Middle-East Journal of Scientific Research 24 (3): 877-886, 2016
ISSN 1990-9233
© IDOSI Publications, 2016
DOI: 10.5829/idosi.mejsr.2016.24.03.23088
Abstract: This paper provides a comprehensive review of various dc-dc converter topologies such as Pulse
Width Modulation converter, hybrid resonant converter, Dual Active Bridge converter, Series and parallel LLC resonant converter. The confusion takes place while selecting a converter as every converter has its merits and demerits. In this paper, detailed description and classification of the resonant converters have made based on the features and its applications. Among all the resonant converters the LLC resonant converter has become more popular due to its special features like wide range of soft switching capability, better efficiency, reduced reverse energy and high power density. By using the new control scheme in the LLC converter by adjusting the power angle between input and output side, the power flow direction and output power of the converter can be changed automatically and continuously, which is attractive for energy storage systems to balance the energy. This review paper is intended to serve as a convenient reference to future resonant converter users.
Key words: Bidirectional converters dc–dc conversion Dual active bridge (DAB) topology Pulse width modulation (PWM) LLC resonant topology
INTRODUCTION active bridge (DAB) converter has attracted lots of research interests due to its simple structure, wide range
The renewable energy resources, such as soft switching capability and high efficiency [7-10].
photovoltaic (PV) and wind power, are the most promising However, it suffers from high reverse energy and high ways for clean electric power generation. However, the intermittent nature of these resources Introduces issues like system stability, reliability and power quality. Energy storage systems (ESSs) are Required to deal with such turn-off power loss which deteriorate the overall efficiency [11]. Several control methods with two or more phase shift angles as control variables were proposed to minimize the reverse energy in [12-14], but the control intermittent outages for grid tied and off grid application.
methods were a little bit complex and the turn-off loss was
The ESSs should have bidirectional power flow capability still high. The turn-off power loss is related to the turn-off to store the excess energy generated by renewable resources and release it when the renewable energy is not sufficient or during peak times of energy consumption
[1-4]. Even though it is common to use power flow in unidirectional, but later due to many applications growing emphasis on small size, compact the two unidirectional way of power flow and efficient power systems. This gives increasing interest to develop a bidirectional type of power flow with bidirectional converter [5].
The concept of power flow in both direction for bidirectional dc-dc converter devices realize current flow in each way [6]. From all the isolated topologies, the dual current, which can be reduced by operating the DAB topology in resonant mode with an extra resonant capacitor, i.e., dual bridge series resonant converter.
However, it can only operate under buck mode which is not suitable for wide input/output range applications like
ESSs [15].
In earlier works, the LLC resonant converter has been used for designing wide output range SMPSs and it has been shown that by using frequency control, this converter can handle a wide-range of regulated output voltage even when input voltage and load have large variations [16-18]. To attain this purpose, zero voltage
Corresponding Author: Dr. P. Sivachandran, Department of Electrical and Electronics Engineering,
Sree Sastha Institute of Engineering and Technology, Chembarambakkam, Chennai-600123, India.
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switching (ZVS) operation even under the worst case conditions is one of the most important subjects that must be realized for reducing electromagnetic interference
(EMI) and improving the efficiency and performance of the converter. Output stage diodes of this converter are always turned ON and OFF with zero current switching
(ZCS) which reduces the switching losses. In MOSFETbased primary-side LLC resonant converter, the ZVS operation causes lower dissipation than ZCS, because the switching losses of the reverse recovery process are eliminated [19].
Generally, bi-directional operation of LLC resonant converter is complicated and difficult to control. Because, the switching frequency has to be changed across fsr
(series resonant frequency) and the switching sequences
(the switching timing between high voltage side and low voltage side) has to be changed due to resonant characteristics as well [20]. In order to simplify the switching scheme between forward and reverse direction, the switching frequency has been moved to higher frequency than series resonant frequency where ZVS condition is maintained. The switching frequency has to be changed largely among forward direction and reverse direction for soft switching operation. This makes the control system complicated as well. In order to reduce the complexity of synchronous driving and control system, then achieve simple seamless operation, this paper propose unique switching scheme of the bi-directional
LLC converter.
In [21-23], input ports are connected in series or parallel in conventional DC-DC converters like flyback, forward and buck/boost, but different ports can’t supply energy synchronously and energy can’t be transferred between different ports either. Isolated three-port DC-DC converters with full bridge or half bridge structure are proposed in [24-26], in which energy is transferred bidirectional between each port and is controlled by the phase shift angle. The switching loss is related to the turn off current, which can be significantly reduced in LLC resonant converter for its ZCS and ZVS capability. Little research has been focused on the multi-port LLC resonant converter, three-port series resonant converters are proposed in [27] and [28], in which phase shift angle is used to control the output voltage with fixed switching frequency, but transfer-coupled structure makes the circulating energy high.
Resonant Converter:
Middle-East J. Sci. Res., 24 (3): 877-886, 2016
The Resonant converter, which were been investigated intensively in the 80's. These converters use resonant action of LC circuits to artistically introduce soft switching into power converters. It can achieve very low switching loss thus enable resonant topologies to operate at high switching frequency.
The characteristic features of resonant power converters can be listed as follows
The increase in frequency of operation can help in the reduction of the size of magnetic components and the C filters. This helps in decreasing the overall size of the converter and improving the overall efficiency of the converter.
Additional inductors and capacitors are necessary to introduce resonant action. Hence additional cost may be incurred in designing these components.
Due to ZVS or ZCS action, the EMI issues are very less in resonant converters.
Classification of Resonant Converter: topologies, Series Resonant Converter (SRC), Parallel
Resonant Converter (PRC) and Series Parallel Resonant
Converter (SPRC, also called LCC resonant converter) are the three most popular topologies.
Fig. 1: Classification of resonant converter
Series Resonant Converter:
In resonant
The resonant inductor Lr and resonant capacitor Cr are in series. They form a series resonant tank. The resonant tank will then in series with the load. From this configuration, the resonant tank and the load act as a voltage divider. By changing the frequency of input voltage Va, the impedance of resonant tank will change. This impedance will divide the input voltage with load. Since it is a voltage divider, the DC gain
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Middle-East J. Sci. Res., 24 (3): 877-886, 2016 of SRC is always lower than 1. At resonant frequency, the impedance of series resonant tank will be very small; all the input voltage will drop on the load. So for series resonant converter, the maximum gain happens.
To minimize the current stress of devices and optimize efficiency for forward mode operation, weighted average rms current is defined to represent the current stress of the converter. Weighted average rms current is contributed by current from primary and secondary side switches, clamp diodes, two primary windings and one secondary winding of high frequency transformer.
Series Parallel Resonant Converter: Its resonant tank consists of three resonant components: Lr,Cs and Cp. The resonant tank of SPRC can be looked as the combination of SRC and PRC. Similar as PRC, an output filter inductor is added on secondary side to math the impedance. For
SPRC, it combines the good characteristic of PRC and
SRC. With load in series with series tank Lr and Cs, the circulating energy is smaller compared with PRC. With the parallel capacitor Cp, SPRC can regulate the output voltage at no load condition. The parameters of SPRC designed for front end DC/DC application.
converter, the maximum gain happens
Fig. 2: Series resonant converter
Parallel Resonant Converter: For parallel resonant converter, the resonant tank is still in series. It is called parallel resonant converter because in this case the load is in parallel with the resonant capacitor. More accurately, this converter should be called series resonant converter with parallel load. Since transformer primary side is a capacitor, an inductor is added on the secondary side to math the impedance.
Fig. 3: Parallel resonant converter
Fig. 4: Series parallel resonant converter
LCC Resonant Converter: LCC resonant converter also could not be optimized for high input voltage. The reason is same as for SRC and PRC; the converter will work at switching frequency far away from resonant frequency at high input voltage. In the DC characteristic of LCC resonant converter, it can be seen that there are two resonant frequencies. One low resonant frequency determined by series resonant tank Lr and Cs. One high resonant frequency determined by Lr and equivalent capacitance of Cs and Cp in series. For a resonant converter, it is normally true that the converter could reach high efficiency at resonant frequency.
For LCC resonant converter, although it has two resonant frequencies, unfortunately, the lower resonant frequency is in ZCS region. For this application, we are not able to design the converter working at this resonant frequency. Although the lower frequency resonant frequency is not usable, the idea is how to get a resonant frequency at ZVS region. By change the LCC resonant tank to its dual resonant network, this is achievable.
Drawbacks of Lcc Resonant Converter:
It is not suitable for optimization of high input voltage.
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Middle-East J. Sci. Res., 24 (3): 877-886, 2016
The converter will work at switching frequency far away from resonant frequency at high input voltage.
Although it has two resonant frequency the lower frequency operating in ZCS condition. so that the converter working at resonant frequency is difficult.
=
( )
2 x
LLC Resonant Converter: A LLC resonant converter is a transformer coupled dc/dc converter whose output voltage is controlled by the switching frequency [29].
One of the key advantages of a LLC resonant converter is that even at light load ZVS is possible. In backward operation mode ZVS is only possible at heavy load – like a traditional phase shifter and also it has the advantages of high efficiency and also its compact size. Though it has several drawbacks like complicated synchronous rectification And Large switching frequency change when it works as bi-directional converter. A new interleaved
LLC-SRC without a complicated controller is proposed in
[30]. It is suitable for the low-voltage and high-current applications by adopting synchronous rectification at secondary side.
Among the resonant converters, the LLC resonant converter has superior performance compared to the series resonant converter (SRC), especially for buck/boost operation capability, narrow switching frequency variation range and higher efficiency [31-34]. A bidirectional LLC resonant topology for vehicular applications was proposed in [35]. The topology was still a traditional SRC during backward operation, which is not preferred for wide voltage range application. In [36], a bidirectional CLLC resonant converter with two resonant tanks in the transformer primary side and secondary side, respectively, was proposed. The extra resonant tank increased both the cost and volume of the converter and the voltage gain was reduced compared with the traditional LLC converter. Furthermore, it uses the deadband control and the output voltage cannot be regulated continuously. And the current in the output side has to flow through the body diodes of the switches which may cause high conduction loss. It is known from traditional
LLC resonant converter, that there is a boundary between the capacitive region (ZCS) and inductive region (ZVS) for the primary side MOSFETs when the switching frequency is below the resonant frequency [37]. Since the converter is always operating below the resonant frequency, the reverse energy is used to achieve voltage gain below 1 like synchronous control method. The power angle of the
LLC resonant converter is that ratio between the output side current and voltage level. It is given (1), Where x=f /f
(1)
Fig. 5: LLC resonant converter
Compared with the popular analog control circuit for traditional LLC converter, the proposed control method is more suitable for digital implementation, which may not be preferred for low power application [38][39]. Generally, bi-directional operation of LLC resonant converter is complicated and difficult to control [40]. The LLC resonant converter has been analyzed and designed, to reduce switching loss, depress electromagnetic interference and improve overall power conversion efficiency, since this converter possesses the softswitching feature as the ZVS. for primary power switches and ZCS for output Rectifiers [41].
In the voltage gain of the bidirectional LLC converter with the synchronous control method the output current is always in CCM with the synchronous control method when fs < fr . And the equivalent output current has a phase angle leading to the output voltage. Therefore, the equivalent load in the fundamental harmonic approximation analysis model is complex impedance instead of a resistor and the phase angle can be used to represent the impedance angle of the equivalent output impedance.
When the switching frequency is above or equal to the resonant frequency, the operation with synchronous control method is the same as a traditional LLC resonant converter with diode rectifier and the equivalent load can also be seen as a resistor, so is 0 when fs = fr.
G =




The voltage gain of the converter is given in (2),
1
+ − kx 2




− Q





( ) ( )(
2


 kx 3
+ − 1
)
+ Q 2
 


 cos
( )
1 −
1
( )
2
2
2
 



2 x 2
(2)
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Middle-East J. Sci. Res., 24 (3): 877-886, 2016
Where K=Lm2/Lr is the inductance ratio, Zr =Lr/Cr is the characteristic impedance, fr = 1/2vLr.Cr is the resonant frequency, fs is the switching frequency, x = fs / fr is the normalized switching frequency, Ro is the equivalent dcload resistance and Q = 2Zr /8n2Ro is the quality factor.
The synchronous control scheme is same as the traditional one when traditional one when
fs = fr
fs < fr
, but it is much lower than the
. Since ?
will increase with the decreasing of the switching frequency, the maximum voltage gain is limited due to large reverse power. The reverse energy exists for the output voltage and output current are not in phase, which means part of the energy is transferred back and forth between the output side and the input side, so the conduction loss will increase. For simplicity, the reverse energy per unit time can also be represented by the reverse power. The ratio of reverse power Pb to the output power Po is given in (3), the reverse power will also increase when ?
Increases, which means more energy is reversed from the output side in a switching period and it will result in higher conduction losses.
Furthermore, the on-time of each switch is the control variable. The duty ratio is changed during one switching period in case of transient response interval.
In this method, not only the switching frequency but also the on-time of each switches is changed in each cycle. when the input voltage fluctuation must be considered, it is confirmed that, in the both input voltage step and load step changes, the transient responses are largely improved even if the output capacitor are large.
The excessive increase of switching frequency in case of transient time is reduced to 1/2 of the conventional method.
P b
P o
= sin −
2 cos cos
(3)
To avoid excessive reverse energy the minimum voltage gain should not be too low which is given in (4).
Fig. 6: Digital controlled LLC resonant dc-dc converter
P
P b o
= tan





2
( )
2





−
1 −
4 x 2 (4)
Digital Pid Control Method: Recently the digital control is gaining more attention because of the advantages to circuit, the output voltage eo is sent to the digital P-I-D control circuit through the A-D converter. Eo is converted realize the high performance characteristics, high energy to the digital value Nn.
management function, high reliability and high flexibility control [42-45]. However, the digital control has the delay time. The conversion time and the processing time of digital controller cause a bad influence on the dynamic characteristics.
This paper presents a new quick response digital modified P-I-D control with flexible duty ratio for LLC resonant dc-dc converter in -48V DC power supply system and the transient response against input voltage step changes. The transient response in this method is largely improved especially in case of small output smoothing capacitor [46].
In this method, the sampling rate is multiple and the calculation process of P-D and I controls are parallel.
Therefore, in this method it is suitable to the digital control LLC resonant converter for DC power supply system [47]. The digital control circuit consists of an A-D converter, a P-I-D control calculator, a digital PFM generator and a timing generator. In the digital control
Fig. 7: Modified PID control method
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Components of Digital Pid Control Method basic parts.
Middle-East J. Sci. Res., 24 (3): 877-886, 2016
The Conditions for Oscillation:
Parts of an Oscillator: Most oscillators consist of three
An Amplifier: This will usually be a voltage amplifier and may be biased in class A, B or C.
Wave Shaping Network: his consists of passive components such as filter circuits that are responsible for the shape and frequency of the wave produced.
Positive Feedback Path: Part of the output signal is fed back to the amplifier input in such a way that the feedback signal is regenerated, re-amplified and fed back again to maintain a constant output signal. Commonly an oscillator is constructed from an amplifier that has part of its output signal fed back to its input. This is done in such a way as to keep the amplifier producing a signal without the need for any external signal input as shown in Fig. 3.4 It can also be thought of as a way of converting a DC supply into an AC signal.
Positive feedback must occur at a frequency where the voltage gain of the amplifier is equal to the losses
(attenuation) occurring in the feedback path. For example if 1/30th of the output signal is fed back to be in phase with the input at a particular frequency and the gain of the amplifier (without feedback) is 30 times or more, oscillation will take place.
The oscillations should take place at one particular frequency.
The amplitude of the oscillations should be constant.
There are many different oscillator designs in use, each design achieving the above criteria in different ways.
Some designs are particularly suited to producing certain wave shapes, or work best within a certain band of frequencies. Whatever design is used however, the way of achieving a signal of constant frequency and constant amplitude is by using one or more of three basic methods.
Positive Feedback: The feedback in the amplifier section of an oscillator must be POSITIVE FEEDBACK. This is the condition where a fraction of the amplifier's output signal is fed back to be in phase with the input and by adding together the feedback and input signals, the amplitude of the input signal is increased. For example, a common emitter amplifier creates a phase change of 180 between its input and output, the positive feedback loop must therefore also produce a 180 phase change in the signal fed back from output to input for positive feedback to occur. The result of a small amount of positive feedback in amplifiers is higher gain, though at the cost of increased noise and distortion. If the amount of positive feedback is large enough however, the result is oscillation, where the amplifier circuit produces its own signal.
Fig. 8: The Essential Elements of an Oscillator
A-D Converter: An analog-to-digital converter (ADC,
A/D, or A to D) is a device that converts a continuous physical quantity (usually voltage) to a digital number that represents the quantity's amplitude. Conversion involves quantization of the input, so it necessarily introduces a small amount of error. Furthermore, instead of continuously performing the conversion, An ADC involves conversion periodically, sampling the input. The result is a sequence of digital values that have been converted from a continuous-time and continuousamplitude analog signal to a discrete-time and discreteamplitude digital signal.
An ADC is defined by its bandwidth (the range of frequencies it can measure) and its signal to noise ratio
(how accurately it can measure a signal relative to the noise it introduces).The actual bandwidth of an ADC is characterized primarily by its sampling rate and to a lesser extent by how it is handles errors such as aliasing.
The dynamic range of an ADC is influenced by many factors, including the resolution (the number of output levels it can quantize a signal to), linearity and accuracy
(how well the quantization levels match the true analog signal) and jitter (small timing errors that introduce additional noise). The dynamic range of an ADC is often summarized in terms of its effective number of bits
(ENOB), the number of bits of each measure it returns that are on average not noise.
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Pulse Frequency Modulation: Pulse Frequency soft switching is achieved for all power devices even
Modulation is a modulation method for representing an analog signal using only two levels (1 and 0). It is analogous to Pulse-Width Modulation (PWM), in which the magnitude of an analog signal is encoded in the duty cycle of a square wave. Normally a signal generator is an invaluable piece of test equipment. The output from a signal generator is a repeating waveform whose characteristics are set by the user. Signal generators can under the worst case conditions, by choosing dead-time value, Cp and maximum switching frequency, properly.
The worst case happens at light- or no-load conditions when output voltage is adjusted at its minimum value and maximum input voltage is applied to the converter. Under these conditions, the resonant inductor current is minimized and Thus, to charge and discharge Cp effectively during the given value of the dead time, be used for research and development purposes, along maximum value of Cp should be limited, properly. By with the servicing and repair of electronic equipment.
In Most operation DC-DC converters are designed to inductor current and ignoring the voltage dependence of move power in only one direction, from input to output, accounting the higher order harmonics of the resonant the drain-sources parasitic capacitances, the necessary however, all switching regulator topologies can be made dead time for achieving the ZVS operation even bi-directional by replacing all diodes with independently under the worst case condition can be found, as reported controlled active rectification. A bi-directional converter can move power in either directional, which is useful in applications requiring regenerative braking.
in.
Reducing the amplitude of the input resonant current as low as possible to minimize the converter conduction losses is another important topic that must be satisfied for
Improving the Converter Performance under the Light-
Load Conditions: ZVT method auxiliary circuit is added to the conventional topology. The added auxiliary circuit consists of one inductor and two capacitors, two switches for bidirectional operation and resonant circuit. In fact, the structure of the ZVT, the auxiliary switch is turned off increasing the LLC resonant converter light-load performance.
Finally, it must be mentioned that the transient analysis and control design of the LLC resonant converter for constant output voltage applications have been discussed. But these analyses do not cover wide output hard switching [48]. The gate drive circuit must be able to sink charges of the MOSFET’s gate-source capacitors and turn them OFF before their drain-source voltages rise significantly above zero. To assist the transistor turn-off process, small capacitor may be added in parallel with the drain-sources of the power MOSFETs. It is used to analyses a novel technique for obtaining a voltage conversion ratio greater than one in a bidirectional series resonant DC/DC converter (SRC) [49].
Cp is the summation of the added capacitor and the parasitic capacitors of the drain-sources of the power
MOSFETs. Soft switching is one of the most important topics that must be satisfied for using the LLC resonant converter as a wide output range voltage source.
To achieve ZVS operation at primary side of the converter at MOSFET turn-on times, the converter should operate in the inductive region and the resonant inductor’s current must be high enough to charge or discharge Cp .
Amplitude of the circulating current should be reduced as much as possible to minimize the conduction losses of the converter; but the reduced circulating current may not be enough to satisfy the ZVS operation.
In the inductive region of the LLC resonant converter, voltage with wide dynamic loading applications and more investigations are necessary. When the output voltage is tried to be adjusted in a new desired value, the incorrect initial conditions for resonant components and the voltage gain mismatch cause high surge current in the circuit.
resonant converter, LCC resonant converter. It can be concluded that the LLC Resonant Converter has the better performance for bidirectional operation in the distribution side. Also this paper discussed about the current research status of LLC resonant converter and its benefits.
CONCLUSION
There is a large incentive to achieve high efficiency and high power density in power delivery systems. The resonant dc-dc converters shows higher efficiency and higher power density characteristics than conventional converters. This paper provides the soft switching capability, reduced reverse energy and higher efficiency performances are discussed with the
LLC resonant converter performance by comparing with the dual active bridge converter, Series and Parallel
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