Ferrite cage RF isolator for power circuitry

RF isolation for power circuitry includes one or more ferrite cages surrounding a pair of coils, one coil connected to power input, and the other coil connected to a load such as a heater. The ferrite cage provides universal isolation for the coils, avoiding the necessity of specially tuned filters or more complicated coil arrangements. A pair of dielectric discs support respective coils. In one aspect, the ferrite cage is constituted by ferrite pieces which fan out from a central portion of the dielectric discs and are connected at an outer periphery of the dielectric discs, and at the central portion of the dielectric discs. In one aspect, the fanned-out ferrite pieces comprises either manganese-zinc or magnesium-zinc ferrites, and the ferrite pieces connecting the fanned-out ferrite pieces comprise nickel-zinc ferrites.

FIELD

The present disclosure relates to substrate processing systems, more particularly to electrostatic chucks for substrate processing systems, and yet more particularly to heating systems for electrostatic chucks in substrate processing systems. Still more particularly, the present disclosure relates to RF isolation for power circuitry used in heaters for electrostatic chucks in substrate processing systems.

BACKGROUND

Substrate processing systems may be used to perform etching and/or other treatment of substrates such as semiconductor wafers. A substrate may be arranged on an electrostatic chuck (ESC), or on a pedestal attached to an ESC, in a processing chamber of the substrate processing system. The ESC may be biased with an RF signal, using RF voltages in the range from tens to thousands of volts and RF frequencies in the range from tens of kHz to hundreds of MHz. Since the ESC also acts as a workpiece holder, proper control of the ESC temperature is an important consideration to ensure repeatable process results.

One or more electric heaters may maintain the ESC's temperature within a desired range. The heaters may be integrated or coupled with the ESC. Electrical power to the heater(s) typically is obtained from line AC voltage via an appropriate control circuit to maintain the ESC within a desired temperature range. By way of example, the electric heater(s) may be powered by DC, line frequency (e.g., 50/60 Hz AC) or kHz range AC power. The heaters may be operated at the same time, or at different times, depending on process requirements, chamber conditions, and the like, to maintain a temperature profile of the process. Maintaining the temperature profile facilitates better uniformity and etch rates in substrate processing.

In this configuration, the DC/low frequency power needs to be coupled to the ESC assembly, which is also simultaneously subject to substantial levels of RF power either by stray coupling or by direct connection. To prevent an undesirable apparent RF short to ground, loss of RF power and high levels of signal interference, even damage via the electric heater power supply and/or control circuitry, RF isolation is required.

Separate filters have been designed to block the RF current path from the heaters to their power source on different tools depending on RF-rejection frequency requirements. Designing RF filters has been tedious because, among other things, it has been necessary to place the parasitic resonances carefully between the harmonic frequencies to avoid unintentional RF-current. In addition, RF-rejection requirements have changed repeatedly as etch-rate and uniformity requirements have changed. Etch rates and uniformity requirements continue to change, so RF-rejection requirements may be expected to undergo continued change as well.

As a result, in lieu of specific filters for specific requirements, a universal solution for RF isolation has been sought. One such solution has been to replace all the RF-filters by designing a broadband and high power RF isolator which filters frequencies by providing a capacitive rejection response.

U.S. Pat. No. 8,755,204 discloses an approach to providing RF isolation. The patent proposes to reduce secondary-to-core capacitive coupling by, for example, providing a shield between a secondary winding and a core of an isolation transformer. The patent also proposes to reduce primary-to-core capacitive coupling by, for example, providing a shield between a primary winding and the core.

FIG. 1shows relevant portions of an isolation transformer implementation, according to the above-referenced patent, to provide high DC or AC line power to a load that is also coupled to one or more high frequency RF signals. InFIG. 1, the load is a heater for an RF coupled chuck in a plasma processing chamber. A source power signal in the form of AC line voltages and frequencies (e.g., 50 Hz or 60 Hz) is supplied via leads202and204to a rectifier/filter circuit206which converts the AC line input power signal to a quasi-DC power signal which may be subsequently filtered into smooth DC if desired.

The DC power signal output by rectifier circuit206is then supplied to a drive circuit208, which converts the DC power signal received on leads210and212to an intermediate signal having an intermediate frequency, for example, in the range of about 10 kHz to about 1 MHz, or in the range of about 10 kHz to about a few hundred kHz, and or in the range of about 10 kHz to about 200 kHz. As a result, the intermediate frequency is intentionally higher than the AC line frequency of 50-60 Hz but lower than the RF frequency to be blocked (which tends to be in the multiple MHz range). Because the intermediate frequency is higher than the AC line frequency, it is possible to use a smaller isolation transformer220.

The intermediate signal output by drive circuit208is then supplied to the primary winding222of isolation transformer220. Primary winding222is shown wound around one segment of a core224. Core224may be formed of manganese zinc (MnZn) or nickel zinc (NiZn) or another suitable high magnetic permeability material (e.g., mu in the 2000 range). An air gap230may be provided in core224, in which case primary winding222may be wound to the sides of the air gap230instead of over air gap230, in order to reduce dissipation in the winding.

Secondary winding236, which is not directly coupled to primary winding222by conduction, is also wound around core224. To reduce capacitive coupling between primary winding222and secondary winding236and provide a high degree of isolation, particularly for the higher frequency RF signals, secondary winding236may be positioned apart from primary winding222. For example, secondary winding236may be positioned opposite primary winding222around core224, asFIG. 1shows.

In the approach just described, the core is machined in a U-shape and requires the following elements to decrease capacitive coupling of the secondary to core, and of the primary to the secondary: i) larger diameter secondary winding, possibly wound on a plastic cylinder—the inner diameter of which will be stuffed with ferrite; ii) primary and secondary windings placed physically apart from each other but still magnetically coupled through the same core.

There are some challenges with respect to this approach. First, the cost to machine a U-shaped ferrite block, large enough to separate the primary winding from the secondary winding, and plastic cylinders (with precise grooves on the outer surfaces and holes for cooling the ferrite inside) can be difficult from a system stand-point. Second, when the ferrite is stuffed into the plastic cylinder on which the secondary winding is wound, it can be challenging to design an efficient cooling mechanism.

It would be desirable to provide RF isolation that is more comprehensive, and that does not require specially tuned circuitry, or intricate and/or expensive assembly and/or manufacture.

SUMMARY

RF isolation for power circuitry in substrate processing systems includes a ferrite cage around coils which supply power. Proper construction of the ferrite cage can eliminate stray capacitances and provide the required RF isolation at a wide range of frequencies. The ferrite cage provides universal isolation for the coils, avoiding the necessity of specially tuned filters or more complicated coil arrangements. A pair of dielectric discs support respective coils. In one aspect, the ferrite cage is constituted by ferrite pieces which fan out from a central portion of the dielectric discs and are connected at an outer periphery of the dielectric discs, and at the central portion of the dielectric discs. In one aspect, the fanned-out ferrite pieces comprises either manganese-zinc or magnesium-zinc ferrites, and the ferrite pieces connecting the fanned-out ferrite pieces comprise nickel-zinc ferrites.

DETAILED DESCRIPTION

FIG. 2shows an RF isolation apparatus according to an aspect of the present disclosure. In one aspect, the apparatus is a ferrite cage100. InFIG. 2, ferrite cage100includes upper and lower dielectric discs110,120, which have respective coils170,180disposed therein, as will be discussed in more detail below. In the following discussion, coil170is a primary winding that draws power from a power source, and coil180is a secondary winding that delivers energy to a load. In one aspect, the load is constituted by heaters for an ESC. For ease of description, coil170will be referred to as a primary and coil180will be referred to as a secondary.

The ferrite cage100also includes ferrite pieces130,140,150, and160. In order to form the ferrite cage100, the ferrite pieces130,140,150, and160are connected to each other, as will be explained, in order to complete a magnetic path. In one aspect, the ferrite pieces130,140, and150are glued to the outer surfaces of upper and lower dielectric discs110,120.FIG. 2also shows ferrite pieces130,140contacting each other, with the ferrite pieces130,140alternating around the circumference of disc-shaped ferrite piece150, and the ferrite pieces130contacting disc-shaped ferrite piece150. In one aspect, ferrite pieces130,140may contact disc-shaped ferrite piece150, but not each other. In another aspect, ferrite pieces130,140may contact each other, but only ferrite pieces130or140contact disc-shaped ferrite piece150.FIG. 2shows ferrite pieces130, but not ferrite pieces140, contacting disc-shaped ferrite piece150.

InFIG. 2, ferrite pieces130,140are disposed on an outer surface of upper and lower dielectric discs110,120. As mentioned previously, primary coil170and secondary coil180are disposed within respective dielectric discs110,120. In one aspect, primary coil170and secondary coil180may be attached on an opposite side respective dielectric discs110,120from ferrite pieces130,140, and150, so that the primary coil170and secondary coil180are inside the ferrite cage100. Another such disc-shaped ferrite piece150is disposed in the center of the outer surface of lower disc120. The ferrite pieces130extend from ferrite piece150toward outer edges of dielectric discs110,120. In one aspect, the ferrite pieces130,140extend all the way to the outer edges of dielectric discs110,120, to facilitate contact with ferrite pieces160and complete the magnetic path. How the ferrite pieces130,140contact ferrite pieces160will be described further below. However, the ferrite pieces130,140do not need to extend all the way to the outer edges of upper and lower dielectric discs110,120in order to contact ferrite pieces160and complete the magnetic path. What it is necessary is that the primary coil170and secondary coil180be inside the ferrite cage100.

In one aspect, the arrangement resembles a flower, with bar-shaped ferrite pieces130extending radially from disc-shaped ferrite piece150. Another way of describing the arrangement of these pieces is a hub-and-spoke configuration, with disc-shaped ferrite piece150being the hub and ferrite pieces130being the spokes.

Depending on operational requirements, the disc-shaped ferrite piece150may have a radius as shown inFIG. 2, relative to a radius of the dielectric discs110,120and/or of the overall ferrite cage100. However,FIG. 2is not necessarily to scale, and disc-shaped ferrite piece150may have a different radius, again depending on operational requirements. As noted earlier,FIG. 2shows inwardly extending ferrite pieces130, but not ferrite pieces140, contacting disc-shaped ferrite piece150, to complete the magnetic path. In one aspect, the same arrangement is provided on an outer surface of upper disc110as on an outer surface of lower disc120.

FIG. 2shows ferrite pieces140disposed between adjacent ferrite pieces130, filling some of the gaps between the adjacent ferrite pieces130. Depending on widths of the ferrite pieces130, ferrite pieces140may be wider or narrower than ferrite pieces130, or equal in length to or shorter than ferrite pieces130.FIG. 2shows the ferrite pieces140being shorter than ferrite pieces130, and also shows the ferrite pieces130,140as rectangular or bar-shaped, with similar widths. Machining the ferrite pieces130,140can be easier if the shapes are similar to each other, particularly if the ferrite pieces130,140are rectangular or bar-shaped. In one aspect, the ferrite pieces130have shapes that differ from those of ferrite pieces140.

In one aspect, the ferrite pieces130may have a shape which is narrower toward a center of dielectric discs110,120, and wider toward an outer diameter of dielectric discs110,120. In the resulting arrangement, ferrite pieces130may have arcuate-shaped segments, or pie-shaped segments, or segments with a shape not dissimilar to flower petals. In one aspect, with the ferrite pieces130having such non-rectangular shapes, ferrite pieces140may not be required, as the ferrite pieces130themselves may cover a sufficient amount of area without needing the ferrite pieces140to “fill in” gaps. In another aspect, in some applications, even if ferrite pieces130have the rectangular or bar shapes described above, ferrite pieces140may not be necessary in order to complete the ferrite cage100appropriately. The number and size of gaps in the ferrite cage100will depend on the frequency range being addressed, and on the stray capacitances that can result.

The ferrite pieces130,140on upper and lower dielectric discs110,120are connected by further ferrite pieces160. One ferrite piece160connects a ferrite piece130or140on disc110to a respective ferrite piece130or140on disc120. In one aspect, one or more ferrite pieces130on one of the dielectric discs110,120may be connected to ferrite pieces140on the other one of the dielectric discs. That is, it is not necessary that the same respective pieces on the upper and lower dielectric discs110,120be connected to each other. The main point is to complete the magnetic path appropriately so that the ferrite cage100functions as intended, to eliminate stray capacitances and provide the desired RF isolation over a range of frequencies.

In order to complete the magnetic path, asFIG. 2illustrates, in one aspect the ferrite pieces160extend through upper and lower dielectric discs110,120so as to contact ferrite pieces130,140on both of the upper and lower dielectric discs110,120. In this aspect, ferrite pieces130,140need not extend all the way to a circumference of upper and lower dielectric discs110,120. It is desirable that the pieces160to be sufficiently inside the outer edge of the dielectric discs110,120to provide appropriate structural integrity for the dielectric discs, and for the overall ferrite cage100.

In one aspect, the ferrite pieces160may be glued around a circumference of upper and lower dielectric discs110,120in order to complete the magnetic path. In this aspect, in order to complete the magnetic path, ferrite pieces130,140extend all the way to a circumference of upper and lower dielectric discs110,120.

One advantage of using ferrite pieces instead of a solid piece of ferrite is ease and cost of fabrication. Assembling ferrite pieces130,140,150, and160into a ferrite cage100such as the one shown inFIG. 2generally is an easier process than machining blocks of ferrite into a suitable shape. Another advantage is that capacitance is reduced.

In one aspect, a diameter of the upper and lower dielectric discs110,120will have a relationship to a diameter of primary coil170and secondary coil180. The primary coil170and secondary coil180will fit within the ferrite cage100, and will have a size that is a function of power transfer requirements. In one aspect, upper and lower dielectric discs110,120have a diameter of approximately 170-200 mm.

FIG. 3shows more interior detail of the arrangement of ferrite pieces130and primary coil170, with upper disc110being shown in slightly transparent form so as to make primary coil170more visible. In one aspect, as noted earlier, primary coil170is disposed inside upper disc110. InFIG. 3, ferrite pieces130are bar-shaped, and extend radially from a central portion of upper disc110. In one aspect, edges or corners of the ferrite pieces130contact each other toward a center of upper disc110, as part of the completion of the magnetic path. The disc-shaped ferrite piece150inFIG. 2is not depicted here, but in one aspect, that ferrite piece150would be arranged similarly to the arrangement inFIG. 2, in order to complete the magnetic path. That disc-shaped ferrite piece150would contact not just the ferrite pieces130, but also the ferrite pieces140, in order to complete the magnetic path.

FIG. 4shows more interior detail of the arrangement of ferrite pieces130,160and secondary coil180, with lower disc120being shown in slightly transparent form so as to make secondary coil180and ferrite pieces130more visible. In one aspect, as noted earlier, secondary coil180is disposed inside lower disc110. InFIG. 4, ferrite pieces130are bar-shaped, and extend radially from a central portion of lower disc120. Ferrite pieces160extend upwardly from ferrite pieces130, not only at an outer radius of lower disc120, but also upwardly from ferrite pieces130toward a center of the lower disc120.

One aspect that appears differently inFIG. 4fromFIG. 2is that inFIG. 4, for ease of illustration, there are only as many vertical ferrite pieces160shown as there are ferrite pieces130. InFIG. 2, there are as many vertical ferrite pieces160at or around the circumference of dielectric discs110,120as there are ferrite pieces130,140, so as to complete the magnetic path.

In one aspect, upper and lower dielectric discs110,120are formed of a dielectric material such as a polyetherimide (PEI) resin. Examples of such resins, particularly amorphous thermoplastic PEI resins, are found in the ULTEM™ family of resins. Other suitable materials can include polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE), and fiberglass, including G7 and G10 versions of fiberglass.

In one aspect, the upper and lower dielectric discs110,120are solid. In another aspect, they are solid except for slots formed for the ferrite pieces160to extend therethrough. In one aspect, one or both of the dielectric discs110,120have air holes to provide laminar air flow without affecting the RF isolation properties of the ferrite cage100.

In one aspect, the ferrite pieces160are nickel-zinc (NiZn) ferrites. In one aspect, the ferrite pieces130,140on the upper surface of disc110and the lower surface of disc120are manganese-zinc (MnZn) ferrites or magnesium-zinc (MgZn) ferrites. In one aspect, the central circular portion150on the upper surface of disc110and the lower surface of disc120comprises a NiZn ferrite, though it also may comprise a MgZn ferrite. Other ferrite materials may be used, depending on such considerations as frequency. Examples of such ferrites include nickel-magnesium (NiMg) ferrites. Generally, permeability of ferrite materials is not linear with frequency. Some ferrite materials have such high permeability as to appear almost like conductors at certain frequencies such as power frequencies.

In one aspect, NiZn may be more appropriate for certain portions of the ferrite cage from the standpoint of frequency dependent permeability, permittivity, and loss characteristics. In this aspect, NiZn ferrites have relatively low permittivity, yielding better isolation between the AC and RF ports. Higher magnetic permeability material can be more advantageous in containing magnetic fields. However, that material also may have a higher dielectric constant, thereby decreasing the isolation between the AC and RF ports. MnZn would be an example of such material. In one aspect, some of the material may be NiZn and some may be MnZn. For example, ferrite pieces130,140(perhaps also ferrite pieces150—overall, the horizontal pieces) may be MnZn ferrites, and ferrite pieces160(the vertical pieces) may be NiZn ferrites. In one aspect, MgZn ferrites in addition to or as an alternative to MnZn ferrites, along with NiZn ferrites, may be advantageous in increasing magnetic coupling between the primary coil170and the secondary coil180without degrading their capacitive isolation significantly.

In one aspect, at higher frequencies, such as frequencies in the range of tens or hundreds of MHz, bars made of ferrites or metal dust are preferred. At frequencies for power transmission, in one aspect 100 kHz to 1 MHz, ferrites such as NiMg or MgZn ferrites, or NiZn ferrites are used. In addition, while ferrites have been the focus of the description and discussion herein, ferrous or ferromagnetic materials, such as powdered irons and steel bars, are classes of materials that may be considered for the ferrite cage100.

Referring again toFIG. 2, primary coil170and secondary coil180are disposed within the ferrite cage100, and in particular, within respective upper and lower dielectric discs110,120. In one aspect, the primary coil170and secondary coil180are planar or pancake coils. In various aspects, the pancake coil configuration could have more than one layer, or could use a printed circuit board (PCB) on either the transmitting end or the receiving end. The primary coil170and secondary coil180are appropriately spaced from each other to permit suitable coupling for power generation (in one aspect, 10-15 kW). Monitoring the amount of current on the primary side will enable monitoring of power going out on the secondary side.

This planar or pancake coil configuration does not rely on magnetic material for flux coupling. This approach also avoids saturation because there is no magnetic material to saturate. The ferrite cage100helps to focus the flux between the primary coil170and secondary coil180.

Notwithstanding the foregoing, the flux coupling achieved may not be optimal. In order to compensate appropriately, either or both of the primary and the secondary side may be resonated in order to tune the system to the operating frequency. This resonating transfers power efficiently, even with loss of flux. Active tuning circuits may be used to track the resonance.

The coil arrangement as shown herein provides better RF isolation compared to the coil arrangement shown in the above-referenced US patent. To the extent there is any tradeoff in flux coupling efficiency, the overall power transfer efficiency of this arrangement is attractive, as will be discussed below. Connector175receives opposing ends172,174of primary coil170. Connector185receives opposing ends182,184of coil180.

The primary coil170will be connected, for example, to an RF power supply, for which the input could be DC, or could be AC at any of a number of frequencies, and converted through appropriate rectifier and other circuitry to power at an appropriate frequency to be supplied to the primary. The secondary coil180will be connected to a load, for example, a heater for an ESC. The ESC may employ multiple heaters, each heater heating a particular zone of the ESC, depending on, among other things, substrate processing requirements and conditions within the substrate processing chamber. The above-mentioned U.S. Pat. No. 8,755,204 provides examples of rectifier and drive circuitry arrangements connected to a primary as shown inFIG. 1. In that Figure, in one aspect, rectifier circuit206is a bridge rectifier and/or may employ triac, SSR, or thyristor controls. Rectifier circuit206converts the AC line input power signal to a quasi-DC power signal which may be subsequently filtered into smooth DC if desired. InFIG. 1, the AC source power signal on leads202/204may be a single phase signal or a 3-phase signal as desired, and rectifier circuit206is correspondingly a single-phase or three-phase rectifier. If a DC power signal is available as input power, then no rectification may be necessary. If high current is drawn from the AC line into the input filter, power factor correction circuitry may be required. In one or more aspects, drive circuit208is a switch-mode power supply, which pulse-width modulates the received DC power signal to the desired intermediate frequency. In one or more embodiments, the duty cycle after pulse-width modulation may vary from slightly above zero to about 50%. If desired, an appropriate drive circuit208may modulate the received DC power signal to an AC sine signal having an intermediate frequency. Reducing the harmonic content in this fashion can prevent interference and noise issues and simplifies any filtering requirements. Alternate power modulation schemes including zero crossing and on/off control may also be implemented either solo, or in combination.

In one or more aspects, filters may be employed to allow the high frequency RF signal (i.e., the RF signal to be blocked) to be presented to isolation transformer220as a common mode signal. Looking back again atFIG. 1, capacitor245is coupled to leads244and246respectively to accomplish the goal of presenting the high frequency RF signal to isolation transformer220as a common mode signal. Filters of other designs well known to those skilled in the art may also be employed.FIG. 1shows stray capacitances, represented by capacitors240and242. In U.S. Pat. No. 8,755,204, these stray capacitances may be dominated by capacitor245for the purpose of insuring that the output signal RF coupling is a common mode signal. However, the RF isolation that the ferrite cage100according to aspects of the present disclosure provides avoids stray capacitances. As a result, if capacitor245is provided in order to present a common mode signal, avoiding resonating at critical frequencies would be the key criterion.

Once the power is transferred across the RF isolation, it can be used to power a passive circuit such as a heater directly, either as AC at the switching frequency, or rectified into deeply modulated DC or filtered back to smoothed DC. It may also be rectified or controlled at the high side if desired.

In one aspect, where the coil geometry is configured to choke certain frequencies, for example, in a range from 400 kHz to 30 MHz, a single ferrite cage100may be sufficient to contain stray fields that the coils170,180may produce. For coil geometries configured to choke still higher frequencies, for example in a range from 400 kHz to 80 MHz, concentric ferrite cages100may be used to provide better isolation and capture more of the stray magnetic fields that coils170,180can produce. The concentric ferrite cages can choke RF frequencies over an even larger range, from 400 kHz to 300 MHz.

In one aspect, ferrite pieces140are located at intervals between adjacent ferrite pieces130. In another aspect, still more such ferrite pieces can be located at different intervals between adjacent ferrite pieces130,140, to capture those stray fields. In still another aspect, the arrangement of ferrite material on upper and lower dielectric discs110,120can be of various designs, including for example a spiral or helical design, resulting in a ferrite cage that is more spiral in shape than cylindrical, while still containing primary coil170and secondary coil180.

Manufacturing planar parts such as dielectric discs110,120tends to be easier than manufacturing cylindrical parts such as are used in the above-referenced US patent. In addition, providing holes for laminar airflow tends to be easier with planar parts than with cylindrical parts. As a result, there can be sufficient air gaps for the ferrite in ferrite cage100to cool, providing a stable design. Taken with the relative ease of machining ferrite bars separately rather than making a ferrite cage out of a ferrite block, overall it is easier to manufacture a ferrite cage100in accordance with aspects described herein.

Another advantage of the planar approach versus the cylindrical approach for the dielectric materials is that core saturation will be at a minimum. In addition, with permittivity of the ferrite material in ferrite cage100in the range 10-1000 according to one aspect, turn-to-turn parasitic coupling is considerably reduced in the planar approach versus the cylindrical approach.

In addition, according to one aspect, ferrite loss enables dampening of unwanted resonances above 100 MHz, without requiring separate dampening elements such as dampening resistors.

Still further, placing primary coil170and secondary coil180in ferrite cage100provides a strong magnetic path between the primary coil170and secondary coil180. Magnetic coupling between the primary coil170and secondary coil180is increased significantly without degrading capacitive isolation between them. As a result, efficiency is substantially independent of load variations. In other approaches, efficiency drops off dramatically as load increases. In operation, there will be periodic increased load, and therefore substantial load variations, as heating elements are turned on and off.

FIG. 5is a graph of RF isolation in dB(S21) versus frequency (in MHz) for designs with and without the ferrite cage. dB(S21) signifies the power received at Port2relative to the power input at Port1. The following table shows the corresponding difference in efficiency of the design including a ferrite cage according to one aspect, as compared to the efficiency of the design without a ferrite cage, at different loads.

With the ferrite cage design described herein, power supply efficiency is better than 90% over a wide range of loads (for example, when multiple heaters are operating at the same time).

According to one aspect, another advantage of the structures and techniques described herein is that, because the coils are not wound around a ferrite bar, as is the case in the above-mentioned US patent, core saturation is reduced, as is turn-to-turn capacitive coupling (which can cause unwanted parasitic resonances).

As an example of the reduction in capacitive coupling, it has been determined that, in the design employing a ferrite cage100as described above, a larger distance between coils (115.4 mm), with only 2.29 pF capacitance between the coils, yields a k-factor (magnetic coupling) of 0.29. In contrast, in order to achieve the same degree of magnetic coupling (k factor of 0.3), the coils must be much closer together (35.4 mm), yielding a much higher capacitance (10.7 pF).

FIG. 6is a graph that shows the difference in k-factor between a design without a ferrite cage100and with a ferrite cage100. Consistent with the results discussed above, with a ferrite cage100according to aspects of the present disclosure, substantial magnetic coupling is achieved even with significant separation between coils170,180. That greater separation results in decreased capacitance between the coils170,180.

FIG. 7depicts field strength lines (in terms of amperes/meter) for coils170,180without a ferrite cage100surrounding them.FIG. 8depicts field strength lines (in terms of amperes/meter) for coils170,180with a ferrite cage100surrounding them. Comparing the field strength lines inFIGS. 7 and 8, it should be noted thatFIG. 8contains field strength lines200which represent guided magnetic field vectors, whereasFIG. 7does not. Provision of the ferrite cage100around coils170,180results in the guided magnetic field vectors200.

FIG. 9depicts field strength lines (in terms of amperes/meter) for coils170,180without a ferrite cage100surrounding them.FIG. 10depicts field strength lines (in terms of amperes/meter) for coils170,180with a ferrite cage100surrounding them. Comparing the field strength lines inFIGS. 9 and 10, it should be noted thatFIG. 10contains field strength lines200which represent guided magnetic field vectors, whereasFIG. 9does not. Provision of the ferrite cage100around coils170,180results in the guided magnetic field vectors200.

The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR, and should not be construed to mean “at least one of A, at least one of B, and at least one of C.” It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure.