SiC-Diodes, SiC-MOSFETs and Gate Driver IC

The best use of SiC devices and applications are shown. Uninterruptible Power Supplies (UPS) will be described in more detail. Additional to SiC, a portfolio of very fast, high output current and high common noise immunity Gate Drivers will be introduced. Those devices can drive Si-SJFET and -IGBTs but are especially perfectly suitable for switching SiC Devices.

By Christopher Rocneanu, BDM, ROHM Semiconductor GmbH

Due to the excellent physical and electrical characteristics of wide bandgap material SiC-devices have several well-known advantages e.g. fast-switching, low-losses, high temperature and high frequency operation as well as high breakdown voltages. Next to its 650V and 1200V SiC Diode and –MOSFET portfolio Rohm Semiconductor is developing e.g. a 1700V 1.2 Ohm SiC MOSFET which enables designers to increase performance and reduce cost in Auxiliary Power Supplies. Like in the past Rohm Semiconductor pioneers again by using D3PAK (TO-268 2L) package with only two leads. This package increases the creepage distance and is easy to mount. Compared to Si devices the low gate charge, low input capacitance and only 1/8 of RDS,on value can lead to heat sink removal which has been needed in the past for slow switching high ohmic, high voltage Si-MOSFETs. Thus total system cost can be decreased while increasing total efficiency. At the same time ROHM is also developing 1700V Diodes for 50A and MOSFETs with an RDS, on of 100mOhm and 57mOhm for high power applications. In the coming years it is even possible to get devices with breakdown voltages of 3,3kV and higher in an acceptable price range.

Figure 1: ROHM Semiconductor SiC Diode portfolio

Diodes: Since 2010 Rohm has a wide portfolio of SiC Diodes with a breakdown voltage of 650V, 1200V and 1700V and a current rating from 5A to 100A as can be seen in figure 1. Standard packages are TO-247, TO-220 and SMD packages. With the industry’s lowest forward voltage drop (VF) those SiC Diodes are not only suitable in PFC, Industrial equipment, Welding, SMPS, and (Battery) Charger Applications but especially suitable in those applications where a low voltage drop is necessary to achieve highest efficiency and lowest total system cost e.g. in Solar, Energy storage or UPS. SiC Diodes don’t suffer from Reverse Recovery like Si-diodes since the SiC Diode is a unipolar device where only a very small, capacitive and thus temperature independent junction capacitance has to be discharged. Another application is for example automotive. Most on-board chargers are at least including a SiC Diode in as PFC Diode and most manufacturers are either using or considering Rohm as preferred source due to its strength in automotive sector.

Another advantage occurs when using SiC Diodes as Freewheeling diodes together with IGBTs e.g. in Bridge configuration. In those configurations the losses of the Diode will be seen at the Transistors. For a RMS current of 300A you usually use a 900A-1000A rate IGBT module due to the high losses of Si-IGBT and Si-Diode. When using SiC Diodes for free-wheeling the total losses can be reduced and thus the derating of the IGBT can be decreased which will save costs. All major third party module manufacturers have tested and qualified Rohm’s SiC diodes. For further information you can either ask your favourite module manufacturer or email the author.

SiC-MOSFET: Since beginning spring of 2012 ROHM started to mass produce its second generation of 1200V, 80mOhm SiC MOSFET. Meanwhile other MOSFETs with RDS,on of 80mOhm, 160mOhm, 280mOhm and 450mOhm have been added to Rohm’s portfolio as can be seen in Figure 2.

Figure 2: ROHM Semiconductor SiC MOSFET portfolio

Si and SiC-MOSFETs contain a parasitic body diode formed by a p-n-junction. Due to the wide bandgap material SiC body diodes have a high threshold voltage (~3V) and a larger forward voltage drop compared to (Ultra-fast) Si-Diodes. The big advantage of a SiC MOSFET is the very good parasitic body diode. Since it is fully qualified one can use the body diode of Rohm’s SiC MOSFET for free-wheeling, which can save significant cost.

Figure 3: Comparison of VF using SCT2080KEC body diode, SCH-2080KEC and SCT2080KEC in reverse conduction mode

Rohm’s SCT2080KEC body diode has a very good Qrr of 44nC but like all SiC body diodes it suffers from a high VF which is a SiC phenomenon. In most applications the higher VF of the body diode doesn’t hurt you compared to the extra cost and space of an additional SiC Diode. For applications where performance but not cost has the highest priority e.g. in the oil and gas industry or for automotive racing Rohm has added an external SiC Diode into the TO-247 package. Now the VF decreases by roughly 70% to 1,4V.

In opposite to other SiC competitors Rohm also introduced SCT2120AF which is a 650V 120mOhm SiC MOSFET in TO-220 package. In the field of 650V competition from Si-devices, especially SJFETs are quite strong since they have low RDS,on and are affordable. The advantage of SiC vs Si is the lower RDS,on dependency versus temperature and the higher temperature capability as well as the good body diode reverse recovery performance. Of course it depends on your application and target cost which devices you want to choose but Rohm’s portfolio also many attractive Si devices like SJ-FETs, IGBT and a so called Hybrid-MOS. The Hybrid-MOS which is a SJFET with IGBT characteristics has lower conduction losses and higher current capability than a SJFET and lower switching losses and higher frequencies are possible than with an IGBT.

(Bidirectional) Application: Not only but especially for bidirectional application like battery charger, regenerative Drives, power supplies, etc. one can also use the SiC MOSFET in reverse conduction. In most topologies the high and low side MOSFETs are driven with a complementary signal which can be seen in figure 4.

Figure 4: Reverse conduction of SiC MOSFET in half-bridge configuration

After the dead time elapsed the gate at commutation side is turned on. Compared to the body-diode the channel has a lower VF and the currents flow mostly through the MOSFET. Thus the MOSFET operates in reverse conduction mode which increases performance and decreases costs by removing the SBD.

With the rising demand of new internet technologies IT centres have to become more energy and cost efficient. Thus one interesting application for SiC is the uninterruptible Power Supply (UPS). There are three types of modern UPS system which are called–on-line, line-interactive and offline or standby topology.

The simplest topology is the Standby UPS which is mostly used in a power range of 0-500VA. Under normal condition a battery charger is used to convert AC to DC and charge the battery while the AC power is bypassed through the UPS to the load. During failure the inverter converts DC to AC to support the load. The offline UPS is normally used in single-phase non critical loads and is called single conversion as the power is converter only once at any point of operation. Thus the efficiency is high. Additionally, the offline UPS is targeting consumer applications e.g. Desktop Computers and can be made very inexpensive but suffers from high and low input voltages due to the lack of power conditioning.

The line-interactive topology is mostly used for server applications and can be found in a typical power range from 500VA-5kVA. This topology is also using a 4-quadrant Converter but compared to standby UPS the DC to AC Inverter is always on and connected to the output of the UPS system. In failure mode the Inverter operates in reverse mode and the battery supplies again to the load. The battery is usually used more than in standby topology which affects the reliability. The use of a tap transformer keeps the energy from dropping abruptly and provides voltage regulation. Additional filtering can be achieved when using an inductance. Thus the line-interactive topology pays a little penalty to the efficiency due to the inductor or transformer losses but on the other hand can increase the range of protection features and power conditioning.

The on-line topology is mainly used from 5kVA-1MVA+ and is also called double conversion. Compared to the standby topology the primary power path is now coming from the inverter instead of the AC Mains. Therefore the battery charger and the inverter are converting the complete load power flow. By using a transformer common mode noise immunity and electrical isolation is provided. Advantageous is the increased range of protection and a very good electrical output performance. By using SiC devices the UPS System can be made more efficient, more reliable and more inexpensive.

Figure 5a: Complex Gate Driver portfolio, A=Available, B=Build in, E=External, S=Shutdown, M=Monitor, ES=Engineering sample, CS=Commercial Sample, MP=mass production

Figure 5: Simple Gate Driver portfolio, A=Available, B=Build in, E=External, S=Shutdown, M=Monitor, ES=Engineering sample, CS=Commercial Sample, MP=mass production

Driving SiC MOSFETs: To obtain less VTH shift Rohm guarantees for its SiC MOSFETs a Gate Source Voltage of 22V/-6V. This is also the Voltage under which all reliability tests (e.g. HTGB) are performed. For more information on reliability test please contact the author or your responsible Rohm Sales Representative. Other competitors have given max. VGS in their datasheet of 25V/-10V but test their device at the given operating VGS =20V/-5V only. Rohm recommends a VGS of 18V/-3V or-4V in order to switch the MOSFET safe and optimize efficiency. At room temperature and 18V the RDS,on =80mOhm while at 15V the RDS,on =100mOhm. Just by not choosing the right positive Voltage you can lose 25% efficiency. You can switch off the SiC MOSFET at 0V but you have to be careful that ringing doesn’t exceed VTH in order to avoid parasitic turn on and failure of your application. At the same time a low negative Voltage gives you a higher margin and decreases switch off time. Rohm’s customers have successfully used 0V as well as low negative Voltage to switch off the device. In general you must adapt to your case and application but you shouldn’t try to “plug and play” the SiC MOSFET with an IGBT This means you need to consider the optimal VGS and you have to consider your design for SiC in terms of EMI, distance between Gate to driver, stray inductances etc… or you can be wasting a lot of money and time.

To support customer as best as possible  ROHM has created a portfolio of industrial and automotive qualified, high performance, simple and complex Gate Driver ICs which are suitable for IGBTs, SJFET, and SiC MOSFETs. Simple means single channel with only a few features like Miller Clamping and complex means 2 channel driver where one channel is used for feedback. The complex drivers have several features the customer can choose like negative power supply, Miller clamping, desaturation, temperature monitor or shutdown and even an integrated flyback controller. All features can be seen in Picture 5. The gate drivers have a magnetic coreless transformer design, featuring an isolation voltage of 2,5kV and a delay time of as low as 90ns. Also they have a peak current of 5A, an ambient temperature -40°C – +125°C and industries best common mode noise immunity of 100kV/us which makes them a perfect suitable to switch fast SiC devices. BM6105FW for example is pin-to-pin compatible to competitors solution.

The drivers are originally designed for high side but since some customers are using negative VGS an isolated gate driver on the low side is useful as well.

Cost, Reliability and Second Sourcing: The main arguments against SiC have been cost, reliability and second sourcing. Reliability issues have been shortly discussed before and all necessary reliability test can be send upon request. While at first there were several manufacturers using different transistor structures like BJT, JFET and MOSFET. Meanwhile the MOSFET has won the race. Rohm Semiconductor and competitors have been manufacturing SiC MOSFETs since 2010/ 2011 and other major players are following with SiC-MOSFETs. The advantage of the MOSFET is the normally-off behaviour as well as the very fast body-diode. Rohm and competitors portfolio are similar with respect to RDS, on which means there is a second source available.

At the same time there are only two major manufacturers of SiC Wafers. One of them is SiCrystal a 100% owned subsidiary of Rohm Semiconductor. SiCrystal is located in Nuremberg and supplies SiC wafers to Rohm’s Japanese factory. SiC is a very hard material (9.6 on the MOHS Scala where diamond has a 10) which requires high cost processing. At the same time while Si wafers are at 8inch, SiC device manufacturers are using 4inch at the moment. An increase in demand will allow ROHM to adapt to 6inch very quickly. Even at 3 and 4 inch wafers the cost of the SiC diodes has been decreased over the years. For 650V diodes the cost per amp is below 0,20USD/Amp at high quantities. For 1200V diode cost is roughly at 0,40USD/Amp. Cost for the MOSFET has come down from roughly 100USD per device in 2011 to below 15USD from online distributor for 1kpcs today for a 1200V 80mOhm MOSFET. The high vertical integration of the supply chain - from wafer to chip to module - allows  Rohm semiconductor to control its quality and decrease the time to market as well as the delivery time.

Summary: Rohm’s SiC Diode and –MOSFET portfolio has been introduced and several advantages have been shown from SiC MOSFETs and -Diodes versus conventional technologies. Besides a wide portfolio of power products Rohm also offers Gate Driver with a high output current, small delay time and high Common Mode Noise Immunity making them especially suitable for switching SiC devices. Since Rohm also offer Si-IGBT and SJFET there are plenty of options for developers. An introduction to UPS has been given.

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