New-Tech Europe Magazine | August 2016 | Digital edition

Fig. 2: Current/voltage AUC while switching on (diagrams above) and off (below) is a measure of switching losses. SiC-FETs (top and bottom right) are more efficient than IGBTs by approximately a factor of four due to the steeper switching edges – at 25°C ambient temperature and taking the losses from one body diode into account

usually 10kHz to 50kHz for IGBTs and somewhat more than 50kHz in SiC- FETs – and steep edges put constant stress on insulation components in the system. Voltage peaks can cause surges that strain the insulation in a transducer to its limits, especially in combination with “hidden” parasitic capacitances and inductances in the circuit. There is no magic formula to calculate voltage peak levels and therefore life expectancy of the system to any degree of accuracy. Measuring equipment can only help to a very limited extent, as even the relatively small capacitance in a high- voltage oscilloscope probe will taint

the result. An voltage spike measured at 2kV, for example, could well be double that without exposure to the probe. There is no final guarantee, and relying on measurement with a converter on insulation that is already tightly dimensioned risks the long-term reliability of the product. Developers therefore tend to allow sufficient safety reserves and use converters with the best possible insulation in dimensioning fast power switches. High-quality DC/DC converters for IGBT and FET SiC applications The RECOM DC/DC converters

range includes a complete range of converters designed specifically for supplying IGBT and SiC-FET drivers. The converters have asymmetric outputs at either +15V/-9 V (IGBT) or +20V/-5V (SiC-FET) at input voltages of 5V, 12V, 15V and 24V. The power required also depends on switching frequency – up to 1W is usually sufficient for around 10 kHz, whereas 2W is required for 50kHz and more. These products operate in power- sharing mode where the rated power can be distributed across the two outputs. Insulation causes additional differences. Products in the RKZ

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