Active filter for electromagnetic interference (EMI) reduction using a single connection point and a negative impedance converter with cross-coupled transistors

An active filter reduces Electro-Magnetic Interference (EMI) created by current flowing through a power line. The active filter connects to the power line at a single node through a connection capacitor. A sense current flows through the connection capacitor when the power line current changes. This sense current is applied to a gain control circuit having cross-coupled PNP transistors that drive currents to both terminals of a variable capacitor. The variable capacitor converts these currents to a voltage that is injected back into the power line through the connection capacitor as an injected compensation voltage that compensates for the sensed current.

FIELD OF THE INVENTION

This invention relates to active filter circuits, and more particularly to single-point sense-and-inject active filters for Electro-Magnetic Interference (EMI) reduction.

BACKGROUND OF THE INVENTION

Filters are used for a variety of circuits. For example, a Switch-Mode Power Supply (SMPS) has power transistors that are rapidly turned on and off, such as at a rate of a few hundred of kHz. These SMPS can deliver energy to home appliances, computers, medical devices, telecom systems, automotive systems, and many other applications.

However, the rapid switching of the transistors in the SMPS can create Electro-Magnetic Interference (EMI) in other devices. Standards have been set to limit such EMI, such as the EN55022 standard that many SMPS must pass.

Passive EMI filters are traditionally added to power supplies and other EMI-generating devices. These passive EMI filters tend to be large and bulky since they rely upon inductors and capacitors. Although passive EMI filters are simple and effective at reducing EMI, they can be bulky, heavy, and have a power loss.

FIG.1shows a prior-art active EMI filter with separate sensor and injection points. Active EMI filters can use an op amp with a more complex circuit to reduce the size and bulk of the EMI filter. A positive input POS_IN is filtered to generate a positive output POS_OUT that has reduced EMI. Capacitor108between POS_OUT and ground GND_OUT helps to maintain a constant supply voltage on POS_OUT when variations occur in the load current.

The power current from POS_IN is sensed by sense inductor102. Current changes such as an increase in the power current flowing through the primary windings of sense inductor102induce a current in the same direction in the secondary windings of sense inductor102. Sense inductor102increases the voltage on the inverting input of op amp110, which generates an opposite voltage at the bottom terminal of capacitor104. The charge on capacitor104causes a positive voltage at the top terminal of capacitor104. Since the current is sinking into the output of the operational amplifier through capacitor104, the voltage on injector inductor106will inject the compensated voltage through the secondary windings of injector inductor106, cancelling the noise voltage in the main. Thus the noise voltage is reduced by the active filter.

The active filter has separate sense and injection points. Sense inductor102senses the current changes in the power supply, while injector inductor106injects a back-current into the power line to compensate for the sensed voltage. While the active filter is smaller than a passive filter, having separate sense and injection points can still required two bulky inductors. Some active filters replace one of sense inductor102, injector inductor106with a capacitor, but the remaining inductor is still bulky and therefore undesirable. Bulky inductors not only occupy much space and weight, but also have bandwidth limitations due to parasitic, non-ideal coupling, and self-resonance of inductors.

FIG.2highlights an active filter with a single point of attachment. The parent application described an active filter with a single point of attachment. Node NO is a single node on line10, which can be a power-supply line in a SMPS or a line that is being filtered to reduce noise such as produced by EMI from line10. Connection capacitor20connects to node NO on line10. The other terminal of connection capacitor20connects to negative capacitance circuit120.

Connection capacitor20acts as both the sensor and the injector of the active filter. Changes in the current flowing along line10cause a sensed current ISENSEto flow through connection capacitor20from line10to negative capacitance circuit120. In response, negative capacitance circuit120generates a voltage change VINJECTthat is applied to connection capacitor20and injected back into line10. VINJECTcompensates for ISENSEto reduce the current fluctuation in line10, and thus reduce the EMI generated by line10.

Negative capacitance circuit120has an impedance in the form of Z=V/I=−1/(sC), where V and I are the AC voltage drop and current on negative capacitance circuit120, s is a scale constant and C is an effective capacitance. In the frequency domain, for a normal positive capacitance, as a current I flows through a capacitor, the voltage drop across the capacitor will be (1/sC)×I, where the coefficient (1/sC) is the impedance of the capacitor, and in time domain the current will lead the voltage by 90 degrees. With a negative capacitance, the voltage drop across the capacitance will be (−1/sC)×I, where the coefficient (−1/sC) is the impedance of the negative capacitance circuit, and in time domain the current will lag the voltage by 90 degrees. In this way, the voltage drops across a positive capacitance and a negative capacitance will be anti-phase (180-degree difference).

When the sensed current ISENSEflows in the direction as shown inFIG.2through connection capacitor20and negative capacitance circuit120to the ground, negative capacitance circuit120acts as a negative capacitance, so negative capacitance circuit120produces a negative voltage drop between its two terminals rather than a positive voltage drop. Negative capacitance circuit120acts as a current-controlled-voltage source with an input impedance of Z=−1/(sC).

Since the bottom terminal of negative capacitance circuit120is ground, the voltage at the top terminal of negative capacitance circuit120is negative. The negative voltage generated by negative capacitance circuit120pulls the voltage of the bottom plate of connection capacitor20lower in voltage. Since connection capacitor20is a regular capacitance, the charge in it generates a voltage difference between Vinject+ and Vinject—. Vinject+ is pulled low because Vinject− is pulled lower by negative capacitance circuit120, helping to reduce noise voltage Vinject+ in line10and thus reduce EMI.

FIG.3shows a high-level block diagram of the active filter of the parent application. Connection capacitor20connects to node NO of line10. Changes in the current flowing through line10are sensed by connection capacitor20and applied to an input of gain control circuit112. The output of gain control circuit is in phase with the input, and is applied to filter capacitor156, which connects gain control circuit112to the input of power amplifier114. Power amplifier114provides a higher current to drive transfer capacitor30.

Gain Control circuit112acts as a resistive voltage divider circuit. Capacitors20,30act as a capacitive voltage divider. The voltage at node N1between capacitors20,30is driven below ground by higher current in line10, as negative capacitance circuit120did inFIG.2. The voltage of node N1going below ground is coupled across connection capacitor20to inject a compensation voltage back into line10to compensate for the higher current in line10.

While the active filter of the parent application is useful and effective, gain control circuit112may require an op amp Integrated Circuit (IC). Performance of the active filter may be limited by the op amp IC. Also, power amplifier114may require a large current. Thus a different circuit for the active filter is desired for added design flexibility and potentially reduced cost.

What is desired is an active EMI filter that has no inductors. An active EMI filter having a single point of connection to the power line is desired. An active EMI filter that has a single-point connection through a capacitor is desired. It is desired that the single-point connection capacitor both senses current changes and injects compensation back into the power line. An active filter that uses a negative capacitance concept is desired to sense and drive feedback the single-point capacitor. An active filter with variable components is desired for gain compensation for temperature and frequency bands is also desired.

DETAILED DESCRIPTION

FIG.4shows a high-level block diagram of the improved active filter. Power amplifier114from the parent application's active filter ofFIG.3has been deleted. Also transfer capacitor30and filter capacitor156ofFIG.3have been replaced by a single capacitor, variable capacitor40. A different circuit is used for gain control circuit121than for gain control circuit112.

Connection capacitor20connects to node NO of line10. Changes in the current flowing through line10are sensed by connection capacitor20as a sense current passing through connection capacitor20that is applied to an input of gain control circuit121. Gain control circuit121is controlled by this sense current.

Gain control circuit121drives an internal current onto variable capacitor40in response to the sense current. Variable capacitor40converts the internal current to an internal voltage. Gain control circuit121responds to the internal voltage on variable capacitor40and injects a compensation voltage back from gain control circuit121through connection capacitor20to line10.

Connection capacitor20generates a sense current in response to noise on line10and injects the compensation voltage back into line10. The sense current flows from line10through connection capacitor20into gain control circuit121, while the compensation voltage generated by gain control circuit121is transferred in the opposite direction through connection capacitor20back to line10.

Gain control circuit121acts as a current-controlled voltage source to provide a trans-impedance function. Gain control circuit121uses variable capacitor40to convert the internal current into the internal voltage, so variable capacitor40acts as a transfer unit for gain control circuit121.

Current flow through connection capacitor20and variable capacitor40are in the same phase. Since current and voltage are 90-degrees out-of-phase with each other when variable capacitor40is used to convert current to voltage, gain control unit121could reverse the polarity of the voltage drop on variable capacitor40by 180 degrees. This reversed voltage is injected between node N1and ground. Thus the active filter circuit acts to compensate for the instantaneous current variations or noise on line10

Bias circuit122provides operating conditions by generating bias voltages or bias currents for internal nodes within gain control circuit121. Bias circuit122generates bias voltages or currents from power and ground that are separate from line10. The resistance of variable capacitor40can be adjusted to adjust the performance of the active filter. The impedance of variable capacitor40can be adjusted to adjust the performance of the active filter.

FIG.5is a schematic of the improved circuit for the single-point active filter with negative capacitance. Bias circuit122and gain control circuit121are shown together inFIG.5as a combined circuit. Bias of the emitter, node NE1, of first PNP transistor24is provided by resistor51to ground, while bias of the base, node NB1, of first PNP transistor24is provided by resistor52to ground and resistor55coupled between the base, node NB1, and collector, node NC1, of first PNP transistor24. Similarly, bias the emitter, node NE2, of second PNP transistor26is provided by resistor54to ground, while bias of the base, node NB2, of second PNP transistor26is provided by resistor53to ground and resistor56coupled between the base, node NB2, and collector, node NC2, of second PNP transistor26.

Resistor57to power provides a current to collector node NC1of first PNP transistor24, while resistor58to power provides a current to collector node NC2of second PNP transistor26. These bias resistors set up a negative current from ground to the emitters and bases, turning on PNP transistors24,26to conduct larger collector currents. Node NE2acts as an AC ground for small-signal analysis, while the left terminals of resistors51,52,53,54connect to a hard DC ground such as a chassis ground or ground pin.

PNP transistors24,26have their bases and collectors cross-coupled to reverse the polarity of the voltage drop on variable capacitor40by 180 degrees. This reversed voltage is injected between node N1and ground, thus the active filter circuit acts to compensate for the instantaneous current variations or noise on line10. Collector node NC1of first PNP transistor24drives the base NB2of second PNP transistor26, while collector node NC2of second PNP transistor26drives the base NB1of first PNP transistor24. Collector nodes NC1, NC2are applied to two terminals of variable capacitor40.

The sense current from line10can be considered to be a small-signal AC signal. For small-signal AC analysis, resistors51-58can be ignored. The AC sense current from line10flows through connection capacitor20, PNP transistor24from NE1to NC1, then through variable capacitor40to node NC2, then through PNP transistor26from NC2to NE2, which is a virtual ground.

Connection capacitor20drives the sense current onto first emitter node NE1. Since the base current is small compared to the collector current, most of this sense current flows to the collector, node NC1. Likewise, the base current of PNP transistor26is small, so most of the collector current flows to the emitter, NE2.

The base-emitter voltage Vbe is fixed by the PN emitter junction. So VNE1=VNB1+Vbe, and since VNC2=VNB1due to cross-coupling, VNE1=VNC2+Vbe. Similarly for PNP transistor26, VNE2=VNC1+Vbe.

The voltage drop across variable capacitor40is:

where ISENSEis the sense current through connection capacitor20, and sC40is the equivalent capacitance of variable capacitor40in the s-domain.

The overall voltage drop across gain control circuit121is VNE1−VNE2. Substituting VNE1=VNB1+Vbe and VNE2=VNB2+Vbe:

Due to cross-coupling, VNB1=VNC2, and VNB2=VNC1, so

which is for a negative capacitance circuit. Thus gain control circuit121acts as a negative capacitance circuit (FIG.2).

Since connection capacitor20and gain control circuit121are connected in series, the equivalent capacitance of the series connection is:

By adjusting the capacitance of variable capacitor40, the active filter circuit can be adjusted.

Also, since VNE1−VNE2=ISENSE(1/sC40), and ISENSEis sensed from line10, any change in total capacitance by adjusting variable capacitor40produces no additional change on line10, node NO. Thus gain control circuit121acts as a current-controlled voltage source.

Variable capacitor40a variable capacitance, allowing adjustment or scaling of the injected voltage. The resistance values of resistors51-58may be selected or adjusted to set the gain and other characteristics of gain control circuit121. Variable resistors could also be substituted. These variable resistances and capacitance can be adjusted to compensate for temperature, or for different operating conditions such as when a different frequency band or switching frequency is selected.

FIGS.6A-6Bare graphs of operation of the active filter. InFIG.6A, the current flowing along line10oscillates, such as at a frequency of switching of the power transistors in a Switch-Mode Power Supply. This current is sensed through connection capacitor20and then through gain control circuit121to variable capacitor40and to AC ground.FIG.6Aplots the sensed current ISENSEthat flows through connection capacitor20and into gain control circuit121.

InFIG.6B, the voltage of line10at node NO, which is the noise voltage level, lags by 90 degrees the line current and the sensed current ISENSEthat flows through connection capacitor20. The network is capacitive.

VNC1−VNC2is the voltage across the terminals of variable capacitor40. Variable capacitor40is a transfer unit that transfers or converts sensed current ISENSEinto voltage information (VNC1−VNC2).

VNE1is the voltage at Node NE1, between connection capacitor20and the emitter of first PNP transistor24. VNE1is 180-degress out-of-phase with line voltage VN0.

After sensing current ISENSE, voltage VNE1is pulled low, below the AC ground by the transistor-based cross-connected circuit and variable capacitor40. Thus VN0is also pulled low. Adjusting the capacitance value of variable capacitor40allows for adjustment to the amount of noise reduction to compensate for the current changes in line10, and the amount of EMI reduction.

FIG.7is a graph showing a reduction in insertion loss using the active filter. Curve402shows the insertion loss for a passive LC filter where C has the same capacitance value as connection capacitor20. Curve400shows the insertion loss for an active LC filter with the active filter such as shown inFIG.5and a theoretical series inductor L. The insertion loss is reduced by about 10 dB at 2 MHz using the active filter ofFIG.5compared with a passive filter of the same capacitor size. This is only about 2 dB worse than the parent application's active filter, but for a potentially lower-cost circuit.

The active filter can also reduce EMI by as much as 75% at 2 MHz. A smaller capacitor in the active filter may be used to achieve the same EMI reduction as a passive filter. The size and bulk of the filter may be reduced.

FIG.8is an alternative active filter with multiple levels of transistors. The sense current from connection capacitor20is applied to the base of NPN transistor64, and this base is also biased by resistors73,75. Changes in the sense current change the emitter-base current through NPN transistor64, changing the collector current from resistor71to node N3that is coupled to the bases of NPN transistor66and PNP transistor68by capacitor80. The emitter of NPN transistor64, node N5, connects to ground through resistor84in parallel with capacitor82.

The output stage bases, node N4, is biased by resistors77,78. The emitters of NPN transistor66and PNP transistor68are connected together at node N6, which is the first terminal of variable capacitor40. Current driven to variable capacitor40by NPN transistor66and PNP transistor68is converted to a compensation voltage by variable capacitor40, which uses its second terminal, node N1, to directly inject this compensation voltage back through connection capacitor20to line10.

The design flexibility of the circuit ofFIG.8is higher compared to circuits using op-amps. The circuit could have a stronger current capability due to this design flexibility, and power amplifier114(FIG.3) is removed. The number of components is also reduced so that the footprint might be smaller, and it could be possible to be integrated into an IC format. The cost may be reduced by roughly 70% since transistors are cheaper.

FIG.9is another alternative active filter with multiple levels of transistors. NPN transistor62is added between the collector of NPN transistor64and resistor71. The base of NPN transistor62is biased by resistors77,78. The base of NPN transistor66, node N3, is driven directly by the collector of NPN transistor62, while the base of NPN transistor68, node N4, is driven directly by the emitter of NPN transistor62. Capacitor80is connected between the bases of NPN transistor66and PNP transistor68, nodes N3, N4.

FIG.10is an active filter with a simplified MOS circuit. A series of resistors90-94form a voltage divider between power and ground to generate a first gate voltage on node N3and a second gate voltage on node N4. Pullup transistor96has a gate driven by node N3, while pull-down transistor98has a gate driven by node N4. The drains of transistors96,98are connected together at node N6to drive one terminal of variable capacitor40, while the other terminal of variable capacitor40is node N1, allowing variable capacitor40to directly inject the compensation voltage back to node N1and connection capacitor20.

The sense current from connection capacitor20is applied to select the resistance of middle resistor92, which can be a variable resistor along with resistors90,94. Pull-up transistor96can be a p-channel transistor while pull-down transistor98can be an n-channel transistor, using standard Complementary Metal-Oxide-Semiconductor (CMOS), Gallium-Nitride (GaN), or other transistor devices.

FIG.11shows active filters being used as differential and common-mode filters. Switched-Transistor Power Supply (STPS)300generates undesirable EMI. To reduce this EMI, active filter70is coupled between positive line P1and negative line G1. Active filter70can be the active filter ofFIG.5with line10being line P1and ground being negative line G1.

Active filter70′ is connected to a common-mode node CM between capacitor72to P1, and capacitor74to G1. Active filter70′ can be the active filter ofFIG.5with line10being the common mode node CM between capacitors72,74, and ground being negative line G1or another ground node.

FIG.12shows active filters being used as differential and single-line filters. Switched-Transistor Power Supply (STPS)300generates undesirable EMI. To reduce this EMI, active filter70is coupled between positive line P1and negative line G1. Active filter70can be the active filter ofFIG.5with line10being line P1and ground being negative line G1.

Active filter70′ is connected to power line P1. Active filter70′ can be the active filter ofFIG.5with line10being power line P1, and ground being an independent ground node such as earth in an AC system or COM in DC-DC systems.

Active filter70″ is connected to negative power line G1. Active filter70″ can be the active filter ofFIG.4with line10being negative power line G1, and ground being an independent ground node.

ALTERNATE EMBODIMENTS

Several other embodiments are contemplated by the inventors. For example various circuits and configurations may be used. Transistors may be cross-coupled in some circuits, but not in other circuits. Feedback may be modified. Variable capacitor40may have both terminals connected to the output of gain control circuit121(FIG.5), or may have only one terminal connected to the output of gain control circuit121(FIGS.8-10), with the other terminal of variable capacitor40connected directly to node N1and coupling capacitor20, so that variable capacitor40provides the injected voltage directly to coupling capacitor20rather than back through gain control circuit121.

A single capacitor may be implemented as several parallel capacitors, and a variable capacitor may be implemented by a switched-capacitor array such as a binary-weighted capacitor array and a decoder. Gain control circuit121may be replaced with other active circuits that have an input impedance in the format of Z=−V/I, such as a power transistor circuit.

Variable capacitor40could be an electronically controlled capacitance, such as voltage tuned capacitance or digitally tuned capacitance (but not limited to this). An active filter with variable components is desired for gain compensation in different temperature conditions and frequency bands, in case the equivalent impedance of variable capacitor40varies for different temperature or frequencies.

While reducing EMI on a power line has been described, EMI may also be reduced in downstream circuits that are powered by the power line. The active filter may be applied to a variety of applications for a variety of purposes other than EMI reduction. The active filter may be applied to internal nodes on an Integrated Circuit (IC) to reduce internal cross-talk interference. The active filter may be attached to an internal clock line to reduce EMI generated by that clock line. The active filter could be connected across differential data lines in a telecom system to filter noise on these lines. Application scenarios that need filters to reduce small signals can benefit from the invention.

A controller or initializer or calibrator could be added to select the value of variable capacitor40or the values of any of resistors51-58if variable resistors are used. A temperature compensator could compensate the bandwidth of the gain-control unit using analog components such as a resistor and a Negative Temperature Coefficient (NTC) device such as a thermistor. The controller could be digital and receive a temperature measurement and look up values in a table to apply to the variable capacitor or variable resistors. The controller could likewise receive a mode bit that indicates a frequency of operation and adjust the variable capacitance or variable resistances according to settings from a look up table. Some of the variable components could be fixed while others are varied. Connection capacitor20could be a variable capacitor rather than a fixed capacitor.

Negative capacitance circuit120could be a negative impedance converter, a negative load, or another circuit that injects energy back into a system rather than stores energy from that system. While a phase shift of 90 degrees between the input sensed current and the output injected voltage has been shown inFIGS.6A-6B, a different phase shift such as 180 degrees may be substituted, and the actual phase shift may depend on circuit delays. Variable capacitor20could be substituted with a transfer impedance that could be an inductor rather than a capacitor. Likewise, connection capacitor20could be replaced by an inductor. A more complex network of capacitors, inductors, and/or resistors could be substituted as well.

Values could be scaled or otherwise operated upon. Parameters such as delays and targets could be adjusted or scaled based on conditions such as device temperature or length of time between detected faults, or statistics or properties of the particular load being driven. For example, large loads with large capacitances could have a longer setting for the TRIP delay and a larger value of the threshold TH than do loads with smaller capacitances or with less switching of load capacitances. When the load uses a higher-frequency clock, TRIP and TH could be increased to allow for more capacitor charging.

Currents may be positive currents or negative currents and may flow in either direction, depending on the carrier polarity. Various theories of operation have been presented to help understand the operation of the system as best understood, but these theories are only approximations of actual circuit behavior and may be incorrect.

Additional components may be added at various nodes, such as resistors, capacitors, inductors, transistors, buffers, dividers, etc., and parasitic components may also be present. Enabling and disabling the circuit could be accomplished with additional transistors or in other ways. Pass-gate transistors or transmission gates could be added for isolation. Inversions may be added, or extra buffering. Separate power supplies and grounds may be used for some components. Various filters could be added. Active low rather than active high signals may be substituted. Various reference voltages or virtual supplies may be used rather than a hard ground.

The background of the invention section may contain background information about the problem or environment of the invention rather than describe prior art by others. Thus inclusion of material in the background section is not an admission of prior art by the Applicant.

Any methods or processes described herein are machine-implemented or computer-implemented and are intended to be performed by machine, computer, or other device and are not intended to be performed solely by humans without such machine assistance. Tangible results generated may include reports or other machine-generated displays on display devices such as computer monitors, projection devices, audio-generating devices, and related media devices, and may include hardcopy printouts that are also machine-generated. Computer control of other machines is another tangible result.