Flyback Converter in DCM

Transcription

1 Flyback Converter n CM m 1:n V O V S m I M m 1 1 V CCM: wth O V I I n and S 2 1 R L M m M m s m 1 CM: IM 2 m 1 1 V 1 Borderlne: O VS I n wth V nv 2 1 R 2 L 1 M m s O S m CM f R > R 2n crt 2 L m 2 (1 ) s 32

2 Flyback Converter n CM Operaton V S v m m v Q L m Q 1:n C R v m V O V S s mmax V O / n 1 s m mmax Q mmax / n V V / n S O V S v Q 33

3 Flyback Converter n CM Operaton m s mmax 1 s m max V L S m s s v m V S V O S s 1s V n (1) s V O / n 1 s mmax / n I 1VS 1 s 1 s 2 Lm n V IO R s O (2) From (1) and(2) V V O S R s 2L 34

4 Optocoupler Fundamentals

5 Optocoupler EMI flter 1:n C F CSN v I Z PWM v con V ramp Z 1 Z 2 Optocoupler 36

6 Structure and Characterstcs of Optocoupler Optcal couplng between a GaAs IR LE and a slcon photo-transstor wthn one package ransmsson of the current sgnal from the LE to photo-transstor wth solaton Galvanc solaton of 2.5 kv 6 kv A substantalparastc capactance between the collector and emtter termnals of the photo-transstor 37

7 CR of Optocoupler Current transfer rato CR CR Forward current Optocouplers exhbt a wde dstrbuton n CR value: 0.2< CR < 2.7 he CR s a crtcal desgn parameter for solated feedback controller he CR needs to be expermentally measured. 38

8 Operatng Pont of Optocoupler CR CR he transster current he s predetermned by the steady-state duty rato of the converter. CR curve of an optocoupler s provded by manufacturers. CR CR he parameters CR and are smultaneously determned by and the CR curve of the gven optocoupler. Forward current 39

9 Parastc Capactance of Optocoupler C CB C CE CR A substantalparastc capactance between the collector and emtter termnals of the photo-transstor C C C : Mller capacatnce: 0.2 nf < C 4 nf j CE CB j A photo-transtor can be modeled as a combnaton of current source and C. C j needs to be expermentally measured. j 40

10 CR and Parastc Capactance CR rade-off between CR and C ) A larger CR wder photon-acceptng Base area larger C ) A smaller CR narrower Base area smaller C j For hgh-bandwdth converters, optocouplers wth a large CR are not recommended because ther wll be large. j j 41

11 Measurement of Parastc Capactor Measurement set-up Measured v ( j) / v ( j) c s V V vc () t R C s vs () t Magntude [db] f p 3 db 1 10 Frequency [khz] 100 Adjust R to obtan the predetermned n the actual converter. Measure of parastc capactor C usng v ( j)/ v ( j) plot. f p 1 1 = Cj= 2C R 2 f R j p j c s 42

12 Alternatve Method to Measure f p Measurement set-up V V R C s vc () t vs () t Sweep the frequency of for the frequency where v f p v s ( t) fromlow frequences to hgh frequences and search c 1 1 = Cj= 2C R 2 f R ( t) reduces by 70% from the low frequency value j p 43

13 On-Resstance of GaAs ode V F R Ideal Pecewse lnear model r d Small-sgnal resstance Small-sgnal resstance r r d I = F : 40 < rd< 150 V affects the small-sgnal transfer functon of the feedback crcut. d of thegaas dode For a small-sgnal resstance r less than 50, the dode current should be larger than 0.6 ma. A not-suffucently small r d d 44

14 L431 Fundamentals

15 L431 EMI flter 1:n C F CSN v I Z PWM V ramp Z 1 Z 2 L431 46

16 L431: hree-ermnal Op Amp wth Voltage Reference v v R (reference) v R K (cathode) K 2.5 V A: anode hree-termnal nternally-compensated transconductance amplfer: v n out emperature-compensated bandgap voltage reference Open-collector confguraton R K 47