Common mode EMI filter

Abstract

Described embodiments include a circuit (500) for filtering electromagnetic interference (EMI) that includes a conductive housing (108) enclosing the circuit, a first power terminal (102) providing a first signal (IN +), and a second power terminal (104) providing a second signal (IN-), the first and second signals forming a differential power input. A filter circuit (250) provides a common mode noise cancellation signal (to 256) at an output in response to first and second inputs (from 252 and 254). The induction choke (120) has first and second coils that are magnetically coupled. The first coil (122) is coupled between the first power terminal and a first converter input. The second coil (124) is coupled between the second power terminal and a second converter input. A third capacitor (256) is coupled between the filter output and the second power terminal. A fourth capacitor (118) is coupled between the first power terminal and the second power terminal, and an inductor (460) is coupled between the housing and a ground terminal.

Description

[0001] The present description relates to power converters and to filtering of electromagnetic interference (EMI) in off-line power converters. Background Art

[0002] Offline power converter applications include kilowatt power converters and on-board car chargers, which have an alternating current (AC) voltage input and produce a direct current (DC) voltage output to charge the battery. These applications may include a relatively large set of inductors and capacitors to filter out EMI. Circuits connected to an AC mains power supply may need to meet certain EMI specifications to limit the amount of electrical noise generated by the circuit being injected into the AC mains power line.

[0003] Electromagnetic interference (EMI) is undesirable noise or disturbance in an electronic device caused by an electromagnetic field generated by the operation of another electronic device. The two modes of EMI in offline power converters are differential mode EMI and common mode EMI. In these converters, common mode EMI is generally more of a concern than differential mode EMI due to the greater size and cost of the components used to filter it. Summary of the invention

[0004] In a first example, a circuit for filtering electromagnetic interference (EMI) in a power converter includes a conductive housing enclosing a circuit, a first power terminal providing a first signal, and a second power terminal providing a second signal, wherein the first and second signals form a differential power input. The filter circuit has first, second, and third filter inputs, and has a filter output. The third filter input is coupled to the housing. The filter circuit is configured to provide a common-mode noise cancellation signal at the filter output in response to the first and second filter inputs.

[0005] A first capacitor is coupled between the first power terminal and the first filter input, and a second capacitor is coupled between the second power terminal and the second filter input. The voltage converter has first and second converter inputs. The inductive choke has first and second coils, the first and second coils being magnetically coupled. The first coil is coupled between the first power terminal and the first converter input. The second coil is coupled between the second power terminal and the second converter input. The third capacitor is coupled between the filter output and the second power terminal. The fourth capacitor is coupled between the first power terminal and the second power terminal, and the inductor is coupled between the housing and the ground terminal.

[0006] In a second example, a method for filtering electromagnetic interference (EMI) from a power converter includes providing a first signal from a positive terminal of a differential voltage input to a first input of a filter circuit via a first capacitor, and providing a second signal from a negative terminal of the differential voltage input to a second input of the filter circuit via a second capacitor. Using the filter circuit, sensing common mode noise present on the first and second signals, and providing a noise cancellation signal at an output of the filter circuit to cancel the common mode noise.

[0007] The noise cancellation signal is added to at least one of the first signal and the second signal. A third input of the filter circuit is coupled to the conductive housing, and the housing is connected to a ground terminal via an inductor.

[0008] In a third example, a circuit for filtering EMI includes a voltage reference terminal. A first input terminal provides a first signal, and a second input terminal provides a second signal. The first and second signals form a differential input voltage. The filter circuit has first, second, and third filter inputs, and a filter output. The third filter input is coupled to the voltage reference terminal. The filter circuit is configured to provide a common-mode noise cancellation signal at the filter output in response to the first and second filter inputs.

[0009] A first capacitor is coupled between the first input terminal and the first filter input. A second capacitor is coupled between the second input terminal and the second filter input. A third capacitor is coupled between the filter output and the second input terminal. A fourth capacitor is coupled between the first input terminal and the second input terminal, and an inductor is coupled between the voltage reference terminal and the ground terminal. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 A block diagram of an example power converter circuit with passive EMI filtering is shown.

[0011] Figure 2 A block diagram of an example power converter circuit with an active EMI filter is shown.

[0012] Figure 3 A schematic showing an example capacitance multiplication circuit.

[0013] Figure 4 A block diagram of an example power converter circuit with an active EMI filter having a chassis-connected inductor is shown.

[0014] Figure 5 A block diagram of an example power converter circuit having an active EMI filter with a chassis-connected inductor for surge protection is shown. DETAILED DESCRIPTION

[0015] In this specification, the same reference numerals depict the same or similar (functionally and / or structurally) features. The drawings are not necessarily drawn to scale.

[0016] Embodiments described herein are directed to filtering of EMI in offline power supplies, and specifically to filters for reducing common-mode EMI noise. Two example modes of EMI in offline power converters are differential-mode EMI and common-mode EMI. The difference between differential-mode EMI and common-mode EMI is the direction of the noise current on the positive power line and the negative power line. The positive power line and the negative power line may also be referred to as the phase line (line) and the neutral line (neutral), respectively. Differential-mode EMI generates noise currents that flow in opposite directions on the positive and negative input terminals of the power supply. In contrast, common-mode EMI generates noise currents that flow in the same direction on the positive line of the power supply and the negative line of the power supply.

[0017] EMI filters suppress the switching noise generated by the power converter from being injected back onto the AC power line. Chokes are inductors used in specific filtering applications. Common mode chokes contain two inductors wound on the same core. The two inductor coils on the choke are electrically isolated but magnetically coupled together, increasing their common mode impedance. The higher the common mode impedance of the choke's coils, the higher the noise attenuation provided by the choke.

[0018] Figure 1 A block diagram of an example power converter circuit 100 with a passive EMI filter is shown. The positive power input terminal of the power converter circuit 100 is IN+102, and the negative power input terminal is IN-104. In one example, IN+102 and IN-104 are the phase and neutral terminals of the AC mains, respectively. The output of the power converter circuit 100 is coupled to differential output terminals OUT+142 and OUT-144. The power converter circuit has a conductive chassis 108. OUT-144 is electrically connected to the chassis 108. The voltage converter 140 provides a differential regulated DC voltage across OUT+142 and OUT-144. The chassis 108 is electrically connected to the earth ground 106.

[0019] Power converter circuit 100 includes chokes 110 and 120. Choke 110 includes coils 112 and 114. Coil 112 is coupled to IN+ 102, and coil 114 is coupled to IN- 104. Coils 112 and 114 are magnetically coupled, but electrically isolated. Choke 120 includes coils 122 and 124. Coil 122 is coupled to a first input of voltage converter 140, and coil 124 is coupled to a second input of voltage converter 140. Coils 122 and 124 are magnetically coupled, but electrically isolated.

[0020] Capacitor 128 is coupled between coil 112 and earth ground 106. Capacitor 130 is coupled between coil 114 and earth ground 106. Capacitor 126 is coupled between coil 112 and coil 114. Capacitor 132 is coupled between coil 122 and earth ground 106. Capacitor 136 is coupled between coil 124 and earth ground 106. Capacitor 134 is coupled between first and second inputs of voltage converter 140.

[0021] The current flows into the choke at the first coil via the terminal with a point, and flows out of the choke at the second coil via the terminal with a point. The current is supplied at IN+102 and flows from the terminal with a point through the coil 112 to the other terminal of the coil 112. Then, the current flows from the terminal with a point into the coil 122 to the other terminal of the coil 122, and then enters the first input of the voltage converter 140. The return path of the current is from the second input of the voltage converter 140 to the coil 124, from the terminal without a point to the terminal with a point. Then, the return current flows into the coil 114 from the terminal without a point to the terminal with a point, and then flows out at IN-104. The current flows in opposite directions in the two coils of the choke. This is a differential mode current, and is the main power current path. When the voltage converter 140 is a switching regulator, the noise ripple current can be applied to the differential mode current and also flows in opposite directions in the two coils of the choke. This noise current is differential mode EMI.

[0022] Common mode EMI is noise current that flows in the same direction in both the positive and negative power lines. Placing a choke in series with the power lines prevents current from flowing in the same direction in the positive and negative lines due to magnetic coupling. An ideal common mode choke would present infinite impedance to common mode current. However, in reality, chokes have a finite common mode impedance because the two coils of the choke are not fully coupled, causing a portion of the magnetic flux to remain uncoupled. This is often referred to as leakage flux and can be represented in a circuit as leakage inductance. Nevertheless, chokes 110 and 120 provide high common mode impedance, which reduces the amount of common mode noise current. Chokes used in combination with capacitors for EMI filtering can take up most of the space in the EMI filtering portion of some power converter circuits.

[0023] The voltage converter 140 has chokes 110 and 120 coupled to its input terminals. Due to the switching noise from the power transistors within the voltage converter 140, ripple current is injected into the chokes 110 and 120. This switching ripple is the main source of differential mode EMI. The noise current from common mode EMI flows in the same direction in both the positive power line and the negative power line. There must be a complete loop for the current to flow. Because the current flows in the same direction in the positive power line and the negative power line, a third terminal is required to return the common mode current. This terminal is the chassis 108 connected to the earth ground 106. Therefore, common mode EMI is any current that flows through the positive and negative lines in the same direction and then returns to the earth ground via the chassis. Common mode EMI current does not flow in the main power current loop. Common mode EMI current flows in a noise loop that is usually caused by parasitic capacitance and inductance in the circuit.

[0024] The voltage converter 140 contains switching field effect transistors (FETs) in its output stage that carry high currents. In some cases, these switching FETs may need to be cooled to prevent overheating, so a heat sink may be added. The heat sink is placed on top of the FETs, and the heat sink is typically connected to the chassis 108, thereby coupling the heat sink to the earth ground 106. The heat sink may have a thermal interface material to ensure that it is electrically isolated from the chassis 108. However, there is a parasitic capacitance between the switch node of the FET and the earth ground 106, which may be tens of pico-farads.

[0025] The parasitic capacitance between the switch node of the FET and the earth ground 106 can cause problems. When the voltage at the switch node swings, noise is injected from the switch node to the earth ground 106 via the parasitic capacitance. If the switch node swings 400-600V, several amps of current can flow through this parasitic capacitance, depending on how fast the switch node voltage swings. This current generates common mode EMI, which is injected into the chassis 108 and therefore into the earth ground 106. The common mode EMI current flows through the chassis and, if the EMI is not filtered, can be coupled back to the positive and negative input lines and injected into the AC mains grid.

[0026] Both common-mode EMI and differential-mode EMI can be filtered using multi-stage inductor-capacitor (LC) filters. The capacitors in differential-mode filters are typically relatively large (i.e., uF), allowing the filter inductance to be relatively small (i.e., uH) while still obtaining the specified filter corner frequency. The relatively small amount of inductance required to obtain the necessary filter characteristics can be obtained from the leakage inductance of the common-mode choke. For common-mode filters, the capacitance is limited to nanofarads by the leakage current requirement, so the inductance needs to be larger (mH) to obtain the same filter corner frequency.

[0027] The multi-stage differential mode filter includes leakage inductance from coils 112, 114, 122, and 124 and capacitance from capacitors 118, 126, and 134. The multi-stage common mode filter includes common mode inductance from coils 112, 114, 122, and 124 and capacitance from capacitors 128, 130, 132, and 136.

[0028] Common mode filters do not provide impedance to differential mode noise. Common mode filters cannot filter differential mode noise because all current flowing into the point terminal of the first coil of the choke is constrained to flow out of the point terminal in the second coil of the choke. However, the non-ideal part of the common mode choke (i.e., the leakage inductance) provides some impedance to the differential mode current because there is no inductive coupling between the coils. Therefore, the leakage inductance forms an LC differential mode filter together with the capacitor.

[0029] The voltage at the switch node of voltage converter 140 can switch hundreds of volts up and down, injecting common mode noise into chassis 108. This noise is filtered by capacitors 128, 130, 132, and 136. The majority of this noise is filtered by capacitors 136 and 132. A proportionally smaller amount of this noise is then filtered by capacitors 130 and 128. The capacitors provide a low impedance path for the noise to return to the input, and the common mode choke provides a high impedance in series to reduce the amount of noise flowing through the input terminals.

[0030] Capacitors ranging from hundreds of nF to several uF can be used in differential mode filters. Using larger capacitors allows smaller inductors to be used to obtain the same filter corner frequency. In one example, a 1uH inductor is used in combination with a 1uF capacitor to achieve a specified filter corner frequency. For this reason, the leakage inductance of the chokes 110 and 120 provides sufficient inductance, so a separate inductor is not required for the differential mode filter.

[0031] Large capacitors cannot be used in common mode filters due to potential safety issues. If chassis 108 is disconnected from earth ground 106, and a person standing on earth ground touches the chassis, the person is now coupled between the capacitor and earth ground. As the capacitance increases, the impedance between the AC voltage source and earth ground decreases. As the impedance between the AC voltage source and earth ground decreases, the amount of current supplied by the capacitor that can flow through the person increases, causing safety issues.

[0032] To mitigate this safety concern, capacitors 128, 130, 132, and 136 can be in the range of 1-10 nF. If capacitors with capacitances several orders of magnitude lower are used, then inductances several orders of magnitude higher are required to obtain the same filter corner frequency. This is why common-mode inductors end up being relatively large, such as several mH, leading to the use of physically larger chokes in the circuit.

[0033] Figure 2 A block diagram of an example power converter circuit 200 with active EMI filtering is shown. In power converter circuit 200, capacitors 128, 130, 132, and 136 of power converter circuit 100 effectively have larger capacitances by using capacitance multiplication circuits (which may also be referred to as capacitance amplification).

[0034] Figure 3 A schematic diagram of an example capacitance multiplication circuit 300 is shown. A voltage source 330 provides a voltage V. An amplifier 310 has an input 308 coupled to the voltage source 330 and receives the voltage V at its input. The amplifier 310 inverts the voltage V and amplifies the gain A, where A is positive and greater than 1. The voltage at the output 312 of the amplifier 310 is equal to -AV. A capacitor 320 has a first terminal coupled to the input of the amplifier 310 at a voltage V, and has a second terminal coupled to the output of the amplifier 310 at a voltage AV. The voltage across the capacitor 320 is (1+A)V.

[0035] The voltage V at the first terminal of the capacitor 320 is sensed. The sensed voltage V is provided as an input to the amplifier 310 for amplification and inversion. The output of the amplifier has a voltage -AV, which is provided to the second terminal of the capacitor 320. The voltage across the capacitor 320 is now (1+A) times the original voltage across the capacitor, which reduces the impedance of the circuit by (1+A) times. Therefore, by providing amplification and inversion of the voltage at the first capacitor terminal, and then injecting the amplified and inverted signal into the second capacitor terminal, the capacitor impedance looks like a smaller impedance, or an effective larger capacitance.

[0036] The first advantage provided by capacitance multiplication circuit 300 is that it produces a larger effective capacitance. Using a smaller capacitor, but effectively making it a larger capacitor in the circuit, allows the use of a proportionally smaller inductor while still maintaining the same filter corner frequency and effectiveness.

[0037] The second advantage provided by the capacitance multiplication circuit 300 is that the frequency at which the capacitor produces an effective larger capacitance can be selected by the transfer function of the amplifier. By placing resistors and capacitors in the feedback loop or input circuit path of the amplifier 310, the frequency response of the amplifier can be shaped. At low frequencies (e.g., 50-60Hz) where the higher voltage from the positive power input signal is located, the amplifier can provide a relatively small gain. This allows the capacitor to present a small effective capacitance at low frequencies, thereby preventing violations of restrictions on having a large capacitance between the earth ground and the current-carrying line. However, at higher frequencies (e.g., tens of kHz), the gain can be increased so that the capacitor has a larger effective capacitance. EMI is usually measured in circuits at frequencies above 150kHz, so the large effective capacitance at those frequencies improves the measurement. If the effective capacitance is 100 times larger at a given frequency, the inductor can be 100 times smaller and achieve the same corner frequency and EMI filter characteristics.

[0038] The positive power input terminal of the power converter circuit 200 is IN+102, and the negative power input terminal is IN-104. The output of the power converter circuit 200 is coupled to the differential output terminals OUT+142 and OUT-144. The power converter circuit has a conductive chassis 108. OUT-144 is electrically connected to the chassis 108, which provides a reference voltage. The voltage converter 140 provides a differential regulated DC voltage across OUT+142 and OUT-144. The chassis 108 is electrically connected to the earth ground 106.

[0039] Power converter circuit 200 includes chokes 110 and 120. Choke 110 includes coils 112 and 114. Coil 112 is coupled to IN+ 102, and coil 114 is coupled to IN- 104. Coils 112 and 114 are magnetically coupled. Choke 120 includes coils 122 and 124. Coil 122 is coupled to a first input of voltage converter 140, and coil 124 is coupled to a second input of voltage converter 140. Coils 122 and 124 are magnetically coupled.

[0040] Capacitor 118 is coupled between IN+ 102 and IN- 104. Capacitor 126 is coupled between coil 112 and coil 114. Capacitor 132 is coupled between coil 122 and earth ground 106. Capacitor 136 is coupled between coil 124 and earth ground 106. Capacitor 134 is coupled between first and second inputs of voltage converter 140. Capacitor 252 is coupled between coil 114 and a first input of active EMI filter 250. Capacitor 254 is coupled between coil 112 and a second input of active EMI filter 250. Capacitor 256 is coupled between coil 114 and an output of active EMI filter 250. A third input of active EMI filter 250 is coupled to chassis 108. Capacitors 252 and 254 sense the voltages on the IN+ and IN- lines, respectively, and capacitor 256 injects a noise cancellation signal onto the IN- line.

[0041] In some cases, an additional capacitor (not shown) is coupled between coil 112 and a second output (not shown) of active EMI filter 250 to inject a signal onto the IN+ line. However, in other cases, no additional capacitor is required between coil 112 and the second output of active EMI filter 250 as long as the capacitance of capacitor 126 is large enough. In this case, capacitor 126 couples the injection current from the output of active EMI filter 250 from the IN- line to the IN+ line, thereby allowing common-mode injection without the need for an additional capacitor.

[0042] The active EMI filter 250 is a filter circuit that reduces common-mode EMI noise. The active EMI filter 250 has one or two outputs and two inputs. The active EMI filter 250 is configured to receive a first AC voltage having a switching frequency at a first input, which is coupled to a capacitor 252. The active EMI filter 250 is configured to receive a second AC voltage having the same switching frequency at a second input, which is coupled to a capacitor 254. The active EMI filter 250 senses noise having a second frequency on at least one of the first or second inputs, and generates an injection signal at its output in response to the detected noise. The output of the active EMI filter 250 is coupled to at least one of the IN+ and IN- lines. An example implementation of an active EMI filter is described in U.S. Patent Application No. 17 / 223,835 filed on April 6, 2021, the entire text of which is incorporated herein by reference.

[0043] The active EMI filter 250 has first and second inputs coupled to capacitors 252 and 254 for sensing common mode noise, and has an output coupled to capacitor 256 for injecting an inverted amplified noise cancellation signal into the IN+ and IN- signal paths. In some examples, an additional injection capacitor is coupled between the second output of the active EMI filter 250 and the coil 112. In other examples where the capacitor 126 is large enough, the additional injection capacitor can be omitted. The injection capacitor is a capacitance amplified within the active EMI filter 250. Capacitors 254 and 252 capacitively sense noise on the IN+ and IN- lines, respectively, to avoid coupling high voltage terminals directly to the input terminals of the active EMI filter 250.

[0044] Common-mode noise current enters power converter circuit 200 at IN+102 and IN-104 terminals. Common-mode noise current flows from IN+102 and IN-104 terminals through coils 112 and 114, respectively, thereby flowing from the terminals with dots to the terminals without dots of coils 112 and 114, respectively. Common-mode noise current flows in coil 112 in the same direction as it flows in coil 114.

[0045] The common-mode noise current from coil 112 flows through capacitor 254 and then flows into the second input of active EMI filter 250. The common-mode noise current from coil 114 flows through capacitor 252 and then flows into the first input of active EMI filter 250. The common-mode noise current that is not filtered by the AEF flows into chassis 108 and then into earth ground 106. A portion of the common-mode noise current then returns to the IN+ and IN- lines.

[0046] Noise is generated by the switching of FETs within voltage converter 140. This noise can be injected into chassis 108 via parasitic capacitance. Active EMI filter 250 senses the noise current on IN+ and IN- lines via capacitors 254 and 252, respectively. Active EMI filter 250 extracts the high frequency common mode component of the noise current while rejecting the line frequency and differential mode components of the signal. The common mode noise component is amplified and inverted. The amplified and inverted signal is then injected onto the IN- line via capacitor 256 and injected onto the IN+ line via capacitors 256 and 126.

[0047] Figure 4A block diagram of an example power converter circuit 400 with an active EMI filter having a chassis-connected inductor is shown. The power converter circuit 400 does not have a choke in series with the IN+ line or the IN- line. The two chokes in the IN+ and IN- lines of the power converter circuit 200 have been replaced by inductors coupled between the chassis 108 and the earth ground 106. Capacitor 254 is directly coupled to the IN+ terminal 102, rather than via a choke coil. Capacitor 252 is directly coupled to the IN- terminal 104, rather than via a choke coil. The other terminals of capacitors 252 and 254 are coupled to the first and second inputs of the active EMI filter 250, respectively.

[0048] Common-mode noise current enters power converter circuit 400 at IN+ 102 and IN- 104 terminals. Common-mode noise current from IN+ 102 flows through capacitor 258. Common-mode noise current from IN- 104 flows through capacitor 256. Common-mode noise current filtered by AEF then flows into chassis 108. Inductor 460 is coupled between chassis 108 and earth ground 106.

[0049] Because inductor 460 is coupled in series between chassis 108 and earth ground 106, the same common-mode noise current that flows through the IN+ and IN- lines flows through earth ground and then returns to the input of active EMI filter 250. If the inductance of inductor 460 is the same as the inductance of coils 112 and 114, power converter circuit 400 will have the same series circuit impedance and the same amount of current will flow in the circuit. However, the difference is that inductor 460 does not carry the large power current carried by coils 112 and 114, but rather carries a smaller common-mode current. Therefore, inductor 460 can be smaller than coils 112 or 114 because it carries a smaller current.

[0050] Power converter circuit 400 provides at least two advantages. The first advantage is that two inductors are replaced by a single inductor because coils 112 and 114 are replaced by inductor 460. The second advantage is that the single inductor is much smaller than either of the two inductors it replaces because it carries less current. Also, because the wire gauge of inductor 460 is smaller than coils 112 and 114, the core of the inductor can be smaller than the choke core.

[0051] The common mode noise current through inductor 460 can range from a few mA to hundreds of mA. However, the power current through IN+ and IN- can be hundreds of amps. Therefore, the coil required to handle hundreds of amps of current is replaced by an inductor required to handle only hundreds of mA. The common mode path inductance is the same, but now the inductor handles three orders of magnitude less current, making the inductor physically smaller, which saves space and cost.

[0052] Figure 5 A block diagram of an example power converter circuit 500 with an active EMI filter having a chassis-connected inductor for surge protection is shown. The power converter circuit 500 is the same as the power converter circuit 400, except that a clamp 462 is added in parallel with the inductor 460 for surge suppression. Although the inductor 460 typically handles only small currents, a surge event may occur, which comes from the earth ground 106 or from the chassis 108. If the inductor 460 is not protected, it may be damaged by this surge event. The clamp 462 is coupled in parallel with the inductor 460 to provide protection against surge events. The clamp 462 can be a transient voltage suppression (TVS) diode or a circuit of some other type designed to limit the voltage to a certain level. The clamp 462 protects the inductor 460 from electrical overstresses, such as those caused by lightning strikes, inductive load switching, and electrostatic discharge.

[0053] In this specification, "terminal", "node", "interconnection", "lead" and "pin" are used interchangeably. Unless specifically stated to the contrary, these terms generally mean the interconnection between device elements, circuit elements, integrated circuits, devices or other electronic or semiconductor components or their ends.

[0054] In this specification, "ground" includes chassis ground, earth ground, floating ground, virtual ground, digital ground, common ground, and / or any other form of ground connection that is applicable or suitable for the teachings of this specification.

[0055] In this specification, even if operations are described in a particular order, some operations may be optional, and the operations need not be performed in the particular order to achieve a specified result. In some instances, multitasking and parallel processing may be advantageous. In addition, the separation of various system components in the embodiments described above does not necessarily require such separation in all embodiments.

[0056] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.

Claims

1. A circuit for filtering electromagnetic interference (EMI) in a power converter, the circuit comprising: a housing, which is electrically conductive, and which encloses the circuit; a first power terminal providing a first signal and a second power terminal providing a second signal, the first and second signals forming a differential power input; a filter circuit having first, second and third filter inputs and having a filter output, the third filter input being coupled to the housing, the filter circuit being configured to respond to the first and second filter input and providing a common mode noise cancellation signal at said filter output; a first capacitor coupled between the first power terminal and the first filter input; a second capacitor coupled between the second power terminal and the second filter input; a voltage converter having first and second converter inputs; an inductive choke having first and second coils, the first and second coils being magnetically coupled, the first coil being coupled between the first power terminal and the first converter input, and the second coil being coupled between the second power terminal and the second converter input; a third capacitor coupled between the filter output and the second power terminal; a fourth capacitor coupled between the first power terminal and the second power terminal; as well as An inductor is coupled between the housing and a ground terminal.

2. The circuit of claim 1, wherein the filter output is a first filter output, the filter circuit includes a second filter output, and the circuit further comprises a fifth capacitor coupled between the second filter output and the first power terminal.

3. The circuit according to claim 1, further comprising: a fifth capacitor coupled between the first converter input and the housing; as well as A sixth capacitor is coupled between the second converter input and the housing.

4. The circuit of claim 1, wherein the circuit further comprises a clamp circuit connected in parallel with the inductor, the clamp circuit being configured to provide protection to the circuit against voltage surges.

5. The circuit of claim 4, wherein the clamping circuit comprises a transient voltage suppression (TVS) diode.

6. The circuit of claim 1, further comprising a fifth capacitor coupled between the first and second converter inputs.

7. The circuit of claim 1, wherein the filter circuit comprises a capacitance multiplication circuit.

8. The circuit of claim 7, wherein the capacitance multiplication circuit comprises: an amplifier having an amplifier input and an amplifier output, the amplifier input being coupled to the voltage source terminal; and A fifth capacitor is coupled between the amplifier input and the amplifier output.

9. The circuit of claim 8, wherein the gain of the amplifier varies depending on the frequency of the signal at the voltage source terminal.

10. A method for filtering electromagnetic interference (EMI) from a power converter, the method comprising: providing a first signal from a positive terminal of a differential voltage input to a first input of a filter circuit via a first capacitor; providing a second signal from a negative terminal of the differential voltage input to a second input of the filter circuit via a second capacitor; using the filter circuit, sensing common mode noise present on the first and second signals, and providing a noise cancellation signal at an output of the filter circuit to cancel the common mode noise; adding the noise cancellation signal to at least one of the first signal and the second signal; coupling a third input of the filter circuit to the conductive housing; as well as The housing is connected to a ground terminal via an inductor.

11. The method of claim 10, further comprising providing surge event protection by connecting a voltage clamp circuit in parallel with the inductor.

12. The method of claim 11, wherein the voltage clamp circuit comprises a transient voltage suppression (TVS) diode.

13. The method according to claim 10, further comprising: providing the first signal to a first terminal of a differential converter input of a voltage converter via a first induction coil; as well as The second signal is provided to a second terminal of the differential converter input via a second inductive coil, the second inductive coil being magnetically coupled to the first inductive coil.

14. The method according to claim 13, further comprising: coupling a third capacitor between the first terminal of the first induction coil and the first terminal of the second induction coil; as well as A fourth capacitor is coupled between the second terminal of the first induction coil and the second terminal of the second induction coil.

15. The method according to claim 10, further comprising: providing a second noise cancellation signal at a second output of the filter circuit; as well as The second noise cancellation signal is added to a second of the first signal and the second signal.

16. A circuit for filtering electromagnetic interference (EMI), the circuit comprising: Voltage reference terminal; a first input terminal providing a first signal and a second input terminal providing a second signal, the first and second signals forming a differential input voltage; a filter circuit having first, second and third filter inputs and having a filter output, the third filter input being coupled to the voltage reference terminal, the filter circuit being configured to provide a common mode noise cancellation signal at the filter output in response to the first and second filter inputs; a first capacitor coupled between the first input terminal and the first filter input; a second capacitor coupled between the second input terminal and the second filter input; a third capacitor coupled between the filter output and the second input terminal; a fourth capacitor coupled between the first input terminal and the second input terminal; as well as An inductor is coupled between the voltage reference terminal and a ground terminal.

17. The circuit of claim 16, wherein the filter output is a first filter output, the filter circuit includes a second filter output, and the circuit further comprises a fifth capacitor coupled between the second filter output and the first input terminal.

18. The circuit of claim 16, wherein the circuit further comprises a clamp circuit connected in parallel with the inductor, the clamp circuit configured to provide protection to the circuit against voltage surges.

19. The circuit of claim 18, wherein the clamp circuit comprises a transient voltage suppression (TVS) diode.

20. The circuit of claim 16, wherein the filter circuit comprises a capacitance multiplication circuit.

21. The circuit of claim 20, wherein the capacitance multiplication circuit comprises: an amplifier having an amplifier input and an amplifier output, the amplifier input being coupled to the voltage source terminal; and A fifth capacitor is coupled between the amplifier input and the amplifier output.

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