DC/DC Converters Single-phase two-stage inverters generally use an intermediate capacitor to buffer the power imbalance between DC input and AC output. However, the resultant low-frequency voltage ripple on this intermediate capacitor may produce low frequency ripple at the source side, especially when the front-end dc/dc converter operates in continuous conduction mode (CCM). Some common solutions to reducing this ripple are feed forward control and power decoupling circuits. Alternatively, this paper analyzes a two-stage inverter where the front-end is a dc/dc converter operating in discontinuous conduction mode (DCM). In general dc/dc converters operating in DCM have inherent natural capability to reduce this low-frequency input current ripple, without needing a sophisticated control or complex circuitry as compared with its CCM operation. Analysis with simulation verification is reported to demonstrate such capability.
- Low-frequency ripple
- Two stage
Fig. 1. A simplified power-stage diagram of a single-phase two-stage inverter.
EXPECTED SIMULATION RESULTS
(a) CCM operation: _vin = 3:3V
(b) DCM operation: _vin = 0:88V
Fig. 2. DCM boost front-end converter has lower voltage ripple than CCM.
Fig. 3. DCM buck-boost front-end converter does not contain low-frequency ripple but only high-frequency ripple.
Fig. 4. SEPIC front-end converter operating in DCM+CCM contains negligible
low-frequency ripple but only high-frequency ripple.
Fig. 5. High-gain front-end converter operating in DCM does contains
significant low-frequency ripple.
This paper analyzes basic and several higher-order front-end dc/dc converters for single-phase two-stage inverter design. Through inspecting the instantaneous average input current of those converters in discontinuous conduction mode (DCM), it has confirmed that buck-boost converter and buck-boost derived converters such as ZETA are free of low-frequency (mainly double ac line frequency) input current ripple due to the lack of direct connection between input and output during switching actions. For boost converter based converters such as SEPIC and C´ uk converters, their input currents contain lower low-frequency content thanks to the cascaded design. For boost converter based high voltage gain converters, its input current may not necessarily reduce the low-frequency content effectively. It depends on how the high-gain sub circuit is constructed and interacts with the input inductor. Further research is necessary to identify suitable converter topologies which have both smooth input current and low frequency content.
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