Title page for ETD etd-07312009-143713

Type of Document Dissertation
Author Dong, Yan
Author's Email Address ydong@vt.edu
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
  • coupled-inductor
  • multiphase buck
Date of Defense 2009-07-24
Availability unrestricted
Multiphase 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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