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Design Space of Flyback Transformers

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Design Space of Flyback Transformers Design Space of Flyback Transformers George Slama Senior Application and Content Engineer [email protected] Würth Elektronik What is the design space? . Background – flyback transformers . Energy storage concept . Minimum energy curve – inductance - discontinuous . Maximum energy curve – inductance - continuous mode . Duty cycle limits . Reflected voltage limits . Mixed mode operation . Tolerances Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 2 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com Background . The term ‘flyback’ comes from the days of cathode ray tube (CRT) in televisions and monitors where the beam had to fly back after each scan to the start position for the next scan line. This required a high horizontal deflection voltage between 10 - 50 kV . Today typically used in universal wide input charging adapters outputting low voltage . Flyback converters come in many flavors DCM, CCM, BCM, QRM, … US Patent US3665288A - 1972 Theodore J Godawski Zenith Electronics LLC Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 3 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com Operation . T1 – switch is on – increasing current flow through primary winding building magnetic D1 V field – no current in secondary winding - IN T1 VOUT load supplied from capacitor C1 C2 . T2 – switch is off – decreasing current flow in secondary winding as magnetic field collapses – no current in primary winding Q1 Current, Q1 on . Only one winding carries current at a time Current, Q1 off . Energy transfer through magnetic field Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 4 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com Variations . DCM – Discontinuous Conduction Mode Current returns to zero variable Variable dead time – fixed frequency . QRM – Quasi Resonant Mode variable Current returns to zero quasi- Small dead time – valley switching fixed Variable frequency . BMO – Boundary Mode Operation variable Current returns to zero none No dead time – variable frequency . CCM – Continuous Conduction Mode Current always flowing Duty cycle varies only with input voltage Current waveforms – Orange=Primary, Violet=Secondary Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 5 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com The zone . Flyback transformers are very common in wide input, low wattage power supplies Turns ratio improves duty cycle usage Provide galvanic isolation to meet safety standards Low component count . App notes usually give you one formula for determining a single inductance value . In reality there a range or a zone of applicability . This zone is bounded by different circuits elements Minimum energy requirements The switching device voltage ratings – MOSFET and diode Applied component margins and deratings Frequency and duty cycle PWM control modes Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 6 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com Things to understand . For efficiency we use the converter efficiency because the transformer has to be able to make up for the lost power (losses) of other devices . Somewhere we set a boundary for CCM/DCM, typically 50% duty cycle . The control scheme changes when crossing DCM/CCM boundary . You can look at it as a dc voltage transfer function or … . You can use load resistance . In CCM duty cycle does not change with load, only input voltage → constant voltage . In DCM duty cycle changes with load and input voltage → constant current Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 7 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com Limitations . Leakage inductance from less than perfect coupling of windings LLKG D1 VIN T VOUT . Snubber dissipates energy, limiting the 1 voltage . Secondary voltage reflects to the primary C1 C2 . MOSFET Vds is fixed typically derated to have a margin Snubber . Secondary diode has similar restrictions . Duty cycle is limited Q1 Current, Q1 on Current, Q1 off . Peak current limits for switch and rectifier Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 8 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com Reflected voltage limits . MOSFET needs to be derated or margined . Allowance for leakage inductance VSPIKE . Reflected output voltage . Maximum input voltage . VIN + VR + VSPIKE + VMARGIN = VDS . Can adjust VR with turns ratio . Can adjust VSPIKE with snubber and leakage specification AN1262 App Note, STMicroelectronics Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 9 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com Worst case conditions - DCM . Worst case vs. nominal conditions Highest load Lowest input voltage Maximum duty cycle . Other corner is No or lowest load Maximum input voltage Minimum duty cycle . Short circuits, tolerances, temperature effects on core material performance Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 10 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com Inductance flavors . Lc - Chord Inductance (or amplitude inductance) Ψ(t) . Ld - Differential Inductance Ψ(t) . L - Reversible Inductance Ld = Ψ/ i Lr = Ψ/ i r Ψ . Lw - Energetic Inductance i Ψ Ψ 2 푖 푑Ψ ׬0 Lc 퐿푤= 2 푖 i 0 i(t) 0 i(t) . Normally Ld < Lw < Lc Bossche, Alex van den., Valchev, 2005. Inductors and Transformers for Power Supplies. Boca Raton, FL, CRC Press Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 11 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com Energy capacity of a core . From the basic energy equation you can show that the energy capacity of a core is 2 1 퐴푒푙푒퐵 dependent on 푊 = 퐿퐼2 W = 2 2휇 휇 B = maximum useable core flux 0 푒 Ae = cross sectional core area AL = which is a function of the gap 휇 휇 퐴 . If you want to store more energy in the core, 퐴퐿 = 0 푒 푒 eventually it must get bigger 푙푒 BSAT is a limited 2 2 Gapping has a practical limit (~< 3% of le) 퐵 퐴 푊 = 푒 2퐴퐿 Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 12 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com B, Ae, Gap(µe) Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 13 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com Material vs. Temperature Ferrite Powder Core Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 14 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com Concepts . One way to look at it is inductance vs. duty cycle . Another way is inductance vs. reflected voltage or turns ratio . You need a minimum amount of energy in each cycle 1 퐷 푊 = 퐿 ∙ 퐼2 퐿 ∙ 푑푖 = 푉 ∙ 푑푡 푑푡 = 2 푃퐾 푓 퐿 ∙ 푉2 ∙ 푑푡2 푉2 ∙ 퐷2∙ 푓2 푊 = = 2 ∙ 퐿2 2 ∙ 퐿 Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 15 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com L vs. DC vs. Size . As duty cycle increases, inductance decreases while peak current increases 2 . Energy storage is proportional to L*IPK while… . Inductor core size is proportional only to L*IPK . Doubling IPK will reduce required inductance to ¼ which… . Reduces the required cores size to ½ . Good design makes duty cycle as large as possible Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 16 © All rights reserved by Wurth Electronics, also in the event of industrial property rights. All rights of disposal such as copying and redistribution rights with us. www.we-online.com Minimum energy curve . Depends on inductance and peak current 1 . This depends on power out, efficiency and frequency 2 푊 = 퐿 ∙ 퐼푃퐾 . Combined 2 . May not know peak current therefore 푃푂푈푇 V 2 D2 푊 = L IN 푓푆푊 ∙ 휂 2 P f 2 ∙ 푃푂푈푇 퐿푀퐴푋 = 2 푓푆푊 ∙ η ∙ 퐼푃퐾 Date 16.03.2020 | APEC 2020 | Public | Topic: Design Space of Flyback Transformers 17 © All rights reserved by Wurth Electronics, also in the event of industrial property rights.
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