Daniel Domes

Stability and performance analysis of a SiC-based cascode switch and an alternative solution

Abstract Wide band-gap semiconductors are most attractive as materials for power devices due to low losses, improved temperature capability and high thermal conductivity. Although silicon carbide Schottky diodes have been commercially available on the market for years, an active wide band-gap switch is still missing. Probably the best performance of upcoming devices is gained with normally-on concepts such as silicon-carbide JFETs and gallium-nitride HEMTs. However, the vast majority of power electronic topologies rely on normally-off switches. An alternative approach is the use of the cascode concept which combines a normally-on wide band-gap device with high blocking capability and a low-voltage normally-off silicon MOSFET. In this work, the performance and stability of such an arrangement is analyzed and an alternative solution is proposed.

1200V SiC JFET in Cascode Light Configuration: Comparison versus Si and SiC Based Switches

A 1200 V SiC JFET has been demonstrated to achieve ultra-low switching losses ten times lower than for industrial grade 1200V Si IGBT. The low switching losses are also shown to compete with the fastest 600V class MOSFET in the market, yielding 1.1% higher PFC stage efficiency for 340 kHz switching frequency, when same device on-resistances were measured. The proposed normally-on JFET also differentiates over the IGBT by its purely Ohmic output characteristics without any voltage threshold, and by a monolithically integrated body diode with practically zero reverse recovery. In this paper we outline as well how the other pre-requisites for a 1200 V SiC switch in applications such as photovoltaic systems and UPS can be fulfilled by the proposed JFET solution: long-term reliability, product cost optimization by low specific on-resistance combined with reasonable process window expectations. Finally, a normally-off like safe operation behavior is ensured by a dedicated driving scheme utilizing a low-voltage Si MOSFET as protection device at system start-up and for system failure conditions.

The effect of different stray inductances on the performance of various types of IGBTs — Is less always better?

In this paper, the effect of different stray inductances on the performance of various combinations of different types of IGBTs and freewheeling diodes was studied. Therefore, a fast switching IGBT4 High-Speed was combined with either a SiC Schottky diode or a Si bipolar diode. It was analyzed how the switching losses are affected by the type of the freewheeling diode, the stray inductance and to what extent a faster switch and diode can be exploited in a low inductance circuit.

SiC JFETs for Power Module Applications

Starting with the production of Infineon´s first silicon carbide (SiC) Schottky diodes in 2001, a lot of progress was achieved during recent years. Currently, a 3rd generation of MPS (merged pn Schottky) diodes is commercially available combining tremendous improvements with respect to surge current capability and reduced thermal resistance. In this work we present the implementation of SiC switches in power modules and a comparison of these units with the corresponding Si-based power modules. Also the frequency dependence of the total losses of the 1200V configurations using Si-IGBTs or SiC-JFETs as active device is shown, indicating that modules solution with a state of the art SiC JFET outperforms all other options for switching frequencies of 20 kHz and beyond. Additionally a total loss vs. frequency study will be presented. Furthermore, it is show that the switching losses of JFET based modules can be further reduced by reducing the internal distributed gate resistivity.

Comparative Simulation Study of Dynamic Behavior of the Body-Diode for 4H-SiC JFET and MOSFET

In switching applications with half-bridge like configurations the load current is commutated to the so-called reverse or body-diode of a switching device once each switching cycle. The bipolar charge generated in the switch in principle leads to a reverse recovery current and to additional losses. Though it is well known, that in silicon carbide these reverse recovery losses are very low compared to e.g. silicon devices, it turns out that depending on device structure and switching conditions the reverse recovery charge for the JFET may become larger than can be explainable by the stored bipolar charge. In this paper therefore we focus on a simulation study comparing the body-diode operation of common lateral channel silicon carbide JFET and MOSFET devices in a so-called double pulse measurement. It is shown, that the MOSFET body-diode operation still remains uncritical under very fast switching conditions, while the JFET body-diode exhibits a pronounced recovery current peak originating from a partial channel turn-on, and thus higher losses.