Potential for optimisation of DC-DC converters for renewable energy by use of high bandgap diodes

Boost converters are commonly used in stand alone renewable energy systems to operate the photovoltaic (PV) generator in its maximum power point (MPP): output voltage of the solar generator is boosted to the higher level of battery voltage. The use of high switching frequency is beneficial to minimise inductor size maintaining a low ripple on DC current This however may lead to considerable switching losses in transistor and diode, which would undesirably deteriorate efficiency of the conversion. A major influence of diodes´ switching behaviour on system efficiency has been identified. This paper describes the results of a current evaluation of different rectifiers in a proven design of boost converter for a voltage level of 100 V and nominal power around 500 W-i. e. of a fast recovery epitaxial diode, of two types of high bandgap gallium arsenide (GaAs) Schottky diodes with different barrier heights and finally of a MOSFET for synchronous rectification with and without soft switching. System losses have been quantified for several operating points-varying current, frequency and switching speed. Those results are compared with the predictions gained from a simplified analytical model. Further, conducted emissions of the converter are evaluated, because diode turn off characteristics are expected to significantly influence EMC behaviour. The paper thus is intended to clarify suitability of the different rectifier technologies for an optimised DC-DC converter by analytical and experimental investigation of system behaviour, leading to a quantification of its major characteristics.