EMI filter and switching power supply with the same

An EMI filter configured for an AC-DC switching power supply, and that is coupled between a DC-DC converter and a rectifier circuit that receives an AC power supply, can include: an input capacitor coupled in parallel to input terminals of the DC-DC converter; a transistor having a first terminal coupled to a first terminal of the input capacitor, and a second terminal coupled to ground; and a control circuit configured to generate a control signal that controls the transistor according to a feedback voltage that represents ripple information of an input voltage to the DC-DC converter, and at least one of a current that flows through the transistor, and a voltage at the first terminal of the transistor, where the control signal is used to regulate the voltage at the first terminal of the transistor to substantially eliminate switching frequency ripples from an input current of the DC-DC converter.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No. 201510975860.1, filed on Dec. 22, 2015, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of power electronics, and more particularly to EMI filters and associated switching power supplies.

BACKGROUND

A switching power supply is a power supply that maintains an output voltage by controlling a ratio between an on time and an off time of a power switch/transistor. Switching power supplies are widely and preferably used in many electronic devices due to their relatively small size, light weight, and high conversion efficiency. In addition, switching power supplies are also finding applications in the rapidly developing electronic information industry.

DETAILED DESCRIPTION

Switching power supplies have been commonly used in various areas due to their relatively high conversion efficiency and diverse power conversion approaches. However, the electromagnetic environment is generally worsening, and normal operation of surrounding electronic equipment and power supplies may be affected due to electromagnetic interference (EMI) that may be caused by the switching power supply itself and power switching devices used inside. In addition, high frequency operation is an inevitable developing direction for switching power supplies as it may miniaturize the switching power supplies. However, high frequency operation may also increase such electromagnetic interference. Passive CLC filters may be utilized in some switching power supplies, in order to avoid harmonic pollution that may be generated when high frequency switching current flows into the power grid.

Referring now toFIG. 1, shown is a schematic block diagram of an example switching power supply. In this example, a switching power supply with a passive EMI filter is shown. The switching power supply can include rectifier circuit1, passive EMI filter2, and DC-DC converter3. An AC mains “AC” can be rectified by rectifier circuit1, and configured to supply a DC operating voltage to DC-DC converter3at a succeeding stage. Passive EMI filter2can connect between rectifier circuit1and DC-DC converter3, and may be used to filter high switching ripples generated in DC-DC converter3. This can prevent high frequency harmonic ripples from flowing into the power grid in a simple and effective way, and may substantially avoid harmonic pollution as a result. However, in such switching power supply, at least two passive components are needed in passive EMI filter2, and a relatively large inductor of a high inductance value is generally required. As such, the miniaturization of such switching power supplies may be very difficult to achieve.

In one embodiment, an EMI filter configured for an AC-DC switching power supply, and that is coupled between a DC-DC converter and a rectifier circuit that receives an AC power supply, can include: (i) an input capacitor coupled in parallel to input terminals of the DC-DC converter; (ii) a transistor having a first terminal coupled to a first terminal of the input capacitor, and a second terminal coupled to ground; and (iii) a control circuit configured to generate a control signal that controls the transistor according to a feedback voltage that represents ripple information of an input voltage to the DC-DC converter, and at least one of a current that flows through the transistor, and a voltage at the first terminal of the transistor, where the control signal is used to regulate the voltage at the first terminal of the transistor in order to substantially eliminate switching frequency ripples from an input current of the DC-DC converter.

Referring now toFIG. 2, shown is a schematic block diagram circuit diagram of an example switching power supply, in accordance with embodiments of the present invention. This particular example switching power supply can include rectifier circuit1that receives an AC mains “AC” with one output terminal “b” being grounded, and an output to provide a DC voltage. DC-DC converter3may have an input to receive the DC voltage as its input voltage VIN, and EMI filter4can connect between rectifier circuit1and DC-DC converter3.

For example, EMI filter4can include input capacitor C1, transistor Q1, and control circuit41. Input capacitor C1can connect in parallel to DC-DC converter3in order to filter input voltage VIN. Transistor Q1may have a first terminal coupled to a second terminal of input capacitor C1, and a second terminal that is grounded, and is also coupled to the second output terminal “b” of rectifier circuit1. A first terminal of input capacitor C1can connect to a first output terminal “a” of rectifier circuit1. In some examples, transistor Q1can be an N channel enhanced type field effect transistor. Transistor Q1can be controlled to dynamically change voltage VQ1_Drainof its first terminal (e.g., the drain of transistor Q1), in order to compensate for the ripples of the input capacitor in the switching power supply, so as to obtain a relatively smooth input current IINwithout switching frequency ripples.

Control circuit41can generate control signal VGof transistor Q1according to a feedback voltage VFBthat represents ripple information of the input voltage, and current IQ1that flows through transistor Q1. Alternatively, control signal VGcan be generated according to feedback voltage VFBthat represents ripple information of the input voltage, and voltage VQ1_Drainat the first terminal of transistor Q1. Control signal VGcan be used to regulate voltage VQ1_Drainat the first terminal of transistor Q1, such that input current IINmay not contain the switching frequency ripples. Feedback voltage VFBthat represents ripple information of the input voltage can be a voltage sampling signal of voltage VQ1_Drainat the first terminal of the transistor, or can be a ripple voltage of the voltage across input capacitor C1.

Control circuit41can include voltage sense circuit42, reference signal generator43, and control signal generator44. For example, voltage sense circuit42can generate voltage sampling signal VSaccording to feedback voltage VFBthat represents the ripple information of the input voltage. Reference signal generator43can generate reference signal VREFaccording to voltage sampling signal VS. Control signal generator44can generate control signal VGaccording to reference signal VREFand current IQ1flowing through transistor Q1. Alternatively, control signal generator44can generate control signal VGaccording to reference signal VREFand voltage VQ1_Drainat the first terminal of transistor Q1. Control signal VGcan be used to regulate voltage VQ1_Drainat the first terminal of transistor Q1, such that input current IINmay not contain the switching frequency ripples.

In particular embodiments, dynamic regulation of the voltage at the first terminal of the transistor can be performed by adopting an active EMI filter, in order to compensate the ripples on the input capacitor of the switching power supply, and to obtain a relatively smooth input current (e.g., to a DC-DC converter) without switching frequency ripples. In this way, high frequency harmonic ripples can be substantially prevented from flowing into the power grid, to substantially avoid harmonic pollution with a simplified implementation. In addition, circuit area can be reduced as compared to approaches that employ passive components, by instead using semiconductor devices to filter the switching frequency ripples. As a result, certain embodiments can be applied to applications that utilize full surface mounted devices. Further example implementation, as well as operating principles of circuit portions of control circuit41of the EMI filter, will be described in more detail below.

Referring now toFIG. 3, shown is a schematic block diagram of an example control circuit in a switching power supply, in accordance with embodiments of the present invention. In this particular example, control circuit41in the EMI filter can include voltage sense circuit421that receives voltage VQ1_Drainat the first terminal of transistor Q1, and generates voltage sampling signal VS1. For example, voltage sense circuit421can include differentiating circuit4211coupled between the first terminal of transistor Q1and ground. Differentiating circuit4211can receive voltage VQ1_Drainfrom the first terminal of transistor Q1, and may perform a differential operation on voltage VQ1_Drain.

In this particular example, differentiating circuit4211can include capacitor C3and resistor R1connected in series, where resistor R1has a terminal that is grounded. Differential voltage signal Vdifmay be provided at a common node of capacitor C3and resistor R1, and a current that flows through resistor R1can be obtained as a differential current signal Idifafter a differential operation of voltage VQ1_Drainis carried out by differentiating circuit4211. It is to be understood that the differentiating circuit may also be implemented by other types of differentiating circuits as long as a suitable differential operation can be performed. Zero-crossing detecting circuit4212can generate zero-crossing detection signal VZEROaccording to differential voltage signal Vdifand/or differential current signal Idif.

Referring now toFIG. 4, shown is a waveform diagram of example operation of a switching power supply, in accordance with embodiments of the present invention. Generally, it is desirable to sample the peak voltage or valley voltage of voltage VQ1_Drainat the first terminal of transistor Q1, which may have a relative large value, by performing a differential operation. Based on such circuit principles, the peak point or valley point of voltage VQ1_Drainat the first terminal of transistor Q1can be obtained when differential voltage signal Vdif(or differential current signal Idif) generated by differentiating circuit4211crosses zero.

For example, the peak point of voltage VQ1_Drainat the first terminal of transistor Q1can be obtained when differential voltage signal Vdif(or differential current signal Idif) crosses zero in a direction from positive to negative. Also, the valley point of voltage VQ1_Drainat the first terminal of transistor Q1can be obtained when differential voltage signal Vdif(or the differential current) crosses zero in a direction from negative to positive. Thus, zero-crossing detection signal VZEROcan be generated when either differential voltage signal Vdifor differential current signal Idifcrosses zero. This facilitates sampling the peak value or the valley value of voltage VQ1_Drainat the first terminal of transistor Q1.

In this particular example, the valley value of voltage VQ1_Drainat the first terminal of transistor Q1can be sampled in order to achieve optimal efficiency. Thus, zero-crossing detection signal VZEROcan be generated when either differential voltage signal Vdifand/or differential current signal Idifcrosses zero in the direction from negative to positive. Referring also toFIG. 3, sample and hold circuit4213can generate voltage sampling signal VS1according to voltage VQ1_Drainat the first terminal of transistor Q1and zero-crossing detection signal VZERO. Sample and hold circuit4213can obtain voltage VQ1_Drainat the first terminal of transistor Q1when zero-crossing detection signal VZEROis active, such that the valley value of voltage VQ1_DrainDrain may be used as voltage sense signal VS1. In other examples, differentiating circuit4211and zero-crossing detecting circuit4212may not be necessary when voltage sense circuit421samples the average value of voltage VQ1_Drain.

Reference signal generator431can generate current reference signal VIREFaccording to voltage sampling signal VS1. For example, reference signal generator431can include operational amplifier AMP1having a non-inverting input terminal that receives voltage sampling signal VS1, and an inverting input terminal that receives reference voltage V1. Compensation circuit4311can be coupled between an output terminal of operational amplifier AMP1and ground, and a terminal of compensation circuit4311that is not connected to ground may provide current reference signal VIREF. Compensation circuit4311can include resistor R2and capacitor C4connected in series between the output terminal of operational amplifier AMP1and ground, and capacitor C5connected between the output terminal of operational amplifier AMP1and ground.

Operational amplifier AMP1can inject or extract a current to/from compensation circuit4311according to a difference between reference voltage V1and voltage sampling signal VS1. Thus, current reference signal VIREFcan be generated at the terminal of compensation circuit4311that is not connected to ground, according to reference voltage V1and voltage sampling signal VS1. Reference voltage V1may represent the valley value of the voltage VQ1_Drainat the first terminal of transistor Q1. It should be understood that compensation circuit4311may alternatively employ other structures, such as a compensation circuit only having a single capacitor.

In this way, the valley voltage of voltage VQ1_Drainat the first terminal of transistor Q1can be obtained in every switching cycle via voltage sampling circuit421. In addition, the valley value of voltage VQ1_Drainbe substantially stable at reference voltage V1with the regulation of reference signal generator431. In the example waveform diagram of voltage VQ1_Drainat the first terminal of transistor Q1, it can be seen that only the envelope of the voltage VQ1_Drainmay be controlled, and reference voltage V1can be selected to be as small/low as possible in order to ensure a certain adjustment. Thus, voltage VQ1_Drainat the first terminal of transistor Q1may only contain a ripple voltage component of the switching frequency, but not too high of DC components. This can reduce losses of transistor Q1, and may substantially avoid bringing in further losses to the overall system.

Control signal generator441can generate control signal VG1of transistor Q1according to current reference signal VIREFand current IQ1that flows through transistor Q1. Control signal generator441can include operational amplifier AMP2having a non-inverting input terminal that receives current reference signal VIREF, an inverting input terminal that receives current IQ1that flows through transistor Q1, and an output terminal that outputs control signal VG1of transistor Q1. Current IQ1that flows through transistor Q1can be obtained by current sampling resistor R3. Control signal VG1of transistor Q1can connect to the control terminal (e.g., gate) of transistor Q1, in order to control the conduction (e.g., on/off) thereof, and to further regulate current IQ1.

For example, current reference signal VIREFthat is generated by reference signal generator431can be a signal that has no switching ripple. Furthermore, by setting the parameters of components in compensation circuit4311, current reference signal VIREFmay be a signal that has the same phase as input voltage VINand is approximately a sinusoidal wave, as shown inFIG. 4. By taking the current reference signal as the reference of current IQ1that flows through transistor Q1, input current IINmay substantially only track the power frequency component of switching current ISW, rather than tracking a switching frequency component.

Thus, a current type EMI filter may be utilized to filter the switching ripples of current reference signal VIREFby sampling the valley voltage of voltage VQ1_Drainat the first terminal of transistor Q1. Current reference signal VIREFmay operate as the reference of current IQ1that flows through transistor Q1. Current IQ1flowing through transistor Q1may be compared against current reference signal VIREF, in order to generate control signal VG1for transistor Q1. Thus, current IQ1that flows through transistor Q1may substantially only contain the power frequency component, and not ripples of the switching frequency. In this way, a high frequency harmonic wave can be substantially prevented from flowing into the power grid, thus avoiding harmonic pollution.

Referring now toFIG. 5, shown is a schematic block diagram of another example control circuit in a switching power supply, in accordance with embodiments of the present invention. In this particular example, control circuit41in the EMI filter can include voltage sense circuit422that receives a voltage at the first terminal of input capacitor C1and the voltage VQ1_Drainat the first terminal of transistor Q1, and generates voltage sampling signal VS2. Voltage sense circuit422can be a ripple voltage sense circuit that receives the voltage at the first terminal of input capacitor C1and voltage VQ1_Drainat the first terminal of transistor Q1, in order to obtain the voltage across input capacitor C1. In the switching power supply, the voltage across the input capacitor may be equal to a voltage difference between the voltage at the first terminal of input capacitor C1and voltage VQ1_Drainat the first terminal of transistor Q1. The ripple voltage sense circuit can take the ripples of the voltage across input capacitor C1as voltage sampling signal VS2.

Reference signal generator432can generate voltage reference signal VVREFaccording to voltage sampling signal VS2. For example, reference signal generator432can include superimpose circuit4321, which can generate voltage reference signal VVREFby subtracting voltage sampling signal VS2from DC bias voltage V2. Thus, voltage sampling signal VVREFcan be obtained by initially generating an inverted signal of voltage sampling signal VS2, and then by adding the DC bias voltage to the inverted signal. That is to say, voltage reference signal VVREFmay have an opposite direction to the ripples of the voltage across input capacitor C1. Initially generating the inverted signal of voltage sampling signal VS2, and the adding DC bias voltage V2to the inverted signal can be performed in order to ensure voltage reference signal VVREFis greater than zero, in order as to avoid the generation of errors, and to facilitate the circuit design.

Moreover, in order to obtain a high efficiency, reference signal generator432may also include DC bias generating circuit4322, which can generate the DC bias voltage V2according to voltage sampling signal VS2. DC bias generating circuit4322can regulate DC bias voltage V2according to voltage sampling signal VS2. For example, when voltage sampling signal VS2increases, DC bias voltage V2can be regulated to increase. Further, when voltage sampling signal VS2decreases, DC bias voltage V2can be regulated to decrease. In this way, voltage sampling signal VVREFmay only contains ripple components, but not DC components, thus reducing system losses and improving operating efficiency.

Control signal generator442can generate control signal VG2for transistor Q1according to voltage reference signal VVREFand voltage VQ1_Drainat the first terminal of transistor Q1. Control signal generator442can include operational amplifier AMP3, which has an inverting input terminal that receives voltage reference signal VVREF, a non-inverting input terminal that receives voltage VQ1_Drainat the first terminal of the transistor, and an output terminal that provides control signal VG2for transistor Q1. Control signal VG2of transistor Q1may be provided to the control terminal (e.g., gate) of transistor Q1, for changing the conduction state of transistor Q1, in order to regulate voltage VQ1_Drain.

Therefore, a voltage type EMI filter can initially filter the switching ripples by sampling the ripples of the voltage across input capacitor C1as voltage sampling signal VS2. Reference voltage signal VVREFas a reference of voltage VQ1_Drainat the first terminal of transistor Q1can be generated by subtracting the ripples of the voltage across input capacitor C1from the DC bias voltage. Reference voltage signal VVREFcan be compared against voltage VQ1_Drainat the first terminal of transistor Q1, and control signal VG2for transistor Q1can be generated. Thus, voltage VQ1_Drainat the first terminal of transistor Q1may counteract the ripples of the voltage across the input capacitor as they have opposite directions. Also, input voltage VINmay not contain ripples of the switching frequency, and input current IINmay be a power frequency signal having no switching frequency ripple. In this way, high frequency harmonic waves can be substantially prevented from flowing into the power grid, the substantially avoiding associated harmonic pollution.