Printer Friendly

Design and verification of proposed operation modes of LLC converter.

I. INTRODUCTION

Purpose the paper is verification of operation modes and the influence of parasitic elements proposed of LLC converter with output power of 1 kW and switching frequency of 500 kHz for telecommunications distributions. LLC resonant converter is multi-resonant converter and is characterized by its unique DC-gain characteristic, which has two resonant frequencies ([f.sub.0] and [f.sub.p]). This converter has several advantages compared to standard serial LC resonant topology. One of them is possibility of stable regulation of output voltage in a wide range of input voltages together with the change of output power from 1% to 100%. The next advantage is achievement of ZVS switching mode during various operational modes. LLC resonant converter is composed of three functional parts (Fig. 1). It deals about pulse generator, resonant circuit with high-frequency transformer and rectifier with capacitive filter.

[FIGURE 1 OMITTED]

II. OPERATION MODES

Operation of LLC converter in different operational modes is described by DC gain characteristic, which should be divided into ZVS and/or ZCS region [1]. ZVS region in dependency on the switching frequency can be further divided into:

--Region with switching frequency equal to resonant ([f.sub.s] =[f.sub.0]);

--Region with switching frequency higher than resonant ([f.sub.s] > [f.sub.0]);

--Region with switching frequency lower than resonant ([f.sub.s] < [f.sub.0]).

[FIGURE 2 OMITTED]

According to the operational modes of resonant converters the operation of LLC resonant converter is rather difficult [2]. The principal waveforms of transformer and output diode during each operating mode are shown in Fig. 2. The impedance of series resonant circuit at the resonant frequency is equal to zero. Therefore the reflected output voltage is equal to the input voltage, what is described by the unity of voltage gain thus the circuit then operates optimally. LLC resonant converter can achieve gain greater, less or equal to 1. If the switching frequency is less than the resonant frequency, magnetization inductance is involved into the resonance of the circuit so the converter can deliver higher gain.

III. DESIGN PARAMETERS OF THE MAIN CIRCUIT

Target of the design is to determine the active and passive elements of the proposed converter. Is the need estimated efficiency, determination of the maximum and minimum input voltage, determination of maximum and minimum voltage gains, determinations of transformer turns ratio, determination of equivalent load resistance and design of the resonant network. The final design of LLC power stage only the three parameters are necessary to be optimally chosen:

--Ratio of magnetizing and resonant inductance

m = [L.sub.m]/[T.sub.r], (1)

where [L.sub.m] is magnetizing inductance and [L.sub.r] is resonant inductance.

--Quality factor

Q = [square root of [L.sub.r]/[L.sub.m]] x [[pi].sup.2]/8[.n.sup.2][.R.sub.L], (2)

where n is transformer ratio and [R.sub.L] is equivalent load.

--Resonant frequencies

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (3)

In terms of design it is important to made the compromise in selection of the inductance ratio m.

[FIGURE 2 OMITTED]

IV. SIMULATION ANALYSIS OF THE OPERATIONAL MODES OF PROPOSED LLC RESONANT CONVERTER

Operational modes of LLC resonant converter has been verified in OrCAD PSpice simulation software.

[FIGURE 3 OMITTED]

Input/Output parameters of LLC resonant:

--Input voltage: 425 V;

--Output voltage: 48 V;

--Output current: 21 A;

--Output power: 1008 W;

--Switching frequency [approximately equal to] 500 kHz.

Simulation waveforms in the Fig.4, Fig.5, Fig.6. confirm the theoretical assumptions. Another simulation test verified the properties of LLC resonant converter at change load and change the input voltage [3].

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Fig. 7 presents the results from the simulation experiment, when LLC resonant converter operates at full load ([P.sub.OUT] = 1008W) when input voltage [U.sub.IN] = 425V. In this mode, transistors are operating with ZVS conditions and output diodes with character of ZCS switching. The unwanted effect of reverse recovery was eliminated by utilization of Schottky rectifier diodes.

Fig. 8 is showing the simulation experiment at 25% of load at the input voltage [U.sub.IN] = 425V. In the picture shows that even at the reduced output power the switching transistors are maintaining excellent operating characteristics of the ZVS mode. In the case of output diodes the ZCS switching character is also still achieved.

Fig. 9 shows the simulation experiment, when LLC resonant converter operates at minimal supply voltage [U.sub.IN] = 325V and at full load condition. Simulation experiment confirmed proper design of converter. Transistor's current has sinusoidal shape until magnetizing inductance became participating in resonance with other circuit parameters. Output diodes are operating in discontinuous ZCS mode. The last experiment has been done at the input voltage [U.sub.IN] = 325V and at output power [P.sub.OUT] = 252W. As can be seen in this operation mode the inverter is still able to achieve ZVS conditions for the main transistors. Soft commutation with ZCS conditions are also achieved for output rectifier diodes, which are operating in discontinuous mode.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

After simulation experiments have been made, we have made multiple graphic interpretation of converter's efficiency in dependency on converter's output power and input voltage. These results are good indicators of converter design and are good starting point for experimental verifications.

[FIGURE 11 OMITTED]

Time waveforms voltage and current may be deformed influence parasitic components. At higher switching frequencies the sensitivity of the parasitic leakage inductances and capacities becomes serious problem. The parasitic components inclusive in the simulation model of main circuit of LLC resonant converter are:

--[C.sub.OSS]: output capacitance of MOSFET;

--[C.sub.TR]: transformer winding capacitance;

--[C.sub.jc]: junction capacitance of rectifier diode;

--[L.sub.lks]: leakage inductance at transformer secondary side.

The simulation model LLC resonant converter include parasitic components is shown in Fig. 12.

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]

[FIGURE 15 OMITTED]

V. CONCLUSIONS

This paper describes the design of the LLC resonant converter, which is done by means of fundamental harmonic approximation (FHA). Output power of proposed converter is 1kW output voltage 48V and switching frequency is 500 kHz. Performance of converter at different operational conditions were verified through simulation analysis by utilization of OrCAD PSpice software. The simulation results of multiple parametrical experiments were obtained and consequently evaluated into graphical interpretation of efficiency of proposed converter. Future work will be focusing on the design of the physical samples, verification activities and influence of the parasitic components on the operation of LLC converter.

http://dx.doi.org/ 10.5755/j01.eee.18.8.2606

REFERENCES

[1] P. Spanik, P. Drgona, M. Frivaldsky, J. Kandrac, "Efficiency increase of switched mode power supply throught optimization of transistor's commutation mode", Elektronika ir Elektrotechnika (Electronics and Electrical Engineering), No. 9, pp. 49-52, 2010.

[2] P. Spanik, P. Drgona, M. Frivaldsky, A. Prikopova, "Design and application of full digital control system for LLC multiresonant convert", Elektronika ir Elektrotechnika (Electronics and Electrical Engineering), no. 10, pp. 75-78, 2010.

[3] B. Dobrucky, M. Pokorny, V. Racek, R. Havrila, "A New Method of the Instantaneous Reactive Power Determination for Single-Phase Power Electronic Systems", in Proc. of EPE'99 Conf., Lausanne, Sept. 1999.

J. Kandrac, M. Frivaldsky, M. Prazenica, A. Simonova

University of Zilina, Univerzitna 1, 010 26, Zilina, Slovakia frivaldsky@fel.uniza.sk

Manuscript received December 15, 2011; accepted April 20, 2012.

The authors wish to thank to project VEGA "Research of Topology and Control of Power Electronic Supply System with Single-Phase HF Input and Two-Phase Orthogonal Output for Two-Phase SM/IM Electrical Motors". Next to grant agency APVV project no. 20-051705 and no. APVV-0535-07. Also we would like to thanks to VMSP-P-0085-09 and LPP-0366-09.
COPYRIGHT 2012 Kaunas University of Technology, Faculty of Telecommunications and Electronics
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2012 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Author:Kandrac, J.; Frivaldsky, M.; Prazenica, M.; Simonova, A.
Publication:Elektronika ir Elektrotechnika
Article Type:Report
Geographic Code:4EXSV
Date:Aug 1, 2012
Words:1291
Previous Article:Power quality issues in dispersed generation and smart grids.
Next Article:Amplitude noise of electrical oscillators based on the LTV model.
Topics:

Terms of use | Privacy policy | Copyright © 2021 Farlex, Inc. | Feedback | For webmasters |