment is reached (at Figure 3 When the control unit IC outputs a low level at the control terminal IO1 (for 1S), the dummy load resistor R6 is not connected to the discharge circuit of the filter capacitor EC3.

[0037] Because the AC voltage Vac varies significantly across different regions' power grids—for example, China's AC voltage Vac is 220V, while the United States' is 110V, a difference of more than double—the voltage output from the rectifier circuit will also differ, meaning the voltage on the bus will vary. When the output power demand at the output terminal Vout is constant, if the connected AC voltage Vac is low, the bus capacitor needs to provide a larger discharge current to meet the output power demand of Vout. If only the first bus capacitor EC1 provides this discharge current, it will cause EC1 to undergo significant charging and discharging, resulting in large ripple on the bus. This, in turn, leads to more harmonics in the AC circuit connected to the rectifier circuit, which reduces the electromagnetic compatibility (EMC) performance evaluation of electronic devices containing this AC-DC circuit.

[0038] Therefore, to mitigate the aforementioned impacts, in some embodiments, the AC-DC circuit further includes a second bus capacitor EC2, a capacitor switch Q3, and an optocoupler. The second bus capacitor EC2 and the capacitor switch Q3 are connected in series between the bus and ground. When the AC voltage Vac is less than the voltage threshold, the control unit IC controls the capacitor switch Q3 to conduct, thereby connecting the second bus capacitor EC2 to the bus's charging and discharging circuit. When the AC voltage Vac is greater than or equal to the voltage threshold, the control unit IC controls the capacitor switch Q3 to disconnect, thereby preventing the second bus capacitor EC2 from being connected to the bus's charging and discharging circuit. In some embodiments, the capacitor switch Q3 can be a MOSFET, such as an N-channel MOSFET. When the AC voltage Vac is less than the voltage threshold, the second bus capacitor EC2 is connected to the bus's charging and discharging circuit by controlling the capacitor switch Q3 to conduct. The second bus capacitor EC2 can also provide a portion of the discharge current. Overall, the first bus capacitor EC1 and the second bus capacitor EC2 only need to perform small-amplitude charging and discharging, resulting in relatively small ripple on the bus and fewer harmonics in the AC circuit connected to the rectifier circuit. When the AC voltage Vac is greater than or equal to the voltage threshold, the voltage on the bus is relatively large. In order to meet the output power requirements of the output terminal Vout and maintain the target voltage, only a small discharge current needs to be provided by the bus capacitor. Providing this discharge current solely through the first bus capacitor EC1 will not cause a large ripple on the bus. Therefore, by controlling the capacitor switch Q3 to be disconnected, the second bus capacitor EC2 is not connected to the charging and discharging circuit of the bus. This avoids more devices such as the second bus capacitor EC2 and the capacitor switch Q3 being connected to the bus, which would lead to more power consumption. At the same time, the ripple caused in the AC circuit is also smaller because the bus ripple is smaller.

[0039] In some embodiments, the busbar is grounded sequentially through a second busbar capacitor EC2 and a capacitor switch Q3. In this case, since the capacitor switch Q3 is connected to the busbar via the second busbar capacitor EC2, it is not directly connected to the busbar, preventing the high voltage on the busbar from being directly applied to the capacitor switch Q3. Therefore, the capacitor switch Q3 will not be damaged by the high voltage of the busbar. From another perspective, the capacitor switch Q3 can be a device with a lower voltage rating, thus saving costs. Furthermore, controlling the capacitor switch Q3 via an optocoupler can achieve isolation between the input and output sides of the DC-DC unit, preventing the output side from being damaged by the high voltage on the input side.

[0040] The AC-DC circuit also includes a first voltage divider resistor R4 and a second voltage divider resistor R5. The optocoupler includes an optocoupler emitting element U1A and an optocoupler receiving element U1B. The control unit IC also has a control terminal IO2. The control unit IC controls the optocoupler emitting element U1A to emit light or not emit light through the control terminal IO2. The optocoupler receiving element U1B and the second voltage divider resistor R5 are connected in series between the driving voltage VCC and the control terminal of the capacitor switch Q3. The control terminal of the capacitor switch Q3 is grounded through the first voltage divider resistor R4. And it satisfies: (VCC-V0)*R4 / (R5+R4)>Vth2. Wherein, Vth2 is the turn-on voltage of the capacitor switch Q3, VCC is the magnitude of the driving voltage, V0 is the forward voltage drop of the optocoupler receiving element U1B, and R4 and R5 are the resistance values ​​of the first voltage divider resistor and the second voltage divider resistor, respectively. Figure 2 The second voltage divider resistor R5 shown is located between the driving voltage VCC and the optocoupler receiving element U1B, but the positions of the second voltage divider resistor R5 and the optocoupler receiving element U1B can be interchanged. Figure 2 The diagram shows one type of optocoupler emitting element U1A and optocoupler receiving element U1B, but other existing types of optocouplers can also be used. During operation, when the AC voltage Vac is less than the voltage threshold, the control unit IC controls the optocoupler emitting element U1A to emit light, and the optocoupler receiving element U1B conducts after receiving light. The voltage at the control terminal of capacitor switch Q3 is maintained at a voltage greater than Vth2 (i.e., (VCC-V0)*R4 / (R5+R4)), thereby controlling capacitor switch Q3 to conduct, thus connecting the second bus capacitor EC2 to the bus charging and discharging circuit. When the AC voltage Vac is greater than or equal to the voltage threshold, the control unit IC controls the optocoupler emitting element U1A not to emit light, and the optocoupler receiving element U1B does not receive light and therefore does not conduct. The voltage at the control terminal of capacitor switch Q3 is pulled down to zero by the first voltage divider resistor R4, thereby controlling capacitor switch Q3 to open, thus preventing the second bus capacitor EC2 from being connected to the bus charging and discharging circuit.

[0041] The present invention also provides an AC-DC circuit, including any of the input voltage detection circuits described above.

[0042] The present invention also provides a switching power supply, including the aforementioned input voltage detection circuit.

[0043] The present invention also provides an input voltage detection method for the aforementioned AC-DC circuit, wherein the control unit IC detects the voltage at the voltage detection terminal Vsen and calculates the magnitude of the input AC voltage Vac based on the relationship between the AC voltage Vac and the voltage at the voltage detection terminal.

[0044] Those skilled in the art will understand that, without conflict, the above-mentioned preferred solutions can be freely combined and superimposed.

[0045] It should be understood that the above embodiments are merely exemplary and not restrictive. Various obvious or equivalent modifications or substitutions that can be made by those skilled in the art regarding the above details without departing from the basic principles of the present invention will be included within the scope of the claims of the present invention.

Claims

1. An input voltage detection circuit for an AC-DC circuit, wherein, The AC-DC circuit includes a rectifier circuit and a DC-DC unit. The DC-DC unit includes a transformer, a filter capacitor, and a rectifier diode. The rectifier circuit rectifies the input AC voltage to obtain a DC voltage and provides the DC voltage to the bus. The DC-DC unit converts the voltage on the bus into the target output voltage. Its characteristic is that... The input voltage detection circuit includes a sampling capacitor, a first sampling resistor, a second sampling resistor, a third sampling resistor, a diode, and a control unit. The control unit also has a reference voltage input terminal and a voltage detection terminal. The first end of the secondary coil of the transformer is connected to the anode of the rectifier diode, and the cathode of the rectifier diode is connected to the positive terminal of the filter capacitor. The second end of the secondary coil of the transformer, the negative terminal of the filter capacitor, and the positive terminal of the sampling capacitor are all grounded. The end of the primary coil of the transformer connected to the busbar is the same terminal as the second end of the secondary coil. The cathode of the diode is connected to the first end of the secondary coil of the transformer, and the anode is connected to the negative terminal of the sampling capacitor. The negative terminal of the sampling capacitor is grounded in sequence through the third sampling resistor and the second sampling resistor. The common terminal of the third sampling resistor and the second sampling resistor is connected to the voltage detection terminal and is connected to the reference voltage input terminal through the first sampling resistor. The reference voltage input terminal receives the reference voltage. The control unit detects the voltage at the voltage detection terminal and calculates the magnitude of the input AC voltage based on the relationship between the AC voltage and the voltage at the voltage detection terminal. The input voltage detection circuit also includes a dummy load resistor; During a set period after the AC-DC circuit is powered on, the control unit controls the dummy load resistor to connect to the discharge circuit of the filter capacitor, and the control unit detects the voltage at the voltage detection terminal and calculates the magnitude of the input AC voltage based on the relationship between the AC voltage and the voltage at the voltage detection terminal. After the set time period, the control unit controls the dummy load resistor to not be connected to the discharge circuit of the filter capacitor, and the control unit stops detecting the voltage at the voltage detection terminal.

2. The input voltage detection circuit according to claim 1, characterized in that, The relationship between the AC voltage and the voltage at the voltage detection terminal is as follows: Vac={-Vsen+[(Vref-Vsen) / R1+Vsen / R2]×R3+VD1}×NP / (NS×k); Where VD1 is the voltage drop of diode D1, NS and NP are the number of turns of the transformer's secondary and primary coils, respectively, Vsen is the voltage detected at the voltage detection terminal, Vref is the reference voltage, R1, R2 and R3 are the resistance values ​​of the first, second and third sampling resistors, respectively, and k is the ratio of the output voltage of the rectifier circuit to the input AC voltage.

3. The input voltage detection circuit according to claim 1, characterized in that, The input voltage detection circuit also includes a resistor switch, and the dummy load resistor and the resistor switch are connected in series between the positive and negative terminals of the filter capacitor; During a set period after the AC-DC circuit is powered on, the control unit controls the resistor switch to turn on so that the dummy load resistor is connected to the discharge circuit of the filter capacitor. After the set time period, the control unit controls the resistor switch to disconnect so that the dummy load resistor is not connected to the discharge circuit of the filter capacitor.

4. The input voltage detection circuit according to claim 3, characterized in that, The AC-DC circuit also includes a first bus capacitor, a second bus capacitor, a capacitor switch, and an optocoupler; One end of the first bus capacitor is connected to the bus and the other end is connected to ground. The second bus capacitor and the capacitor switch are connected in series between the bus and ground. When the AC voltage is less than the voltage threshold, the control unit controls the capacitor switch to turn on via an optocoupler, thereby connecting the second bus capacitor to the charging and discharging circuit of the bus; when the AC voltage is greater than or equal to the voltage threshold, the control unit controls the capacitor switch to turn off via an optocoupler, thereby preventing the second bus capacitor from being connected to the charging and discharging circuit of the bus.

5. An AC-DC circuit, characterized in that, Includes the input voltage detection circuit as described in any one of claims 1-4.

6. A switching power supply, characterized in that, Includes the AC-DC circuit as described in claim 5.

7. A method for detecting the input voltage of an AC-DC circuit as described in claim 5, characterized in that, The control unit detects the voltage at the voltage detection terminal and calculates the magnitude of the input AC voltage based on the relationship between the AC voltage and the voltage at the voltage detection terminal.

8. The input voltage detection method as described in claim 7, characterized in that, Using the input voltage detection circuit as described in claim 1, During a set period after the AC-DC circuit is powered on, the control unit controls the dummy load resistor to connect to the discharge circuit of the filter capacitor, and the control unit detects the voltage at the voltage detection terminal and calculates the magnitude of the input AC voltage based on the relationship between the AC voltage and the voltage at the voltage detection terminal. After the set time period, the control unit controls the dummy load resistor to not be connected to the discharge circuit of the filter capacitor, and the control unit stops detecting the voltage at the voltage detection terminal.

9. The input voltage detection method as described in claim 7, characterized in that, The input voltage detection circuit as described in claim 4 is used. When the AC voltage is less than the voltage threshold, the control unit controls the capacitor switch to turn on, thereby connecting the second bus capacitor to the charging and discharging circuit of the bus; when the AC voltage is greater than or equal to the voltage threshold, the control unit controls the capacitor switch to turn off, thereby preventing the second bus capacitor from being connected to the charging and discharging circuit of the bus.

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