Gallium Nitride (GaN) based transistors have been in the spotlight for power electronics due to promising properties like high bandgap, high breakdown field, high electron mobility, and high-frequency applications. While there are some commercial devices based on these transistors available, there is still room for improvement in these devices for widespread usage. In this project, GaN-based transistors fabricated at RISE AB were investigated. These devices had previously shown high leakage current. Different approaches taken to reduce the said leakage current were analysed. The main scope of the thesis was static electrical testing of a new batch of these transistors at room temperature, mainly investigating their leakage current. The new transistors were subjected to surface treatments and also a new in-situ dielectric layer was used. The surface treatments did not show much improvement but the in-situ grown dielectric showed almost half of the initial leakage current. In addition to this different device architectures with varying gate length, gate width, and gate to drain distance were tested and compared. It was found that devices with 3 μm gate length and 12 μm gate to drain distance showed the best performance. The blocking characteristic of the transistors was also tested and the devices could withstand up to 350V. Suggestions to further identify the sources of the leakage current are presented. Possible improvement in the design of the transistors to increase the blocking voltage is also described. / Transistorer baserade på galliumnitrid (GaN) har varit i strålkastaren för kraftelektronik på grund av lovande egenskaper som högt bandgap, högt nedbrytningsfält, hög elektronmobilitet. Dessa egenskaper gör materialet synnerligen lämpligt för komponentapplikationer vid höga effekter och, framför allt, höga frekvenser. Även om det finns några kommersiella applikationer baserade på dessa transistorer finns det fortfarande stort utrymme för förbättringar. I detta projekt undersöktes GaN-baserade transistorer tillverkade vid RISE AB. Dessa komponenter hade tidigare visat hög läckström och olika tillvägagångssätt för att minska nämnda läckström har analyserats. Transistorerna i detta projekt var ytbehandlade på ett nytt sätt och dielektirkat i styrelektroden var ocskå tillverkat på ett nytt sätt. Ytbehandlingarna visade inte mycket förbättring men det dielektrikat visade nästan hälften av den initiala läckströmmen. Utöver detta testades och jämfördes olika layouter med varierande geometri, gate-längd, gate-bredd och avstånd mellan gate/source. Det visade sig att komponenter med 3 μm gate-längd och 12 μm mellan gate och drain visade bästa prestanda. Transistorernas blockeringskaraktäristik testades också och visade sig tåla upp till 350V. Förslag för att ytterligare identifiera källorna till läckströmmen presenteras. Eventuell förbättring av utformningen av transistorerna för att öka blockeringsspänningen beskrivs också.

Gallium nitride (GaN) based high electron mobility transistors (HEMTs) are promising for power electronic applications due to their high breakdown voltage and power efficiency compared to Si-based power devices. As known, the design of the HEMT has high impact on the performance of the devices. In this project various GaN HEMTs on SiC substrate with different design configurations are characterized and investigated. These HEMTs are designed and fabricated by the Research Institutes of Sweden (RISE). The important properties of the HEMTs such as contact resistance, current density, capacitance, and breakdown voltage are characterized and emphasized. The uniformity of the contact resistance of the devices located across a 4’’ wafer is investigated, which reveals the lowest contact resistance of 4.3Ω·mm at the center of the wafer. The highest maximum current density of the devices is 1.15A/mm, and the maximum current scales with the gate dimensions of the devices. The gate capacitance of the devices is between 0.1 and 0.6pF under 1MHz. The gate insulation breakdown voltage of the devices is above 40V and the drain to source breakdown voltage is higher than 360V. Based on the results, discussions about the effects of the designs on the device performance are provided. Suggestions for further improvement of the device performance are given. / Galliumnitrid (GaN) baserade högelektronmobilitetstransistorer (HEMTs) är lovande för kraftelektroniska applikationer på grund av deras höga nedbrytningsspänning och effektivitet jämfört med Si-baserade kraftenheter. Som känt har designen av HEMT stor inverkan på enheternas prestanda. I detta projekt karakteriseras och undersöks olika GaN HEMTs på SiC-substrat med olika designkonfigurationer. Dessa HEMTs är designade och tillverkade av Sveriges forskningsins titut (RISE). De viktiga egenskaperna hos HEMTs såsom kontaktmotstånd, strömtäthet, kapacitans och nedbrytningsspänning karakteriseras och betonas. Enhetligheten i kontaktmotståndet för enheterna som är placerade över en 4'' skiva undersöks, vilket avslöjar det lägsta kontaktmotståndet på 4.3 Ω·mm i mitten av skivan. Den högsta maximala strömtätheten för enheterna är 1.15A/mm, och den maximala strömskalan med enheternas grindmått. Portens kapacitans för enheterna är mellan 0.1 och 0.6pF under 1MHz. Enhetsspänningen för grindisoleringen för enheterna är över 40V och avloppsspänningen till källan är högre än 360V. Baserat på resultaten ges diskussioner om designens effekter på enhetens prestanda. Förslag för ytterligare förbättring av enhetens prestanda ges.

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This work explores how GaN devices and advanced control can improve the power density of battery chargers for the plug-in hybrid electric vehicle. Gallium nitride (GaN) devices are used to increase switching frequency and shrink passive components. An innovative DC link reduction technique is proposed and several practical design issues are solved. A multi-chip-module (MCM) approach is used to integrate multiple GaN transistors into a package that enables fast, reliable, and efficient switching. The on-resistance and output charge are characterized. In a double pulse test, GaN devices show fast switching speed. The loss estimation based on the characterization results shows a good match with the measurement results of a 500 kHz GaN-based boost converter. Topology selection is conducted to identify candidates for the PHEV charger application. Popular topologies are reviewed, including non-isolated and isolated solutions, and single-stage and two-stage solutions. Since the isolated two-stage solution is more promising, the topologies consisting of an AC/DC front-end converter and an isolated DC/DC converters are reviewed. The identified candidate topologies are evaluated quantitatively. Finally, the topology of a full bridge AC/DC plus dual active bridge DC/DC is selected to build the battery charger prototype for fixed switching-frequency, low loss, and low realization complexity. The DC link capacitor is one of the major power density barriers of the charger, as its size cannot be reduced by increasing the switching frequency. This work proposed a charging scheme to reduce the DC link capacitance by balancing the ripple power from input and output given that the double-line-frequency current causes minor impact to the battery pack in terms of capacity and temperature rise. An in-depth analysis of ripple power balance, with converter loss considered, unveils the conditions of eliminating the low-frequency DC link capacitors. PWM-zero-off charging where the battery is charged by a current at double-line-frequency and DC/DC stage is turned off at the zero level of the waveform, is also proposed to achieve a better tradeoff between the DC link capacitor size and the charger efficiency. The practical design issues are outlined and the solutions are given at different levels of implementations, including the full bridge building block, the AC/DC stage, and the DC/DC stage. The full bridge section focuses on the solution of a reliable driving and sensing circuitry design. The AC/DC stage portion stresses the modulator improvement, which solves the often-reported issues of the current spike at the zero-crossing of the line voltage for the high frequency totem-pole bridgeless converter. In the DAB section, analytical expressions are given to model the converter operation at various operating conditions, which match well with the measurement results. The overall charging-system operation including the seamless transition of bi-directional power flow and the charging-profile control is verified on a laboratory GaN charger prototype at 500 kHz and 1.8 kW with an efficiency of 92.4%. To push the power density, some bulky components including the control board, the cooling system, and the chassis are redesigned. Together with other already-verified building blocks including full bridges, magnetics, and capacitors, a high-density mock-up prototype with 125 W/in3 power density is assembled. / Ph. D.

Motivated by the recent commercialization of gallium-nitride (GaN) switches, an effort was initiated to determine whether it was feasible to switch the flyback converter at 5 MHz in order to improve the power density of this versatile isolated topology. Soft switching techniques have to be utilized to eliminate the switching loss to maintain high efficiency at multi-megahertz. Compared to the traditional modeling of zero-voltage-switching quasi-square-wave converters, a numerical methodology of parameters design is proposed based on the steady-state model of zero-voltage switching quasi-square-wave flyback converter. The magnetizing inductance is selected to guarantee zero-voltage switching for the entire input and load range with the trade-off design for conduction loss and turn-off loss. A design methodology is introduced to select a minimum core volume for an inductor or coupled inductors experiencing appreciable core loss. The geometric constant Kgac = MLT/(Ac2WA) is shown to be a power function of the core volume Ve, where Ac is the effective core area, WA is the area of the winding window, and MLT is the mean length per turn for commercial toroidal, ER, and PQ cores, permitting the total loss to be expressed as a direct function of the core volume. The inductor is designed to meet specific loss or thermal constraints. An iterative procedure is described in which two- or three-dimensional proximity effects are first neglected and then subsequently incorporated via finite-element simulation. Interleaved and non-interleaved planar PCB winding structures were also evaluated to minimize leakage inductance, self-capacitance and winding loss. The analysis on the trade-off between magnetic size, frequency, loss and temperature indicated the potential for a higher density flyback converter. A small-signal equivalent circuit of QSW converter was proposed to design the control loop and to understand the small-signal behavior. By adding a simple damping resistor on the traditional small-signal CCM model, it can predict the pole splitting phenomenon observed in QSW converter. With the analytical expressions of the transfer functions of QSW converters, the impact of key parameters including magnetizing inductance, dead time, input voltage and output power on the small-signal behavior can be analyzed. The closed-loop bandwidth can be pushed much higher with this modified model, and the transient performance is significantly improved. With the traditional fix dead-time control, a large amount of loss during dead time occurred, especially for the eGaN FETs with high reverse voltage drop. An adaptive dead time control scheme was implemented with simple combinational logic circuitries to adjust the turn on time of the power switches. A variable deadtime control was proposed to further improve the performance of adaptive dead-time control with simplified sensing circuit, and the extra conduction loss caused by propagation delay in adaptive dead-time control can be minimized at multi-megahertz frequency. / Ph. D.

Power converters used in high reliability radiation hardened space applications trail their commercial counterparts in terms of power density and efficiency. This is due to the additional challenges that arise in the design of space rated power converters from the harsh environment they need to operate in, to the limited availability of space qualified components and field demonstrated power converter topologies. New radiation hardened Gallium Nitride (GaN) Field Effect Transistors (FETs) with their inherent radiation tolerance and superior performance over Silicon Power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are a promising alternative to improve power density and performance in space power converters. This thesis presents the considerations and design of a practical implementation of the Phase Shifted Full Bridge DC-DC Isolated converter with synchronous rectification for space applications. Recently released radiation hardened GaN FETs were used in the Full Bridge and synchronous rectifier power stages. A survey outlining the benefits of new radiation hardened GaN FETs for space power applications compared to current radiation hardened power MOSFETs is included. In addition, this work presents the overall design process followed to design the DC-DC converter power stage, as well as a comprehensive power loss analysis. Furthermore, this work includes details to implement a conventional hard-switched Full Bridge DC-DC converter for this application. An efficiency and component stress comparison was performed between the hard-switched Full Bridge design and the Phase Shifted Full Bridge DC-DC converter design. This comparison highlights the benefits of phase shift modulation (PSM) and zero voltage switching (ZVS) for GaN FET applications. Furthermore, different magnetic designs were characterized and compared for efficiency in both converters. The DC-DC converters implemented in this work regulate the output to a nominal 20 V, delivering 500 W from a nominal 100 V DC Bus input. Complete fault analysis and protection circuitry required for a space-qualified implementation is not addressed by this work. / MS / Recently released radiation-hardened Gallium Nitride (GaN) Field Effect Transistors (FETs) offer the opportunity to increase efficiency and power density of space DC-DC power converters. The current state of the art for space DC-DC power conversion trails their commercial counterparts in terms of power density and efficiency. This is mainly due to two factors. The first factor is related to the additional challenges that arise in the design of space rated power converters from the harsh environment they need to operate in, to the limited availability of space qualified components and field demonstrated converter topologies. The second factor lies in producing reliable radiation hardened power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs). GaN FETs not only have better electrical performance than power MOSFETs, they have also demonstrated inherent tolerance to radiation. This results in less structural device changes needed to make GaN FETs operate reliably under high radiation compared to their MOSFETs counterparts. This work outlines the design implications of using newly released radiation hardened GaN FETs to implement a fixed frequency isolated Phase Shifted Full Bridge DC-DC converter while strictly abiding to the design constraints found in space-power converter applications. In addition, a one-to-one performance comparison was made between the soft-switched Phase Shift modulated Full Bridge and the conventional hard-switched Full Bridge DC-DC converter. Finally, different magnetic designs were evaluated in the laboratory to assess their impact on converter efficiency.