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Posted on 02 July 2019

Silicon Carbide MOSFETs Provide Ultimate Energy Efficiency and Easy Design In

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The SiC MOSFET also has significant advantages including simple drive circuit

The silicon carbide (SiC) MOSFET has unique capabilities that make it a superior switch when compared to its silicon counterparts. By nature of its material advantages, SiC MOSFETs provide lower switching loss, lower on-resistance across its operating temperature range, and superior thermal properties. Furthermore, the SiC MOSFET is the easiest to use wide bandgap switch presently demonstrated. Best of all, SiC MOSFETs from Cree are now available for commercial use.

By Bob Callanan, SiC Power Applications Manager, Cree, Inc.

 

In this article the characteristics of a typical 1.2 kV, 80 mΩ SiC MOSFET will be discussed. For comparison purposes the following silicon devices were utilized:

• 1.2 kV, 20 A trench/field stop (TFS) Si IGBT Fairchild FGA20N120FGD [3]
• 1.2 kV, 20 A non-punch though (NPT) Si IGBT International Rectifier IRGP20B120U [4]
• 1.2 kV, 0.30 Ω Si MOSFET (Si MOS8) Microsemi APT34M120J [5]

The key to optimal application of the SiC MOSFET requires an understanding of the device’s unique operating characteristics. The forward conduction characteristics of the SiC MOSFET along with the Si MOS8, TFS, and NPT IGBTs are presented in Figure 1.

Forward conduction characteristics comparison

The relatively high temperature coefficient of RDS(on) for the Si MOS 8 has considerable effect on its conduction losses. At 150 °C, the RDS(on) of the SiC MOSFET increases only about 20% from 25 °C to 150 °C whereas the Si MOS8 device increases by 250% as shown in Figure 2. This has a significant effect on system thermal design. For systems operating in the higher end of their temperature range, the increase in RDS(on) can be critically important where degradation in conduction loss must be avoided.

Normalized RDS(on) vs. temperature

The inductive turn-off losses versus temperature of the SiC MOSFET compared with the TFS and NPT IGBTs are shown in Figure 3. The freewheeling diode used with all devices was a 1.2 kV, 10A SiC Schottky diode. The turn-off losses of the IGBTs are significantly higher than the SiC MOSFET and strongly increase with temperature. This is due to the tail loss inherent with IGBTs. The NPT IGBT is significantly better than the TFS IGBT. However, the NPT IGBT conduction losses are much higher than the SiC MOSFET. The TFS IGBT conduction loss is lower than the NPT IGBT, but the switching loss is the highest of the three. In all cases the SiC MOSFET switching losses are significantly better than its silicon competitors.

Switching loss vs. temperature comparison (VDD = VCC = 800V, ID = IC = 20A, RG = 10Ω)

To realize the considerable benefits if the SiC MOSFET there are a few characteristics of the device that need to be understood. The output characteristics of a typical 1.2 kV, 80 mΩ SiC MOSFET is shown in Figure 4. The modest amount of transconductance causes the transition from triode to saturation to be spread over a wider range of drain current. Therefore, the SiC MOSFET behaves more like a voltage controlled resistance than a voltage controlled current source. The lowest RDS(on) is achieved with a +20V gate drive. The modest transconductance and short-channel effects are important to consider when applying the device. SiC MOSFETs need to be driven with a higher gate voltage swing than what is customary with Si MOSFETs or IGBTs. The rate of rise of gate voltage will have a greater effect on the rate of rise of the drain current due to the lower transconductance.

SiC MOSFET forward characteristics (TJ = 150 °C)

The recommended gate drive voltage for the SiC MOSFET is 20V. However, the amount of gate charge required to switch the device is low. The ramifications of the modestly higher gate voltage and lower gate charge can be reconciled by using the product of gate charge and gate voltage as a measure of gate energy. The gate energy comparison is shown in Figure 5. The results of this comparison show that the SiC MOSFET gate energy is comparable to or lower than the other devices. Therefore, the higher voltage swing does not adversely affect gate drive power requirements.

Gate energy comparison

The gate driver for the SiC MOSFET is simple and uses existing commercially available components; standard 35V MOSFET/IGBT gate driver chips are ideal. One recommended line of gate drivers is available from Clare [6]. The SiC MOSFET does require a modest amount (-2V to -5V) of negative bias. This is easily accomplished using very simple techniques. A schematic of a simple gate driver circuit is shown in Figure 6.

Typical gate driver circuit to provide +20/-2V gate pulses

To achieve fast switching time, the gate drive interconnections need to have minimum parasitics, especially inductance. The gate driver must be located as close as possible to the SiC MOSFET.

In addition to the performance advantages over competing silicon switches, SiC MOSFETs have distinct advantages when compared with other SiC switching devices as well. The competing devices are normally-on and normally-off SiC junction field effect transistors (JFETs). Reported specific on-resistance of the normally-on SiC JFETs tends to be the lowest of all SiC majority carrier switches. However, the device has the inherent drawback of being normally-on. This causes system complications; notably lack of a ‘fail-safe’ feature. If the gate bias is lost due to a failure in the housekeeping supplies, the SiC JFET will be on and could cause a damaging shoot-through. This can be mitigated with a SiC JFET – Si MOSFET cascode circuit. In this approach, a low voltage Si MOSFET is used to switch the JFET source. Being a cascode, the MOSFET will conduct full load current and therefore adds to the overall switch conduction loss partially offsetting the low specific on-resistance. Providing gate drive for the Si MOSFET and gate bias for the SiC JFET requires a custom gate driver design.

The normally-off SiC JFET also has very low reported specific onresistance. Lowest on-resistance requires the gate junction to be hard forward biased for the device to operate at its maximum rated current at normal operating temperatures. The magnitude of the gate current in this condition is about 200 mA to 1A and must be applied when the device is conducting. The result is additional system losses on the order of 0.5W to 3W adversely affecting overall system efficiency. Supplying this current requires another custom gate driver design. Unlike the SiC MOSFET, both normally-on and normally-off SiC JFETs normalized RDS(on) versus temperature is very similar to a silicon MOSFET; more than doubling from 25 °C to 150 °C. In most cases the reported RDS(on) for SiC JFETs are measured at 25 °C junction temperature. Therefore, the lower specific on-resistance advantage is lost at routine operating junction temperatures. Lastly, the vertical SiC JFETs have very limited avalanche capability whereas the SiC MOSFET has very high avalanche capability [7]. This makes the SiC MOSFET a very robust switch. A summary of this comparison is shown in Table 1.

SiC Device Comparison

Conclusions

Switches employing wide bandgap materials have significant advantages over their silicon counterparts. The 1.2 kV SiC MOSFET has definite system advantages over competing Si switching devices. These advantages include lower conduction loss and lower switching loss. Of the competing SiC switch architectures, the SiC MOSFET also has significant advantages including simple drive circuit requirements and high avalanche capability. These factors make the SiC MOSFET a nearly ideal power switch.

 

References:

1) Bob Callanan, “Application Considerations for Silicon Carbide MOSFETs”, Power Electronics Europe, Issue 3, April 2010, pp. 40-43.
2) R. J. Callanan, A. Agarwal, A. Burk, M. Das, B. Hull, F. Husna, A. Powell, J. Richmond, Sei-Hyung Ryu, and Q. Zhang, “Recent Progress in SiC DMOSFETs and JBS Diodes at Cree”, IEEE Industrial Electronics 34th Annual Conference – IECON 2008, pp 2885 – 2890, 10 – 13 Nov. 2008.
3) Fairchild FGA20N120FGD Datasheet, Rev A, December 2007: www.fairchildsemi.com/ds/FG%2FFGA20N120FTD.pdf
4) International Rectifier IRGP20B120U-E Datasheet, PD-94117, 3/6/2019: http://www.irf.com
5) Microsemi APT34M120J Datasheet, 050-8088 Rev A, 2-2007, www.microsemi.com/datasheets/APT34M120J_A.PDF
6) www.clare.com/Products/IGBT-MOSFETDvr.htm
7) J. Palmour, Sei-Hyung Ryu, Q. Zhang, L. Cheng, “Silicon Carbide Switching Devices: Pros and Cons for MOSFETs, JFETs and BJTs”, Power Electronics Europe, Issue 5, July/August 2009, pp.19-22.

 

 

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