A thesis submitted to attain the degree of
DOCTOR OF SCIENCES of ETH ZURICH
(Dr. sc. ETH Zurich)
presented by
GUSTAVO CARLOS KNABBEN
MSc EE, UFSC
born on 23.05.1992
citizen of Joinville, Brazil
accepted on the recommendation of
Prof. Dr. Johann W. Kolar, examiner
Prof. Dr. Marcelo Lobo Heldwein, co-examiner
The increasingly-electric future requires next-generation power supplies
that are compact, efficient, low-cost, and ultra-reliable, even across
mains failures, to power mission-critical electrified processes. Hold-up time
requirements and the demand for ultra-high power density and minimum
production costs, in particular, drive the need for DC/DC power converters
with (i) a wide input voltage range, to reduce the size of the hold-up capacitor,
(ii) soft-switching over the full input-voltage and load ranges, to achieve low
losses that facilitate a compact realization, and (iii) complete PCB-integration
for low-cost manufacturing. Wide-bandgap power semiconductors, with
excellent on-resistance properties and low switching and reverse-recovery
losses, come along these requirements toward the conceptualization of nextgeneration
power-supply modules, but cannot alone catapult state-of-theart
converter technology to the performance baseline of future automotive,
automated manufacturing and hyperscale data-center applications. Instead,
the combination of wide-bandgap devices with proper converter topology,
control and magnetics design has proven to be the real enabler of power
supplies for the increasingly-electric future.
This thesis makes a case for the combination of these three features (widebandgap
devices, proper topology/control and advanced magnetics) as the
keys for paving the way toward next-generation power-supply modules.
Therefore, a suitable low-complexity circuit topology with improved control
scheme that operates across a wide-input-voltage range with complete softswitching
is identified, which switches efficiently at higher frequencies and
high output currents with PCB-integrated magnetics, improving significantly
power density compared to state-of-the-art designs. This topology embeds a
sophisticated PCB-integrated matrix transformer that has a single path for the
magnetic flux, ensuring equal flux linkage of parallel-connected secondary
windings despite possible geometric PCB-layout asymmetries or reluctance
imbalances. The so-called snake-core transformer avoids the emergence of
circulating currents between parallel-connected secondary windings and
guarantees proper operation of parallel-connected, magnetically-coupled
converter modules.
The benefits of the proposed topology, control scheme and transformer design
are validated by three fabricated 300 V-430 V-input, 12 V-output DC/DC
hardware demonstrators. The converters utilize an LLC-based control scheme
for complete soft-switching and the snake-core transformer to divide the output
current with a balanced flux among multiple secondary windings. First,
a 3 kW DC/DC series-resonant converter achieves 350Win3 (21”4 kWdm3)
vii
Abstract
power density and 94 % peak efficiency, validating control and transformer
operation. Then, a second hardware prototype with 1”5 kW showcases a peak
efficiency close to 96 % and a power density of 337Win3 (20”6 kWdm3), with
full PCB-integration and zero-voltage switching even down to zero load. Finally,
the third demonstrator—a magnetically-coupled, input-parallel/outputparallel,
two-1”5 kW-module DC/DC converter—achieves a peak efficiency
of nearly 97 % and a power density of 345Win3 (21”1 kWdm3) with ideal
current sharing among modules and stable operation, important characteristics
enabled by the novel snake-core transformer. Detailed loss models are
derived for every converter’s component and the measurement results are in
excellent agreement with the calculated values. These loss models are used
to identify improvements to further boost efficiency, the most important of
which is the minimization of delay times in synchronous rectification with
either synchronous rectifier ICs embedded into the power-device’s package
or, at a minimum, Kelvin-source connections on high-current MOSFETs.
The results accomplished in this thesis indicate the necessity of careful
topology/control selection and advanced-magnetics design for enabling WBGbased
industrial power supplies that will outperform state-of-the-art solutions
and catapult them to the next-generation performance standards. None of
these features—be it WBG devices, wide-gain-range resonant converters,
or advanced PCB-integrated magnetics—will alone enable next-generation
power-supply modules, but the thoughtful combination of these technologies
and their careful application to the particular application, with emphasis to
high-frequency PCB magnetics and soft-switching topologies, which enable
compact and cost-effective converters with competitive efficiencies.
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