Voltage collapse in low-power low-input voltage converters - A critical comparison

Transcription

1 Proeedgs of the 6th WSEAS/IASME Int. Conf. on Eletri Power Systems, High oltages, Eletri Mahes, Tenerife, Spa, Deember 6-8, oltage ollapse low-power low-put voltage onverters - A ritial omparison MOHAMMADEZA AFIEI, TEYMOO GHANBAI, And AHMAD DAABI Faulty of Eletrial and obotis Engeerg Shahrood University of Tehnology Shahrood IAN rafiei@ieee.org, teymoor_ghanbari@yahoo.om, darabi_ahmad@hotmail.om Abstrat- Low power onverters with low put voltage are vestigated this paper. Analysis and design of these onverters are today of quite importane beause of their widespread appliations. A key pot suh appliations is the need for onverters with speifi features that is small size, high effiieny, and low ost. Furthermore, suh onverters we are usually onerned with some other important issues regardg the onverter s operation that is the startup regime and the voltage ollapse phenomena. In this paper we maly fous on voltage ollapse issues, a onept reently trodued to the literature. In this regard, besides providg omparative studies between Boost, Buk-Boost, and Flybak onverters, a new topology improvg voltage ollapse is proposed. Extensive simulation results are also presented to ompare and prove the superiority of the suggested topology. Key Words: oltage ollapse, Fuel ells, Low power onverters, d/d onverters, Boost onverter Introdution Nowadays, low power onverters are widely used various portable eletroni devies suh as mobile phones, alulators, portable media systems, et. In some ases, due to eonomial issues, the ma supply of the onverter is either a sgle fuel ell (or sgle solar ell), or a sgle thermoeletri devie. Owg to high prie of the thermoeletri devies, when used, they are usually implemented as a sgle element as well. Individual fuel ells naturally yield a low voltage, typially with peak voltage less than v for no load and around v for full load onditions. That of ourse results very lower put voltage for the onverter use. In almost all ases we usually must onsider some additional important issues that are effiieny, size, weight, and ost. As for effiieny, onsiderg the fat that the range of the put power suh devies is usually low, even a low rate of loss is ruial. As for size and weight, both obviously need to be mimized for any smart design. The last issue refers to mimum ost requirement that requires a design with mimum number of swithes and omplexity. The ma funtion of a DC-DC onvener suh appliations is to produe a stiff put voltage from a non-regulated and low put voltage. Low voltage onverters have reently found muh attention the literature []-[6]. There are some well-known basi topologies suh as Boost, Buk-Boost, and Flybak onverters an potentially be used for suh appliations thanks to their simpliity and ability to boost the put voltage. So far, many topologies have been vestigated for low voltage appliations [],[5],[6],[3]. For example, [3] appliation of a thermoeletri devie with put voltage around 300 m is trodued. On the other hand, the voltage ollapse onept is fairly new to the field, and only few papers have addressed the subjet the reent years. In [] voltage ollapse a Boost Converter has been trodued. In that referene an analytial approah to predit the voltage ollapse was also presented. In this paper we present an extensive omparative study on various basi topologies suh as Boost, Buk-Boost, and Flybak onverters. The same method appeared [] is extended to evaluate the other topologies terms of voltage ollapse problem. An enhaned Boost topology with improved safe operatg region preventg ollapse, suitable espeially for low power appliations, is also presented and vestigated.

2 Proeedgs of the 6th WSEAS/IASME Int. Conf. on Eletri Power Systems, High oltages, Eletri Mahes, Tenerife, Spa, Deember 6-8, oltage Collapse Phenomena This onept has reently been addressed the field of low power d/d onverters []. In simple words, when the load of d-d onverter supplied by a low voltage soure, is signifiantly high (exessive put power), it naturally reases the put urrent of the onverter. That turn an rease the ondution loss of the onverter dramatially. Havg a low effiieny under suh put power situations, results signifiant redution of the put voltage. That ultimately auses voltage ollapse. In pratie, beause of the exessive urrent, the put voltage redues too muh that annot even drive the ontrol iruit of the onverter. As an example, Figure depits the voltage ollapse phenomena for a typial Boost onverter when the normal put voltage is redued to 0.3 at the time t=0. s and auses signifiant drop the put voltage. As an itial study, we assume that the synhronous Boost onverter shown Figure is operatg under Contuous Condution Mode (CCM) ondition. The put urrent always flows through the dutor and one of two Time (s) Fig.: Typial voltage ollapse Boost Converter Fig.: Synhronous Boost onverter MOSFETs as well as part of the prted iruit board traes. This total d resistane produes a power loss equals. I. where I is the put urrent of the onverter. There is also another loss aused by the equivalent series resistane ( ES ) of the apaitor. In this regard, we an approximate the squared rms value of the apaitor urrent as: I = I (I -I ) () Where I is the d put urrent and I is the rms value of the apaitor urrent. The onverter s swithg loss an also be approximated as []: off. I on. t swith W swith = () Where W swith is the energy loss per swithg yle for eah swith and off is the swith s off- state voltage. Also, I on is the on state urrent and t swith is the swithg time (the sum of turn on and turn off times). Hene, the swithg loss an be simplified as: P = KI (3) lossswith g Where K = f.. t. Based on the above swithg equations, the power balane equation for the iruit is represented as follows: ES I I = P KI I P oh (4) Poh is the power loss assoiated with the ontrol iruit, and P is the onverter s put power. Solvg above equation for I results the followg quadrati equation []: I K ES P I P oh (5) P P ES = 0 oots of equation (5) predit the voltage ollapse. If the roots are omplex, that implies that there is no physial response for the put urrent and therefore the voltage ollapse has ourred []. On the other hand, the dutor s loss is usually the major part of the total loss; therefore the dutor design needs signifiant attention. With a good design of the iruit s lay, it is also possible to redue the overall value of. As addressed [3], most pratial onverters usually utilize high-power MOSFETs as their ma swithg devies with low on-state resistane and low gate urrent. By reasg the swithg speed of MOSFETs, the d esistane and its assoiated loss are dereased. For many appliations, the rate of the swithg frequeny is around 00 KHZ (medium frequeny). And it is also possible to fd MOSFETs with low d resistane. Hene, the swithg losses for suh levels of frequeny not signifiant, so there is no need to

3 Proeedgs of the 6th WSEAS/IASME Int. Conf. on Eletri Power Systems, High oltages, Eletri Mahes, Tenerife, Spa, Deember 6-8, employ soft swithg tehniques. Soft swithg an rease the onverter s ondution loss and may even ause voltage ollapse. Sometimes, P oh and the loss trodued by ES are more signifiant ompare with other losses. Besides preditg the voltage ollapse, equation (5) provides some other useful formation on how to hoose MOSFETs and Indutors with other passive omponent. Where a low power DC_DC onverter is use, for example a mobile phone, the ollapse an easily our at erta low level of the put voltage when the onverter s effiieny rashes dramatially as shown Figure 3. Fig.3: Effiieny versus put voltage To quantify the voltage ollapse predition, the designer must test equation (5) for its roots []. In general, a low enough is required preventg omplex roots for worst ases with highest level of the load power and the lower possible put voltage. 3 Collapse Buk-Boost and Flybak In this setion, we drive similar equations for Buk- Boost and Flybak onverters to make a omparison between them. A typial synhronous Buk-Boost onverter supposed to operate CCM mode is shown Figure 4. In the first subterval of eah swithg period, the urrent I flows through the dutor together with one of the swithes, whereas the urrent flows through dutor and the other swith the seond subterval. Similar to the Boost onverter we have: I = I ( I I ) (6) Fig.4: Buk-Boost onverter 3 Fig.5: Flybak onverter Usg similar equation used for the Boost onverter we obta: P = D.. I D..( I I ) (7) Where D = D and P is the loss assoiated with the equivalent resistane. The orrespondg power balane is governed by (8): D.. I ( ( K ES D. ). P ). I (8) Poh P ES ( P ) = 0 A similar equation an also be developed for Flybak onverter shown figure 5. If N & and N & are the turn numbers and resistanes of the transformer wdgs respetively, then we defe: = N (9) N = N N (0) where N is the transformer s turn ratio. Then, the orrespondg power balane is derived as follows: I = D I i D ( I I ) () KI P I P P ES oh Eq. () an be simplified to (). This equation is used to predit the voltage ollapse. DI (( D ) P ES K ) I () P Pore Poh ES ( P ) = 0 As appears, the turn ration N orporates oeffiients of () and so affets the voltage ollapse risk It is also worth mentiong that as beause the Boost onverter is an trsially unstable topology, usg Flybak topologies an relief the problem with just proper designg the duty yle and the turn ratio N. 4 An Enhaned Boost Topology Notg the advantages of the Flybak topology terms of voltage ollapse and stability as addressed the previous setion, a new Boost type topology equipped with transformer is proposed. Figure 6.a ore

4 Proeedgs of the 6th WSEAS/IASME Int. Conf. on Eletri Power Systems, High oltages, Eletri Mahes, Tenerife, Spa, Deember 6-8, Fig.6: Boost onverter with self-drive a) Ciruit b) the transformer s model shows the proposed topology. The operation on this iruit is quite similar to the lassi one. In addition, its self-drive feature an signifiantly redue the swithg and therefore total loss of the onverter. It also retas most advantages of the Flybak ludg the stability features. That is beause it an usually operate at lower duty yles ompare with the lassi Boost Converter. Figure 6.b shows the iruit equivalent of the transformer used where L m is the magnetizg dutane. It is easy to show that the equivalent dutane seen between termals a and, whih exatly take role the onverter s operation is: N N L = ( ) * l m (3) N A similar approah gives the relation between the put and put voltages steady-state as governed by (4 ). That equation learly desribes how for any given D, the onverter s ga is greater than the lassi one. Hene, it an be set for lower duty yles. N = ( )( D) (4 ) D N Steady state operation of this topology is similar to the onventional boost onverter. I flows through the first subterval, and then through and next subterval, where = N, and and are the primary and seondary resistanes of the transformer s wdgs respetively. With adequate seletg the turn ratio N, it is possible not only to boost the put voltage up to the required 4 level, but also to edue the loss. For this onverter, the power balane equation an be presented as follows: P = D D ( N D I (5) ( ) ) N Substitutg P by I P ( D ( N ) ) I ( ( ) ) K ES I N Poh P Pore ES ( P ) equation (5) results : (6) Comparg (6) with (5), reveals that due to derease the oeffiient of I, we now naturally expet ourrg ollapse for higher levels of the put power P. That means that the risk of ollapse is redued. 5 Simulation esults and Comparison To evaluate and ompare the topologies presented the previous setions with respet to voltage ollapse problem, a set of low power DC-DC onverters for mobile phone appliation is onsidered. The onverters are supposed to be supplied by a sgle fuel ell unit. 5. Converter s parameters Most of the onverters parameters are taken based on []. The onverters maximum put power is.5w and the put voltage must be kept at 4, while the put voltage ranges between and. (see fuel ell harateristis shown figure ). The other parameters are as follow: -The on-state resistane of the MOSFETs is 4 m Ω -The swithg frequeny is set at 00 khz -The dutor s apaity for Boost and Buk-Boost Converters is 0 µ H with 0 mω ternal resistane -The apaitor s apaity is 50 µ F with a high ES exeedg 3 mω. - K is equal to P oh is supposed to be equal to 0 mw whih is signifiant for this study. - P ore that is the transformer s loss equals 0mW 5. Boost, Buk-Boost, and Flybak onverters The effiieny performane of the Boost, and the Buk-Boost Converters under study are presented figure 7. These urves learly explore the signifiant drop the onverter s effiieny ours under high load situations, whih means the voltage ollapses. Also, as seen, the Boost topology features a slightly higher effiieny than the Buk-Boost one for high

5 Proeedgs of the 6th WSEAS/IASME Int. Conf. on Eletri Power Systems, High oltages, Eletri Mahes, Tenerife, Spa, Deember 6-8, powers and low put voltages. The simulation results of the Flybak Converter for different turn ratios are also urved figure 8. It an easily be reognized that the voltage ollapse risk is improved ompare to the other topologies. Also, the results demonstrate impat of the turn ratio N on the effiieny and n y e e boost onverter:=4v =v =v=.v =v =v P(w) buk-boost onverter:=4v =v =v =v =.v =v P(w) Fig.7: Effiieny versus put power a) Boost onverter b)buk-boost onverter therefore ollapse risk. Turn ration N fluene both the loss and put voltage of the onverter. The figure also explas how the Flybak onverter is of higher effiieny with a flatter urve. 5.3 New Boost onverter Assumg P ore = 0mW, the new Boost Converter looks to have an overall performane lose to the Flybak, the fat is shown by Figure 9. However, Flybak has higher effiieny and so the lowest risk of ollapse off ourse for high put powers, whereas the suggested Boost onverter features higher effiieny for low powers. For example, as seen figures 8 and 9, the Boost topology has apparently higher effiieny than the Flybak one for put powers less than W. Hene, that is quite an terestg iruit espeially for low power appliations. For further evaluation, the effiieny versus put voltage for the aforementioned onverters ludg the new Boost topology are represented figure 0. In this figure, improvement terms of 5 voltage ollapse is apparent. For a given P = W the onventional Boost Converter ollapses at put voltage equals 0.3v. It is terestg that the voltage ollapse ours at for the Flybak onverter. Aordg to figure, the Flybak Converter may look superior at P=W, as it ollapses at lower put voltage. However, it would not be true for suffiiently lower powers as already disussed. Complete omparison of the Flybak and the proposed Boost Converters an be arried by omparg the orrespondg the oeffiients appear equations (6) and (_) flybak onverter:=4v & n=4, n=8 = =v = =.v =v P(w) 5 5 flybak onverter:v=4v & n=, n=8 = = = = =.v P(w) Fig. 8: Effiieny versus put power for Flybak Converter, N=8 a) N=4 b) N= boost onverter with suggested topology =v =v =.v =v =v P(w) Fig.9: Effiieny versus put power for the Boost Converter with suggested topology 5.4 Fuel ell harateristis As another issue needs suffiient attention. Considerg the -I harateristis of a typial fuel

6 Proeedgs of the 6th WSEAS/IASME Int. Conf. on Eletri Power Systems, High oltages, Eletri Mahes, Tenerife, Spa, Deember 6-8, boost, - buk-boost 3- flybak 4-boost with suggested topology data data data3 data (v) Fig.0: The effiieny versus put voltage for different onverters Fig. : -I harateristi of a typial fuel ell boost onverter with modeled FC P(w) flybak onverter (n=, n=8 ) with modeled FC P(w) Fig.: Effet of mismath between fuel ell and onverter. a) Boost b) Flybak ells (FC) shown figure, there are problems may arise due to signifiant mismath between the fuel ell and the onverter harateristis. Figure says ab 6 a dramati ollapse may our even at low power levels when the FC is workg beyond its resistive region. Therefore, designg an adequate operatg pot for the fuel ell is quite important. 6 Conlusion In this paper, voltage ollapse issues of some basi DC-DC onverters supplied by low voltage fuel ells was analytially studied and ompared. It was shown that the Flybak onverter potentially demonstrates better performane terms of voltage ollapse when ompared to Boost and Buk-Boost onverters. A modified self-drive Synhronous Boost Converter omprisg higher effiieny and lower risk of ollapse was proposed and studied. Through extensive numerial and analytial studies, it was shown that the proposed onverter, besides havg an improved performane with respet to voltage ollapse, provides higher effiieny espeially for low powers with a flat urve over a wide range of put power. Further studies an be arried to improve the existg DC- DC onverters with lower risk of ollapse and higher effiieny. eferene: [] J. W.Kimball, T. L. Flowers, and P. L. Chapman, Low-put-voltage, Low-power Boost onverter design issues, IEEE Power Eletronis Letters, ol., Issue 3, Sept. 004 pp [] M. Saidani, A.Meyer, and M. A. M. Gijs, Hybrid MEMS dutor for miaturized autonomous power onverter, IEEE 6-th Annu. Conf. on Miro-Eletro- Mehanial Systems, Kyoto, 003, pp [3] M. Saidi and M. A. M. GUS, "Cubi millimeter power dutor fabriated bath wafer tehnology," Journal of Miro-eletromehanial Systems, vol., No., pp. 7-78, 003. [4] J. M. Damashke, Design of Low-put-voltage onverter for thermoeletri generator, IEEE Transations on Industry Appliations, ol. 33, Issue 5, Sept.-Ot. 997, pp [5] X. Duan, H. Deng, N. X. Sun, A. Q. Huang, and D. Y. Chen A high performane tegrated boost dd onverter for portable power supply, Center for Power Eletronis Systems, 004. [6] M. Shepard and. C. Williamson, "ery lowvoltage, power onversion," IEEE International Symposium on Ciruits and Systems, olume 3, 6-9 May 00, pp