New-Tech Europe Magazine | August 2016 | Digital edition

potential in the IGBTs or SiC-FETs; insulated DC/DC converters provide an especially convenient solution in the latter. Commercially available driver modules are asymmetrically controlled by positive and negative voltage. However, IGBT and SiC-FET drivers differ in the voltage levels required. Data sheets quote threshold voltages of +3V for For IGBTs. In practice, however, quick, clean switching requires +15V, even requiring a negative bias voltage on the gate to switch off securely and prevent an undesired restoration of power due to the high dV/dt load. In practice, -9V has proven to be a safe bias voltage. Converters with voltages from +15V and -9V are therefore required for supplying IGBT drivers (Fig. 3, left). SiC-FETs raise a somewhat different situation as their switching thresholds are between +1V and +2V. A gate- source voltage of +20V makes sense to keep conductive losses (RDSON) to a minimum – settling for a conventional +15V value for IGBTs would increase RDSON losses by up to 30%. A negative voltage of -5V has established itself as ideal for them to switch off reliably (Fig. 3, right). A more negative voltage would result in a change in gate-source parameters, leading to impaired stability. High dV/dt places a strain on the insulation barrier A DC/DC converter’s insulation can be visualised as a dam dimensioned for a certain maximum level – a dam that is high enough will withstand the waves in a storm surge, but flooding even at places will cause a breach. A DC/DC converter behaves the same way. High switching frequencies –

Fig. 1 Highly insulated DC/DC converters optimise power supply to IGBT and SiC-FET drivers in power electronics

was eliminated by the development of IGBTs or insulated date bipolar transistors. As the name suggests, this is a combination of MOSFET at the input and bipolar transistor at the output, providing the best of both worlds. Like MOSFETs, IGBTs operate almost without control current, and have low voltage drops on the collector and emitter paths in their ON state as is typical of bipolar transistors allowing them to switch high voltages and currents while keeping losses very low. SiC-FETs are ideal for higher power and higher frequencies IGBTs use monocrystalline silicon as a semiconductor material compared to silicon carbide in SiC-FETs. Silicon carbide has a higher melting point and allows much thinner insulation

layers at the gate, which improves thermal conductivity and allows higher power density. The switching losses in SiC-FETs are lower than IGBTs by a factor of at least four (Fig. 2) – an advantage that especially pays off in higher performance ranges and higher switching frequencies. SiC-FETs will still only dominate in the high-performance range due to cost, and will not be able to drive IGBTs out of the mass market. optimised power supply As mentioned above, control quality is the main determinant in switching losses, so gate drivers and their power supply deserve special attention. Input signal and power supply to driver ICs need very effective insulation as they are directly coupled to the high Insulated DC/DC converters ensure

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