Step-Up DC–DC Converters: A Comprehensive Review of Voltage-Boosting Techniques, Topologies, and Applications

TL;DR: In this paper, the authors comprehensively review and classify various step-up dc-dc converters based on their characteristics and voltage-boosting techniques, and discuss the advantages and disadvantages of these voltage boosting techniques and associated converters.

Abstract: DC–DC converters with voltage boost capability are widely used in a large number of power conversion applications, from fraction-of-volt to tens of thousands of volts at power levels from milliwatts to megawatts. The literature has reported on various voltage-boosting techniques, in which fundamental energy storing elements (inductors and capacitors) and/or transformers in conjunction with switch(es) and diode(s) are utilized in the circuit. These techniques include switched capacitor (charge pump), voltage multiplier, switched inductor/voltage lift, magnetic coupling, and multistage/-level, and each has its own merits and demerits depending on application, in terms of cost, complexity, power density, reliability, and efficiency. To meet the growing demand for such applications, new power converter topologies that use the above voltage-boosting techniques, as well as some active and passive components, are continuously being proposed. The permutations and combinations of the various voltage-boosting techniques with additional components in a circuit allow for numerous new topologies and configurations, which are often confusing and difficult to follow. Therefore, to present a clear picture on the general law and framework of the development of next-generation step-up dc–dc converters, this paper aims to comprehensively review and classify various step-up dc–dc converters based on their characteristics and voltage-boosting techniques. In addition, the advantages and disadvantages of these voltage-boosting techniques and associated converters are discussed in detail. Finally, broad applications of dc–dc converters are presented and summarized with comparative study of different voltage-boosting techniques.

Summary

Introduction

• To provide researchers with a global picture of the array of step-up dc–dc converters proposed in the literature, numerous boosting techniques and topologies are surveyed and categorized.
• These categorizations should assist researchers in understanding the advantages and disadvantages of various voltage-boosting techniques and topologies in terms of their applications.

II. CATEGORIES OF STEP-UP DC–DC CONVERTERS

• Fig. 1 illustrates a general categorization of step-up dc–dc converters.
• In following subsections, the details of each class of converter with respective major circuits are described in the following general form.

A. Nonisolated/Isolated

• A basic method for stepping-up a dc voltage is to use a PWM boost converter, which comprises only three components (an inductor, a switch, and a diode).
• Analogous to a PWM boost converter, other nonisolated dc–dc structures are usually amenable to relatively low-power levels with reduced cost and size [10], [11].
• Isolated dc– dc converters can be single- or two-stage structures and can be implemented using either a coupled inductor or transformer.
• The coupled inductor will store energy in one cycle and then power the load in the other cycles; such converters usually operate at high frequency in order to reduce the size of the magnetic components.
• This auxiliary circuit, which can be a single dc–dc converter with separate modulation and control [225] or can comprise an impedance (Z-) source network, benefits from integrated modulation and control [226], [229]–[231].

B. Unidirectional/Bidirectional

• Most of the fundamental dc–dc converter types are used to transfer unidirectional power flow, in which the input source should only supply the load (in generation) or absorb the energy (in regeneration) [43]–[212], [229]–[277].
• Boost-derived applications differ from buck-derived applications in that, while they may not have large output current, their voltage may be very high, e.g., > 600 V, in which case the diode voltage drop might not be as dominant in the power loss calculation.
• Fig. 3(c) shows a schematic of an isolated unidirectional converter along with an example of a unidirectional dc–dc converter.
• As demonstrated in [234], its magnetic components can be integrated into a single core in order to reduce the size and cost of the converter.
• Fig. 4(c) shows an example of the well-known current-fed full-bridge converter, which consists of an input inductor and a capacitive output filter.

D. Hard Switched/Soft Switched

• A main drawback of hard-switched converters is their higher switching power loss.
• Soft-switching converters can be classified as load resonant with resonant networks, active snubber switch cells, and isolated structures with auxiliary assisted circuits.
• Proper operation of these converters is quite dependent on the operating point and resonant frequency, making them not suitable for wide range of operating conditions.
• Fig. 5(b) illustrates some of these switch cell types implemented in dc–dc converters.
• Auxiliary circuits can consist of an auxiliary transformer/coupled inductor or an active network (AN).

E. NMP/Minimum Phase

• Systems with RHP zeros are called NMP systems.
• The former is intrinsically more difficult owing to the effects of the RHP zero.
• Various techniques can be employed to alleviate the effect of the RHP zero in boost converters.
• For higher voltage gain, high-order-derived KY converters can be exploited but at the expense of additional switches for each stage.
• Another improved boost converter type with no RHP zero and ripple-free input and output current is shown in Fig. 7(h).

III. DIFFERENT VOLTAGE-BOOSTING TECHNIQUES

• Step-up converters are used to implement various voltage boost techniques in dc–dc converters.
• Fig. 8shows a broad categorization of the voltage-boosting techniques that can be found in the literature.
• Five major subsections are included, namely SC (CP), voltage multiplier, switched inductor and VL, magnetic coupling, and converters with multistage/-level structures.
• In the following section, the general structures of these techniques are first illustrated and then major circuits are shown to illustrate their underlying concepts in detail.

A. Switched Capacitor (Charge Pump)

• Voltage-level enhancement in a CP circuit comes solely from capacitive energy transfer and does not involve magnetic energy transfer.
• In the second phase, capacitor C1 is placed in series with the input source, which ideally doubles the output voltage level [68].
• By changing the input voltage node in the lower ladder of capacitors, different voltage gains can be obtained from this type of SC.
• This converter employs the distributed stray inductances of each SC module to provide zero current turn ON and OFF to the devices; as a consequence, voltage and current spikes are reduced, power losses are minimized, and efficiency is increased.
• CW-VMRs, as shown in Fig. 13(d), are popular for their simple cascading structures that can provide high-voltage levels [168].

C. Switched Inductor and Voltage Lift

• The VL technique is another useful method that is broadly used in dc–dc converters to increase output voltage level.
• To further increase the VL, a multiple-lift circuit using an n-stage basic diode capacitor VL circuit was demonstrated in [171].
• Various A-SL networks are shown in Fig. 17.
• In the ANs shown in Fig. 17(a)–(c), the shared operation of inductors allow for integration into a single core to potentially decrease the size and weight of the converters [92].
• QA-SLs can provide high voltage gain and low-voltage stress on S1 and S2 with a small coupledinductor core size.

D. Magnetic Coupling

• Magnetic coupling is a popular voltage-boosting technique that is used in both isolated and nonisolated dc–dc converters.
• Some examples of general built-in transformer-based converter structures are shown in Fig. 21.
• As many applications do not require electrical isolation, the use of coupled inductors provides a helpful alternative boosting technique in dc–dc converters that can be achieved by tapping or simply coupling the inductors.
• On the other hand, as the root mean square (RMS) current of the switches, RMS current of inductors, and diode-blocking voltages all increase when inductor tapping is utilized [106], designing a clamp/snubber circuit is sometimes necessary [102] and [107].
• Fig. 25(b) shows a Γ-source impedance network that also utilizes a coupled inductor.

E. Multistage/-Level

• One well-known method for increasing the voltage gain of a dc–dc converter is to employ several stages of converter modules connected in various ways.
• Several basic quadratic boost converter structures are shown in Fig. 27(d)–(g) [16].
• An extra boost converter is used to suppress the voltage stresses across the switches caused by leakage inductance.
• A comparison between various interleaved dc– dc converters with different boosting techniques is presented in Table IX.
• Fig. 32(d) shows a conventional dc–dc boost converter as a single-converter module for integration within a cascaded multilevel converter [41].

IV. APPLICATIONS AND COMPARISON OF STEP-UP DC–DC CONVERTERS

• Step-up dc–dc converters have been used for wide range of power conversion applications from the milliwatt scale upward, e.g., from energy harvesting to MW-level high-voltage dc transmission systems.
• High step-up dc–dc converters are popular in portable electronic devices, in which battery storage systems or standalone renewable sources like PV and FC are typically employed as input sources to supply dc bus for electronic devices [117], [118].
• This technology permits thin TV displays and lower costs but requires higher current LEDs.
• Typically, bidirectional dc–dc converters are employed in the battery back-up systems of aircraft in order to convert low-voltage inputs to a high-voltage dc bus in the boost mode [281], [288], [293], [304].
• The appropriate connection of HVDC transmission using high-voltage dc–dc converters is an important issue; in these systems, MMCs have become popular owing to their significant reduction of harmonic content and their scalability in terms of voltage levels [308], [309].

V. SUMMARY AND CONCLUSION

• The ongoing technological progress in high-voltage step-up dc–dc converter has five primary drivers—energy efficiency, power density, cost, complexity, and reliability—all of which also influence each other to some extent.
• Q. Zhao and F. C. Lee, “High-efficiency, high step-up DC-DC converters,” IEEE Trans.
• A. Richelli, S. Comensoli, and Z. M. Kovacs-Vajna, “A DC/DC boosting technique and power management for ultralow-voltage energy harvesting applications,” IEEE Trans.
• D. M. Bellur and M. K. Kazimierczuk, “DC-DC converters for electric vehicle applications,” in Proc. Electr.
• He is the Editor-in-Chief of the IEEE TRANSACTIONS ON POWER ELECTRONICS and was highlighted in the inaugural edition of the book The 300 Best Professors (Framingham, MA, USA: Princeton Review, 2012).

Figures

Fig. 4. Voltage- and current-fed dc–dc converter structures. (a)Common structure in isolated and nonisolated forms. (b) Voltage-fed full-bridge dc–dc converter. (c) Current-fed full-bridge dc–dc converter.

Fig. 8. Broad categorizations of voltage boost techniques used for dc–dc converters.

Fig. 10. Various switched capacitor dc–dc converters. (a), (b), and (e) are two switched capacitors with diode-capacitor stages. (c) and (d) are modular switched capacitors.

Fig. 9. Basic charge pump and switched capacitor circuits. (a) Basic charge pump. (b) Basic switched capacitor. (c) Doubler. (d) Series–parallel. (e) Ladder. (f) Dickson. (g) Makowski or Fibonacci.

Fig. 31. Multilevel modular dc–dc converters with single dc source. (a) General structure. (b) and (c) Two example dc–dc modules.

Fig. 30. Multilevel modular dc–dc converters with voltage multipliers. (a) General structure with several submodules. (b) and (c) Two examples for the multilevel dc–dc converter with voltage multiplier cells.

Fig. 32. Multilevel cascaded dc–dc converters with multiple dc sources. (a) General cascaded structure. (b)–(f) Various converter modules.

TABLE V COMPARISON AMONG DIFFERENT A-SL CIRCUITS

Fig. 16. General topological view of an A-SL circuit (AN SL) in a step-up converter.

TABLE IV VOLTAGE STRESS FOR VARIOUS VMRS

Fig. 15. Voltage lift cells. (a) General placement of the voltage lift cell in stepup converters. (b)–(e) Various voltage lift switched inductor cells. [(a) Basic SL cell. (b) Elementary-lift cell. (c) Self-lift SL cell. (d) Double self-lift SL cell.]

Fig. 26. General layout of the cascaded dc–dc converter.

Fig. 5. Different soft switching dc–dc converters. (a) General resonant tank networks. (b) Various soft switching cells and soft switching isolated dc–dc converters with auxiliary circuits at (c) primary side and (d) secondary side.

Fig. 28. Hybrid cascaded dc–dc converters. (a) General layout of the quadratic hybrid cascaded converter. (b) and (c) Some examples of quadratic-based hybrid cascaded converters. (d) General layout of the cascaded connection of two different converters. (e) and (f) Some examples of cascade connection of two different converters.

Fig. 27. Quadratic dc–dc converters. (a) Two cascaded boost converters. (b) Quadratic boost converter. (c) Three-level quadratic boost converter. (d)–(g) Some basic quadratic boost dc–dc converters.

TABLE I SUMMARY OF DIFFERENT DC–DC CONVERTER STRUCTURES

Fig. 14. Various full-wave VMRs. (a) Voltage doubler rectifier. (b) Voltage quadrupler rectifier. (c) VMR (even group). (d) Voltage tripler rectifier. (e) Voltage quintupler rectifier. (f) VMR (odd group).

Fig. 13. Various half-wave VMRs. (a) Greinacher voltage doubler. (b) Improved Greinacher voltage doubler. (c) Greinacher voltage quadrupler. (d) CW voltage doubler. (e) CW voltage multiplier.

Fig. 1. Categorization of step-up dc–dc converters.

Fig. 21. Step-up dc–dc converter consisting of built-in transformer concept. (a) General layout with the horizontal structure. (b) Comprising voltage multiplier [207]. (c) Comprising both coupled inductor and built-in transformer [130]. (d) General layout with the vertical structure. (e) Boost integrated stacked converter [208]. (f) Integrated with a transformer-assisted auxiliary circuit [210].

Fig. 22. Tapped-inductor based converters. (a) General configuration of a tapped inductor boost converter and (b) double-tapped inductor boost converter.

Full text

Aalborg Universitet
Step-Up DC-DC converters
A comprehensive review of voltage-boosting techniques, topologies, and applications
Forouzesh, Mojtaba; Siwakoti, Yam P.; Gorji, Saman A.; Blaabjerg, Frede; Lehman, Brad
Published in:
IEEE Transactions on Power Electronics
DOI (link to publication from Publisher):
10.1109/TPEL.2017.2652318
Publication date:
2017
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Citation for published version (APA):
Forouzesh, M., Siwakoti, Y. P., Gorji, S. A., Blaabjerg, F., & Lehman, B. (2017). Step-Up DC-DC converters: A
comprehensive review of voltage-boosting techniques, topologies, and applications. IEEE Transactions on
Power Electronics, 32(12), 9143-9178. [7872494]. https://doi.org/10.1109/TPEL.2017.2652318
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 32, NO. 12, DECEMBER 2017 9143
Step-Up DC–DC Converters: A Comprehensive
Review of Voltage-Boosting Techniques,
Topologies, and Applications
Mojtaba Forouzesh, Student Member, IEEE,YamP.Siwakoti, Member, IEEE,
Saman A. Gorji, Student Member, IEEE, Frede Blaabjerg, Fellow, IEEE, and Brad Lehman, Senior Member, IEEE
Abstract—DC–DC converters with voltage boost capability are
widely used in a large number of power conversion applications,
from fraction-of-volt to tens of thousands of volts at power lev-
els from milliwatts to megawatts. The literature has reported on
various voltage-boosting techniques, in which fundamental energy
storing elements (inductors and capacitors) and/or transformers in
conjunction with switch(es) and diode(s) are utilized in the circuit.
These techniques include switched capacitor (charge pump), volt-
age multiplier, switched inductor/voltage lift, magnetic coupling,
and multistage/-level, and each has its own merits and demerits
depending on application, in terms of cost, complexity, power den-
sity, reliability, and efficiency. To meet the growing demand for
such applications, new power converter topologies that use the
above voltage-boosting techniques, as well as some active and pas-
sive components, are continuously being proposed. The permuta-
tions and combinations of the various voltage-boosting techniques
with additional components in a circuit allow for numerous new
topologies and configurations, which are often confusing and dif-
ficult to follow. Therefore, to present a clear picture on the gen-
eral law and framework of the development of next-generation
step-up dc–dc converters, this paper aims to comprehensively
review and classify various step-up dc–dc converters based on
their characteristics and voltage-boosting techniques. In addi-
tion, the advantages and disadvantages of these voltage-boosting
techniques and associated converters are discussed in detail. Fi-
nally, broad applications of dc–dc converters are presented and
summarized with comparative study of different voltage-boosting
techniques.
Index Terms—Coupled inductors, multilevel converter, multi-
stage converter, pulse width modulated (PWM) boost converter,
switched capacitor (SC), switched inductor, switched mode step-up
dc–dc converter, transformer, voltage lift (VL), voltage multiplier.
Manuscript received May 24, 2016; revised November 15, 2016; accepted
January 7, 2017. Date of publication March 6, 2017; date of current version
August 2, 2017. (All papers from Northeastern University, Boston, are handled
by Editor-at-Large, Prof. P. T. Krein, in order to avoid conflict of interest.)
Recommended for publication by Associate Editor D. J. Perreault.
M. Forouzesh, Y. P. Siwakoti, and F. Blaabjerg are with the Department
of Energy Technology, Aalborg University, Aalborg 9220, Denmark (e-mail:
m.forouzesh.ir@ieee.org; yas@et.aau.dk; fbl@et.aau.dk).
S. A. Gorji is with the School of Software and Electrical Engineering,
Swinburne University of Technology, Hawthorn, VIC 3122, Australia (e-mal:
sasgharigorji@swin.edu.au).
B. Lehman is with the Department of Electrical and Computer Engineer-
ing, Northeastern University, Boston, MA 02115 USA (e-mail: lehman@ece.
neu.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPEL.2017.2652318
I. INTRODUCTION
S
WITCHED-MODE step-up dc–dcconverters originated
with the development of pulse width modulated (PWM)
boost converters. Step-up dc–dc topologies convert lower dc
voltage levels to higher levels by temporarily storing the input
energy and then releasing it into the output at a higher volt-
age level. Such storage can occur in either magnetic field stor-
age components (single inductor/coupled inductor) or electric
field storage components (capacitors) through the use of vari-
ous active or passive switching elements (power switches and
diodes). With the introduction of semiconductor switches in the
1950s, step-up dc–dc converters achieved steady performance
advancements and their use accelerated through the 1960s when
semiconductor switches became commercially available with
allied manufacturing technologies [1]. The rise of the aerospace
and telecommunication industries further extended the research
boundaries of boost converters, especially in applications where
efficiency, power density, and weight were of major concern.
Efficiency has steadily improved since the late 1980s owing to
the use of power field-effect transistors (FETs), which are able
to switch more efficiently at higher frequencies than power bipo-
lar junction transistors while incurring lower switching losses
and requiring a less complicated drive circuit. In addition, the
FET replaces output rectifying diodes through the use of syn-
chronous rectification, whose “on resistance” is much lower
than and further increases the efficiency of the step-up dc–dc
converter, which requires a higher number of diodes for voltage
boosting [1]–[3].
A PWM boost converter is a fundamental dc–dc voltage step-
up circuit with several features that make it suitable for various
applications in products ranging from low-power portable de-
vices to high-power stationary applications. The widespread
application of PWM boost dc–dc converters has been driven
by its low number of elements, which is a major advantage
in terms of simplifying modeling, design implementation, and
manufacturing. The voltage step-up capability of a PWM boost
dc–dc converter is enabled by an inductor at the input side that
can operate either with a continuous current—in the so-called
continuous conduction mode (CCM)—or including a zero cur-
rent state in the discontinuous conduction mode (DCM). In
general, CCM operation is more prevalent owing to the load
dependent voltage gain, high current ripple, and low efficiency
0885-8993 © 2017 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution
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9144 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 32, NO. 12, DECEMBER 2017
Fig. 1. Categorization of step-up dc–dc converters.
of DCM operation. However, the higher stability characteris-
tics of the boost converter and smaller inductor implementa-
tion in the DCM mean that, occasionally, DCM operation of
step-up dc–dc converters is preferable [4]–[5]. In addition to
the abovementioned features, a PWM boost converter also has
several shortcomings: hard switching and severe reverse recov-
ery in the output diode, both of which cause lower efficiency;
nonminimum-phase (NMP) characteristics owing to the pres-
ence of a right half plane (RHP) zero, which leads to difficult
high-bandwidth control design; low voltage gain with moderate
duty cycle switching; and low power density, which may lead to
inefficient operation in high-voltage/power applications. Some
of the shortcomings in conventional boost converters have led
researchers to investigate and discover new topologies and op-
erational methods, especially when high input-to-output boost
ratio and better dynamics, stability, and reliability, along with
higher power density and efficiency, are sought. Furthermore,
improvements have been made in improving the power supply
rejection ratio and input voltage and current ripples while low-
ering electromagnetic interference (EMI) and costs [6]–[11].
Within the literature discussed later in the paper, there is
a consistent demand for reliable, efficient, small-sized, and
lightweight step-up dc–dc converters for various power appli-
cations. Some of these demands can be simply achieved by
using second-, third-, and fourth-order fundamental PWM dc–
dc converters, e.g., boost, SEPIC,
´
Cuk, and Zeta converters.
Furthermore, flyback, forward, push–pull, half-, and full-bridge
converters are still popular and are employed for use at var-
ious voltage and power levels in which galvanic isolation is
required. However, the literature also presents more compli-
cated newer t opologies that utilize different voltage-boosting
techniques such as using multilevel, interleaved, or cascaded
topologies, or using voltage multiplier cells (VMC), perhaps
even combined with switched capacitors (SCs) and/or coupled
inductors [12]–[309]. Each topology has its own advantages
and disadvantages and should be selected based on the appli-
cation and its requirements, e.g., isolated/nonisolated, unidirec-
tional/bidirectional, voltage-fed/current-fed, hard/soft switched,
or with/without minimum-phase characteristics.
The permutations and combinations of various voltage-
boosting techniques form an immense number of topologies and
configurations. This can be both confusing and difficult to s ur-
vey and implement for particular applications. In this paper, to
provide researchers with a global picture of the array of step-up
dc–dc converters proposed in the literature, numerous boosting
techniques and topologies are surveyed and categorized. Indeed,
a great part of this paper is devoted to demonstrating recent con-
tributions and possibilities in terms of providing step-up voltage
gain. The paper provides a “one-stop” information source with
various categorizations of voltage-boosting techniques for step-
up power conversion applications. These categorizations should
assist researchers in understanding the advantages and disad-
vantages of various voltage-boosting techniques and topologies
in terms of their applications. With this intention, a broad topo-
logical overview based on the characteristics of step-up dc–dc
converters i s first presented in Section II. To discuss different
voltage-boosting techniques, namely—SC [charge pump (CP)],
voltage multiplier, switched inductor/voltage lift (VL), mag-
netic coupling, and multistage/-level—a comprehensive review
based on the respective major circuits is presented in Section III.
Finally, an applicational overview of step-up dc–dc converters
is presented in Section IV and concluded in Section V.
II. C
ATEGORIES OF STEP-UP DC–DC CONVERTERS
Fig. 1 illustrates a general categorization of step-up dc–dc
converters. In following subsections, the details of each class
of converter with respective major circuits are described in the
following general form.
A. Nonisolated/Isolated
A basic method for stepping-up a dc voltage is to use a PWM
boost converter, which comprises only t hree components (an
inductor, a switch, and a diode). A PWM boost converter is
a simple, low cost, and efficient nonisolated step-up converter
suitable for many dc applications. Fig. 2(a) illustrates a gen-
eral view of a nonisolated dc–dc converter along with a PWM
boost converter. Analogous to a PWM boost converter, other
nonisolated dc–dc structures are usually amenable to relatively
low-power levels with reduced cost and size [10], [11]. Owing
to their broad applicability and simplicity of implementation
and design, much research has been dedicated to the subject of
nonisolated dc–dc converters [32]–[224]. These circuits can be
with used with shared ground between the input and output or
with a floated output, and Fig. 2(b) shows a general view of a
nonisolated dc–dc converter with floated output along with a
three-level boost converter. A shared connection between the
input and output of nonisolated dc–dc converters can be used to
improve the system performance of applications such as trans-
formerless grid connected PV systems [12], and in addition to
special applications in which a common ground between the
input source and load is not necessary, the output of nonisolated
dc–dc converters can also be floated in a manner similar to that
in a three-level boost converter [13]. Furthermore, nonisolated

FOROUZESH et al.: STEP-UP DC–DC CONVERTERS: A COMPREHENSIVE REVIEW OF VOLTAGE-BOOSTING TECHNIQUES 9145
Fig. 2. Different nonisolated and isolated dc–dc converter structures. (a) Common grounded and (b) floated output nonisolated dc–dc converters. (c) Single-stage
and (d) two-stage isolated dc–dc converters.
dc–dc converters can be built with or without magnetic cou-
pling. If high-voltage step-up is not considered and efficiency
is not a major concern, nonisolated structures without magnetic
coupling and comprising only switching devices and passive
components can be a useful solution that simplifies converter
design by eliminating the need for coupled magnetic design.
However, in high-power systems, it is often beneficial to utilize
magnetic coupling if high voltage gain is required, and doing so
can improve both efficiency and reliability. Both the transformer
in its nonisolated form (built-in) and the coupled inductor can
be employed in nonisolated dc–dc structures [32]–[224].
Electrical isolation is an important feature for grid-tied dc–dc
converters and for some other applications that require reliable
power transfer with low noise and reduced EMI. The applicable
safety standard indicates the voltage level of electrical isolation
between the input and output of a dc–dc converter, which can
be achieved by means of either transformer or coupled inductor
[225]–[297]. Some sensitive loads such as those used in medi-
cal, military, and avionics applications are vulnerable to faults
and noise; as safety is also a major concern for these appli-
cations, electrical isolation is typically necessary [239]–[241],
[247], [248], [263], [281], [282], [288], and [293]. Isolated dc–
dc converters can be single- or two-stage structures and can be
implemented using either a coupled inductor or transformer. Fig.
2(c) shows schematics of single-stage isolated dc–dc converters
and an isolated dc–dc converter with a coupled inductor. In this
category, the coupled inductor will store energy in one cycle
and then power t he load in the other cycles; such converters
usually operate at high frequency in order to reduce the size of
the magnetic components. The literature reports on several iso-
lated dc–dc converters that employ coupled inductors for various
applications [242], [258]–[261], [271], [272], [275], [282], and
[283]. In a high-frequency transformer, the voltage of an input dc
source is converted to an ac voltage, often a square/quasi-square
wave voltage, and then passed through the transformer. The
switching concept in isolated dc–dc converters varies by topol-
ogy, with forward, push–pull, half-, and full-bridge converters
being examples of well-known transformer-based isolated dc–
dc structures [225]. Furthermore, there is a family of three-level
transformer isolated dc–dc structures [233] that benefits from
smaller current ripple and reduced voltage stresses compared
with corresponding conventional converters. Flyback convert-
ers are a type of isolated buck-boost dc–dc converters that use
a coupled inductor instead of an isolation transformer and store
energy in the ON state of the switch while transferring it to the
load in the OFF state of the switch. As shown in Fig. 2(d), an
auxiliary converter can be employed in the first stage of a two-
stage isolated dc–dc converter to preregulate the voltage level
demanded. This auxiliary circuit, which can be a single dc–dc
converter with separate modulation and control [225] or can
comprise an impedance (Z-) source network, benefits from in-
tegrated modulation and control [226], [229]–[231]. Impedance
source networks are an emerging technology in various power
conversion applications, in which no additional active switches
are required to provide step-up capability [20].
B. Unidirectional/Bidirectional
Most of the fundamental dc–dc converter types are used to
transfer unidirectional power flow, in which the input source
should only supply the load (in generation) or absorb the en-
ergy (in regeneration) [43]–[212], [229]–[277]. Unidirectional
converters would be usable for this purpose in on board loads
such as sensors, utilities, and safety equipment. A typical layout
of such a converter, which is usually implemented via unidi-
rectional semiconductors such as power MOSFETs and diodes,
is shown in Fig. 3(a), in which conventional buck and boost
converters are also depicted as basic examples of unidirectional
dc–dc converters. In converters such as those shown, the power
flow is unidirectional because single-quadrant switches are used,
i.e., there is no path for the current to be conducted in the re-
verse direction in the diodes. By contrast, Fig. 3(b) shows the
bidirectional structure of a nonisolated dc–dc converter, which
can be realized by replacing the one-way direction semiconduc-
tors used in unidirectional topologies with current-bidirectional
two-quadrant switches [278]. When unidirectional power flow

9146 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 32, NO. 12, DECEMBER 2017
Fig. 3. Unidirectional and bidirectional dc–dc converters. (a) Nonisolated
unidirectional and (b) nonisolated bidirectional dc–dc converters, (c) isolated
unidirectional, and (d) isolated bidirectional dc–dc converters.
is desired, unidirectional converters are preferred owing to their
lower number of controllable switches and correspondingly sim-
pler control implementation.
An interesting design aspect of unidirectional boost convert-
ers is that, unlike high-power step-down applications, diodes
can sometimes have minimized effects on the circuit’s power
efficiency. When the output voltage is much higher than the
rectifying diode voltage drop, in many applications, design-
ers may decide to retain a diode instead of replacing it with a
synchronous rectifier, even as power increases. On the other
hand, buck-derived applications such as in voltage regula-
tor modules have been trending toward lower output voltages
that would be dominated by the diode power loss and there-
fore incorporate synchronous rectification and have topolog-
ical bidirectional current flow capabilities in their switches.
Boost-derived applications differ from buck-derived applica-
tions in that, while they may not have large output current, their
voltage may be very high, e.g., > 600 V, in which case the
diode voltage drop might not be as dominant in the power loss
calculation.
As discussed in the previous subsection and as will be ad-
dressed in the following sections, isolation transformers may
be employed either to augment the boost ability of a dc–dc
converter or to provide other requirements (e.g., electrical iso-
lation between the input and the output or meeting special
standards for particular systems). Fig. 3(c) shows a schematic
of an isolated unidirectional converter along with an exam-
ple of a unidirectional dc–dc converter. The full-bridge dc–
dc converter is a popular topology of this family, particularly
when dealing with high-power levels such as in industrial ap-
plications. This type of converter comprises a dc–ac stage—a
high-frequency isolation transformer—followed by a rectifica-
tion stage. As an example of its various applications, the out-
put voltage of a full-bridge dc–dc converter may supply an
ac–dc inverter through a dc-link capacitor for use in a power
supply system, ac motor, etc. [234], [245]–[247], [252]–[255],
[264]–[267], [293].
The growing demand for applications with the storage sys-
tem and bidirectional energy transfer capability will result in the
increased use of bidirectional dc–dc converters. These convert-
ers are used in renewable energy systems, railway transportation
(e.g., train and tramway), automotive transportation (e.g., hybrid
electric vehicles (HEV) and vehicle to grid), aerospace applica-
tions, elevators and escalators, uninterruptable power supplies,
batteries, supercapacitors, smart grid applications, and many
other applications [213]–[223], [278]–[297]. Although, in prin-
ciple, energy storage and bidirectional transfer can be achieved
by implementing two unidirectional dc–dc converters—one to
transfer power from the input to the output, and another to
transfer power in the opposite direction—in practice, as men-
tioned previously, replacing unidirectional semiconductor ele-
ments with bidirectional switches will result in a bidirectional
topology. Fig. 3(d) shows a schematic of an isolated bidirec-
tional converter along with a popular example of a bidirectional
dc–dc converter, dual active bridge (DAB), which is one of
the most promising types of isolated bidirectional dc–dc con-
verter derived from unidirectional full-bridge dc–dc converter
topology. DAB converters are useful in high-voltage/power-
level applications [280], [287]–[289]. I n the DAB topology,
energy transfer is controlled by adjusting the phase shift be-
tween two ac voltage waveforms across the windings of the
isolation transformer, and control strategy is one of the more
important subjects of research with regard to such converters
[280].

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Step-up dc–dc converters: a comprehensive review of voltage-boosting techniques, topologies, and applications" ?

The literature has reported on various voltage-boosting techniques, in which fundamental energy storing elements ( inductors and capacitors ) and/or transformers in conjunction with switch ( es ) and diode ( s ) are utilized in the circuit. The permutations and combinations of the various voltage-boosting techniques with additional components in a circuit allow for numerous new topologies and configurations, which are often confusing and difficult to follow. Therefore, to present a clear picture on the general law and framework of the development of next-generation step-up dc–dc converters, this paper aims to comprehensively review and classify various step-up dc–dc converters based on their characteristics and voltage-boosting techniques. In addition, the advantages and disadvantages of these voltage-boosting techniques and associated converters are discussed in detail. 

Q2. What is the important subject of research with regard to DAB converters?

In the DAB topology, energy transfer is controlled by adjusting the phase shift between two ac voltage waveforms across the windings of the isolation transformer, and control strategy is one of the more important subjects of research with regard to such converters [280]. 

Q3. What is the basic method for stepping up a dc voltage?

A basic method for stepping-up a dc voltage is to use a PWM boost converter, which comprises only three components (an inductor, a switch, and a diode). 

Q4. What is the effect of the modular SC converter?

This converter employs the distributed stray inductances of each SC module to provide zero current turn ON and OFF to the devices; as a consequence, voltage and current spikes are reduced, power losses are minimized, and efficiency is increased. 

Q5. What is the purpose of step-up converters?

Step-up dc–dc converters have been used for wide range of power conversion applications from the milliwatt scale upward, e.g., from energy harvesting to MW-level high-voltage dc transmission systems. 

Q6. What is the use of a capacitor charging power supply?

In pulsed power applications such as pulsed lasers and radar systems, in which high-voltage pulses are required, the use of a capacitor charging power supply (CCPS) is a potential solution. 

Q7. What is the switching concept in isolated dc–dc converters?

The switching concept in isolated dc–dc converters varies by topology, with forward, push–pull, half-, and full-bridge converters being examples of well-known transformer-based isolated dc–dc structures [225]. 

Q8. What is the way to alleviate the NMP characteristics of step-up converters?

Additional methods for alleviating the NMP characteristics of step-up converters include an interesting two-phase interleaved inverse-coupled-inductor boost converter without RHP zeros, as proposed in [202]. 

Q9. What is the effect of the leakage inductance on the semiconductor?

As mentioned previously, all dc–dc converters with magnetic coupling are vulnerable to the detrimental effects of the leakage inductance, i.e., voltage ringing and high spiking on semiconductors. 

Q10. What type of converters have a single inductor and a capacitor?

In general, cascaded boost-type converters, such as those in Fig. 27, usually have four switches, with at least one of them active. 

Q11. Why do current-fed converters have a higher switching power loss?

Because switching losses increase as the switching frequency increases, there often is a limit to the maximum switching frequency of such converters. 

Q12. What is the safety standard for step-up dc–dc converters?

The applicable safety standard indicates the voltage level of electrical isolation between the input and output of a dc–dc converter, which can be achieved by means of either transformer or coupled inductor [225]–[297]. 

Q13. What is the typical layout of a dc–dc converter?

A typical layout of such a converter, which is usually implemented via unidirectional semiconductors such as power MOSFETs and diodes, is shown in Fig. 3(a), in which conventional buck and boost converters are also depicted as basic examples of unidirectional dc–dc converters. 

Q14. What is the solution to the problem of switch voltage spiking?

Passive clamping may not effectively eliminate switch voltage spiking, and several solutions have been proposed to address this problem. 

Q15. What is the difference between unidirectional and buck-derived converters?

When unidirectional power flowis desired, unidirectional converters are preferred owing to their lower number of controllable switches and correspondingly simpler control implementation. 

Q16. What is the main difference between load-resonant converters and passive magnetic/electric field?

Load-resonant converters are suitable for high-power applications because they allow reductions in the size/weight of the converter owing to their high-frequency operation without conversion efficiency degradation. 

Q17. What is the difference between a normal and an interleaved boost converter?

In addition to zero reverse-recovery of output diodes, an interleaved boost converter with coupled input inductors has lower current ripple and a smaller switching duty cycle than a normal boost [58].