This article shows the proposal of a current-mode one-cycle control for linear-assisted DC/DC converters. Linearassisted DC/DC converters are structures that allow to take advantages of the two classic alternatives in the design of power supply systems: voltage linear regulators (classic NPN topology or LDO –low dropout–) and switching DC/DC converters. The current-mode one-cycle control technique is proposed in order to obtain the duty cycle of the linear-assisted converter switch. The proposed structure can provide an output with suitable load and line regulations. Thus, the paper shows the design and simulation results of the proposed current-mode one-cycle linear-assisted converter.Postprint (published version

Martínez García, Herminio

English

This article shows the proposal of a current-mode one-cycle control for linear-assisted DC/DC converters. Linearassisted DC/DC converters are structures that allow to take advantages of the two classic alternatives in the design of power supply systems: voltage linear regulators (classic NPN topology or LDO –low dropout–) and switching DC/DC converters. The current-mode one-cycle control technique is proposed in order to obtain the duty cycle of the linear-assisted converter switch. The proposed structure can provide an output with suitable load and line regulations. Thus, the paper shows the design and simulation results of the proposed current-mode one-cycle linear-assisted converter

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Linear-assisted DC/DC converters with modified current-mode control applied to photovoltaic solar systems

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 International Conference on Renewable Energies and Power Quality (ICREPQ’14) Cordoba (Spain), 8th to 10th April, 2014 exÇxãtuÄx XÇxÜzç tÇw cÉãxÜ dâtÄ|àç ]ÉâÜÇtÄ (RE&PQJ)  ISSN 2172-038 X, No.12, April 2014     Linear–Assisted DC/DC Converters with Modified Current-Mode Control Applied to Photovoltaic Solar Systems  Herminio Martínez-García  Barcelona College of Industrial Engineering (EUETIB) Department of Electronics Engineering Technical University of Catalonia (UPC). BarcelonaTech C/ Comte d’Urgell, nº 187 08036 - Barcelona. SPAIN Phone Number: +34.93.413.72.90; Fax Number: +34.93.413.74.01; E-mail: herminio.martinez@upc.edu      Abstract. This article shows the proposal of a current-mode one-cycle control for linear-assisted DC/DC converters. Linear-assisted DC/DC converters are structures that allow to take advantages of the two classic alternatives in the design of power supply systems: voltage linear regulators (classic NPN topology or LDO –low dropout–) and switching DC/DC converters. The current-mode one-cycle control technique is proposed in order to obtain the duty cycle of the linear-assisted converter switch. The proposed structure can provide an output with suitable load and line regulations. Thus, the paper shows the design and simulation results of the proposed current-mode one-cycle linear-assisted converter.  Key words  Power Electronics; DC-DC switching power converters, linear-assisted DC-DC converters.  1. Introduction  Linear-assisted DC/DC converters (also known as linear-switching hybrid converters) are circuital structures that present an increasing interest for the implementation of power supply systems that require two demanding design specifications: (1) high slew-rate of the output current and (2) high current consumption by the output load. This it is the case of the systems based on the modern microprocessors and DSPs, where both requirements converge [1], [2].  These linear-switching hybrid converters are able to combine the well-known advantages of the two existing typical alternatives for the implementation of DC/DC voltage regulators or converters, diminishing as well their disadvantages. These two alternatives are known largely: (1) the use of voltage series linear regulators (classic standard NPN –or nMOS– topologies and LDO) have been widely used for some decades [3]-[5], and (2) the DC/DC switching converters, thanks to which high current power supply systems can be obtained [6]-[8]. Linear-assisted DC/DC converters can be implemented on printed circuits using discrete components. Nevertheless, they are also an attractive alternative susceptible to be integrated in on-chip power supply systems as a part of power management systems.  An important part of these converters is their controller. Some alternatives are present [9]-[12]. However, the nonlinear control technique known as one-cycle control ([13], [14]) is proposed in current article. One-cycle control takes advantage of the pulsed and nonlinear nature of switching converters and achieves instantaneous dynamic control of the average value of a switched variable; more specifically, it takes only one switching cycle for the average value of the switched variable to reach a new steady state after a transient. There is no steady-state error or dynamic error between the control reference and the average value of the switched variable. This technique provides fast dynamic response, excellent power source disturbance rejection, robust performance, and automatic switching error correction.  Though one-cycle control has so many advantages, it is infirmness for load disturbance. However, the inclusion in the linear-assisted converter of a voltage linear regulator provides the suitable load regulation thank to a second loop included in the linear regulator block.  2. Basic Topology of a Linear-Assisted DC/DC Converter  The basic scheme of a single output linear-assisted converter is shown in Figure 1 [11], [12]. This structure consists, mainly, of a voltage linear regulator in parallel with a step-down switching DC/DC converter. In this type of converters, the value of the output voltage, supposed constant, is fixed with good precision by the voltage linear regulator. The current flowing through this linear regulator is constantly sensed by the current sense element Rm. Based on its value, the controller activates or not the output of comparator CMP1 that, as well, leads to the switch element of the DC/DC switching converter. Therefore, notice that the current through the linear regulator constitutes a measurement of the error of the power supply system.  The power stage (that is, the switching converter) injects in the output the necessary current to force to a minimum value (not necessarily zero) the current flowing through the linear regulator. As a consequence, it is obtained, altogether, a power supply system where the switching frequency comes fixed, among other parameters (such as the possible hysteresis of the analog comparator), by the value of the current through the linear regulator. On the other hand, the output voltage value is fixed by the voltage linear regulator.  Vout Vin Ireg Iout IL VC CMP1 L1 IQ Rm  RL Voltage Linear Regulator Vref + – + – Driver DC-DC Switching Converter Current Sensing   Fig. 1. Block diagram of the proposed linear-assisted converter.  In the linear-assisted converter in Figure 2, in which the switching converter is a step-down type [11], [12], the linear regulator consists of a push-pull output stage (transistors Q2a and Q2b). In this strategy, the main objective of the DC/DC switching converter is to provide almost all the load current in steady-state conditions (to obtain a good efficiency of the whole system). Thus, in steady state, the linear regulator provides a little part of the load current, maintaining the output voltage to an acceptable constant value.  As a matter of fact, if the current demanded by the load Iout is inferior to a maximum value of current, which we will denominate switching threshold current, Iγ, the output of comparator CMP1 will be at low level, disabling the DC/DC switching converter and, thus, the current flowing through inductor L1 will be zero (Figure 3). Therefore, the voltage linear regulator supplies the load RL, providing all the output current (Ireg=Iout). However, when the current demanded by the load overpasses this current limit Iγ, automatically the output of the comparator will pass to high level, causing that the current through the inductance L1 supplies almost the total load output current.  3. One-Cycle Control Concept  One cycle control was proposed by Keyue M. Smedley in 1991 and realized initially in buck PWM converter [13]-[15]. DC/DC switching converters with one-cycle control reject input perturbations in only one switching cycle and follow the control reference instantly. It is said that one-cycle control is universal and can be applied directly to switching converters in either PWM or quasi resonant modes [15], [16]. From then on, with one-cycle control, boost converter, Cûk converter, three-phase PFC converter and three-phase boost rectifier have been presented. During this period, a general-purpose feedfodward one cycle controller was also advanced.  One-cycle control theory is shown in figure 4, and its operating waveforms are shown in figure 5.  Vout Vin Iout IL VC L1 D1 Q1 Vin CMP1 Q2a Q2b Ireg Vin VZ OA1 Rm  RLVref+ – + – R3R1R2Voltage Linear Regulator + – Current Sensing   Fig. 2. Basic structure of the proposed linear-assisted DC/DC converter.  iL(t) ireg(t) I TON TOFF Linear block enabled Switching block disabled Both linear and switching blocks enabled 0 IH IL I (A)  t Iout  Fig. 3. Principle of operation of the proposed linear-assisted DC/DC converter.  4. Topology of the Current-Mode One-Cycle Linear-Assisted DC/DC Converter  Based on the linear-assisted DC/DC converter, the structure of the proposed linear-assisted DC/DC converter with current-mode one-cycle control strategy is shown in Figure 6. In this figure we can observe the linear regulator, the switching DC/DC converter and the control loop that fixes the duty cycle of the switch element Q1.  OA1 y(t) CMP1 x(t)  – + Ci Ri vint(t) reset – + One-cycle controlled constant frequency switch Controller vREF(t) Clock vcmp(t)   Fig. 4.  Basic scheme of one-cycle control.   x(t) t y(t) t vint(t) t vREF(t) clock t vcmp(t) t   Fig. 5.  Typical waveforms of one-cycle control.  The basic idea of the proposed control is to achieve an average inductor current during the time interval TON equal to the output current. Thus, having into account that:  ( ) ( )L reg outi t i t I    , (1) the average value provided by the linear regulator block will tend to be zero. In addition, notice that the main difference with one-cycle control is that the linear regulator fixes the output voltage. In fact, the output voltage is given (and fixed) by the linear regulator. Therefore, according to before detailed, the new proposed technique can reject not only power disturbances, but load disturbances too.  The main idea of the proposed control is to achieve that the average value of the current flowing across the inductor during the time interval TON (= ( ) Sd t T ) equals to the average value of the current through the load during this interval. Therefore, considering that the output current is given by Iout=Ireg+IL, the average current flowing through the linear regulator will tend to zero.  Vout Vin Iout iL(t) VC D1 Q1 CMP1L1 ireg(t) Q2a Q2b VZ OA1 OA1 VC Rm=–1 Rm=–1 MUL1 RL +–Driver RS Flip–Flop R S Q VCLK /Q  ( )d tVoltage Linear Regulator +–– + Ci Ri vint(t) Low Pass Filter reset Current Sensing Current Sensing vmult(t) Switching DC/DC Converter vcmp(t)   Fig. 6.  Basic structure of a current-mode one-cycle linear-assisted DC/DC converter.  On the one hand, the low-pass filter extract the average value of the duty cycle, ( )d t , that multiplied by the output current Iout (considered constant), provides a measure of the average value of the current flowing through the load during the interval TON. Thus, considering the gain factor of the current sensing element Rm=1, we have:  ( ) ( )mult outv t d t I  (2) On the other hand, the analog integrator provides the measure of the average value of the current flowing across the inductor during the time interval TON, considering that the time constant is given by:  i i SR C T  (3) Therefore:  int1 1( )  ( )  ( )t tL Lo oi i Sv t i t dt i t dtR C T    (4) where the gain factor of the current sensing element Rm=–1 (the negative sign compensates the additional inversion provided by the integrator).  The average value of the switched variable at the switch output is guaranteed to be:  1  ( ) ( )SdT L outoSi t dt d t IT  (5) in each cycle. Figure 7 shows the operating waveforms of the circuit when vREF(t) is constant.   i(t) ty(t) tvint(t) tvREF(t)= ( ) ( )mult outv t d t I  clock tvcmp(t) t  Fig. 7.  Typical waveforms of the proposed linear-assisted converter with current-mode one-cycle control.  5. Simulation Results  In order to validate the presented structure of the current-mode one-cycle linear-assisted DC-DC converter in figure 6, simulation results have been obtained from a system that provides 5.0 V at the output Vout. The value of the inductor is L=100 H. Figure 8 shows the circuit the start-up transient waveforms when the current-mode one-cycle linear-assisted DC/DC converter provides 2 A at the output Vout. Notice that the one-cycle control loop assures a value of the average current flowing through the linear stage near zero. Figure 9 shows the transient response of the linear-assisted DC/DC converter to a step when the reference voltage increases from 5 V to 8 V at 500 µs.  6. Conclusions  In the presented paper, the design and simulation of a current-mode one-cycle control linear-assisted DC/DC converter has been carried out. The article has shown that linear-assisted DC/DC converters are suitable structures that allow to take advantages of the two classic alternatives in the design of power supply systems (voltage linear regulators and switching DC/DC converters). In addition, starting from this linear-assisted topology, and thanks to the general idea of one-cycle control, a current-mode one cycle linear-assisted DC/DC converter has been proposed. As a whole, the proposed DC/DC converter provides a good line regulation, thanks to the excellent power source disturbance rejection shown by the one-cycle control loop, a suitable load regulation thanks to the excellent load disturbance shown by the linear regulator and, finally, high efficiency shown by the switching converter.  Acknowledgment  This work has been partially funded by project TEC2010-15765/MIC from the Spanish Ministerio de Ciencia e Innovación (MICINN) funds.  References   [1] X. Zhou, P. L. Wong, P. Xu, F. C. Lee, A. Q. Huang. “Investigation of Candidate VRM Topologies for Future Microprocessors”. IEEE Transactions on Power Electronics, vol. 15 (nº 6): pp. 1172–1182, November 2000. [2] B. Arbetter, D. Maksimovic. “DC–DC Converter with Fast Transient Response and High Efficiency for Low–Voltage Microprocessor Loads”. IEEE Applied Power Electronics Conference, pp. 156-162. 1998. [3] C. K. Chava, J. Silva-Martínez. “A Frequency Compensation Scheme for LDO Voltage Regulators”. IEEE Transactions on Circuits and Systems–I: Regular Papers, vol. 51 (nº 6): pp: 1041-1050, June 2004. [4] R. J. Milliken, J. Silva-Martínez, E. Sánchez-Sinencio. “Full On-Chip CMOS Low-Dropout Voltage Regulator”. IEEE Transactions on Circuits and Systems–I: Regular Papers, vol. 54 (nº 9): pp: 1879-1890, September 2007. [5] V. Grupta, G. A. Rincón–Mora, P. Raha. “Analysis and Design of Monolithic, High PSR, Linear Regulator for SoC Applications”. Proceedings of the IEEE International SoC Conference: pp. 311–315, 2004. [6] R. W. Erickson, D. Maksimovic. “Fundamentals of Power Electronics”. 2nd edition, Ed. Kluwer Academic Publishers, 2001. [7] J. G. Kassakian, M. F. Schlecht, G. C. Verghese. “Principles of Power Electronics”. Ed. Addison–Wesley, 1991. [8] N. Mohan, T. M. Underland, W. P. Robbins. “Power Electronics: Converters, Applications and Design”. Ed. John Wiley & Sons, 1989. [9] P. Midya, F. H. Schlereth. ‘Dual Switched Mode Power Converter’. IECON’89. Industrial Electronics Society: pp. 155–158, 1989. [10] F. H. Schlereth, P. Midya. ‘Modified Switched Power Convertor with Zero Ripple’. Proceedings of the 32nd IEEE Midwest Symposium on Circuits and Systems (MWSCAS’90): pp. 517–520, 1990. [11] A. Conesa, H. Martínez, J. M. Huerta. “Modeling of Linear & Switching Hybrid DC–DC Converters”. 12th European Conference on Power Electronics and Applications (EPE 2007), September 2007. [12] H. Martínez, A. Conesa,. “Modeling of Linear-Assisted DC–DC Converters”. European Conference on Circuit Theory and Design 2007 (ECCTD 2007), 26th-30th August 2007. [13] K. M. Smedley, S. Cuk. “One-Cycle of Switching Converters”. Proceedings of the 22nd Power Electronics Specialists Conference. (PESC’91): pp. 888-896. 1991. [14] K. M. Smedley, S. Cuk. “One-Cycle of Switching Converters”. IEEE Transactions on Power Electronics, vol. 10 (nº 6): pp: 625-633, November 1995. [15] Z. Hu, B. Zhang, W. Deng. “Feasibility Study on One Cycle Control for PWM Switched Converters”. Proceedings of the 5th 35th Annual IEEE Power Electronics Specialisrs Conference: pp. 3359-3365, 2004. [16] Y. Wang, S. Shen. “Research on One-Cycle Control for Switching Converters”. Proceedings of the 5th World Congress on Intelligent Control and Automation: pp. 74-77, June 2004.               Time0s 50us 100u s 150us 200us 250 us 300us 350us 400usI(Rs ens) I(RL1 ) I(L1)-1.0A0A1.0A2.0A3.0A           Time0s 50us 100u s 150us 200us 2 50us 300u s 350us 400usV(V out)0V2.0V4.0V6.0V Iout IL(t) Ireg(t) Vout   Fig. 8.  Start-up transient waveforms when the current-mode one-cycle linear-assisted DC/DC converter provides 2 A at the output Vout. Notice that the one-cycle control loop assures a value of the average current flowing through the linear stage near zero.             Time0s 100us 200us 300us 400us 500us 600us 700us 800usV(Vout)0V5.0V10.0V           Time0s 100us 200us 300us 400us 500us 600us 700us 800usI(Rsens) I(RL1) I(L1)0A2.0A4.0A-1.0A   Iout IL(t) Ireg(t) Vout   Fig. 9.  Transient response of the linear-assisted DC/DC converter to a step when the reference voltage increases from 5 V to 8 V at 500 µs.  

Los convertidores DC/DC asistidos linealmente (linear-assisted DC/DC converters) son estructuras que permiten aprovechar las ventajas de las dos alternativas clásicas en el diseño de sistemas de alimentación: reguladores de tensión lineales (ya sean éstos con topología clásica NPN o del tipo LDO

–baja tensión de dropout–), y convertidores DC/DC conmutados. En este trabajo se muestra la propuesta de un control de un ciclo en modo corriente modificado (modified current-mode one-cycle control technique) para convertidores DC/DC conmutados asistidos linealmente. Se propone esta técnica de control de un ciclo en modo corriente modificado con el fin de obtener el ciclo de trabajo del interruptor del convertidor conmutado. La estructura propuesta puede proporcionar una salida con adecuadas regulaciones de carga y de línea. El  trabajo  muestra el diseño  de las propuestas del regulador DC/DC asistido linealmente, así como los resultados de simulación que validan la propuesta presentada en el artículo.Peer Reviewe

Linear–Assisted DC/DC Converters with Modified Current-Mode Control Applied to Photovoltaic Solar Systems

Los convertidores DC/DC asistidos linealmente (linear-assisted DC/DC converters) son estructuras que permiten aprovechar las ventajas de las dos alternativas clásicas en el diseño de sistemas de alimentación: reguladores de tensión lineales (ya sean éstos con topología clásica NPN o del tipo LDO
–baja tensión de dropout–), y convertidores DC/DC conmutados. En este trabajo se muestra la propuesta de un control de un ciclo en modo corriente modificado (modified current-mode one-cycle control technique) para convertidores DC/DC conmutados asistidos linealmente. Se propone esta técnica de control de un ciclo en modo corriente modificado con el fin de obtener el ciclo de trabajo del interruptor del convertidor conmutado. La estructura propuesta puede proporcionar una salida con adecuadas regulaciones de carga y de línea. El  trabajo  muestra el diseño  de las propuestas del regulador DC/DC asistido linealmente, así como los resultados de simulación que validan la propuesta presentada en el artículo.Peer Reviewe

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Linear-assisted DC/DC converters with modified current-mode control applied to photovoltaic solar systems

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