How GaN can boost wireless power transfer

in English

Gallium Nitride (GaN) is transforming the semiconductor industry, particularly in wireless power transfer (WPT) systems.

Originally used in LEDs and satellite solar cell arrays, GaN is now making significant advances in charger technology due to its superior heat management. The reduced heat generation allows for more compact charger designs without compromising power or safety. As silicon technology reaches its physical limits, GaN stands out for its ability to handle higher electric fields, conduct electrons more efficiently, and operate under extreme conditions. This makes GaN a key material for the next generation of power devices. Its high-power density, better heat dissipation, and higher frequency capabilities result in smaller, more efficient, cost-effective solutions. These advantages are crucial for meeting the demands of advanced electronics, 5G base stations, and various power conversion and RF applications, making GaN a cornerstone in the evolution of wireless power transfer.

Principles of wireless power transfer

Wireless charging technology relies on two main principles: magnetic induction and magnetic resonance. Magnetic induction, the most common wireless charging method, uses coils to generate an electromagnetic field that induces current in the receiver coil, charging the battery. Standardized by Qi, it operates at 100-300 kHz and requires precise alignment and proximity between the charging pad and the device. Magnetic resonance, in contrast, uses multiple coils to create a resonant electromagnetic field. This method enables charging over greater distances and through surfaces like tables or backpacks. Championed by the AirFuel Alliance, it operates at 6.78 MHz and allows simultaneous charging of multiple devices in any orientation.

Figure 1: Wireless power transfer system blocks (Source)

GaN (Gallium Nitride) technology and why it is preferred over Si and SiC (Silicon Carbide) in WPT (Wireless Power Transfer)

Gallium Nitride (GaN), a wide-band gap (WBG) semiconductor, is transforming power electronics with its superior electrical properties. GaN power devices outperform traditional silicon-based devices in efficiency, managing higher voltages, switching frequencies, and temperatures, outperforming traditional silicon-based devices in efficiency. In wireless charging, GaN power devices offer several benefits:

Wide bandgap: Enables higher electric field handling.
Higher power densities and power transfer efficiency: Allows for faster charging times.
Higher switching frequencies: Reduces the need for bulky capacitors and inductors, making charging pads smaller and more portable.
Lower driving voltage: This leads to lower gate charge loss.
Reduced heat generation: Enhances energy efficiency and minimizes heat dissipation, protecting both devices and charging pads.
Cost-effectiveness: More economical to manufacture and more reliable than Silicon, lowering repair and replacement costs.

Wide bandgap technology:

Gallium nitride (GaN), with a 3.4 eV bandgap compared to Silicon’s 1.1eV, handles higher electric fields and operates at higher temperatures. This reduces on-state resistance and improves performance in small signal and high-power applications. GaN also enhances the figure of merit (FOM) for power MOSFETs, leading to higher efficiency in high-frequency DC-DC converters compared to Silicon.

Property	Si	4H-SiC	GaN
Bandgap (eV)	1.12	3.2	3.4
Critical field Ecr (MV/cm)	0.25	3	4
Dielectric constant ε	11.8	9.7	9.5
Saturation velocity (10⁷ cm/s)	1	2	3
Electron mobility μ (cm²/Vs)	1350	800	2000
Thermal conductivity k (W/cm K)	1.5	4.9	1.3

Table 1: Si vs. SiC vs. GaN (physical and electronic comparison)

Figure 2: FOM comparison between gallium nitride (grey) and silicon technology (different vendors; green, pink, cyan) (Source)

Learn more about Wide-Bandgap semiconductors from click here.

Higher power transfer efficiency:

GaN High Electron Mobility Transistors (HEMTs) significantly improve wireless power transfer (WPT) efficiency. Conventional Qi wireless charging operates between 80 and 300 kHz for medium power applications and between 110 and 205 kHz for low power levels (up to 5 W). The effectiveness of WPT inversely correlates with the Quality factors (Q1, Q2) of transmitting and receiving coils at a given separation distance, which are maximized in the 5–15 MHz range. GaN HEMTs' high frequency switching capabilities enable highly efficient WPT with several benefits:

Higher power transfer: Transfer power levels exceed 1000 W, far surpassing the traditional 15 W at kHz frequencies.
Increased spatial freedom: Allows for greater flexibility in the positioning of devices in all three axes.
Reduced Weight: Thinner PCB antenna and transmit/receive components, reducing weight for EV applications like in-cabin charging.
Larger Z-Axis Separation: Allows for easier placement of transmitters under surfaces and behind walls, simplifying installation for security cameras and in-vehicle charging boxes.
Enhanced Safety: It avoids heating metal objects, enhancing safety.

Figure 3: The trend for the energy stored in the output capacitance across three consecutive generations of superjunction devices compared to GaN HEMTs (Source)

As shown in Figure 3, low-voltage GaN's output capacitance is lower, but stored energy is similar to that of Superjunction devices. This energy is lost as heat during hard switching, making half-bridge circuits ideal for GaN while single-ended topologies may be less advantageous.

GaN devices provide fast switching, low resistance, and efficient current handling and are packaged for optimized PCB layouts. They feature robust common-mode voltage rejection, active Miller clamping, negative off-state driving voltage to prevent induced turn-on, and tight propagation delay tolerances for shorter lead times and enhanced performance.

𝑷𝑫𝑻 = (𝑽𝑺𝑫 ∗ 𝑰𝑶𝑼𝑻 ∗ 𝒇𝑺𝑾 ∗ 𝑻𝑫𝑻)

Learn more about enhancing industrial energy efficiency with GaN click here.

Figure 4: Structure of a HEMT GaN transistor (Source)

Higher switching frequency:

GaN has superior material properties, such as high carrier mobility, low on-resistance, and low parasitic capacitance. These properties allow GaN to operate at much higher switching frequencies than Silicon, reaching up to tens of megahertz. This enables power electronics for designers to create smaller, more efficient, and higher-performing systems. This reduces the size and weight of circuit components and improves overall efficiency in applications such as electric vehicles, satellites, and consumer electronics.

In power electronics and audio amplifiers, GaN's high switching frequency reduces distortion and noise, improves audio quality, and minimizes switching losses, resulting in increased efficiency and reduced heat generation. The reduced switching losses also result in longer battery life for portable devices and significant energy savings in data centres. Additionally, GaN's fast switching reduces heat generation and simplifies thermal management. Its capability to lower EMI levels helps meet regulatory requirements, making GaN an ideal solution for advanced power electronics.

Lower driving voltage leads to lower gate charge loss:

GaN devices operate with a 5V gate drive voltage, compared to 10V for standard silicon MOSFETs. Their gate charge (QG) is about one-fifth that of MOSFETs with similar RDSon (drain-to-source on-state resistance) and VBRR( breakdown voltage). This results in a much lower gate drive current and reduced losses in the gate driver IC. To minimize gate charge loss, selecting low QG devices with a low gate threshold voltage is ideal. This enables the use of lower driving voltages, further reducing overall driving circuitry losses. The gate charge losses can be calculated as:

𝑷𝑮𝑨𝑻𝑬 = (𝑸𝐆_𝐒𝐘𝐍𝐂 ∗ 𝒇𝑺𝑾 ∗ 𝑽𝒅𝒓)

The QG_SYNC is the gate charge at voltage Vdr without the QGD (since it is assumed a ZVS transition), fSW is the switching frequency, and Vdr is the driving voltage.

Better thermal performance:

GaN devices significantly enhance heat dissipation due to their unique properties. Traditional MOSFET faces challenges with heat concentration near the gate region, leading to high temperatures and reduced performance. Introducing GaN micropits makes heat more evenly distributed, lowering peak temperatures and improving thermal management. This structural innovation allows heat to spread along the device length rather than concentrating in one area, resulting in better performance and higher current density. Consequently, devices with GaN micropits demonstrate significantly lower peak channel temperatures and enhanced electrical characteristics, making them highly efficient for high-power applications.

GaN-Based Wireless Power Transfer for 5G Applications

GaN technology is revolutionizing wireless power transfer (WPT) for 5G applications by meeting high power density, efficiency, and thermal management demands. 5G networks, which offer speeds up to 20 times faster than 4G with significantly reduced latency, rely on high-performance power semiconductors. GaN FETs excel in these environments, providing higher power density and better heat dissipation than silicon-based devices. This enables more compact designs, reduced costs, and extended base station coverage.

GaN's high power density and low thermal resistance enable power amplifiers to operate at higher temperatures with less reliance on heatsinks, thus minimizing network power consumption and enhancing efficiency. GaN devices can achieve up to 100 GHz operating frequencies, making them ideal for 5 G's broad bandwidth and massive MIMO systems.

Optimizing Coupling Coils for WPT

A high coupling factor (Q) is crucial to optimizing coupling coils. The transmitter coil's Q must be large enough to transfer significant power through a wall. According to GaN Systems' calculations, a 200 × 200 mm coil can transfer power over a 250 mm distance. Engineers used a Class EF2 amplifier and a T-type and PI-type impedance matching mix. For balanced thermal performance, the 200 × 200 mm Tx coil has five turns, a 4 mm track width, and 3 mm spacing. Both Tx and Rx coils are identical in size.

Efficiency Calculation

The figure of merit (FOM), or U, is calculated as:

k√(Q1 × Q2); with Q1 = Q2, a 200 mm gap achieves over 87% efficiency when U is 14, and Tx/Rx impedances are around 30 Ω. The efficiency is calculated using this equation-

Future trends

As 5G technology evolves, GaN continues to replace older LDMOS devices in telecommunications, radar, and IoT applications, setting new standards in performance and efficiency. To learn more about Power GaN FETs fro reduced power loss in 5G.

GaN in DC/DC Converters

GaN HEMTs offer significant advantages over silicon FETs, including smaller gate charges, faster switching, and no reverse recovery loss. These characteristics make them ideal for switching-mode power supplies. In this design, GaN power HEMTs and a GaN half-bridge driver are used to create a high-efficiency multi-MHz synchronous buck converter. This converter achieves a minimum on time of three nanoseconds and can operate at frequencies up to 50 MHz. It also includes features such as undervoltage lockout (UVLO) and overtemperature protection.

This design enhances power density and efficiency while providing a wide control bandwidth, making it perfect for space-constrained, fast-response applications. Examples include high-speed synchronous buck converters, 5G telecom power, 48-to-point-of-load (POL) server power, industrial power supplies, Class D audio amplifiers, and envelope tracking.

Figure 5: Block Diagram of the Synchronous Buck Converter (Source)

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Conclusion

Gallium Nitride (GaN) semiconductors are a promising solution for enhancing the performance of wireless power transfer systems. GaN technology enables high-frequency operation, improves power conversion efficiency, and reduces power losses. It can also significantly improve the capabilities of wireless charging solutions.

Incorporating GaN devices into wireless power transfer systems results in higher power transfer efficiency, increased range, and greater power transfer capabilities. This makes them a versatile charging solution across various industries.