a zero current switching resonant buck converter

A ZERO CURRENT SWITCHING RESONANT BUCK CONVERTER
1
1,2
P. SAI PRASANNA, 2V. LAVANYA
PG Students, GMR institute of Technology, Rajam
Abstract- In this paper a zero current switching auxiliary circuit used for soft switching of DC-DC buck converter
is presented. Buck converters are high frequency dc-dc converters operating on PWM principle. The switching devices are
made to turn on and turn off the entire load current at high di/dt and withstands high voltage stress across them.
This propels the need for soft switching techniques which provides an effective solution and eliminates electromagnetic
interference. The proposed model of zero current switching(ZCS) resonant buck converter is simulated in matlab/
Simulink. The performance of the converter is improved by introducing PID controller and fuzzy logic controller. The
ZCS buck converter obtained has achieved improved efficiency along with reduced ripple output. The feasibility of
circuit is confirmed by waveforms that were found to be in precise proximity of the theoretical waveforms.
Index Terms- Buck converter, Soft switching, Zero current switching.
current to zero before the switch voltage rises, making
it more effective than zvs in reducing losses [3].
I. INTRODUCTION
Buck converter is a step down DC-DC converter that
is widely being used in different electronic devices like
laptop, PDAs, cell phones and also in electric vehicles
to obtain different levels of voltages. A few
applications of DC-DC converters are where 5V DC
on a personal computer motherboard must be stepped
down to 3V or less [2]. In all of these applications,
power conversion is required retaining high efficiency
as much as possible. The main advantage of switching
regulators is that the energy stored by inductor and
capacitor can be transformed to output voltage that
can be less than the input, can be greater than the input
[2]. These converters essentially just change the input
energy into a different impedance level [2]. However,
the higher input voltages and lower output voltages
have brought about very low duty cycles, increasing
switching losses and decreasing conversion efficiency.
Power switches have to cut off the load current within
the turn-on and turn – off times under the hard
switching conditions.
This paper introduces a new auxiliary circuit for buck
converter to provide ZCS condition. Soft switching is
achieved by reducing the switch current to zero just
before switching instants and thus the switching losses
would be reduced to zero. The gate driver circuit for
the switch is controlled by PID controller and Fuzzy
logic controller.
II. PROPOSED ZERO CURRENT SWITCHING
BUCK CONVERTER
In this section the proposed converter is introduced
and analyzed. Consider the buck converter circuit
shown in fig1.
Hard switching techniques refer to the stressful
switching behaviour of the power electronic devices.
So, the efficiency of buck converter is improved by
adopting soft switching techniques and thus
minimizing the switching losses [7]. Soft switching is
achieved by reducing the switch current or voltage to
zero just before the switching instants and thus the
switching losses would be reduced to zero. If the
switch voltage is reduced to zero at switching instants,
it is commonly called zero-voltage switching (ZVS)
and if the switch current is reduced to zero it is called
zero-current switching (ZCS) [1]. The major
limitation in ZVS technique is that a large external
resonating capacitor is needed to lower the turn off
switching loss [3]. Conversely zcs eliminates the
voltage and current overlap by forcing the switch
Fig 1: Circuit diagram of proposed converter
Mode 1(t1-t2): Initially, Sm is turned on and Cr
charges through a resonance with Lm. Also, during
this mode, power is transferred from input to output.
Mode 2(t2-t3): At t2, ILm has reached zero and Sm can
be turned off under ZCS. In this mode, the resonance
voltage stays constant equal to 2Vs-Vout. Because of
the topology structure this interval can be eliminated.
Proceedings of Fourth IRF International Conference, 13th July 2014, Goa, India, ISBN: 978-93-84209-36-0
66
A Zero Current Switching Resonant Buck Converter
Mode 3(t3-t4): At t3, Sa is turned on and a resonance
between Cr and La until its voltage reaches the output
voltage.
Mode 4(t4-t5): At t4, diode D is turned on
and La energy is transferred to the load and its current
linearly decreases to zero. The voltage across Cr
remains
constant
and
equal
toVout.
Mode 5(t5-t6): At t5, Sa can be turned off under ZCS.
In this interval the load is supplied by output
capacitor. All the equivalent circuits corresponding
different modes are shown in fig 2.
At steady state condition, the converter voltage gain
can be calculated by considering the converter
efficiency equal to 100%. In this converter, energy is
absorbed from the input source only in the first
interval. Thus, the voltage gain can be calculated
using this fact. For the first interval the following
relations can be derived.
Fig 3: steady state waveforms of buck converter
t6
Ε in

Ε out
E in
2
 V s I Lm dt  2C r V s
t1
2 Τ
V out

R
2Rf C r
From buck converter design rules,
L m 
(2)
(3)
 Ε out
Α  E out E in 
(1)
V o (1  D)
f ΔI
ΔI
Cm 
8f ΔV
(4)
(5)
(6)
By increasing the frequency, the power transferred
ascends, but the frequency is limited to the two
resonant periods.
Fig 2: Equivalent circuit of each operating mode
(From top to bottom: Mode 1, 3, 4, 5)
Proceedings of Fourth IRF International Conference, 13th July 2014, Goa, India, ISBN: 978-93-84209-36-0
67
A Zero Current Switching Resonant Buck Converter
T min  α 1min  α 2max  π
LmCr π
LaCr 
1
f max
(4) When the set point is reached and the output is
still changing, the duty cycle must be changed a little
bit to prevent the output from moving away.
(5) When the set point is reached and the output is
steady, the duty cycle remains unchanged.
(6) When the output is above the set point, the sign of
the change of duty cycle must be negative.
(7) When the sign of change of duty cycle is
negative, the output is above the set point.
(7)
The input and output voltages of the proposed
converter are 100V and 50V respectively. The output
current is about 1.2A. The switching frequency is
about 25KHZ. The main and auxiliary inductors are
about 2.77mH and 0.28mH. The resonant capacitor is
0.12µF. The output filter capacitor is chosen as 50µF
for 1% ripple in output voltage.
2.2.1 MEMBERSHIP FUNCTIONS:
The membership function is a graphical
representation of the magnitude of participation of
each input. It associates a weighting with each of the
inputs that are processed, define functional overlap
between inputs and ultimately determines an output
response. The rules use the input membership values
as weighting factors to determine their influence on
the fuzzy output sets of the final output conclusion.
Once the functions are scaled and combined, they are
defuzzified into a crisp outputs which drives the
system [15].
2.1 DESIGN OF CONTROL CIRCUIT USING PID
CONTROLLER:
The reference voltage is the desired output voltage that
is 50V. Vout is the output voltage of the converter
which is less than 50V. The output is fed back to the
summing point and an error signal is generated. The
PID controller modifies the error and generates a
steady output signal. This signal is compared with the
repeating sequence which is a saw tooth wave
[10-13].During one switching cycle the amplitude of
output signal of PID controller is less than that of
amplitude of repeating sequence the outcome would be
a high state pulse. During next switching cycle the
amplitude of output signal of PID controller is greater
than that of amplitude of repeating sequence the
outcome would be low state pulse. Thus an on-going
pulse and off-going pulse is formed. This signal fed to
gate terminal of the IGBT and this drives main circuit.
The switch of the auxiliary circuit is driven by
applying NOT gate to the signal at gate terminal of
IGBT in the main circuit [10-13].
Three continuous membership functions are chosen to
model, analyze and simulate the fuzzy controller. It
has been defined taking into account the conditions of
normality and convexity of fuzzy sets. It embodies the
mathematical representation of membership in a set
and is required to have uniform shapes, parameters
and functions for the sake of computational efficiency
[16]. The membership functions for the input and
output are shown in figure (5).
Fig 4: Control circuit using PID controller
2.2 DESIGN OF CONTROL CIRCUIT USING
FUZZY LOGIC CONTROLLER:
The derivation of the fuzzy control rules is heuristic in
nature and based on the following criteria [15]:
(1) When the output of the converter is far from the
set point, the change of duty cycle must be large so as
to bring the output to the set point quickly.
(2) When the output of the converter is approaching
the set point, a small change of duty cycle is necessary.
(3) When the output of the converter is near the set
point and is approaching it rapidly, the duty cycle
must be kept constant so as to prevent overshoot.
Fig 5: Membership functions: (a) for error e and difference
error de (b) for output du
Proceedings of Fourth IRF International Conference, 13th July 2014, Goa, India, ISBN: 978-93-84209-36-0
68
A Zero Current Switching Resonant Buck Converter
2.2.2DEVELOPMENT OF RULE BASE:
The collection of rules is called a rule base and it
expresses input output relationship in linguistic terms.
They are typically written as antecedent – consequent
pairs of IF THEN structure and the inputs are
combined by AND operator. The antecedent
(condition part) and consequent (operation part) are
the description of process state and control output
respectively in terms of logic combination of a fuzzy
propositions. The generic linguistic control rule has
the form as IF x is A AND y is B THEN z is C where x,
y are the input linguistic variables and z is output
linguistic variables. 49 rules as shown in table are
formed depending on the number of membership
functions to play a key role in the improvement of
system performance [15]. NB (Negative Big), NM
(Negative Medium), NS (Negative Small), ZE (Zero),
PS (Positive Small), PM (Positive Medium), and PB
(Positive Big) [15].
III. SIMULINK MODELS AND RESULTS:
Fig.7: Open loop model for ZCS buck converter
THE LINGUISTIC LABELS REPRESENTATION
OF RULE BASE
Fig.8: Input voltage, output current and output voltage of open
loop ZCS buck converter
Fig.9: Voltage and current of the main switch
Fig.10: Voltage and current of the auxiliary switch
Fig. 6: Control circuit using fuzzy logic controller
Proceedings of Fourth IRF International Conference, 13th July 2014, Goa, India, ISBN: 978-93-84209-36-0
69
A Zero Current Switching Resonant Buck Converter
Fig.11: closed loop model for ZCS buck converter using PID
controller
Fig.15: closed loop model for ZCS buck converter using Fuzzy
logic controller
Fig.12: Input voltage, output current and output voltage of
closed loop ZCS buck converter using PID controller
Fig.16: Input voltage, output current and output voltage of
closed loop ZCS buck converter using fuzzy logic controller
Fig.17: Voltage and current of the main switch
Fig.13: Voltage and current of the main switch
Fig.18: Voltage and current of the auxiliary switch
Fig.14: Voltage and current of the auxiliary switch
Proceedings of Fourth IRF International Conference, 13th July 2014, Goa, India, ISBN: 978-93-84209-36-0
70
A Zero Current Switching Resonant Buck Converter
COMPARISON OF PERFORMANCE OF BUCK
CONVERTER WITH PID AND FUZZY LOGIC
CONTROLLER
It is clear that the settling time with fuzzy logic
controller is less than the settling time with PID
controller. The ripple content gets reduced with fuzzy
logic controller. The main switch Sm and auxiliary
switch Sa achieved the ZCS condition.
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CONCLUSION:
Summing up in simple terms, soft switching
techniques not only reduces the switching losses but
also allows the switching frequency be typically 100
-500 KHz. In these soft switched converters resonance
is allowed to occur just before and during the turn-on
and turn-off processes so as to ensure ZVS and ZCS
conditions. For high efficiency power conversion, the
ZCS topologies are most frequently adopted.The
performance of the proposed converter is improved by
PID controller and fuzzy logic controller.
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simulation of digital PID controller for open loop and closed
loop control of buck converter”, IJCST vol 1, issue 2,
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converter for efficiency regulation”, IOSR journal of
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Mohammad Mahdavi, Amin Emrani, Hosein Farzanehdard, “A
New Zero Current Switching Resonant Buck Converter”,
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Proceedings of Fourth IRF International Conference, 13th July 2014, Goa, India, ISBN: 978-93-84209-36-0
71

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