DC-DC Switch-Mode Converters

Transcription

1 - Swich-Mde nverers - cnverers are used : egulaed swich-mde pwer supplies, nrmally wih HF elecrical isla Mr drives, nrmally wihu an isla ransfrmer We will lk a he w basic dc-dc cnverer plgies: Sep-dwn (buck) cnverer Sep-up (bs) cnverer We will als cver he buck-bs cnfigura which is a cmba f he basic plgies. Fally we will als lk a w islaed dc-dc cnverer plgies: Frward cnverer derived frm buck cnverer Flyback cnverer derived frm buck-bs cnverer A vlage Uncnrlled ide ecifier unregulaed Filer apacir unregulaed - nverer regulaed ad v cnrl Basic ncep f swich-mde regula nrller vu v reference

2 sla bundary A vlage EM filer ecifier plus Filer unregulaed ecifier plus Filer u HF ransfrmer Gae rive nrl Opcupler PWM nrl E/A - + v ref Schemaic f a swich-mde dc pwer supply shwg dc-dc cnverer wih isla and feedback cnrl

3 Sep-wn (Buck) nverer i S i v d v (lad) v d n ff v - - v di d di d v i ΔQ v dv i d id + K 3

4 n ur iial analysis f he buck cnverer, we assume ha: he upu capacir is large such ha is cnsan nducr curren is cnuus nsiderg seady-sae wavefrms fr buck cnverer: dd 0 v 0 0 v v Nw, 0 n v d vd d d herefre, We can herefre cnrl by cnrllg duy-cycle. Neglecg pwer lsses wih he dc-dc cnverer (i.e. assumg ideal cmpnens): P P where is he average pu curren and. Buck cnverer is equivalen a dc ransfrmer Peak-peak Analysis fr ducr Δ pp and curren Δ pp ripple is given by: n di di Δ pp d d pp ( ) d Δ 0 n d Peak-peak upu vlage ripple is given by: Δ and pp ΔQ Δ pp ( Δ pp )( ) Δ pp ( ) 8 f s 8 8 4

5 iscnuus nduc Mde (M) Bundary beween M & M crrespnds wih he cndi when ducr curren jus uches zer: Δ pp ( ) f s ( ) he fllwg wavefrms apply his case: n seady-sae, v 0 and v sill apply: d v d + Δ - v Δ + Δ - v d fd we can assume zer cnverer lsses: Δ P P i Δ i max imax herefre we have: ( ) f s fd we need slve his quadraic equa. 5

6 Frward nverer (wih sla ransfrmer) i : : n i A v A B v v v v (lad) v B 3 v E v 3 : : n i B i n BJ ON: m A i 3 ideal ransfrmer ransfrmer equivalen circui v A and herefre v n. is FB and is B. v n Auxiliary wdg B develps v B > 3 is B wih v 3 npu curren i has w cmpnens: magneizg curren i m and i ni. Magneisg curren i m creases a a rae f / m wih BJ n. BJ OFF: A BJ urn-ff, wdg A develps emf maa magneisg curren i m. his emf is als duced wdg B and secndary. 3 becmes FB and cnducs when v B reaches. Magneisg curren i m is ransferred wdg B which has he same magneisg ducance as wdg A and BJ curren falls zer. While 3 cnducs A - and v E. Als v -n and is B. cnducs allw freewheelg f curren. Magneisg curren, i m, wdg B decreases a a rae f - / m. Magneisg curren MUS reurn zer durg each perid avid cres saura > BJ duy cycle mus n exceed 50%. Behaviur f cmpnens,, and same as buck cnverer. ils A and B are bifilar (wund geher) mimise leakage ducance. issipaive snubber is n needed and cil B reurns magneic energy supply avidg energy lsses. 6

7 Wavefrms fr Frward nverer M i i i ni v E i m (A) i m (B) v 3 v n + n v n 7

8 M Wavefrms () - reurns zer afer i m i i ni i m (A) i m (B) v E v 3 v n + n v n 8

9 M Wavefrms () - reurns zer befre i m i i ni i m (A) i m (B) v E v 3 v n + n v n 9

10 Sep-up (Bs) nverer i d i v v S S v d (lad) Assume: is very large such ha upu vlage ripple is very small and can be assumed negligible nverer peraes cnuus-cnduc mde urg n : S is ON and is reverse-biased: i v creases a a rae f / and energy sred creases. urg ff : When S is urned OFF, dide urns ON prvide a pah fr leadg energy ransfer he lad: i v n seady-sae, energy crease durg n mus balance u wih energy decrease durg ff wih a crrespndg decrease durg ff > seady-sae, upu vlage MUS exceed

11 urg n : urg ff : i i i v v v S n seady-sae, v 0 v - ( - ) n ff v and S ( ) varies frm as varies frm 0 ( ) i + i Prvided ha > : - ΔQ Δ Δ Q Δ Δ Neglecg pwer lsses cnverer, we have and herefre: ( )

12 iscnuus nduc Mde (M) Bundary beween M & M crrespnds wih he cndi when ducr curren jus uches zer: 0 and, herefre ( ) ( ) his implies ha maximum lad resisance fr pera M is f s given by ( ) he fllwg wavefrms apply fr M: - ( - ) v S v i Δ We need ba an express fr rder derive relaship beween and : ( )( + Δ) i ( + Δ) Frm v 0 we have Δ Δ herefre ( ) ( ) - +

13 Neglecg pwer lsses cnverer, we have and herefre: givg: 0

14 Buck-Bs nverer i s S i d v v d (lad) i his cnverer is a cmba f he buck and he bs cnverer plgies. he upu vlage can be higher r lwer han, bu i has a negaive plariy relaive. his plgy fds applica swich-mde pwer supplies. Assume: is very large such ha upu vlage ripple is very small and can be assumed negligible nverer peraes cnuus-cnduc mde urg n : S is ON and is reverse-biased by +. v i creases a a rae f / and energy sred creases. urg ff : When S is urned OFF, dide urns ON prvide a pah fr leadg energy ransfer he lad. decreases a a rae f / i d v v d i

15 urg n : urg ff : i d v v i i c v d i v - n ff n seady-sae, v 0 ( ) and varies frm 0 as varies frm 0 Fr: 0 0.5, 0 i 0.5, - ΔQ Prvided ha > : Δ Δ Q Δ Neglecg pwer lsses cnverer, we have :

16 iscnuus nduc Mde (M) Bundary beween M & M crrespnds wih he cndi when ducr curren jus uches zer: 0 and, herefre ( + ) ( ) ( ) his implies ha maximum lad resisance fr pera M is f s given by ( ) elaship beween and M n M, Neglecg pwer lsses cnverer, f s

17 he fllwg wavefrms apply fr M: v - Δ i - v S + v d +

18 Flyback nverer (wih sla ransfrmer) i p : n i s i s v p v s p v n v E BJ ON: v p and herefre v s n. is B wih v n and herefre n curren can flw secndary wdg. i p creases a a rae f / p wih BJ n. BJ OFF: A BJ urn-ff, primary wdg develps emf maa curren i p. A crrespndg emf is duced he secndary. When v s reaches, dide urns n and a secndary curren i s flws maa he cre flux: i s iial i p fal n Secndary curren i s falls a a rae f - / s Magneic energy sred cre is ransferred (parially M) lad and we d n need a demagneizg wdg as frward cnverer avid cre saura. n his case, we jus need ensure ha i p des n exceed he value crrespndg maximum cre flux densiy.

19 Flyback nverer Wavefrms fr M i p p p i s s s v S +( /n) v d n + n v p - /n v s n - Wavefrms abve drawn fr n 0.8

20 Flyback nverer Wavefrms fr M i p p i s s v Sw +( /n) v d n + n v p - /n v s n - Wavefrms abve drawn fr n 0.8