11-04-2012, 11:32 AM
A ZERO VOLTAGE SWITCHTNG BOOST CONVERTER USING A SOFT SWITCHING AUXILIARY CIRCUIT
INTRODUCTION
In exarnining previous ZVT converters it is found that many [22]-[25] do not offer
a lossless tuni-on and tum-off of the awtiliary switch which results in lower efficiency in
the converter. These converters also have to incorporate a capacitor as a snubber across
the main switch in order to achieve its zero voltage turn-off. The addition of this
capacitor results in higher rms current in the auxiliary switch that results in more
conduction losses in the auxiliary circuit. Also it is seen fkom [31] that adding this
capacitor also results in lesser ZVS tum-on interval of the main switch if other
parameters in the auxiliaxy circuit are kept the same.
functional description
ZVT converter that is being presented and analysed in this
thesis. The circuit cm be assumed to be made up of two parts:
1). The main power circuit consisting of a diode bridge, main boost switch SI, boost
inductor Lin the boost diode DI, and the output capacitor Co.
2). The auxiliary circuit consisting of the resonant inductor L,, resonant capacitor Cr and
another capacitor Cb,a u x i l i q switch & and diodes DI,D 3, Dga nd Ds.
The output load is represented by an output resistance The diode bridge
rectifies the variable input AC source voltage at 60 Hz into an uncontrolled DC voltage.
The boost inductor L,, main switch SI and boost diode DI fonn a simple boost converter
which converts the uncontrolled DC into a controlled DC bus voltage at the output
capacitor Co. Capacitor Co filters the second harmonic current and prevents its appearing
at the load. Switch Sz is tumed on just before SI and serves to achieve a zero current tuni
off of the diode Dr and also discharges the parasitic capacitance across SI to ensure ZVS
of SI.
converter feature
The main feature of this converter is the simple auxiliary circuit containing few
components. A floating gate drive for the auxiliary switch S2 is not required as it is
connected to ground,
The auxiliary switch S2 has a soft --off in this converter which is a feature not
found in many ZVT converter topologies. This soft tum-off is important as without it
some of the reduction in switching losses of the main switch is offset by increased
switching loss in the auxiliary switch. Most ZVT topologies use a dissipative snubber in
the auxiliary circuit to minimise the oscillations caused by resonance between inductor
and output capacitance inside the auxiliary switch.
study state analysis
This section describes the steady state analysis of the auxiliary circuit of the
converter during one switching cycle. The purpose of this analysis is to obtain
characteristic curves of the converter which aid in designing the converter.
conclusion
The proposed ZVT Boost converter was introduced in this Chapter. This
converter has an auxiliary circuit which has low conduction losses and simple
construction.