IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 21, NO. 2, MARCH 2006
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Three-Level Buck Converter for Envelope Tracking Applications Vahid Yousefzadeh, Student Member, IEEE, Eduard Alarcón, Member, IEEE, and Dragan Maksimovic´, Member, IEEE
Abstract—This letter proposes a three-level buck converter for tracking applications such as envelope-tracking in radio frequency power amplifiers (RFPAs). It is shown that the three-level buck converter can offer advantages in terms of switching ripples, losses, bandwidth, or the size of magnetic components compared to a standard buck or a two-phase buck converter. Experimental results illustrate improved efficiency and ripple rejection in an RFPA envelope-tracking application representative for low-power battery-operated systems. Index Terms—Envelope elimination and restoration (EER), radio frequency power amplifier (RFPA), three-level buck converter.
Fig. 1. Envelope-tracking technique for RFPAs.
I. INTRODUCTION
E
NVELOPE elimination and restoration (EER) techniques or, more generally, polar modulation techniques, have been proposed to improve the efficiency and linearity of radio frequency power amplifier (RFPA) systems [1]. In such systems, as shown in Fig. 1, an efficient, wide-bandwidth envelope-tracking power supply modulates the supply voltage for the RFPA. Various approaches have been proposed to address the tradeoff between the wide-bandwidth tracking capability and the efficiency, including pulsewidth modulated (PWM) [2]–[4] or delta–sigma modulated buck converters [5], a single-ended primary inductance converter (SEPIC) with average current-mode control [6], a cascade of buck and boost converters [7], a multiphase converter [8], or linear-assisted switched-mode converters [9], [10]. Multilevel converters with flying capacitors, such as the three-level (i.e., two-cell) buck converter shown in Fig. 2, have been proposed for high-voltage high-power applications [11]. In this letter, we propose the use of the three-level buck converter configuration to achieve favorable tradeoffs in terms of the switching ripple, efficiency, bandwidth, or decreasing filter element sizes in envelope-tracking power supplies, including RFPA systems in low-power, battery-operated electronics. Operation of the three-level converter is briefly summarized in Section II. Section III compares the three-level converter to the two-phase buck converter. An example of time-varying
Manuscript received June 11, 2005; revised June 28, 2005. This work was supported by the Defense Advanced Research Projects Agency (DARPA) under the Intelligent RF Front Ends (IRFFE) Program under Grant N00014-02-10501. This letter was presented in part as “Three-Level Buck Converter for Envelope Tracking in RF Power Amplifiers” at APEC’05. Recommended by Associate Editor J. A. Cobos. V. Yousefzadeh and D. Maksimovic´ are with the Colorado Power Electronics Center, Electrial and Computer Engineering Department, University of Colorado, Boulder, CO 80309 USA (e-mail:
[email protected]). E. Alarcón is with the Department of Electronic Engineering, Technical University of Catalunya, Barcelona 08034, Spain. Digital Object Identifier 10.1109/TPEL.2005.869728
Fig. 2.
Three-level buck converter [11].
modulation is presented in Section IV. Experimental results are shown in Section V. II. THREE-LEVEL BUCK CONVERTER OPERATION The power stage of the three-level buck converter is shown and in Fig. 2. Two pairs of complementary switches, , are operated at the same duty cycle , and phase shifted by 180 (similar to the operation of a two-phase converter), as illustrated by the waveforms in Fig. 3. Assuming equals 2, the switch that the flying capacitor voltage 2, or node voltage can take one of three possible levels: 0, . Furthermore, by phase shifting the switching of the two pulses is 2 , pairs of transistors, the frequency of the where is the switching frequency. The three-level operation, in combination with the effective doubling of the switching frequency, results in favorable trade offs in terms of decreasing the switching ripples, decreasing the switching frequency, reducing the size of the filter elements, increasing the converter open-loop bandwidth, or increasing the converter efficiency [12]. For example, assuming the same switching frequency and the same maximum switching ripples, the three-level converter requires 4 times smaller inductance and two times smaller capacitance compared to the standard buck converter. In our experimental prototypes, described in more detail in Section V, the switching frequency of the standard buck converter must be increased by a factor of 2 2 from 200 to 560 kHz to obtain the same maximum output voltage ripple of 12 mV as in the three-level con-
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 21, NO. 2, MARCH 2006
g ,g
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Fig. 4.
Two-phase buck converter.
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