Considerations when designing with wide bandgap devices
In power conversion electronics, the perfect semiconductor switch with minimal losses is what every designer wants and wide bandgap silicon carbide (SiC) and gallium nitride (GaN) devices are often presented as being close. “Perfect” has to include more than low loss though. The switch has to be easy to drive, it should not cause unintended effects like high EMI, it should be robust and, of course, low cost.
Gate drive is critical
Driving the gates of SiC and GaN devices is possibly the most critical consideration and, in most ways, it is more difficult than with the IGBTs and MOSFETs. These older technologies could be simply driven with around 0 – 12 V for blocking to full saturation, with a large margin to the absolute maximum of typically +/-20 V. The stable threshold might be around 5 V, giving good noise immunity while peak current is reasonable and gate drive power is modest, at least at the frequencies that the devices are usually switched.
In comparison, SiC-MOSFETs and GaN high electron mobility transistor (HEMT) cells particularly can have lower gate thresholds with some hysteresis. Also, thresholds for both reduce substantially at high temperatures and therefore gates often have to be driven with a negative off-state voltage to avoid spurious phantom turn on. This is a direct result of one of the benefits of the devices – the very fast di/dt achievable induces voltage transients in connection inductances, which can couple into the gate circuit. Great care is therefore necessary to avoid any common connection in the switched current and gate drive loops. Even package inductance can be problematic, so many manufacturers offer Kelvin connections to the source to mitigate the effect.
The very fast dV/dt of SiC and GaN devices has a similar effect, coupling transients into the gate through the drain-gate Miller capacitance. In practice, current and voltage edge rates are often deliberately slowed through added series gate resistance and/or snubbers, particularly in motor control where the switching frequency is often kept low. In this application, dynamic losses are not too significant but the low conduction loss of wide bandgap devices is an advantage. Devices are also typically slowed from their maximum edge rates to keep electromagnetic interference within bounds.
SiC-MOSFETs need a high gate voltage for full enhancement, typically around 15-18 V — close to the absolute maximum — which can be as low as 20 V. The limit for a negative drive is also typically lower than with a silicon MOSFET at around -7 to -10 V. Also, at the maximum recommended operating gate drive voltage, the short-circuit withstand of the devices reduces, so SiC-MOSFET gate drives need accurate control and voltage limiting for reliable performance.
GaN HEMT cells have quite different gate characteristics to Si- and SiC-MOSFETs with their isolating gate oxides. GaN gates appear like a diode which has a drop of about 3-4 V, which is forward-biased with a few milliamps to turn the cell on. The threshold is about 1.3 V nominally but could be closer to 0.5 V at high junction temperature, so a negative off-state drive is commonly recommended. This can be sourced from a bipolar supply to a driver but can more simply be achieved with a series capacitor in the gate circuit as recommended by Infineon and shown in the circuit diagram below. The resistors serve to control dV/dt separately on leading and falling edges and ensure stable operation.
The diode-like input provides natural clamping of the voltage, but the current must not exceed a value typically averaging less than 20mA to avoid damage. The appearance of GaN in IC packages like the Nexperia cascode GaN or STMicroelectronics MasterGaN platform with integrated driver effectively moves the GaN-driving challenge into the package level, so the designer does not have to deal with it. Dedicated GaN drivers also help in more complicated driving schemes to achieve the target gains in efficiency and power density and to protect the GaN device itself.
Avalanche and short-circuit performance are considerations
IGBTs and silicon MOSFETs have an avalanche rating for ability to withstand energy from an overvoltage at the collector/drain. Values for SiC-MOSFETs are not as yet easily available, with some manufacturers only offering estimates. GaN cells inherently do not have the ability to survive overvoltage and will immediately fail so are offered with very conservative maximum operating voltage ratings.
Series capacitor at gate circuit
Typical gate drive circuit for a GaN cell. Source: Infineon
SiC-MOSFETs do have stated short-circuit rating, comparable with silicon types, while GaN is less well defined with some variability and possible cumulative degradation. As the mechanisms of failure become better characterized, datasheets are likely to begin to include short circuit ratings. However, wide bandgap materials do have the inherent advantage of much higher maximum die temperature rating, and in the case of SiC, much better material thermal conductivity, which helps with transient power dissipation under fault conditions.
Reverse conduction effects are quite different
In many power converter circuits, reverse conduction of the switch is required for voltage clamping, as in motor drives for soft switching converter topologies or as an enabler for bidirectional energy flow, for example. IGBTs cannot do this and require an external lossy anti-parallel diode. Si- and SiC-MOSFETs can conduct through their channel in reverse with low loss, but they also have a body diode that can commutate or conduct naturally during dead times before the channel is driven on. This time is typically minimized by the control circuitry, but dissipation can be high for the duration. In this respect, SiC body diodes with a forward drop of around 4 V are typically much worse than silicon types at less than 1 V. However, the reverse recovery energy of a SiC body diode is much lower than silicon. This means dynamic losses are much lower in “hard” switched converter topologies, where the diode is taken from forward to reverse bias with appreciable forward current flowing, GaN HEMT cells, like IGBTs, do not have a body diode and have no reverse recovery losses at all. The devices do naturally commutate though with a reverse voltage with conduction through the channel occurring. The voltage drop before the channel is actively driven on, however, is complex. It is the sum of the drop across the channel resistance plus the threshold voltage, plus any negative off-state gate drive voltage. This can easily add up to more than 5 V. A summary of some of the key comparisons between an IGBT, SiC-MOSFET, GaN HEMT cell and a Si-MOSFET are given in the figure below for typical 600 V-class devices.
Switch comparisons
Comparisons between switch technologies for typical 600V class devices
Wide bandgap devices are different but still promise major efficiency gains
Having discussed some of the characteristics of SiC and GaN devices, a few pitfalls are apparent. However, manufacturers are working to make the parts more robust and easier to use, with novel variants such as cascode combinations of Si-MOSFETs and SiC junction-based normally on transistor types (JFETs) addressing most of the issues. The headline advantages of wide bandgap devices remain though: potentially much higher efficiency of power converters and switching at higher frequencies with energy, size, weight and consequent cost savings in magnetics and heatsinks.