White Paper – Tuning Qi® and AirFuel®/PMA® Inductive
`
`Resonance Circuits for Optimal Efficiency
`
`By Nicholaus Smith, Manager Wireless Power Product Applications Engineering, Integrated Device Technology, Inc.
`
`Abstract: Wireless power transfer is accomplished using two isolated halves of an air-core transformer (a transmitter pad and a mobile
`receiver device, such as a cell phone). Due to the lack of a continuous core, accurate mathematical models that can predict the performance
`of a wireless power transfer system are challenging to create. The magnetic losses are substantially increased due to the low coupling
`caused by the lack of continuous core material. This article will discuss the challenges of simulation and the methods that can develop a
`sufficiently close approximation in order to generate inductor samples. The standard method to determine the resonance point in order to
`improve the efficacy of the system is described. A demonstration is provided for the best practice methods for optimally tuning the resonance
`circuits to achieve peak efficiency while charging a mobile device with a Qi® and/or PMA® inductive resonance circuit.
`
`Introduction
`
`Wireless power transfer is accomplished using two isolated halves of an air-core transformer (a transmitter pad and a mobile receiver device,
`such as a cell phone). Due to the lack of a continuous core, accurate mathematical models that can predict the performance of wireless
`power transfer systems are challenging to create, and it is difficult to represent all real-use cases. Rough estimates are possible using
`standard mutual inductance calculations, but often due to limited resources or time, these estimates do not account for physical geometries of
`the actual coils that will be used. Other challenges that are difficult to account for are the magnetic losses and energy that will be consumed
`by ferrous materials necessary for the final system transmitter and receiver designs. This article will discuss the challenges of simulation and
`focus on the methods that can develop an adequate approximation in order to generate inductor samples. Finally with consideration for the
`Qi® and AirFuel® inductive resonance standards, a demonstration is provided for the best practice methods for empirically determining the
`resonance point of the system and for tuning the receiver resonance tank to maximize the efficiency based on the efficacy of the system.
`
`As the market leader supplying integrated circuits for wireless power transfer, Integrated Device Technology, Inc. (IDT®) has accumulated a
`great deal of experience in tuning such circuits to optimize efficiency. A quick introduction is in order for the two most prominent inductive
`resonance standards that wireless power designs can follow, the Wireless Power Consortium’s Qi (“Chi”) and AirFuel (a merger of two prior
`standards groups: PMA and A4WP). Qi and PMA both call for designs that operate within the frequency range of 110kHz to 300kHz. Typical
`systems are based on input voltages of 5VDC, 12VDC, and 18 to 19 VDC. In general, the higher the input voltage, the higher the Tx coil
`inductance, and the Tx side of the system is typically designed to resonate at 100kHz. In order to control the amount of power being
`transferred, the operating frequency is adjusted and is always kept above the resonance frequency of the Tx LC tank. A frequency change
`toward resonance (lower frequency than the current set point) results in more power delivered, and a change away from resonance (increase
`in frequency) results in less power delivered to the receiver.
`
`In these systems, the Rx demand (which is typically, but not exclusively, the amount of charge required to fully charge the battery plus any
`system needs while charging) is used to control the Tx by communication from the Rx to the Tx. Systems currently available in the market are
`designed to provide 5W up to 15W and are designed to charge one device per charging pad. An in-depth analysis of the system should
`quickly produce a large matrix of dependent variables inclusive of second and third order derivatives and nonlinear circuits, which can lead to
`confusion and distraction. For example, the Tx side is composed of active semiconductors that control a DC-to-AC inverter that drives the Tx
`series LC resonance tank, which is composed of a coil with a given area, inductance, and number of turns, which is firmly attached to a ferrite
`shield with some permeability, connected in series with a resonance capacitor bank. It is designed to operate below the saturation point of the
`ferrite material.
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`Then there is the space inherent to the Tx design above the surface of the coil and some ferrous materials plus a stylish case, which is then
`coupled to an Rx device, such as a cell phone. When it is placed in close proximity to the center of the Tx coil, a magnetic coupling is created
`and used to transfer energy from the Tx to the Rx device. The Rx device is composed of a ferrous lithium-ion battery, any other permeable
`materials necessary to the device (such as copper PCBs and metallic cases), a secondary Rx coil with a given area and number of turns, and
`a second ferrite shield in series with another resonance capacitor. The Rx resonance tank feeds active semiconductor devices that convert
`the AC energy into DC in order to charge the Rx device. The mutual inductance is a function of the coupling, which depends on the distance
`between coils; the angle of the magnetic flux intersection with the Rx coil; remaining battery life (output load); and even the temperature of the
`system. To control the transmitted power, the receiver communicates with the transmitter by modulating the incoming AC magnetic field,
`which results in Rx impedance changes that are reflected across the mutual inductance and are detectable by the Tx. The Rx sends a series
`of digital communication messages that are used to adjust power levels or terminate power transfer as well as to authenticate valid devices.
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`© 2017 Integrated Device Technology, Inc.
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`May 10, 2017
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`White Paper – Tuning Qi® and AirFuel®/PMA® Inductive Resonance Circuits for Optimal Efficiency
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`Application Design Methodology
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`It is the simple break-down of first-order variables during steady-state operation that influences the system, and if properly investigated, it
`should be determined that there are many other variables that can influence the performance at higher orders in addition to the necessary
`inclusion of non-steady-state variables, such as transients, temperature, spatial positioning, and rapid coupling changes in addition to the
`non-linear effects of the semiconductors themselves. While the analysis can be conducted and is always encouraged, under the influence of
`time to market, it is often productive to simply build a prototype based on educated first-order assumptions and test the performance to
`advance the system design toward release. In Figure 1, some of the key items associated with the magnetic system are shown and will be
`discussed in further detail.
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`Figure 1. Cross Section of a Typical Wireless Power Transmitter (Tx) and Receiver (Rx)
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`Tx and Rx Coil Design
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`The most important parts of the system are the Tx and Rx coils, and these are positioned such that they are facing the interface. In order to
`maximize the efficacy of the system, the Tx and Rx devices need a spacer consisting of the cases (i.e. back cover of the phone and the
`plastic casing of the charger) that is 2 to 4 mm in thickness so that the system coupling is adequate for power transfer, but the devices are not
`so close to each other as to transfer too much energy to the Rx for proper conversion to DC.
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`In general, the Tx coil designs are well defined by the WPC and PMA specifications, which should be consulted when starting a design to
`determine the appropriate Tx coil and resonance capacitors. Standard coils are readily available. A common system is operated using a
`5VDC input voltage with a 6.3µH Tx coil and 400nF resonance capacitors. If powered by IDT’s P9239A Tx IC, a mobile device designed to
`comply with WPC or PMA standards can be wirelessly charged with this system. Each coil must have a ferrite sheet with high permeability in
`order to shield the device from magnetic fields and to concentrate the flux behind the respective coil during operation, which boosts system
`efficiency. Typically on the Tx, the ferrite is placed adjacent to and immediately above the main circuit board (PCB) with the Tx coil above the
`ferrite facing up toward the interface. Typically, the Rx device is stacked in the opposite order with the Rx coil facing down under the ferrite,
`which is typically next to the battery.
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`© 2017 Integrated Device Technology, Inc.
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`PCB
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`Battery
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`Rx Coil
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`Tx Coil
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`PCB
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`Mobile Device
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`Charger
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`Ferrite Rx
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`Spacer Rx
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`Interface
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`Spacer Tx
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`Ferrite Tx
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`White Paper – Tuning Qi® and AirFuel®/PMA® Inductive Resonance Circuits for Optimal Efficiency
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`Wireless Tx and Rx Controller IC Design
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`The primary components of the wireless power system are the integrated circuits (ICs) used to convert a DC power source to AC that
`energizes the Tx resonance tank, an Rx resonance tank that collects the AC power, and another set of ICs used to convert the AC power
`back to DC to be used to charge the battery or provide power to the receiver. Since different IC manufacturers will have different
`characteristics, they will not be specifically considered based on performance characteristics, but will be generalized so that the main power
`transfer elements, i.e., the Tx and Rx resonance tanks, can be considered in isolation.
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`In Figure 2, the wireless Tx and Rx controllers have been reduced to basic blocks. The TX controller is a DC-to-AC inverter and
`communication demodulator. The Rx controller is an AC-to-DC rectifier and communication generator.
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`Figure 2. Basics of a Wireless Power System – Example using the P9235A Wireless Tx and the
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`P9221 Wireless Rx
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`Designing the Inductive Resonance Circuit
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`The components under discussion shown in Figure 2 are the Tx resonance tank comprised of CP and LTX, the mutual inductance based on
`the spacing (total of Tx and Rx spacers when the device to be charged is placed on the charger), and the resonance tank of the Rx composed
`of CS, LRX, and Cd. Additional attributes of significance are the inner diameters of the Tx and Rx coils and the number of turns for each. As
`stated earlier, most Tx resonance tanks are well defined and designed to resonate at a certain frequency (100kHz typically). The Tx will
`always operate above 110kHz (or above resonance) to guarantee that the Tx does not stimulate the resonance tank at its resonant frequency
`and so that any frequency change can have a predictable effect on transmitted power (when operated at frequencies greater than the
`resonance frequency, additional increases in operating frequency will result in less transferred power and frequency decreases will result in
`more transmitted power).
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`In most Tx designs, there is a spacer above the Tx resonance coil that is 1.5 to 2 mm thick composed of non-ferrous material (Tx spacer)
`between the uppermost surface of the windings; below the Rx coil, there is commonly 0.75 to 1.5 mm of non-ferrous material between the
`bottom of the Rx coil windings and the interface. These two dimensions are critical to the performance of the system since the total thickness
`of the two spacers determines the effective k (coupling factor) that will govern the amount of energy transmitted at any given frequency based
`on the Tx coil attributes (inductance, Tx spacer, ferrite permeability, and number of turns) and the same features for the Rx coil.
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`For common Rx coil designs with 5W to 15W power levels, the most effective coils tend to have 7 to 10 turns and are 7µH to 11µH. An
`important attribute of the Rx coil design is to keep the windings tightly coupled and close together and to ensure that the inner diameter (ID,
`area in center of coil windings) is in the range of 75% to 125% of the size of the ID of the Tx coil most likely to be used with the Rx under
`development. It is fairly easy to narrow down the expected Tx types based on the power level that the Rx will demand by following the
`standard specifications. The Rx coil allows design flexibility, and the optimal component values are not always easily calculated depending on
`the constraints. Figure 3 and Figure 4 demonstrate a typical A11 Tx coil and Rx coil, respectively, that can be used with IDT controller ICs.
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`© 2017 Integrated Device Technology, Inc.
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`VIN
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`GND
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`Wireless Tx
`Controller
`(DC à AC)
`(Comm. Decoder)
`(Full-Bridge)
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`CP
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`CS
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`Cd
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`LTX
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`LRX
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`Lm(k)
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`Wireless Rx
`Controller
`(AC à DC)
`(Comm. Generator)
`(Rectifier)
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`VOUT
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`GND
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`White Paper – Tuning Qi® and AirFuel®/PMA® Inductive Resonance Circuits for Optimal Efficiency
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`Figure 3. Example: Würth 760308100111 WPC A11 Tx Coil (6.3µH) Used for 5W Wireless Power
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`Systems
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`Image used with permission from Würth Elektronik GmbH.
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`© 2017 Integrated Device Technology, Inc.
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`Note: All dimensions are in mm.
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`Inner Diameter
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`Coil Winding
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`Ferrite Shielding
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`Outer Diameter
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`White Paper – Tuning Qi® and AirFuel®/PMA® Inductive Resonance Circuits for Optimal Efficiency
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`Figure 4. Example: Amotech ASC-504060M22-S00 Rx Coil (8.2µH) Used for 5W Wireless Power
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`Systems
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`Image used with permission from AMOTECH Corp., Ltd.
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`Tuning the Inductive Resonance Circuit
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`Most wireless power systems have the majority of the geometric limitation placed on the receiver design. As such, it is the receiver that plays
`the most critical role in fine-tuning the system. Normally the mechanical engineers for the Rx device will assign the physical dimensions that
`the Rx coil can occupy; once the available space is known, the Rx coil design should be undertaken. For very thin designs (< 0.7mm), flex
`PCB circuit types are the main solution due to the thin copper thickness in order to keep the ferrite as thick as possible. In general, thicker
`ferrites in the Rx will provide a more stable response across the power delivery range and thin ferrites tend to allow more flux to pass through
`their volume at higher power levels, which can result in degraded system performance at higher loads, such as diminished communication
`strength and poor transient step-load performance. In choosing the copper material, it should be considered that the copper wire types are
`thicker than the flex PCB type but have the advantage of having lower DC losses and thus offer higher efficiencies.
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`© 2017 Integrated Device Technology, Inc.
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`Note: All dimensions are in mm.
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`Coil Winding
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`Inner Diameter
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`Outer Diameter
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`Ferrite Shielding
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`White Paper – Tuning Qi® and AirFuel®/PMA® Inductive Resonance Circuits for Optimal Efficiency
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`As for the permeability, it is necessary to have a relative permeability in the range of 60. If the permeability is increased, it will allow thinner
`ferrites to reduce leakage flux, but if it is increased too much (> approximately 80), the ferrite losses start to increase and offset the gain of the
`higher permeability. It is highly recommended that the ferrite is a continuous sheet of material that is uninterrupted under the coil windings.
`With the available space for the coil known, the decision to use copper wires or flex circuit should be made based on the remaining thickness
`available for the ferrite shield. The ferrite shield should be kept at 0.5mm or greater. Generally, copper wires have lower cost and higher
`efficiencies due to the fact that they are producible at higher yields and are easily produced while the flex-PCB coil types require very thick
`copper foil weights and tend to require more time to produce at lower yields, which leads to higher cost, but can fit into thinner designs more
`readily.
`
`A third option is to purchase stamped coil types, which generally offer higher resistance but can achieve lower cost and can be physically
`thinner than wire-wound designs.
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`At this point in the design stage, there are several variables that have been decided: the Rx coil ID and the ferrite type and thickness. The
`next step is to determine the optimal number of turns based on the copper winding type that will be used. As stated, the number of turns
`should be between 7 and 10 turns, and each additional turn will have a corresponding increase in inductance of the coil. As such, it should be
`determined how many windings it will take to reach approximately 8µH. At this point, a prototype that can be used for testing the system can
`be produced by a major vendor for a nominal non-recurring engineering (NRE) fee.
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`Designing the Final Hardware and Testing
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`Building at least three samples is recommended: one with approximately 8µH of inductance and two others with plus or minus one winding
`each. Other variables that can be explored are ferrite permeability and copper-winding type. The number of samples will have an impact on
`development time and cost, so the number of variables should be limited based on the resources of the design team. Another method is to
`contact vendors, such as Würth Elektroniks, Amotech, TDK, Sunlord, NuCurrent, or AKStamping, and request standard samples that have
`the correct ID and approximate inductance to test with the integrated circuits that will be used to convert the wireless energy to DC, such as
`IDT’s P9221 or P9025 receivers.
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`It is recommended that the next steps be determining the total space (Tx + Rx spacer thickness) and commencing with testing. The
`recommended test is to conduct efficiency tests by measuring the input and output voltages and currents while varying the load current from
`0A up to the full output load current required at the voltages that will be used for the design.
`
`The efficiency can be calculated with Equation 1:
`
`𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (%) =
`
`𝑉𝑂𝑈𝑇 𝑥 𝐼𝑂𝑈𝑇
`𝑉𝐼𝑁 𝑥 𝐼𝐼𝑁
`
` 𝑥 100
`
`Equation 1
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`At this stage, the space should be fixed and the resonance capacitance should be fixed at 400nF or chosen by calculating the resonance
`frequency for each coil that will be tested (see IDT Application Note AN816 for details to determine Rx resonance values). Then the efficiency
`should be measured at 5 to 10 load current levels ranging across the expected load range of the application, and a comparison of each coil
`should be made. The top two performers that meet price targets and offer the highest efficiency should be selected for further testing.
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`Next, the Rx resonance capacitance (CS); see Figure 2) should be increased by 47nF or 100nF and the efficiency should be tested again. If
`the efficiency result improves at the same load levels, space, and coils, then another increase should be made of the same step size (47nF to
`100nF). This should be continued until the efficiency decreases. If the efficiency decreases when the first increase is made, then the
`resonance capacitors (CS) should be decreased and the test repeated. Normally, Cd should be between 2.2nF and 4.7nF, and 3.3nF is a
`common value. Once the point that the efficiency stops increasing and begins decreasing has been determined, finer steps should be taken
`to incrementally approach the point of optimal efficiency. At this point, the optimal combination of capacitors should be calculated based on
`standard values, and an effort should be made to keep each value as close to the others as possible. The previous method should be
`repeated for both WPC and PMA transmitters for dual-mode receivers. For example, if the optimal value is 389nF, then 3x100nF and one
`82nF may be the best option to minimize component count and match impedance to promote equally distributed current densities through
`each capacitor. Once the optimal values are found and the best coil/capacitor combination has been selected, exhaustive system level tests
`should be conducted to guarantee that operation is satisfactory in the application.
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`© 2017 Integrated Device Technology, Inc.
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`White Paper – Tuning Qi® and AirFuel®/PMA® Inductive Resonance Circuits for Optimal Efficiency
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`The final step is to pick the dielectric and voltage rating. 50V capacitors are the minimum recommendation in all cases, and C0G components
`are the best. If X7R or X5R types are used, the component with the lowest ESR at 100kHz should be identified and used. When optimizing
`the system, it is recommended to do so on hardware that is as close to final as possible to reduce the opportunity for system influence when
`the final hardware is available. If testing is done on standalone boards or the final hardware changes, verifying the results of the preliminary
`testing on the final hardware is recommended.
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`Summary of the Design Sequence
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`The following steps summarize the process of designing and tuning Qi® and AirFuel® or PMA® inductive resonance circuits for optimal
`efficiency:
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` Pick the optimal Tx to be used with the system (such as WPC A11 or PMA Type 3).
`
` Set the Tx resonance to 100kHz using the Tx coil inductance and C0G capacitors or an alternative resonance value based on the target
`specification of the design.
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` Determine the Rx coil physical volume allowance.
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` Design the Rx coil such that the ID is 80% to 120% of the Tx ID. Typically, inductance is 7µH to 11µH.
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` Get prototypes or samples that meet design constraints.
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` Test with CS selected at resonance for each coil based on the presence of the Tx coil.
`— Find coil(s) that offer the highest efficiency.
`
`
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`Increase and/or decrease CS while running iterative efficiency sweeps with the best coil(s) to find quasi-peak efficiency with fairly large
`capacitor value steps (47nF or 100nF).
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`— Fine-tune step size to find the optimal resonance capacitance.
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` Make final selection of capacitors by matching values as much as possible and minimizing the ESR for each capacitor by inspecting ESR
`curves.
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` Design final hardware and conduct tests of final assembled system to guarantee satisfactory performance.
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`DISCLAIMER Integrated Device Technology, Inc. (IDT) and its affiliated companies (herein referred to as “IDT”) reserve the right to modify the products and/or specifications described herein at any time,
`without notice, at IDT's sole discretion. Performance specifications and operating parameters of the described products are d etermined in an independent state and are not guaranteed to perform the same
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`Integrated Device Technology, Inc. All rights reserved.
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