WBG devices pose problems for gate driver power supplies

Jun 14, 2024
RECOM's R24C2T25 series with blue background
Gate driver power supplies for Silicon Carbide (SiC) and Gallium Nitride (GaN) devices must address the unique bias requirements of these wide-bandgap semiconductors. This blog will discuss key considerations when designing power supplies for gate drivers in SiC and GaN applications.

Table of contents

RECOM series RxxP22005D, RA3/SMD and RxxP21503D
Fig. 1: A few of the many RECOM DC/DC modules optimized for WBG gate driver applications (source: RECOM)
SiC and GaN are wide-bandgap (WBG) semiconductor materials that offer advantages over traditional silicon (Si) in power electronics applications because they can switch faster with lower losses. They are gaining significant market share in applications where high efficiency and high power density are key considerations.

The early adopters vary by technology. SiC, the more mature technology, has largely replaced Si IGBTs for powering traction inverters in electric vehicles, and GaN has found much success in chargers for laptops and similar devices.

Gate driver requirements for WBG devices

The gate drivers for SiC and GaN transistors must be tailored to the specific characteristics of each device. For high side gate drivers, the gate driver and its DC supply must be isolated.

First-generation SiC MOSFETs typically require a +20V VDD gate drive during the on−state to provide lowest on-resistance. Since the gate turn−on threshold can be less than 2V, SiC MOSFETs drivers typically swing to a negative gate voltage during the turn-off phase for optimal switching reliability.
SiC MOSFET and GaN HEMT circuit diagrams
Fig. 2: Driver voltage comparison for SiC & GaN devices (source: RECOM)
For next generation devices, the optimal turn-on and turn-off voltages are +15 or +18V and -3 or -4V, respectively. The gate driver must be capable of very fast rise and fall times on the order of a few ns, but otherwise most gate drivers can be used with asymmetric VDD and VEE supply voltages without any problems. The power consumption of the gate driver increases with higher switching frequency, but the peak gate drive currents are supplied by capacitors placed close to the driver power supply pins, and only low power 2W to 3W DC/DC converters needed.

A GaN high electron mobility transistor (HEMT) has a typical full enhancement voltage of 7V but will be damaged if VGS exceeds 10V, which is much lower than the gate voltages required for SiC gate drivers. Due to the extremely fast rise and fall times of the HEMT structure’s low-capacitance gate channel, any excessive inductance in the external gate drive could cause spikes or voltage ringing, leading to these voltage limits being exceeded. Therefore, a 6V gate drive voltage is a good compromise between high efficiency and staying within a safe operating range. Unlike the SiC MOSFET, the low capacitance of the HEMT gate channel allows the turn-off voltage to be zero volts.

Key specifications for WBG gate driver power supplies

A power supply for SiC or GaN gate drivers must be able to supply the required voltage levels discussed above. It consists of an isolated DC/DC converter that supplies typically +20/-5V for SiC MOSFETs (+15/-3V for 2nd Gen. devices) or +6V and +9V for GaN HEMTs.

The high-side drive circuits in many SiC and GaN applications are often floated by hundreds of volts above ground, requiring galvanic isolation for the drivers and their associated circuits. Optocouplers provide isolation for the control signal, but there is a need for isolation for the power supply.

The simplest (functional) isolation of a DC/DC converter can withstand 1kVDC for one second but this is often not sufficient for the high-side gate drivers in a bridge configuration. The DC/DC isolation voltage should be at least twice the working voltage, but the high ambient temperature and fast switching generated by high-power SiC and GaN transistors impose additional stresses on the insulation barrier. As a result, highly isolated DC/DC converters are needed to power the gate drivers.

RECOM solutions for SiC and GaN gate drivers

RECOM’s DC/DC converters for gate drivers are drop-in modules for that provide a reliable, long-life solution for gate drivers in WBG applications. We offer a range of isolated DC/DC converter modules that provide a simple power solution for SiC MOSFET or GaN HEMT power devices. Features of these converters include asymmetric output voltages, high insulation voltage, and low isolation capacitance.

What are the optimum RECOM DC/DC converter families for SiC and GaN drive technologies?

SiC MOSFETs

The RxxP22005D and RKZ-xx2005D series feature asymmetric outputs of +20V and -5V to switch the SiC MOSFET efficiently and effectively.

The RxxP21503D series provides asymmetric output voltages of +15 and -3V, which are needed to efficiently switch second generation SiC MOSFETs.

GaN HEMTs

High slew-rate GaN transistor drivers reach optimal performance switching at +6V from RECOM’s DC/DC converters RP-xx06S and RxxP06S series featuring high isolation voltage and low isolation capacitance. In GaN applications where higher noise and interference must be considered, RECOM also offers converters with +9V output which can be split up via a Zener diode to +6V and -3V to provide a negative gate voltage on turn-off ensuring that the gate voltage stays below the turn-on threshold.

RA3 Series is optimized for gate drivers

RECOM’s RA3 family of unregulated 3-watt DC/DC converters is especially designed to power transistor gate drivers. The modules are available with input voltages of 5, 12, or 24VDC with single or dual asymmetric outputs to cover the latest Si, SiC, and GaN transistors.

The compact SMD design ensures that required board space is minimal - especially on multilayer PCBs. The modules offer a robust 5.2kVDC/1min isolation and an isolation capacitance less than 10pF. With typical efficiencies from 78% to 82%, the operating temperature range of -40C to +85°C at full load meets harsh environmental requirements for solar inverters as well as induction heating, telecom, EV battery chargers, and motor drives.

Futureproofing WBG designs

Semiconductor gate drive voltage ranges
Fig. 3: Recommended and absolute maximum driver voltage comparison for different IGBT, SiC & GaN devices (source: RECOM)
The optimum gate driver supply voltage combination varies according to the transistor type (IGBT, SiC or GaN), the manufacturer, the technology generation and whether a cascode configuration is used or not (Figure 3). This has caused some concern in the electronics industry. How can I futureproof my design so that future generations or alternate source transistor suppliers with different optimal turn-on and turn-off voltages can be easily accommodated?

The solution to this dilemma is RECOM’s new R24C2T25 converter. This is an isolated SMD DC/DC converter in an IC-style SSOP package specially designed to power isolated WBG gate drivers. The regulated positive and negative output voltages can be independently adjusted in the range from +2.5V to +22.5V / -2.5V to -22.5V using preset resistors, so all existing gate driver voltages for IGBT, MOSFET, SiC (all generations) or even GaN can be covered with just one part (Figure 4).

The output is Basic Grade isolated to 3kVrms/1 min. and will deliver 1.5W over the full operating temperature range of -40°C to +105°C without derating (2W up to +85°C). The CMTI (Common Mode Transient Immunity) value exceeds 150kV/µs, so the power supply can be used with very fast switching edges. The part uses an SMD SSOP package that can be mounted close to the switching transistors and the outputs are fully protected (UVLO, OTP, SCP, OLP) for worry-free installation.
RECOM R24C2T25 schematic and product

Fig. 4: R24C2T25 adjustable asymmetric output voltage gate driver power supply

Conclusion

Wide-bandgap transistors provide unique advantages in high-power applications, but they pose challenges to designers of SiC and GaN gate driver power supplies. The standard isolation level of a DC/DC converter is not sufficient for these circuits, so specialized solutions are needed. RECOM combines the required high insulation voltage, asymmetric output voltages, and low isolation capacitance into several families of DC/DC modules that are optimized for ease of use in SiC and GaN gate drive designs.
Applications
  Series
1 DC/DC, 2.5 W, Single/dual Output, SMD R24C2T25 Series
Focus
  • 2W isolated DC/DC converter
  • Programmable asymmetrical output voltages
  • Ideal for IGBT/Si/SiC/GaN gate drive bias voltages
  • High 3kVAC/1min isolation
2 DC/DC, 3.0 W, SMD (pinless) RA3 Series
Focus
  • 3W isolated DC/DC converter
  • High 5.2kVDC/1min isolation
  • Wide operating temperature range: -40°C to +85°C
  • Ideal for IGBT/Si/SiC/GaN gate drive power
3 DC/DC, 2.0 W, Dual Output, THT RKZ-xx2005 Series
  • Power sharing
  • High isolation 3kVDC & 4kVDC for 1 second
  • Efficiency up to 87%
  • Wide operating temperature range from -40°C to +85°C
4 DC/DC, 1.0 W, Single Output, THT RP-xx06 Series
  • 6V Output for GaN driver applications
  • Pot-Core transformer with separated windings
  • High 5.2kVDC isolation In compact Size
  • Low isolation capacitance (10pF max.)
5 DC/DC, 1.0 W, Single Output, THT RxxP06 Series
  • 6V Output for GaN driver Applications
  • Pot-Core Transformer with separated windings
  • High 5.2kVDC Isolation in compact size
  • Low isolation capacitance (10pF max.)
6 DC/DC, 2.0 W, Dual Output, THT RxxP21503 Series
  • +20/-5V & +15/-3V asymmetric outputs for SiC driver applications
  • Qualified with 65kV/µs @ Vcommon mode =1KV
  • +15/-9V asymmetric outputs for IGBT driver applications
  • Pot-core transformer with separated windings
7 DC/DC, 2.0 W, Dual Output, THT RxxP22005 Series
  • +20/-5V & +15/-3V asymmetric outputs for SiC driver applications
  • Qualified with 65kV/µs @ Vcommon mode =1KV
  • +15/-9V asymmetric outputs for IGBT driver applications
  • Pot-core transformer with separated windings