Theoretical Study and Implementation of a Fast ... - IEEE Xplore

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 19, NO. 4, JULY 2004

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Theoretical Study and Implementation of a Fast Transient Response Hybrid Power Supply Andrés Barrado, Member, IEEE, Ramón Vázquez, Emilio Olías, Member, IEEE, Antonio Lázaro, Member, IEEE, and Jorge Pleite, Member, IEEE

Abstract—This paper presents a theoretical and experimental study of the hybrid sources capabilities to improve both the dynamic response and the stability of switching power supplies. The hybrid sources are composed by both, a linear and a switching source connected in parallel. The reached improvements have been possible without affecting, significantly, the efficiency of the whole circuit. This solution is checked in low voltage sources. The obtained experimental results show that these power supplies present high dynamical performance, and therefore they can be used to feed disgital signal processors and microprocessors. Index Terms—Digital signal processors (DSPs), hybrid sources, switching power suplies.

I. INTRODUCTION

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T IS WELL known that the switching power supplies are characterized by their high efficiency and large density of power by volume unit, surpassing actual possibilities of the linear sources. However, the switching power sources present some limitations that the linear sources do not have. The switching power supplies have limited their bandwidth by the switching frequency, which is limited by the switching losses. The switching losses are meaningful for frequencies greater than 500 kHz. Also, high switching frequencies and large currents increase the electromagnetic emissions, which create interference in near electronic circuits. Another handicap, in the switching power circuits, is the dependence the small-signal gain of both, the conduction mode and the external parameters such as input voltage and output current. These limitations are important when switching sources supply some devices such as microprocessors and digital signal processors (DSPs), which require a high stability of the output voltage, even when these devices vary their consumption from some milliamperes to several tens of amperes, in a few nanoseconds [1]. An alternative to solve this problem is the use of hybrid power supplies. This kind of supply is based on the utilization of two current sources connected in parallel. One source is linear and the other one is a switching power supply. The final system takes the advantages of each supply. The linear

Fig. 1. Simplified diagram of a hybrid power supply.

source provides good stability, fast transient response and high bandwidth. The switching source provides its high efficiency. The hybrid systems have been presented in other papers. In [2]–[4] this solution was implemented in order to eliminate the ripple of the output voltage in a power supply, in which a high efficiency and a very low ripple are required. In [5] and [6], the hybrid systems allow to eliminate the output filter capacitor in power supplies with variable output voltage, increasing closed loop bandwidth. In [7]–[9] the hybrid source are used in an audio-amplifier, with the objective of improving efficiency. The main goal of the present paper is to prove the theoretical conclusions comparing both the efficiency and the dynamical response for two sources. A first source is a classical buck converter and the second one is the same buck converter together with a linear source, making up a hybrid source. In order to reach this goal the following has been developed. 1) A theoretical study which main aspect is the small-signal analysis of the hybrid power supply and its comparison with the classical frequency response for the buck converter. 2) Experimental results to validate the conclusions of the theoretical study. II. HYBRID POWER SUPPLY OPERATION PRINCIPLE

Manuscript received October 1, 2001; revised December 1, 2003. Recommended by Associate Editor B. Fahimi. The authors are with the Departamento de Tecnología Electrónica, Universidad Carlos III de Madrid, Madrid 28911, Spain (e-mail: barrado@ ing.uc3m.es). Digital Object Identifier 10.1109/TPEL.2004.830034

The hybrid power supply, Fig. 1, is made up of two sources, ) and a switching one (block ). a linear source (block The linear power supply is composed of both, a low power amplifier (OA1) and an output power block. This

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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 19, NO. 4, JULY 2004

Fig. 2. Transformations of small-signal block diagram of the hybrid power supply shown in Fig. 1.

output is directly connected to the block (resistor and the capacitor ). The reference voltage is applied to the noninverting input of the linear power supply. Its inverting input is connected to the output voltage. The linear supply is designed to have a closed loop bandwidth much higher than the switching one. Furthermore, the linear source must be able to inject current into the load, for short periods of time, equal to the maximum current delivered by the hybrid source. The switching source is a buck converter. In this case, the injected current from the linear source to the output, , constitutes the error signal of the switching power supply.

Therefore, the switching power stage injects, into the output, the current necessary to force to “zero” the current contributed by the linear source. Actually, the current contributed by the linear source reaches a few milliamperes, which are needed to maintain the switching source reference, in steady-state operation. When a load current step occurs, the linear supply provides the necessary current to keep up constant the output voltage. of the linear source, forces The increase of the current an increase of the current provided, to the output, by the to “zero.” Therefore, switching source, reducing the current the linear supply injects current to the output for small period of time; thus, its power dissipation is low.

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III. SMALL-SIGNAL BLOCK DIAGRAMS THE HYBRID POWER SUPPLY AND FREQUENCY ANALYSIS Analyzing Fig. 1, the simplified closed loop block diagram of block is the hybrid source can be deduced [Fig. 2(a)]. The constituted by the switching power source. Its small-signal gain is defined by (1) In the same way, the block source. Its gain is defined by

is constituted by the linear

(2) is the error voltage obtained as the difference between the and output voltages. reference , the output block, the voltage is sensed to be In block in the input of the linear source. compared with the and the load resistor constiThe output capacitor tute the output block. The small-signal gain of this block is defined by

Fig. 3. Bode plots of the closed loop small-signal gain of the switching block, the linear block and the whole hybrid source.

TABLE I MAIN BLOCKS GAINS FOR DIFFERENT FREQUENCY RANGES

(3) In order to simplify the analysis of the contribution of each source, to the output voltage and the dynamical behavior, some block transformations are carried out (Fig. 2). In Fig. 2(e), we can see the final block diagram. This diagram is composed of two big blocks, the “ ” block and the “ ” one. It is very important take into account that “ ” block is constituted by several blocks besides the block that represents . Likewise, “ ” block is comto the linear source posed of several blocks besides the block that represents to the . switching source The gain , in (4), defines the contribution of the linear , in (5), the block “ ” to the output voltage, and the gain contribution of the switching block “ ,” Fig. 2(e) (4)

(5) is defined in (6). It represents The total gain of the system the sum of the contribution of each block

linear block is much greater than the open loop crossover . frequency of the switching block From the analysis of (4)–(6), it can be concluded that both the dynamic behavior and the small-signal stability of the linear source define the small-signal stability of the hybrid power supply. In actual circuits, in order to become stable the hybrid source, both of them, the switching and linear source, must be stable, although ideally is not necessary. It is due to the fact that, the actual linear source has limited its output power to the full load current. Then, if the switching source is unstable, the linear source will not be able to compensate the instability have large amplitude, particularly when the oscillations of becomes negative (using synchronous rectifiers). if Also, from the Table I, it can be concluded that for low frequencies the output power is almost totally delivered by the switching source and at high frequencies the output power is supplied by the linear source. IV. EXPERIMENTAL RESULTS

(6) An analysis of each block and the total system, for different frequency ranges, is shown in Table I, and their conclusions are clarified by means of the Fig. 3. This small-signal analysis supposes that the bandwidth of the linear source is much greater than the bandwidth of the switching source. This condition necessarily implies that the open loop crossover frequency of the

In order to check, experimentally, the exposed idea in this paper, a hybrid power supply and a buck converter have been designed, developed and tested for two output voltages, 3.3 V and 5 V. Both power supplies have been tested under the same conditions to check the actual advantages and disadvantages of the hybrid solution versus the classical solution of the buck converter. The same developed circuit to implement the buck converter has been used to make the switching block of the hybrid source.

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TABLE II MAIN DESIGN PARAMETERS OF THE PROTOTYPES

Ch1 ) 10 mV=div; Ch2 ) 2 V=div, Ch4 ) 10 A=div (1 V = 1 A). Fig. 5.

Fig. 4. Load current step in the typical buck converter for CCM. Ch1 500 mV=div; Ch2 ) 2 V=div; Ch4 ) 10 A=div (1 V = 1 A).

Load current step in the hybrid power supply for CCM.

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In this way, we have tried to minimize the differences that could appear between both studied systems during their assemblies. The characterization of both sources has been carried out by means of static and dynamic measurements. The values of the main design parameters of the prototypes are shown in Table II. The transient responses under the same load current step, for both power supplies with output voltage of 3.3 V, have been measured. The obtained results for the buck converter are shown in Figs. 4 and 6, for continuous conduction mode (CCM) and from discontinuous conduction mode (DCM) to CCM, respectively. The obtained results for the hybrid power supply, under the same load current step, are shown in Figs. 5 and 7, in CCM and from DCM to CCM, respectively. Fig. 8 zooms in the edge up of the load current step for the hybrid power supply working from DCM to CCM. In all cases, the obtained results for the hybrid source are significantly better than the same results for the buck converter. Nevertheless, in Fig. 8 we can notice that the first-spike voltage is higher than in Fig. 7, (15 mV versus 0.22 V). This is due, basically, to the layout and components parasitic effects, which are captured by the scope with more detail in zoomed waveforms. But this first-spike in the hybrid source can reduce by improving the components and the layout. However, an improving of the components and layout in the buck converter will not affect to the 1-V spike, Fig. 6, since this voltage depends mainly on the control bandwidth. In Fig. 9, we can see the output current delivered by the ; the current corresponding to the whole hybrid source,

Ch1 ) 500 mV=div; Ch2 ) 2 V=div; Ch4 ) 10 A=div (1 V = 1 A). Fig. 6. Load current step in the typical buck converter from DCM-CCM.

linear source, , and finally, the current provided by the . It is easy to notice that the linear source switching supply, provides current to the output during the edges of the load current step, and that the current provides by the linear source during the steady-state operation is negligible. It confirms the previously described operation. If Figs. 5 and 9 are compared, the high frequency ringing in Fig. 5 can be explained. This ringing appears in the output voltage when the current contributed by the linear source reaches a few milliamperes, and therefore its steady-state operation. To eliminate this ringing the current sense and the operational amplifier AO2 must be improved. In Fig. 10, the bode plots of the closed loop small-signal gain in the buck converter are shown. The trace 1 shows the operation in CCM and the trace 2 the operation in DCM. It is a wellknown fact that the gain in the buck topology decreases when the operation of converter goes from CCM to DCM.

BARRADO et al.: FAST TRANSIENT RESPONSE HYBRID POWER SUPPLY

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Fig. 7. Load current step in the hybrid power supply from DCM-CCM. Ch1 10 mV=div; Ch2 2 V=div; Ch4 10 A=div (1 V = 1 A).

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Fig. 9. Output currents in the hybrid power supply during a load step. 1-Load ), M 2 5, 9 A/div. 2-Output current of the linear power supply current (i (i ), M 3 7 A=div; 4-Output current of the switching power supply (i ), Ch4 10 A=div.

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Fig. 10. Bode plot of closed loop small-signal gain in the buck converter for CCM (1) and DCM (2).

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Fig. 8. Edge up of the load step in the hybrid power supply, crossing from 100 mV=div; Ch2 2 V=div; Ch4 DCM to CCM. V = 1 A. Ch1 5 A=div.

Fig. 11 presents the same traces for the hybrid source. We can notice how the bandwidth is almost constant for CCM and DCM. This is because in the hybrid power supply the bandwidth is imposed by the linear source. The implemented buck converter presents a small bandwidth, taking into account the capabilities of this topology and the selected switching frequency, 250 kHz. This fact is determined by the large value of output capacitor. The bulk capacitor is needed in order to avoid the significant notches in the output voltage when high load current steps appear. Now, with the hybrid source, even with the output capacitor value of 5, 4 mF the obtained bandwidth is close to 200 kHz. Furthermore, the hybrid source allows us to decrease the value of the bulk capacitor without affecting the dynamical behavior of the system and keeping up the output voltage ripple in a permissible range.

Fig. 11. Bode plot of closed loop small-signal gain in the hybrid power supply for CCM (1) and DCM(2).

In Figs. 12 and 13, the efficiency in steady-state operation, for different load current in both prototypes, is shown. The measurements of the efficiency have been carried out for output voltage values of 3.3 and 5 V. The difference between the switching and hybrid sources efficiencies is due to the current (some milliamperes) that the linear source contributes to the output, in steady-state operation, in order to maintain the reference of the switching source. Nevertheless, this difference is not meaningful, particularly for 3.3-V

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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 19, NO. 4, JULY 2004

Fig. 12.

Efficiency of the converters for 3.3 V in output.

Fig. 13.

Efficiency of the converters for 5 V in output.

[2] F. H. Schlereth and P. Midya, “Modified switched power convertor with zero ripple,” in Proc. 32nd Midwest Symp. Circuits and Systems, vol. 1, 1990, pp. 517–20. [3] H. Ertl, J. W. Kolar, and F. C. Zach, “A new 1 kW class-D supported linear power amplifier employing a self-adjusting ripple cancellation scheme,” in Proc. PCIM’96 Conf., 1996, pp. 265–74. [4] P. Midya, “Linear switcher combination with novel feedback,” in Proc. PESC’00 Conf., 2000, pp. 1425–1429. [5] J. F. Silva, A. Galhardo, and J. Palma, “High-efficiency ripple-free power converter for nuclear magnetic resonance,” in Proc. PESC’00 Conf., 2000, pp. 384–389. [6] H. Ertl, J. W. Kolar, and F. C. Zach, “Basic consideration and topologies of switched-mode assisted linear power amplifiers,” in Proc. APEC’96 Conf., 1996, pp. 207–213. [7] S. Funada and H. Akida, “A study of high efficiency audio power amplifiers using a voltage switching method,” J. Audio Eng. Soc., vol. 32, no. 2, pp. 755–761, 1984. [8] R. A. R. Van der Zee and E. van Tujil, “A power-efficient audio amplifier combining switching and linear techniques,” IEEE J. Solid-State Circ., vol. 34, July 1999. [9] A. E. Ginart, R. M. Bass, and W. M. Leach jr, “High efficiency class AD audio amplifier for a wide range of input signals,” in Proc. IEEE Industry Applications Conf., vol. 3, 1999, pp. 1845–50.

Andrés Barrado (M’95) was born in Badajoz, Spain, in 1968. He received the M.Sc. degree in electrical engineering from the Universidad Politécnica de Madrid, Spain, in 1994 and the Ph.D. degree from the University Carlos III, Madrid, Spain, in 2000. Since 1994, he has been an Associate Professor with the University Carlos III, Madrid. His research interests focus on switching-mode power supplies, multiple output dc–dc converters, modeling of dc–dc and ac–dc converters, low-voltage fast transient response dc–dc converters, EMC, ballasts, and high

input voltage, since the control was designed for this voltage. The low values of the efficiencies with the high currents are associated, mainly, with the losses in the diode of the switching source since synchronous rectification has not been used. V. CONCLUSION This paper shows a theoretical and experimental study of the advantages of the hybrid sources respect to the traditional switching sources. An increase in the bandwidth and the system stability has been achieved. Furthermore, the gain of the proposed hybrid power supply results less sensitive to the variation of external parameters of the system such as the input voltage, and the load current even when the conduction mode is affected. The dynamic improvements of the hybrid sources have been reached without a significant reduction in the efficiency of the whole circuit, although the efficiency depends on the load step frequency. Moreover, in the hybrid sources is eliminated the narrow link between the switching frequency and the bandwidth of the system. In this way, sources with low switching frequency and high bandwidth can be implemented. This fact results very useful to reduce the electromagnetic emissions of the switching power supplies. A handicap, for the implementation of the hybrid sources, is that the complexity of the final circuit increases. From the point of view of the assembly, the linear source may be placed near to the load, and the output filter could be reduced without affecting to the features of the hybrid source. The hybrid supply could become an alternative solution to satisfy the growing demand of high dynamic performance, high efficiency and low voltage power supplies for microprocessors and DSPs. REFERENCES [1] X. Zhou, P.-L. Wong, P. Xu, F. C. Lee, and A. Q. Huang, “Investigatión of candidate VRM topologies for future microprocessor,” IEEE Trans. Power Electron., vol. 15, pp. 1172–1182, Nov. 2000.

power factor rectifiers.

Ramón Vázquez was born in Placetas, Cuba, in 1962. He received the M.Sc. degree in electronic engineering from the Polytechnical Institute of Baku, Baku, Azerbaijan, in 1986 and the Ph.D. degree from the University Carlos III, Madrid, Spain, in 2003. From 1986 up to 1994, he was a Researcher in the field of the semiconductor technology. Since 1994, his research interest is focused on switching-mode power supplies and low voltage dc/dc converters.

Emilio Olías (M’97) received the M.S. and Ph.D. degrees in industrial engineering from the Universidad Politécnica de Madrid, Spain, in 1981 and 1983, respectively. Since 1996, he has been a Professor in the Department of Electronic Technology, Universidad Carlos III de Madrid, Spain, heading the Power Electronics Systems Group and collaborating with Power Electronics Companies in different subjects around Research Projects (financed by private and public founds). He is working on Power Electronics (dc–dc converters with new topologies, control strategies and power factor correction), Alternative energy systems (photovoltaic and hybrid systems), and electromagnetic compatibility. He is also interested in CAD tools applied to the design and analysis of power systems operating in low and medium frequency. In Universidad Carlos III de Madrid, he is a Staff Member of the Escuela Politécnica Superior. Named First Vice-Dean in 2000, he handles the educational and administrative duties for Industrial Engineering Studies, Universidad Carlos III de Madrid.

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Antonio Lázaro (M’03) was born in Madrid, Spain, in 1968. He received the M.Sc. degree in electrical engineering from the Universidad Politécnica de Madrid, Spain, in 1995 and the Ph.D. degree in electronic engineering from the Universidad Carlos III de Madrid, in 2003. He has been an Assistant Professor with the Universidad Carlos III de Madrid since 1995. He has been involved in power electronics since 1994, participating in more than ten research and development projects and has published nearly 40 papers in IEEE conferences. His research interest is switching mode power supplies, power factor correction, modeling of dc–dc and ac–dc converters, and low-voltage fast transient response dc/dc converters.

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Jorge Pleite (M’01) was born in Madrid, Spain, in 1966. He received the M.Sc. degree in electronics and automation engineering from the Polytechnic University of Madrid, in 1995 and the Ph.D. degree from the Carlos III University of Madrid, in 2000. He was an Assistant Professor at the Carlos III University of Madrid from 1995 to 2000, and where, since 2000, he has been an Associate Professor. His research interests are mainly power electronics and magnetic components.