Abstract
A DC-DC hybrid switched-capacitor LLC resonant converter integrating a ladder cell at the input of the LLC resonant converter using frequency modulation is proposed in this paper. This converter has six switches that are subjected to half the voltage of the input source. All switches commutate at zero voltage over the entire load range, and the diodes of the output rectifier bridge commutate at zero current. The proposed converter has the following characteristics: (a) symmetrical operation, (b) simple frequency modulation, (c) commutation of all switches at zero voltage, (d) all switches subjected to half the input voltage, and (e) static gain practically immune to load variations. Theoretical analysis, design, and experimental results in the laboratory for a prototype of 2 kW, 1000 VDC input, 48 VDC output, and 90 kHz switching frequency are included in this study. The maximum efficiency measured was 97.3%.
Highlights
INTRODUCTIONResonant converters are widely used when a high energy density is desired, as they facilitate soft commutation of their switches, which enables the use of high switching frequencies and thereby reductions in the weight and volume of magnetic devices
The pioneer device among resonant converters was the resonant series converter [1] proposed by Schwarz in the 1970s, which has been widely used since due to its characteristics of a high switching frequency, a high power density, low switching losses, and sinusoidal currents in the components, which means low EMI emission
Entire operating range in primary MOSFETs and ZCS-type soft switching of output diodes operating below resonance, and all of the parasitic elements being used to achieve ZVS commutation of switches [9]–[14]. Despite these advantages of the LLC converter over other isolated DC-DC converters, the switches are subjected to the input voltage, which makes its use difficult in applications where this voltage is high, for example, when the voltage comes from the output of a three-phase PWM rectifier with active power factor correction or in PV generation and direct current microgrid applications
Summary
Resonant converters are widely used when a high energy density is desired, as they facilitate soft commutation of their switches, which enables the use of high switching frequencies and thereby reductions in the weight and volume of magnetic devices. The LLC converter is widely used due to its main characteristics, such as high power density, high efficiency, possibility of regulating the output voltage through small variations in the switching frequency, ZVS-type soft commutation over the. We will demonstrate that the proposed converter has the main characteristics of the LLC converter, such as soft commutation (ZVS) of all switches, high immunity of the static gain to load variations, and symmetrical operation of the isolation transformer, with the advantage of semiconductors being subjected to half the voltage of the input source, which allows the use of low-voltage switches. The commutation of rectifier diodes occurs at zero current, further contributing to the converter efficiency Another important advantage of the proposed solution, regarding the solutions employing the three-level technique, is the natural equalization of the voltages across the capacitors associated in series, without the need for voltage sensors or control of individual voltages.
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