13-02-2013, 11:52 AM
Buck and Boost Converters With Transmission Lines
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Abstract
The switch mode power conversion circuits buck, boost, and buck–boost incorporate a power inductor as an en- ergy storage device. If the power inductor in these fundamental circuits is replaced with a transmission line, new power conversion circuits will emerge. By introducing microwave properties, such as propagation delay and characteristic impedance, new functions may be feasible in the area of power conversion. Examples of such functions could be inverting and noninverting voltage polarity abil- ities or circuits, which share switch components between multiple output voltages reducing the number of semiconductors needed. Alternatively, the buck–boost power converter circuit may give rise to new high-efficiency radio circuits in the area of microwave technology.
INTRODUCTION
OWER conversion using transmission lines (TLs) as a means of dc–dc, ac–dc, and dc–ac conversion has been studied [1]–[7] as an alternative to the use of power inductors. The electrical circuits evaluated were mainly amplifier circuits adapted to perform power conversion. An energy accumulation and discharge cycle synchronous with the TL’s self-resonance frequency was completed. The TL or wave propagation medium could be a coaxial cable, a printed circuit board microstrip, or built using multiple lumped inductors and capacitors forming
an LC network [8].
In this paper, energy accumulation in TLs with frequencies an order of magnitude lower than their self-resonance frequency is discussed. This mode of operation may result in high-efficiency power conversion, without using any amplifier variants, but in- stead derived directly from the original buck, boost, and buck– boost circuits.
SUBSAMPLING
The low-frequency properties of a TL are studied by plotting voltage and current waves against time, when a dc voltage source is applied momentarily to its input, by means of an ideal switch. Fig. 1 shows a time-space diagram where the voltage (hatched area) and current (cross-hatched area) along the length of the TL are plotted horizontally on the vertical time axis at different time instances.
OVERSAMPLING
The option of accumulating and discharging energy syn- chronous to one of the TL’s self-resonance frequencies is called oversampling mode in this paper. To visualize this mode of op- eration, a second switch SB is connected to an arbitrary load XLD as shown in Fig. 4.
Energy is accumulated in the TL by briefly turning ON switch
SA as shown in Fig. 5(a). A current and a voltage wave will con- sequently start to propagate into the TL
MIXED SUB- AND OVERSAMPLING
The option of operating switch STL synchronous with one of the TL’s self-resonance frequencies, without the transitions from TL energy accumulation to energy discharge or vice versa, is called mixed sub- and oversampling mode. The switch state and current waveforms in Fig. 6 again refer to the circuit dia- gram presented in Fig. 2. One of the TL’s resonance frequencies is excited, but contrary to oversampling mode, the resonance frequency is used for energy accumulation during several reflec- tions, again controlling the current fluctuation by low-frequency inductance L .
PULSED AMPLIFIER EXAMPLE
A. Simulation Verification
The prime boost circuit Oa1, presented in Fig. 7, is simulated in this section by means of a lumped TL designed using 40 dis- crete LC elements. The main components are listed in Table II. Inductor models include dc resistance and parallel stray ca- pacitance derived from the inductor self-resonance frequency. Capacitor models include equivalent series inductance (ESL) and equivalent series resistance (ESR).
The circuit, detailed in Fig. 8, is operated in alternating sub- and oversampling mode. This circuit can be used in amplifier applications where the load impedance R is defined and giving efficiencies close to conventional power converters. The circuit generates a pulsed output voltage that is, for example, applicable in radar transmitters where RF signals are amplitude-modulated according to the system’s pulse repetition frequency (PRF). A continuous modulated output voltage can be achieved by parallel coupling of multiple Ba2 circuits, shown in Fig. 7, as in a con- ventional multiphase power converter. A continuous modulated output voltage is, for example, applicable in envelope tracking radio transmitters where RF signals are amplitude-modulated to carry information.