Appliance Design - February 2008 - (Page 40) COOKING TECHNOLOGY Fig. 2. Cross-section view of a TrenchStop-cell of a RC-IGBT, second generation. It is optimized for soft-switching applications, realizing a low saturation voltage and low switching losses. called Reverse Conducting (RC), as these devices have a monolithic body diode for clamping reverse currents. The reverse conducting diode is formed using the intrinsic diode of the MOSFET structure with the IGBT P-layer shorted in specific areas to allow reverse current flow. This device is most applicable for soft switching applications such as induction cooking plates, microwave ovens, or rice cookers. On one hand, these RC-IGBTs combine a low saturation voltage and low switching losses of the IGBT. On the other hand, the integrated reverse diode has a low forward voltage drop and a low thermal resistance. Test results have achieved blocking voltages up to 1,600 V and static parameters and switching waveforms show the improvement of this technology. The first products with this feature were introduced in 2003 with 600 V devices. Further implementations of this technology in 1,200 V devices followed in 2005. Now, Infineon Technologies offers a full range of voltage classes (600 V, 900 V, 1,000 V, 1,200 V and 1,600 V) with devices having the integrated reverse diode. Fig. 2 shows a cross-section view of a TrenchStop-cell of IHW20N120R2 (Inom = 30 A, VBRces = 1,200 V), one of Infineon’s second generation of RC-IGBTs. “Soft switching” topologies use external components such as inductors or capacitors in order to force either a low current or a low voltage during the switching transition. Design engineers use two major topologies, half-bridge and single-ended. These topologies dominate the market for induction cooking. A half-bridge topology operates a resonant circuit shown in Fig. 3a, which consists of the inductance Lres and the capacitance Cres. Additionally, there is a snubber capacitance CS, which dominates the voltage rate of rise over the IGBT. The larger the capacitance CS, the slower the voltage increase dVce/dt during turn-off. The current shape in the resonant circuit is approximately sinusoidal. Both of the IGBTs take over the current for a half sinewave period according to Fig. 3b. The IGBT is turned on during the conduction period of the diode, so that the turn-on energy of the IGBT is virtually zero. During turn-off, the snubber capacitor forces a slow increase of the collector-emitter voltage, so that the turn-off energy is also highly reduced. A single-ended topology circuit and its Fig. 3a. Half-bridge topology for soft switching. 40 applianceDESIGN February 2008 Fig. 3b. Typical waveform www.applianceDESIGN.com http://www.appliancedesign.com
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