Abstract
M.Ing. (Electrical And Electronic Engineering)
DC to AC converters are used daily in industry, shipping and aviation as power ..
supplies, standby power supplies or uninterrupted power supplies. Power levels
vary from a few watts to several megawatts.
This thesis investigates a composite converter with a high frequency link as such a
DC to AC converter. There are two basic building blocks, the primary inverter as a
push-pull inverter and the secondary converter as a cyclo-converter.
A existing 2.5kVA system with bipolar transistors in the primary inverter and
asymmetrical thyristors in the secondary converter is investigated. In the primary
inverter, regenerative snubbers are used to minimise switching losses. A second
2.5kVA system with mosfets in the primary inverter and IGBT's in the secondary is
developed. Switching losses in the primary inverter are minimised by including a
resonant circuit concisting of a non-linear inductor and linear capacitor.
This resonant circuit in the primary inverter, was originally simmulated by a PSPICE
simmulation program, after which it was build up experimentally. The simmulated
results and experimental results coincided, and development of the secondary
converter started. The switches in the secondary had to comply to two conditions
viz, low conducting lossses and the ability to switch at switching frequencies higher
than 20kHz. IGBT's were consequently chosen and were implemented with the
necessary gate drive circuits.
With the higher switching frequency of 25kHz to the 5kHz of the first system, the
ripple on the load current was significantly reduced. Furthermore, the construction
of the composite converter with the gate drive switches was easy and compact,
and the total cost of developing such a converter were reduced.