Silicon Carbide Switches and their drivers
The power electronics industry is looking for a SiC switch solution to replace the ubiquitous Silicon (Si) IGBT in many motor drive and industrial automation applications. After many years of research and development, SiC-based switches are rapidly becoming commercially available. These include devices from many major switch families- SiC Junction Transistors (SJTs); Power MOSFETs – including planar DMOSFET as well as trench-MOSFETs; JFETs – including normally-ON, and normally OFF.
The power electronics industry is looking for a SiC switch solution to replace the ubiquitous Silicon (Si) IGBT in many motor drive and industrial automation applications. After many years of research and development, SiC-based switches are rapidly becoming commercially available. These include devices from many major switch families- SiC Junction Transistors (SJTs); Power MOSFETs – including planar DMOSFET as well as trench-MOSFETs; JFETs – including normally-ON, and normally OFF.
Unlike Silicon, where IGBTs offered much superior drive and switching performance and drive as compared to BJTs, and much higher current carrying capability than MOSFETs, it may not be obvious which of the new SiC device families may offer the best performance, circuit efficiency and usability. This is because SiC material properties offer a different set of advantages and disadvantages as compared to Silicon devices. For example, unlike Si BJTs and IGBTs, SiC Junction Transistors are completely free of any minority carriers in the Drain region, making them operate at very high frequencies like majority carrier devices, and are completely free of dynamic breakdown (reverse bias safe operating area, RBSOA) issues. Additionally, contemporary SiC MOSFETs suffer from much poorer channel mobilities (10-20X) as compared to Silicon MOS-devices. Further, realizing normally-OFF SiC JFETs has proven to be an extremely difficult endeavor from a manufacturing standpoint because high dopings are typically in their Drain regions.
Most motor control and power supply applications presently use voltage controlled drivers due to the dominance of Si IGBTs in these applications. Modern Gate drivers generally switch at +15V levels, and their current sourcing/sinking capabilities have recently increased to many amperes to accommodate the high operating switching frequencies and large gate capacitances in these IGBTs as well as high current MOSFETs. Contemporary SiC MOSFETs require +20 V gate drive voltage to achieve a sufficiently low on-resistance due to the poor transconductance realized due to the poor channel mobilities of SiC. Junction-based devices like SiC Junction Transistors (SJTs) and Normally-OFF JFETs require a +4 V drive, but can require continuous Gate currents that are non-zero. Normally-ON JFETs can require a high negative bias (up to -30 V) to turn them completely OFF. From an initial assessment, it may appear that each of these devices require a non-standard gate drivers, and indeed, many SiC device manufacturers are actively working on optimum gate drivers for their switch offerings. A few Si IGBT/MOSFET gate drivers may offer up to +20 V, making them compatible with SiC MOSFETs. In addition, SiC Junction Transistors offer current gains in excess of 100, enabling the use of off-the-shelf IGBT drivers because their continuous gate current requirement can be supplied by them. A series Gate Resistance, similar to that used in IGBT drive can easily control the amount of gate current, as well as provide the requisite Gate-Source voltage (3-4V) required for operating SJTs.
A simplified gate drive for driving all these SiC switches, isn’t much different from those used in standard IGBTs. They consists of an isolated input signal in series with a commercial IGBT gate driver IC and a resistor-capacitor output network for improved dynamic performance. The individual component specifications depend on the type of switch used. For example, in comparison to SiC MOSFETs, which require a non-standard +20 V gate bias due to poor transconductance characteristics, the SJTs can be driven with gate voltages as low as 8–10 V. The SJT also does not require a negative gate voltage to remain off.
The gate drive IC must be capable of supplying a sufficient amount of continuous current to the SJT gate during on-state operation, but can be a lower power IC for MOSFETs, and Normally-ON JFETs. The external parallel gate resistor should be adjusted to meet this requirement. As will be described in this document, the external parallel capacitor can be appropriately chosen to ensure an optimum level of dynamic gate current during turn-on and turn-off initial transients. This dynamic current is essential for fast charging of the internal gate-source capacitance. The presence of this paralleled resistor and capacitor on the output of the gate driver can increase device switching speed, reduce device switching loss, and reduce driver losses as well. The selection of these component values is addressed later in this document.
Due to the high voltages being switched, an optocoupler or isolator should be used to protect the input signal source from potential high drain voltages. The isolation rating should greatly exceed the predicted DC voltages in use, particularly with an inductive load present. Also, choke coils are shown to be effective in reducing common-mode noise in the circuit on voltage supplies and gate driver inputs and outputs, they may be used when and where necessary.
While these gate drive considerations take into account only steady state on-state operation, it is much more important to consider dynamic losses at high operating frequencies. This is because SiC switches make commercial sense only when operating frequencies exceed many 10s or 100s of kHz. At these operating conditions, the charging/discharging of Gate-Source and Miller capacitances may play a dominant role in determining the driver, as well as overall losses. The driver switching losses are directly proportional to the Gate-Source (CGS) capacitance, and the SQUARE of the voltage swing. For SJTs, and normally-OFF JFETs, the voltage swing is only 4-5V; while for MOSFETs and Normally-ON JFETs, it may be 20-30 V, with correspondingly higher driver losses. Device switching loss is a product of Gate-Drain (CGD, Miller) capacitance and square of the device voltage swing (for example 800 V). Presently, SJTs and normally-OFF JFETs can offer 2-3X smaller CGD as compared to a MOSFET of a similar current rating.
SJTs are high-performance SiC switches capable of fast switching speeds with ultra-low losses without the drawbacks of other SiC transistors or bipolar Si devices. A simple, IGBT-compatible gate driver can be used to operate them. In applications where switch-rectifier anti-parallel configurations are used (for example front end rectification), having a fast switching SiC switch along with a similarly fast SiC rectifier are essential to achieve an overall high operating frequency. Many Si IGBTs are preferred over MOSFETs because MOSFETs suffer from a slow switching reverse PiN body diode in its structure, which competes with an external fast diode in the flyback configuration of a switch-rectifier pair. Similarly SiC MOSFETs have a slower SiC PiN body diode which can compete with an external SiC Schottky diode at higher operating temperatures. However, SJTs do not have a body diode, and like Si IGBTs, are quite compatible for use with a fast SiC Schottky diode. SiC JFETs also do not have a body diode.
As various families, and ratings of SiC switches become available in the marketplace, it is up to power circuits engineering community to analyze all the choices before deciding which SiC switch technology works best for their application. The decision to replace Si IGBTs with SiC switches may not be as easy as most may have anticipated.
