Published March 4, 2013

Power losses in all-SiC, hybrid Si/SiC, and Silicon Switch-Rectifiers Circuits

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GeneSiC is aggressively developing Silicon Carbide (SiC) “Super” Junction Transistors (SJTs) in voltages ranging from 1.2 kV to 10 kV for high efficiency power conversion in aerospace, defense, down-hole oil drilling, geothermal, Hybrid Electric Vehicle (HEV) and inverter applications . The SiC SJT is a normally-off “Super-High” current gain SiC BJT that exhibits a square reverse biased safe operating area (RBSOA), high temperature (> 300 °C) operation capability, low VDS(on) as well as faster switching capability (10’s of MHz) as compared to any other competitor SiC switch. Unlike SiC MOSFETs, the SiC SJT is free from metal oxide semiconductor (MOS) interface reliability concerns and high channel resistance, which have limited the SiC MOSFET to less than 150 °C operation temperatures and >15 V Gate biases. The SiC SJTs display a lower positive temperature coefficient of RDS(on). GeneSiC’s SJTs are packaged in industry standard commercial packages. When incorporated in power electronic circuits, the SJTs can improve the circuit efficiencies significantly while reducing the overall system size, component count, cooling requirements and cost. Here, we investigate the high temperature (>250 °C) blocking, on-state, and switching characteristics of 1200 V/ 7 A SiC SJTs by comparing their static, dynamic characteristics and the associated losses with the following best-in-class 1200V Si IGBTs:

• NPT1: 125 °C/1200 V rated Si Non Punch Through IGBT
• NPT2: 150 °C/1200 V rated Si Non Punch Through IGBT
• TFS: 175 °C/1200 V rated Si Trench Field Stop IGBT

All the above co-packs have Si FREDs associated in anti parallel direction with the Si IGBTs.

The temperature independent blocking characteristics of a 1200 V/7 A SiC SJT is shown in Figure 1. The SJT blocks its rated 1200 V even at 325 °C with a low leakage current (< 100 µA ). Figure 2 shows the leakage currents measured on a SJT and the three Si IGBTs at a blocking voltage of 1200 V at various temperatures. Extremely high leakage currents deteriorate the performance of even the best Si IGBTs beyond 175 °C. On the other hand, the operational temperature of the SJTs is limited (to 325 °C) in this present study only by the capability of the power package. As compared to the Si IGBTs, the SJT also displays a lower positive temperature co-efficient of leakage current.

 

Figure 1: Open-Gate Blocking characteristics of a representative 1200 V/ 7A SiC SJT.

 

Figure 2: Comparison of leakage currents of SiC SJT and Si IGBTs as a function of temperature

The output I-V characteristics of the SiC SJTs (shown in Figure 3) feature a near-zero Drain-Source offset voltage, distinct lack of a quasi-saturation region and the merging of the different Gate current I-V curves in the saturation region. The last two features imply lack of charge storage in the drift region of the SiC SJT and clearly distinguish it from a “bipolar” Si BJT. This inherent property of the SiC SJT enables temperature independent fast switching transients, as will be evidenced in the next section. At a given temperature, the on-state voltage drops of SJT are relatively smaller than the existing same current/voltage rated Si IGBTs with VDS(on) values of 1.5 V at 25 °C and 2.6 V at 125 °C at 7 A of drain current. Like a typical majority carrier device, the SJT displays a positive temperature coefficient of VDS(on) which is a desirable feature for reliable paralleling of multiple SJTs for high current configurations. An on-resistance of 220 mΩ (calculated at a Gate current of 400 mA) was measured at 25 °C operating temperature on a 7 A SJT. As the junction temperature is increased from 25 °C to 325 °C, the maximum current gain decreased from 63 to 34 (see Figure 4).

Figure 3: Output characteristics of a representative 1200V/7A SiC SJT as a function of temperature.

Figure 4: Temperature variant On-resistance and (maximum) Common Source Current Gain of a representative 1200V/7A SiC SJT.

An inductively clamped double pulse switching setup was used to investigate the switching performance of the SJT and Si IGBTs. A GeneSiC 1200 V/ 7A SiC Schottky diode and Si IGBT co-packs were used as Free Wheeling Diodes (FWDs) in the switching test circuit. The Gate and Source terminals of the Si IGBT co-pack (FWD) were tied together (VGS = 0 V) to avoid the IGBT conduction during the dynamic testing. A 1 µF charging capacitor, a 150 µH inductor, 22 Ω Gate resistor and a supply voltage of 800 V were used in the testing process. A commercially available IGBT gate driver with an output voltage swing from -8 V to 15 V is used for driving all the devices. While driving the SiC SJT, a 100 nF dynamic capacitor connected in parallel with the Gate resistor generated high initial dynamic Gate currents of 4.5 A and -1 A during turn-on and turn-off switching respectively, while maintaining a constant Gate current of 0.52 A during its turn-on pulse. These large initial dynamic Gate currents charge/discharge the device input capacitance rapidly, yielding a faster switching performance .

A comparison of the turn-on (Figure 5) and turn-off (Figure 6) energy losses of the various Si and SiC device configurations was performed at temperatures ranging from 25 °C to their respective maximum operating temperatures. Si TFS + SiC FWD represents Si TFS IGBT as the DUT and SiC Schottky diode as FWD respectively where as Si TFS + Si TFS represents Si TFS IGBT as DUT and Si TFS IGBT co-pack as FWD respectively. The SiC SJTs displayed a temperature independent (up to 250 °C) Drain current rise time as low as 12 ns and a fall time as low as14 ns for switching at 800 V and 7 A, resulting in significantly lower switching losses as compared to any of the all-Si or Si IGBT+SiC FWD configurations.

 

Figure 5: Turn Off Switching Energy comparison of the SiC SJT and SiIGBTs at various operating temperatures.

Figure 6: Turn On Switching Energy comparison of the SiC SJT and SiIGBTs at various operating temperatures.

Figure 7 shows an overall power loss comparison of all the devices, extracted from the measured dynamic and static characteristics for a 100 kHz switching frequency and a0.7 Duty Cycle (D). The measured gate drive, conduction and switching losses of the SJT are 5.25 W, 26.65 W and 20 W respectively at 250 °C. Though the gate driver losses of SJT are higher than Si IGBTs, their contribution to the overall losses is insignificant. Replacing a Si FRED with a SiC Schottky diode for FWD applications alone reduces the overall switching losses by more than 30%. An All-SiC SJT/Schottky rectifier solution as opposed to an all-Si IGBT/PIN rectifier solution resulted in more than 50% power loss reduction.

Figure 7: Overall loss comparison of SJT and Si IGBTs at their maximum operating temperature.

Significantly lower overall power losses are extracted from the temperature variant static and dynamic characterization performed on SiC SJTs in comparison with three best-in-class Si IGBTs. The extremely low conduction and temperature independent switching losses of the SiC SJTs make them ideal candidates for replacing Si IGBTs in high temperature and high frequency applications.  

G. Yan, A.Q. Huang, Sumi Krishnaswami, J. Richmond, A.K. Agarwal, “Comparison of Static and Switching Characteristics of 1200V 4H-SiC BJT and 1200V Si-IGBT” in IEEE 2006 Industry Applications Conference, 2006, pp. 325-329.
GeneSiC Semiconductor, Inc. Available: http://www.genesicsemi.com/index.php/sic-products/schottky
A. Lindgren, M. Domeij, “Degradation free fast switching 1200 V 50 A Silicon Carbide BJT’s” in IEEE 2011 Applied Power Electronics Conference and Exposition, 2011, pp. 1064-1070.