High-power wireless power transmission technology for industrial environments

1.Introduction

As wireless power transmission becomes more and more popular in consumer electronics, the focus is shifting to the technology and its inherent advantages in the industrial and medical sectors. As communication interfaces become increasingly wireless, driven by wireless technologies such as WLAN and Bluetooth, wireless power transmission is becoming a corresponding option. The adoption of 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 that need to resist harsh environments such as aggressive cleaning agents, severe pollution and high mechanical stress, such as ATEX, medicine, construction machinery, etc. For example, it can replace expensive and fragile collector rings or contacts. Another application area is transformers that must meet special requirements such as reinforced or double insulation.

This application note aims to demonstrate that wireless power transfer solutions of hundreds of watts or more can be easily realized using circuit techniques without the need for software or controllers.

Figure 1: Würth Elektronik wireless charging coil.

2.ZVS Oscillator (Differential Mode Resonant Converter)

In this application note a classic resonant converter is used as the clock circuit.

This oscillator offers many advantages:

• Independent oscillation, only one DC power supply required

• The current and voltage curves are very close to sinusoidal curves

• No active components or software required

• Flexible and scalable from 1 W to 200 W

• MOSFET switching near zero crossing

• Scalable to accommodate a variety of voltages/currents

2.1.Basic Circuit/Schematic:

Figure 2: Basic resonant converter circuit.

The basic circuit shown in Figure 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 will be slightly more conductive than the other. The positive feedback of the two MOSFET gates and the opposite drain of the less conductive MOSFET creates 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 ends of the parallel resonant circuit, charging the resonant circuit periodically.

Another feature of this circuit topology is that the voltage is always close to the zero crossing, 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 leads to relatively high power consumption in the idle state. Therefore, ideally, the resonant converter should only be operated under load. It should also be considered that the frequency of the resonant circuit changes with the coupling factor on the receiving side. This is caused by the reflected impedance on the receiving side affecting the magnetizing inductance on the transmitting side, because the two sides are connected in parallel. As the magnetizing inductance on the transmitting side decreases, the coupling factor 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 components used. When the input voltage is above 20 V, contact protection must be observed because the voltage in the resonant circuit is already π times or more above the SELV (Safety Rated Low Voltage) threshold of 50 VAC/120 VDC.

Figure 3: The voltage across the transmitter coil is shown in blue and red. The gate voltage is shown in yellow and green.

(These voltage curves were measured with reference to circuit ground GND; Vin = 20 V; Pout = 100 W; optimized gate drive, see Application Examples)

In practice, the efficiency of the entire wireless power transfer circuit can exceed 90%. This is very rare, because the coupling losses through the air gap are already taken into account and a stable DC voltage is available at the input. The efficiency remains stable for air gaps in the range of 4-10 mm. A large part of the energy in the magnetic field that is not coupled to the receiving side returns to the "resonant tank". Depending on the specific application, the maximum distance can be 18 mm, but there will be some sacrifices in coupling coefficient and EMC.

The same circuitry from the transmit side can be used on the receive side, with the resonant converter acting as a synchronous rectifier. It is important to consider here that the resonant frequency on the receive side should be very close to the resonant frequency on the transmit side. This also results in a maximum "snubber circuit effect". The parallel connection of C and L means that the secondary side acts as a constant current source similar to the load, which can significantly increase the overall efficiency of the circuit. In addition, the capacitor also compensates for the stray inductance of the wireless power coil. If the circuit is built properly (i.e. ...), the receiver can feed energy back to the transmitter (i.e. with Linear Technology's "ideal" diode at the load).

Figure 4: Transmitter coil voltage (not referenced to circuit GND; Vin = 20 V/Pout = 100 W).

Figure 5: Reflected ripple and noise of the input supply on the transmitter side (Vin = 20 V/Pout = 100 W)  Low ESR polymer and ceramic capacitors can be used to reduce voltage ripple.

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

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

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

The load frequency should generally not exceed 150 kHz, otherwise the losses in the parallel capacitors, transmitting and receiving coils will be too high. In addition, the EMC limits below 150 kHz are also higher (e.g. CISPR15 EN55015 9 kHz - 30 MHz). 105-140 kHz is the best frequency range obtained by weighing all the tests conducted so far. Based on the currently approved inductive power transmission frequency band (100-205 kHz), this frequency range ensures that you are in a safe frequency range.

If the final product will be marketed in multiple countries, the regulations in each country as well as the permitted frequency bands should be determined in advance to shorten the development phase.

Figure 6: 6.5 mm air gap measurement circuit (Vin = 20 VDC; Pout = 100 W).

Figure 7: 6.5 mm air gap measurement circuit (Vin = 20 VDC; Pout = 100 W).

Figure 8: Temperature rise of the circuit/coil for Pout = 100 W (Vin = 20 V) (upper side = filter + capacitor).

Figure 9: Temperature rise of the circuit/coil for Pout = 100 W (Vin = 20 V) (lower side = MOSFET + gate drive).

2.2.1EMC characteristics of wireless power transformers

Complying with EMC limits when transmitting power via various wireless power applications is not an easy task. The challenge is that the transmit and receive coils act like a transformer with a poor coupling coefficient and a large air gap, which results in high stray electromagnetic fields in the vicinity of the coils. EMC measurements have shown 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 a certain margin), it can be assumed that the interference field strength will also be kept below the limit. Overall, limits such as EN55022 Class B can be a challenge in development that should not be underestimated.

Figure 10: Example of frequency spectrum in interference voltage measurement (9 kHz - 30 MHz/Class B limit).

Figure 11: Suppression measures for common-mode and differential-mode interference sources.

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 (e.g. copper-clad PCB) facing the circuit should be placed under the WPT coil (especially the transmitting coil). The circuit must be grounded or connected to the circuit housing via a capacitor (e.g. 1-100 nF/2000 V WE-CSMH). This short-circuits most of the E field to the power supply and no longer propagates through the ground.

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

C.If leakage current allows, Y capacitors (max 2x4.7 nF) can be used to reduce broadband interference levels (WE-CSSA)

.

D.To filter out common mode interference sources in the low frequency range (50 kHz – 5 MHz), the following current compensated (common mode) choke series can be used, depending on the operating voltage and current: WE-CMB/WE-CMBNC/WE-UCF/WE-SL/WE-FC

E.To filter out common mode interference sources in the high frequency range (5 MHz – 100 MHz), the following current compensated choke series can be used, depending on the operating voltage and current: WE-CMB NiZn / WE-CMBNC / WE-SL5HC / WE-SCC

F.Depending on the operating voltage, the following series of capacitors that can suppress differential mode interference can be connected between +/- L/N: WE-FTXX/WE-CSGP

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