Electronics Guide

Magnetic Field Characteristics

The magnetic field produced by an IPT transmitter coil has characteristic spatial distribution that depends on coil geometry:

Circular coils produce fields that are strongest at the coil center and decrease with distance according to the inverse cube law in the far field. Near the coil, the field distribution is more complex, with the axial field component strongest along the coil axis and radial components becoming significant at the edges.

Rectangular and DD coils (double-D configuration) produce different field patterns optimized for specific coupling characteristics. DD coils create a flux path that closes through two adjacent coil halves, concentrating the field in the coupling region and potentially reducing far-field emissions.

The field strength at the operating frequency must be sufficient for efficient power transfer, typically requiring hundreds of microtesla to several millitesla in the coupling region. This is orders of magnitude stronger than typical EMC emissions limits, necessitating careful attention to field containment.

Harmonic Emissions

Beyond the fundamental operating frequency, IPT systems generate harmonic emissions from several sources:

• Inverter switching: The power electronic inverter driving the transmitter coil produces harmonics related to switching frequency and PWM modulation. These harmonics appear in both conducted and radiated emissions
• Core saturation: If magnetic cores are used, saturation effects generate odd harmonics of the fundamental frequency
• Rectifier harmonics: The receiver-side rectifier generates harmonics that can couple back through the magnetic link
• Control circuit interference: Communication and control circuits operating at different frequencies can create intermodulation products

Harmonic content typically decreases with frequency but may find resonances in cables, structures, or victim equipment that amplify specific harmonic orders. Third harmonic emissions often require particular attention as they can fall within protected frequency bands.

Field Containment Strategies

Several approaches help contain magnetic fields to the coupling region:

Ferrite backing: Ferrite plates behind the coils provide a low-reluctance return path for magnetic flux, reducing field extension behind the coils and improving coupling efficiency. The ferrite effectively creates a magnetic mirror that enhances flux linkage between coils while reducing stray fields.

Aluminum shielding: Aluminum plates positioned around the coil assembly induce eddy currents that oppose and attenuate the magnetic field beyond the shield. However, aluminum shields reduce efficiency by absorbing energy, requiring careful trade-off analysis. Shield placement must balance emission reduction against acceptable efficiency loss.

Coil geometry optimization: Coil shapes can be designed to create flux patterns that naturally cancel at distance. Bipolar coil arrangements, where adjacent coil sections carry opposite currents, create quadrupole-type field patterns that decay more rapidly with distance than simple dipole fields.

Active field cancellation: Auxiliary coils driven with appropriate phase and amplitude can generate fields that cancel stray emissions in specific directions. This technique is most effective for cancelling fields at particular locations or in specific directions rather than achieving omnidirectional reduction.

Measurement Considerations

Measuring near-field magnetic emissions presents unique challenges:

Standard EMC test methods using antennas assume far-field conditions that may not apply at typical IPT operating frequencies (often below 500 kHz). Near-field probes calibrated for magnetic field measurement in specific units (typically amperes per meter or tesla) are more appropriate than antenna measurements in volts per meter.

The high field gradients near IPT coils mean that small changes in probe position can significantly affect readings. Measurement procedures should specify precise probe locations referenced to the equipment under test.

Some standards require measurement at the fundamental operating frequency and harmonics separately, while others use broadband receivers. Understanding which approach applies to a specific product is essential for accurate compliance assessment.

Exposure Standards Overview

Two primary organizations establish electromagnetic field exposure guidelines:

ICNIRP (International Commission on Non-Ionizing Radiation Protection) publishes guidelines adopted by most countries outside North America. The guidelines establish reference levels for external field strength and basic restrictions on internal field strength and specific absorption rate (SAR). Reference levels are derived from basic restrictions with conservative assumptions, so compliance with reference levels guarantees compliance with basic restrictions.

IEEE C95.1 establishes exposure limits used primarily in North America. While based on similar scientific foundations, the specific numerical limits differ from ICNIRP in some frequency ranges and exposure scenarios.

Both frameworks distinguish between occupational and general public exposure, with more restrictive limits for general public exposure to account for potentially vulnerable individuals and uncontrolled exposure conditions.

Frequency-Dependent Limits

Exposure limits vary significantly with frequency, reflecting the different biological interaction mechanisms at different frequencies:

Below 100 kHz: The primary concern is induced electric fields and currents in tissue, which can stimulate nerves and muscles. Limits are expressed in terms of internal electric field (V/m) or derived reference levels for external magnetic field (A/m or T).

100 kHz to 10 MHz: Both nerve stimulation and thermal effects are considered. Limits may be expressed as induced current density or SAR, with reference levels for both electric and magnetic fields.

Above 10 MHz: Thermal effects dominate, and SAR becomes the primary metric. This range is less relevant for most IPT systems, which typically operate below a few megahertz.

Most IPT systems operate in the tens to hundreds of kilohertz range, where nerve stimulation limits apply. The transition region around 100 kHz requires particular attention as both thermal and stimulation effects may need assessment.

Exposure Assessment Methods

Demonstrating compliance with exposure limits requires either measurement or calculation:

Direct field measurement: Magnetic field probes can measure field strength at locations where humans might be exposed. Measurements must account for spatial averaging (ICNIRP specifies averaging over the whole body for some metrics) and temporal averaging (time-varying fields require appropriate averaging). Measurement uncertainty must be considered when comparing to limits.

Numerical simulation: Computational electromagnetic modeling can calculate field distributions and induced fields in anatomical human body models. This approach enables assessment of internal fields and SAR that cannot be directly measured in humans. Validated simulation tools with appropriate human body models (such as the Virtual Population models) are used for regulatory submissions.

Compliance distance: Some products define a compliance distance beyond which field levels meet reference limits. Users are instructed to maintain this distance during operation. This approach requires clear labeling and user instructions.

Practical Design Implications

Human exposure requirements influence IPT system design in several ways:

• Power level limits: Higher power systems generate stronger fields, potentially requiring larger exclusion zones or more aggressive shielding
• Operating frequency: Lower frequencies have less restrictive magnetic field limits but require larger coils for equivalent power transfer
• Coil placement: Keeping coils away from areas where humans will be present reduces exposure. Floor-mounted vehicle chargers benefit from vehicle ground clearance providing inherent separation
• Shielding effectiveness: Shields must reduce fields to compliant levels while maintaining acceptable efficiency
• Operational modes: Systems may reduce power or cease operation when presence detection indicates humans are nearby

Heating Mechanisms

Objects in the IPT field heat through two primary mechanisms:

Eddy current losses: Time-varying magnetic fields induce circulating currents in conductive objects. These currents dissipate power as heat proportional to the square of the field strength and frequency. Small metallic objects like coins, keys, or metal clips can reach dangerous temperatures within seconds when exposed to IPT fields.

Hysteresis losses: Ferromagnetic materials experience additional heating from magnetic domain realignment during each field cycle. The combination of eddy current and hysteresis losses can make ferromagnetic objects heat more rapidly than non-magnetic conductors of similar size.

The heating rate depends on object size, material properties, position within the field, and the IPT system's power level and frequency. Small objects may heat faster than large ones because eddy current heating scales with the square of object dimension while thermal mass scales with the cube.

Detection Technologies

Several technologies are used for foreign object detection:

Inductive sensing: Small sensing coils distributed across the charging surface detect the presence of metallic objects through changes in inductance or induced voltage. An array of coils can provide spatial resolution to locate objects. This approach is widely used in consumer electronics charging systems.

Quality factor monitoring: The presence of a lossy object in the magnetic field reduces the system Q-factor by absorbing energy. Monitoring Q-factor or efficiency can detect objects, though distinguishing foreign objects from the intended receiver can be challenging.

Temperature sensing: Infrared sensors or thermistors can detect heating of the charging surface caused by object heating. This provides a direct measurement of the hazard condition but may not respond quickly enough for all scenarios.

Capacitive sensing: Capacitive touch sensing can detect objects on the charging surface regardless of their magnetic properties. However, capacitive sensing cannot detect objects that do not touch the surface and may be affected by environmental conditions.

Radar or imaging: High-frequency radar or optical imaging systems can detect objects with high reliability but add significant cost and complexity.

FOD System EMC Considerations

Foreign object detection systems create their own EMC challenges:

Interference with power transfer: FOD sensing signals must not interfere with the main power transfer function or the communication link between transmitter and receiver. Frequency separation, time-division operation, or careful filtering may be required.

Immunity to IPT fields: The FOD system must operate reliably in the presence of strong magnetic fields from the IPT system. Sensing coils and electronics must be designed to reject the fundamental frequency and harmonics while remaining sensitive to small object perturbations.

Emissions from FOD: If the FOD system uses active sensing (rather than passive Q-factor monitoring), its emissions must comply with applicable EMC limits. High-frequency sensing systems may fall under different emission standards than the main IPT system.

Detection reliability: FOD systems must maintain reliable detection across the full range of operating conditions including temperature extremes, humidity, and electromagnetic interference. False positives reduce user convenience while false negatives create safety risks.

Standards Requirements

Product safety and wireless power standards specify FOD requirements:

The Wireless Power Consortium Qi specification defines FOD requirements for consumer device chargers, including test objects, detection thresholds, and response times. Compliance testing uses standardized test objects and positioning.

SAE J2954 for electric vehicle wireless charging includes FOD requirements scaled for the higher power levels and larger charging areas involved. Test procedures account for the vehicle environment and typical debris that might be present.

IEC 61980 series standards for electric vehicle wireless power transfer systems include general FOD provisions applicable across different system designs.

Coupling and Efficiency

Power transfer efficiency in IPT systems depends primarily on the coupling coefficient between transmitter and receiver coils and the Q-factors of the resonant circuits:

The coupling coefficient k represents the fraction of transmitter flux that links the receiver coil. Higher coupling enables more efficient power transfer but typically requires the coils to be close together and well aligned. Designs that achieve high coupling through tight coil spacing and ferrite flux concentration may have good efficiency but limited tolerance for misalignment.

The Q-factor of the transmitter and receiver resonant circuits determines how much energy is lost in resistance versus transferred to the load. Higher Q enables efficient operation at lower coupling but makes the system more sensitive to frequency accuracy and component tolerances.

Shielding Trade-offs

Adding shielding to reduce emissions inevitably affects efficiency:

Ferrite shielding improves efficiency by providing a low-loss return path for magnetic flux while also reducing field extension behind the coils. The efficiency improvement can offset core losses in well-designed systems, making ferrite shielding generally beneficial for both efficiency and EMC.

Aluminum shielding reduces far-field emissions effectively but absorbs energy through eddy current losses. The efficiency penalty depends on shield placement and design. Shields positioned to intercept primarily stray flux (rather than coupling flux) minimize efficiency impact while providing emission reduction.

Shield geometry optimization uses electromagnetic simulation to find configurations that maximize shielding effectiveness for a given efficiency penalty. This often involves partial shields that selectively block emissions in problematic directions while leaving the coupling region relatively unaffected.

Frequency Selection

Operating frequency significantly affects both efficiency and EMC:

Lower frequencies (tens of kilohertz) allow use of larger, lower-loss coils and reduce core losses in ferrites. However, larger coils may produce more extended field patterns, and some harmonics may fall in protected frequency bands.

Higher frequencies (hundreds of kilohertz) enable smaller coils and components but increase core losses and skin effect losses in conductors. Higher operating frequencies may encounter more restrictive emissions limits but allow harmonics to fall above sensitive frequency bands.

ISM (Industrial, Scientific, and Medical) frequency bands provide regulatory advantages for IPT systems. Common ISM frequencies used for wireless power include 6.78 MHz and 13.56 MHz, though many automotive and consumer systems operate in the 80-90 kHz or 100-300 kHz ranges.

Power Level Effects

Higher power systems face greater EMC challenges:

• Field strengths scale with power, increasing both emissions and human exposure
• Higher currents in power electronics create stronger conducted emissions
• Thermal management requirements may increase electromagnetic radiation from heat sinks
• Component stresses may limit operating frequency, affecting frequency selection trade-offs

Scaling IPT systems to higher power levels requires proportionally more aggressive EMC measures to maintain compliance.

Coil Topologies

Various coil topologies offer different performance characteristics:

Circular coils are the simplest design, with field patterns well understood from basic electromagnetic theory. They offer reasonable efficiency with modest shielding requirements but are sensitive to lateral misalignment.

DD (Double-D) coils use two D-shaped coil halves wound in opposite directions. The flux pattern creates strong horizontal components that improve tolerance to lateral misalignment. The opposing currents in the two halves provide partial far-field cancellation.

DDQ coils combine a DD coil with a quadrature (Q) coil, enabling power transfer regardless of lateral position. The additional coil adds complexity but improves positioning flexibility significantly.

Bipolar coils arrange multiple coil sections with alternating current directions. Like DD coils, this creates field patterns with improved far-field cancellation compared to simple circular coils.

Each topology creates different field distributions, affecting both coupling characteristics and emissions patterns. Coil selection for a specific application requires evaluating EMC performance alongside efficiency and misalignment tolerance.

Coil Size and Turns

Coil dimensions and turn count affect electromagnetic performance:

Larger coils spread the magnetic field over a wider area, potentially reducing peak field strengths but extending the field footprint. For a given power level, larger coils can operate at lower field intensities, potentially improving human exposure compliance.

More turns increase inductance and voltage while reducing current for a given power level. Higher turns count may reduce conducted emissions related to high currents but increases voltage stress and interwinding capacitance effects.

Litz wire construction, using many individually insulated fine strands, reduces AC resistance at high frequencies by mitigating skin and proximity effects. Lower resistance improves efficiency and reduces heating but has minimal direct effect on emissions.

Ferrite Design

Ferrite cores and shields are essential components in most IPT systems:

Core geometry shapes the magnetic field pattern. Flat ferrite plates behind the coils create a flux path that enhances coupling and reduces rear-side emissions. More complex ferrite structures can shape fields for specific applications.

Material selection involves choosing ferrite grades appropriate for the operating frequency. Materials with high permeability and low losses at the operating frequency provide the best performance. Power ferrites designed for tens to hundreds of kilohertz are typically suitable for IPT applications.

Saturation limits must be respected to avoid harmonic generation. Local flux concentration at coil edges or ferrite discontinuities can cause saturation even when average flux density is below material limits.

Thermal management of ferrites is important because permeability and losses are temperature-dependent. Systems must remain within acceptable operating ranges across temperature extremes.

Parasitic Effects

Real coils exhibit parasitic effects that influence EMC:

Interwinding capacitance creates high-frequency resonances that may amplify harmonic emissions. Winding layouts that minimize capacitance improve high-frequency EMC performance.

Coil-to-shield capacitance provides a coupling path for common-mode noise onto shielding structures, potentially making them radiating antennas for inverter switching noise.

Lead inductance and connection parasitics can create additional resonances. Connections between the coil and power electronics should be designed to minimize parasitic inductance and its effects.

Magnetic Shielding Principles

Magnetic field shielding at IPT frequencies relies on two mechanisms:

Flux shunting uses high-permeability materials to provide an alternative path for magnetic flux. Ferrite or other soft magnetic materials placed around the coil assembly attract flux that would otherwise extend into space. This approach is most effective at lower frequencies where skin depth in the shield material is large compared to shield thickness.

Eddy current cancellation uses conductive shields that develop eddy currents opposing the incident magnetic field. Aluminum or copper sheets positioned around the coil assembly reduce external field strength through the opposing field created by eddy currents. This approach is more effective at higher frequencies where eddy currents are confined to a thin skin depth.

Practical IPT shielding often combines both mechanisms: ferrite backing to shape the flux and improve coupling, with aluminum shielding around the periphery to reduce far-field emissions.

Shield Configuration Options

Several shield configurations are used in IPT systems:

Back shields placed behind the coil reduce field extension in that direction. For floor-mounted chargers, back shields direct the field upward toward the receiver. Back shields typically have minimal efficiency impact because they redirect rather than absorb coupling flux.

Edge shields around the coil periphery reduce lateral field extension. These shields may have greater efficiency impact because they are closer to the coupling region, but they are essential for controlling lateral emissions.

Complete enclosures around the coil assembly provide maximum shielding but require apertures for the coupling field. The aperture size and position must be optimized for coupling while minimizing leakage in other directions.

Selective shielding addresses emissions in specific directions or at specific frequencies. This approach, guided by simulation and measurement, can achieve required emission reduction with minimum efficiency impact.

High-Frequency Considerations

While the fundamental IPT frequency may be in the tens to hundreds of kilohertz range, harmonic emissions extend to much higher frequencies:

Aluminum and copper shields become increasingly effective at higher frequencies as skin depth decreases. A 1 mm aluminum sheet provides substantial attenuation at frequencies above a few megahertz.

However, apertures, seams, and cables penetrating the shield can leak high-frequency emissions. Standard EMC shielding practices for aperture control, gasket selection, and cable filtering apply to IPT enclosures just as they do to other electronic equipment.

Ferrite materials have frequency-dependent permeability that typically decreases above a few megahertz. Ferrite shields effective at the fundamental frequency may provide little attenuation at harmonic frequencies.

Integration with Vehicle or Product Structure

In many applications, the IPT coils integrate with a larger structure that influences shielding:

Electric vehicles have metal floor pans that can serve as part of the receiver shielding. The vehicle body provides significant attenuation of fields above the floor level. Transmitter-side shielding must be designed considering the vehicle ground clearance and floor pan geometry.

Consumer electronics often have plastic enclosures that provide no shielding. Metal components within the device (battery, frame elements) may provide incidental shielding or may act as secondary radiators.

Industrial equipment may have metal enclosures that can be utilized for shielding, but apertures for cooling, cables, and user interface must be managed.

Types of Misalignment

Three types of misalignment affect IPT system performance:

Lateral offset: The receiver coil is displaced horizontally relative to the transmitter. This reduces coupling and may create asymmetric field patterns that affect emissions.

Gap variation: The distance between transmitter and receiver coils varies from nominal. Increased gap reduces coupling significantly; reduced gap may cause overheating or excessive coupling.

Angular misalignment: The receiver coil is tilted relative to the transmitter. This reduces the effective coupling area and may create localized high-field regions.

Practical systems must tolerate combined misalignments within specified limits while maintaining safe and efficient operation.

EMC Effects of Misalignment

Misalignment influences EMC performance through several mechanisms:

Reduced coupling causes the system to increase drive current or voltage to maintain power transfer, potentially increasing emissions proportionally. Systems should limit maximum drive levels to prevent emissions exceedances under worst-case alignment.

Field pattern changes occur as the transmitter and receiver fields no longer align optimally. Regions of constructive interference may create local emission hot spots not present under aligned conditions.

Efficiency reduction means more power is lost as heat in coils and shields, potentially affecting temperature-dependent EMC characteristics.

Control system behavior may change as the system adjusts operating parameters to compensate for misalignment. Frequency adjustments, duty cycle changes, or phase shifts all affect emissions characteristics.

Design for Alignment Tolerance

Several design approaches improve alignment tolerance while managing EMC:

• Coil topology selection: DD and DDQ coils offer better lateral alignment tolerance than simple circular coils
• Oversized transmitter: A transmitter coil larger than the receiver provides consistent coupling over a range of lateral positions
• Position detection: Active position sensing enables the system to adjust operating parameters or guide the user to better alignment
• Power derating: Reducing power at poor alignment maintains efficiency and limits emissions increases
• Worst-case EMC testing: EMC compliance should be verified across the full alignment tolerance range, not just at optimal alignment

EMC Standards

Electromagnetic emissions from IPT systems fall under various standards depending on application:

CISPR 11 applies to industrial, scientific, and medical (ISM) equipment. Most IPT systems classify as ISM equipment with limits depending on operating frequency and whether emission is intentional or unintentional.

CISPR 32 applies to multimedia equipment including consumer electronics chargers. The limits and test methods differ from CISPR 11 in some respects.

FCC Part 18 in the United States covers ISM equipment with requirements similar to CISPR 11 but with some differences in limits and measurement procedures.

Automotive EMC standards including CISPR 25 and ISO 11452 series apply to vehicle-mounted components. Electric vehicle wireless charging systems must meet these automotive requirements.

Classification of IPT emissions as intentional versus unintentional affects applicable limits. The fundamental frequency field may be considered intentional while harmonic emissions are unintentional, potentially with different limits applying to each.

Human Exposure Standards

Human exposure compliance is addressed through:

ICNIRP Guidelines provide exposure reference levels adopted by most jurisdictions. The 2020 guidelines update previous versions with revised limits in some frequency ranges relevant to IPT.

IEEE C95.1 establishes US and Canadian exposure limits with some differences from ICNIRP in numerical limits and assessment methods.

IEC 62311 provides assessment methods for demonstrating compliance with human exposure requirements. This standard is harmonized with European regulatory requirements.

Product-specific standards including IEC 62233 for household appliances incorporate exposure assessment requirements appropriate to the product type.

Wireless Power Standards

Interoperability and safety standards specific to wireless power include:

Qi Specification from the Wireless Power Consortium defines requirements for consumer device inductive charging up to 15W and extended power profile systems. Qi includes both interoperability requirements and safety provisions including FOD.

SAE J2954 defines wireless power transfer for light-duty electric vehicles at power levels from 3.7 kW to 22 kW (and potentially higher in future revisions). The standard includes alignment methods, interoperability requirements, and EMC provisions.

IEC 61980 series covers electric vehicle wireless power transfer systems with general requirements, specific requirements for magnetic field systems, and test methods.

ISO 19363 provides test procedures for wireless power transfer systems, complementing IEC 61980 with detailed measurement methods.

Compliance Strategy

Successful compliance requires a systematic approach:

1. Identify all applicable standards early in product development
2. Design for compliance from the start rather than treating EMC as a late-stage fix
3. Use simulation to predict EMC performance before hardware prototyping
4. Conduct pre-compliance testing during development to identify issues early
5. Test across the full range of operating conditions including alignment variations and power levels
6. Document design decisions and test results for regulatory submissions
7. Consider certification body requirements for specific markets