Comparison of DC/DC Converters in DCM for Reducing Low-Frequency Input Current Ripple of Single-Phase Two-Stage Inverters


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.


  1. Dc/ac
  2. Low-frequency ripple
  3. Single-phase
  4. Two stage



dc/dc converters

Fig. 1. A simplified power-stage diagram of a single-phase two-stage inverter.


comparison dc dc converters

  • (a) CCM operation: _vin = 3:3V

  • comparison dc dc converters

(b) DCM operation: _vin = 0:88V

Fig. 2. DCM boost front-end converter has lower voltage ripple than CCM.

comparison dc dc converters

Fig. 3. DCM buck-boost front-end converter does not contain low-frequency ripple but only high-frequency ripple.

comparison dc dc converters                                               Fig. 4. SEPIC front-end converter operating in DCM+CCM contains negligible

low-frequency ripple but only high-frequency ripple.

comparison dc dc converters

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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A Power Quality Improved Bridgeless Converter Based Computer Power Supply



Poor power quality, slow dynamic response, reduced output voltage regulation, high device stress, harmonic rich, periodically dense, peaky, distorted input current are the major problems which are frequently encountered in conventional switched mode power supplies (SMPSs) used in computers. To mitigate these problems, it is proposed here to use a non-isolated bridgeless buck-boost single ended primary inductance converter (SEPIC) in discontinuous conduction mode (DCM) at the front end of an SMPS. The bridgeless SEPIC at the front end provides stiffly regulated output dc voltage even under frequent input voltage variations and loads. The output of the front end converter is connected to a half bridge dc-dc converter for isolation and also for obtaining different dc voltage levels at the output that are needed in a computer. Controlling a single output voltage is able to regulate all the other dc output voltages as well. The design and simulation of the proposed power supply is carried out for obtaining improved power quality which is verified through the experimental results.


  1. Bridgeless converter
  2. PFC
  3. Input current
  4. Computer power supply



Fig.1 Schematic diagram of PFC converter based SMPS


Fig.2a Performance of the computer power supply at rated condition

Fig. 2b Input current and its harmonic spectrum at full load condition

Fig. 2c Waveform of various components of bridgeless converter

Fig.3a Performance of the computer power supply at light load condition

Fig. 3b Input current and its harmonic spectrum at light load condition


A bridgeless non-isolated SEPIC based power supply has been proposed here to mitigate the power quality problems prevalent in any conventional computer power supply. The proposed power supply is able to operate satisfactorily under wide variations in input voltages and loads. The design and simulation of the proposed power supply is initially carried to demonstrate its improved performance. Further, a laboratory prototype is built and experiments are conducted on this prototype. Test results obtained are found to be in line with the simulated performance. They corroborate the fact that the power quality problems at the front end are mitigated and hence, the proposed circuit can be a recommended solution for computers and other similar appliances.


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