Novel pwm modulation technique for minimizing circulation current and zero crossing distortion

1. INTERNATIONAL Proceedings of the 2nd International JOURNAL Conference on OF Current ELECTRICAL Trends in Engineering and ENGINEERING Management ICCTEM -2014
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17 – 19, July 2014, Mysore, Karnataka, India
TECHNOLOGY (IJEET)
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 5, Issue 8, August (2014), pp. 132-140
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IJEET
© I A E M E
NOVEL PWM MODULATION TECHNIQUE FOR MINIMIZING
CIRCULATION CURRENT AND ZERO CROSSING DISTORTION IN DUAL
BUCK INVERTERS
V. Nandakumar1, Prof. M. R. Manjunath2
1M.Tech Student, Dept. of EEE, The National Institute of Engineering, Mysore, India
2Professor, Dept. of EEE, The National Institute of Engineering, Mysore, India
ABSTRACT
The trend for inverter design is towards low cost, high efficiency, high reliability, low ground
leakage current and low-output ac-current distortion. This paper introduces a high efficiency and low
cost dual buck inverter, which has no reverse recovery problem for MOSFET and is suitable for
Super-junction power MOSFET application. However, the duty cycle would be 50% at zero-crossing,
current will have a significant distortion when it adopts bipolar PWM for PV applications.
This paper proposes a novel modulation method for bipolar PWM, to achieve low circulation current
and with this modulation method; the current zero-crossing distortion will be minimized. All the
simulation results support the analysis and the proposed method for distortion correction in the Dual
buck PV inverter.
Keywords: Inverter, High Efficiency, Transformerless, Zero Crossing Distortion.
1. INTRODUCTION
Along with gradually reducing photovoltaic (PV) cost and worldwide demand for growing
electric energy, there has been a great spurt in PV market. In recent years, there is a considerable
innovation on PV products. One innovation is transformer-less topology, which eliminate the 50Hz
transformer to achieve lower cost and higher efficiency, such as Heric of Sunways inverters. The
overall trend for transformer-less PV inverters is towards low cost (Silicon device), high efficiency
(less high frequency switches, using Super-junction power MOSFETs), low circulation current and
low-output ac-current distortion.
One way to improve inverter efficiency is to develop new inverter circuit; another way is to
use advanced device, such as SiC devices and Super-junction power MOSFET, which has low

2. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
switching loss and conduction loss and has been widely applied in PFC (Power Factor Correction)
circuit, battery charger and so on. Due to bad reverse recovery of body diode, the application of
Super-junction power MOSFET in inverter is always a challenge. This paper introduces a dual buck
inverter, which has no reverse recovery problem for MOSFETs and is suitable for super-junction
power MOSFETs application [1-6].
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Due to leakage current requirement in transformer-less PV inverter, the only choice for dual
buck PV inverter is Bipolar PWM method. This paper is organized as follows: Part-A introduces the
high efficiency and high reliability dual-buck full-bridge transformerless inverter with traditional
bipolar PWM (TB-PWM) method; the TB-PWM would have a circulating current in inverter. Part-B
presents an improved bipolar PWM (IB-PWM) method, which can solve the leakage current on PV
panel and circulating current in inverter. However, this PWM method results in distortion when
current is at zero-crossing. Part-C proposes a novel nonlinear bipolar PWM method to solve the
current distortion at zero-crossing. Part-D gives the THD analysis of all these inverter topologies.
2. DUAL BUCK INVERTER
The dual-buck full-bridge PV inverter as shown in Fig.1, doesn’t need dead time, and totally
eliminates the shoot-through and body diode reverse recovery concerns, which means high
reliability. To utilize the benefits of power MOSFETs, such as lower switching loss, resistive
conduction voltage drop, and fast switching speed that allows reduction of current ripple and the size
of passive components, dual-buck full-bridge inverter had been proposed. As in Fig.1, the inductor
current is unidirectional and only works for positive or negative half-cycle current. The only
drawback is that four separate inductors are needed [1-12].
Fig.1: High efficiency transformer-less dual-buck full bridge PV inverter
2.1 Traditional Bipolar PWM Method
Dual to common mode voltage from traditional bipolar PWM (TB-PWM) method is always
Vdc/2, so the proposed dual-buck full-bridge inverter could adopt the TB-PWM for PV application.
The block diagram of TB-PWM is shown in Fig.2.
Fig. 2: Traditional bipolar PWM (TB-PWM) method
The operating mode and equivalent circuit under TB PWM is shown in Fig.3 (a) and (b)
S1+S4 and S2+S respectively, would conduct complementarily, so each inductor has current. When

3. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
S1 and S4 are conducting, freewheeling current of L2 and L3 would circle back to DC bus through
D1 and D4. When S2 and S3 are conducting, freewheeling current of L1 and L4 flow back to DC bus
through D2 and D3, which leads to a circulating current and decrease efficiency.Fig.4 shows the
simulation results of dual buck inverter using TB-PWM, for clear understanding the results without
filter are shown, where inductor current is not zero in another half cycle i.e. in negative half cycle,
this shows the presence of circulating current.
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Fig. 3(a): Operating mode and equivalent circuit under TB-PWM method during S1 and S4 turn on
Fig. 3(b): Operating mode and equivalent circuit under TB-PWM method during S2 and S3 turn on
Fig. 4: Simulation results with TB-PWM method without Filter

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2.2 Improved Bipolar PWM Method
In a dual-buck inverter, the operation principle is to let each inductor operate under positive
or negative half-cycle current, so PWM should be fed to the switches considering the current
polarity. The improved bipolar PWM method (IB PWM) method takes into account of the current
polarity, and the block diagram is shown in Fig.5 [3-5].
Fig. 5: Improved bipolar PWM (IB-PWM) method
The operating model and equivalent circuit are shown in Fig.6 (a), (b) ,(c) and (d), S1 and S4
will switch during positive half-cycle current, and freewheeling current goes through D2 and D3; and
S2 and S3 will switch during negative half-cycle current, freewheeling current goes through D1 and
D4.
As shown in Fig.6 and simulation result in Fig.7, IB-PWM reduces the switching power loss
and eliminates the circulating current. However, the current in zero-crossing region would have an
obvious distortion, which originates from the discontinuous-conduction mode (DCM) condition.
(a) Charging Interval during Positive Half Cycle (b) Freewheeling Interval during Positive Half Cycle
(c) Charging Interval during Negative Half Cycle (d) Freewheeling Interval during Negative Half Cycle

5. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
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Fig. 7: Simulation result under IB-PWM method
The Volt-second balance equation on inductor is,
When inductor current is under continuous-conduction mode (CCM) condition, TB-PWM
method and IB-PWM method have same Volt-second balance equation, which is [5],
(2)
However, during DCM condition at unity PF operation, Volt-second equation for IB-PWM
method will be
So, if Vref = 0, switching on and off time would be:
(4)
And ripple current and average current would be:
(5)

6. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
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Assume Vref and output current Iac are in phase, when current is positive and S1-S4 are
conducting, the relationship between the duty cycle (D) and Vref is:
When Vref=0, D will be 50%, it will let current in boundary mode [ibm] which depends on
current ripple.
When current is negative and S2-S3 are conducting, the relationship between the duty cycle
(D) and Vref is:
When Vref = 0, D will be 50%, it will let current in boundary mode [-ibm ] which depends on
current ripple.
As shown in the simulation results in Fig.7, the current would jump between +ibm and –ibm
when current changes the polarity at zero crossing, and the duty cycle will be 50% at zero crossing,
and the inductor L1 current will be in boundary mode condition. The Total harmonic distortion
(THD) of output AC current is 29.66%, which is mainly coming from zero-crossing distortion [3-4].
2.3 Novel PWM Modulation Method For Current Zero-Crossing
As discussed in last section, the current distortion happened at DCM region, and the reason is
that without active switch working during turn off, AC output current could only be either positive or
negative in each cycle, and polarity changing of current results in current distortion. Proposes
method combines the advantage of TB-PWM (no distortion) and IB-PWM (high efficiency). Fig.8
shows the realization of Novel PWM modulation method which adopts the unipolar method, but the
PWM gating pattern is still bipolar.
Fig. 8: Proposed Novel PWM Modulation Method in Dual Buck Inverter
This paper proposed a novel modulation method for bipolar PWM, which provide 0% duty
cycle at zero-crossing. With the 0% duty cycle at zero-crossing, the inductor current would go into
DCM, rather than boundary mode. So zero current would be got at zero-crossing.
The proposed modulation method is shown in Fig.8. As shown in the Fig.8, the modulation
method adopts the unipolar method, but the PWM gating pattern is still bipolar. With this method,

7. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
the duty cycle could go to zero at the zero-crossing. So, the current distortion at zero crossing would
be improved.
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Fig. 9: Simulation Results with Proposed Bipolar PWM Method in Dual Buck Inverter
3. THD ANALYSIS AND COMPARISON FOR ALL THE INVERTER TOPOLOGIES
THD calculations can be obtained from the SIMULINK. The switching pattern that is used in
this project for all of the inverters topologies are harmonic elimination method.
Where,
V1= Fundamental Voltage magnitude
V2 = Magnitude of 2nd Harmonic
V3 = Magnitude of 3rd Harmonic
V4 = Magnitude of 4th Harmonic
.
.
.
Vn = Magnitude of nth Harmonic

8. Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
17 – 19, July 2014, Mysore, Karnataka, India
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TABLE-1: Total Harmonic Distortion Analysis
METHODS TOTAL HARMONIC DISTORTION
IB-PWM 29.66%
NOVEL-PWM 6.35%
Fig.10: THD comparison between IB-PWM method and Novel PWM method
4. CONCLUSION
The trend for transformerless inverter is high-efficiency and low cost. This paper introduces a
high-efficiency dual buck inverter, which reduces circulation current and distortion at zero crossing,
and which is highly suitable for Super-junction power MOSFET.
Detail discussion and analysis point out:
1. Traditional bipolar PWM will have circulating current, but no current distortion.
2. Improved bipolar PWM does not have circulating current, but has significant current distortion.
With proposed Novel PWM method, the current distortion can be solved, and the circulating
current will be significantly reduced, which will not affect the efficiency significantly.
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17 – 19, July 2014, Mysore, Karnataka, India
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