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In this dissertation, switching oscillators for power conversion applications
are investigated in depth. The contents includes the principles of energy-st
orage oscillations, the effects of different switching devices on switching os
cillators, the role of parasitic capacitance on energy-storage oscillations, a
nd the input current shaping of bridge switching oscillators. Two switching o
scillators suited for specific power conversion applications are adopted for i
nvestigation. One is the bridge oscillator which transmits the source energy
to loads directly, and the other is the blocking oscillator which transmits th
e source energy to loads indirectly.The bridge oscillator is commonly utilized
for ballasting the current of discharge lamps in off-line illumination applic
ations. Its oscillation frequency is mainly determined by a saturable driving
transformer which is in series with the energy-storage inductor. Due to the
parasitic capacitance across the switching devices, the bridge oscillator can
easily achieve the zero-voltage-switching and clamping voltage (ZVS-CV) functi
on. The study results show that bipolar switching devices are more suitable t
han unipolar switching devices in the transformer-driven switching oscillators
, under the conditions of using the same circuit topology but different switch
ing devices. Moreover, in order to achieve the energy-storage oscillations, t
he bridge oscillator always behaves like a low-quality-factor load-resonant in
verter.The blocking oscillator usually acts as the auxiliary power supply for
external-drive switching circuits. Its oscillation frequency is determined by
its output loads. Since the choke current is always discontinuous and rings
with the parasitic capacitance of circuit devices, this type of converters is
also referred to as ringing choke converter (RCC). Due to the indirect energy
transmission, the blocking oscillator can limit the output power naturally.
Under the the same energy-storage circuit, different control strategies result
in different oscillation characteristics. The study results show that the ou
tput-current foldback protection and the soft-start function can be simultaneo
usly fulfilled by a simple resistor-capacitor charging-discharging feedback-co
ntrol circuit.The role of parasitic capacitance on both of the studied switchi
ng oscillators can be qualitatively interpreted by state-plane analysis. The
state trajectories of switching oscillators show that the parasitic resonance
resulted from parasitic capacitance is indeed a part of energy-storage oscilla
tions. For the bridge oscillator, the parasitic capacitance across switching
devices is useful for the ZVS operation and the reduction of switching losses.
Similarly, the parasitic capacitance across the transformer choke can effect
ively prolong the oscillation cycle of the ringing choke converter. Hence, th
e voltage regulation can be improved and the required dummy load can be decrea
sed.Finally, two types of single-stage high-power-factor electronic ballasts w
hich utilize the bridge oscillator to conduct boost-type power-factor-correcti
on operations are discussed. By adding an input boost inductor, the bridge os
cillator can easily obtain the function of input current shaping. The study r
esults show that the charging-discharging current of the input boost inductor
deteriorates the ZVS operation and increases the requirement of powerline filt
ering. Therefore, the studied self-excited single-stage power-factor-correcti
on configurations are only suitable for low power applications. The future re
searches are directed at the improvement of the ZVS characteristics and the re
duction of the powerline filtering burden.
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