High Frequency (MHz) Resonant Converters using GaN HEMTs and Novel Planar Transformer Technology

Sammanfattning: The increased power consumption and power density demands of moderntechnologies have increased the technical requirements of DC/DC and AC/DC powersupplies. In this regard, the primary objective of the power supply researcher/engineeris to build energy efficient, high power density converters by reducing the losses andincreasing the switching frequency of converters respectively. Operating the convertercircuits at higher switching frequencies reduces the size of the passive componentssuch as transformers, inductors, and capacitors, which results in a compact size,weight, and increased power density of the converter. Therefore, the thesis work isfocussed on the design, analysis and evaluation of isolated converters operating in the1 - 5MHz frequency region with the assistance of the latest semi conductor devices,both coreless and core based planar power transformers designed in Mid SwedenUniversity and which are suitable for consumer applications of varying power levelsranging from 1 – 60W.In high frequency converter circuits, since the MOSFET gate driver plays a prominentrole, different commercially available MOSFET gate drivers were evaluated in thefrequency range of 1 - 5MHz in terms of gate drive power consumption, rise/fall timesand electromagnetic interference (EMI) and a suitable driver was proposed.Initially, the research was focused on the design and evaluation of a quasi resonantflyback converter using a multilayered coreless PCB step down transformer in thefrequency range of 2.7 – 4MHz up to the power level of 10W. The energy efficiency ofthis converter is found to be 72 - 84% under zero voltage switching conditions (ZVS).In order to further improve the energy efficiency of the converter in the MHzfrequency region, the new material device GaN HEMT was considered. Thecomparisons were made on a quasi resonant flyback DC-DC converter using both theSi and GaN technology and it was found that an energy efficiency improvement of 8 –10% was obtained with the GaN device in the frequency range of 3.2 – 5MHz. In orderto minimize the gate drive power consumption, switching losses and to increase thefrequency of the converter in some applications such as laptop adapters, set top box(STB) etc., a cascode flyback converter using a low voltage GaN HEMT and a highvoltage Si MOSFET was designed and evaluated using a multi-layered coreless PCBtransformer in the MHz frequency region. Both the simulation and experimentalresults have shown that, with the assistance of the cascode flyback converter, theswitching speeds of the converter can be increased with the benefit of obtaining asignificant improvement in the energy efficiency as compared to that for the singleswitch flyback converter.In order to further maximize the utilization of the transformer, to reduce the voltagestress on MOSFETs and to obtain the maximum power density from the convertercircuit, double ended topologies were considered. Due to the lack of high voltage highside gate drivers in the MHz frequency region, a gate drive circuitry utilizing themulti-layered coreless PCB signal transformer was designed and evaluated in both ahalf-bridge and series resonant converter (SRC). It was found that the gate drive powerconsumption using this transformer was around 0.66W for the frequency range of 1.5 -v3.75 MHz. In addition, by using this gate drive circuitry, the maximum energyefficiency of the SRC using multilayered coreless PCB power transformer was found tobe 86.5% with an output power of 36.5W in the switching frequency range of 2 –3MHz.In order to further enhance the energy efficiency of the converter to more than 90%,investigations were carried out by using the multiresonant converter topology (LCCand LLC), novel hybrid core high frequency planar power transformer and the GaNHEMTs. The simulated and experimental results of the designed LCC resonantconverter show that it is feasible to obtain higher energy efficiency isolated DC/DCconverters in the MHz frequency region. The peak energy efficiency of the LCCconverter at 3.5MHz is reported to be 92% using synchronous rectification. Differentmodulation techniques were implemented to regulate the converter for both line andload variations using a digital controller.In order to realize an AC/DC converter suitable for a laptop adapter application,consideration was given to the low line of the universal input voltage range due to theGaN switch limitation. The energy efficiency of the regulated converter operating inthe frequency range of 2.8 – 3.5MHz is reported to be more than 90% with a loadpower of 45W and an output voltage of 22Vdc. In order to determine an efficient powerprocessing method on the secondary side of the converter, a comparison was madebetween diode rectification and synchronous rectification and optimal rectification wasproposed for the converters operating in the MHz frequency range for a given powertransfer application. In order to maintain high energy efficiency for a wide load rangeand to maintain the narrow switching frequency range for the given input voltagespecifications, the LLC resonant converter has been designed and evaluated for theadapter application. From the observed results, the energy efficiency of the LLCresonant converter is maintained at a high level for a wide load range as compared tothat for the LCC resonant converter.Investigations were also carried out on isolated class E resonant DC-DC converter withthe assistance of GaN HEMT and a high performance planar power transformer at theswitching frequency of 5MHz. The simulated energy efficiency of the converter for theoutput power level of 16W is obtained as 88.5% which makes it feasible to utilize thedesigned isolated converter for various applications that require light weight and lowprofile converters.In conclusion, the research in this dissertation has addressed various issues related tohigh frequency isolated converters and has proposed solution by designing highlyenergy efficient converters to meet the current industrial trends by using coreless andcore based planar transformer technologies along with the assistance of GaN HEMTs.With the provided solution, in the near future, it is feasible to realize low profile, highpower density DC/DC and AC/DC converters operating in MHz frequency regionsuitable for various applications.