A class of robust switched-mode power amplifiers with highly linear transfer characteristics

On the elimination of zero-crossing distortion in switching converters

When looking at the trends of the technology road map in the semiconductor industry, it can be recognized that the high-power switching amplifiers used in lithographic equipment are rapidly becoming a limiting factor. In order to satisfy the requirements for the next decade, the generated noise and other disturbances are required to decrease by at least one order of magnitude. A further complicating factor is that due to the increasing complexity the number of amplifiers in such systems is growing. As a result, the reliability of power amplifier systems should also improve. Similar trends are visible in medical imaging, where improved accuracy is needed for higher resolution images, and in high-power studio-quality audio amplification, where high efficiency switched-mode power amplifiers have to compete with less efficient linear amplifiers that do not suffer from distortion due to switched-mode operation.

Switched-mode amplifiers process electrical power efficiently by controlling the energy flow between intermediate storage components. Modern switched-mode amplifiers are mostly based on the conventional switching leg, which, in turn, consists of two stacked semiconductor switches that are operated complementarily. Several sources of distortion can be attributed to the pulse width modulated (PWM) switching leg, the most significant being blanking time. This blanking time, also referred to as dead time, is essential to avoid fatal cross conduction and results in a current-dependent voltage error, typically several percent of the input power supply voltage.

Extensive studies can be found in the literature on blanking time in PWM converters, aiming at elimination, minimization, and compensation of its effects. The proposed techniques achieve mitigation of the problem but are not capable of completely eliminating it. Alternative topologies have been proposed that do not suffer from blanking-time effects. However, the proposed solutions have either a nonlinear relation between input and output, or are not robust for shoot-through currents.

The conventional switching leg can be transformed to a topology that does not suffer from distortion due to blanking time, is robust for shoot-through current, and has a linear relation between input and output. This topology is based on parallel-complementary unidirectional switching legs, and is known as the dual-buck (DB) converter. The DB exhibits a linear relation between input and output only for continuous inductor currents, which is accomplished with a bias current.

The DB topology does not suffer from blanking-time-related effects. However, other sources of distortion also affect switched-mode amplifiers. The impact of semiconductor switch and diode
parameters on the output quality of the DB is investigated. It is shown that the forward voltages of the diodes and switches have no effect on the output quality.

A prototype of the selected topology has been realized in order to validate the approach. More than 100 times improvement of the open-loop spurious-free dynamic range compared to conventional PWM converters has been demonstrated experimentally.

In this presentation the advantages and disadvantages of the DB topology are discussed and further highlighted with practical examples.