Comparisons between CRM and CCM PFC *

Transcription

1 Energy and Pwer Engineering, 13, 5, di:1.436/epe.13.54b165 Published Online July 13 ( Cmparisns between CRM and CCM PFC * Weiping Zhang,Wei Zhang,Jianb Yang 1,Faris Al-Naemi 1 1 Materials and Engineering Research Institute (MERI, Sheffield Hallam University (SHU, Sheffield, UK Lab f Green Pwer & Energy System (GPES, Nrth China University f Technlgy (NCUT, Beijing, China jumb-yang@htmail.cm Received September, 1 ABSTRACT The paper presents detailed cmparisns between CRM (critical cnductin mde and CCM (cntinuus cnductin mde cntrl schemes used fr Bst PFC cnverter. The tw schemes are analyzed and cmpared under the chips f L6561 and UC 3854 which are cmmnly used fr CRM and CCM respectively. Bth schemes are based n multiplier; hwever, the CCM is mre cmplex and needs mre periphery cmpnents which increase the cst. The Bst PFC under CRM is easier t be implemented. Nevertheless, the variable switch frequency makes the system (including the pwer-stage inductr and capacitr hard t design. It seems that the CRM PFC is mre attractive in lw pwer applicatins which nly need t meet IEC61-3- D standard. Sme experiment results are als presented fr the cmparisn. Keywrds: CRM; CCM; PFC; Bst 1. Intrductin * Prject supprted by Natural Science fundatin f China (N The Imprtatin and Develpment f High-Caliber Talents Prject f Beijing Municipal Institutins (N.IDHT1351 In cnventinal AC-DC cnversin, a capacitr fllwing a bridge rectifier is used t derive DC vltage frm the AC pwer surce. With this capacitr, hwever, the input current pulsates. This pulsating current increases the input current harmnics and results in a lw pwer factr less than.64[1]. T reduce the input current harmnics and increase the pwer factr, a high pwer factr technique is desired. Pwer factr crrectin is t make the input t a pwer supply lk like a simple resistr. Therefre, an AC-DC cnverter based n pwer stage f bst cnfiguratin as shwn in Figure 1 has been studied as a high-pwer-factr pre-regulatin circuit []. As shwn in Figure 1, accrding t the current f inductr L, the peratin mdes can be specified as: CCM (cntinuus cnductin mde, DCM (discntinuus cnductin mde and CRM (critical cnductin mde. The critical cnductin mde perates at the bundary f CCM and DCM. In recent years, PFC with different cntrl schemes has been prpsed. Nevertheless, a thrugh cmparisn is seldm reprted. In this paper, a detailed cmparisn between CCM and CRM with cnstant n time is presented including applicatin area, cmpnents selectin and small signal analysis f the entire system. Sme experiments are carried ut fr the experimental cmparisn. The experimental results match the theretical analysis well.. Cmparisn f Cntrl.1. Cntrl Schemes The cntrl schemes f the tw perating mdes fr the identical bst PFC cnverter are depicted in Figures and 3 respectively. Figure illustrates the bst PFC under CRM. Crrespndingly, a bst PFC under CCM is detailed in Figure 3. One pint shuld be made clear that CRM and CCM are nly cncern with the minimum f the input inductr current. CRM means the input inductr current tuches withut maintenance in every switching circle. While, the input inductr current is always abve zer in CCM perating mde. As depicted, PFC under CRM is actually the current cntrl. The current is sensed frm switch and cmpared with the prgramming current. When the input Figure 1. Bst PFC cnverter.

2 J. B. YANG ET AL. 865 Figure. CRM (Critical Cnductin Mde. diagrams f the tw implementatins can be detailed as Figures 4 and 5 [3,4]. As shwn, G 1 (s is fr the variatins in the utput f cmpensatr t variatins t the utput f multiplier. G (s in Figure 4 is the rati f utput variatins f the multiplier t the inductr current variatins. G ci (s in Figure 5 means the rati f variatins f input current t the variatins f the utput f current cmpensatr. The means f ther blcks are discussed in details in [4,5]. Obviusly, there is ne mre lp in the CCM PFC under average current cntrl. This increases the cmplexity. The inner lp shuld be decided befre design the uter lp. Hwever, as the input current is cntrlled directly, the distrtins f the input current are much better than the CRM PFC. The lp gain withut the cmpensatr f the bth cntrl scheme have just ne ple [5,6]. This decreases the cmplexity f design f the cmpensatr. The cmpensatr can have the cnfiguratins and the frequency respnses simulated by Matlab are presented in Figures 6 and 7. Figure 4. Cntrl blck f CRM. Figure 3. CCM (Cntinuus Cnductin Mde. current equals t the prgramming signal, the switch will be turned ff. The switch turned n signal cmes frm a ZCD (zer current detectin functin blck. This ZCD blck will send a turned n signal t switch when the input current reaches zer. Therefre, the input current will tuch zer in every switch cycle. In cntrast, the PFC under CCM is the average current cntrl and has different cnfiguratin. The input current is sensed frm inductr instead f the switch. This sensed current is still cmpared with the prgramming signal; hwever, a current errr amplifier rather than a cmparatr is emplyed here. Thus, the errr signal is amplified and cmpared with a PWM generatr. It is clear that there is ne cmpensatin netwrk in CRM PFC; nevertheless, tw cmpensatrs exist in the CCM PFC. Mre cmpnents are needed in the average current CCM cntrl scheme. This increases the cmplexity and cst f the CCM average current cntrl... Cntrl Blcks Accrding t Figures and 3, the small signal blck Phase (deg Magnitude (db Figure 5. Cntrl blck f CCM. Bde Diagram Frequency (HZ Figure 6. One-zer, tw-ple cmpensatr.

3 866 J. B. YANG ET AL. Bde Diagram Magnitude (db Figure 8. Input current f CRM. -8 Phase (deg Frequency (HZ Figure 7. One-ple cmpensatr. Bth f the cmpensatrs are suitable fr the CRM and CCM PFC except that the CCM PFC with average current cntrl has tw lps and tw cmpensatrs. This makes the CCM PFC mre cmplicated t design. The CCM design prcess is detailed in [7]. The CRM PFC is designed in detail in [8]. 3. Cmparisn f Wavef As discussed in sectin, the switch f CRM bst PFC is turned n when the inductr current reaches zer and turned ff when the inductr current equals t the prgramming signal. Therefre, the envelpe f the input current is the rectified AC line signal since the prgramming signal is derived frm the rectified AC line vltage. Things turn t be different in the CCM PFC. As a large gain amplifier is implemented fr the input current signal, the input current is frced t fllw the prgramming signal which is als derived frm the rectified AC line vltage. As a result, the average value f input current will fllw the prgramming signal [7]. The inductr wavef and duty circle are presented in Figures 8 and 9. As shwn, the f input current fllws a rectified AC line signal under CRM PFC. Furthermre, the average input current equals t half f the prgramming signal. The average f the input current under CCM PFC equals t the prgramming signal. S the CRM PFC is nt suitable fr the high pwer applicatins. Fr pwer applicatins higher than 3w, CCM PFC is widely used. Fr pwer applicatins lwer than 3w, CRM PFC is mre ppular [1]. There are actually tw cnstants in bth implementatins respectively. In CRM PFC, when switch is n: vt i n R4 Rs kvi k1kv i (1 l R R 3 4 Figure 9. Input current f CCM. where Vi is the input vltage, Rs is the sensing resistr, l is the input inductr and R3, R4 refers t the Figure ; k is utput f vltage amplifier (E/A in Figure which is nt change under stable state. T n is the n-time f the R4 switch. k1, thus, R R 3 4 T kkl 1 n ( Rs When the pwer stage circuit is determined, Tn is determined t. As a result the turned n time f switch f CRM PFC is cnstant. This is quite different with the CCM PFC. Fr the average current cntrl with UC3854; the switch frequency is cnstant [9]. As fr Bst PFC cnverter, V 1 1 (3 vin 1 d Tn 1 T Therefre, fr CRM bst PFC, the switch frequency will be minimum when input vltage reaches its. The max frequency happens when the input vltage ges acrss zer. The minimum switch frequency f the CRM PFC can be btained as, [8] f min 1 V V V LP V in in, ( in, where P in is the input pwer, V in, is the RMS value f the input vltage, V is the utput vltage. The variable frequency will bring a little cmplexity in design f the (4

4 J. B. YANG ET AL. 867 circuit. This will be discussed next. 4. Input Inductr Selectin The selectins f cmpnents f CCM PFC circuit have been prvided in [7]. The apprach f selectins is almst the same with CRM PFC circuit except the input inductr (L in Figure 1. Fr CRM PFC, the minimum switch frequency dminates the design f the input inductr. Mathematically manipulating f (4, the input inductr can be btained as. L max 1 V V V f P V min in in, ( in, V in, is the value f the lw input line vltage. Such as 9 v when the input vltage is required between 9 6 v. The input current distrtin is the mst imprtant cnsideratin fr the design f input inductr f CCM bst PFC. When the pwer is cnstant, the input current will be higher fr the lw input vltage (fr universal input range. Thus, I in,max in(min, (5 P in (6 v I in,max means the maximum input current. The t- ripple current is assumed t be % f the input current. Refers t (3, the input inductr can be btained as, vin(min, D L fsi (7.5 vin(min, ( V vin(min, fpv s in where V in(min, is the f the lw input line vltage. P in is the input pwer. V is the utput vltage and f s is the switching cycle. As discussed, the selectin f the inductr fr CRM is mainly cncerns the minimum switch frequency. Whereas, fr CCM, the input inductr selects frm the maximum -t- ripple current. 5. Experimental Results The perfrmances f the tw implementatins are verified with tw 1w prttypes. The CRM PFC is carried ut by L6561 and the CCM PFC is implemented with UC3854. The utput vltage is set t be 36v. The results are shwn belw. As shwn in Figure 1, Bth f the perating mdes have almst the same pwer factr. The pwer factr f CCM is.98 which is a little higher than the ne f CRM, which is.96. The efficiency f CCM PFC is 84%, while the efficiency f CRM PFC is abut 8%. As the input current f the input current perates in critical cnductin mde, the current falls t zer nce during every switching cycle as shwn in Figure 11. This cause the current distrtin f CRM is mre (a CRM: input vltage: vac; current:.57 ma (; pwer factr:.96 (b CCM: input vltage: vac; input current:.54 ma (; pwer factr:.98 Figure 1. Input wavef. (a Switching cycle (b Line frequency Figure 11. Input inductr current f CRM (L in Figure.

5 868 J. B. YANG ET AL. (a CRM: v (-t- during each switch cycle and nly has a rectified sinusidal envelpe. Whereas, the inductr current is almst the same with the input current except sme high harmnics. 3CCM PFC has the cnstant switch frequency and variable turned-n time. While CRM PFC has cnstant switch turned-n time, variable switch frequency which makes the design f pwer stage cmpnents mre cmplicated. 4 CRM PFC has nly ne cntrl lp and needs fewer external cmpnents which is easy t be implemented. There are tw cntrl lps in the CCM PFC. the CCM PFC needs mre external cmpnents which will increase the csts, hwever, the input current f CCM PFC has fewer harmnics and higher efficiency. Bth PFC perating mdes have their wn advantages and disadvantages. Selectin is depending n the applicatins. (b CCM: 15 v (-t- Figure 1. Output vltage ripple. serius than the CCM PFC, which leads t a lwer efficiency. Besides, the CRM PFC requires EMI filters t eliminate the high frequency harmnics f the input current which further reduces the efficiency and cmplicates the design. Figure 1 shws the utput vltage ripples. The t- value f the vltage ripple f CRM PFC is abut v, and the cunterpart f CCM is abut 15v, which is 5 v lwer. It seems that the CCM PFC is a better chice than CRM PFC with respect t the pwer factr and efficiency. Hwever, there are tw cntrl lps which need t be design in the CCM PFC. This increase the cmplexity. 6. Cnclusins Bst PFC cnverters under CRM and CCM are cmpared with each ther in this paper. Sme cnclusins can be btained as: 1 The inductr current f the CRM PFC is twice higher than the average input current, which makes the CRM PFC mre suitable fr lw pwer applicatins (lwer than 3 w. The CCM PFC dminates the high pwer applicatins (higher than 3 w. The inductr current f CRM PFC reaches zer REFERENCES [1] X. W. Ma and D. W. Zhu, The Cntrl IC, Principles and Applicatins f PFC, 7, 1st editin, China Electric Pwer PRESS [] R. Keller and G. Baker, Unity Pwer Factr ff-line Switching Pwer Supply, in IEEE INTELEC recrd, 1984, pp [3] W. P. Zhang, F. Chen, X. S. Zha and Y. C. Liu, A Discrete Mdeling fr Pwer Factr Crrectin Circuit, PEDS 9, pp [4] K. I. HWu, C. H. Wu and C. F. Chuang, Develpment f AC-DC Cnverter fr Labratry Pwer Amplifier, PEDS 9, pp [5] W. P. Zhang, The Mdeling and Cntrl f Switch Cnverter, M, first editin, [6] J. Sun, R. M. Bass, Mdeling and Practical Design Issues fr Average Current Cntrl, in Cnf. Rec, APEC Furteenth annual, 1999, pp [7] P. C. TODD, UC3854 Cntrlled Pwer Factr Crrectin Circuit Design, UNITRODE applicatin nte, U134 [8] S. L. Xie, L. F. Liu and J. B. Liu, 1 W Pwer Factr Crrectin f Basing On L6561, AIMSEC 11, nd cnference, pp [9] S. L. Liu and Y. G. Yan, UC3854A/B Pwer Factr Bst Cntrllers Prvide the Imprvements t UC3854 and Pwer Limiting with Sinusidal Input Current fr PFC Frnt Ends, Electrnic Cmpnents Applicatin, Vl. 3, N. 1, Mar 1, pp [1] J. W. Kim, S. M Chi and K. T Kim, Variable On-time Cntrl f the Critical Cnductin Mde Bst Pwer Factr Crrectin Cnverter t Imprve Zer-Crssing Distrtin PEDS 5, pp