IEEE Solid-States Circuits Magazine - Fall 2021 - 104

from - VRECT
Recall that, in FBR, CP
to + VRECT
is charged
while it is
charged from 0 V to V2 OC
in SECE.
By adopting the aforementioned synchronous
switching techniques, it is
possible to precharge CP
to some VP
higher than 0 V right before the positive
current cycle begins. This allows
CP
than V2 OC
to be charged to voltages higher
so that more power can
be extracted. This precharging technique
is also called predamping in the
literature [16].
The series SSHI (S-SSHI) [17] shown
in Figure 4(e)-(g) is one such predamping
interface, which is similar to
Figure 4(a) and (b) except that the positions
of the FBR and switch L2
+ are
swapped. The effective series combination
stays the same, while two pads
are saved for the external inductor,
and VRECT
is dc with this specific implementation.
At positive peaks ofV ,CP
is turned on to transfer energy in
CP to CRECT through L .2 As VCP
S1
drops
below V ,RECT current in L2 starts to decrease,
while it continues to discharge
C .P
off S1
when CP
operation turns off S1
Different from the SECE that turns
is fully discharged, this
when the inductor
current drops to zero, obviating the
need for a de-energizing diode
OC2 RECT
D .5
In systems withVV , the operation
can discharge CP
voltage -V ,P as shown with the waveforms
in Figure 4(f). This, in turn, results
in a positive predamped voltage
VP
44 OC
OC +
=
CV CV Vf
the output
PP P
2
$
VV ,
OC1 RECT
before the positive current cycle
begins. The extracted power can be
calculated as () .
Notice that, with V ,0P
power reduces to that from the SECE.
However, in cases with
with L2
connected between the transducer
and C ,RECT VCP remains larger
than 0 V after the discharging switching
activity, failing to " predamp " the
transducer. In [18]-[20], a two-step operation
is proposed that performs an
additional voltage inversion after the
energy extraction. Figure 4(h) and (i)
shows the specific implementation in
[18] that uses only one inductor. The additional
voltage inversion is enabled by
S ,2
resulting in the desired predamping
effect. Furthermore, the energy-extrac104
FALL
2021
to a negative
tion switch S1 is placed after the rectifier.
This allows easy implementation of
the switch because S1
sees dc voltage
during the interface operation.
As S-SSHI is essentially the same
as the SECE in circuit architecture and
differs only in how long switch S1
is
turned on, predamping can be applied
for the various implementations of
SECE by lengthening the ON time of S1
and
S .2 Yang [16] reported an output
power that is 7.8 times higher than
that from an FBR using the same architecture
in Figure 4(d).
Notice that predamping by discharging
CP
to a negative voltage VP
-
may not always improve the performance.
There are systems where predamping
leads to overdamping [9],
and " less-damping, " such as the VCP
waveform shown in Figure 4(g), is desired
[18].
Energy Investing or Pileup
In the previous section, we described
how predamping can be utilized to
boost the extracted power in the
open-circuit type of interface. In a
similar attempt, [5] proposed an energy-investing
operation, as shown in
Figure 4(j) and (k). As VCP
reaches positive
peaks, Vdc is connected across L2
first for a short period of time before
CP
is then connected to C ,P
and the
is being discharged. This sets an
initial inductor current, equivalently
investing some energy. The inductor
L2
nonzero initial current helps drain CP
to a more negative voltage at the end
of the switching activity when the inductor
current drops to zero, boosting
the damping and extracted energy.
Notice that the proposed operation
is asymmetric, as the energy is extracted
only at negative VCP
peaks, and only
two switches are required. However,
the invested energy comes from the energy
buffer and therefore experiences
two more transfers and the associated
loss than the harvested energy. While
the control circuit consumes only
630 nW, the reported conversion efficiency
is 69%, and the maximum output
power increasing rate is 247% [5].
The authors of [21] and [22] proposed
another voltage-boosting techIEEE
SOLID-STATE CIRCUITS MAGAZINE
nique called energy pileup, as shown
in Figure 4(l). The proposed design
operates in energy pileup mode first by
turning on S1
when VCP
peaks. With wellcontrolled
ON time, this SSD-I operation
inverts the interface voltage through
L .2
This voltage inversion is performed
for several consecutive peaks without
extracting the energy, piling up VCP
progressively. After VCP
reaches the
desired value, typically limited by the
breakdown voltage of interface transistors,
the operation is switched to the
conventional S-SSHI energy transfer
mode. Without the loss associated
with energy investment, the design in
[21] reported a maximum output power
increasing rate of 422% [21].
Chamanian et al. [23] further proposed
a self-adapting SSH (SA-SSH)
interface using only three switches, as
shown in Figure 4(m). The proposed
operation performs energy pileup
at positive peaks and energy extraction
as well as investment at negative
peaks. With the asymmetric operation,
a more symmetric waveform that incurs
less concern on mechanical stability
can be maintained. An extracted
power of five times higher than that of
an FBR is reported. In [24], a thermoelectric
generator (TEG) is combined
with the piezoelectric transducer to
speed up the energy pileup process.
Short-Circuit Interface Circuits
As is described in the previous section,
with the open-circuit type of SSH
interface, the transducer sees an opencircuit
load most of the time, and
the energy extraction happens only
during a short period of time at IS
zero-crossings and VCP
peaks. In this
section, we are going to cover the second
short-circuit type of SSH interface
that, on the other hand, performs energy
transfer during almost the entire
cycle. Synchronized switching activities
are employed to help maximize
the energy transfer duration, therefore
maximizing the extracted power.
With Inductors
The most famous short-circuit interface
operation is called P-SSHI [25]
or bias-flip interface [4]. From the
IEEE Solid-States Circuits Magazine - Fall 2021

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