I. INTRODUCTION
Buck converters have been widely used in the industry, especially in low-voltage and high-current applications. With the development of power electronics technology, it is imperative to demand small-sized, lightweight, and high-reliability qualities and power density for the converters. To achieve these, high-switching frequency is used to the converters. However, the increase of switching losses results in an increase of switching frequency, if converters operate under hard-switching conditions, and consequently, adversely affects the efficiency of the overall circuits. Then, soft-switching techniques are applied to the converters, which will considerably decrease switching losses, improve efficiency, and enhance stability. In addition, soft switching can reduce electromagnetic interference and the size of heat sinks.
In recent years, to achieve soft switching, many researchers have proposed a great amount of methods. Zero-voltage switching (ZVS) and zero-current switching (ZCS) are the most popular methods of soft switching, which can be realized by quasi-resonant circuits [1]-[11]. While some auxiliary components are normally added to the converter to obtain quasi-resonant circuits, such as switches, diodes, inductors, capacitors and so on. In [4], [5], the loss of main switch is decreased by the quasi-resonant circuits, but some additional elements work under hard-switching conditions, which generate a large amount of power losses. Therefore, it is not obvious that the total efficiency of the converters has improved. High-peak voltage or current of the main power switches and the diodes also happened [6]-[8]. Consequently, higher ranks of devices must be adopted for the converters, and additional power losses will also be generated. In any case, the control algorithm is more complicated than that of conventional pulse width modulation converters because of the auxiliary switches being added to the converters.
Coupled inductor also has been applied to the conventional converters in the early researches to realize soft switching that can obtain high efficiency [12]-[18]. In [13], even though the efficiency of the proposed converter can be improved under heavy-load conditions, it is worse than that of the conventional ones under light-load conditions because the auxiliary circuits generate a large number of additional conduction losses at light load. To make the main switch achieve ZCS condition, it is demanding the converter to operate under discontinuous conduction mode in [15]. When the current of the main small inductor is discontinuous, the coupled inductor can supply power to the loads. However, the additional diode and the copper losses of the coupled inductor itself have an adverse effect on the total efficiency. Although the topology of the buck converter in [17] is very simple, the switching frequency is variable, which makes the control method much more complex.
In this paper, a revolutionary control method is proposed to achieve soft switching, based on the extended topology in [17]. The topology of proposed ZVS-ZCS buck converter is shown in Fig. 1. As is evident from the figure, the filter inductor of the conventional converter is replaced by a coupled inductor. The main power switch can work under ZVS conditions at turn on and turn off. The freewheeling diode can also operate under ZCS conditions at turn on and turn off, i.e., soft switching of the proposed converter can be achieved. Moreover, there are not any auxiliary components or quasi-resonant circuit branches, which often generate additional power dissipations. Hence, the control method is very facile, similar to that of the conventional converter. A 200-W prototype is also built to verify the theory of the proposed converter.
Fig. 1.The topology of proposed ZVS-ZCS buck converter.
The rest of this paper is organized as follows: Section II takes a brief description of the proposed converter, then the key waveforms and the equivalent circuits of each mode are presented. In Section III, the requirements of achieving soft switching and the specific parameter design of proposed converter are given. Section IV presents some simulation results and the detailed power dissipations. In Section V, the experimental results are obtained to illustrate the proposed converter. Finally, some conclusions are included in Section VI.
II. CONVERTER DESCRIPTION AND OPERATING PRINCIPLES
A. Description of the Converters
The topology of proposed ZVS-ZCS buck converter is shown in Fig. 1. S1 is the power MOSFET, and DS is an anti-parallel diode that integrates in the power MOSFET. D1 is the freewheeling diode, C1 is the filter capacitor, and Cr is the parasitic capacitor. L1 and L2 are tightly coupled on the same ferrite core that constitutes a coupled inductor. The coupled inductor L1 is so small that its current can be bidirectional. Because of the resonance between the parasitic capacitor Cr and the coupled inductor L1, switch S1 can be turned on and off under ZVS conditions. The coupled inductor L2 creates ZCS conditions for the freewheeling diode D1 that is turned on and off.
B. Operation Principles and Analysis
The operation processes of switching circuits are repeated by the switching period, and any particular time during the switching period can be chosen as a starting point to analyze them. The analysis processes can be simplified by selecting the appropriate starting point. This paper chose a starting point at switch S1 turned-off moment. The key ideal waveforms of the proposed ZVS-ZCS buck converter are shown in Fig. 2. The operation of the proposed converter in one switching period can be divided into six, and the equivalent circuits of each stage are presented in Fig. 3. The detailed analyses of each mode are described as follows:
Fig. 2.Key ideal waveforms of the proposed ZVS-ZCS buck converter.
Fig. 3.Proposed ZVS-ZCS buck equivalent circuits of each operation mode. (a) Mode 1, t0-t1. (b) Mode 2, t1-t2. (c) Mode 3, t2-t4. (d) Mode 4, t4-t5. (e) Mode 5, t5-t6. (f) Mode 6, t6-t0.
1) Mode 1 [t0-t1, Fig. 3(a)]: The freewheeling diode D1 turns on in this interval. Before t0, the switch S1 is turned on, and uCr is equal to zero. The freewheeling diode D1 is turned off, and i2 is also equal to zero. At t0, switch S1 turns off under a ZVS condition, and the current of the coupled inductor L1 reaches maximum value, i.e., iS = i1 = I1max. At the same time, the freewheeling diode D1 turns on automatically under a ZCS condition. After t0, the coupled inductor L1 discharges, and i1 starts decreasing from I1max, while the coupled inductor L2, parasitic capacitor Cr charge, i2, and uCr increase from zero. At t1, iS drops to zero, and i1 and i2 are equal to It1.The voltage across Cr reaches steady value, and the charging is completed.
According to Magnetism Chain Conservation Theorem, this process can have
According to Magnetism Chain Conservation Theorem, this process can have
Simplifying (1), i1 and i2 at t1 can be obtained as follows:
2) Mode 2 [t1-t2, Fig. 3(b)]: The coupled inductor L1 and L2 discharge in this interval. After t1, i1 and i2 are equal, and decrease linearly. The voltage across Cr remains steady value, and iS is equal to zero.
The conduction voltage drop uD1 of the freewheeling diode D1 is ignored, and based on KCL and KVL, we can obtain
The voltage equations of the coupled inductor L1 and L2 can be described as follows:
By substituting (4) into (3), the slopes of i1 and i2 are derived as follows:
The slope of the filter inductor current of the conventional buck converter in this interval is equal to
Obviously, we can obtain
Consequently, it demonstrates that the discharging of the proposed buck converter is slower than that of conventional buck converter in this mode, and it contributes to decrease the ripple of output voltage Vo.
Combining (3) and (4), the voltage across coupled inductor L1, L2, and parasitic capacitor Cr can be written as follows:
3) Mode 3 [t2-t4, Fig. 3(c)]: The resonance between the parasitic capacitor Cr and coupled inductor L1 occurs in this interval. At t2, i1 and i2 are equal to zero. It provides a necessary condition for the freewheeling diode D1 turned off under a ZCS condition. After t2, the parasitic capacitor Cr discharges through coupled inductor L1, and i1 changes its direction and is equal to iS. At t3, i1 reaches negative maximum value. Then, i1 starts to decline negatively until the voltage uCr drops to zero at t4.
Let us make an assumption that the output capacitor C1 is large enough, or the output voltage Vo is constant. Based on KCL and KVL, we can obtain
The current equation of coupled inductor L1 and the voltage equation of parasitic capacitor Cr can be expressed as follows:
Combining (9) and (10), the following resonant equation can be written as follows:
The initial conditions of the resonant circuit at t2 are i1 = 0, and . Some assumptions are made in this interval as follows:
According to the abovementioned equations in this mode, i1, iS, and uCr are derived as follows:
where the constraint condition is t2 ≤ t ≤ t4.
4) Mode 4 [t4-t5, Fig. 3(d)]: The anti-parallel diode DS is turned on in this interval. At t4, the discharging of parasitic capacitor Cr is completed, and uCr is equal to zero. After t4, the anti-parallel diode DS turns on. As a result, it makes uCr stay at zero. Meanwhile, i1 is negative and is equal to iS, which declines linearly. The conduction voltage drop of the anti-parallel diode DS can be neglected, and based on KVL equation, the voltage uL1 across coupled inductor L1 is given by
Then, the slope of i1 can be obtained as follows:
At the same time, the voltage uL2 across coupled inductor L2 can be derived as follows:
According to KVL, the voltage uD1 of the freewheeling diode can be written as follows:
5) Mode 5 [t5-t6, Fig. 3(e)]: The switch S1 is turned on, and the current i1 is negative in this interval. Before t5, the current i1 flows through anti-parallel diode DS, and the voltage uCr of the parasitic capacitor Cr is equal to zero. Therefore, a ZVS condition of the switch S1 turned on can be obtained at t5. After t5, it is the same as Mode 4, except that i1 flows through switch S1. The current i1 decreases negatively with the slope(Vin-Vo)/L1 until it reaches zero at t6.
6) Mode 6 [t6-t0, Fig. 3(f)]: The switch S1 is turned on, and the current i1 is positive in this interval. At t6, the current i1 changes its direction. After t6, this mode is the same as Mode 4 and Mode 5, except that the current i1 is positive. Then, i1 increases linearly with the slope(Vin-Vo)/L1 until switch S1 turns off at t0. At the end of this mode, the next operating cycle begins.
III. SOFT SWITCHING ANALYSIS AND DESIGN PARAMETERS
A. Analysis of the Soft Switching
The proposed buck converter can easily achieve ZCS conditions of the freewheeling diode D1, as long as the coupled inductor L1 is so small that it can reduce the current i1 to zero and become negative, i.e., the coupled inductor L1 works under a discontinuous conduction mode (DCM). Then, some assumptions are made as follows:
where R is load resistor, TS is switching period, N is turn ratio, and the dimensionless parameter K is a measure of the tendency of a converter to operate in the DCM.
Therefore, the following formula must be satisfied to make the proposed converter operate under DCM
where D is the duty cycle.
However, to obtain ZVS conditions of the switch S1, the switch S1 must be turned on between t4 and t6, as shown in Fig. 4.
Fig. 4.ZVS condition analyses particularly.
According to the voltage across parasitic capacitor Cr in equation (15), t4 can be obtained as follows:
Then, we can obtain
where It4 is the current iS at t4.
Consequently, ΔT that is between t4 and t6 can be derived as follows:
B. Design of the Proposed Circuits
As presented in formula (22), the coupled inductor L1 must be chosen a small one to satisfy it. However, to achieve ZVS better, it demands ΔT as long as possible.
The voltage conversion ratio MD is the ratio of the output to the input voltage of the converter, and can be obtained under DCM as follows:
Thus, the equation (25) can be simplified as follows:
As we can see in equation (27), the ΔT is closely related to ω0, i.e., the coupled inductor L1 and resonant capacitor Cr. Therefore, when the design of the coupled inductor L1 is completed, and the voltage conversion ratio MD and turn ratio N are constant, it can be chosen a large resonant capacitor Cr to increase ΔT. But at the same time, the current i1 at t3 is -U0/Z0, which also increases. As a matter of fact, we expect to decrease the value of i1 at t3. Hence, the volume of Cr must be appropriate. The specific values and others parameter values are shown in Table I.
TABLE IRELATED SPECIFICATIONS OF THE PROPOSED CONVERTER
IV. SIMULATION ANALYSIS
A. Soft Switching Waveforms of Simulation
To illustrate the operation of the proposed ZVS-ZCS buck converter, it has been accomplished through a simulation with Multisim software. Using the parameters in Table I, the soft-switching waveforms are obtained, in which the turn ratio N is equal to 1, as shown in Fig. 5.
Fig. 5.The soft switching waveforms of simulation. (a) ZVS conditions of switch S1 (magnification: voltage is 0.5 and current is 1). (b) ZCS conditions of freewheeling diode D1 (magnification: voltage is 0.2 and current is 1). (c) The voltage and current of coupled inductor L1 (magnification: voltage is 0.833 and current is 1).
The resonant circuit, which consists of the coupled inductor L1 and parasitic capacitor Cr, provides a necessary condition for switch S1 turned on under a ZVS condition. Furthermore, the ZVS condition of switch S1 turned off is obtained by the parasitic capacitor Cr, as shown in Fig. 5(a). Since the proposed converter works under DCM, the current i2 is equal to zero at the freewheeling diode D1 both turned on and turned off. Therefore, the ZCS conditions of the freewheeling diode D1 that is turned on and turned off are achieved, as shown in Fig. 5(b). The voltage uL1 and current i1 of coupled inductor L1 are supplied in Fig. 5(c).
B. Analysis of Power Losses
The power losses of the proposed ZVS-ZCS buck converter can be divided into three segments, i.e., switch losses, diode losses, and others. When the turn ratio N is equal to 1, power losses at different output power are shown in Fig. 6. As we can see in the figure, the main factors that affect the total efficiency of the proposed converter are the switch and diode losses.
Fig. 6.Power losses of the proposed converter.
As shown in Fig. 5(a), the switch losses are closely related to the voltage uCr, i.e., the switch losses will decrease as the voltage uCr declines and when the switch S1 turns off. The voltage of parasitic capacitor Cr in equation (8) can be simplified as follows:
The normalized parameter uCr_N is
Similarly, Fig. 5(b) shows that the diode losses will reduce as the current i2 declines at freewheeling diode D1 turned on moment. The maximum value I1max of the current i1 at t0 can be derived under hard-switching conditions as follows:
Therefore, the current i2 of coupled inductor L2 at t1 in equation (2) can be simplified as follows:
Normalized Current It1_N is defined as
The contours of uCr_N and It1_N are shown in Fig. 7(a) and (b), respectively, in which K is constant value. The x axis represents the duty cycle D, and the y axis represents the turn ratio N. As shown in Fig. 7, the increase of duty cycle D results in a decrease of uCr_N. However, It1_N increases first, then decreases, while uCr_N and It1_N will both decrease as the turn ratio N increases. In Fig. 8, the switch losses and freewheeling diode losses are presented with different turn ratio N at diverse output power, respectively. Both the switch losses and freewheeling diode losses can be decreased by increasing the turn ratio N. Hence, the total power losses can be decreased by increasing the turn ratio N, i.e., the overall efficiency of the proposed converter can be improved this way.
Fig. 7.Contour graphs of the proposed converter. (a) uCr_N contour graph. (b) It1_N contour graph.
Fig. 8.Switch and freewheeling diode losses of the proposed converter. (a) Switch losses. (b) Freewheeling diode losses.
C. Evaluation of Output Voltage
The ripple of output voltage Vo, an important index to evaluate the performance of proposed buck converter, is affected by the slopes of i1 and i2 when the freewheeling diode D1 turns on. The equation (5) can be simplified as follows:
In any case, the value of coupled inductor L1 is so small that the current i1 can be negative. Hence, the ripple of output voltage also closely associates with the current i1. At t3, the negative maximum of the current i1 is
According to the couple of equations above, when the coupled inductor L1, parasitic capacitor Cr, and output voltage Vo are constant, |k| and |It3| will both decrease as turn ratio N increases. That is to say, the ripple of Vo can be decreased by increasing turn ratio N. In Table II, the ripples of Vo are presented at different turn ratio N by simulation.
TABLE IIRIPPLE OF OUTPUT VOLTAGE AT DIFFERENT TURN RATIO
V. EXPERIMENTAL RESULTS
To verify the theoretical and simulated results of the proposed ZVS-ZCS buck converter, a 200-W and 50-kHz prototype has been built in the laboratory. The photograph of the proposed converter prototype is shown in Fig. 9. The used parameter values are the same as those specified in the simulation, and the semiconductors used are
Fig. 9.The prototype of the proposed converter.
- Switch S1: MOSFET IRL2910S
- Freewheeling diode D1: MBR30200PT.
A. Soft Switching Waveforms of Experiment
The experimental soft-switching waveforms of the proposed ZVS-ZCS buck converter at medium load, light load and full load are shown in Fig. 10, 11, and 12, respectively. It is presented that the ZVS operations of the switch S1 at turned on and off moment are achieved in Fig. 10(a). The coupled inductor L1 and parasitic capacitor Cr constitute a resonant circuit that provides a ZVS-turned on condition for the switch S1. The parasitic capacitor Cr is parallel with the switch S1, and makes a necessary condition for the switch S1 turned off under a ZVS condition. In Fig. 10(b), the ZCS conditions of the freewheeling diode D1 turned on and off also happen. Because of the proposed converter working under DCM, the current i2 dropped to zero before the freewheeling diode D1 turned off. Furthermore, the current i2 keeps at zero until the freewheeling diode D1 turns on. The experimental voltage uL1 and current i1 of coupled inductor L1 are shown in Fig. 10(c). In comparison to Fig. 11 and 12, the proposed buck converter can successfully achieve ZVS and ZCS conditions, as well.
Fig. 10.Experimental waveforms at medium load. (a) ZVS conditions of switch S1 (current iS: 5A/div. and voltage uCr: 10V/div.). (b) ZCS conditions of freewheeling diode D1 (current i2: 5A/div. and voltage uD1: 20V/div.). (c) The voltage and current of coupled inductor L1 (current i1: 5A/div. and voltage uL1 : 10V/div.).
Fig. 11.Experimental waveforms at light load. (a) ZVS conditions of switch S1 (current iS: 5A/div. and voltage uCr: 20V/div.). (b) ZCS conditions of freewheeling diode D1 (current i2: 2A/div. and voltage uD1: 20V/div.). (c) The voltage and current of coupled inductor L1 (current i1: 5A/div. and voltage uL1 : 20V/div.).
Fig. 12.The experimental waveforms at full load. (a) ZVS conditions of switch S1 (current iS: 10A/div. and voltage uCr: 20V/div.). (b) ZCS conditions of freewheeling diode D1 (current i2: 10A/div. and voltage uD1: 20V/div.). (c) The voltage and current of coupled inductor L1 (current i1: 10A/div. and voltage uL1 : 10V/div.).
B. Efficiency
The efficiency curves of the buck converters are shown in Fig. 13. As we can observe in the figure, the overall efficiency values of the proposed buck converter are relatively high with respect to those of the rest buck converters. Moreover, the efficiency reaches 97.3% at full load. The figure also shows that even at light load (about 10% of the full power) the measured efficiency is as high as 91%.
Fig. 13.Measured efficiency.
VI. CONCLUSION
In this paper, a simple structure of ZVS-ZCS buck converter with coupled inductor has been proposed. Both the switch working under ZVS conditions and freewheeling diode working under ZCS conditions at turned on and off are achieved. The detailed theoretical analyses of the operating principle at steady state have also been provided. The main factors of power losses are discussed. The prototype of the proposed buck converter was built, and the simulation and experimental results confirm the related theoretical analyses. Since no additional component is added in this topology, the proposed converter presents a simple structure and also enjoys a very simple control method, as well as that of the conventional buck converter.
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