Pulse Density Modulated Soft-Switching Single-Phase Cycloconverter Taufik Taufik and Jesse Adamson Electrical Engineering Department California Polytechnic State University San Luis Obispo, California, USA Anton Satria Prabuwono Faculty of Information Science and Technology Universiti Kebangsaan Malaysia UKM Bangi, Selangor D.E., Malaysia Abstract-Single stage cycloconverters generally incorporate hard-switching at turn on and soft-switching at turn off which limit limits their use in lower frequency applications. This paper presents a proposed solution to this problem using a pulse density modulated soft-switching cycloconverter or abbreviated PDMSS cycloconverter. Unlike standard cycloconverters, the controller in PDMSS cycloconverter lets only complete half cycles of the input waveform through to the output. This requires a much greater frequency step down from the input to the output. The analysis and design of the PDMSS cycloconverter will be described using a simulink model whose results demonstrate the ability of PDMSS to produce the sinusoidal waveforms. I. INTRODUCTION Many electrical loads use power electronics. These include cell phones, computer, stereos, televisions and various home appliances. It is common for these devices to use power electronics extensively. For example, different circuitry in various parts of a computer requires different voltage and current levels that must be regulated by advanced power electronics. Cars use power electronics as well in their computer systems, auxiliary equipment and alternators. Beyond cars, expanding in size, power electronics are seen in airplanes, trains and ships. Stationary applications include power plants, sub stations, wind turbines and other centralized and distributed generation. Power electronics creates and adjusts voltage level in the traditional way using AC generated magnetic fields. The advancement is in techniques to adjust voltage and current levels as well as convert between DC and AC of various frequencies. With power electronics, solid state switches are used to generate AC signals at high frequencies. The higher frequencies are often needed to reduce magnetic component size and improve efficiency. Cycloconverter is one commonly used AC-AC conversion technique that lowers the frequency and may also be used to change the number of phases of electrical power as it is being transferred from one power bus to another. In this paper, a new modulation technique is applied to conventional single-phase cycloconverter. The technique attempts to improve converter’s efficiency by implementing soft-switching. This provides several potential benefits as well as tradeoffs. The design is simulated to explore its strengths, weaknesses and potential for further development. II. BACKGROUND The simplest cycloconverter is the single-phase to single- phase topology. This is implemented with four bi-directional switches in an H-bridge configuration as shown in Figure 1. In this configuration, the switches are operated in pairs. Switching on only S1 & S4 causes the current through the load to flow in forward polarity with AC-IN, while switching on only S2 & S3 causes the current to flow in reverse polarity with AC-IN. This operation allows current to flow either direction through the load, for any given polarity of the AC-IN source. The switches can, therefore, be activated in such a way that the average or filtered output waveform is the desired wave shape (sinusoidal) and frequency. Fig. 1. Single Phase Cycloconverter with Bidirectional Switches Standard power Field Effect Transistors (FET’s), and Insulated Gate Bipolar Junction Transistors (IGBT’s) are unipolar; that is, they only block and allow current flow in one direction. For this reason, they are not viable realizations of S1 through S4. Triacs are standard bidirectional switches sometimes used for small scale cycloconverters. They are not currently available for large current applications. Standard transistors can be arranged to form a bidirectional switch as seen in Figure 2 [1]. This can work for medium applications like electric vehicles and industrial motor drives. Their advantage is the high switching speeds of standard FET’s or IGBT’s. Their disadvantage is the extra voltage drop created by the diode in series with the switch. This diode is essential for two reasons: first to create a path for current to flow around which ever transistor happens to reverse biased, and second to prevent the transistor from being reverse biased with significant voltage. Fig. 2. Bidirectional Transistor Switch FET’s have an inherent diode in the reverse direction making them incapable of reverse blocking. This inherent diode is poor quality, however, so an additional fast diode is usually designed into the silicon substrate as well. If this is 2011 IEEE Applied Power Electronics Colloquium (IAPEC) U.S. Government work not protected by U.S. copyright 189