Infineon Introduces First GaN Power FET to Integrate Schottky Diode

What you'll learn:

• GaN: Fast times in high-frequency power electronics.
• Understanding the bidirectional current capabilities of GaN.
• How Infineon’s integrated Schottky saves power.

Gallium nitride (GaN) is fast becoming a foundational technology in power electronics, offering higher power density and better efficiency than traditional silicon. By bringing faster switching speeds, reduced output charge, and lower conduction losses to the table, the wide-bandgap (WBG) power semiconductor is displacing silicon in everything from smartphone fast chargers to EV onboard chargers to data center power supply units (PSUs).

Infineon Technologies tries to push the envelope further with what it claims is the first GaN power transistor with an integrated Schottky diode. Part of its CoolGaN G5 family, the company said these industrial power devices can reduce undesired dead-time losses in hard-switching power systems. The result, according to Antoine Jalabert, VP of Infineon’s medium-voltage GaN products, is a significant leap in system power efficiency.

While GaN power FETs are already physically smaller than silicon MOSFETs, this new integrated solution further simplifies power system design and lowers cost compared to using an external diode.

GaN: Fast Times in High-Frequency Power Electronics

GaN power FETs have a wide range of advantages over silicon when it comes to power conversion.

In MOSFETs, the total gate capacitance—including the capacitance between gate and source (C_GS) and between gate and drain (C_GD)—will impact the switching speed at the gate of the power device, slowing it down and reducing its efficiency. The gate capacitance of a GaN power FET is generally about 10X less than silicon. Thus, less charge is required to flow into the device to turn it on or off. That, in turn, reduces the gate charge (Q_G) needed for each switching event, resulting in higher efficiency.

GaN power FETs also have a relatively high mobility of electrons in the channel—the area under the device’s gate through which current flows between source and drain. This characteristic means that power electronics based on GaN can run efficiently at frequencies of more than 1 MHz as opposed to about 100 kHz for silicon. GaN power FETs also have minimal on-state resistance (R_DS(on)), which cuts down on conduction losses.

The higher efficiency of GaN power devices equates to smaller heatsinks in power electronics, while the ability to operate at high frequencies means inductors, capacitors, and other passives can be very small.

These power devices also stand out for their reduced output capacitor charge (Q_OSS). It plays a vital role in facilitating zero-voltage switching (ZVS), particularly in line-commutated converters (LCCs) and other soft-switching DC-DC converters.

Another advantage of GaN power devices lies in their lack of internal body diodes. Without these components, the devices can avoid reverse-recovery losses, enabling very fast switching speeds compared to silicon MOSFETs. This leads to more efficient operation in half-bridges and other hard-switching power topologies, such as the totem-pole power-factor-correction (PFC) stage of a data center power supply, which often runs in continuous conduction mode (CCM) when handling kilowatts of power.

Understanding the Bidirectional Current Capabilities of GaN

One of the fundamental characteristics of a power MOSFET is its ability to conduct current in both directions. Bidirectional current flow is largely due to the body diode that resides between the source and drain of the power device.

In normal operation, current flows from the drain to the source when the gate of the power transistor is completely turned off, applying a positive voltage to the drain. This is also called “first quadrant” operation in the parlance of the power electronics industry.

However, when the gate of the transistor is completely turned off and the drain voltage becomes negative relative to the source, current flows in the other direction through the body diode, enabling current flow from source to drain. This is referred to as “third quadrant” operation.

Bidirectional current flow is crucial in areas like synchronous rectification (SR), a technique that replaces rectifier diodes with power MOSFETs to improve the efficiency and performance of DC-DC buck converters.

One of the main drawbacks relates to the reverse recovery of the power MOSFET. When the power transistor changes direction, reverse current continues to flow unintentionally while the charge that’s been building up inside the body diode is depleted. The problem is it takes some time—also called the reverse-recovery time (T_rr)—for the diode inside the MOSFET to do that. This interval is tied to the reverse recovery charge (Q_rr).

This delay negatively impacts switching speeds and increases power losses along with the resultant heat that’s generated. These issues are inevitably more prevalent at high switching frequencies, such as those used in modern DC-DC converters.

GaN power FETs avoid this issue entirely. By eliminating the body diode, GaN power devices effectively eliminate reverse-recovery losses. By switching significantly faster than silicon MOSFETs, GaN can also reduce ringing and EMI.

Despite lacking a traditional body diode, GaN can conduct current in both directions. In contrast to silicon MOSFETs, GaN power FETs are lateral devices, comprised of a source and drain connected by a symmetrical channel (for more details, see “How Enhancement-Mode GaN Transistors Work”).

In reverse conduction, source and drain effectively switch roles due to the symmetry of the device. The drain voltage is less than the gate turning on the power transistor, enabling reverse current flow without the need for a separate diode.

The Diode Inside: How Infineon’s Integrated Schottky Saves Power

There are some tradeoffs to this architecture, which Infineon is trying to address with its latest GaN power transistor.

In hard-switching topologies, GaN FETs can experience higher power losses due to the larger effective body diode voltage (V_DS) of the devices. Reducing these power losses requires tight control of dead times. Dead times are the delays between turning off one MOSFET and turning on the other in a half-bridge or other hard-switching DC-DC converter. Keeping these dead times as short as possible is key to improving the system’s performance.

Typically, power engineers reduce these dead times by placing a separate Schottky diode in parallel with the GaN power FET. However, this solution can add complexity, time, and cost to the development process. The alternative is to upgrade the power supply’s controller to tighten the dead times or even do adaptive dead-time control, which reduces power loss by adjusting the dead time based on the system’s operating conditions.

Infineon said it can reduce undesirable dead times by integrating the Schottky diode directly into the power device. The integrated solution is appropriate for use in DC-DC converters, synchronous rectifiers for USB-C battery chargers, IBCs for servers and telecommunications, data center PSUs, and motor drivers. In addition to reducing complexity, cost, and heat, Infineon said the solution also gives engineers more flexibility when choosing a controller for the power supply.

Due to the lack of a body diode, the reverse-conduction voltage (V_RC) of the GaN power device depends on several factors: the threshold voltage (V_TH) of the GaN power device—the turn-on voltage of the transistor—and the gate bias voltage (V_GS) when the transistor is turned off. Moreover, the turn-on voltage of a GaN power FET tends to be higher than the turn-on voltage of a silicon diode, leading to a disadvantage during reverse-conduction operation, according to Infineon.

By integrating the diode into the GaN transistor, the company said it can reduce these reverse-conduction losses. The diminished dead times also help ensure compatibility with a wider range of high-side gate drivers.

The first device in the new family is a 100-V, 1.5-mΩ CoolGaN device housed in a 3- × 5-mm PQFN package.