Mid Sweden University

The authors report the feasibility of low profile, high power density and energy efficient isolated DC/DC converter in 3 – 4 MHz frequency region corresponding to low line input voltage of 90 – 110 V_ac suitable for 45W laptop adapter application. Using novel hybrid power transformer, GaN HEMTs, the simulation and experimental results of LLC resonant converter for DC input voltage range of 127 – 155V_dc with a regulated output voltage of 22V_dc was presented. The maximum energy efficiency of converter at the full load condition is reported to be approximately 91% whereas light load efficiency was improved using pulse skip modulation technique.

Power supply is an inevitable component for all electronic instruments as a constant DC source. Almost all power supplies use feedback loop to achieve constant DC output voltage. In a feedback loop, the galvanic isolation is required between input and output. It can be achieved either by implementation of opto-coupler feedback on the secondary side or magnetic feedback. In this paper both opto-coupler and auxiliary feed-back techniques are implemented and analyzed in a high frequency half bridge converter using coreless Printed Circuit Board (PCB) power transformer. The implementation of opto-coupler feedback is temperature sensitive and has relatively high cost. The auxiliary winding of transformer can also be used to provide feedback signals as alternate to opto-coupler. Auxiliary feedback implementation is cheaper, temperature stable and insensitive to temperature variations. In this paper, the feasibility of feedback using auxiliary winding of coreless PCB power transformer is investigated. It is observed that this technique can be used as an alternative to opto-coupler feedback.

The increased power consumption and power density demands of modern

technologies have increased the technical requirements of DC/DC and AC/DC power

supplies. In this regard, the primary objective of the power supply researcher/engineer

is to build energy efficient, high power density converters by reducing the losses and

increasing the switching frequency of converters respectively. Operating the converter

circuits at higher switching frequencies reduces the size of the passive components

such as transformers, inductors, and capacitors, which results in a compact size,

weight, and increased power density of the converter. Therefore, the thesis work is

focussed on the design, analysis and evaluation of isolated converters operating in the

1 - 5MHz frequency region with the assistance of the latest semi conductor devices,

both coreless and core based planar power transformers designed in Mid Sweden

University and which are suitable for consumer applications of varying power levels

ranging from 1 – 60W.

In high frequency converter circuits, since the MOSFET gate driver plays a prominent

role, different commercially available MOSFET gate drivers were evaluated in the

frequency range of 1 - 5MHz in terms of gate drive power consumption, rise/fall times

and electromagnetic interference (EMI) and a suitable driver was proposed.

Initially, the research was focused on the design and evaluation of a quasi resonant

flyback converter using a multilayered coreless PCB step down transformer in the

frequency range of 2.7 – 4MHz up to the power level of 10W. The energy efficiency of

this 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 MHz

frequency region, the new material device GaN HEMT was considered. The

comparisons were made on a quasi resonant flyback DC-DC converter using both the

Si 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 order

to minimize the gate drive power consumption, switching losses and to increase the

frequency 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 high

voltage Si MOSFET was designed and evaluated using a multi-layered coreless PCB

transformer in the MHz frequency region. Both the simulation and experimental

results have shown that, with the assistance of the cascode flyback converter, the

switching speeds of the converter can be increased with the benefit of obtaining a

significant improvement in the energy efficiency as compared to that for the single

switch flyback converter.

In order to further maximize the utilization of the transformer, to reduce the voltage

stress on MOSFETs and to obtain the maximum power density from the converter

circuit, double ended topologies were considered. Due to the lack of high voltage high

side gate drivers in the MHz frequency region, a gate drive circuitry utilizing the

multi-layered coreless PCB signal transformer was designed and evaluated in both a

half-bridge and series resonant converter (SRC). It was found that the gate drive power

consumption using this transformer was around 0.66W for the frequency range of 1.5 -

v

3.75 MHz. In addition, by using this gate drive circuitry, the maximum energy

efficiency of the SRC using multilayered coreless PCB power transformer was found to

be 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 (LCC

and LLC), novel hybrid core high frequency planar power transformer and the GaN

HEMTs. The simulated and experimental results of the designed LCC resonant

converter show that it is feasible to obtain higher energy efficiency isolated DC/DC

converters in the MHz frequency region. The peak energy efficiency of the LCC

converter at 3.5MHz is reported to be 92% using synchronous rectification. Different

modulation techniques were implemented to regulate the converter for both line and

load 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 the

GaN switch limitation. The energy efficiency of the regulated converter operating in

the frequency range of 2.8 – 3.5MHz is reported to be more than 90% with a load

power of 45W and an output voltage of 22V

dc. In order to determine an efficient power

processing method on the secondary side of the converter, a comparison was made

between diode rectification and synchronous rectification and optimal rectification was

proposed for the converters operating in the MHz frequency range for a given power

transfer application. In order to maintain high energy efficiency for a wide load range

and to maintain the narrow switching frequency range for the given input voltage

specifications, the LLC resonant converter has been designed and evaluated for the

adapter application. From the observed results, the energy efficiency of the LLC

resonant converter is maintained at a high level for a wide load range as compared to

that for the LCC resonant converter.

Investigations were also carried out on isolated class E resonant DC-DC converter with

the assistance of GaN HEMT and a high performance planar power transformer at the

switching frequency of 5MHz. The simulated energy efficiency of the converter for the

output power level of 16W is obtained as 88.5% which makes it feasible to utilize the

designed isolated converter for various applications that require light weight and low

profile converters.

In conclusion, the research in this dissertation has addressed various issues related to

high frequency isolated converters and has proposed solution by designing highly

energy efficient converters to meet the current industrial trends by using coreless and

core 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, high

power density DC/DC and AC/DC converters operating in MHz frequency region

suitable for various applications.

The authors report the utilization of the core based transformer for power transfer applications with high power density and high energy efficiency in the MHz frequency region. A custom made POT core center tapped transformer of 4:1:1 turn’s ratio using novel winding strategy with the core diameter of 16mm is designed and evaluated. The designed transformer has been characterized using sinusoidal excitation for a given output power in the frequency range of 1 – 10MHz and determined the operating frequency region of the transformer. The power tests of the transformer has been carried out up to the power level of 62W at an operating frequency of 6.78MHz with a peak energy efficiency of 98.5% resulting in the record power density of ~1100W/in³. The designed transformer has been characterized using class E isolated DC-DC converter topology at an output power of approximately 18W. The simulated energy efficiency of the converter is 88.5% under the full load condition. This work provides the significant step for the development of next generation high power density isolated converters (both AC/DC and DC/DC) in MHz frequency region

In this paper, design and analysis of an isolated low profile, series resonant converter (SRC) using multilayered coreless printed circuit board (PCB) power transformer was presented. For obtaining the stringent height switch mode power supplies (SMPS), a multilayered coreless PCB power transformer of approximately 4:1 turn’s ratio was designed in a four layered PCB laminate which can be operated in MHz switching frequency. The outermost radius of the transformer is 10mm with achieved power density of 16W/cm². The energy efficiency of the power transformer is found to be in the range of 87-96% with the output power level of 0.1-50W operated at a frequency of 2.6MHz. This designed step-down transformer was utilized in SRC and evaluated. The supply voltage of converter is varied from 60-120V DC with a nominal input voltage of 90V and has been tested up to the power level of 34.5W. The energy efficiency of converter under Zero Voltage Switching (ZVS) condition is found to be in the range of 80-86.5% with the switching frequency range of 2.4-2.75MHz. By using constant off-time frequency modulation technique, the converter was regulated to 20V DC for different load conditions. Thermal profile with converter loss at nominal voltage is presented.

The significant problems in increasing the switching frequency of isolated converters to achieve low cost and high power density are increased magnetic and switching losses. This paper presents solution with the investigation of newly designed high frequency, low profile, hybrid power transformer together with commercially available GaN MOSFETs. For achieving stringent power converters, a center tapped 4:1:1 half bridge transformer was designed in a multilayered PCB laminate with a total height of transformer as 4.2mm. The maximum obtained energy efficiency of transformer is 98% in the frequency region of 1 – 5MHz. The designed transformer possesses better high frequency characteristics in the frequency range of 3 - 5MHz with tested power density of 47W/cm³ at an operating frequency of 3MHz. This transformer has been utilized in the multi resonant zero voltage switching (ZVS) half bridge converter with synchronous rectification using GaN MOSFETs and then evaluated. The maximum achieved efficiency of multi resonant ZVS half bridge converter with this transformer is 92% in 3 – 4.5 MHz switching frequency range with characterized output power of 40W. This is to the author’s knowledge, the best energy efficiency ever reported for an isolated DC-DC converter in this frequency range and power levels. At a specified input voltage, the converter has been regulated to 15V within 1% for both line and wide load variations. This work provides considerable step for the development of next generation ultra flat low profile isolated DC-DC/AC-DC converters operating in MHz frequency region.

The fabrication of emerging power semiconductor devices and high frequency PCB power transformers has made it possible to design the power converters in MHz switching frequency range. However, the higher switching frequency, di/dt loops and dv/dt nodes in power stages of these converters generate higher order harmonics which causes Electro Magnetic Interference (EMI). It is commonly believed that the EMI has worst affect in the converters switching in MHz frequency range than the converters operating below 150 kHz. Thus, it is important research direction to investigate the consequences of implementing a line filter to suppress the conducted EMI in high frequency power converters. In this paper, the measurements, and analysis of the conducted EMI in emerging power converters, switching in MHz frequency range, and the design of the filter for its suppression is presented. The design of LISN and its PCB implementation for EMI measurements is presented. The measurement of conducted EMI of a half bridge DC-DC converter switching at          3.45 MHz and the analysis of the frequency spectrum is discussed. The design, PCB implementation and characterization of the EMI filter and the measurement of the suppressed conducted noise by applying the filter are also discussed.

A half bridge transformer with novel winding strategy was designed and tested up to the output power level of 50W with a maximum energy efficiency of 98% in 1-6 MHz frequency region. In this paper, the effect of air gap on the performance of the designed transformer for high frequency (MHz) Switch Mode Power Supplies (SMPS) was studied and analyzed. The air gap of the transformer was varied from 0 – 2mm and the effect on self, leakage, mutual inductances, coupling coefficient and power transfer capability was recorded in 1 - 6MHz. The parameters of the transformer with different air gaps were extracted using network analyzer. The transformer was excited with sinusoidal voltages using power amplifier and the performance characteristics such as input impedance, power transfer capability and hence the energy efficiency was measured. At a particular operating frequency of 3MHz, the maximum power transferred for the given excitation voltage with 0mm air gap was found to be 22W whereas it is only 8W with the 2mm air gap . The measured energy efficiency of the transformer at 3MHz with 0mm and 2mm air gap was found to be 93% and 79% respectively. With this study, for the given power transfer application, at a particular high frequency operation of converter, an optimum air gap without sacrificing the energy efficiency and core saturation was proposed

In this paper, the effect of two different dielectric materials in planar printed circuit board (PCB) transformers applicable for power transfer applications in MHz frequency region is discussed. The 2-D model of the planar transformer was developed and the effect of the dielectric material on the magnetic field and the current distribution in the transformer was analyzed. Based on the FEA analysis, the power transformers of two different dielectric materials were designed, manufactured and characterized by using ‘S’ parameters obtained from network analyzer in order to determine the performance of dielectrics at high frequencies. The electrical parameters such as inductances, capacitances and resistances of the transformers were obtained and the changes in these parameters with the variation of the dielectric material were analyzed in the MHz frequency. This paper discusses the effect of dielectric material on the magnetic field distribution and the current density which results in the variation of the coupling coefficient and the eddy current losses of the transformer. By changing the dielectric material from a traditional FR-4 to a Rogers, the coupling coefficient was improved by a maximum of 5% and the copper losses of transformer were also decreased in the MHz frequency region. The power gain/energy efficiency obtained from the network analyzer with a resonant capacitor is verified with those obtained using a power amplifier and the simulations. An overall energy efficiency improvement of about 2-5% was achieved with a Rogers’s material in MHz frequency region compared to traditional FR-4 laminate.

In this paper the comparative results of two layered and three layered coreless Printed Circuit Board (PCB) step down 2:1 power transformers operating in MHz frequency were addressed. The  two different step down transformers approximately having same self inductances, one in two layer and the other in three layer were designed and evaluated for the given power transfer application. The performance characteristics of these transformers under similar conditions were measured and the comparative parameters of these transformers in terms of their resistances, self, leakage, mutual inductances, and coupling coefficient are analyzed. For the given output power, the measured energy efficiency of the three layered transformer is improved by 3% and the area is reduced by 32% compared to two layered transformer. The efficiency of the three layered transformer is 94.5% approximately for an output power level of 25W at an operating frequency of 2.5MHz