Soft switching ac-link three-phase ac–ac buck–boost converter

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Soft-Switching AC-Link Three-Phase AC-AC
Buck–Boost Converter
Abstract
Different types of ac–ac converters have been proposed
over the years. These converters can be classified as direct or
indirect depending on their power conversion type. Matrix
converters and cyclo converters are examples of the direct ac–ac
converters, whereas the dc-link and ac-link converters are
classified as indirect ac–ac converters. Cyclo converters and
matrix converters have several limitations that hinder their
widespread use in industry. Among these limitations are the
poor input displacement, low input power factor (PF), and
limited output frequency in the cyclo converters, and the low
output to input voltage ratio in the matrix converters. The dc-
link converters are the most common type of ac–ac converters.
This type of converters is formed by a three-phase boost rectifier
and a three-phase buck inverter. Regardless of the type of the
rectifier or the inverter, dc electrolytic capacitors are integral
part of these converters. Electrolytic capacitors are very
sensitive to temperature and can cause severe reliability
problems at higher temperatures. Therefore, converters that
contain dc electrolytic capacitors have higher failure rates and
shorter lifetimes compared to the other converters. This is not

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the only problem with dc-link converters. In these converters,
galvanic isolation can be provided by three-phase low-frequency
transformers. Therefore, another limitation of the dc-link
converters is the large size and the heavy weight of the low-
frequency transformers employed.
Existing system
A new zero voltage switching partial-resonant link AC/AC
converter which overcomes most of the aforementioned
disadvantages and consists of 12 unidirectional switches is
proposed. The converter synthesizes output ac waveforms by
integrating high-frequency current pulses like series-resonant
converters. But switching transients does not occur at zero
current instants but at zero voltage instants. The partial-resonant
link normally does not resonate during entire operation interval
but operates only during the switching transients to ensure zero
voltage switching condition. Thus this converter has low VAR
ratings of resonant elements L, and C, and has low dissipations.
In particular, the converter can control individual current pulse
amplitude very easily in each switching cycle, which enables
improving the output voltage waveform and solving the
instability problem usually caused by parasitic resonance
between output capacitor CO and load inductance Lo.
Symmetrical arrangement of power circuit in the converter also

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provides fully regenerative operation together with buck-boost
capability.
Proposed system
In this paper, a soft-switching ac-link ac–ac buck–boost
converter is introduced, studied, analyzed, and evaluated. In this
converter, the link current and voltage are both alternating, and
their frequency can be very high. This eliminates the need for
the dc electrolytic capacitors and the low-frequency
transformers. In case galvanic isolation is required, a single-
phase high frequency transformer may be added to the link. By
adding a small capacitor to the link, the converter can provide
soft switching as well. This converter transfers power entirely
through the link inductor. The link is charged through the input
phase pairs and then discharged into the output phase pairs.
Charging and discharging take place alternately. The frequency
of charge/discharge is called the link frequency and is typically
much higher than the input/output line frequencies. Between
each charging and discharging, there is a resonating mode
during which none of the switches conduct and the LC link
resonates to facilitate the zero voltage turn-on and the soft turn-
off of the switches. Due to the existence of the bidirectional
switches, charging and discharging of the link in a reverse
direction is feasible, leading to an alternating link current.

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Advantages
 Better utilization of the inductor.
Applications
 Wind power generation and variable-speed drives.

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Block diagram
Tools and software used:
 MPLAB – microcontroller programming.
 ORCAD – circuit layout.
 MATLAB/Simulink – Simulation.
THREE PHASE
AC-AC BUCK
BOOST
CONVERTER
INPUT AC
SUPPLY
DRIVER CIRCUIT
PIC CONTROLLER WITH
BUFFER
5V DC
12V DC
THREE PHASE
LOAD

Soft switching ac-link three-phase ac–ac buck–boost converter

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Soft switching ac-link three-phase ac–ac buck–boost converter