Filter circuit with harmonic compensation

Abstract

A filter circuit with harmonic compensation is coupled to a bridge rectifier circuit receiving an AC voltage and an input capacitor. The bridge rectifier circuit includes a live terminal, a neutral terminal, a positive terminal, and a negative terminal. The filter circuit also includes a freewheeling capacitor, a freewheeling switch, and a unidirectional switch. One end of the freewheeling capacitor is coupled to the live terminal of the bridge rectifier circuit, and one end of the freewheeling switch is coupled to the other end of the freewheeling capacitor. The other end of the freewheeling switch is coupled to the positive terminal of the bridge rectifier circuit and the input capacitor. One end of the unidirectional switch couples the freewheeling capacitor and the freewheeling switch, and the other end of the unidirectional switch receives a voltage source. A first conduction voltage of the freewheeling switch is greater than a second conduction voltage from the live terminal to the positive terminal of the bridge rectifier circuit.

Description

Technical Field

[0001] The present invention relates to a filter circuit with harmonic compensation, and more particularly to a filter circuit with harmonic compensation that can be selectively constructed using only passive electronic components while simplifying circuit control and reducing costs. Background Art

[0002] Total harmonic distortion (THD) is a measure of harmonic distortion in electrical signals. It is commonly defined as the ratio of the sum of the power of all harmonic components to the power of the fundamental frequency signal. It is sometimes expressed as the distortion factor. A higher total harmonic distortion indicates a greater proportion of harmonic components. Harmonic distortion refers to the addition of unwanted signals to the input signal due to nonlinear factors. These unwanted signals are multiples of the original input signal's frequency.

[0003] Most current electronic devices operate directly or indirectly on DC voltage. Therefore, most electronic devices use bridge rectifiers at their inputs to rectify AC voltage to generate DC voltage. The drawbacks of bridge rectifiers are a phase difference between the input voltage and current, which reduces the power factor, and the input current contains high harmonics. Therefore, if harmonic compensation is not performed on the input voltage and current, the resulting input current waveform is severely distorted, leading to power system instability and, in severe cases, the risk of power outages.

[0004] Therefore, how to design a filter circuit with harmonic compensation, and further selectively achieve the harmonic compensation effect when the filter circuit is composed entirely of passive electronic components, is a major research topic that the inventors of the present disclosure want to conduct. Summary of the Invention

[0005] In order to solve the above problems, the present invention provides a filter circuit with harmonic compensation to overcome the problems of the prior art. Therefore, the filter circuit of the present invention is coupled to a bridge rectifier circuit that receives an AC voltage and an input capacitor, and the bridge rectifier circuit includes a live wire terminal, a neutral wire terminal, a positive terminal, and a negative terminal, wherein the filter circuit includes: a freewheeling capacitor, one end of which is coupled to the live wire terminal of the bridge rectifier circuit. A freewheeling switch, one end of which is coupled to the other end of the freewheeling capacitor, and the other end of the freewheeling switch is coupled to the positive terminal of the bridge rectifier circuit and the input capacitor. And a unidirectional switch, one end of which is coupled to the freewheeling capacitor and the freewheeling switch, and the other end of which receives a voltage source. Wherein, the first conduction voltage of the freewheeling switch is greater than the second conduction voltage from the live wire terminal to the positive terminal in the bridge rectifier circuit.

[0006] The primary purpose and technical effect of the present invention is to utilize a clamping circuit structure comprising a first diode in series with a second diode to ensure that the voltage across the two diodes is greater than the voltage across the bridge diode. This prevents the freewheeling capacitor from generating a negative voltage when both the bridge diode and the freewheeling switch are forward conducting, thereby improving the harmonic compensation performance of the filter circuit.

[0007] In order to further understand the technologies, means and technical effects adopted by the present invention to achieve the intended purpose, please refer to the following detailed description of the present invention and the accompanying drawings. It is believed that the purposes, features and characteristics of the present invention can be understood in depth and in detail. However, the accompanying drawings are provided for reference and illustration only and are not intended to limit the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 A circuit block diagram of a filter circuit with harmonic compensation according to the present invention;

[0009] Figure 2A A current path diagram of the filter circuit with harmonic compensation of the present invention in the negative half cycle of the AC voltage;

[0010] Figure 2B A current path diagram of the filter circuit with harmonic compensation of the present invention in the positive half cycle of the AC voltage; and

[0011] Figure 3 Schematic diagram of the waveform of the AC voltage and input current after harmonic compensation in the present invention.

[0012] The description of the accompanying drawings is as follows:

[0013] 100…Filter circuit

[0014] Cf…freewheeling capacitor

[0015] 10…Freewheeling switch

[0016] D1…first diode

[0017] D2…second diode

[0018] D…Diode

[0019] 20…Power generation circuit

[0020] 22…Auxiliary winding

[0021] Cs…voltage stabilizing capacitor

[0022] Dc…rectifier switch

[0023] 200…bridge rectifier circuit

[0024] L…Firewire terminal

[0025] N…Neutral terminal

[0026] A…positive terminal

[0027] B…Negative terminal

[0028] Db…bridge diode

[0029] Cin…input capacitance

[0030] GND…ground point

[0031] 300…Electronic devices

[0032] IC…Controller

[0033] Vac…AC voltage

[0034] Vh…half-wave voltage

[0035] Vs…voltage source

[0036] Vaux…Auxiliary voltage

[0037] Vcf…capacitor voltage

[0038] Iin…input current

[0039] Lnh…Negative half-cycle harmonic compensation path

[0040] Lnr…negative half cycle rectification path

[0041] Lph…positive half-cycle harmonic compensation path

[0042] Lpr…positive half-cycle rectification path

[0043] T1~T8…time period

[0044] I~IV…Waveform DETAILED DESCRIPTION

[0045] The technical content and detailed description of the present invention are described as follows with reference to the accompanying drawings:

[0046] See also Figure 1This is a block diagram of a filter circuit with harmonic compensation according to the present invention. The filter circuit 100 with harmonic compensation performs harmonic compensation on the input current Iin to reduce its harmonic distortion. A bridge rectifier circuit 200 (hereinafter referred to as bridge rectifier circuit 200) receives an AC voltage Vac from a live terminal L and a ground terminal N. After rectifying the AC voltage Vac, it provides a half-wave voltage Vh from a positive terminal A and a negative terminal B. The half-wave voltage Vh is filtered by an input capacitor Cin to provide a stable DC voltage Vdc to power a connected electronic device 300. The bridge rectifier circuit 200 includes four bridge rectifier diodes Db. The junction between each pair of bridge rectifier diodes Db is respectively coupled to the live terminal L and the neutral terminal N of the AC voltage Vac, as well as the positive terminal A and the negative terminal B that provide the rectified half-wave voltage. The filter circuit 100 couples the bridge rectifier circuit 200 to the input capacitor Cin and includes a freewheeling capacitor Cf, a freewheeling switch 10, and a unidirectional switch D. One end of the freewheeling capacitor Cf is coupled to the live wire terminal L, and the other end of the freewheeling capacitor Cf is coupled to one end of the freewheeling switch 10 and one end of the unidirectional switch D. The other end of the freewheeling switch 10 is coupled to the positive terminal A and one end of the input capacitor Cin. The other end of the unidirectional switch D is coupled to the voltage source Vs, which can charge the freewheeling capacitor Cf through the unidirectional switch D. In one embodiment of the present invention, the unidirectional switch D is, for example but not limited to, a diode. Any element that enables the voltage source Vs to charge the freewheeling capacitor Cf and prevents energy from flowing back to the voltage source Vs when the freewheeling capacitor Cf discharges should be included in the scope of this embodiment, such as using a switch in conjunction with a corresponding control loop. The filter circuit 100 may also include a power generation circuit 20, and the power generation circuit 20 can serve as a provider of the voltage source Vs. One end of the power generation circuit 20 is coupled to the other end of the unidirectional switch D, and the other end of the power generation circuit 20 is coupled to the negative terminal B (i.e., ground point GND).

[0047] Specifically, the freewheeling switch 10 includes a first diode D1 and a second diode D2 coupled in series. The anode of the first diode D1 is coupled to the other end of the freewheeling capacitor Cf and one end of the unidirectional switch D, and the cathode of the second diode D2 is coupled to the positive terminal A and the input capacitor Cin. The power generation circuit 20 includes an auxiliary winding 22 and a stabilizing capacitor Cs. One end of the auxiliary winding 22 is coupled to the negative terminal B. One end of the stabilizing capacitor Cs is coupled to the other end of the auxiliary winding 22, and the other end of the stabilizing capacitor Cs is coupled to the negative terminal B. The auxiliary winding 22 generates an auxiliary voltage Vaux, which charges the stabilizing capacitor Cs, thereby establishing a voltage source Vs across the two ends of the stabilizing capacitor.

[0048] Furthermore, the auxiliary winding 22 is primarily coupled to an electronic device 300 (typically a power converter) having a transformer (not shown), and an auxiliary voltage Vaux is sensed via the coupling transformer. A voltage-stabilizing capacitor Cs is connected in parallel to ground on the output path of the auxiliary voltage Vaux to provide a stable voltage source Vs to the controller IC, enabling the controller IC to control the stable operation of the electronic device 300. In other words, the power generation circuit 20 serves as the power source (voltage source Vs) for the controller IC. However, in one embodiment of the present invention, the power generation circuit 20 utilizes its ability to provide a stable voltage source Vs and simultaneously supplies the voltage source Vs to the filter circuit 100 for harmonic compensation. This reduces the number of circuit components in the filter circuit 100 and eliminates the need for an additional power generation circuit. However, if required by the actual circuit application, the voltage source Vs can also be provided by an external power supply or by dividing the voltage at a node within the electronic device 300 using a voltage divider circuit (not shown).

[0049] The power generation circuit 20 may further include a rectifier switch Dc, one end of which is coupled to the auxiliary winding 22 and the other end of which is coupled to the stabilizing capacitor Cs. Specifically, depending on the type of electronic device 300 (e.g., but not limited to, a flyback converter or a converter operating in discontinuous conduction mode) or the polarity of the auxiliary winding 22, the auxiliary voltage Vaux sensed by the auxiliary winding 22 may include a negative voltage. However, since the controller IC is typically only capable of withstanding positive voltages, and the filter circuit 100 also requires a positive voltage source Vs for harmonic compensation, the rectifier switch Dc not only provides rectification for the auxiliary winding 22 during voltage conversion, but also ensures the correct power polarity is provided to the controller IC and the filter circuit 100.

[0050] See also Figure 2A The current path diagram of the filter circuit with harmonic compensation in the negative half cycle of the AC voltage of the present invention is shown in FIG. Figure 2B This is a current path diagram of the filter circuit with harmonic compensation in the positive half cycle of the AC voltage of the present invention, and is also referred to in conjunction with Figure 1 .At Figure 2AIn the filter circuit 100, during the negative half-cycle of the AC voltage Vac, two current paths are provided. One is the negative half-cycle harmonic compensation path Lnh, and the other is the negative half-cycle rectification path Lnr. During the negative half-cycle of the AC voltage Vac, current paths are generated at only two points in time. One is when the AC voltage Vac is greater than the DC voltage Vdc on the input capacitor Cin, generating the negative half-cycle rectification path Lnr. This path follows the AC voltage Vac, the neutral terminal N, the bridge rectifier diode Db, the positive terminal A, the input capacitor Cin, the negative terminal B, the bridge rectifier diode Db, the live terminal L, and returns to the AC voltage Vac. This path is also the current path for the AC voltage Vac to charge the input capacitor Cin via the bridge rectifier. The other current path, primarily provided by the filter circuit 100, is the negative half-cycle harmonic compensation path Lnh, generated when the AC voltage Vac is greater than the DC voltage Vdc plus the voltage on the stabilizing capacitor Cs (i.e., the voltage source Vs). Therefore, the negative half-cycle harmonic compensation path Lnh is the current path from the AC voltage Vac, the neutral terminal N, the bridge rectifier diode Db, the positive terminal A, the input capacitor Cin, the voltage stabilizing capacitor Cs, the unidirectional switch D, and the freewheeling capacitor Cf back to the AC voltage Vac. This path is the current path for the filter circuit 100 to perform harmonic compensation on the negative half-cycle of the input current Iin. During the negative half-cycle harmonic compensation path Lnh, since the capacitor voltage Vcf on the freewheeling capacitor Cf (i.e., the voltage across the freewheeling capacitor Cf) has been completely discharged during the first half cycle (when the positive half-cycle harmonic compensation path Lph is generated in the positive half-cycle), the freewheeling capacitor Cf at this time has no stored energy. Therefore, during the negative half-cycle harmonic compensation, the voltage source Vs provided by the power generation circuit 20 charges the freewheeling capacitor Cf through the unidirectional switch D, causing the freewheeling capacitor Cf to establish the capacitor voltage Vcf.

[0051] At Figure 2BIn the filter circuit 100, during the positive half-cycle of the AC voltage Vac, two current paths are also included. One is the positive half-cycle harmonic compensation path Lph, and the other is the positive half-cycle rectification path Lpr. During the positive half-cycle of the AC voltage Vac, current paths are generated at only two points in time. One of these paths is when the AC voltage Vac is greater than the DC voltage Vdc on the input capacitor Cin, generating the positive half-cycle rectification path Lpr. This path follows the AC voltage Vac, the hot terminal L, the bridge rectifier diode Db, the positive terminal A, the input capacitor Cin, the negative terminal B, the bridge rectifier diode Db, the neutral terminal N, and then returns to the AC voltage Vac. This path is also the current path for the AC voltage Vac to charge the input capacitor Cin via the bridge rectifier. The other current path, primarily provided by the filter circuit 100, is the positive half-cycle harmonic compensation path Lph generated when the AC voltage Vac plus the capacitor voltage Vcf on the freewheeling capacitor Cf (i.e., the voltage across the freewheeling capacitor Cf) is greater than the DC voltage Vdc. Therefore, the positive half-cycle harmonic compensation path Lph is the current path from the AC voltage Vac, the freewheeling capacitor Cf, the freewheeling switch 10, the input capacitor Cin, the negative terminal B, the bridge diode Db, and the neutral terminal N back to the AC voltage Vac. This path is the current path for the filter circuit 100 to perform harmonic compensation on the positive half-cycle of the input current Iin. Since the freewheeling capacitor Cf establishes a capacitor voltage Vcf during the first half cycle (when the negative half-cycle generates the negative half-cycle harmonic compensation path Lnh), the energy stored in the freewheeling capacitor Cf is discharged to the input capacitor Cin via the freewheeling switch 10 during the positive half-cycle harmonic compensation.

[0052] Furthermore, in one embodiment of the present invention, in certain electronic products, the product only needs to comply with the specification of harmonic distortion of the input current Iin, but not the specification of power factor of the input current Iin (for example, but not limited to, IEC6100-3-2 Class A). Since this specification only requires that the harmonic distortion rate must be less than a certain ratio, the back end of the input capacitor Cin does not need to be coupled with a power factor corrector (PFC) with a relatively expensive circuit cost. Instead, the filter circuit 100 only needs to be used to make the following adjustments during the positive and negative half cycles of the AC voltage Vac: Figure 3 By modifying the current waveforms I and III, harmonic distortion can be improved. Furthermore, the invention selectively utilizes only inexpensive and readily available passive electronic components, completely eliminating the need for active control components and corresponding control technologies. This reduces circuit costs while achieving the technical effect of improving harmonic distortion.

[0053] See also Figure 3 The waveform diagram of the AC voltage and the input current after harmonic compensation of the present invention is shown in FIG. Figures 1-2B , and see Figure 2A 、 Figure 2B 、 Figure 3. During the time period T1, the AC voltage Vac is in the negative half cycle. During this period, the input capacitor Cin has the voltage of the first half cycle (i.e., when the AC voltage Vac is in the positive half cycle), so there is no input current Iin path. At this time, the unidirectional switch D is not conducting, and the power generation circuit 20 only provides the voltage source Vs to maintain the stable operation of the controller IC. During the time period T2, the freewheeling capacitor Cf has no stored energy (the previous cycle has been discharged), and the voltage on the series circuit of the input capacitor Cin and the voltage stabilizing capacitor Cs (i.e., the DC voltage Vdc plus the voltage source Vs) is lower than the AC voltage Vac. Therefore, a negative half-cycle harmonic compensation path Lnh is generated to charge the freewheeling capacitor Cf, so that the freewheeling capacitor Cf establishes a capacitor voltage Vcf.

[0054] During time period T3, the freewheeling capacitor Cf has established a capacitor voltage Vcf, causing the negative half-cycle harmonic compensation path Lnh to be disconnected. At this time, since the AC voltage Vac is still greater than the DC voltage Vdc on the input capacitor Cin, a negative half-cycle rectifier path Lnr is generated, causing the AC voltage Vac to charge the input capacitor Cin. During time period T4, since the AC voltage Vac is less than the DC voltage Vdc on the input capacitor Cin, the negative half-cycle rectifier path Lnr is disconnected, so there is no path for the input current Iin. Since the voltage source Vs charges the freewheeling capacitor Cf during the negative half-cycle harmonic compensation path Lnh, the waveform I of the first compensation current is generated during time period T2. The waveform I of the first compensation current just compensates for the missing section of the input current Iin before the negative half-cycle rectifier path Lnr is generated (i.e., the missing portion before the negative half-cycle rectifier current waveform II), making the negative half-cycle waveform of the compensated input current Iin (i.e., waveform I plus waveform II) close to the negative half-cycle waveform of the AC voltage Vac.

[0055] At the junction of time periods T4 and T5, the voltage waveform of the AC voltage Vac turns to the positive half cycle. During time period T5, because the input capacitor Cin has a voltage in the first half cycle (i.e., when the AC voltage Vac is in the negative half cycle), there is no path for the input current Iin. At this time, the unidirectional switch D is not conducting, and the power generation circuit 20 only provides the voltage source Vs to maintain the stable operation of the controller IC. During time period T6, the freewheeling capacitor Cf has established a capacitor voltage Vcf in the first half cycle, and the AC voltage Vac plus the capacitor voltage Vcf is greater than the DC voltage Vdc on the input capacitor Cin. Therefore, when performing positive half-cycle harmonic compensation, the energy stored in the freewheeling capacitor Cf is discharged to the input capacitor Cin via the positive half-cycle harmonic compensation path Lph until the freewheeling capacitor voltage Vcf plus the AC voltage Vac is less than the DC voltage Vdc on the input capacitor Cin.

[0056] During time period T7, because the freewheeling capacitor voltage Vcf plus the AC voltage Vac is less than the DC voltage Vdc on the input capacitor Cin, the positive half-cycle harmonic compensation path Lph is disconnected. At this point, because the AC voltage Vac is still greater than the DC voltage Vdc on the input capacitor Cin, a positive half-cycle rectifier path Lpr is generated, causing the AC voltage Vac to charge the input capacitor Cin. During time period T8, because the AC voltage Vac is less than the DC voltage Vdc on the input capacitor Cin, the positive half-cycle rectifier path Lpr is disconnected, resulting in no path for the input current Iin. Since the freewheeling capacitor Cf discharges the input capacitor Cin during the positive half-cycle harmonic compensation path Lph, waveform III of the second compensation current is generated during time period T6. Waveform III of the second compensation current precisely compensates for the missing segment of the input current Iin before the positive half-cycle rectifier path Lpr is generated (i.e., the missing portion before the positive half-cycle rectifier current waveform IV), making the compensated positive half-cycle waveform of the input current Iin (i.e., waveform III plus waveform IV) close to the positive half-cycle waveform of the AC voltage Vac.

[0057] See also Figures 1 to 2BThe reason why the freewheeling switch 10 uses the first diode D1 in series with the second diode D2 is that the forward conduction voltage of the bridge diode Db is generally about 1V, and the forward conduction voltage of an ordinary diode is about 0.7V. Therefore, if the freewheeling switch 10 only uses a single diode, when the bridge diode Db between the live terminal L and the positive terminal A and the single diode in the freewheeling switch 10 are both forward-conducting, the voltage across the bridge diode Db will be greater than the voltage across the single diode in the freewheeling switch 10. Therefore, a negative voltage will be generated on the freewheeling capacitor Cf, causing adverse effects when the filter circuit 100 performs harmonic compensation, and even shortening the life of the components. For the above reasons, in one embodiment of the present invention, the freewheeling switch 10 uses the first diode D1 in series with the second diode D2, which can increase the voltage across the two diodes to approximately 1.4V, which is greater than the voltage across the bridge diode Db (1V). That is, the first conduction voltage of the freewheeling switch 10 (e.g., 1.4V) is greater than the second conduction voltage (e.g., 1V) of the bridge diode Db between the live terminal L and the positive terminal A. This prevents the freewheeling capacitor Cf from generating a negative voltage when both the bridge diode Db and the freewheeling switch 10 are forward-conducting, thereby achieving the technical effect of improving the harmonic compensation effect of the filter circuit 100. It is worth noting that, in one embodiment of the present invention, the freewheeling switch 10 is not limited to a circuit structure that only uses the first diode D1 in series with the second diode D2. Any circuit structure and application that can ensure that the conduction voltage of the freewheeling switch 10 is greater than the conduction voltage of the bridge diode Db when conducting (e.g., by utilizing a switch and its control technology) is included within the scope of this embodiment. For example, a single diode with a conduction voltage greater than the conduction voltage of the bridge diode Db may be used, or multiple diodes may be connected in series to ensure that the total conduction voltage is greater than the conduction voltage of the bridge diode Db, or any component whose voltage across the freewheeling switch 10 is greater than the voltage across the bridge diode Db when conducting may be used.

[0058] On the other hand, since the voltage-stabilizing capacitor Cs must stably and continuously provide the power supply controller IC, the freewheeling capacitor Cf only needs to provide short-term power to generate the corresponding current path during time periods T2 and T6. Therefore, the capacitance value of the voltage-stabilizing capacitor Cs must be greater than the freewheeling capacitor Cf, and the voltage-stabilizing capacitor Cs is preferably an electrolytic capacitor. The freewheeling capacitor Cf can use an electrolytic capacitor, a ceramic capacitor, or a tantalum capacitor. In addition, the capacitance value of the freewheeling capacitor Cf will affect the current waveform (i.e., waveforms I and III) during time periods T2 and T6. The smaller the capacitance value of the freewheeling capacitor Cf, the less energy is stored and released, making the time duration of time periods T2 and T6 shorter, and waveforms I and III steeper. Otherwise, the time is longer, and waveforms I and III are relatively gentle. Therefore, the designer can adjust the capacitance value of the freewheeling capacitor Cf according to actual needs to obtain the harmonic compensation effect of the filter circuit 100 at different expected values.

[0059] The above description is only a detailed description and drawings of preferred specific embodiments of the present invention, and the features of the present invention are not limited thereto and are not intended to limit the present invention. The full scope of the present invention shall be based on the following claims. All embodiments that are consistent with the spirit of the claims of the present invention and similar variations thereof shall be included in the scope of the present invention. Any changes or modifications that can be easily conceived by any person skilled in the art within the field of the present invention shall be covered by the following claims of the present disclosure.

Claims

1. A filter circuit with harmonic compensation, coupled to a bridge rectifier circuit receiving an AC voltage and an input capacitor, the bridge rectifier circuit comprising a live terminal, a neutral terminal, a positive terminal, and a negative terminal, wherein the filter circuit comprises: a freewheeling capacitor, one end of which is coupled to the live wire end of the bridge rectifier circuit; a freewheeling switch, one end of which is coupled to the other end of the freewheeling capacitor, and the other end of which is coupled to the positive terminal of the bridge rectifier circuit and the input capacitor; and a unidirectional switch, one end of which is coupled to the freewheeling capacitor and the freewheeling switch, and the other end of which receives a voltage source; A first conduction voltage of the freewheeling switch is greater than a second conduction voltage between the live wire end and the positive end in the bridge rectifier circuit, and the voltage source is provided to a node between the freewheeling capacitor and the freewheeling switch through the unidirectional switch.

2. The filter circuit according to claim 1, wherein The freewheeling switch comprises: a first diode, coupling the freewheeling capacitor and the unidirectional switch; and A second diode is coupled in series between the first diode and the input capacitor.

3. The filter circuit according to claim 2, wherein: When the first diode is turned on, a first forward conduction voltage is generated, and when the second diode is turned on, a second forward conduction voltage is generated, and the sum of the first forward conduction voltage and the second forward conduction voltage is the first conduction voltage; a bridge diode is included between the live wire end and the positive end, and the first conduction voltage is greater than the second conduction voltage of the bridge diode.

4. The filter circuit as claimed in claim 1 , further comprising a power generation circuit, the power generation circuit being coupled to the unidirectional switch and the negative terminal and comprising: an auxiliary winding, one end of which is coupled to the unidirectional switch, and the other end of which is coupled to the negative terminal; and a voltage-stabilizing capacitor coupled in parallel to the auxiliary winding; The auxiliary winding generates an auxiliary voltage as the voltage source.

5. The filter circuit according to claim 4, further comprising: A rectifier switch is coupled to the auxiliary winding and the voltage-stabilizing capacitor.

6. The filter circuit according to claim 4, wherein: The capacitance value of the voltage stabilizing capacitor is greater than the capacitance value of the freewheeling capacitor.

7. The filter circuit according to claim 4, wherein: The input capacitor is coupled to an electronic device having a transformer, and the auxiliary winding generates the auxiliary voltage by being coupled to the transformer.

8. The filter circuit according to claim 7, wherein: The voltage stabilizing capacitor is coupled to a controller, which controls the electronic device, and the voltage source provides power required for the controller to operate.

9. The filter circuit according to claim 4, wherein: In the negative half cycle of the AC voltage, the power generation circuit charges the freewheeling capacitor to generate a first compensation current. The first compensation current compensates for a missing segment of a negative half cycle of an input current received by the bridge rectifier circuit, so that the negative half cycle waveform of the compensated input current is close to the negative half cycle waveform of the AC voltage.

10. The filter circuit according to claim 4, wherein: A negative half-cycle harmonic compensation path of the filter circuit is the positive terminal, the input capacitor, the power generation circuit, the unidirectional switch, the freewheeling capacitor to the live wire terminal.

11. The filter circuit according to claim 1, wherein: In the positive half cycle of the AC voltage, the AC voltage charges the freewheeling capacitor to generate a second compensation current. The second compensation current compensates for a missing segment of a positive half cycle of an input current received by the bridge rectifier circuit, so that the positive half cycle waveform of the compensated input current is close to the positive half cycle waveform of the AC voltage.

12. The filter circuit according to claim 1, wherein: A positive half-cycle harmonic compensation path of the filter circuit is the live wire end, the freewheeling capacitor, the freewheeling switch, the input capacitor, and the negative terminal.

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