Estimation of Reliability of a Interleaving PFC Boost Converter

Transcription

1 SRBIAN JOURNAL OF LCTRICAL NGINRING Vol. 7, No. 2, November 2010, UDK: stimation of Reliability of a Interleaving PFC Boost Converter Gulam Amer Sandeudi 1, Srinivasa Rao 2 Abstract: Reliability lays an imortant role in ower sulies. For other electronic equiment, a certain failure mode, at least for a art of the total system, can often be emloyed without serious (critical) effects. However, for ower suly no such condition can be acceted, since very high demands on its reliability must be achieved. At higher ower levels, the continuous conduction mode (CCM) boost converter is referred toology for imlementation a front end with PFC. As a result, significant efforts have been made to imrove the erformance of high boost converter. This aer is one of the efforts for imroving the erformance of the converter from the reliability oint of view. In this aer, interleaving boost ower factor correction converter is simulated with single switch in continuous conduction mode (CCM), discontinuous conduction mode (DCM) and critical conduction mode (CRM) under different outut ower ratings. Results of the converter are exlored from reliability oint of view. Keywords: Boost Converter, Reliability, Power factor correction (PFC), Simulation of converter and MIL-HDBK Introduction Reliability is the robability of oerating a roduct for a given eriod of time without failure under secified conditions and within secified erformance limits. It lays an imortant role in ower electronic systems by which the number of system failures, reair costs, guarantee etc. are estimated [1]. Interleaved converters [2-5] are a result of a arallel connection of switching converters. They usually share the same outut filter. Interleaved converters offer several advantages over single-ower stage converters; a lower current rile on the inut and outut caacitors, faster transient resonse to load changes and imroved ower handling caabilities at greater than 90% ower efficiency. An interleaved converter can be realized by interleaving the control singles to each of the aralleled converters resulting in an effective increase in its switching frequency. They are used in alications where the loads demand low rile tolerances. Such requirements are found in the new 1 Research Scholar, Deartment of lectrical ngineering, National Institute of Technology, Warangal, India 2 Associate Professor, Det. of lectrical ngineering, National Institute of Technology, Warangal, AP India 205

2 G. Amer, S.S. Rao generation of ersonal comuters, in which core voltages and currents of the central rocessing units are aroaching 1V and 130A. Interleaving converters also find alications in switching audio amlifiers by interleaving series or arallel combinations of ower inverters. Additionally, interleaving enables the converter to sread its comonents and the dissiated ower over a larger area. Further, interleaving enables the converter to sread its comonents and the dissiated ower over a larger area. Performance comarison of CCM and DCM fly-back converters is referred in [6]. Asects comared to comonent stress, outut voltage regulation and transient resonse due to ste load and efficiency. The comarison was carried out exerimentally on a 5V/25W, 50kHz rototye CCM and DCM fly-back converters which have been designed and built with similar circuit lay outs, comonents and ower ratings. Tests erformed on the rototye converter have shown that devices in the CCM fly-back converter sustain the same voltage stress, but lesser current stress than its DCM counterart, when delivering the same outut ower. In this comarison, reliability is absent and is not considered, whereas reliability is a key necessity in ower electronic devices which the life time, number of failures and associated cost are estimated. In [7, 8] effect of leakage inductance on reduction of reliability of switching ower sulies is discussed. In [9], a boost converter is considered as the PFC art of a switching ower suly the simulation and reliability calculation of boost converter within DCM and CCM modes oerating in three different ower ratings is done. In this aer, a boost ower factor correction converter is simulated with single switch interleaving technique in CCM, DCM and CRM modes at different outut ower ratings and results are comared. Reliability calculation is based on MIL-HDBK217 standard and in art counting method [10]. Results have shown that for an Interleaving boost converter as a PFC, working in CCM mode is better than DCM and CRM mode with reference to reliability. 2 Concet of Reliability A Reliability definition Reliability is a disciline that combines engineering design, manufacture, and test. An efficient reliability rogram emhasizes early investment in reliability engineering tasks to avoid subsequent cost and schedule delays. The reliability tasks focus on revention, detection, and correction of design deficiencies, weak ans, and workmanshi defects with the goal of influencing the roduct develoment rocess and roducing roducts which oerate successfully over the required life [1]. 206

3 stimation of Reliability of a Interleaving PFC Boost Converter B Reliability function The reliability of a comonent can be described as an exonential function. The robability of finding a comonent oerating after a time eriod is defined as: Rt () = e t, (1) where, is the constant failure rate during the useful life eriod. The mathematical mean value of R() t occurs at t equal to 1. 1 is the mean time elased until a failure occurs or the Mean Time To Failure, MTTF. C MTBF (Mean Time between Failures) As reair time (MTTR) normally can be neglected comared to MTTF for electronics, MTBF can be found as: MTBF = MTTF + MTTR MTTF =1. MTBF or the failure rate can be calculated using different kinds of inut data. D Calculation of MTBF for equiment [1] When calculating the MTBF for equiment, its total failure rate e must be found. By assuming the equiment or aaratus containing n comonents. The robability to find n comonents in oeration after the time t is: e t n e t R RR R e t = e t n = = (2) and = n. (3) The total failure rate for the equiment at secified conditions is accordingly achieved as: = c + c + + c. (4) e b1 1 b2 2 bn n By simly inverting this value, the MTBF figure for the equiment is found: 1 MTBF =. (5) In this aer, the MOSFT is chosen as IXFH12N100Q/IXS, the Diode MUR850 and the inut bridge KBPC_35_06. e 207

4 G. Amer, S.S. Rao 3 Interleaved Boost Converter Interleaved converters offer several advantages over single-ower stage converters; a lower current rile on the inut and outut caacitors, faster transient resonse to load changes and imroved ower handling caabilities at greater than 90% ower efficiency. Another imortant advantage of interleaving is that it effectively increases the switching frequency without increasing the switching losses. The obvious benefit is an increase in the ower density without the enalty of reduced ower-conversion efficiency. There is still a enalty, however. Interleaving requires increased circuit comlexity (greater number of ower-handling comonents and more auxiliary circuitry), leading to higher arts and assembly cost and reduced reliability. A Continuous conduction mode ven though the inductor currents in I L1 and I L2 are discontinuous the inut current which is the sum of two inductor currents is continuous [7]. So interleaving virtually eliminates discontinuity in the inut current. The simulation of interleaved boost converter is shown in Fig. 1 and inductor current waveform after zooming is shown in Fig. 2. Fig. 1 CCM Interleaved Boost converter. 208

5 stimation of Reliability of a Interleaving PFC Boost Converter Fig. 2 Inductor current waveforms after zoom. B Discontinuous conduction mode To oerate interleaving configuration in discontinuous mode of oeration the hase shift of 180 o is roerly incororated between the two inductor currents by using the delay. The simulation schematic of DCM interleaved converter is shown in Fig. 3 and inductor current waveform is shown in the Fig. 4. Fig. 3 DCM Interleaved Converter. 209

6 G. Amer, S.S. Rao Fig. 4 Inductor currents waveforms. C Critical conduction mode The interleaved switching converter, comosed of arallel connection of switching converters, oerating at same switching frequency, but each switching hase is sequentially shifted over equal fractions of the switching eriod [4]. The simulation schematic of CRM interleaved boost converter is shown in Fig. 5 and inductor current waveform is shown in Fig. 6. Fig. 5 Simulation schematic of CRM interleaved converter. 210

7 stimation of Reliability of a Interleaving PFC Boost Converter 4 Reliability Calculations Fig. 6 Inductor Current waveform. In this section, reliability of the boost converter for different outut ower is calculated and resented in detail. For different outut owers and oerating modes, the results of reliability calculations are shown in Table 1. The art counting method [10] is used to calculate reliability. In this aroach, first the failure rate of each element in the converter structure is obtained individually and then the value of the converter s MTBF is calculated from equations (4) and (5), where N is the number of consisting arts. For these calculations the following assumtions are made: The ambient temerature is 27 o C; The control structures of these converters are not the same whose reliability can be neglected for comaring the reliability of main comonents; To calculate the reliability, first the dynamic and static losses of MOSFT and Diodes should be calculated for different outut owers working in three oerating modes namely CCM, DCM and CRM; Pdynamic = Vavg Iavg tol f s, (7) Pstatic = Von Iavg ton f s, (8) Ploss = Pstatic + P switching, (9) 211

8 G. Amer, S.S. Rao It should be noted that if the converter is oerating in DCM mode, then before further turn-on of the switch, the inductor current should reach zero. So there will not be turn-on loss. But in CCM oerating mode, the current should be transferred from diode to the switch. The turn ON overla and turn OFF overla is shown in Figs. 7 and 8 resectively. Fig. 7 Turn on Overla. Fig. 8 Turn off Overla. 212

9 stimation of Reliability of a Interleaving PFC Boost Converter Table 1 Reliability calculations for CCM, DCM and CRM oerating modes of interleaving boost PFC. Outut Power 800W 1000W Oerating Mode CCM DCM CRM CCM DCM CRM (MOSFT) (Outut Diode) (Inut Bridge) (Inut Inductor) (Outut Caacitor) (Outut Resistor) Total MTBF (hours) Results From the results noted in the Table-I, the following oints are observed. The boost converter has highest reliability in CCM oerating mode comared to DCM and CRM modes of oeration. The eak and rms values of current are higher in DCM and CRM modes, when comared with CCM, as a result, they offer higher current stress on switching devices. Hence, switching devices have highest failure rate in DCM and CRM modes when comared with CCM mode. 6 Conclusion In this aer, a boost PFC converter is simulated under three outut ower ratings and CCM/CRM/DCM oerating modes. Reliability calculation of the converter is done based on MIL-HDBK-217 and in art count method. Results have shown that switches have the highest failure rate in the converter structure in both CCM/DCM oerating modes and different outut owers. It is observed that, with reference to reliability, boost converter as a PFC, working in CCM mode is better when comared with DCM and CRM modes of oeration. 7 Aendix In this section, the samle calculation for failure rate for each comonent is resented: a) Calculation of Failure rate for MOSFT (IXFH12N100Q/IXS): V = 1000V, n 0 θ jc = 0.42 C W, θ = ca 0 1 C W 213

10 G. Amer, S.S. Rao Tc = Ta +θ ca Ploss = = Tj = Tc +θ jc Ploss = = π T = ex 1925 = T j b = 0.012, π = 6, π A = 10, π Q = 5.5 = = = b Q A T b) Calculation of failure rate ( ) for Ouut diode: V = 1000V, n 0 θ jc = 2 C W, P = 0 θ ca = 1 C W, loss W Tc = Ta +θ ca Ploss = = Tj = Tc +θ jc Ploss = = π T = ex 1925 = T j b = 0.069, π = 6, π Q = 5.5, π C = 1 V S 90 = = π = V = S S = b Q S T C = = = c) Calculation of failure rate ( ) for Inut Bridge: V = 1000V, n 0 θ jc = 1.6 C W, P loss = W Tc = Ta +θ ca Ploss = = Tj = Tc +θ jc Ploss = = π T = ex 1925 = T j

11 stimation of Reliability of a Interleaving PFC Boost Converter V S b = 0.069, π = 6, π Q = 5.5, π C = = = π = V = S S = b Q C S T = = = d) Calculation of failure rate ( ) for Inductor: T = T Δ T = = 39.1 HS A 15.6 T HS b = = m 329 π = 6, π = 20 3 = = Q e) Calculation of failure rate for Caacitor: CV ( ) 0.18 = = μ = C π = 2, π = 10 = = = b Q CV Q f) Calculation of failure rate for Resistor: π R = 1, π = 2, π Q = 10, b = = = = b Q R Therefore the total system failure rate will be: system N = = n= 1 art ( ) failures /10 hours 1 MTBF = =

12 G. Amer, S.S. Rao 8 References [1] Reliability asects on Power Sulies, Design Note 02 N/LZT RIA (C. ricsson Microelectronics AB). [2] Simon Ang, Alejdro Olivia: Power Switching Converters, 2nd dition, Taylor & Francis 2002, age [3] A. Pressman: Switching Power Suly Design, 2nd edition 1998, McGraw-Hill. [4] JW Kim, SM Choi and KT Kim: Variable On-time Control of the Critical Conduction Mode Boost Power Factor Correction Converter to Imrove Zero-crossing Distortion, PDS 2005, Vol 2, [5] Brett A. Miwa david, M. Otten martin, F. Schlecht: High fficiency Power Factor Correction using Interleaving Techniques, APC 1992, [6] S. Howimanorn, C. Unlaksananusorn: Performance comarison of Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM) flyback converter. PDS, 2003, Vol. 2, [7] Dao-Shen Chen, Jih-Sheng Lai: A Study of Power Correction Boost Converter Oerating at CCM-DCM Mode. I Proceedings SOUTH AST CON '93, [8] B. Abdi, M. B. Menhaj, L. Yazdanarast, J. Milimonfared: The ffect of the Transformer Winding on the Reliability of Switching Power Sulies, P-PMC 2006, [9] B. Abdi, A. H. Ranjbar, J Milimonfared, G.B. Gharehetian: Reliability Comarison of Boost PFC Converter in DCM and CCM Oerating Modes, SPDAM 2008, International Symosium on Power lectronics. [10] MIL-HDBK-217: Reliability Prediction of electronics equiment. 216