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EET426 Power Electronics II
FLYBACK CONVERTER Prepared by : Mohd Azrik Roslan EET 426 – Power Electronis II
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What you should know after this lecture
Buck boost converter recap Flyback converter concept Flyback converter equivalent circuit Flyback application CCM DCM Classification Flyback Transformer RCD Clamp EET 426 – Power Electronis II
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EET 426 – Power Electronis II
OFF-LINE CONVERTERS This is the general topology for switch mode power supply EET 426 – Power Electronis II
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EET 426 – Power Electronis II
BUCK-BOOST CONVERTER C R L Ein Vout Refresh back on NOTE VOLTAGE INVERSION EET 426 – Power Electronis II
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EET 426 – Power Electronis II
BUCK-BOOST CONVERTER VOLTAGE TRANSFER FUNCTION INVERTED STEP DOWN (<1) INVERTED STEP UP (>1) EET 426 – Power Electronis II
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BUCK-BOOST ISOLATED CONVERTER
Ein R Vout 2L 2L NOTE VOLTAGE INVERSION Use 2L in parallel to get L EET 426 – Power Electronis II
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BUCK-BOOST ISOLATED CONVERTER
ISOLATION C Ein R Vout 2L 2L NOTE VOLTAGE INVERSION EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Dot convention If primary voltage is positive at the dotted end of the winding with respect to undotted end, then the secondary voltage will be positive at the dotted end also. Voltage polarities are the same with respect to the dots in each side of the core If the primary current of the transformer flows into the dotted end of the primary winding, the secondary current will flow out of the dotted end of the secondary winding. EET 426 – Power Electronis II
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EET 426 – Power Electronis II
FLYBACK CONVERTER C R Vout 2L 2L Ein NOTE VOLTAGE INVERSION EET 426 – Power Electronis II
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EET 426 – Power Electronis II
FLYBACK CONVERTER C R Vout 2L 2L Ein NOTE NO VOLTAGE INVERSION EET 426 – Power Electronis II
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Flyback Converter Equivalent Circuit
EET 426 – Power Electronis II
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Flyback Converter Equivalent Circuit
EET 426 – Power Electronis II
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EET 426 – Power Electronis II
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commonly used power circuit in household & consumer electronics
FLYBACK APPLICATIONS Flyback: commonly used power circuit in household & consumer electronics color TVs printers PCs monitors battery chargers ac adapters EET 426 – Power Electronis II
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HIGH output current 12A 15A LARGE OUTPUT CAPACITOR RIPPLE CURRENT
FLYBACK POWER RATINGS HIGH OUTPUT VOLTAGE 400 V LOW OUTPUT POWER 20 W OUTPUT POWER 150 W ok HIGH output current 12A 15A LARGE OUTPUT CAPACITOR RIPPLE CURRENT EET 426 – Power Electronis II
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WHY CHOOSE FLYBACK ? when low-cost ‘isolated’ solution required
least components and smallest footprint NO output inductor or FWD when multiple output voltages required add extra transformer secondary windings a single output is normally regulated FLYBACK output voltages track one another with input voltage and load changes better than in the FORWARD converter EET 426 – Power Electronis II
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EET 426 – Power Electronis II
FLYBACK DCM WAVEFORMS or EET 426 – Power Electronis II
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transformer primary and secondary current waveforms
DEFINING DCM & CCM DCM CCM BDY trapezoidal in CCM triangular in DCM primary current and secondary current NOT continuous transformer primary and secondary current waveforms EET 426 – Power Electronis II
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FLYBACK CONVERTER CCM or DCM ????
DCM FLYBACK LOW POWER APPLICATIONS mosfet (switch) and rectifier currents considerably higher in DCM compared to CCM higher peak output capacitor current EET 426 – Power Electronis II
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FLYBACK CONVERTER CCM or DCM ????
DCM FLYBACK good transient line and load response single pole feedback loop is easy to stabilise CCM FLYBACK feedback loop has 2 poles and a right half plane zero more difficult to stabilise EET 426 – Power Electronis II
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EET 426 – Power Electronis II
FLYBACK DCM versus CCM ILM EET 426 – Power Electronis II
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MATERIAL: b-H characteristic
flux density v magnetic field intensity slope: permeability CORE: f-F characteristic total flux v magnetomotive force slope: permeance F f CIRCUIT ELEMENT: characteristic defines equivalent electrical characteristic (core + N turns) slope: inductance EET 426 – Power Electronis II
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HYSTERESIS CURVE and ENERGY
MAGNETIC SYSTEM ENERGY INPUT HYSTERESIS LOSS STORED ENERGY RETURNED EET 426 – Power Electronis II
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HYSTERESIS CURVE and ENERGY
MAGNETIC SYSTEM ENERGY INPUT HYSTERESIS LOSS STORED ENERGY RETURNED EET 426 – Power Electronis II
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EET 426 – Power Electronis II
Ideal Magnetic Core the vertical slope of the ideal magnetic core curve represents an apparent infinite inductance but as there is no stored energy only energy loss the characteristic is therefore resistive! Only Power Loss. EET 426 – Power Electronis II
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Real Magnetic Core magnetic core curve slope reduction
reduced inductance but energy storage EET 426 – Power Electronis II
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greater energy storage
AIR GAPPED CORE lower slope lower inductance greater energy storage The introduction of an air gap ‘skews’ but linearises the characteristic curve and results in lower inductance but higher energy storage thereby providing a better solution for inductor design. The core stored energy is virtually zero; the core gap may only be a few mm but it stores almost all the recoverable energy. EET 426 – Power Electronis II
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IFC vs Flyback Converter Transformer
ISOLATED FORWARD CONVERTER (IFC) Transformer Performs as a normal transformer transferring energy No energy storage ideally Gapless core structure leakage inductance represents energy stored in non-magnetic regions EET 426 – Power Electronis II
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IFC vs Flyback Converter Transformer
The Flyback Converter transformer is basically a coupled inductor the primary side stores energy during the switch on-time and releases it to the load during the rectifier on- time. EET 426 – Power Electronis II
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The “flyback transformer”
A two winding inductor Symbol is same as transformer but function differs significantly from ideal transformer Energy is stored in magnetizing inductance Magnetizing inductance is relatively small Current does not simultaneously flow in primary and secondary windings. Instantaneous winding voltage follow turns ratio Instantaneous (and rms) winding currents do not follow turns ration Model as (small) magnetizing inductance in parallel with ideal transformer. EET 426 – Power Electronis II
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FLYBACK DEMAGNETISATION
NO requirement for demagnetisation energy stored during switch on time (DswTsw) is transferred to the output during the rectifier on time EET 426 – Power Electronis II
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LEAKAGE INDUCTANCE Lleak C R Vout Ein a high voltage VDS spike
VOR a high voltage VDS spike that can lead to mosfet avalanche breakdown occurs due to leakage inductance Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. EET 426 – Power Electronis II
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EET 426 – Power Electronis II
RCD CLAMP Lleak C R Vout Ein energy stored in the leakage inductance is transferred to the snubber capacitor (Csn) at mosfet turn-off, and is eventually dissipated in the snubber resistor (Rsn) Lleak energy increases VDS until it becomes greater than VCsn at which point the diode conducts and the drain is clamped Rsn is based on the energy dissipated at the nominal clamp voltage (Vclamp) per cycle being equal to Csn stored energy EET 426 – Power Electronis II
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approximately 50% higher than reflected output voltage (VOR)
RCD CLAMP Lleak C R Vout Ein VCLO is normally approximately 50% higher than reflected output voltage (VOR) EET 426 – Power Electronis II
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EET 426 – Power Electronis II
RCD CLAMP Lleak C R Vout Ein LOW COST: small number of low cost components EFFICIENT: only the transformer loop is used to charge Csn other clamp versions require source energy to charge Csn EET 426 – Power Electronis II
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EET 426 – Power Electronis II
energy stored in the leakage inductance transferred to the snubber capacitor (Csn) EET 426 – Power Electronis II
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EET 426 – Power Electronis II
PARASITICS Parasitic capacitance Also known as stray capacitance Unwanted capacitance that exist between the parts of an electronics component or circuit due to the short distance between each other Inductors, diodes and transistors Cause the behaviour differ from ideal condition Parasitic inductance Also known as stray inductance Produced by wire or component leads that have current flow through them if they are long enough. can result in a disruption of normal current flow within a circuit EET 426 – Power Electronis II
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EET 426 – Power Electronis II
PARASITICS EET 426 – Power Electronis II
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EET 426 – Power Electronis II
PARASITICS Cprim Primary winding stray capacitance Csec Secondary winding stray capacitance Cprim-sec Stray capacitance linking the primary and secondary winding Crect Diode stray capacitance Cout,mos MOSFET output stray capacitance Lpcb PCB stray inductance LM Magnetizing inductance Lleak Leakage inductance Parasitics are those nasty little gremlins that creep into your PCB (quite literally) and wreak havoc within your circuit. They are the hidden stray capacitors and inductors that infiltrate high-speed circuits. They include inductors formed by package leads and excess trace lengths; pad-to-ground, pad-to-power-plane, and pad-to-trace capacitors; interactions with vias, and many more possibilities. EET 426 – Power Electronis II
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EET 426 – Power Electronis II
PARASITICS - Effect EET 426 – Power Electronis II
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