13-04-2013, 02:41 PM
An Adaptive Energy Storage Technique for Efficiency Improvement of Single-Stage Three-Level Resonant AC/DC Converters
An Adaptive Energy Storage.pdf (Size: 1.36 MB / Downloads: 62)
Abstract
The use of single-stage power-factor-corrected
(SSPFC) three-level resonant ac/dc converters solves many
problems that present SSPFC converters face today, namely, high
component stresses, high circulating currents, and low efficiency.
This makes single-stage three-level resonant ac/dc converters a
good candidate for high-power applications. These converters
provide the flexibility of simultaneously using two control
variables. They can operate with a combined variable-frequency
and asymmetrical pulsewidth modulation or with a combined
variable-frequency phase-shift modulation. This provides good
regulation of the output voltage, dc-bus voltage, and input
current. The drawback of these methods is that the efficiency of
the converter drops as the load is reduced because the converter
starts to drift away from its resonance frequency, thus leading
to more circulating currents and conduction losses. Therefore,
a load-adaptive energy storage technique is proposed in this
paper to guarantee the converter operation near its maximum
efficiency point for a wide range of loading. This leads to almost
constant converter efficiency from full load to 40% load. The
use of interleaved converters is also proposed to extend the
constant efficiency range of operation to lighter loads (15%–20%
of full load). Analytical simulation and experimental results are
presented to verify the proposed methods.
INTRODUCTION
ACHIEVING high conversion efficiency has been a significant
challenge for single-stage power-factor-corrected
(SSPFC) topologies. Low efficiency in these converters can be
attributed to the high voltage and current stresses imposed on
switches and other circuit components. Furthermore, in many
cases, in order to achieve proper input current shaping, circulating
currents pass through the converter, causing excessive
conduction losses [1]–[11].
VFAPWM-Controlled Converter
The circuit construction here is the same but with no need
for additional auxiliary circuit. The boost inductor Lin operates
in continuous conduction mode (CCM). The control of this
converter is achieved via a combined APWM and VF controller.
The duty cycle obtained from the APWM controller serves to
regulate the dc-bus voltage for all input and output conditions,
whereas the VF controller allows for a tightly regulated output
voltage.
In this circuit, the upper and lower pairs of switches operate
complementarily. The switching sequence is shown in Fig. 2.
When S3 and S4 are on, energy from the supply is stored in the
boost inductor (Lin), while the bulk capacitor (Cb2) supplies
energy to the load through the resonant circuit. Therefore, the
voltage across the resonant circuit during this period is −V2.
Switch S4 is made to turn off a very short time before S3
to allow the voltage across S4 to be clamped to V2 through
clamping diode Dc2. This stage of operation ends at t = DTs,
where D is the duty cycle of the boost stage and Ts is the
switching period. During the remaining part of the switching
cycle, the energy is transferred from the boost inductor to the
storage capacitors through body diodes of S1 and S2 and then
through S1 and S2.
INTERLEAVED CONVERTERS
The load-adaptive dc-bus voltage reference is capable of
providing an almost constant efficiency from full load down to
about 40%–50% of full load due to restrictions on the duty ratio
and dc-bus voltage level. At lighter loads, if Vbus is too low, the
duty ratio also becomes too low, causing undesirable voltage
dips at the output. Therefore, a minimum value for Vbus has to
be set to ensure this does not happen. Consequently, at lighter
loads, the converter switching frequency moves further away
from the resonance frequency, leading to a drop in efficiency
in the range below 40% of full load. For high-power systems,
in order to improve the conversion efficiency at light-load
conditions, the use of interleaved converters is proposed. With
proper switching algorithm, this can lead to expanding the highefficiency
region of the converter so that it can be achieved at a
much lighter load condition.
CONCLUSION
In this paper, the use of load-adaptive dc-bus voltage method
for improving the efficiency of single-stage three-level resonant
ac/dc converters operating with VFAPWM/VFPSM control has
been proposed. This method allows the resonant converter to
operate very close to its resonance frequency down to almost
40%–50% of full load. This leads to a reduction in circulating
currents in the resonant circuit, thus reducing the conduction
losses. For the lighter load conditions, the use of interleaved
converters gives an almost constant efficiency curve for a
much wider loading range. For two converters operating with
VFAPWM and load-adaptive dc-bus voltage, the efficiency
remains constant down to 20% of full load. The use of more
converters in parallel and designing them for different power
levels can further enhance the efficiency curve at light loads.