International Conference on Current Research in Engineering Science and Technology (ICCREST-2016)
A NOVEL METHOD FOR RESONANT AND SOFTCHARGING OPERATION OF SWITCHED
CAPACITOR CONVERTER
Ms.E.Padmapriya, Power Electronics And Drives,EEE Department, JJCET., Guided by
Mr. S. Karthikeyen, M.E.,EEE Depart.,JJCET,Trichy.
ABSTRACT
Traditionally, switched-capacitor (SC)
converters have suffered from high transient
currents, which limit both the efficiency and
power density of such converters. Softcharging operation can be employed to
eliminate the current transients and greatly
improve the power density of SC converters.
In this approach, a second-stage magnetic
converter is cascaded with the SC stage to act
as a controlled current load. Another approach
is to use resonant SC converters with zerocurrent switching. Still the output power is too
low, such as conversion takes only at the ratio
of 8 to 1. This project shows that resonant and
soft-charging operations of SC converters are
closely related, and a technique will be
proposed, which achieves either operation by
adding a inductor to existing SC topologies
and an another inductor is added to improve
the output power also to act as a boost
converter. And a dc motor is connected as load
and the speed of the motor is controlled by
controlling the voltage level at converter
circuit.
INTRODUCTION
In the traditional switched capacitor
converter circuit, capacitors are directly
charged/discharged by other capacitors or
voltage sources, hence large transient current
spikes can occur, which limit the efficiency
and power density of the converter. Moreover,
these transient effects increase the device
stress
and
can
cause
undesirable
electromagnetic interference problems. To
reduce the current spikes, either large
capacitors or higher switching frequency has
to be employed, neither of which is a
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satisfactory solution. The quasi-SC converter
manages to reduce the peak of the current
transient, but results in the same power loss as
conventional SC converters. Resonant SC
converters (or soft-switching SC converters),
which incorporate one or more inductors, have
been proposed to eliminate the current
transients. Because of the resonant inductor,
the capacitor current becomes sinusoidal, and
if the switching takes place at the moment
when the current reaches zero, resonant SC
converters can operate in zero-current
switching (ZCS) mode. This mode of
operation enables such converters to operate at
higher frequencies and achieve higher power
density than their hard-switched counterparts.
Another drawback of SC converters is that
high efficiency is only achieved at one or a
few conversion ratios. This limits the
application of SC converters to mostly low
power applications. In higher power
applications, the solution is usually to cascade
a magnetic converter to act as a post regulation
stage. However, the overall converter size may
increase and the peak conversion efficiency
may be reduced.
PROPOSED SYSTEM
In this thesis , the origin of the
transient current and associated losses is first
revisited and the concept of soft-charging
introduced. The requirements imposed on the
SC converters by soft charging operation are
postulated. Resonant SC converters and softcharging SC converters are then analyzed.
This makes it possible to use similar
techniques to analyze and synthesize both
types of converters. Here a design is presented
to achieve soft charging or resonant mode of
operation with inductor added to existing SC
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International Conference on Current Research in Engineering Science and Technology (ICCREST-2016)
converter topologies. In addition a motor is
added as load and the speed of the motor is
controlled by the PI controller.
DC SOURCE
SWITCHED
CAPACITOR
CONVERTER
SINGLE
PHASE
INVERTER
PWM
SINGLE PHASE
INDUCTION
MOTOR
PI
CONTROLLER
BLOCK DIAGRAM OF PROPOSED
SYSTEM
SOFT-CHARGING OPERATION WITH
AN INDUCTOR
Since an inductor allows instantaneous
change of its terminal voltage, it can also act
as a controlled current load. In fact, the buck
converter is able to facilitate soft charging
operation precisely because of the inductor it
contains. In this section, the technique of
adding an inductor alone to achieve softcharging is presented. Furthermore, it will be
shown that resonant operation can also be
achieved using the same technique. To
illustrate the technique, a simple SC converter
is shown in Fig. 1(b), and a modified structure
is shown in Fig. 1(a). As can be seen in Fig.
1(b), the technique to achieve resonant and
soft-charging operation is to add an inductor at
the output of the SC converter, immediately
before the output capacitor. The simple circuit
structure in Fig. 1(b) allows direct circuit
analysis using differential equations. First,
with the additional inductor, the modified
converter is able to reach the same minimum
impedance at a much lower switching
frequency, due to the elimination of the current
transient and associated loss. Therefore, the
proposed converter can achieve the same
efficiency as conventional SC converters,
while using significantly lower switching
frequency, or equivalently, significantly
smaller flying capacitor values. The minimum
frequency at which the converter is able to stay
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in FSL operation can be defined as fcrit , and
for the modified converter in Fig. 1(b), it is
given by
1
2 √
where L is the added inductance and C is the
collective capacitance in series with the
inductor. This naturally coincides with the
design goal of the soft-charging SC converters.
In discrete implementations, the addition of
the inductors often results in overall
improvement in energy utilization of the
passive components. While the inductor is
more difficult to integrate than the capacitors
given the current IC technology, the energy
density, and quality of integrated inductors are
improving as more advanced processes, and
the proposed converter is able to take
advantage of the progress and advancement of
technologies in inductors, capacitors, and
switches simultaneously.
(a)
Fig. 1 (b) Switched capacitor converter circuit
(a) modified switched capacitor converter
circuit
ZERO VOLTAGE SWITCHING (ZVS)
A switch that operates with ZVS has
an anti-parallel diode and a capacitor across it.
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International Conference on Current Research in Engineering Science and Technology (ICCREST-2016)
If negative current is forced to flow through
the anti-parallel diode then voltage across
switch reduces to zero and then the switch is
turned on with ZVS. During turn-off the
capacitor across switch reduces the rate of rise
of voltage across the device as current reduces
to zero.
Zero voltage switching
ZVS is preferred over ZCS because
with ZVS the parasitic switch capacitance
dissipates the energy into the load. If there
were no ZVS this parasitic capacitance
dissipate as heat in the switch which lowers
the efficiency of the system.
In the thesis, the converter employs
another LC resonant circuit designed to
resonant at switching frequency so that ZVS
condition is achieved during buck operating
modes.
CONTROLLER
Controllers are to perform a controlled
operation to obtain the desired output. Mainly
controllers have 3 modes they are PProportional, I-Integral, D-Derivative mode
combine with another mode to create various
controllers. There are 3 main controllers used
for controlling. They are
 P controller
 PI controller
 PID controller
P CONTROLLER
In general it can be said that P
controller cannot stabilize higher order
processes. For the 1st order processes, meaning
the processes with one energy storage, a large
increase in gain can be tolerated. Proportional
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controller can stabilize only 1st order unstable
process. Changing the controller gain K can
change closed loop dynamics. A large
controller gain will result in control system
with:
a) Smaller steady state error, i.e. better
reference following.
b) Faster dynamics, i.e. broader signal
frequency bands of the closed loop
system and larger sensitivity with
respect to measuring noise
c) Smaller amplitude and phase margin
When P controller is used, large gain is
needed to improve steady state error. Stable
systems do not have problems when large gain
is used. Such systems are system with one
energy storage (1st order capacitive systems).
PI CONTROLLER
PI controller will eliminate forced
oscillation and steady state error resulting in
operation of on-off controller and P controller
respectively. Introducing integral mode has a
negative effect on speed of the response and
overall stability of the system. Thus, PI
controller will not increase the speed of
response. It can be expected since PI controller
does not have means to predict what will
happen with the error in near future. This
problem can be solved by introducing
derivative mode which has ability to predict
what will happen with the error in near future
and thus to decrease a reaction time of the
controller.
PI controllers are very often used in
industry, especially when speed of the
response is not an issue. A control without D
mode is used when:
a) Fast response of the system is not
required
b) Large disturbances and noise are
present during operation of the process
c) There is only one energy storage in
process(capacitive or inductive)
d) There are large transport delays in the
system
PID CONTROLLER
PID controller has all the necessary
dynamics; fast reaction on charge of the
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International Conference on Current Research in Engineering Science and Technology (ICCREST-2016)
controller input (D mode), increase in control
signal to lead error towards zero (I mode) and
suitable action inside control error area to
eliminate oscillations (P mode). Derivative
mode improves stability of the system and
enables increase in gainK and decrease in
integral time constant Ti, which increases the
speed of the controller response. PID
controller is used when dealing with higher
capacitive processes (processes with more than
one energy storage) when their dynamic is not
similar to the dynamics of the integral (like in
many thermal processes). PID controller is
often used in industry, but also in the control
of mobile objects (course and trajectory
following included) when stability and precise
following are required. Conventional autopilot
is for the most part PID type controllers.
Transfer function for most basic form of PID
controller
( )=
+
+
Where Kp-proportional gain, Ki-integral gain,
Kd-derivative gain
EFFECTS OF PROPORTIONAL,
INTEGRAL AND DERIVATIVE
There is always a steady state error in
proportional control. The error will decrease
with increasing gain, but the tendency towards
oscillation will also increase. It follows from
the strength of integral action increases with
decreasing integral time Ki. The steady state
error disappears when integral action is used.
The tendency for oscillation also increases
with decreasing Ki.
Parameter
Speed of
response
Stability
Accuracy
Increasing K
Increasing
Ki
Increasing
Kd
Increases
Decreases
Deteriorate
Deteriorate
Improves
Improves
Increases
Improves
No impact
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Effects of coefficient
The parameters K and Ki are chosen so
that the closed loop system is oscillatory.
Damping increases with increasing derivative
time, but decreases again when derivative time
becomes too large. Recall that derivative
action can be interpreted as providing
prediction by linear extrapolation over the time
Kd. Using this interpretation it is easy to
understand that derivative action that does not
help if the prediction time Kd is too large. A
derivative action ceases to be effective when
Kd is larger than done sixth of the period. Also
notice that the period of oscillation increases
when derivative time is increased to obtain a
good PID controller it is also necessary to
consider
 Noise filtering and high
frequency roll-off
 Set point weighting
 Windup
 Tuning
 Computer implementation
PID TUNING
Tuning is adjustment of controller
parameters to optimum values for the desired
control response. Stability is the basic
requirement. However, different systems have
different behavior, different applications have
different requirements, and requirements may
conflict with one another. There are various
methods for tuning, some of them
 Manual tuning method
 Ziegler-Nichols tuning method
 PID tuning software method
In this thesis, Ziegler-Nichols tuning
method is used and which is explained below.
ZIEGLER-NICHOLS TUNING METHOD
The Ziegler-Nichols closed loop
method is based on experiments executed on
an established control loop (a real system or a
simulated system). The tuning procedure is as
follows:
 From the response of the process as
shown in fig.14. If the plant involves
neither integrator nor dominant
complex-conjugate poles, then such a
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unit step response curve may be look
as S-Shaped.
 From the S-Shaped curve, delay
time(L) and time constant (T)
determined by drawing a tangent line
at the inflection point of the curve.
 And from which controller parameters
may be found and thus given in below
tabulation.
Simulink model of motor in open loop
S-Shaped response curve
Type of
controller
P
PI
PID
Kp
0.9
1.2
Ki
Kd
∞
0
0
0.3
2L
0.5L
Ziegler-Nichols, gain parameters calculation.
SIMULINK MODEL OF RESONANT
CONVERTER WITH ZVS OPERATION
Fig. 3 Speed characteristics of the motor in
open loop
From fig.3 delay time (L) and constant time
(T) have been found out and the values are
given as
Delay time (L)
Constant time (T) = 0.02
We know that,
Simulink model of motor in open loop
Open loop circuit of a motor is
designed in order to find the characteristics of
the motor from which the control parameters
may be found out by Ziegler-Nichols PI
controller and which is explained below.
( )=
+
+
Where,
= 0.9
=
.
= 0.9
=
.
.
( )=
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= 0.015
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.
.
=
1.2
= 0.05
1.2 + 24
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International Conference on Current Research in Engineering Science and Technology (ICCREST-2016)
Ziegler-Nichols chart for the response.
Simulink model of resonant converter(ZVS) driving a motor
Simulink model.
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International Conference on Current Research in Engineering Science and Technology (ICCREST-2016)
Converter stage output Voltage waveform
REFERENCES
CONCLUSION
In this thesis, the soft-charging operation is
achieved with the addition of a single inductor.
Using the proposed technique, ZCS resonant
operation for SC converters can also be
achieved at an appropriate switching
frequency. Also the output voltage is boosted
with the additional inductor added next to the
source. Experimental results are shown above.
The proposed method expands the family of
both resonant and soft-charging SC converters
and makes SC converters suitable for an
increasing number of applications.
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