Enhanced PFM-PSM Hybrid Control of CLLC Resonant Converter for Electric Vehicles: Comprehensive Study and Verification

The conventional
CLLC
resonant converters have been generally operated by the pulse-frequency modulation (PFM) method, which changes switching frequency according to the load conditions. However, it causes high switching losses and the first harmonic approximation error, particularly for step-down operations. To handle these issues, several studies, which combines the PFM and phase-shift modulation (PSM) methods, have been reported. Nevertheless, they still have the problem with so-called
inoperative region
, which degrades overall efficiency and reliability during mode changes in wide ranges of voltage. To overcome this limitation, this article proposes the new voltage conversion ratio (VCR) based PFM-PSM hybrid control (VFSHC) method for the
CLLC
resonant converters applied to electric vehicles. It can effectively eliminate the
inoperative region
by changing the PFM and PSM methods based on the reference VCR, which is mathematically determined. Therefore, two methods can be changed seamlessly and successfully without causing undesired transients. Also, it is able to improve overall efficiency by applying the PSM method when the required VCR is lower than the reference VCR. The comprehensive study of proposed VFSHC method is theoretically made, and its practical effectiveness is verified by experimental tests on the hardware prototype of 1.5 KW.