Mid Sweden University

The Power Supply Unit (PSU) plays a vital role in almost all electronic equipment. The continuous efforts applied to the improvement of semiconductor devices such as MOSFETS, diodes, controllers and MOSFET drivers have led to the increased switching speeds of power supplies. By increasing the switching frequency of the converter, the size of passive elements such as inductors, transformers and capacitors can be reduced. Hence, the high frequency transformer has become the backbone in isolated AC/DC and DC/DC converters. The main features of transformers are to provide isolation for safety purpose, multiple outputs such as in telecom applications, to build step down/step up converters and so on. The core based transformers, when operated at higher frequencies, do have limitations such as core losses which are proportional to the operating frequency. Even though the core materials are available in a few MHz frequency regions, because of the copper losses in the windings of the transformers those which are commercially available were limited from a few hundred kHz to 1MHz. The skin and proximity effects because of induced eddy currents act as major drawbacks while operating these transformers at higher frequencies. Therefore, it is necessary to mitigate these core losses, skin and proximity effects while operating the transformers at very high frequencies. This can be achieved by eliminating the magnetic cores of transformers and by introducing a proper winding structure.

A new multi-layered coreless printed circuit board (PCB) step down transformer for power transfer applications has been designed and this maintains the advantages offered by existing core based transformers such as, high voltage gain, high coupling coefficient, sufficient input impedance and high energy efficiency with the assistance of a resonant technique. In addition, different winding structures have been studied and analysed for higher step down ratios in order to reduce copper losses in the windings and to achieve a higher coupling coefficient. The advantage of increasing the layer for the given power transfer application in terms of the coupling coefficient, resistance and energy efficiency has been reported. The maximum energy efficiency of the designed three layered transformers was found to be within the range of 90%-97% for power transfer applications operated in a few MHz frequency regions. The designed multi-layered coreless PCB transformers for given power applications of 8, 15 and 30W show that the volume reduction of approximately 40-90% is possible when compared to its existing core based counterparts. The estimation of EMI emissions from the designed transformers proves that the amount of radiated EMI from a three layered transformer is less than that of the two layered transformer because of the decreased radius for the same amount of inductance.

Multi-layered coreless PCB gate drive transformers were designed for signal transfer applications and have successfully driven the double ended topologies such as the half bridge, the two switch flyback converter and resonant converters with low gate drive power consumption of about half a watt. The performance characteristics of these transformers have also been evaluated using the high frequency magnetic material made up of NiZn and operated in the 2-4MHz frequency region.

These multi-layered coreless PCB power and signal transformers together with the latest semiconductor switching devices such as SiC and GaN MOSFETs and the SiC schottky diode are an excellent choice for the next generation compact SMPS.

The most essential unit required for all the electronic devices is the Power Supply Unit (PSU). The main objective of power supply designers is to reduce the size, cost and weight, and to increase the power density of the converter. There is also a requirement to have a lower loss in the circuit and hence in the improvement of energy efficiency of the converter circuit. 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. At present the switching frequency of the converter circuit is limited due to the increased switching losses in the existing semiconductor devices and in the magnetic area, because of increased hysteresis and eddy current loss in the core based transformer. Based on continuous efforts to improve the new semi conductor materials such as GaN/SiC and with recently developed high frequency multi-layered coreless PCB step down power transformers, it is now feasible to design ultra-low profile, high power density isolated DC/DC and AC/DC power converters. This thesis is focussed on the design, analysis and evaluation of the converters operating in the MHz frequency region with the latest semi conductor devices and multi-layered coreless PCB step-down power and signal transformers.

An isolated flyback DC-DC converter operated in the MHz frequency with multi-layered coreless PCB step down 2:1 power transformer has been designed and evaluated. Soft switching techniques have been incorporated in order to reduce the switching loss of the circuit. The flyback converter has been successfully tested up to a power level of 10W, in the switching frequency range of 2.7-4 MHz. The energy efficiency of the quasi resonant flyback converter was found to be in the range of 72-84% under zero voltage switching conditions (ZVS). The output voltage of the converter was regulated by implementing the constant off-time frequency modulation technique.

Because of the theoretical limitations of the Si material MOSFETs, new materials such as GaN and SiC are being introduced into the market and these are showing promising results in the converter circuits as described in this thesis. Comparative parameters of the semi conductor materials such as the

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energy band gap, field strengths and figure of merit have been discussed. In this case, the comparison of an existing Si MOSFET with that of a GaN MOSFET has been evaluated using a multi-layered coreless PCB step-down power transformer for the given input/output specifications of the flyback converter circuit. It has been determined that the energy efficiency of the 45 to 15V regulated converter using GaN was improved by 8-10% compared to the converter using the Si MOSFET due to the gate drive power consumption, lower conduction losses and improved rise/fall times of the switch.

For some of the AC/DC and DC/DC applications such as laptop adapters, set-top-box, and telecom applications, high voltage power MOSFETs used in converter circuits possess higher gate charges as compared to that of the low voltage rating MOSFETs. In addition, by operating them at higher switching frequencies, the gate drive power consumption, which is a function of frequency, increases. The switching speeds are also reduced due to the increased capacitance. In order to minimize this gate drive power consumption and to increase the frequency of the converter, a cascode flyback converter was built up using a multi-layered coreless PCB transformer and this was then evaluated. Both simulation and experimental results have shown that with the assistance of the cascode flyback converter the switching speeds of the converter were increased including the significant improvement in the energy efficiency compared to that of 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 power converter, double ended topologies were chosen. For this purpose, a gate drive circuitry utilising the multi-layered coreless PCB gate drive transformer was designed and evaluated in both a Half-bridge and a Series resonant converter. It was found that the gate drive power consumption using this transformer was less than 0.8W for the frequency range of 1.5-3.5MHz. In addition, by using this gate drive circuitry, the maximum energy efficiency of the series resonant converter was found to be 86.5% with an output power of 36.5W.

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 -

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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.