Active electromagnetic interference cancellation circuits

Methods and apparatus for active EMI cancellation in a switch mode power supply are provided herein. For example, an apparatus comprises an active EMI cancellation circuit coupled to a switch mode power supply circuit comprising an isolation transformer, wherein the active EMI cancellation circuit is positioned such that current flow through an EMI coupling capacitor substantially matches displacement current flow through a primary-to-secondary interwinding capacitance of the isolation transformer.

BACKGROUND

Field of the Disclosure

Embodiments of the present disclosure relate generally to in switch mode power supplies, and for example, to active electromagnetic interference (EMI) cancellation circuit for use in switch mode power supplies.

Description of the Related Art

A switch mode power supply (SMPS) is an electronic power supply that utilizes semiconductor switching techniques to provide a required output voltage. SMPSs provide greater efficiency than linear regulators, are generally small in size, and are widely used in computers and other sensitive electronic equipment.

In order for an SMPS to utilize smaller components, the SMPS must operate at higher switching frequencies. Increasing the SMPS switching frequency, however, results in increased transformer displacement currents, which are the fundamental mechanism for creating electromagnetic interference (EMI) in the SMPS. In addition, while the use of planar transformers in SMPSs offers advantages over the use of conventional transformers, such as low profile and high-power densities, they result in much greater transformer displacement currents.

Conventional techniques for mitigating transformer displacement currents include passive EMI cancellation of transformer displacement currents and active EMI cancellation of transformer displacement currents. With passive EMI cancellation, careful design analysis of an SMPS can identify a source of any transformer displacement currents and lead to a passive cancellation design. For example, the passive cancellation design aims to create a displacement current return path that is deliberately kept as close to the transformer with a view of minimizing the physical size of the loop that the displacement currents flow. Accordingly, an effectiveness of the passive solution, however, is limited.

Active EMI cancellation uses an amplifier as an active circuit. Rather than trying to engineer a solution around identifying the source of EMI in an SMPS, such a generic approach aims to directly measure the EMI being generated to create an equal and opposite signal to cancel the EMI. Accordingly, an effectiveness of the active (generic) solution, however, is also limited.

Therefore, the inventors have provided herein improved methods and apparatus using active EMI cancellation circuitry specifically targeted at a source of the EMI being generated.

SUMMARY

In accordance with at least aspects of the disclosure, an apparatus for active EMI cancellation in a switch mode power supply comprises an active EMI cancellation circuit coupled to a switch mode power supply circuit comprising an isolation transformer, wherein the active EMI cancellation circuit is positioned such that current flow through an EMI coupling capacitor substantially matches displacement current flow through a primary-to-secondary interwinding capacitance of the isolation transformer.

In accordance with at least aspects of the disclosure, a single-phase switched mode power supply comprises a DC side comprising a first plurality of switches and a DC component, an AC side comprising a second plurality of switches and connected to an AC line, an active EMI cancellation circuit coupled to a switched mode power supply circuit comprising an isolation transformer, wherein the active EMI cancellation circuit is positioned such that current flow through an EMI coupling capacitor substantially matches displacement current flow through a primary-to-secondary interwinding capacitance of the isolation transformer, and a controller coupled to the active EMI cancellation for controlling operation of the first plurality of switches and the second plurality of switches to generate a differential mode voltage across the isolation transformer such that a voltage applied across ends of a primary winding of the isolation transformer generates a corresponding voltage across ends of a secondary winding of the isolation transformer to achieve power conversion from the DC component to the AC line or vice versa.

In accordance with at least aspects of the disclosure, a method of controlling a single-phase switched mode power supply comprises operating the single-phase switched mode power supply and controlling a first plurality of switches and a second plurality of switches of an active EMI cancellation circuit positioned such that current flow through an EMI coupling capacitor substantially matches displacement current flow through a primary-to-secondary interwinding capacitance of an isolation transformer of a switched mode power supply circuit to generate a differential mode voltage across the isolation transformer such that a voltage applied across ends of a primary winding of the isolation transformer generates a corresponding voltage across ends of a secondary winding of the isolation transformer to achieve power conversion from a DC component of the single-phase switched mode power supply to an AC line of the single-phase switched mode power supply or vice versa.

DETAILED DESCRIPTION

Embodiments of the present disclosure generally relate to an active electromagnetic interference (EMI) cancellation circuit for switched mode power supplies (SMPS). In some embodiments, such as for use in a single-phase SMPS as described below, the active EMI cancellation circuit comprises two small autotransformers (a primary autotransformer and a secondary autotransformer) and two small capacitors (a blocking capacitor and a coupling capacitor). In one or more other embodiments, a single 3-limb transformer can be used in place of the two autotransformers. The number of turns for each autotransformer is determined based on peak flux density considerations and requirements to provide the correct magnitude of the transformer displacement current cancellation current that will flow through the EMI coupling capacitor. For example, the EMI coupling capacitor can be (nominally) chosen to equal the primary-to-secondary interwinding capacitance that is present in the isolation transformer of the SMPS. The value of the blocking capacitor is chosen so that the value of the blocking capacitor is much larger than avalue of the EMI coupling capacitor.

The two individual autotransformers (or single 3-limb transformer) in the active EMI cancellation circuit can be extremely small transformers as the actual power the two individual autotransformers need to process can be minute compared to the power processed through the main isolation transformer of the SMPS. The autotransformers can be located in close proximity to the main isolation transformer and may be constructed using a conventional ‘magnet wire’ winding design or, alternatively, they may be implemented as small planar transformer designs.

Although the active EMI cancellation circuit is described below with respect to use in a single-phase SMPS, the configuration of the active EMI cancellation circuit may be engineered to work with other SMPS topologies. For example, the active EMI cancellation circuit may be engineered by analyzing a source of any transformer displacement currents and deriving an appropriate cancellation circuit to mitigate the EMI resulting from these displacement currents. To apply this concept to an SMPS, the specific details of the primary and secondary switch modulation schemes can be considered along with the isolation transformer construction design. One skilled in the art should be able to derive the specific design details required to be able to apply this design concept to any types of SMPS.

FIG.1is a block diagram of a single-phase SMPS100(switched mode power supply) in accordance with embodiments of the present disclosure. The SMPS100is a single-phase bidirectional DC-AC resonant converter comprising a DC side102and an AC side104coupled via an isolation transformer124. The SMPS100further comprises a controller140for operably controlling power conversion by the SMPS100. One or more additional components not shown may be coupled to the SMPS100for enabling the power conversion, such as voltage and/or current monitors which measure voltage and/or current at various points and coupled the measured data to the controller140.

The DC side102comprises a capacitor122coupled across both an input bridge118and a primary winding P of the isolation transformer124. The input bridge118comprises multiple switches106-1,106-2,106-3, and106-4(e.g., MOSFETS) configured as a full H-bridge, although in other embodiments the input bridge118may be a half bridge. The input bridge118is coupled across a DC component120, such as one or more renewable energy sources (e.g., photovoltaic (PV) modules, wind farms, hydroelectric systems, or the like), batteries, fuel cells, or any suitable DC component which can provide and/or receive DC power.

The AC side104comprises the secondary winding S of the isolation transformer124coupled in series with an inductor Lr and a capacitor Cr. The series combination is coupled across a cycloconverter160that is further coupled to a single-phase AC line. The cycloconverter160comprises two four-quadrant (4Q) fully bidirectional switches128-1and128-2(collectively referred to as 4Q switches128). The 4Q switches128-1and128-2are coupled to capacitors108-1and108-2, respectively, to form respective first and second legs A and B. The first and second legs A and B are coupled in parallel, with a first AC line terminal coupled between the 4Q switch128-1and the capacitor108-1, and a second AC line terminal coupled between the 4Q switch128-2and the capacitor108-2. As depicted inFIG.1, the cycloconverter160is a half-bridge cycloconverter; in some other embodiments, a full-bridge cycloconverter may be alternatively used.

The 4Q switches128-1and128-2are each fully-controlled native 4Q bidirectional switches, for example gallium nitride (GaN) high mobility electron transistor (HEMT) switches built as native 4Q switch devices. Examples of the such native 4Q switches may be found in commonly assigned U.S. Patent Application No. 63/214,260, titled “Gallium Nitride Bi-directional High Electron Mobility Transistor in Switched Mode Power Converter Applications”, which is herein incorporated by reference in its entirety. In some other embodiments, the 4Q switches may be any other suitable structure that acts as a 4Q switches, such as two source-connected metal—oxide—semiconductor field-effect transistor (MOSFET) switches.

The AC side104comprises an individual resonant inductor Lr coupled in series with a single resonant capacitor Cr, and this series combination is further coupled in series with the end of the isolation transformer secondary winding S that is coupled to the 4Q switches128. Additionally, a ‘virtual neutral’ point is created by splitting the ‘X-capacitor’ in the EMC filter into two capacitors (Cx)—this ‘virtual neutral’ point is connected to the other end of the secondary winding S of the isolation transformer124.

An active EMI cancellation circuit126is coupled across the primary winding P, between the switches106-2and106-4, and between the secondary winding S and the capacitor108-1.

The controller140is communicatively coupled to the input bridge switches106-1,106-2,106-3, and106-4, collectively referred to as switches106, and the 4Q switches128for operatively controlling the switches to generate the desired output power.

The controller140comprises a CPU184coupled to each of support circuits183and a memory186. The CPU184may comprise one or more conventionally available microprocessors or microcontrollers; additionally or alternatively, the CPU184may include one or more application specific integrated circuits (ASICs). The support circuits183are well known circuits used to promote functionality of the CPU184. Such circuits include, but are not limited to, a cache, power supplies, clock circuits, buses, input/output (I/O) circuits, and the like. The controller140may be implemented using a general purpose computer that, when executing particular software, becomes a specific purpose computer for performing various embodiments of the present disclosure.

The memory186is one or more non-transitory storage media comprising read only memory, random access memory, or a combination thereof for storing software and data. In one embodiment, the software comprises an OS187(operating system), if necessary, of the controller140that can be supported by the CPU capabilities. In some embodiments, the OS187may be one of a number of commercially available operating systems such as, but not limited to, LINUX, Real-Time Operating System (RTOS), and the like.

The memory186may store various forms of application software, such as a conversion control module189for controlling power conversion by the SMPS100, for example maximum power point tracking (MPPT), switching, and the like. The memory186may further store a database199for storing various data. The controller140further processes inputs and outputs to external communications194(e.g., gateway) and a grid interface188.

During operation of the SMPS100, each of the switches106and128are activated and deactivated by the controller140, according to the conversion control module189, to achieve power conversion from the DC component120to the AC line (and/or vice versa). For the purpose of power conversion, the switches106and128are controlled to generate a differential mode voltage across the isolation transformer124—e.g., a voltage applied across the ends of primary winding P will generate a corresponding voltage to be generated across the ends of the secondary winding S.

A consequence of this switching action is that, in addition to the desired differential mode voltage, a common mode voltage is generated across the primary P to secondary S of the isolation transformer124. This common mode voltage serves no useful purpose in regard to power conversion and is directly responsible for generating undesirable transformer displacement currents which will flow from the primary P to the secondary S via the primary-to-secondary interwinding capacitance that results from the physical proximity of the primary winding P and the secondary winding S. For the specifics of the isolation transformer design in various embodiments, this primary-to-secondary interwinding capacitance can be modeled by one skilled in the art as a single equivalent lumped capacitor Cy connected from a specific point on the primary winding P to a specific point on the secondary winding S.

In one or more embodiments where the isolation transformer124has a planar transformer structure, the entire surface area of the primary winding P is physically located such that it covers the entire surface area of the secondary winding S; this configuration results in the interwinding capacitance being evenly distributed over the entire area of the primary and secondary windings P and S, resulting in an equivalent lumped capacitance Cy connected from the mid-point of the primary winding P to the mid-point of the secondary winding S, as depicted by the horizontal line cross the primary winding P and the secondary winding S inFIG.1. In some embodiments, other winding configurations for the isolation transformer124may differ from this design—for example, a multi-layered design might result in only part of the primary winding area covering part of the secondary winding area.

The arrangement of the power components Lr, Cr and Cx is required to ensure that the common mode voltage generated across the primary P to secondary S windings of the isolation transformer124is reduced to a simple waveform that can be fed into the active EMI cancellation circuit126. In other embodiments where an SMPS has a topology different from the SMPS100, one skilled in the art would be able to design the required circuit rearrangement needed to make the SMPS compatible with the active EMI cancellation circuit126.

FIG.2is a schematic diagram of the active EMI cancellation circuit126in accordance with embodiments of the present disclosure. The active EMI cancellation circuit126comprises two autotransformers (e.g., a primary autotransformer201and a secondary autotransformer202) and two small capacitors (e.g., a blocking capacitor203and an EMI coupling capacitor204).

The number of turns for each of the primary autotransformer201and the secondary autotransformer202is selected based on peak flux density considerations and the requirements to provide the correct magnitude of the transformer displacement current cancellation current that will flow through the EMI coupling capacitor204. The value of the EMI coupling capacitor204is (nominally) chosen to equal the primary-to-secondary interwinding capacitance Cy that is present in the isolation transformer124. The value of the blocking capacitor203is selected so that it is much larger than the value of the EMI coupling capacitor204.

In other embodiments, other configurations of the active EMI cancellation circuit126may be derived to work with other types of SMPS converters. For example, different numbers of the blocking capacitor203and/or the EMI coupling capacitor204may be used, and/or different transformer configurations may be used (e.g., a single 3-limb transformer may be used in place of the two autotransformers).

FIGS.3A-3Hdepict a series of waveforms300produced during operation of the SMPS100in accordance with embodiments of the present disclosure. The series of waveforms300relate to the operation of the active EMI cancellation circuit126in one or more embodiments.

The series of waveforms300comprises waveforms304-1,304-2,304-3,304-4,304-5,304-6,304-7, and304-8, which may be collectively referred to as “waveforms304”. For each of the waveforms304, a depiction of the SMPS100is shown with a corresponding indicator (i.e., arrows302-1,302-2,302-3,302-4,302-5,302-6,302-7, and302-8) identifying the location within the SMPS100across which the corresponding waveforms is present.

During operation, the switching modulation scheme (implemented by the controller140) repeatedly cycles through the four states in the order shown (continually). The waveform304-1depicts the resulting primary waveform that is applied to the isolation transformer124, shown by the arrow302-1, and is also applied to the primary autotransformer201of the active EMI cancellation circuit126(FIG.3A).

The primary autotransformer201inverts the voltage applied to the primary winding P of the isolation transformer124and scales the waveform according to the primary-to-secondary turns ratio of the isolation transformer124(main transformer), resulting in the waveform304-2at the location shown by the arrow302-2(FIG.3B).

The phase shift modulation scheme results in each end of the isolation transformer winding being connected to the negative end of the DC input for 50% of the time, and connected to the positive end of the DC input the remaining 50% of the time, as shown by the waveform304-3at the location shown by the arrow302-3(FIG.3C).

Adding the waveforms304-2and304-3results in the waveform304-4that exists between the negative end of the DC input and the input of the secondary autotransformer, shown by the arrow302-4(FIG.3D).

The phase shift modulation scheme results in each end of the isolation transformer winding being connected to the negative end of the DC input for 50% of the time and connected to the positive end of the DC input the remaining 50% of the time. This in turn will result in a DC voltage being established across the blocking capacitor203which will be equal in magnitude to half of the DC input voltage—e.g., Vdc/2. Taking into consideration this voltage generated across the blocking capacitor203, the voltage applied to the secondary autotransformer202, depicted by the arrow302-5, can be derived and is shown as the waveform304-5(FIG.3E).

The secondary autotransformer202inverts this signal to result in the waveform304-6and the location shown by the arrow302-6(FIG.3F).

Based on all the analysis described above with respect toFIGS.3A-3F, the common mode voltage being generated across the primary-to-secondary of the isolation transformer124(the waveform304-7(FIG.3G) at the location shown by the arrow302-7) is equal and opposite to the voltage being generated across the EMI coupling capacitor204(the waveform304-8(FIG.3H) at the location shown by the arrow302-8). As a result of the EMI coupling capacitor204being selected to have a value (nominally) equal to the primary-to-secondary interwinding capacitance Cy of the isolation transformer124, the current that flows through the EMI coupling capacitor204will substantially match the displacement current that flows through the primary-to-secondary interwinding capacitance Cy of the isolation transformer124. For example, in at least some embodiments, the current that flows through the EMI coupling capacitor204can be within +/−10% of the displacement current that flows through the primary-to-secondary interwinding capacitance Cy of the isolation transformer124. Thus, in at least some embodiments, matching the currents can completely cancel out the transformer displacement current and, thereby, eliminate the need for this displacement current to be filtered out by an EMC filter.

FIG.4is a schematic diagram illustrating design considerations for the active EMI cancellation circuit126in accordance with embodiments of the present disclosure. The embodiment of the active EMI cancellation circuit126shown inFIG.4is the same as the embodiment shown inFIG.2.

As previously described, the primary autotransformer winding configuration of the active EMI cancellation circuit126requires the number of turns being chosen to match the primary-to-secondary turns ratio for the isolation transformer124. In addition, the number of turns on the autotransformers must be chosen to achieve a desired maximum flux density—the number of turns r for the primary autotransformer201and number of turns q for the secondary autotransformer202are illustrated in the schematic ofFIG.4. It is desirable to incorporate the design freedom that will allow an EMI coupling capacitor value to be chosen that is different to the primary-to-secondary interwinding capacitance Cy for the isolation transformer124(i.e., the nominal value). The scale factor m shown inFIG.4allows for the scaling of the EMI coupling capacitor204. Combining all these design considerations together leads to the culmination of the indicated design details in the circuit schematic shown inFIG.4, where: Cy=EMI Coupling Capacitor, m=EMI Coupling Capacitor scaling factor, p=Number of primary winding turns on the isolation transformer, q=Secondary autotransformer turns required to achieve desired transformer core peak flux density, r=Primary autotransformer turns required to achieve desired transformer core peak flux density, and s=Number of secondary winding turns on the isolation transformer.

FIG.5is a schematic diagram of an active EMI cancellation circuit126in accordance with one or more other embodiments of the present disclosure. In the embodiment shown inFIG.5, the autotransformers have been replaced with a 3-limb transformer502. The transformer design parameters described above (m, p, q, r, & s) must be used to derive the number of windings for the three individual limbs on the 3-limb transformer502. One skilled in the art would be able to determine this design detail.

FIG.6shows a drawing of the 3-limb transformer502in the active EMI cancellation circuit126ofFIG.5in accordance with embodiments of the present disclosure. The 3-limb transformer502comprises a core610having limbs602,604, and606. Windings612,614, and616are wound on the limbs602,604, and606, respectively, and connected as shown inFIG.6. Using the transformer design parameters described above (m, p, q, r, and s), one skilled in the art would be able to determine the design detail of the number of windings for the limbs602,604, and606.

FIG.7is a flowchart of a method700of controlling a single-phase switched mode power supply in accordance with embodiments of the present disclosure.

For example, at702, the method700comprises operating the single-phase switched mode power supply. For example, under the control of the controller140, the single-phase switched mode power supply can be operated.

Next, at704, the method700comprises controlling a first plurality of switches and a second plurality of switches of an active EMI cancellation circuit positioned such that current flow through an EMI coupling capacitor substantially matches displacement current flow through a primary-to-secondary interwinding capacitance of an isolation transformer of a switched mode power supply circuit to generate a differential mode voltage across the isolation transformer such that a voltage applied across ends of a primary winding of the isolation transformer generates a corresponding voltage across ends of a secondary winding of the isolation transformer to achieve power conversion from a DC component of the single-phase switched mode power supply to an AC line of the single-phase switched mode power supply or vice versa. For example, as noted above with respect toFIGS.3A-3H, the controller140can control one or more of the switches106(or switches128) so that the EMI coupling capacitor204can be selected to have a value (nominally) equal to the primary-to-secondary interwinding capacitance Cy of the isolation transformer124, such that the current that flows through the EMI coupling capacitor204will exactly match the displacement current that flows through the primary-to-secondary interwinding capacitance Cy of the isolation transformer124. The matching of the currents will completely cancel out the transformer displacement current and, thereby, eliminate the need for this displacement current to be filtered out by an EMC filter.

The multiple examples described herein have been given to illustrate various features and are not intended to be so limiting. Any one or more of the features may not be limited to the particular examples presented herein, regardless of any order, combination, or connections described. In fact, it should be understood that any combination of the features and/or elements described by way of example above are contemplated, including any variation or modification which is not enumerated, but capable of achieving the same. Unless otherwise stated, any one or more of the features may be combined in any order.

As above, figures are presented herein for illustrative purposes and are not meant to impose any structural limitations, unless otherwise specified. Various modifications to any of the structures shown in the figures are contemplated to be within the scope of the disclosure presented herein. The disclosure is not intended to be limited to any scope of claim language.

Where “coupling” or “connection” is used, unless otherwise specified, no limitation is implied that the coupling or connection be restricted to a physical coupling or connection and, instead, should be read to include communicative couplings, including wireless transmissions and protocols.

Any block, step, module, or otherwise described herein may represent one or more instructions which can be stored on a non-transitory computer readable media as software and/or performed by hardware. Any such block, module, step, or otherwise can be performed by various software and/or hardware combinations in a manner which may be automated, including the use of specialized hardware designed to achieve such a purpose. As above, any number of blocks, steps, or modules may be performed in any order or not at all, including substantially simultaneously, i.e., within tolerances of the systems executing the block, step, or module.

Where conditional language is used, including, but not limited to, “can,” “could,” “may” or “might,” it should be understood that the associated features or elements are not required. As such, where conditional language is used, the elements and/or features should be understood as being optionally present in at least some examples, and not necessarily conditioned upon anything, unless otherwise specified.

Where lists are enumerated in the alternative or conjunctive (e.g., one or more of A, B, and/or C), unless stated otherwise, it is understood to include one or more of each element, including any one or more combinations of any number of the enumerated elements (e.g. A, AB, AC, ABC, ABB, etc.). When “and/or” is used, it should be understood that the elements may be joined in the alternative or conjunctive.

The foregoing description of embodiments of the disclosure comprises a number of elements, devices, circuits and/or assemblies that perform various functions as described. These elements, devices, circuits, and/or assemblies are exemplary implementations of means for performing their respectively described functions.