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The Flyback Converter Lecture notes ECEN De rivation of the flyback converter a tran formerisolated The Flyback Converter Lecture notes ECEN De rivation of the flyback converter a tran formerisolated

The Flyback Converter Lecture notes ECEN De rivation of the flyback converter a tran formerisolated - PDF document

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The Flyback Converter Lecture notes ECEN De rivation of the flyback converter a tran formerisolated - PPT Presentation

Its derivation is illustrated in Fig 1 Figure 1a depicts the basic buckboost converter with the switch realized using a MOSFET and diode In Fig 1b the i nductor winding is constructed using two wires with a 11 turns ratio The basic function of the i ID: 18131

Its derivation illustrated

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The Flyback Converterrivation of the flyback converter: a tranformer-isolated verson of thebuck-boostDerivation of the flyback converterThe flyback converter is based on thebuck-boost converter. Its derivation is illustrated inFig. 1. Figure 1(a) depicts the basic buck-boost converter, with theswitchrealizedusing aMOSFET 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 parallelwindingsare equivalent to a single winding constructed of larger wire. In Fig. 1(c), the – L–V VgQ1D1 +– L–V VgQ1D1 c)d) – LM–V VgQ1D1 1:1 +– LM+VVgQ1D1 1:n turns ratio and p ositive out p ut. current is now distributed between the windings differently. The magnetic fields inside theinductor in both cases are identical. Although the two-windingmagnetic device issymbol as thetransformer, a more descriptive name is “two-winding inductor”. This ce is sometimes alsocalled a “ftransformer”. Unlikethe ideal transformer, current does not flow simultaneously in both windings of the flybacktransformer. Figure 1(d) illustrates the usual configuration of the flyback converter. TheMOSFETsconnected to the primary-side ground,simplifying the gate drivecircuit. The transformer polarity marks are reversed, to obtain a positive output voltage. Aconverters can be adequately understoodby modeling the physical transformer with a simple equivalent circuit consisting of an idealtransformer in parallelwith themagnetizing inductance. The magnetizinginductance must then followall of theusual rules for inductors; in particular,volt-secondbalancemust hold when theimplies that the average voltage appliedvery winding of the transformerLet us replace the transformer ofFig. 1(d) withthe equivalent circuitbove.The circuit of Fig.2(a)is then obtained.The functions in the samemanner as inductor of the oribuck-boost converter of Fig. 1(a). Whenonducts, energy from the. When diodeconducts, this stored energy isto theload, with the inductorvoltage and current scaled according to – LM+vVgQ1D1 1:n C transformer model iig R iC+vL– – LM+vVg 1:n C transformer model iig R iC+vL– – +vVg 1:n C transformer model i R iC +vL–ig2.Flyback converter circuit, (a) withdurin subinterval 1, (c) durin g subinterval 2. , while transistor conducts, the converter circuit model, and dc source current With the assumption that the converter operates with small inductor current ripple and smallcapacitor voltage ripple,the magnetizing current and output capacitor voltage can be During the second subinterval, the transistor is in the off-state, and the diode conducts. Theequivalent circuit of Fig. 2(c) is obtained. The primary-side magnetizing inductance voltage iC=in –vR It is important to consistently define side of thetransformer for allsubintervals. Upon making the small-ripple iC=In –VR , awaveforms aresketched in Fig. 3.Application of the principle of volt-balance to the primary-side '(– vLiCigt Vg0Ts Q1D1devices: –V/RI/n – V/RIg. 3. 4(DVg =nDD' onversionratio of the flyback converter is similar to that of the buc =D(–VR n –VR yieldsI=nVD'R This is the dc component of the magnetizing current, referred to theprimary.The dc which models the dc components of the flyback converterwaveforms can be constructed. The resulting dc equivalent circuit of the flyback converteris given in Fig. 4. It contains a 1:buck-type conversion ratio,followed by a (1 – conversion ratio, and an addedfactor of 1:arising from theThe flyback converter issed at the50-100Wpower range, as well as in high-voltagepower televisions and computer monitors.It has the advantage of very lowparts count. Multipleoutputs canbe obtained using a minimumeach additional output requires only an additional winding, diode, andcapacitor. The peak transistor voltage equal to the dc input voltage plus the ; in practice, additional voltage is observed due to ringing associated withransformerleakageinductance. A snubbercircuitmay be required to clamp themagnitude of this ringing voltage to a safe level that is within the peak voltage rating of the – +– R+VVg ' In D ' Vn – Ig – R+VVg I Ig 1 : D D' : n 4.Flyback converter equivalent circuit model: (a) circuitscorresponding to Eqs. (5), (7), and (9); (b) equivalent circuit