Semiconductors have come a long way since the early days of the Minuteman project, and the traitorous eight wondered if improved office air filtering would increase the yield of their production. There has been a significant push in recent years toward energy-efficient conversion.
With this push, silicon MOSFETs with RDSon values in the low m range became commonplace in low voltage devices, but the higher the blocking voltage of MOSFETs goes, the higher the RDSon. Despite their high RDSon, these devices were still unsuitable for high voltage and high power applications, necessitating the use of IGBT devices.
Silicon Carbide has proven to be a material that can be used to create MOSFET-like components that enable circuits to be more efficient than was previously possible with IGBTs.
SiC is gaining popularity these days, not only because of its properties, but also because the devices are becoming increasingly cost competitive with IGBTs, and manufacturers are implementing a long-term investment strategy at the system level to secure supplies.
Device Properties And Their Gate Driving
In high energy applications, the SiC material properties outperform Si in terms of parameters. It’s time to take a closer look at the devices and applications. STMicroelectronics, as mentioned above, is a major player in the SiC market, so let’s take a look at their product portfolio in figure 1.
Looking at the voltage ranges covered by the portfolio, it is clear that silicon carbide mosfets compete with Si MOSFETs in one range and IGBTs in another.
Si MOSFETs compete with SiC devices in the lower voltage range, but SiC devices have a lower gate charge and better thermal performance. IGBT devices are located at the opposite end of the spectrum. In this case, it is clear that silicon carbide mosfets are unquestionably superior to IGBTs due to their low RDSon, not to mention their lower gate charge.
When turning off SiC MOSFETs with 0V, one effect must be considered: the Miller effect, which is well known from Si MOSFETs. This effect can be problematic when the device is used in a bridge configuration, particularly when one silicon carbide mosfet is turned on and the second experiences a surge on its Drain and accidentally switches on due to parasitics. This activation causes a short circuit from high voltage to ground, causing the circuit to be damaged.
Figure 1: SiC Power MOSFET product placement in the ST portfolio, STPOWER.
How to Drive SiC MOSFETS
Given the superior material properties, the question becomes how these parts must be controlled in order to perform optimally. To begin with, Si MOSFETs require a positive gate voltage, which is typically around 12V or less, and a negative gate voltage, which should be ground potential.
IGBTs have asymmetrical gate driving voltages, which means the positive gate voltage is around 15V and the negative voltage is around -5V.
Figure 2 shows the output characteristics of the SCT30N120 (Tj = 25 °C).
A silicon carbide mosfet works with the voltage levels of a Si MOSFET or IGBT, but not at their best. A SiC MOSFET should ideally have a gate voltage of 20V in order to be switched on at the lowest RDSon.
When turning off SiC MOSFETs with 0V, one effect must be considered: the Miller effect, which is well known from Si MOSFETs. This effect can be problematic when the device is used in a bridge configuration, particularly when one silicon carbide mosfet is turned on and the second experiences a surge on its Drain and accidentally turns on due to parasitics. This activation causes a short circuit from high voltage to ground, causing the circuit to be damaged.
However, SiC devices can be operated at lower gate voltages than the previously mentioned 20V, but the output characteristics change significantly, as illustrated in figure 2. A lower gate voltage results in a lower overall system efficiency, as can be concluded. Optimising the silicon carbide mosfet gate driving circuit for low RDSon with a high enough gate voltage is only half the job. Another aspect that can be optimised is switching losses.
STGAPxx MOSFET drivers were used to operate the SiC MOSFETs. As illustrated in Figures 3 and 4, STGAPxx MOSFET drivers are available in two flavors. The schematic in Figure 3 shows how to gate drive SiC MOSFETs with a bipolar gate driver supply.
This bipolar gate driving voltage is not required as described above, but it aids in minimizing the Miller effect and creating more controllable on/off switching. As a result, Figure 4 depicts a schematic that includes an active Miller clamp. As a result, the designer is able to use a unipolar gate driver supply.
Figure 3: STGAP2SICS in half bridge mode with separate gate driving paths.
Figure 4 shows the STGAP2SICS in a half bridge configuration with a combined gate driving path and an additional miller clamp.
Figures 5 and 6 show the difference in energy losses when the resistors in the gate driving circuit are optimised. The difference between a 10 and a 1 is 250J in losses that can be avoided. It also emphasises that switching losses are not symmetrical, implying that switching on losses differ from switching off losses.
It’s also worth noting that if a longer switch off time is required for better EMI performance, the efficiency is not hit as hard as it is for switching on because the slope is lower. And there’s one more thing to consider when comparing IGBT to silicon carbide mosfets. When the devices are switched off, there is a significant difference between silicon carbide mosfets and IGBTs.
To be completely switched off, the IGBT must sweep out all of its minority carriers. This last transport occurs when the IGBT is already turned off and the voltage across the collector and emitter is at its maximum, and it contributes significantly to the switching losses of an IGBT. This effect, known as tail current, does not exist in SiC MOSFETs, and switching off requires less energy.