|Year : 2008 | Volume
| Issue : 6 | Page : 414-420
Modeling, Simulation and Development of Isolated Cuk AC-DC Converter in DCM and CCM Operation
Bhim Singh, Ganesh Dutt Chaturvedi
Department of Electrical Engineering, Indian Institute of Technology, Delhi, New Delhi - 110 016, India
Department of Electrical Engineering, Indian Institute of Technology, Delhi, New Delhi - 110 016
This paper presents modeling, simulation and development of high frequency isolation based single-phase
buck-boost AC-DC Cuk converter, which consists of only one switch, resulting in simplicity in design and manufacturing, and reduction in input current total harmonic distortion (THD). Design and analysis are carried out for a 13.5 V output voltage and 20.25 W output power. To validate the design of the converter at the primary stage, the simulation is performed in discontinuous conduction mode (DCM) and continuous conduction mode (CCM) of operation, using PSJM 6.0 platform, which shows high-level steady state and dynamic performance. A laboratory prototype of the proposed single switch Cuk buck-boost converter in DCM operation is developed and test results are presented to validate the proposed design and developed model of the system.
Keywords: Power factor correction, Power quality improvement, Single switch Cuk converter.
|How to cite this article:|
Singh B, Chaturvedi GD. Modeling, Simulation and Development of Isolated Cuk AC-DC Converter in DCM and CCM Operation. IETE J Res 2008;54:414-20
| 1. Introduction|| |
With the advent of new technologies in power processing systems, the improvement of power factor becomes necessary. Line currents drawn by the conventional diode rectifier filter capacitor are peaked pulse currents  , which result in distortion of the utility line. The popular boost converter requires complex control  . Ripple currents are limited only by the size of the inductor. As the boost converter is operated in continuous conduction mode (CCM), the inductor required is large. This in turn increases the crossover distortion. The boost converter in discontinuous conduction mode (DCM) also acts as an automatic current wave shaper. However, it requires a high conversion gain to reduce distortion. The flyback converter in DCM can also be used for power factor correction. As the ripple currents in DCM are high, the line harmonics have to be filtered. This paper deals with the Cuk converter in DCM, which acts as an automatic current wave shaper. The Cuk converter presents an attractive solution for AC-DC conversion, with high frequency isolation. It offers all the benefits of buck-boost topology, with high level of power quality. The Cuk converter with integrated magnetics, when used for input current shaping, exhibits advantages over other topologies  .
A comparative analysis of a Cuk converter is presented in both modes of operation, DCM and CCM, from the point of view of steady state and dynamic behavior, power quality, simplicity, control scheme, device rating and converter size. The simulation results show source current THD of 4.2% in CCM, as compared to 4.5% in DCM at full load. It is observed that CCM is more suitable for higher power applications in which it requires little complex control and sensing the additional variables. However, for lower power rating, DCM is the first choice, owing to its simplicity and less cost.
The conventional single switch Cuk buck-boost converter is available generally for more than 100 W rating and power factor close to 0.99. This paper deals in detail with the design of this AC-DC converter for less than 25 W output power and the experimental result confirms that this AC-DC converter can achieve power factor up to 0.995 and efficiency close to 80%. The simulation is carried out using PSIM6.0 platform for different load conditions of 20 and 100% of rated power. These results show the fast response of PI (proportional integral) controller, where the output voltage is regulated to 13.5 V, after sudden application and removal of the load.
The hardware implementation of Cuk buck-boost converter in DCM operation is carried out using the parameters designed and verified by the simulation results. The Cuk buck-boost converter prototype in DCM is tested for different loading conditions with a wide range of input voltage, to demonstrate its improved steady state performance. The components are selected with the closest specifications available in the market and satisfactory performance is obtained from experimental results.
| 2. Circuit and Operation|| |
[Figure 1] shows the proposed single-phase, single switch Cuk buck-boost converter. This converter consists of an AC power source, V mf input EMI filter of inductor Lf and capacitor Cf, full-wave rectifier FWR, inductor Li, switch SW, capacitor C/, high frequency transformer, capacitor C, high frequency diodes D/, output inductor La, output capacitor C 0 and load resistance R L .
The input filter is required to reduce the ripple in the input current and power factor correction. For the PWM control of the converter, the voltage follower approach in discontinuous conduction mode, and the average current mode control approach in continuous conduction mode are applied.
| 3. Design of Single Switch CUK Buck-Boost Converter|| |
For the desired maximum duty ratio (D max) at minimum input voltage, turn ratio (n) for Cuk converter can be obtained by satisfying the following inequality as:
where V in , is the input AC voltage and V o is the output DC voltage and d is the duty ratio of the converter.
Now, the condition for operation in DCM and CCM can be obtained:
Sign must ensure the DCM operation, for which the following inequality must hold well:
where K e is the conduction parameter and n is the transformer primary to secondary turn ratio. In DCM, K e is calculated for the maximum value of M, which occurs at the maximum output voltage, and the minimum input voltage for a given range of specification. The value K e chosen corresponds to the maximum output power condition, so that DCM of operation is also ensured at full load.
For CCM operation, the following condition must be satisfied, to ensure the continuous conduction mode of operation
The value is chosen for minimum value of load resistance (R Lmin ), which is given as:
where P omax represents the maximum output power of Cuk buck-boost converter. Equivalent inductance (L eq ), which is the parallel combination of L 1 and L 2 , is given as:
where R L is the load resistance and T s is one cycle time of converter, which is equal to 20µsec for 50 kHz operation.
The duty ratio for given power (load resistance) in DCM is obtained as:
Output capacitor (C o ) is chosen according to the specified ripple, allowed in the output voltage. It can be achieved by applying the following formula:
where r v is the per unit ripple in output voltage, wL is line frequency and R Lmin is the minimum load resistance of Cuk converter.
| 4. Modeling and Simulation|| |
To verify and investigate the design and performance at the preliminary stage, simulation study of Cuk buck-boost converter is accomplished in discontinuous conduction mode (DCM) and continuous conduction mode (CCM) operations, using the PSIM6.0 platform, which are shown in [Figure 2] and [Figure 3] respectively. When the load current is low enough to allow the inductor current to run out during the off-time of the switch, DCM operation occurs. T t uses a simple control scheme, which requires sensing of output voltage only . At higher power level, CCM operation, which uses little complex technique to regulate the output voltage, can be preferred. This technique can be implemented for reducing die output voltage ripple, to improve the source current THD and for better converter efficiency.
The Cuk converter is an attractive solution for AC-DC conversion with high frequency isolation. It offers all the benefits of buck-boost topology, with high level of power quality. The Cuk converter with integrated magnetics, when used for input current shaping, exhibits advantages over other topologies. The EMI filter is investigated through simulations and verified through hardware design. Different combinations of the EMI filter, introduced through changing the values of inductor and capacitor, are designed, and their effect on power quality indices is analyzed. Simulation is carried out for steady state performance from 20 to 100% loading conditions and dynamic performance for sudden application of 100% load and then removal of load (20-100-20% load change) in DCM and CCM operations. In order to observe the circuit performance at lower as well as at higher loads, simulation studies are made in two categories:
- Steady state operation with 100% load
- Steady state operation with 20% load
The specifications of the single switch Cuk buck- boost converter are summarized in [Table 1]. Simulated results are shown in [Figure 4],[Figure 5],[Figure 6],[Figure 7],[Figure 8],[Figure 9]. [Table 2] and [Table 3] summarize the design parameters of the Cuk converter.
| 5. Hardware Implementation|| |
A 20.25 W isolated single switch Cuk buck-boost converter prototype with 13.5 V output voltage, having transformer isolation with 50 kHz switching frequency and PFC in DCM operation, is developed. Hardware implementation for single-switch Cuk buck-boost converter is carried out using the parameters designed and verified by the simulation results. A MOSFET (switch) gate driver circuit is developed and it is used in the normal circuit of the converter. After proper functioning of the power circuit, closed loop implementation is carried out by feedback circuit. The proportional integral (PI) controller for different combinations is tested and, after suitable selection, it is verified experimentally. Current limiting feature has been studied and used in these converters. Finally, the converter is tested for supply voltage disturbance over a wide range.
For improving the efficiency of the converter transformer and the inductor, they are designed in a different mode, such as using normal copper wire, combination of six, ten, twelve, sixteen and twenty parallel thin copper wires, copper foil, litz wire etc. Finally, the efficiency of the converter is improved by careful design of the transformer and its leakage inductance using sandwiched winding. The transformer and inductor are also designed in different core gap with different magnetizing inductance. The photograph of the experimental setup of the Cuk buck-boost converter is shown in [Figure 10]. The use of the EMI filter is also tested and demonstrated by conducting tests with and without EMI filter. The input and output ground designed, using optocoupler IC4N35 and performance improvement, is carried out by different experimental tests. Extensive tests are conducted on the developed prototype of the converter and the test results are shown in [Figure 11],[Figure 12],[Figure 13],[Figure 14],[Figure 15]. The components used in the hardware implementation of Cuk buck-boost converter are summarized in [Table 3] with a detailed description.
| 6. Result and Discussion|| |
In order to show the converter performance at light and full load conditions, simulation is performed at 20 and 100% load, i.e. at 4 W and 20.25 W, in DCM and CCM operations. [Figure 4] shows the source voltage and the source current for foil load condition (20.25 W) in DCM and the power factor is observed to be 0.993. From this curve, it is clear that the input current follows the input voltage and the circuit behaves as a resistor emulator, where power factor is identified as being close to unity and THD Iess than 5%. The simulated output voltage waveforms with 1.2% peak-to-peak voltage ripple at foil load is shown in [Figure 5] for DCM operation. The source voltage and source current for 20% loading condition (4 W) is shown in [Figure 6] for DCM operation.
The simulation is carried out in CCM operation and the waveforms are shown in [Figure 7],[Figure 8],[Figure 9]. [Figure 7] shows the source voltage and source current for full load condition. The THD of the input current is observed to be 4.2% and PF is around 0.994, which is quite high. The enlarged view of output voltage at 100% load in CCM is shown in [Figure 8] and ripple is observed to be 1.1%, which is less than DCM operation. To steady the performance of Cuk converter at light load condition, the simulation is carried at 20% (4 W) load. The source voltage and current waveform in CCM operation is shown in [Figure 9] at 20% load and power factor observed here is 0.978. These waveforms verify the performance of average current mode control circuit, which is used to regulate the output voltage. The various waveforms given for DCM and CCM clearly demonstrate that for low power application, DCM operation is more suitable, as compared to CCM operation.
[Figure 10] shows the photograph of the experimental setup of the Cuk buck-boost converter. It includes the control supply and protections such as overload shutdown, input under voltage and output over voltage. In order to examine the switching noise effect at the AC side, the tests are also conducted without EMI filter. [Figure 11] and [Figure 12] show the source voltage and current waveforms for 220 V input supply, without using EMI filter at 100 and 20% loading condition respectively. To verify with simulation input voltage and current waveforms at 100 and 20% load, the experimental waveforms are shown in [Figure 13] and [Figure 14], with EMI filter. Practically, the power factor achieved is 0.995, which is better than simulated value at full load and close to unity. Thus, it shows a good agreement with simulated results in DCM, where it behaves like a resistor emulator. The enlarged view of output DC voltage is shown in [Figure 15], with 135 mV or 1% peak-to-peak output voltage ripple in practical implementation. Consequently, it is clear that the voltage follower control scheme is quit effective to suppress the output voltage ripple in the proposed single switch Cuk buck-boost converter in DCM operation.
Finally, the converter is tested for supply voltage disturbance. Source voltage is suddenly changed from 100 V to 260 V. [Table 4] summarizes the power quality observation via comparison between the experimental and simulation results of the Cuk buck-boost converter. The converter THIX PF, Efficiency and Output voltage ripple at 10-120% load are summarized in [Table 5], which demonstrates the improved power quality and reduced THD of the Cuk buck-boost converter.
| 7. Conclusions|| |
The modeling, simulation and development of Cuk converter in DCM and CCM for 13.5 V, 20.25 W output shows that power factor at AC input mains is observed to be close to unity and efficiency is close to 80%. High power quality is obtained with design parameters, with PF of order of 0.99 and THD less than 8%, from light load to full load conditions. In addition, a detailed comparison of DCM and CCM is presented in order to select the proper mode at different power levels for different applications, from the points of view of low cost, simplicity and device stresses. To verify the simulation results, the prototype board of the Cuk buck boost converter in DCM is successfully implemented and the converter shows good steady state performance from 10 to 120% loading conditions. The full load source current total harmonic distortion is observed to be 7.9% with power factor of 0.995, which is close to unity. Full load efficiency is observed to be 79.9%. The hardware implementation has shown reduced THD, improved power factor and reduced output voltage ripple as compared to simulation results.
Bhim Singh (SM'99) was born in Rahamanpur, U.P. India in 1956. He received a B.E. (Electrical) degree from the University of Roorkee, India in 1977 and M.Tech. and Ph.D. degrees from the Indian Institute of Technology (IIT), New Delhi, in 1979 and 1983, respectively. In 1983, he joined as a Lecturer and in 1988 became a reader in the department of Electrical Engineering, University of Roorkee. In December 1990, he joined as an assistant Professor, became an Associate Professor in 1994 and full Professor in 1997 in the Department of Electrical Engineering, IIT Delhi. His fields of interest include power electronics, electrical machines and drives, active filters, static VAR compensators, and analysis and digital control of electrical machines. Prof Singh is a Fellow of the Indian National Academy of Engineering (INAE), the Institution of Engineers (India) (IE) (I) and the Institution of Electronics and Telecommunication Engineers (IETE), a life Member of the Indian Society for Technical Education (ISTE), the System Society of India (SSI) and the National Institution of Quality and Reliability (NIQR) and Senior Member of IEEE (Institute of Electrical and Electronics Engineers).
Ganesh Dutt Chaturvedi was born in Chanderi (Ashoknagar), M.P. India in 1976. He received a B.E. (Electronics and Communication) degree from R.K.D.F Institute of Science and Technology, Bhopal, India in 1999. In 2000, he joined as a Research Trainee and in 2002 became a Senior Design Engineer at the Associated Electronics Research Foundation, Noida. Presently he is a Research Scholar in the Department of Electrical Engineering, IIT Delhi, pursuing his MS (Research) degree. His fields of interest include power quality, low power converter design; reliability analysis, analog control, and microcontroller based digital control.
| References|| |
|1.||D. Costa, and C. Manoel, " The ZVS-PWM Active-Clamping CUK Converter", in IEEE Trans, on Industrial Electronics, Vol. 51, No.l, pp. 54-60, Feb. 2004. |
|2.||B. R. Lin, and C. L. Huang," Zero voltage switching active clamp buck- boost stage cuk converter," in IETElectrical power Appl, 2007, pp. 173-82. |
|3.||A. A. Aboulnaga, and A. Emadi, " High Performance Bidirectional Cuk Converter for Telecommunication Systems," in Proc. IEEE Trans., 2004, pp. 182-9. |
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]
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