EET426 Power Electronics II FLYBACK CONVERTER Prepared by : Mohd Azrik Roslan EET 426 – Power Electronis II

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

EET 426 – Power Electronis II OFF-LINE CONVERTERS This is the general topology for switch mode power supply EET 426 – Power Electronis II

EET 426 – Power Electronis II BUCK-BOOST CONVERTER C R L Ein Vout Refresh back on NOTE VOLTAGE INVERSION EET 426 – Power Electronis II

EET 426 – Power Electronis II BUCK-BOOST CONVERTER VOLTAGE TRANSFER FUNCTION INVERTED STEP DOWN (<1) INVERTED STEP UP (>1) EET 426 – Power Electronis II

BUCK-BOOST ISOLATED CONVERTER Ein R Vout 2L 2L NOTE VOLTAGE INVERSION Use 2L in parallel to get L EET 426 – Power Electronis II

BUCK-BOOST ISOLATED CONVERTER ISOLATION C Ein R Vout 2L 2L NOTE VOLTAGE INVERSION EET 426 – Power Electronis II

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

EET 426 – Power Electronis II FLYBACK CONVERTER C R Vout 2L 2L Ein NOTE VOLTAGE INVERSION EET 426 – Power Electronis II

EET 426 – Power Electronis II FLYBACK CONVERTER C R Vout 2L 2L Ein NOTE NO VOLTAGE INVERSION EET 426 – Power Electronis II

Flyback Converter Equivalent Circuit EET 426 – Power Electronis II

Flyback Converter Equivalent Circuit EET 426 – Power Electronis II

EET 426 – Power Electronis II

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

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

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

EET 426 – Power Electronis II FLYBACK DCM WAVEFORMS or EET 426 – Power Electronis II

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

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

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

EET 426 – Power Electronis II FLYBACK DCM versus CCM ILM EET 426 – Power Electronis II

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

HYSTERESIS CURVE and ENERGY MAGNETIC SYSTEM ENERGY INPUT HYSTERESIS LOSS STORED ENERGY RETURNED EET 426 – Power Electronis II

HYSTERESIS CURVE and ENERGY MAGNETIC SYSTEM ENERGY INPUT HYSTERESIS LOSS STORED ENERGY RETURNED EET 426 – Power Electronis II

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

Real Magnetic Core magnetic core curve slope reduction  reduced inductance but energy storage EET 426 – Power Electronis II

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

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

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

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

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

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

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

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

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

EET 426 – Power Electronis II energy stored in the leakage inductance transferred to the snubber capacitor (Csn) EET 426 – Power Electronis II

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

EET 426 – Power Electronis II PARASITICS EET 426 – Power Electronis II

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

EET 426 – Power Electronis II PARASITICS - Effect EET 426 – Power Electronis II