High-power wireless power transmission technology for industrial environment

With the increasing popularity of wireless power transfer technology in consumer electronics, the industrial and medical industries are also turning their attention to the technology and its inherent advantages. Driven by various wireless technologies such as WLAN and Bluetooth (Bluetooth), communication interfaces are increasingly developing wirelessly, and wireless power transmission technology has also become a corresponding choice.

by ANDREAS NADLER AND CEM SOM

1 Introduction

With the increasing popularity of wireless power transfer technology in consumer electronics, the industrial and medical industries are also turning their attention to the technology and its inherent advantages. Driven by various wireless technologies such as WLAN and Bluetooth (Bluetooth), communication interfaces are increasingly developing wirelessly, and wireless power transmission technology has also become a corresponding choice. Adopting some completely new solutions not only brings obvious technical advantages, but also opens up more possibilities for new industrial designs. This technology offers many new concepts, especially in industrial fields such as ATEX, pharmaceuticals, construction machinery, etc., where resistance to harsh environments such as aggressive cleaning agents, severe pollution and high mechanical stress is required. For example, it can replace expensive and fragile slip rings or contacts. Another area of ​​application is transformers that have to meet special requirements such as reinforced or double insulation.

The purpose of this application note is to demonstrate that wireless power transfer solutions of hundreds of watts and beyond can be easily implemented using circuit technology without the use of software or controllers.

High-power wireless power transmission technology for industrial environment
Figure 1: Würth Elektronik wireless charging coil.

2. ZVS oscillator (differential mode resonant converter)

A classic resonant converter is used as the clock circuit in this application note.

This oscillator offers many advantages:

• Independent oscillation requiring only one DC power supply
• Current and voltage curves very close to sinusoidal
• No active components and software required
• Scalable from 1 W – 200 W
• MOSFET switch close to zero crossing
• Scalable to accommodate many different voltages/currents

2.1. Basic circuit/schematic diagram:

High-power wireless power transmission technology for industrial environment
Figure 2: Basic resonant converter circuit.

The basic circuit shown in FIG. 2 is the transmitting side, including the transmitting coil LP. The same basic circuit can be used on the receiving side (see Chapter 3.1).

2.2. Function

Resonant converters usually operate at a constant operating frequency, which is determined by the resonant frequency of the LC parallel resonant circuit. Once a DC voltage is applied to the circuit, it starts to oscillate based on the MOSFET device tolerance. For a short period of time, one of the two MOSFETs conducts slightly more than the other. The positive feedback of the gates of the two MOSFETs and the opposing drains of the less conductive MOSFETs create a 180° phase shift. Therefore, the two MOSFETs are always driven out of phase and can never be turned on at the same time. The two MOSFETs alternately ground the two parallel resonant circuits alternately, so that the resonant circuits are periodically charged.

High-power wireless power transmission technology for industrial environment

Another feature of this circuit topology is that the voltage is always close to the zero-crossing point, which means that the switching losses of the MOSFET are extremely low. The disadvantage of this switching topology is that the reactive current flowing in the resonant circuit results in relatively high power consumption in the idle state. Therefore, ideally, the resonant converter should only operate under applied load. At the same time, it should be considered that the frequency of the resonant circuit will vary with the coupling coefficient on the receiving side. This is due to the reflected impedance on the receiving side affecting the magnetizing inductance on the transmitting side because the two sides are in parallel. As the magnetizing inductance on the transmit side decreases, the coupling coefficient decreases, resulting in an increase in frequency.

The basic circuit in Figure 1 can operate from 3.3 V to over 230 V, depending on the device used. When the input voltage is higher than 20 V, care must be taken to protect against touch, because the voltage in the resonant circuit is already 50 VAC/120 VDC π times or more above the SELV (Safety Low Voltage) threshold.

High-power wireless power transmission technology for industrial environment
Figure 3: The voltage of the transmit coil is shown in blue and red. The gate voltage is shown in yellow and green.
(These voltage curve measurements are referenced to circuit ground GND; Vin = 20 V; Pout = 100 W; optimized gate drive, see application example)

In fact, the efficiency of the entire wireless power transfer circuit may exceed 90%. This is hard to come by because coupling losses through the air gap are accounted for and a stable DC voltage is available at the input. Efficiency remains stable for air gaps in the range of 4-10 mm. A significant portion of the energy in the magnetic field that is not coupled to the receiving side is returned to the “resonant tank”. Depending on the application, the maximum distance can be up to 18 mm, but there are sacrifices in coupling factor and EMC.

The circuit on the transmit side can also be used on the receive side, while the resonant converter acts as a synchronous rectifier. It needs to be considered here that the resonant frequency of the receiving side should be very close to the resonant frequency of the transmitting side. This also produces the greatest “snubber effect”. Connecting C and L in parallel means that the secondary side acts as a constant current source similar to the load, which can significantly improve the overall efficiency of the circuit. In addition, capacitors can also compensate for the stray inductance of the wireless power coil. If the circuit is constructed properly (ie…), the receiver can feed energy back to the transmitter (ie Linear Technology’s “ideal” diode at the load).

High-power wireless power transmission technology for industrial environment
Figure 4: Transmitter coil voltage (not referenced to circuit GND; Vin = 20 V/Pout = 100 W).

High-power wireless power transmission technology for industrial environment
Figure 5: Reflected ripple and noise from the input supply on the transmit side (Vin = 20 V/Pout = 100 W) à Low ESR polymer and ceramic capacitors can be used to reduce voltage ripple.

Efficiency can be improved by using a smaller MOSFET to drive the gate instead of a Schottky diode, or using a bipolar push-pull circuit (see application example).

For supply voltages above 20 V, a capacitive voltage divider can be used to drive the gate of a MOSFET or a DC/DC converter (such as the highly efficient and compact Würth Elektronik MagI³C power module) can be used as an auxiliary voltage source (see application example in Section 3) .

Likewise, on the receiving side, the resonant converter can also be replaced by a classical bridge rectifier. The advantages include higher output voltage, lower cost, and less space, but diode losses result in lower efficiency.

The load frequency should generally not exceed 150 kHz, otherwise the losses in the parallel capacitors, transmit and receive coils will be too high. In addition, the EMC limits are higher below 150 kHz (eg CISPR15 EN55015 9 kHz – 30 MHz). 105-140 kHz is the best frequency range based on the trade-offs of all the experiments conducted so far. This frequency range ensures that you are within a safe frequency range according to the currently approved frequency band for inductive power transfer (100-205 kHz).

If the final product will be available in multiple countries, the regulations and allowed frequency bands for each country should be determined in advance to shorten the development phase.

High-power wireless power transmission technology for industrial environment
Figure 6: 6.5 mm air gap measurement circuit (Vin = 20 VDC; Pout = 100 W).

High-power wireless power transmission technology for industrial environment
Figure 7: 6.5 mm air gap measurement circuit (Vin = 20 VDC; Pout = 100 W).

High-power wireless power transmission technology for industrial environment
Figure 8: Temperature rise of circuit/coil for Pout = 100 W (Vin = 20 V) (upper side = filter + capacitor).

High-power wireless power transmission technology for industrial environment
Figure 9: Temperature rise of circuit/coil for Pout = 100 W (Vin = 20 V) (lower side = MOSFET + gate drive).

2.2.1 EMC characteristics of wireless power transformers

Complying with EMC limit requirements is not an easy task when transferring power through various wireless power applications. The challenge is that the transmit and receive coils act like a transformer with poor coupling coefficients and an extremely large air gap, which results in high stray electromagnetic fields near the coils. EMC measurements show that broadband interference can occur in the frequency range from the fundamental spectrum to 80 MHz. If the measured interference level is kept below the limit (with some margin), it can be considered that the interference field strength can also be kept below the limit. Overall, the EN55022 Class B equivalent limit can be a challenge that cannot be underestimated in development.

High-power wireless power transmission technology for industrial environment
Figure 10: Example of spectrum in interference voltage measurement (9 kHz – 30 MHz/class B limits).
High-power wireless power transmission technology for industrial environment

High-power wireless power transmission technology for industrial environment
Figure 11: Suppression measures for common-mode and differential-mode interferers.

Since E-fields (stray fields) are the main cause of EMC problems in WPT applications, appropriate measures must be taken:

A. To reduce eddy currents, a slotted metal plane (eg copper clad PCB) should be placed under the WPT coil (especially the transmitter coil) facing the circuit. The circuit must be grounded or connected to the circuit enclosure through a capacitor (eg 1-100 nF/2000 V WE-CSMH). This shorts most of the E-field to the power supply and no longer propagates through ground.

B. Shield the transmit and receive coils and their drivers with adequate metallic shielding and/or absorbing material (WE-FAS/WE-FSFS).

C. If the leakage current allows, a Y capacitor (maximum 2×4.7 nF) can be used to reduce the broadband interference level (WE-CSSA).

D. In order to filter out the common mode interference source in the low frequency range (50 kHz C 5 MHz), according to the specific operating voltage and current, the following current compensation (common mode) choke coil series can be used: WE-CMB/WE-CMBNC/ WE-UCF/WE-SL/WE-FC

E. In order to filter out the common mode interference source in the high frequency range (5 MHz C 100 MHz), according to the specific operating voltage and current, the following current compensation choke coil series can be used: WE-CMB NiZn/WE-CMBNC/WE- SL5HC/WE-SCC

F. According to the specific operating voltage, the following capacitor series that can suppress differential mode interference can be connected between +/- L/N: WE-FTXX/WE-CSGP

G. Due to the very high AC currents in the entire circuit, depending on the application, a compact low-inductance PCB layout is critical to successfully overcome EMC issues. Power stage devices and resonant circuits should be placed in close proximity and connected with low inductance using large copper areas (polygons).

High-power wireless power transmission technology for industrial environment
Figure 12: Example of a setup where a Y-capacitor to ground cannot be used due to maximum allowable leakage current limitations (eg medical devices, handheld devices, ATEX).

During development, it is often recommended to work with a qualified EMC lab to measure performance throughout the design process. Making changes after large-scale production often incurs higher costs and additional work (EM field strength limits).

2.2.2 Transmitting and receiving coils

In order to find a suitable wireless power transfer coil, the following aspects should be considered first:

• What is the expected maximum current (reactive rated current) in the coil?

§What is the maximum allowable package size (L/W/H)?

To avoid unnecessary saturation or overheating of the coil, a safety buffer of 30% should always be reserved. If more than one coil can be fitted, the coil with the highest inductance should be used so that the capacitor of the resonant circuit can be smaller. In addition, this measure also reduces the reactive currents generated in the resonant circuit. Reducing the current in the resonant circuit reduces self-heating and improves EMC characteristics.

High-power wireless power transmission technology for industrial environment

Optimal coupling is achieved when the transmit and receive coils have the same package size, so a 1:1 size ratio is recommended. WE-WPCC series devices (e.g. 760 308 102 142(53 mm x 53 mm), 760 308 100 143(ø 50 mm), 760 308 100 110(ø 50 mm)) are developed for high power applications and can be used as transmitters and receiving coils. They are characterized by a very low Rdc value, a very high Q value, and a very high saturation current IR.

2.2.3. Parallel capacitors

Due to the higher currents flowing in parallel resonant circuits, not all capacitor technologies are suitable for this project. Depending on the application, only three different types are available: MKP (eg WE-FTXX) (WE-FTBP), NP0 (eg WE-CSGP) or FKP. These capacitor types have a low loss factor and are therefore able to sustain high AC currents without overheating. However, depending on the specific power of the resonant converter, it is also very common to spread the current and self-heating by connecting multiple capacitors in parallel. Care should be taken to avoid allowing the temperature of any one capacitor to exceed 85°C. Capacitors such as X7R, X5R, MKS, etc. are not suitable for resonant converters due to their high loss coefficients (especially dielectric losses). Considering package size, cost, and the need to minimize reactive currents in the resonant circuit, the capacitor should be chosen as low as possible. The limiting factors here are the maximum operating frequency of the converter and the inductance levels of the transmit and receive coils. Voltage stability should be at least π Vin with a 20% safety margin. It must also be taken into account that the maximum allowable VACrms voltage of the MKP capacitor drops significantly below 5 kHz.

Capacitor loss factor (%): DF = 2•π•f•ESRcap•C•100

2.2.4. Filter inductors

Two filter inductors moderately separate the “AC” resonant circuit from the power supply. At the same time, they can also be used as constant current sources and filter elements. The allowable load current must be adapted to the maximum rated current of the circuit. Be sure to use classic power inductors with an air gap and high quality factor (eg WE-HCI; WE-PD; WE-LHMI). Its rated inductance should be at least 5 times higher than the inductance of the WPT coil to reload enough energy into the resonant circuit. If the input/output ripple is still too high, the value of the filter Inductor or capacitor can be increased. The ESR of the filter device can also be reduced, and the ripple can also be reduced. Flat wire power inductors (WE-HCF/WE-HCI) help keep AC and DC losses as low as possible at high currents. Since these inductors must constantly reload high AC currents to the resonant circuit, the hysteresis and eddy current losses of the core material can generate considerable self-heating. The required inductance level is directly related to the capacitance of the filter capacitor. See Section 2.2.7 for more information.

High-power wireless power transmission technology for industrial environment

2.2.5MOSFET

Choosing the right N-MOSFET mainly depends on the size of the supply voltage. For example, if you only have 5 V, you must use a logic level type in order to drive the gate reliably. Since the maximum gate voltage of most power MOSFETs is +/- 20 V, measures must be taken to protect the gate when the supply voltage is higher than 20 VDC. A grounded Zener diode or capacitive divider can be used to keep the gate voltage within the optimum range. Also, care must be taken that the gate voltage cannot be too low, or the MOSFET in the resonant converter will be stuck in linear amplifier operation, causing the circuit to latch up. This usually causes one of the two MOSFETs to overheat. In addition, care must be taken to prevent voltage overshoot higher than π times the supply voltage. For example, at 20 Vcc, the MOSFET must withstand a drain-source voltage of at least 63 V, in which case the 100 V_DS type should be used. The efficiency of the circuit depends to a large extent on the “on” resistance (Rdson) of the MOSFET and the magnitude of the gate charge (total gate charge). A trade-off must be made here because the total gate charge is generally higher when the Rdson of the MOSFET is lower.

High-power wireless power transmission technology for industrial environment

2.2.6. Diodes and pull-up resistors

Since the MOSFET must be charged relatively quickly, the charging and discharging of the gate quickly generates large currents. These charge/discharge currents must pass through pull-up resistors and diodes, and the resulting losses are not negligible. Therefore, the maximum allowable power dissipation (Pv) and current-carrying capacity of these devices must be properly selected. Likewise, the diode must have the same voltage stability as the MOSFET. In addition, the body diode of the MOSFET can also be used instead of a classical diode or Schottky. Depending on the model, they can exhibit advantageous properties at elevated temperatures, which are usually noted in the data sheet. Reverse recovery losses should also not be underestimated and must be considered.

Pull-up resistor/diode power loss: Pv = (U diode•I) + (I2•R pull-up)

2.2.7 Input and output capacitors

These capacitors are used in conjunction with power inductors, primarily as filters. When the resonant frequency is lower than 200 kHz, the capacitance must be increased accordingly. Tests have shown that the capacitance value is expected to be between 10 and 1000 μF, depending on the application and the power inductor used. The -6 dB cutoff frequency produced by the LC filter should be approximately 1/10 the frequency of the resonant circuit. Theoretically, the attenuation is expected to be 40 dB/dec. Considering parasitic component effects, the actual attenuation value should be 30 dB/dec. Depending on the filter coil used, a higher AC current component can be superimposed on the DC current. If the current is too high, aluminum polymer capacitors can be used instead of aluminum electrolytic capacitors to withstand high AC currents. Low ESR polymer and ceramic capacitors also offer the potential to significantly reduce the reflected voltage ripple amplitude. Smaller voltage ripple means lower disturbance levels in EMC disturbance measurements. Best results are achieved by using an aluminum electrolytic capacitor in parallel with a polymer or ceramic capacitor such as WCAP-PTHR/WCAP-PSLC.

High-power wireless power transmission technology for industrial environment

2.3. Disadvantages of resonant converters

In practice, this circuit topology must consider two factors to prevent a latch-up condition of the MOSFET.

1. Power supply of the transmitter when switching

If the power supply cannot provide enough current during transient oscillations of the circuit, one of the two MOSFETs may be stuck in linear amplification mode and the input voltage will be permanently shorted to ground. This can cause the MOSFET to overheat and cause permanent damage. It should also be noted that the input filter capacitor size should not be too large, otherwise this “latch-up” effect may be further exacerbated by the larger charging capacitance of the power supply.

In practice, this effect can be avoided by connecting the capacitor and resonant circuit to the operating voltage before the rest of the circuit. The gate of the MOSFET can then be switched by an optocoupler or transistor. The gate can also be driven by switching an independent voltage source delayed from the power supply, such as the Würth Elektronik MagI³C power module.

2. Reflected impedance from the receiving side to the transmitting side

Considering a large load jump on the receiving side or a sudden change in the coupling coefficient of the two coils, the reflected impedance may cause a short circuit in the part of the magnetizing inductance on the transmitting side. This, in turn, can cause the oscillations to break and put the circuit into a “latch-up” state.

High-power wireless power transmission technology for industrial environment

To deal with this, another capacitor in parallel (10-20% higher frequency than the transmitter) can be used to slightly lower the frequency of the receiver resonant circuit. Alternatively, another inductor (power inductor) can be connected in parallel to the transmit coil which is not magnetically coupled to the transmission path. This parallel inductance must be equal to or less than the magnetizing inductance of the transmit coil. This shunt inductor stores energy during ZVS, helping to maintain oscillation during adverse load transients.

High-power wireless power transmission technology for industrial environment

During the first prototype stage, all possible load cases must be tested where possible to ensure that the design is reliable and functional.

2.4.WPT coil environment optimization

If the WPT coil is fixed to metal, inductive losses may occur due to induced eddy currents caused by stray magnetic fields. Additionally, nearby metals, such as copper on a PCB, may be passively heated. Circuits may also be affected by strong stray magnetic fields. This effect increases as the WPT coil spacing increases.

Appropriate countermeasures include increasing the distance between the coil and the PCB/metal, and using a high permeability ferrite foil such as WE-FSFS. In this way, the magnetic flux is clearly controlled and not converted into heat. At the same time, the coupling coefficient can also be increased, thereby improving the efficiency. This self-adhesive flexible ferrite foil is available in a variety of sizes and thicknesses.

High-power wireless power transmission technology for industrial environment
Figure 13: WE-FSFS model 374 006 (µ’ is the real part of the loss, µ ” is the imaginary part of the loss).

3. Application example

3.1. Simple receiver circuit

High-power wireless power transmission technology for industrial environment
Figure 14: Bridge rectifier circuit using Schottky diodes and anti-ripple current aluminum polymer SMD capacitors. The output power of this receiver circuit is about 20 W, depending on the cooling surface.

3.2. Standard resonant converters (up to ~10 W for transmitter and receiver)

High-power wireless power transmission technology for industrial environment

Figure 15: Example of a simple transmitter/receiver resonator circuit for up to 10 W. Input current monitoring should be implemented for all transmitters. This prevents thermal overloading of the power FET. If the oscillation fails to start properly or fails during operation, one of the power FETs will be permanently controlled by GND, resulting in high temperature damage. Logic level FETs can only be used when the supply voltage is below 9 V.

PLEASE NOTE: USE PRECAUTIONS AND CONTACT PROTECTION FOR VOLTAGE OVER 50 VAC/120 VDC!

3.3. Modified Receiver Circuit Example (up to about 50 W)

High-power wireless power transmission technology for industrial environment

Figure 16: Replacing the filter inductor with a power Schottky diode increases the output voltage by double rectification at the receiver; output filter (C7/C8/L1) is required; to power the gate, with the help of LDO/buck conversion The voltage divider and 10 diodes can be removed from the design by generating a low voltage from the high DC voltage at L1. This circuit can only be used on the receiving side up to 50 W. If logic level FETs are not used, the gate supply voltage needs to be at least 9 V for safe and reliable transfer.

PLEASE NOTE: USE PRECAUTIONS AND CONTACT PROTECTION FOR VOLTAGE OVER 50 VAC/120 VDC!

3.4. Push-Pull Gate Control Example (Transmitter and Receiver up to ~100 W)

High-power wireless power transmission technology for industrial environment

Figure 17: Power MOSFET gates are controlled by push-pull switching of the gates instead of half-sine; this circuit can be used on both the transmit and receive sides. With the help of LDO or WE power modules (171 012 401), an auxiliary voltage of 8-10 V can be generated from the operating voltage. The input terminal must be set to overcurrent cut-off. If the oscillation fails to start properly or fails during operation, one of the power FETs will be permanently connected to GND, resulting in high temperature damage. Logic level FETs can only be used when the supply voltage is below 9 V.

PLEASE NOTE: USE PRECAUTIONS AND CONTACT PROTECTION FOR VOLTAGE OVER 50 VAC/120 VDC!

3.5. Push-Pull Gate Control Example (Transmitter and Receiver up to ~60 W)

High-power wireless power transmission technology for industrial environment

Figure 18: Power MOSFET gates are controlled by push-pull switching of the gates instead of half-sine; this circuit can be used on both the transmit and receive sides. With the help of LDO or WE power modules (171 012 401), an auxiliary voltage of 8-10 V can be generated from the operating voltage. The input terminal must be set to overcurrent cut-off. If the oscillation fails to start properly or fails during operation, one of the power FETs will be permanently connected to GND, resulting in high temperature damage. Logic level FETs can only be used when the supply voltage is below 9 V.

PLEASE NOTE: USE PRECAUTIONS AND CONTACT PROTECTION FOR VOLTAGE OVER 50 VAC/120 VDC!

3.6. Push-Pull Gate Control Example (Transmitter and Receiver up to ~30 W)

High-power wireless power transmission technology for industrial environment

Figure 19: Power MOSFET gates are controlled by push-pull switching of the gates instead of half-sine; this circuit can be used on both the transmit and receive sides. With the help of LDO or WE power modules (171 012 401), an auxiliary voltage of 8-10 V can be generated from the operating voltage. The input terminal must be set to overcurrent cut-off. If the oscillation fails to start properly or fails during operation, one of the power FETs will be permanently connected to GND, resulting in high temperature damage. Logic level FETs can only be used when the supply voltage is below 9 V.

PLEASE NOTE: USE PRECAUTIONS AND CONTACT PROTECTION FOR VOLTAGE OVER 50 VAC/120 VDC!

3.7. Dual resonant converter applications (up to 20 V/8 A)

High-power wireless power transmission technology for industrial environment
Figure 20: Transmitter and receiver about 100 W.

3.8. Resonant converter application with center tap (up to 30 W)

High-power wireless power transmission technology for industrial environment

Figure 21: Center-tapped coil resonant converter. The advantage of this circuit is that only one filter coil is required. Due to the center tap, the frequency is doubled and the voltage amplitude is reduced. This allows the use of smaller filter coils. Also, an array with two overlapping coils is easier to control. Using an LDO or a WE Magic power module (171 012 401), an auxiliary voltage of 8-10 V can be generated from the operating voltage.

PLEASE NOTE: USE PRECAUTIONS AND CONTACT PROTECTION FOR VOLTAGE OVER 50 VAC/120 VDC!

High-power wireless power transmission technology for industrial environment
Figure 22: The structure of the coil array (760 308 104 119) transmitter/receiver 3.9 application example.

4. Summary

The resonant converter is flexible enough to meet the requirements of many different applications and is currently the most efficient way to transfer hundreds of watts wirelessly. Hardware developers can extend this circuit as a base circuit if application requirements increase in terms of safety, on/off, state-of-charge monitoring, etc. Instead of the resonant converter topology, a classical H-bridge circuit with active regulation can also be used as the base circuit. In any case, EMC measurements should be performed on the first prototypes at an early stage of development.

Achieving high efficiency, the most compact package, and good EMC characteristics depends primarily on the clock circuit and the transmit and receive coils. In addition to a wide range of products, Würth Elektronik offers coils with the highest quality factor in their class, which enables higher inductance values ​​and thus reduces the package size of capacitors.

In addition, high power products use only HF stranded wire (low AC losses) and high quality ferrite material (high permeability). This means that the final product can achieve the highest efficiency and the best EMC performance.

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