Tweet

Posted on 17 July 2019

Progress in Using Normally-off SiC JFET Power Transistors – The First Year

Free Bodo's Power Magazines!

 

 

 

Since the initial release of the normally-off SiC JFET in September 2008, much has been learned

The 1200 V normally-off SiC JFET released in 2008 has generated market interest in applications including solar or photovoltaic (PV) inverters, server/telecom power supplies, industrial welding inverters, future electric drive automotive inverters, and even audio amplifiers. The normally-off SiC 1200 V JFET was released in a TO-247, in the form of a 100 m-Ohm (SJEP120R100) and a 63 m-Ohm (SJEP120R063) variety. Some of common market drivers behind adopting these JFETs are better efficiency, higher power density, and potentially less system cost.

By Jeff Casady, SemiSouth

 

For these competing drivers, it is important to move to higher switching voltages, higher frequencies, and higher efficiencies simultaneously. These three requirements are conflicting, and are limited by the conventional power transistors and diodes in silicon, which in turn is driving advanced designs to use SiC power transistors and diodes. The relatively new, normally-off SiC JFET has proven to be costeffective compared to previous SiC transistor developments. To use the high-speed, efficient, SJEP120R063, it is important to keep parasitic capacitance and inductance in the circuit to a minimum. For example, to effectively measure the switching energy in these devices, very close spacing is used to minimize the parasitic inductance between the JFET and the driver chip. An example of a standard test fixture used to measure switching energy losses in the normally- off JFET, shown in Figure 1, a standard half bridge circuit is shown with a SJEP120R063 from SemiSouth, an IXYS 509 driver chip, and a 1200 V, 20 A SiC diode (SDP20S120 from SemiSouth).

Example of a standard test fixture in a half-bridge configuration used to measure switching energies in a normally-off SiC JFET

Aside from close spacing in the circuit to minimize parasitic inductance because of the high-speed (high di/dt) nature of the device, it’s important to understand the gate drive philosophy of the normally-off SiC JFET. This device enjoys many similarities to a MOSFET, with a standard output family of curves for gate voltages ranging from 1.0 to 3.0 V. A nominal threshold of +1 V is typical for the normally-off SiC JFET, but because of the PN junction diode between gate and source, and gate and drain, there are differences between the SiC JFET and the MOSFET. The gate – drain/source diodes both have variable capacitance, and when VGS increases enough the SiC gatesource PN junction diode will conduct. The diode current is a function of VGS, chip size (area), external gate resistance, and temperature. For the conduction mode of SJEP120R063, the diode (IGS) will conduct 150-200 mA at a minimum RDSON. When switching, IGS will be increased for a very short time period to charge and discharge the variable capacitances between the gate and D/S terminals [R. Kelley, et al.].

In Figure 2 an example driver schematic is shown in more detail with the corresponding gate pulses (turn-on and turn-off), and a small (~ 100-200 mA) gate current when the device is in the conduction mode.

Example of SJEP120R063 SiC JFET drive circuit topology illustrating dual pulse and conduction through pnp

In the topology of Figure 2, the turn-on and turn-off is handled through a dual-stage gate driver chip (IXDD 509) with a bipolar supply of +/- 15 V. The turn-on, as shown in the upper right graph of Figure 2, is a 5.5 A, 100 ns IGS pulse used to turn the SJEP120R063 JFET on with minimal losses (EON of 131 µJ as shown in the lower left graph of Figure 2). Once the device is turned on, the logic chip turns on a simple, low-cost pnp bipolar transistor to provide a low level of IGS (200 mA) with a 6 V supply (stepped down from + 15 V through a simple DC-DC converter). The IXDD 509 then supplies the short (less than 40 ns) negative IGS pulse of a few amps to discharge the internal device capacitances and turn off the JFET. The turn-off energy, as shown in the waveforms from 25 A, and 600 V in the SJEP120R063 is only 94 µJ, which results in a total EON + EOFF of 225 µJ.

How do the SJEP120R063 results compare to Si IGBT technology at 1200 V?

Examining the previous measured results above discussed in Figure 2 for the SJEP120R063, one can compare its performance with a popular Fairchild Non-Punch-Through (NPT) IGBT, the FGL40N. In the table below, the key device characteristics are compared, with particular differences seen in improved conduction losses at light load or low-frequency (obviously no collector-emitter saturation voltage in the unipolar SJEP120R063), and improved switching losses for higher-frequencies (measured total energy losses compared in the SJEP120R063 with datasheet numbers from the FGL40N result in 7-10 times improvement).

Comparison Between silicon N Type IGBT and SiC JFET

The silicon IGBT therefore has limited ability to allow the inverter designer to improve its performance. The designer can over-size the device to improve conduction efficiency, which in turn will decrease the switching efficiency. To aggressively push the voltage, frequency, and efficiency of the inverter up, the SJEP120R063 offers substantial new options.

The applications impact of the available SiC power transistors

In 2009, Fraunhofer Institute for Solar Energy Systems examined the impact of dropping in SiC power transistors on state-of-the-art solar inverter systems. In their work with the SJEP120R063 SiC JFET, they were able to achieve a world record solar inverter efficiency of > 99% in a Heric® single phase solar inverter module by replacing an Infineon silicon generation four IGBT [B Burger et al.]. From Figure 3, in a full three phase full bridge topology (right), an efficiency increase of ~ 1.2% using the same JFET in place of the IGBT was achieved, even though the JFET operated at three times higher frequency (48 kHz). This shows the ability to increase inverter efficiency while also cutting the cost, size, and weight of the unit through reduction in magnetics at the higher frequencies.

Left is a comparison of the SJEP120R063 SiC JFET from SemiSouth, Right is a comparison of the SJEP120R063in a three phase fullbridge inverter up to 5 kW

The next generation SiC power transistors

Additionally, there are newer, higher power normally-off SiC JFET’s in development. A 1200 V, 25 m-Ohm SiC JFET, normally-off, has been recently reported. The DC family of curves, shown in Figure 4 (left graph), has a saturation current approaching 120 A at room temperature [A. Ritenour, et al.]. Although it will be offered in bare die form for modules as well as TO-247 packages, a 15 mm2, 25 m-Ohm JFET packaged in a low-parasitic Kyocera package for characterization is shown in Figure 4 and the switching performance is shown in Figure 5.

At left are the pulsed output characteristics at Vgs= 3 V, At right is a picture of the 15 mm2 normally off SiC JFET

The switching performance of the 15 mm2 die in a single-switch clamped inductive load test circuit was characterized using a dualstage, bipolar gate drive (± 15 V) similar to that described above. Four 10 A SiC Schottky diodes were paralleled at the clamping diode position in the test circuit. Figure 5 shows switching energies at half rated voltage (Vds= 600 V) and 25 °C as a function of drain current. At 20 and 40 A, the total losses are 0.31 and 0.74 mJ respectively. This device was benchmarked against a similarly rated Si trench IGBT (Infineon IGW40T120). The IGBT switching losses were taken from the datasheet at Vcc= 600 V, Ic= 40 A, and Tj= 25 °C. The use of a SiC Schottky diode in the IGBT inductive load test circuit would reduce the IGBT losses so the turn-on and turn-off losses were scaled to 40% and 70% of their respective datasheet values in order to provide a more fair comparison with the results for the SiC JFET. Figure 5 (right graph) compares the switching losses at V= 600 V, I= 40 A, and Tj= 25 °C. The total switching losses for the Si IGBT are 4.8x higher than for the SiC JFET (3.56 mJ compared to 0.74 mJ).

Above left are the switching energies, On the right is a comparison of switching energies

The significance of the SiC power transistor availability in the power electronics market

What has limited silicon carbide in the past has been the technology – making the technology affordable, reliable, and available in volume to take advantage of SiC’s performance advantages. With the release of the SiC normally-off JFET from SemiSouth, an important missing piece of the SiC power component business is now available, and we will see new market penetration by SiC power transistors in solar inverter, power supply, and eventually automotive electric drive markets. The SemiSouth JFET has demonstrated lower conduction losses and up to 7-10 times lower switching energies than comparable rated Si IGBT technology. Other vendors such as Cree, Infineon, Rohm, and others are also beginning to offer various versions of SiC power transistors at 1200 V.

Acknowledgements

The author gratefully acknowledges the engineering technical staff at SemiSouth which contributed significantly to this work, including, but not limited to, Dr. D.C. Sheridan, Dr. A. Ritenour, R. Kelley, F. Rees, T. Francis, W. King, A. Mulkana, B. Robinson, D. NullI E. Bowman, I.R.B. Casady and S. Sunkari.

 

References:

[1] R. Kelley, et al., PCIM, Nuremberg, Germany, May 2009.
[2] B Burger et al., EPE, Barcelona, Spain, September 2009.
[3] A. Ritenour, et al., ICSCRM, Nuremberg, Germany, October 2009.

 

 

VN:F [1.9.17_1161]
Rating: 0.0/6 (0 votes cast)

This post was written by:

- who has written 791 posts on PowerGuru - Power Electronics Information Portal.


Contact the author

Leave a Response

You must be logged in to post a comment.