IEEE Solid-States Circuits Magazine - Fall 2021 - 102

As is explained in the following
sections, SSH techniques can be categorized
into two major groups: the opencircuit
and short-circuit types. The
former, such as SECE and series SSHI,
operates in a way such that the transducer
sees an open-circuit load most
of the time. With CP
electrodes, the current IS
of the voltage source VS
of L ,1 C ,1 and CP
between the two
that flows out
can, therefore,
be maximized if the system operates
at OC~
when the series combination
reaches zero impedance.
On the other hand, the shortcircuit
type of synchronized switching
interface circuits, such as parallel SSHI
and SSH on a capacitor (SSHC), operate
by regulating the voltage amplitude
across
C ,P
and
tential energy into Joule heat, the operation
converts the potential energy
completely into magnetic energy in L2
by turning on the switches for a wellcontrolled
duration. Once the switches
S1
L2
and S2
converting the
durinductor's
magnetic energy into C .BUF
From the waveforms in Figure 4(b),
VCP is charged from 0 V to V2 OC
ing the positive current cycle. In
other words, a potential energy of
.( )CV
05 2P
$
which is achieved by shorting
the electrodes through the rectifier.
Due to the voltage regulation, the
transducer ideally does not see ,CP
the extracted power peaks at the shortcircuit
resonant frequency SC~ when
L1
resonates with C .1
Open-Circuit Interface Circuits
Synchronous Electric
Charge Extraction
A popular open-circuit type of interface
operation called SECE was first proposed
in [10], as shown in Figure 4(a) and (b).
In the basic SECE architecture, the transducer
sees the series combination of an
FBR, two switches S1
ductor L .2
and S ,2 and an inNotice
that, as opposed to the
basic FBR architecture, no capacitor is
placed at the rectifier's output (in theory,
a rectifier provides alternating current
paths for rectifying its input voltage; depending
on the load condition, VRECT
is
not necessarily constant), and a buffer
capacitor CBUF
in parallel with the effective
loading from the application circuits
RLOAD
switch S3
is connected to L2
and a diode D .5
with another
During the operation, the switches
are off most of the time, and IS
and discharges CP
and S2
across CP
charges
in an open-circuit
manner. The same as the SSD-S operation,
SECE turns on S1
tralize VCP
of I ,S
to neuat
zero-crossings
waveforms of VCP
resulting in the same interface
and I .S
However,
instead of dissipating the electrical po102
FALL
2021
$$ which is four times higher
than the maximum output power from
an FBR or HBR. Active switching with
precise timing control consumes circuit
power and degrades the extracted
power. Fortunately, this power cost is
usually much smaller than the additional
power that can be harvested by
applying the switching operation. As
demonstrated in [11], the control circuit
consumes 300 µW, and the extra
harvested power is 3.85 mW.
In the specific implementation
P OC
2
shown in Figure 4(a), the diode D5
prevents negative current and ensures
complete energy transfer from L2
It can be replaced with a switch
to
C .BUF
in sync with S3
to reduce the loss due
to voltage drop V .D However, accurate
timing control is therefore needed to
ensure complete energy transfer.
Furthermore, in systems where
VOC is larger than the buffer voltenergy
transfer from CP
age V ,dc
to
L2 can be achieved by turning on S1
and S ,3
and S3
leading to a buck converterlike
implementation that takes away
the switches of S2
altogether
[12]. This improves the conversion
efficiency by reducing the series loss
during energy transfer.
In a similar attempt to reduce the
loss in energy transfer, [13] proposed
an implementation of SECE as shown
in Figure 4(c), in which the rectifier is
completely removed. During the
switching activity, the switches S1
S2
and
energy in CP
are first turned on to transfer the
In the subsequent
to L .2
operation, one of the two switches is
IEEE SOLID-STATE CIRCUITS MAGAZINE
OC
2 on CP
can be harvested
during each switching activity, corresponding
to an extracted power of
4 CV ,f
turned off depending on the signal
polarity, and either D1
or D2
provides
the current path to convert the inductor
energy into
C .BUF This implemenare
turned off, disconnecting
from the transducer, a subsequent
operation turns on ,S3
tation eliminates the loss in rectifier
and reaches a mechanical-to-electrical
conversion efficiency of 78% [13] at
the cost of two limitations: 1) VOC
to be less than Vdc
has
so that D2
S .1
remains
off during positive current cycles.
This can be addressed by applying
dynamic body biasing on
2) Some
of the node voltages become negative
during the operation.
Shareef et al. [14] proposed another
rectifierless SECE implementation, as
shown in Figure 4(d), in which two
capacitors C1
and C2
are used for
positive and negative cycles, respectively,
achieving 73% of conversion efficiency.
However, this operation also
results in negative voltages, which require
negative supplies.
Another way to reduce the conduction
loss in switches is to decrease the
current during the energy transfer.
This can be achieved by increasing the
inductance of
L ,2
which is undesirable
due to the large form factor. Gasnier
et al. [15] proposed a multishot SECE
operation, where the potential energy
in CP
is converted into L2
in multiple
steps. This reduces the peak inductor
current and the associated conduction
loss. This design claimed to extract
25% more power compared to traditional
SECE [15].
The key advantage of SECE is that it
decouples the interface operation from
the load voltage of
V .dc
This is due to
the fact that the operation always discharges
CP
completely, leading to an ex2
P
OC
tracted
power of 4 CV .f$$ It can be
shown that the extracted power reaches
PAVL
whenVV$16 S
OC = r
only when RC /16= ^h orSPr~
(/ ). As a result,
SECE rarely harvests the transducer's
available power [17].
Predamping
As explained previously, the extracted
power of SECE increases with V2
and reaches PAVL
r
OC
when VOC
is exactly
(/ ).V16 S However, in typical piezois
usually
[17].
r
electric transducers, VOC
much smaller than (/ )V16 S
IEEE Solid-States Circuits Magazine - Fall 2021

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