Switching Frequency Modulation of a 27 kW DC-DC Converter with Si-IGBTs in Light-Load Operation

For power management, fuel cell hybrid vehicles use a DC-DC converter in front of the battery. In the literature, the authors recommend SiC-MOSFETs and present Si-IGBTs in DC-DC converters as power semiconductors with high losses, high volume, and poor thermal performance. However, high efficiencies are usually only achieved in the nominal power range. Moreover, in light-load operation, the efficiency of DC-DC converters decreases, whereby - according to the worldwide harmonized light vehicles test procedure - vehicles rarely operate in the nominal power range. Increasing the efficiency of DC-DC converters in light-load operation is possible by switching frequency modulation. For output powers above 10 kW and Si-IGBT modules, it is necessary to reduce the switching frequency to enhance efficiency as switching losses of Si-IGBTs usually dominate converter losses. However, by decreasing the switching frequency, DC-DC converters could reach discontinuous conduction mode, and in terms of controller design, it is crucial to avoid this operation mode. This paper shows that by using powder cores, switching frequency modulation for critical conduction mode is useful for DC-DC converters for power ranges above 10 kW. This study uses a 27 kW bidirectional cascaded buck and boost converter with Si-IGBT half-bridge modules to validate the proposed modulation method. In light-load operation, the results prove power loss improvement by up to 16% in buck mode and 33% in boost mode. However, the high core volume indicates that DC-DC converters based on Si-IGBTs for power ranges above 10 kW are unsuitable for fuel cell hybrid vehicles.