Optimum design and novel control techniques for isolated, resonant and quasi-resonant, DCDC converters

Resumen

MOTIVATION AND OBJECTIVES The power supplies or power converters are required in many applications to convert between the AC of the distribution grid to AC or DC at different voltage and current levels required by the electric loads. Each applica-tion may include different conversion steps and at different voltage levels. In every conversion step part of the energy is lost and dissipated in the form of unwanted heat. The conversion efficiency in every step is the result of dividing the utilized output power by the total input power in the converter. The total efficiency of the system is the result of multiplying the individual efficiencies of the stages, and will always be necessarily lower than the minimum of the individual efficiencies in the conversion chain. Traditional linear power supplies have been extensively replaced by the more efficient and smaller Switched Mode Power Supplies (SMPS). SMPS makes use of semiconductor switches commutating at a relatively high frequency and reactive circuit elements (inductors and capacitors) to perform the power conversion. The semi-conductor switches in SMPS alternate between their on state, where their voltage drop is minimal, and their off state, where they don’t conduct current. However, during the switches turn-on transition and turn-off transition, there is certain overlap of voltage and current which causes switching losses. For the currently available semiconductor power switching devices the resonant and quasi-resonant convert-ers are the most attractive options for achieving high efficiency, high power density or a combination of both at a reasonable cost. The key advantage of the resonant and quasi-resonant converters is the reduction of the switching losses. The switching losses can be reduced achieving Zero Voltage Switching (ZVS) for the turn-on transition and Zero Current Switching (ZCS) for the turn-off transition. Among the isolated, resonant and quasi-resonant DCDC converter topologies, the most common ones in medium-high power and high-voltage applications are the series-parallel resonant converter (LLC), the Phase Shifted Full Bridge (PSFB) and the Dual Active Bridge (DAB) due to their simplicity and high efficiencies. Among those topologies each one has its own advantages and disadvantages, which makes each of them best suited for different applications, power and voltage ranges. This thesis is focused in the study of the LLC and the PSFB. Both of this topologies are very promising and well suited for any of the available power switch semiconductor technologies in the market, and more specifi-cally for Si Super Junction (SJ) MOSFETs, which are the most mature devices and the most competitive in cost currently and in the foreseeable future. The general objective of this thesis is the optimization, improved reliabil-ity and functionality of the LLC and the PSFB converters maintaining as much as possible the simplicity of the standard circuit configuration. Therefore, this thesis comprises the design and construction of several DCDC converters with similar specifi-cations and similar magnetic structures to study the achievable performance of the two main topologies objec-tive of this work: LLC and PSFB. More specifically, the fundamental research objectives of this thesis include: • The development of very high-efficiency DCDC converters for server applications, including the design optimization of quasi-resonant converters experimentally demonstrated with a 1.4 kW PSFB converter from 400 V to 12 V. • The development of very high-efficiency DCDC converters for telecommunitation applications, including the design optimization of quasi-resonant converters, experimentally demonstrated with a 3.3 kW PSFB converter from 380 V to 54.5 V, and the design optimization of resonant converters, experimentally de-mostrated with a 3.3 kW half-bridge LLC converter from 400 V to 52 V. • The development of bidirectional DCDC converters for Electrical Energy Storage (EES) applications, includ-ing the bidirectional operation of traditionally considered non-bidirectional PSFB converters, experimentally demonstrated with a 3.3 kW bidirectional PSFB from 380 V to 54 V and from 50 V to 400 V. • The integration of a very high-efficiency DCDC converter in a complete two-stage off-line Power Supply Unit (PSU) for server applications, experimentally demonstrated with a 3 kW totem-pole ACDC converter followed by a 3 kW half-bridge LLC DCDC converter from 400 V to 50 V. Once the objectives have been outlined and after the previous brief theoretical background, in the following we summarize the several contributions of this thesis together with the most relevant results obtained and the conclusions that could be derived from them. RESULTS AND CONCLUSIONS It has been found that, although PSFB can achieve higher efficiencies than what is commonly expected thanks to the newest devices and the newly proposed control techniques, the LLC is still potentially superior and capable of higher efficiencies with less components at a lesser cost. However, in bidirectional applications the large signal gain range of the PSFB makes it arguably superior to the LLC. A 3300 W bidirectional PSFB DCDC converter from 380 V to 54 V has been designed and built achieving 98 % efficiency in forward or buck mode and 97% in reverse or boost mode. The achieved power density is in the range of 4.34 W/cm³ (71.19 W/in³), which is enabled by the use of Surface Mount Device (SMD) packages, the innovative stacked magnetic construction and the innovative cooling solution. This DCDC converter proves the feasibility of PSFB topology as a high efficiency topology at the level of fully resonant topologies when com-bined with the latest SJ MOSFET technologies. This DCDC converter proves as well that the PSFB topology can be used as a bidirectional DCDC stage without changes in the standard design or construction of a traditional and well known topology, but only through innovations in control techniques powered by digital control. A 3300 W LLC DCDC converter from 400 V to 51.5 V has been designed and built achieving 98.1 % peak of efficiency. The achieved power density is in the range of 4 W/cm³ (66 W/in³). The optimized layout and an optimized driving circuitry achieves benchmark performance with minimum stress on the devices, enabled also by the innovative cooling concepts presented in this converter. This DCDC converter proves the feasibility of the half-bridge LLC as a high-efficiency topology for a 3300 W converter, at the level of full-bridge LLC or a dual stage LLC. This DCDC converter also proves that digital control is not only capable of controlling the LLC topology but the most effective way to overcome its difficulties and pitfalls. Moreover, the included protections mechanisms and control schemes further boost the reliability and performance of the converter achieving the best possible efficiency. A 1400 W PSFB DCDC converter from 400 V to 12 V has been designed and built achieving 97 % peak effi-ciency. The achieved power density is in the range of 3.70 W/cm³ (60.78 W/in³) thanks to the full SMD solution. This DCDC converter proves that an analytical optimum design procedure can further boost the performance of the PSFB near fully resonant topologies for low voltage outputs. Finally, a complete 3 kW two stage off-line PSU for server applications has been designed and built achieving 97.5 % peak efficiency. The overall outer dimensions of the PSU are 73.5 mm x 520 mm x 40 mm, which yields a power density in the range of 32 W/inch³ (1.95 W/cm³). Due to the outer dimension limits, a planar transformer with reduced height and volume was preferred for this design. In the next paragraphs the described contributions correspond to the published works that constitute this the-sis by compendium of publications. In the first contribution, a novel modulation scheme is proposed for the bidirectional operation of the PSFB DCDC converter reducing the secondary side rectifiers overshoot without requiring additional circuitry or lossy snubbering techniques. The novel modulation scheme is experimentally demonstrated in a 3.3 kW PSFB DCDC converter with nominal 380 V input and nominal 54.5 V output. M. Escudero, D. Meneses, N. Rodriguez and D. P. Morales, "Modulation Scheme for the Bidirectional Opera-tion of the Phase-Shift Full-Bridge Power Converter," in IEEE Transactions on Power Electronics, vol. 35, no. 2, pp. 1377-1391, Feb. 2020. In the second contribution, a novel modulation scheme is proposed for the reduction of the overshoot in the secondary side rectifiers in PSFB DCDC converters operating in Discontinuous Conduction Mode (DCM). Fur-thermore, other secondary side rectifiers overshoot causes are analyzed and solutions proposed for each of the scenarios. The novel modulation scheme and the design principles are experimentally demonstrated in a 3.3 kW PSFB DCDC converter with nominal 380 V input and nominal 54.5 V output. M. Escudero, M. Kutschak, D. Meneses, D. P. Morales and N. Rodriguez, "Synchronous Rectifiers Drain Volt-age Overshoot Reduction in PSFB Converters," in IEEE Transactions on Power Electronics, vol. 35, no. 7, pp. 7419-7433, July 2020. In the third contribution, a detailed set of design criteria for PSFB DCDC converters is proposed. The design optimization procedure is experimentally demonstrated with the design of a new 1.4 kW PSFB DCDC converter further exceeding the efficiency and power density of a previous 1.4 kW PSFB DCDC converter with similar specifications, input voltage (400 V) and output voltage (12 V). Escudero, M.; Kutschak, M.-A.; Meneses, D.; Rodriguez, N.; Morales, D.P. A Practical Approach to the De-sign of a Highly Efficient PSFB DC-DC Converter for Server Applications. Energies 2019, 12, 3723. In the fourth contribution the impact in the performance and the design of resonant ZVS converters of the non-linear distribution of charge in the capacitances of semiconductor devices is analyzed. A Si SJ device is compared to a SiC device of equivalent Coss(tr), and to a GaN device of equivalent Coss(er), in single device topol-ogies and half-bridge based topologies, in full ZVS and in partial or full hard-switching. A prototype of 3300 W resonant LLC DCDC converter, with nominal 400 V input to 52 V output, was designed and built to demon-strate the validity of the analysis. M. Escudero, M. Kutschak, N. Fontana, N. Rodriguez and D. P. Morales, "Non-Linear Capacitance of Si SJ MOSFETs in Resonant Zero Voltage Switching Applications," in IEEE Access, vol. 8, pp. 116117-116131, 2020. In the fifth contribution the design of a new complete power supply for server applications is discussed. The main constraints of the new solution are the very high peak efficiency, the long hold-up time and the maximum outer dimension. The PSU is comprised of a front-end ACDC bridgeless totem-pole PFC and a back-end DCDC half-bridge LLC. Due to the extended hold-up time, the PSU requires large intermediate energy storage and an extended gain range for the LLC. Due to the outer dimension limits, a planar transformer with reduced height and volume is preferred. Finally, the very high efficiency target requires a careful design based on the accurate modeling of the overall losses of the converter. The analysis is demonstrated experimentally with a 3 kW PSU achieving 97.47 % of efficiency at 230 VAC. M. Escudero, M. -A. Kutschak, D. Meneses, N. Rodriguez and D. P. Morales, "High Efficiency, Narrow Output Range and Extended Hold-Up Time Power Supply with Planar and Integrated Magnetics for Server Applica-tions," PCIM Europe digital days 2021; International Exhibition and Conference for Power Electronics, Intel-ligent Motion, Renewable Energy and Energy Management, 2021, pp. 1-8.