Type of Document Dissertation Author Dong, Yan Author's Email Address email@example.com URN etd-07312009-143713 Title Investigation of Multiphase Coupled-Inductor Buck Converters in Point-of-Load Applications Degree PhD Department Electrical and Computer Engineering Advisory Committee
Advisor Name Title Lee, Fred C. Committee Chair Baumann, William T. Committee Member Boroyevich, Dushan Committee Member Suchicital, Carlos T. A. Committee Member Xu, Ming Committee Member Keywords
- multiphase buck
Date of Defense 2009-07-24 Availability unrestricted AbstractMultiphase interleaving buck converters are widely used in today’s industrial point-of-load
(POL) converters, especially the microprocessor voltage regulators (VRs). The issue of today’s
multiphase interleaving buck converters is the conflict between the high efficiency and the fast
transient in the phase inductor design. In 2000, P. Wong proposed the multiphase coupledinductor
buck converter to solve this issue. With the phase inductors coupled together, the
coupled-inductor worked as a nonlinear inductor due to the phase-shifted switching network, and
the coupled-inductor has different equivalent inductances during steady-state and transient. One
the one hand, the steady state inductance is increased due to coupling and the efficiency of the
multiphase coupled-inductor buck converter is increased; on the other hand, the transient
inductance is reduced and the transient performance of the multiphase coupled-inductor buck is
improved. After that, many researches have investigated the multiphase coupled-inductor buck
converters in different aspects. However, there are still many challenges in this area: the
comprehensive analysis of the converter, the alternative coupled inductor structures with the
good performance, the current sensing of converter and the light-load efficiency improvement.
They are investigated in this dissertation.
The comprehensive analysis of the multiphase coupled-inductor buck converter is
investigated. The n-phase (n>2) coupled-inductor buck converter with the duty cycle D>1/n
hasn’t been analyzed before. In this dissertation, the multiphase coupled-inductor buck converter
is systematically analyzed for any phase number and any duty cycle condition. The asymmetric
multiphase coupled-inductor buck converter is also analyzed.
The existing coupled-inductor has a long winding path issue. In low-voltage, high-current
applications, the short winding path is preferred because the winding loss dominates the inductor
total loss and a short winding path can greatly reduce the winding loss. To solve this long
winding path issue, several twisted-core coupled-inductors are proposed. The twisted-core
coupled-inductor has such a severe 3D fringing effect that the conventional reluctance modeling
method gives a poor result, unacceptable from the design point of view. By applying and
extending Sullivan’s space cutting method to the twisted core coupled inductor, a precise
reluctance model of the twisted-core coupled-inductor is proposed. The reluctance model gives
designers the intuition of the twisted-core coupled-inductors and facilitates the design of the
twisted-core coupled-inductors. The design using this reluctance model shows good correlation
between the design requirement and the design result. The developed space cutting method can
also be used in other complex magnetic structures with the strong fringing effect.
Today, more and more POL converters are integrated and the bottleneck of the integrated
POL converters is the large inductor size. Different coupled-inductor structures are proposed to
reduce the large inductor size and to improve the power density of the integrated POL converter.
The investigation is based on the low temperature co-fire ceramic (LTCC) process. It is found
that the side-by-side-winding coupled-inductor structure achieves a smaller footprint and size.
With the two-segment B-H curve approximation, the proposed coupled-inductor structure can be
easily modeled and designed. The designed coupled-inductor prototype reduces the magnetic
size by half. Accordingly, the LTCC integrated coupled-inductor POL converter doubles the
power density compared to its non-coupled-inductor POL counterpart and an amazing 500W/in3
power density is achieved.
In a multiphase coupled-inductor converter, there are several coupled-inductor setups. For
example, for a six-phase coupled-inductor converter, three two-phase coupled inductors, two
three-phase coupled-inductors and one six-phase coupled inductors can be used. Different
coupled-inductor setups are investigated and it is found that there is a diminishing return effect
for both the steady-state efficiency improvement and the transient performance improvement
when the coupling phase number increases.
The conventional DCR current sensing method is a very popular current sensing method for
today’s multiphase non-coupled-inductor buck converters. Unfortunately, this current sensing
method doesn’t work for the multiphase coupled-inductor buck converter. To solve this issue,
two novel DCR current sensing methods are proposed for the multiphase coupled-inductor buck
Although the multiphase coupled-inductor buck converters have shown a lot of benefits,
they have a low efficiency under light-load working in DCM. Since the DCM operation of the
multiphase coupled-inductor buck converter has never been investigated, they are analyzed in
detail and the reason for the low efficiency is identified. It is found that there are more-than-one
DCM modes for the multiphase coupled-inductor buck converter: DCM1, DCM2 …, and DCMn.
In the DCM2, DCM3…, and DCMn modes, the phase-currents reach zero-current more-thanonce
during one switching period, which causes the low efficiency of the multiphase coupledinductor
buck converter in the light load. With the understanding of the low efficiency issue, the
burst-in-DCM1-mode control method is proposed to improve the light load efficiency of the
multiphase coupled-inductor buck converter. Experimental results prove the proposed solution.
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