L1, L2, N1 N2. + Vout. C out. Figure 2.1.1: Flyback converter

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1 page 11 Flyback converer The Flyback converer belongs o he primary swiched converer family, which means here is isolaion beween in and oupu. Flyback converers are used in nearly all mains supplied elecronic equipmen for low power consumpion, up o approximaely 300W. Examples of which are elevisions, personal compuers, priners, ec.. Flyback converers have a remarkably low number of componens compared o oher SMPSs, hey also have he advanage ha several isolaed oupu volages can be regulaed by one conrol circui. Cin V 1 L1, L, V Cou Vou Load Vcon V DS Figure.1.1: Flyback converer Fig..1.1 shows he basic circui of a flyback converer. The ransisor works as a swich, which is urned on and off by he pulse-widh-modulaed conrol volage V con. During he on-ime of he ransisor he primary volage of he ransformer V 1 is equal o he inpu volage V in which resuls in he curren I 1 increasing linearly. During his phase, energy is sored in he ransformer core. During he on-phase he secondary curren is zero, because he diode is blocking. When he ransisor is urned off he primary curren I 1 is inerruped and he volages a he ransformer inver due o Faraday's Law ( v = L di ), he diode conducs and d he energy moves from he ransformer core via he diode o he oupu capacior. During he on-phase of he ransisor he drain-source volage V DS is equal o zero. During he off-ime of he ransisor, he oupu volage V ou will be ransformed back o he primary side and he drain-source volage heoreically seps up o V DS = V in V ou. If a mains volage of 30V/50Hz is used V DS will jump up o approximaely 700V. In pracice his volage will be even higher due o he self inducion of he leakage inducance of he ransformer. To allow for his effec he minimum raed drain-source breakdown volage of he ransisor mus be 800V. The ransformer is no a "normal" ransformer, because is funcion is o sore energy during he on-ime of he ransisor and o deliver his energy during he off-ime via he diode o he oupu capacior. In effec he ransformer is a sorage inducor (ofen called a choke) wih a primary and secondary winding. To sore energy he ransformer core needs an air gap (normal ransformers do no have an air gap). An imporan consideraion for his ransformer is, ha primary and secondary windings are closely coupled o achieve a minimum leakage inducance. I should be noed ha he energy of leakage inducance canno be ransfered o he secondary side and is herefore disipaed as hea on he primary side. C ou

2 page 1 Vcon 1 VDS Vou( ) T V 1 -Vou I I Î secondary curren calculaed o he primary side: I L I I L I L = I Figure.1.: Volages and currens a he flyback converer Design of he flyback converer: For he primary volage of he ransformer V 1 he average V 1 mus be equal o zero for seady sae condiions (if no, he curren will increase o infiniy). This leads o: V in 1 = V ou (T 1 ) and: V ou = V in 1 T 1 The urns raio of he ransformer should be choosen so ha for he raed oupu power he on-ime (energy charge ime) 1 is equal o he off-ime (energy discharge ime) T 1. This leads o he urns raio: = V in V ou

3 page 13 The breakdown volage of he ransisor and he reverse volage of he diode mus be for his case: Transisor: V DS = V in V ou V in Diode: V R = V ou V in V ou I should be noed ha he raed breakdown volage of he ransisor mus be chosen significanly higher, because a he urn-off insan he energy of he leakage inducance L s will no be aken over by he secondary winding. To keep he overvolage in an accepable range a snubber circui is required, see Fig.1.3. A he insan of urn-off he curren of he leakage inducance L s is divered hrough by he diode D and charges he capacior C. The power is dissipaed in resisor R. If R and C are required o operae a 30V AC,, a value of R has o be deermined experimenally o ensure ha he dc volage across C falls wihin he region of 350V o 400V. C R D Ls Vou Figure.1.3: Snubber circui o limi he peak volage across he ransisor To design he ransformer he primary inducance L 1 has o be calculaed firs. L 1 has o sore energy during he on-ime of he ransisor, which is he energy required a he oupu. This energy is given by: W = P ou T, where T is he periodic ime of he swiching frequency and P ou is he raed power. This energy is sored in he primary inducance during he firs half of he period ime and is ransfered o he oupu capacior during he second half of he swiching period. As before he swiching period is divided ino wo equal pars, one par o sore he energy and he oher par o ransfer he energy. During he on-ime of he ransisor he volage across he primary inducance is equal o V in and he curren is a ramp waveform. For every cycle of he inpu energy i follows ha : I 1 I W = V 1 T in (see Fig..1.4) This energy is sored in L 1 and can be calculaed as: W = 1 L 1I 1 For he size of he primary inducance his leads o: V in L 1. 8 P ou f

4 page 14 The calculaion above assumes an efficiency of 100 %. If we consider an efficiency of η, i means ha we have o sore more energy in L 1 and no all of his energy is delivered o he oupu, hen can be calculaed as follows: L 1 L 1 V in 8 P ou f η η has o be esimaed because is value is no known a his poin of calculaion. ( η 0.75 is normally a good esimae.) Î1 T/ T Fig..1.4: Shape of he inpu curren I 1 for raed power The peak value of he curren I 1 is: I 1 = 4 P ou V in η The RMS-value of he curren I 1 is: I 1RMS = I 1 6 The core of he ransformer and he windings can now be calculaed wih he help of Chaper 5: "Calculaion of inducors and high frequency ransformers" The oupu capacior C ou is charged by pulses (Ref Fig..1.). The ripple V ou of he oupu volage resuls from he pulsaing charge curren I and is mainly deermined by he impedance of he capacior. can be verified from he capacior daa shee. Z max Z max The magniude of he ripple volage is given as follows: V ou I Z max The inpu capacior C in can be calculaed for 30V/50Hz-mains as follows: C in 1 µf W P in A special feaure of he flyback converer is he possilbiliy of conrolling several isolaed oupu volages wih only one conrol circui (Fig..1.5).

5 page 15 Vou N3 Vou3 conrol circui Fig..1.5: Flyback converer for several oupu volages One oupu volage is regulaed (in Fig..1.5 V ou 3 ). Volage V ou is coupled o V ou 3 via he V ou urns raio: =. The energy which is sored in L 1 ( ) during he on-ime of he V ou3 N 3 ransisor moves during he off-ime o he oupus. These oupu volages mainain heir values in relaionship o he urns raio. The oupu volages in relaion o he he urns raio from he primary side appear o be in parallel. Therefore he energy from he primary side ransfers o he oupu where he lowes volage appears.