Competition in Wide-Bandgap Devices: a Technical Session at PCIM 2012
One of the highlights of the technical program at this year’s PCIM Europe event was a session on "Competition in Wide-Bandgap Devices" that focused on considerations for selecting SiC devices for power conversion applications. Among the companies making presentations were: Danfoss Solar Inverters, EADS, Fairchild Semiconductor and STMicroelectronics.
A team of engineers from Danfoss Solar Inverters in Denmark analyzed the switching and conducting performance of two types of SiC normally on JFETs – a SiC normally-off JFET, two types of SiC MOSFETs and a SiC-BJT from a variety of manufacturers including Cree, Fairchild, Infineon, SemiSouth and Rohm. They used measurements at exactly the same boundary conditions to compare the devices. In addition to comparing the switching and conducting behaviors of the devices, the team also compared the requirements for driving the devices. They concluded that identification of the "best" device depends strongly on the application, since all devices show advantages, but in different ways. Therefore a universally valid recommendation for one of these power semiconductors cannot be given.
The following is a portion of the Danfoss team’s analysis of characteristic for the depletion mode JFET (Normally-On JFET) is that it is turned on at a gate voltage of 0V: This characteristic leads to considerable efforts for the driver: if the JFET is directly driven, it must be ensured that the driver is also supplied with power in case of a failure on the ac or dc side, to hold the JFET in blocking state. If the JFET is directly driven, a gate voltage of -15V is required to block the device and 0V to hold the device in conducting state. For further reduction of the on resistance a positive voltage of up to 2V can be applied.
An alternative to drive the JFET directly is to operate it in a cascode. The advantage of the cascode is that the JFET can be driven as a conventional MOSFET. However this topology also has two disadvantages: first, a positive gate-source voltage cannot be applied to the JFET and therefore the optimum conducting performance cannot be achieved; second, the full dynamic potential cannot be utilized, because the switching performance is mainly given by the Si-MOSFET. This analysis noted that Semisouth and Infineon have introduced driver concepts in their application notes that combine the advantages of the cascode with the direct driven approach. Danfoss found that the effort for the driver thus increases considerably.
A team of researchers from EADS and Friedrich-Alexander University in Erlangen, Germany, focused on the behavior and performance of SiC body diodes and all-SiC switch/diode combinations regarding reverse recovery. To compare the influence of the different devices on the recovery behavior of the combined switch, several switch/diode-pairs are used. For the active switch the Cree SiC MOSFET technology was chosen. It is readily available in two different chip sizes: 80m (CMF20120D) and 160m (CMF10120D). For the diodes, SiC Schottky diodes from Semisouth were chosen with current ratings of 5A (SDA05S120), 10A (SDP10S120D), and 30A (SDP30S120).
Reverse recovery tests were performed with single SiC MOSFET body diodes and pairs of SiC MOSFETs with SiC Schottky diodes at different conditions. Key reverse recovery parameters were extracted and compared. The manufacturer of the selected MOSFETs does not recommend the use of their body diode because of its high forward voltage and accompanying conduction losses. The researchers found that this disadvantage could be omitted if the FET channel was used for reverse conduction. This work investigated the recovery performance of the body diode and also showed that it is advisable the use a Schottky diode in parallel with the FET. This significantly decreases the required switching energy, in extreme cases by nearly 37%. It was also shown that for further improved performance, a small Schottky diode is preferable.
Fairchild teamed up with the Center of Competence for Distributed Electric Power Technologies at the University of Kassel to study SiC bipolar transistors (BJTs). The team concluded that the BJT offers one of the best alternatives among the diverse existing SiC switch technologies, given the low specific chip resistance and low temperature dependence. In addition to this they verified that common drawbacks associated with Si-BJTs – like poor dynamic behavior, low gain and limited SOA – are not applicable when SiC is used as the base material.
The application of SiC BJTs in a switching power converter demonstration resulted in very good levels of efficiency without resorting to over dimensioning the system, i.e. connecting several devices in parallel. The use of a higher switching frequency along with a new core material enabled a significant reduction of the filter inductor, which was about one-quarter of the size of the reference system. The team claimed a potential for savings in hardware by having lower chip expenditure, lower level of losses (smaller heatsink) and smaller filter elements.
In the paper from STMicroelectronics, the company’s first 20A, 1,200V SiC MOSFET prototype was presented and compared with a 1200V normally-off SiC JFET and a 1200V SiC BJT. A complete dynamic comparison analysis among the three switches in the range 1 to 7A was presented with a special focus on each device driving needs at T=25°C. According to the paper, the results in terms of total dynamic losses (driving effort is not included) are clear and impressive: the SiC MOSFET shows around 55% less total dynamic losses than the SiC JFET and around 60% less than the SiC BJT driven with a negative voltage (percentages are calculated at Id=7A,T=25°C). An additional evaluation was performed at T=125°C and Id=7A: no significant degradation was observed, except for a slight worsening of the SiC JFET turn on energy (+20% if compared to the 25°C data).
The STMicroelectronics analysis found that despite its high Ron*A value, the SiC MOSFET was demonstrated to be the most promising switch among the others under evaluation, exhibiting total dynamic losses far lower than the SiC BJT and SiC JFET normally-off and a very simple drive approach, which makes the SiC MOSFET the best candidate in the high frequency, high efficiency power conversion context.