Very High Frequency Galvanic Isolated Offline Power Supply

Jeppe Arnsdorf Pedersen

Research output: Book/ReportPh.D. thesis

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During the last decades many researchers have turned their attention to raising the operation frequency of power converters to the very high frequency (VHF) range going from 30 MHz to 300 MHz. Increasing the operating frequency of a power converter leads to smaller energy storing components and hence smaller volume. The smaller volume comes from the passive components that scales inversely with frequency, and thereby decrease in value and size as the frequency is increased. Air-core magnetics are the preferred choice for VHF converters as there is no core losses and the selection of magnetic materials are still very limited. Ceramic capacitors are very suitable for VHF converters and with new semiconductor materials, this area will only grow in the near future.
Increasing the frequency to the VHF range also increases the switching losses, there-fore soft switching techniques are used to eliminate the switching losses. The topologies used, are inspired by radio frequency amplifiers, which are used to generate high fre-quency current for an antenna. In VHF converters this antenna is replaced by a rectifier to generate a DC voltage. Driving these type of converters can be a challenge as hard gating of traditional silicon MOSFETs generates losses that are frequency depended. Several soft gating solutions have been presented that limit the losses at these fre-quencies. In this thesis, one gate drive solution is described and used for multiple implemented converters. The gate drive is self-oscillating and consists of a few passive components ensuring a low cost. This gate drive is used as both a low side gate driver for an inverter and a synchronous rectifier in a bidirectional converter achieving a peak efficiency of 80% at 37 MHz. This gate drive is also implemented with a coupled induc-tor to drive a half-bridge based converter achieving 81% efficiency at an input voltage of 80 V.
The subject of this thesis is off line VHF converters, which means input voltages of several hundred volts. As most VHF topologies have relatively high voltage stresses with peak voltages reaching multiple times the input voltage, the resonant currents needed to achieve soft switching will generate high AC losses. A solution to this problem is presented in this thesis where several inverters have their input connected in series to split the high input voltage. An off-line converter build for the US mains is presented build with three inverters with a single combined rectifier. The converter designed to deliver 9 W to a 60 V LED load and is achieving an efficiency of 89.4% and a power density of 2.14 W3 . The development of this converter proof that offline VHF converter can be implemented with high efficiencies even for low power applications.
VHF converters are also subject to EMC regulations and the need for galvanic isolation as well as other standards. Galvanic isolation is usually implemented with transform-ers, however as mentioned earlier there are only limited magnetic materials for the VHF range. In this thesis PCB transformers are described together with the possibility of using capacitors as the power galvanic isolation, both methods of creating galvanic isolation are implemented in converters. Regarding EMC a series of converters with different filter implementations are examined. The results from the conducted mea-surement from 150 kHz to 30 MHz shows no peaks as the switching frequency is above the measured range. However, the radiated measurement shows high peaks with no filter, these peaks are reduced to 6 dB below the limits from EN 55022 with filtering and a EMC shield. The filter component scales with frequency, and therefore the implemented filter were still small compared to a traditional EMC filter.
Due to the high frequencies of VHF converters it is difficult to use traditional pulse width modulation (PWM) control, instead the most used are burst mode (on/off) control which can be implemented with commercial available controllers. A new imple-mentation of burst mode together with a self-oscillating gate drive is presented.
Another approach is frequency control where the switching frequency is controlled in respect to the output or input. A converter topology is analyzed with the first harmonic approach to evaluate the operation, and to create a new frequency control method to ensure a good power factor (PF). A prototype is implemented with the frequency control method, which achieves a PF of 0.99 and a THD of 5.68%. This is for a converter, switching at traditional frequencies, however the same method can be used for VHF converters.
During this Ph.D. thesis, different areas of an offline VHF converters are described, dur-ing the project different areas have been investigated such as, gate drive, synchronous rectifiers, PCB transformers, control of a resonant converter, galvanic isolation, EMC performance, power factor and stacking of converters.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages199
Publication statusPublished - 2017

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