Buck-boost-pwm-converters Having Two Independently Controlled Switches

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<ul><li><p>7/28/2019 Buck-boost-pwm-converters Having Two Independently Controlled Switches</p><p> 1/6</p><p>1Buck-Boost PWM Converters Having</p><p>Two Independently Controlled Switches</p><p>-LQJTXDQ&amp;KHQ'UDJDQ0DNVLPRYLDQG5REHUW(ULFNVRQ</p><p>Colorado Power Electronics Center</p><p>Department of Electrical and Computer EngineeringUniversity of Colorado at Boulder</p><p>Boulder, CO 80309-0425, USA</p><p>1 This work is supported by Philips Research, Briarcliff Manor, NY, through Colorado Power Electronics Center</p><p>Abstract Single-switch step-up/step-down converters, such</p><p>as the buck-boost, SEPIC and Cuk, have relatively high</p><p>voltage and current stresses on components compared to the</p><p>buck or the boost converter. A buck-boost converter with</p><p>two independently controlled switches can work as a boost or</p><p>as a buck converter depending on input-output conditions,</p><p>and thus achieves lower stresses on components. Using the</p><p>converter synthesis method from [1], families of two-switch</p><p>buck-boost converters are generated, including several newconverter topologies. The two-switch buck-boost converters</p><p>are evaluated and compared in terms of component stresses</p><p>in universal-input power-factor-corrector applications.</p><p>Among them, one new two-switch converter is identified that</p><p>has low inductor conduction losses (50% of the boost</p><p>converter), low inductor volt-seconds (72% of the boost</p><p>converter), and about the same switch conduction losses and</p><p>voltage stresses as the boost converter.</p><p>I. INTRODUCTION</p><p>Dc-dc converters with step-up/step-down characteristic are</p><p>required in all applications where the input and the output</p><p>voltage ranges overlap. For example, in power factorcorrection (PFC) applications, the use of step-up/step-down</p><p>converters such as the buck-boost, SEPIC or Cuk, allows one</p><p>to set the output dc voltage to an arbitrary intermediate value.</p><p>For one given dc operating point, it is well known that the</p><p>buck (if the input is greater than the output), or the boost</p><p>converter (if the input is lower than the output) perform</p><p>conversion with lower component stresses and energy storage</p><p>requirements than the single-switch step-up/step-down</p><p>converters.</p><p>Paralleling [2] and multilevel techniques [3][4] can be used</p><p>to share current or voltage stresses at the expense of more</p><p>switching components. However, neither of these approaches</p><p>aims at reducing the current and voltage stresses at the sametime. In this paper we show how converters with buck-boost</p><p>characteristic can be constructed using two active switches to</p><p>achieve low component stresses, low energy storage</p><p>requirements, and therefore size and efficiency performance</p><p>comparable to the performance of the simple buck or boost</p><p>converters.</p><p>In Section II, we begin with an introduction of how the</p><p>power transfer mechanisms in switching converters affect the</p><p>component stresses. The converter synthesis method</p><p>described in [1] is adopted to derive all possible two-switch</p><p>buck-boost topologies that are capable of achieving minimum</p><p>indirect power. The synthesis method is briefly reviewed in</p><p>Section III. Families of two-switch buck-boost converters are</p><p>presented in Section IV. Selected topologies are compared</p><p>against the boost converter and the buck-boost converter inSection V, and new converters that outperform previously</p><p>known topologies are highlighted.</p><p>II. POWER TRANSFER MECHANISMS IN</p><p>SWITCHING CONVERTERS</p><p>In the boost and buck converters, there are two</p><p>mechanisms that cause transfer of power from the converter</p><p>input to the load, and hence the dc output power P is</p><p>composed of two components [5]. A part of the power,</p><p>Pindirect, is processed by the switching devices using the</p><p>(a)</p><p>D (Q)</p><p>Energy Storage</p><p>ElementsQ D</p><p>Boost (buck)</p><p>Converter</p><p>Pdirect</p><p>Pindirect</p><p>Pindirect</p><p>P P</p><p>Input Load</p><p>(b)</p><p>Energy Storage</p><p>Elements</p><p>Pindirect</p><p>Pindirect</p><p>P P</p><p>Single-switch Buck-</p><p>Boost Converter</p><p>Input LoadDQ</p><p>Fig. 1 Energy flow chart (a) boost and buck converter; (b) single-switch</p><p>buck-boost converter.</p></li><li><p>7/28/2019 Buck-boost-pwm-converters Having Two Independently Controlled Switches</p><p> 2/6</p><p>inductor for intermediate energy storage. The remainder of</p><p>the input power, Pdirect, flows directly to the output, bypassing</p><p>the intermediate process. Fig. 1(a) illustrates the energy flow</p><p>process in the boost and buck converters. The ability of</p><p>providing direct energy path leads to lower componentstresses, lower energy storage and higher efficiency. In</p><p>single-switch step-up/step-down converters, such as the buck-</p><p>boost, SEPIC and Cuk, the direct power is equal to zero. All</p><p>of the input power is processed by the switching devices, as</p><p>illustrated in Fig. 1(b). As a result, component stresses and</p><p>energy storage requirements are higher. Figure 2(a) illustrates</p><p>the relative indirect power Pindirect/P for the dc-to-dc buck,</p><p>boost and single-switch step-up/step-down converters, as a</p><p>function ofVin/Vo.</p><p>In universal-input (85Vrms-264Vrms) power-factor-</p><p>correction (PFC) applications, the boost converter is usually</p><p>preferred because of its simplicity, relatively low component</p><p>stresses and relatively high efficiency. However, an output</p><p>voltage higher than the peak input voltage must be chosen to</p><p>satisfy the functional limit of the boost converter. Single-</p><p>switch step-up/step-down converters can be used in</p><p>applications that require an intermediate output voltage level,</p><p>but since the direct power is equal to zero, component</p><p>stresses and energy storage requirements are higher. For a</p><p>converter in the PFC application, the theoretical minimum</p><p>indirect power is shown in Fig. 2(b) as a function of the</p><p>Vm/Vo, where Vm is the peak ac line input voltage and Vo is the</p><p>output dc voltage. From the discussion above, it follows that</p><p>voltage and current stresses can be reduced provided that</p><p>there is a direct path for energy delivery. It istherefore of practical interest to find buck-boost</p><p>configurations that process minimum indirect power and have</p><p>reduced component stresses.</p><p>Two simple examples of cascade connection of the buck</p><p>and the boost converter in Fig. 3 have the ability to provide</p><p>direct energy path and have a widely adjustable output</p><p>voltage. In both cases, if the transistors are driven by the</p><p>same control signal, there is no direct energy path. To</p><p>approach the minimum indirect power process, the transistors</p><p>must be independently and optimally controlled. When the</p><p>instantaneous input voltage is less than the dc output voltage,</p><p>the transistor of the boost converter operates with PWM,</p><p>while the transistor of the buck converter is always on. Whenthe instantaneous input voltage is greater that the dc output</p><p>voltage, the buck converter is PWM controlled and the boost</p><p>transistor is always off. This can lead to a converter system</p><p>with capability of intermediate output voltage and with the</p><p>theoretically minimum indirect power characteristic shown in</p><p>Fig. 2(b).</p><p>Although the circuits of Fig. 3 can approach the theoretical</p><p>lower limit of indirect power and have lower semiconductor</p><p>voltage stresses, the converter in Fig. 3(b) exhibits increased</p><p>(a)</p><p>Ro</p><p>L2</p><p>C2Vg</p><p>D1</p><p>D2Q1</p><p>Q2</p><p>C1</p><p>L1</p><p>(b)</p><p>Ro</p><p>L</p><p>CVg D2</p><p>D1</p><p>Q2</p><p>Q1</p><p>Fig. 3 Cascaded two-switch buck-boost topologies: (a) boost-buck-</p><p>cascaded, (b) buck-boost-cascaded.</p><p>(a)</p><p>0</p><p>0.2</p><p>0.4</p><p>0.6</p><p>0.8</p><p>1</p><p>1.2</p><p>0 0.5 1 1.5 2 2.5 3</p><p>Vin/Vo</p><p>P indirect/P</p><p>Boost</p><p>Buck</p><p>single-switch buck-boost, flyback, Cuk or SEPIC</p><p>(b)</p><p>0</p><p>0.2</p><p>0.4</p><p>0.6</p><p>0.8</p><p>1</p><p>0 0.5 1 1.5 2 2.5 3</p><p>Vm/Vo</p><p>P indirect/P</p><p>Fig. 2 (a) Relative indirect power for dc-to-dc converters; (b) minimum</p><p>relative indirect power for low harmonic rectifiers.</p><p>(a)</p><p>S11 S12</p><p>S12</p><p>S11</p><p>(b)</p><p>S21 S22</p><p>S21</p><p>S22</p><p>Fig. 4 Ac (left) and dc circuits (a) boost cell, (b) buck cell</p></li><li><p>7/28/2019 Buck-boost-pwm-converters Having Two Independently Controlled Switches</p><p> 3/6</p></li><li><p>7/28/2019 Buck-boost-pwm-converters Having Two Independently Controlled Switches</p><p> 4/6</p><p>conduction loss at low ac line input because of the additional</p><p>conduction loss of the buck transistor. It is desired to find out</p><p>other two-switch buck-boost converter topologies and</p><p>compare their performances in terms of component</p><p>conduction losses and stresses.</p><p>III. SYNTHESIS OF PWM DC-TO-DC</p><p>CONVERTERS</p><p>The synthesis method introduced in [1] is based on the</p><p>equivalent circuits of a PWM dc-to-dc converter at ac and dc,</p><p>which are, in the limit, valid for switching frequency</p><p>components and dc components, respectively. In ac</p><p>equivalent circuits, the voltage sources and filter capacitors</p><p>are shorted, while the current sources and filter inductors are</p><p>removed. In dc equivalent circuits, the filter capacitors are</p><p>removed and the filter inductors are shorted. Therefore, only</p><p>the switches remain in both equivalent circuits of a PWM dc-</p><p>to-dc converter. For example, Fig. 4 represents the ac and dc</p><p>circuits of simple boost and buck converters. Compared to</p><p>earlier synthesis methods [6][7], instead of dealing with thelarge number of possible connections of switches, reactive</p><p>elements, supplies and loads, this method considers possible</p><p>ac and dc circuits having only switch elements. Furthermore,</p><p>there are formulation rules of ac circuits and topological</p><p>connections between ac and dc circuits that can quickly</p><p>narrow down the scope. Also, a method of inserting the</p><p>minimum number of inductors and capacitors to realize</p><p>complete PWM converters from respective ac and dc circuits</p><p>is described in [1].</p><p>IV. DERIVATION OF TWO-SWITCH BUCK-BOOST</p><p>TOPOLOGIES</p><p>The two-switch converters investigated in this paper can</p><p>work functionally as either a boost or a buck converter</p><p>depending on the input/output conditions. Such converters</p><p>can therefore be considered connections of the buck and the</p><p>boost converter. For example, the converters in Fig. 3 are</p><p>cascade connections of the buck and the boost converter.</p><p>Their equivalent dc circuits, shown in Fig. 6(a),(b)</p><p>respectively, are cascade connections of those of the buck and</p><p>the boost cells of Fig. 4. Their ac circuits, shown in Fig. 5(a),</p><p>are those of the boost and the buck cells connected at a single</p><p>node.</p><p>Following the considerations above, new buck-boost</p><p>converters that meet the minimum indirect power objectivecan be found by identifying other possible connections of the</p><p>boost and the buck cells, together with the appropriate control</p><p>schemes. In addition to the cascade connections, we have</p><p>found that interleaved and superimposed connections lead to</p><p>several new two-switch buck-boost converters. Cascaded,</p><p>interleaved and superimposed classes of two-switch buck-</p><p>boost converters are summarized in this section. Their ac and</p><p>dc circuits are presented in Fig. 5 and Fig. 6 respectively.</p><p>A. Cascaded connectionIn addition to the converters shown in Fig. 3, there are two</p><p>other configurations shown in Fig. 7 and 8 respectively,</p><p>having the same equivalent ac and dc circuits, and two</p><p>inductors. For converters of these two families, the following</p><p>control sequence is applied to achieve the minimum indirect</p><p>power delivery: (1) when the input voltage is smaller than the</p><p>output voltage, PWM control applies to the boost cell, while</p><p>the transistor (S21) of the buck cell is always on. The</p><p>converter works as a boost converter; (2) when the input</p><p>voltage is greater than the output voltage, PWM control</p><p>applies to the buck cell, while the diode (S12) of the boost cell</p><p>is always on. The converter works as a buck converter. All</p><p>these converters share the same overall conversion ratio:</p><p>1</p><p>21</p><p>d</p><p>dM</p><p>= (1)</p><p>where d1 and d2 are the duty ratio of the boost and the buckcell, respectively.</p><p>B. Interleaved connectionTwo families of topologies are derived from interleaved</p><p>connection. In the dc circuits of this class, shown in Fig.</p><p>6(c),(d), the buck (boost) cell is separated from the boost</p><p>(buck) cell, and would regain its functionality provided that</p><p>one of the boost (buck) switches is closed. The interleaved</p><p>topologies have the same ac circuits as the cascaded</p><p>topologies and thus have the same control sequence applied</p><p>to achieve the minimum indirect power. The family of</p><p>converters in which the boost cell is separated is named Boost</p><p>Interleaved Buck-Boost converter (BoIBB), and has the</p><p>following overall conversion ratio:</p><p>1</p><p>12</p><p>1 d</p><p>ddM</p><p>+= (2)</p><p>There is only one BoIBB with two inductors. The</p><p>converter is shown in Fig. 9.</p><p>The family of converters where the buck cell is separated is</p><p>named BuIBB, and has following overall conversion ratio:</p><p>)1</p><p>(</p><p>1</p><p>1</p><p>12</p><p>d</p><p>dd</p><p>M</p><p>+</p><p>= (3)</p><p>There is only one BuIBB with two inductors. It is shown in</p><p>Fig. 10.</p><p>C. Superimposed connectionFig. 5(b) shows another possible ac circuit and the control</p><p>sequence that meet the requirement of minimum indirect</p><p>power delivery. In each subinterval, there is one and only one</p><p>switch conducting. The duty ratios obey:</p><p>122211211 =+++ dddd (4)</p></li><li><p>7/28/2019 Buck-boost-pwm-converters Having Two Independently Controlled Switches</p><p> 5/6</p><p>where d11, d12, d21, and d22 are the duty ratio of S11, S12, S21and S22 switch, respectively. Fig. 6(e) is the equivalent dc</p><p>circuit that can be identified as superimposed connection of</p><p>the buck and the boost cells. Notice that in both ac and dc</p><p>circuits, S12 and S21 are in parallel. One of these switches is</p><p>redundant. Fig. 5(c) and Fig. 6(f) show the ac and dc circuits</p><p>obtained by removing this redundant switch. The control</p><p>scheme then becomes:</p><p>1321 =++ ddd (5)</p><p>with d1 to d3 representing duty ratio of the switches S1 to S3 in</p><p>Fig. 5(c) and Fig. 6(f). The switch S2 is playing the role ofS12when the boost cell is active, and S21 when PWM control is</p><p>applied to the buck cell. The overall conversion ratio is:</p><p>32</p><p>21</p><p>dd</p><p>ddM</p><p>+</p><p>+= (6)</p><p>The results from the realization procedure are two two-</p><p>switch converters with two inductors shown in Fig. 11. A</p><p>voltage-bidirectional switch is needed to realize S2. The</p><p>converter with continuous output current is named BuSBB,</p><p>while the converter with continuous input current is named</p><p>BoSBB.</p><p>V. PERFORMANCE COMPARISONS IN PFC</p><p>APPLICATIONS</p><p>In this section, the performance of two-switch buck-boost</p><p>converters as universal-input power-factor-correctors will be</p><p>evaluated and compared to performance of the boost and the</p><p>single-switch buck-boost converters in terms of component</p><p>stresses, conduction losses and size of magnetics. It is</p><p>assumed that converters are operating in continous</p><p>conduction mode (CCM).</p><p>A. InductorsTwo items are considered here: (1) volt-seconds applied</p><p>during a switching period and (2) rms current. These are the</p><p>main factors that determine the inductor size.</p><p>An inductor in a two-switch buck-boost converter can play</p><p>one of three possible roles: (1) as the input inductor of the</p><p>boost cell, (2) as the output inductor of the buck cell, (3) asan inactive low-frequency filter. Table I shows the</p><p>functionality that the inductors take in two parts of the ac line</p><p>input: boost mode, when the input voltage is lower than the</p><p>output, and buck mode, when the input voltage is higher than</p><p>the output. It is interesting to note that L1 in all topologies</p><p>works as the boost inductor in the boost mode, and L2 as a</p><p>buck inductor in the buck mode. Their roles in the other</p><p>mode are quite different. For those converters where L1 and</p><p>L2 always have the same functionality, the inductors can be</p><p>coupled on the same...</p></li></ul>


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