Silicon Carbide (SiC) MOSFETs are being used more commonly in certain power-switching applications at voltages higher than 500V, especially in those that benefit from the higher switching speeds that SiC devices can support when compared to an equivalent silicon (Si) MOSFET.
In fact, there are many similarities between SiC MOSFETs and Si MOSFETs: both are enhancement-mode devices with body diodes, and they are much faster than IGBTs. In addition, the on-resistance of a SiC MOSFET increases with temperature just as the on-resistance of an Si MOSFET does, but not by as much.
And in both types of device, the switching speed is governed mostly by the speed at which the gate charge is removed or added. The speed limit is determined by the output capacitance and the ability of the devices to source high currents.
There are, though, important differences between SiC and Si MOSFETs:
• SiC MOSFETs have a low on-resistance. They are normally driven at a higher gate voltage, in the range -5V to +20V, to keep on-resistance to a minimum and increase switching speed.
• The body diode of a SiC MOSFET has a high voltage drop of around 4V, but a low minority carrier lifetime. They have a significantly faster recovery time and a lower recovery charge than those of silicon MOSFETs.
• The output switching current, dI/dt, is considerably higher in SiC MOSFETs than in Si MOSFETs. This affects DC bus ringing, EMI, and output-stage losses.
• The slew rate at the output of a SiC half-bridge can be much higher than when using a Si MOSFET. SiC power stages can easily switch at a dV/dt of 30-50kV/μs. This should be considered in the design of the gate drive’s signal isolation, gate power isolation, and EMI mitigation.
SiC MOSFETs, then, offer considerable benefits in the operation of high-voltage and high-speed power-conversion circuits. Two particular aspects of a SiC MOSFET-based circuit’s operation are discussed in this Design Note.
The effect of temperature on on-resistance
Figures 1-3 show the different effect of temperature on on-resistance in SiC and Si MOSFETs. The behaviour of electron mobility in Si MOSFETs is governed by thermal scattering. Figure 1 shows that, from 25°C to 150°C, on-resistance increases by around 2.7 times.
Figure 2 is typical of a Microsemi 1,200V SiC MOSFET. There are two scattering mechanisms affecting electron mobility, with the resulting benefit that from 25°C to 175°C, on-resistance only varies typically from about 1.5 to 1.8. A Microsemi 700V SiC MOSFET’s behaviour is slightly different, as shown in Figure 3.
This behaviour means that parallel operation of SiC MOSFETs is possible. A positive on-resistance slope makes it easier to share current in parallel operation. If the slope is negative, as with the 700V devices, there is a need for thermal coupling between devices to be sure that current is shared at low temperatures. In most cases, as long as parallel devices are on the same heat-sink, current sharing should not present any problems.
Gate drive considerations
The design of a SiC MOSFET gate driver is much like the design of a standard IGBT or Si MOSFET driver. Most of the features and practices are the same:
• Asymmetrical high gate drive current keeps switching losses to a minimum
• Secondary-side power monitoring and short-circuit protection might both be useful
Due to the higher speed of SiC MOSFETs, the capacitance of the gate- drive interface should be kept to a minimum. At 50V/ns, 5pF of interface capacitance results in 250mA current pulses. On the control side, these current pulses should be directed away from cables or control electronics by bypassing to a chassis or other filtering structure.
The power supply that supplies the gate-drive power should be rated for at least 50V/ns. Most are not, but a good option is RECOM’s RxxP22005D series of DC-DC power supplies, which are intended for use with SiC MOSFETs. The gate driver itself should support bipolar operation, and should also support >50V/ns speeds.