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Isolated dc-dcConverters

EE514Switched Mode Power Supplies

Yrd.Do.Dr.Mutlu Boztepe

E.. Elektrik-Elektronik Mh.Mart 2007

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Transformer isolation

Objectives: Isolation of input and output ground connections, to meetsafety requirements Reduction of transformer size by incorporating high

frequency isolation transformer inside converter Minimization of current and voltage stresses when alarge step-up or step-down conversion ratio is needed use transformer turns ratio

Obtain multiple output voltages via multiple transformersecondary windings and multiple converter secondarycircuits

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A Simple Transformer Model

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The magnetizing inductance LMModels magnetization oftransformer core material Appears effectively in parallel withwindings If all secondary windings aredisconnected, then primary windingbehaves as an inductor, equal to themagnetizing inductance At dc: magnetizing inductance tendsto short-circuit. Transformers cannot

pass dc voltages Transformer saturates whenmagnetizing current iMis too large

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Volt-second balance in LMThe magnetizing inductance is a real inductor,obeying

integrate:

Magnetizing current is determined by integral of

the applied winding voltage. The magnetizingcurrent and the winding currents are independentquantities. Volt-second balance applies: insteady-state, iM(Ts) = iM(0), and hence

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Transformer reset

Transformer reset is the mechanism by which magnetizinginductance volt-second balance is obtained The need to reset the transformer volt-seconds to zero by the end ofeach switching period adds considerable complexity to converters

To understand operation of transformer-isolated converters: replace transformer by equivalent circuit model containingmagnetizing inductance analyze converter as usual, treating magnetizing inductance asany other inductor

apply volt-second balance to all converter inductors, includingmagnetizing inductance

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Full-bridge and half-bridgeisolated buck converters

Full-bridge isolated buck converter

Sekonder 2 sargl olarak dnlrse , sarm oran 1:n:n olan 3 sargl bir

transformatr gibi kabul edilebilir.

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Full-bridge, with transformerequivalent circuit

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During first switching period:transistors Q1and Q4conductfor time DTs, applying voltsecondsVg DTsto primary

winding During next switching period:transistors Q2and Q3conductfor time DTs, applying voltseconds

VgDTsto primary

winding Transformer volt-secondbalance is obtained over twoswitching periods Effect of nonidealities?

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Effect of nonidealitieson transformer volt-second balance

Volt-seconds applied to primary winding during first switching period:

(Vg (Q1and Q4forward voltage drops))( Q1and Q4conduction time)

Volt-seconds applied to primary winding during next switching period:

(Vg (Q2and Q3forward voltage drops))( Q2and Q3conduction time)

These volt-seconds never add to exactly zero.

Net volt-seconds are applied to primary windingMagnetizing current slowly increases in magnitudeSaturation can be prevented by placing a capacitor in series withprimary, or by use of current programmed mode (chapter 11)

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Operation of secondary-side diodes

)()()(11

tititiM

During second (D) subinterval, bothsecondary-side diodes conduct

)(2

1)(

2

1)(

)(2

1)(

2

1)(

6

5

tin

titi

tin

titi

MD

MD

0)()()( 651 tnitniti DD

)()()()( 651 titnitniti MDD

Output filter inductor current divides approximately equally between diodes

Secondary amp-turns add to approximately zero Essentially no net magnetization of transformer core by secondary winding currents

i1(t)=0 ise;

iM

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Volt-second balance on output filterinductor

Inductor average voltageshould be zero, so;

nDDM

)(

gnDVV

sVV

Buck converter with turns ratio

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Full bridge

Output voltage can be controlled by duty cycle.

Duty cycle range 0

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Replace transistors Q3 and Q4 withlarge capacitorsVoltage at capacitor centerpoint is

0.5Vgvs(t) is reduced by a factor of twoM = 0.5 nDTransistor currents two times of fullbridge.

Half bridge isolated

buck converter

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Forward converter

Buck-derived transformer-isolated converter Single-transistor and two-transistor versions Maximum duty cycle is limited (for n1=n2 , 0

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Forward converterwith transformer equivalent circuit

Magnetizing current, in conjunction with diode D1, operates in discontinuousconduction mode (transformer reset) Output filter inductor, in conjunction with diode D3, may operate in either CCMor DCM

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Forwardconverter:waveforms

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Magnetizing inductance volt-secondbalance

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Transformer reset

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What happens when D > 0.5

magnetizing currentwaveforms,

forn1 = n2

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Conversion ratio M(D)

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Maximum duty cycle vs.transistor voltage stress

Maximum duty cycle limited to

which can be increased by increasing the turns ratio n2 / n1. But thisincreases the peak transistor voltage:

Forn1 = n2

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The two-transistor forward converter

Subinterval 1 : Both transistor conduct

Subinterval 2 and 3: Both transistor are off. Magnetizing current flows through D1and D2. So, primary voltage isVg.

Transistor blocking voltage limited to Vg

Duty cycle limited to D

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The two-transistor forward converter

P h ll

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Push-pullisolated buckconverter

Used with low-voltage input. It exhibits lowprimary losses, because one transistorconducts at any given instant. Secondary-side circuit identical to full bridge As in full bridge, transformer volt-secondbalance is obtained over two switching periods Effect of nonidealities on transformer volt-second balance? Current programmed control can be used to

mitigate transformer saturation problems. Dutycycle control not recommended.

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Flyback converter

buck-boost converter:

construct inductorwinding using twoparallel wires:

Isolate inductorwindings: the flyback

converter

Flyback converter having a 1:n turns

ratio and positive output:

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The flyback transformer

A two-winding inductor Symbol is same astransformer, but functiondiffers significantly from ideal

transformer Energy is stored inmagnetizing inductance Magnetizing inductance isrelatively small

Current does not simultaneously flow in primary and secondary windings Instantaneous winding voltages follow turns ratio Instantaneous (and rms) winding currents do not follow turns ratio Model as (small) magnetizing inductance in parallel with ideal transformer

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Subinterval 1

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Subinterval 2

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CCM Flyback waveforms andsolution

Similar to Buck-Boost converter

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Discussion: Flyback converter

Widely used in low power (50W to 100 W range)and/or high voltage applications

Low parts count

Multiple outputs are easily obtained, with minimumadditional parts

Cross regulation is inferior to buck-derived isolatedconverters

Often operated in discontinuous conduction mode

DCM analysis: DCM buck-boost with turns ratio