Why the Industrial Industry Is Embracing High Power Wireless Power Transfer

As the presence of wireless power transfer technology increases in consumer electronics, the industrial and medical industries are shifting focus toward this technology and its inherent advantages.

Read on to learn why the industrial environment is embracing high power wireless power transfer and why we recommend using a classical resonant converter as the clocking circuit for these applications.

Introduction to High Power Wireless Power Transfer for Industrial Applications

As communication interfaces are becoming increasingly wireless with technologies like WLAN and Bluetooth, wireless power transfer has become a relevant option. Completely new approaches can be taken that not only offer obvious technical advantages, but also open up possibilities for new industrial design.

This technology offers new concepts — especially in industrial sectors struggling with tough environmental conditions, aggressive cleaning agents, heavy soiling, and high mechanical stresses (e.g. ATEX, medicine, construction machines). For instance, expensive and susceptible slip rings or contacts can be substituted. Another field of application is transformers, which have to satisfy special requirements, such as reinforced or doubled insulation.

In this blog series, we want to demonstrate that easy-to-achieve solutions for wireless power transfer of one hundred Watts or more are reachable using circuit technology — without the need of software or controllers

Benefits of the ZVS Oscillator (Differential Mode Resonant Converter)

At Würth Elektronik, we recommend using a classical resonant converter as the clocking circuit in high power wireless power transformers for industrial applications.

This oscillator offers multiple benefits:

  • It oscillates independently and only requires a DC source.
  • The current and voltage profile is almost sinusoidal.
  • No active components and no software are needed.
  • It is scalable from 1 W – 200 W The MOSFETs switch close to the zero crossover point.
  • It is scalable for many different voltages/currents.

Functionality of the Wireless Power Resonant Converter

The resonant converter usually operates at a constant working frequency, which is determined by the resonant frequency of the LC parallel resonant circuit. As soon as a DC voltage is applied to the circuit, it starts to oscillate based on the MOSFETs component tolerances. Within a fraction of a second, one of the two MOSFETs is slightly more conductive than the other. The positive feedback of the two MOSFET gates and the opposite drain of the less conductive MOSFET gives rise to a 180° phase shift.

The two MOSFETs are therefore always driven out of phase and can never conduct simultaneously. The MOSFETs alternately connect both ends of the parallel resonant circuit to ground allowing the resonant circuit to be periodically recharged with energy.

Another feature of this circuit topology is that the voltage always switches close to the zero crossover point, meaning the switching losses in the MOSFETs are very low. The disadvantage of this switching topology is that the power consumption in the idle state is relatively high due to the reactive currents circulating in the resonant circuit.

For this reason, the resonant converter should ideally only be operated with a load. It should be considered that the frequency of the resonant circuit changes with the coupling factor of the receiver side. This arises due to the reflected impedance from the receiver side, which influences the magnetizing inductance of the transmitter side, as both sides are in parallel. A decreasing coupling factor causes the frequency to rise, as the magnetizing inductance of the transmitter side drops.

Efficiency of the Wireless Power Resonant Converter

The efficiency of the entire wireless power transfer circuit may exceed 90% in practice. This is quite remarkable as the coupling losses via the air gap are already included and a steady DC voltage is available at the input. The efficiency remains stable within an air gap range of 4-10 mm. A large share of the energy in the magnetic field, which is not coupled to the receiver side, is returned to the “tank circuit”. A maximum distance of up to 18 mm is possible depending on the application however, concessions are made in terms of coupling factor and EMC. The circuit on the transmitter side can be used identically for the receiver side.

The resonant converter then works as a synchronous rectifier. Here it needs to be considered that the resonant frequency of the receiver side should be very closely matched to that of the transmitter side. This also generates a maximum “absorption circuit effect”. The parallel connection of C and L means that the secondary side behaves like a constant current source for the load. This allows the overall efficiency of the circuit to be raised significantly. In addition, the capacitor compensates the stray inductance of the wireless powercoil. If the circuit is prepared properly, the receiver can feed energy back to the transmitter (i.e. “Ideal” diode from Linear Technology at the load).

The efficiency can be raised by using smaller MOSFETs rather than Schottky diodes for driving the gate or by using a bipolar push-pull stage (see application examples).

For supply voltages over 20 V, a capacitive voltage divider can be used to drive the MOSFET gates or a DC/DC converter (like the highly efficient and compact Würth Elektronik MagI³C Power Module) as an auxiliary voltage source (see application examples in section 3).

Likewise, on the receiver side, instead of a resonant converter, a classical bridge rectifier can also be used. The advantage is a higher output voltage, lower costs and space savings at the cost of efficiency, due to diode losses.

The frequency under load should generally not exceed 150 kHz, otherwise the losses in the Parallel capacitors, transmitter and receivers coils become too high. Additionally, the EMC limit values are higher beneath150 kHz (e.g. CISPR15 EN55015 9 kHz - 30 MHz). The frequency range 105 – 140 kHz emerged as the best compromise in tests carried out so far. This range also ensures that you remain in a safe range according to the currently approved frequency band for inductive power transfer (100 – 205 kHz).

If the end product will be launched in several countries, the regulations and permissible frequency bands should be ascertained for each country beforehand to speed up the development phase.

With high power wireless power transformers being used more frequently in industrial applications, we recommend using a classical resonant converter like the ZVS Oscillator as the clocking circuit for these applications.

If you have any questions or would like us to point you in the right direction for your specific application, please contact us, or view our ANP032 offerings for wireless power transfer for the industrial environment!

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