This paper presents a LLC resonant converter as an interface for a renewable ... widely used for the demand of light weight and small size converter system ...
IJETSE International Journal of Emerging Technologies in Sciences and Engineering, Vol. 5, No. 3, March 2012 ⓒ2012 IJETSE
High Efficiency LLC Resonant Interface Converter for a Renewable Generation N. H. Saad1, A. A. El-sattar1, N. M. Rady2 1
Electrical Power and Machines Department, Faculty of Engineering, Ain Shams University, Cairo, Egypt 2 North Cairo electric distribution company, Cairo, Egypt
Abstract - With the use of power electronic interface converters, renewable sources can be connected directly to a distribution network or combined with other local generators and loads to form an independent power system. As a promising topology for this application, resonant converters offer low switching losses due to zero voltage switching (ZVS) making them popular for high frequency applications. The basic idea of a resonant converter is to operate the MOSFETs with either a sinusoidal voltage or by running a sinusoidal current through it. The switching instant must be selected in proximity to the zero crossing of the sinusoidal voltage or current. The dissipated power will then be very small. This paper presents a LLC resonant converter as an interface for a renewable generation. The performance characteristics of the system are studied using a MATLAB/SIMULINK simulation without a resonant filter, with a resonant filter, and a resonant filter with an active damping resistance. The converter can be operated at resonance as well as below and above resonance. The results prove the validity and advantages of the proposed methodology.
Key words: Zero Voltage switching,, LLC Resonant Converter, DC-DC Interface Converter, Renewable Generation.
1. Introduction Clean and sustainable electricity generation from renewable resources has experienced a significant development during the last decades. Distributed renewable sources can be harnessed by using powerelectronic converters. Renewable sources may be linked directly with the existing power system, or combined with local loads to form an independent power system [1]. For many years, Pulse Width Modulation (PWM) technology has remained at relative plateau in regard to the advances in performance, although their power density (watts per cubic mm) and their frequency of operation remained low. The main advantages of using PWM method include constant frequency operation which allows optimal design of the magnetic filter components. Also they have minimum voltage current stresses and good control range. However, the increase in device switching losses as the frequency increases and the high voltage stress induced by the parasitic inductances following diode reverse recovery are major drawbacks of this method. Resonant type converters can achieve high efficiency and wide input voltage range capability because of its voltage gain characteristics and small switching loss. They are being widely used for the demand of light weight and small size converter system because of its ability of high switching frequency operation with low switching losses [2, 3]. This paper investigates a LLC resonant converter for renewable generation applications.
The paper deals with soft-switching techniques for resonant converter which increase their efficiency and further increase of the switching frequency while reducing the size, cost and electromagnetic interference (EMI) problems. The main drawback with resonant converter is that the LLC-filter will introduce a resonance frequency into the system. Harmonic components in the output voltage can lead to resonance oscillations and instability problems unless they are properly handled. One way of reducing the resonance current is by adding a passive damping circuit to the filter. This damping circuit can be purely resistive. Compared with PWM topologies, the resonant converter can achieve higher efficiency. It has ZVS turn-on and low turn-off loss for primary side switches over the whole load range which is very important for applications with high input voltage. The paper also focuses on the detailed comparison between the resonant and the conventional converters.
2. Analysis of Full Bridge LLC Resonant Converter with Passive Damping Resistance The circuit of LLC resonant DC-DC converter is shown in the Fig. 1. The switching circuit is implemented with a full-bridge circuit. The output voltage is swing between -12V and +12V. The resonant tank circuit is made up of a capacitor, an inductor, a magnetizing inductor, a damping resistance and an isolating transformer.
2
In some cases the magnetizing inductor and the resonant inductor are included in the transformer parameters. This configuration would reduce converter size, cost, and complexity. 12 V DC voltage output of photovoltaic module is converted into high frequency AC voltage using an inverter. The output of the inverter is filtered using LLC filter. This voltage is stepped up to 70 V by using a step up transformer. The full bridge inverter applies a square wave of voltage to the resonant network. The system was designed and the tank components were selected as given in table I, so that, nominally, the switching frequency of the MOSFETs nearly equals the tank resonant frequency. Therefore, if the tank circuit is “well designed”, it will have the effect of filtering the higher harmonic voltages so that a sine wave of current is obtained. The resonance oscillations in a network can be damped by connecting a resistor in series with the filter [4]. In this case, the classical AC analysis techniques can be used [5]. For this case the fundamental component of the square wave voltage is applied to the resonant network, and the resulting sine waves current and voltage in the resonant circuit are computed. A rectifier, with a low pass output filter, is used to obtain dc output voltage.
Table I system parameters. Resonance frequency = 8.7 KHz
Sampling time = 4 μ sec
Switching frequency = 10 KHz
DC-link voltage = 70 V
PV Module parameters
Filter parameters
V pv 12 V
Lr 95 μH
Pmax 60 W
Lm 500 μH C r 3.5 μF R3 Ω
Circuit operation is divided into two time intervals, where the two MOSFETS Q1 and Q2 operate in Complimentary mode and also, MOSFETS
Q3 and Q4
as follows:-
1-Time interval (a)
Q2 ,OFF ,Q3,OFF ,Q1,ON ,Q4 ,ON Q2 and Q3 are turned off. At this moment, resonant inductor Lr current (tank current) is negative. This current will flow through diodes D2 and D3 , which creates a ZVS condition for Q1 and Q4 . Gate signals of Q1 and Q4 should be applied at this This interval begins when
instant when tank current passes through zero, and this current begins to rise, this will force secondary diodes D5 and D8 conduct while D6 and D7 is not conducting, and the output current begins to increase. Also, at this instant the transformer sees output voltage on the secondary side and Lm is charged with constant voltage. Fig. 1 The LLC resonant converter.
2- Time interval (b) Soft switching of the switches is done using LLC circuit at the output of the inverter. The even harmonics in the output of the rectifier are filtered using the LC filter. Driving pulses are applied to the MOSFET in such a way that the switching frequency is very near to the tank resonant frequency.
Q1,OFF ,Q4 ,OFF ,Q2 ,ON ,Q3,ON This interval begins when Q1 and Q4 are turned off and at this instant, gate signals of Q2 and Q3 are applied. During this mode, secondary diodes D6 and D7 conduct.
Lm is linearly charged with output voltage, so it
doesn't participate in the resonance during this period. This interval ends when tank current is the same as magnetizing current as shown in Fig. 2. Output current reaches zero and transformer secondary voltage is lower than output voltage.
At the end of this instant, Q2 and Q3 are turned off and Lm participates in the resonance.
5
tank current magnetizing current
4 3
current(Amp.)
2 1 0 -1 -2
resonant converter voltage withoutdamping resistance(V)
3
15
10
5
0
-5
-10
-15 2
2.1
2.2
2.3
2.4
time(sec)
2.5 -3
x 10
-3
Fig. 4 Resonant filter output voltage without damping resistance.
-4 -5
0.15
0.15
0.15
0.15 0.1501 0.1501 0.1501 0.1501 0.1501 0.1501
time(sec)
Fig. 2 Currents flowing in the primary side.
3. Simulation Results
15
15
10
10
resonant filter voltage(V)
inverter voltage(v)
The simulation circuit of LLC resonant converter has been implemented with MATLAB/SIMULINK and tested with a resistive load. The output voltage of the inverter is shown in Fig. 3.
When using a damping resistance R = 3 Ω, the output voltage is nearly sinusoidal as shown in Fig. 5. The effectiveness of the damping resistor becomes clear as the resonance is well damped. The voltage spectra with and without damping resistor is shown in Fig. 6. This figure illustrates a good resonance damping. The resonance oscillations are damped fast and effectively by the damping resistor. Output DC voltage with LC filter is shown in Fig. 7.
5
0
-5
0
-5
-10
-10
-15
5
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
time(sec)
0.9
1 -3
x 10
Fig. 3 Inverter output voltage.
Passive damping is achieved by adding a resistance in series or in parallel with the capacitance or the inductance of the filter. With different damping resistor values varying from R = 0 Ω to R = 3 Ω and constant system resonance frequency. The presence of the resonant filter introduces resonance oscillations and the output voltage is distorted with R = 0 Ω as shown in Fig.4.
-15 2
2.1
2.2
2.3
2.4
time(sec)
Fig. 5 Resonant filter output voltage with damping resistance R = 3 Ω.
2.5 -3
x 10
4
Fig. 8 demonstrates ZVS on the low-side MOSFET. This is achieved with magnetizing current, which is not related to load current, so ZVS could be realized even with zeroload. Since this magnetizing current is also the turn off current of MOSFET. Choosing different magnetizing inductance could control it. The turn off current could be much smaller than load current, so turn off loss can be reduced. In this case it is confirmed that, the switching loss of the converter is very small.
90 80
load voltage(v)
70 60 50 40 30 20
4. Circuit Operation below Resonance
10
Circuit operation below resonance is calculated by Equation (1).
0 -10
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
f sw f T sw T r r
0.2
time(sec)
where:
Fig. 6 DC output load voltage. 25%
20%
15%
10%
ت ت تت
5%
0% 3 rd
5 th
7 th
9 th
R= 0Ω
R = 3Ω
Ω
Ω
(1)
f sw is the switching frequency. f r is the resonance frequency.
Essentially, the circuit behavior is similar to that at resonance. However, there are some significant differences that directly influence the secondary side switching behavior. Fig. 9 shows the relevant waveform and should be compared to Fig. 5. When operating below resonance, for a given input voltage, the LLC resonant full-bridge will provide an output voltage higher than that available at resonance. In other words, the conversion ratio, intended as previously mentioned, is > 1, then the LLC is said to have a "stepup" characteristic when operating below resonance. Since the tank voltage fundamental sine wave has a shorter period compared to the switching period, the output AC voltage will have higher periodic time than that at resonance.
Fig. 7 The DC output voltage spectra with and without damping resistor. 10
2
resonant filter voltage(v)
MOSFET gate to source voltage MOSFET drain to source voltage at base10
voltage (V)
1.5
1
5
0
-5
0.5 -10 2
0
2.1
2.2
2.3
time(sec) 0.1
0.1001
0.1002
time(sec)
Fig. 8 Zero voltage switching.
2.4
2.5 -3
x 10
0.1003
Fig. 9 Filter output voltage when operating below resonance.
5
5. Circuit Operation above Resonance Circuit operation above resonance is calculated by Equation (2).
f sw f T sw T r r
(2) The circuit behavior is somehow reversed compared to the operation below resonance. Fig. 10 shows the relevant waveform and should be compared to Fig. 5. It can be seen that, when operating above resonance, for a given input voltage the LLC resonant full-bridge will provide an output voltage lower than that available at resonance. In this case the conversion ratio, intended as previously mentioned is < 1, then the LLC is said to have a "step-down" or "buck" characteristic when operating above resonance. Since the resonant period is longer than the switching period, the output voltage will have smaller periodic time than that at resonance.
resonant filter voltage(v)
10
5
0
-5
-10 2
2.1
2.2
2.3
2.4
time(sec)
2.5 -3
x 10
Fig. 10 Filter output voltage when operating above resonance.
6. Conventional PWM Converters and Resonant Mode Converters In this part, the efficiency of LLC resonant converter is compared with the conventional PWM converter shown in Fig. 11. Conventional PWM converter is choosing as candidate because this topology is the most common and widely available for power designer [8]. Comparison is based on simulation results. Simulation models of both converters were built with same design specifications. The design data for both converters are: Vin = 12 V, Vout = 70 V and Pout = 60 W, switching frequency for PWM 10 kHz and switching frequency for LLC is from 10 kHz.
Fig. 11 Conventional full Bridge Converter.
Fig. 12 shows the simulated input current waveform for two converters at 12 V input voltage at full load. It can be seen that for conventional full Bridge converter, the current waveform is highly asymmetrical. This will increase both the conduction loss and switching loss. So the efficiency of the converter will be hurt by wide input range. This asymmetrical waveform will also increase the voltage stress of the secondary rectifier. Higher voltage rating devices have to be used which have higher forward voltage drop. Secondary conducting loss will be a large part of total loss. For LLC resonant converter, the input current has lower peak value and RMS value, so the conduction loss is much lower. Also, the secondary side voltage stress is lower than that of the conventional full bridge. For LLC resonant converter, high current stress or voltage stress is always a concern since the increase of conduction loss will compensate the benefit get from reduced switching loss. In fact, for conventional converter, the energy stored in leakage inductance is used to achieve ZVS, so at light load, the converter will loss ZVS capability. But for LLC resonant converter, the magnetizing inductor is used to achieve ZVS, so ZVS is obtained in whole load range. 30
input current waveform input voltage waveform 20
10
0
-10
-20
-30 2
2.1
2.2
2.3
time(sec)
(a)
2.4
2.5 -3
x 10
6
30
0.97
input current waveform input voltage waveform
Conventional converter LLC converter
0.96
20
0.95
Efficiency
10
0
0.94
0.93
-10 0.92
-20
0.91
-30 2
2.1
2.2
2.3
2.4
time(sec) (b)
2.5
efficiency
can
be
calculated
Po
by:
(3)
Conventional converter LCC converter
Efficiency
0.95
0.94 0.93
0.92
0.91
20
30
40
50
60
70
80
10
12
14
16
18
20
22
24
Fig. 14 shows the measured efficiency versus output load currents for the two different topologies. It is revealed that the LLC exhibits the best efficiency at heavy loads. It is showing very high values at maximum load level, higher than 96%. Also at light load, the converter efficiency is very good, reaching a value better than 93%.
7. Conclusions
0.97
10
8
Fig. 14 Measured efficiency of LLC and conventional converters at different loading conditions and constant input voltage.
The circuit efficiency has been calculated at different input dc voltage, and is plotted in Fig. 13. Compared with conventional converter, the LLC resonant converter could improve the efficiency at normal operation point by more than 3%.
0.9
6
Load current (Amp.)
Pi where: Po is the load output power. Pi is the input power.
0.96
4
x 10
Fig.12 Simulated input current waveform of (a) Conventional full Bridge and (b) LLC resonant converter at the same input voltage.
Measured
0.9
-3
90
100
110
120
input DC voltage(V) Fig. 13 Measured efficiency of LLC and conventional converters at different input voltage and full load.
LLC resonant topology was introduced for interfacing renewable generation application. It introduces a viable alternative to the existing converters. Ultimately, make the switching power losses (dissipated power) very small, and reduce di/dt, the best solution is to try to operate the system in such a way that during the switching time, the MOSFET voltage and/or current is as close to zero as possible. This constraint would introduce low switching losses with high efficiency. MATLAB circuit model for the closed loop system is developed and it is successfully used for simulation studies. From analysis and simulation, LLC resonant converter is proved to be able to improve the performance of front end DC/DC converter significantly, where the output of the inverter is nearly sinusoidal when switching frequency is the same as the resonance frequency. Design the LLC filter with a damping resistance, is shown to provide adequate damping of the resonance oscillations. The output of rectifier is also pure DC due to the presence of the law pass filter at the output. LLC resonant converter is shown to exhibit variant voltage gain by varying the switching frequency. It has voltage gain =1 at resonance. The gain will be less than one above resonance and more than one below resonance.
7
This paper also focuses on the comparison between the LLC resonant converter and the conventional full bridge converter when they are used as the front-end DC/DC converter of the renewable generation. The LLC resonant converter could cover wide input range with much higher efficiency compared with conventional PWM converter. Also, for different loading conditions with constant input voltage, the LLC exhibits better efficiency.
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