The Flyback Converter - Lecture Notes | ECEN 4517, Study notes of Electrical and Electronics Engineering

Download The Flyback Converter - Lecture Notes | ECEN 4517 and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! The Flyback Converter Lecture notes ECEN4517 ! Derivation of the flyback converter: a transformer-isolated version of the buck-boost converter ! Typical waveforms, and derivation of M(D) = V/Vg ! Flyback transformer design considerations ! Voltage clamp snubber Derivation of the flyback converter The flyback converter is based on the buck-boost converter. Its derivation is illustrated in Fig. 1. Figure 1(a) depicts the basic buck-boost converter, with the switch realized using a MOSFET and diode. In Fig. 1(b), the inductor winding is constructed using two wires, with a 1:1 turns ratio. The basic function of the inductor is unchanged, and the parallel windings are equivalent to a single winding constructed of larger wire. In Fig. 1(c), the connections between the two windings are broken. One winding is used while the transistor Q1 conducts, while the other winding is used when diode D1 conducts. The total current in the two windings is unchanged from the circuit of Fig. 1(b); however, the a) b) + – L – V + Vg Q1 D1 + – L – V + Vg Q1 D1 1:1 c) d) + – LM – V + Vg Q1 D1 1:1 + – LM + V – Vg Q1 D11:n C Fig. 1. Derivation of the flyback converter: (a) buck-boost converter, (b) inductor L is wound with two parallel wires, (c) inductor windings are isolated, leading to the flyback converter, (d) with a 1:n turns ratio and positive output. 2 current is now distributed between the windings differently. The magnetic fields inside the inductor in both cases are identical. Although the two-winding magnetic device is represented using the same symbol as the transformer, a more descriptive name is “two- winding inductor”. This device is sometimes also called a “flyback transformer”. Unlike the ideal transformer, current does not flow simultaneously in both windings of the flyback transformer. Figure 1(d) illustrates the usual configuration of the flyback converter. The MOSFET source is connected to the primary-side ground, simplifying the gate drive circuit. The transformer polarity marks are reversed, to obtain a positive output voltage. A 1:n turns ratio is introduced; this allows better converter optimization. Analysis of the flyback converter The behavior of most transformer-isolated converters can be adequately understood by modeling the physical transformer with a simple equivalent circuit consisting of an ideal transformer in parallel with the magnetizing inductance. The magnetizing inductance must then follow all of the usual rules for inductors; in particular, volt-second balance must hold when the circuit operates in steady-state. This implies that the average voltage applied across every winding of the transformer must be zero. Let us replace the transformer of Fig. 1(d) with the equivalent circuit described above. The circuit of Fig. 2(a) is then obtained. The magnetizing inductance LM functions in the same manner as inductor L of the original buck-boost converter of Fig. 1(a). When transistor Q1 conducts, energy from the dc source Vg is stored in LM. When diode D1 conducts, this stored energy is transferred to the load, with the inductor voltage and current scaled according to the 1:n turns ratio. a) + – LM + v – Vg Q1 D11:n C transformer model iig R iC+ vL – b) + – LM + v – Vg 1:n C transformer model iig R iC+ vL – c) + – + v – Vg 1:n C transformer model i R iC i/n – v/n + + vL – ig =0 Fig. 2. Flyback converter circuit, (a) with transformer equivalent circuit model, (b) during subinterval 1, (c) during subinterval 2. t Iyboack transformer design Far “Ais Tab, apn are gen Phe Sllaotis flyback trauslorwer design tusks : «sel Ly such Hat at = SOR of I e use tins rho ne 72 = 1S 1 e use a PQ Ba/a0 core DAL, MET, Ay ane. gree celect turns on, suck That thal less ts wintmizad | 1 mimize Pyar = Pre + Poy ens ms core fess iu veststances, less, of primory ond secondary wn sags + dalermine air 5p Yeng th 8s «deerme. primary aud secondary wie Jauges us fll Lace Ky = ob . + check ty wake sure Hit de peck B dres «st cause fe core A scharctde , Choong te white. ot Vie can wlale Ba. to ny and AL using Ba baste. Somme Ae Lis ae = nA, Bae = Lan AL so AF bi AU n, A 1 ie Once we ave toled Ha camerte, cent 4 fed Be desved volves of Ln and at, dan La > Si, aud A, ave Kuoun . Reuce this penton vleles = Bho a Trereasiig 4, Aeerars Poe, ubich decreases te cove loss Poe» Compathng careless Core loss Py depend s en te peak vale «f He 26 compa + ‘. Flee densihy Bae. Manutnchwers published dats shaks combin ~ pls of F Hat Pillow fuwchens of te farm Pre = Ke at ad, Sea TDR WIC1 fenile, uth, — course website wks ty dete sleds Ke isa cousttat & pre porhauali, thet doped s on surkling ev ond cre mattered p (saw exponent tat depeads on come wearer ia | A fu is He velne of He core Gr wick werd | B= ae _ Cie at Sok Ke ~ 2 at bere to gk to kia wrk ALB, expressed ~« ow Bre expressed 7 Tasha Ne expressad watts 8 B-H bop caused by Currest vigpe al magnet 2004 curreat i) -t power] \Ue esd Prololena * meveasins, D> copper Resistance <f wudwas Wwe. avens Primary wwe avn, Secondary Wwe, Oren Wading, vesicharars Privary twding sesihnass Seeowdor ny winding, reashuce, R= “y loss Au, = % Kwa “ flog = %hua Me Re pm (mcr) =P “he e Me (MET) for ® Ba Se use ds of tums * ty chereare core less , "\ peveasas — veststances of word 3 increase S ‘ wend more tums of smaller, wink tor awd S A,+a, =41 d= dachon of A allecclal ty Primary a= Fmchon af ley albered +p Sheerdarny wake hens yelie ows Se + “1 o sen Pralechiour £ Q using a vothage clamp saubber Srubber OT ssmubler provides a pal Le ip de fhe afer Q bes honed off + bnerany shred Le = z Le . = tle I” is Franserred ty Cy and Hon 13 dsipded Ly Re fhemegp poe = dhe Tf © peck drausishr ve Hage is clamped ty Vt % > Vath DYTs Ts An appreocdh t selecdr R, ad Cs? + Use lange Ss, 36 het utes bas nage ripple Cy >> s DS YlO 2 Vs * Voltooe Ve rises until power drosipaled by Reo ogee to average postr austere fom Le! Vst = 4 Ler, => choose Ry such Het Veo is acceptably tou o Nek Hat Le depends ow onading sponehny aud fs wot Known whl ctmastr wer i umd Brmasure Le vin short cirenct Ast or ques ty wale to. Example - a Sect ~ pass selechm, Rs and Cs Gwen Wy= ISOV | VE ISV nz oad f= oa Lae dnl T= USA WesFeT peak volhage wathng = Yaov Tr ts desived ty linct “peak vp ts 3aS¥ Estiwde bei iw a sed, carchully toound Jrmns former, it way be pssble ho adiee bp= 32 of La = Boat Eneray, ahred iy Lg during Ofte BTS, 3 Wy = ELeTt= (4) (Bemh) (USAY = 33,75, Anensy pover trewslened fem Le fo saulber ; P,= Wel, = (33,253) (lookRe) = aasw To tet peak vy de BO5V, ue weed Va= (peak ve) ~ Ny = 35 ~ ise = 1sV¥ Se choose ge ML Cosy & = 4074 Sh Gzsey) > we wight use a jek Sw resishir, Then Coo Ths (ens) = 40F Rs Gon) ~ A goed choice wight be Cy = HI uF, a50V. The above coleulahons are. Lased on the echwale Les 3m Aha, aud, sheuld be considered Sod - pass eshwales y