IEEE Power Electronics Magazine - March 2016 - 47

converters, and 3) switched-capacitor converters (charge
pumps). A typical portable product, such as a cell phone or
a digital camera, may contain a complex power-management system based on point-of-load (POL) dc supplies combining all three of the aforementioned techniques [19].
In linear regulators, a power semiconductor is used to
effect a voltage reduction in the series path between the input and the output. A control loop compares a sample of
the output voltage with a dc reference source, and feeds
the control input of the power semiconductor to maintain
the output dc value constant. First developed in the 1960s,
linear regulators were very inefficient, with typical efficiencies in the range of 30-60%. However, they have very useful
properties, such as low-noise output with a rapid transient
response to fast changes of load current (high current slew
rate). Another advantage of these linear regulators is the
low component count and, hence, the very low cost and
small printed circuit board (PCB) real estate required in
portable electronics.
As a solution to the low efficiency of the linear regulators, during the last four decades, high-frequency switchmode converters containing inductors and capacitors
were developed. While the basic switch-mode topologies and techniques are well developed, product designers have to overcome their well-known issues, such as
complexity, radio-frequency interference (RFI)/electromagnetic interference (EMI) issues, and the high component count. When there is very little PCB real estate
in a portable product, switched-capacitor converters or
charge pumps are also used. This approach eliminates
the requirement of bulky inductors within the switchmode converters. However, charge pumps are not suitable for high-current dc rails. In general, a portable electronic product, which runs from a battery pack, usually
combines all the three techniques based on a POL design
approach to achieve a compromise between critical en-

Activated Carbon
Electrode

Load

Aqueous
Electrolyte

Power
Source

Electrolyte

Charged
Activated Carbon
Electrode

Anion
Cation
Collector
Electrodes
(Activated Carbon)

Discharged

(a)
Rs

Cdl V
V

C1
C2

C1R2

R1
R2

Cdl

Ls Rs

Rm Rs

Cm Cs

(b)
C

(c)
FIG 2 An SC based on activated carbon and an aqueous
electrolyte and equivalent circuits: (a) an EDLC concept based
on activated carbon, providing high surface area, (b) selected
forms of equivalent circuits, and (c) a simplified model useful
for design calculations.

gineering design specifications [19]. Table 2 compares
the popular dc-dc converter techniques. More details are
available in [20]-[25].

Table 1. A comparison of SCs and electrolytic capacitors based on engineering specifications.

Engineering specification
dc Voltage
short-circuit currating (V)
rent capability (a)

Energy-storage
capability (J)

capacitor
type

Manufacturer

capacitance

<1 J

Electrolytic

RSS

2,200 nF

104

153

1-5 J

Maxwell

2.7

3.85

700

Cap-XX

2.4 F

2.3

115

Electrolytic

Cornell Dubillier

2,200 nF

704

Maxwell

10 F

2.5

180

5-50 J

Electrolytic
>50 J

Esr (mX)

Cap-XX

1.2 F

4.5

112.5

Nesscap

10 F

2.3

Cornell Dubillier

82,000 nF

1,441

11.1

Vicor

270 nF

200

325

614

Maxwell

350 F

2.7

840

3.2

Cap-XX

120 F

2.3

144

March 2016

z IEEE PowEr ElEctronIcs MagazInE

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - March 2016