WBG devices pose problems for gate driver power supplies

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.

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 V_DD 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.

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 V_DD and V_EE 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 V_GS 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.

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.