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Flyback Transformer Flyback transformer Speaker: WE Technical ACADEMY Ing. Andrea De Gruttola [email protected] Mob: 366 6169566 Author: Alain LaFuente WE Tech Academy WE Group: transformer players 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Transformer Design: General sequence 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Design of a Custom Flyback transformer Compile specification Define Turns ratio Define inductance Calculate peak and rms currents Select core Calculate and define number of turns and calculate core loss no Core loss ok yes Define wire size and calculate copper loss no Copper loss ok yes Build samples and test in circuit 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Basic Principles 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Flyback Converter ° CCM − The primary current never drops to zero during all operating conditions − Primary Inductance value is relatively bigger. − Two states per switching cycle: • The ON state: Transistor is ON and Diode is OFF • The OFF state: Transistor is OFF and Diode is ON − Lower ripple currents − Right hand plane zero − Core losses are lower − Flux swing is smaller − Improved EMC properties − Worst case at low Vin 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Flyback Converter (CCM) ° Switch closed (Transistor ON): − Primary winding current rises from Ip_min to Vout Ip_max in t on = DT ∆= = + _ 2 I p Ip_max Ip_min Time Vpri Vin di V Vo x N s/N p Time = L = in ton VLp Vin toff dt Lp 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Flyback Converter (CCM) ° Switch opened (Transistor OFF): − Secondary winding current falls from Is_max to Is_min in t off = (1-D)T. = _ = _ Is Is_max Is_min Time Vpri Vin Vo x N s/N p Time ton t di −V off V = −V Ls = out Ls out dt Ls 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Flyback Converter (CCM) ° Critical inductance value (L critical ): – At Boundary of CCM/DCM – Minimum inductance to obtain a continuous conduction mode at minimum output power 2 1 L > V out × Critical 2P ⋅ f 2 out _ min switch N s + V out N p V in _ max ° Turns Ratio: = % !_" ! !_"#$ 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Flyback Converter (CCM) ° CCM Design Equations: *+ = = - ./0102 = VCR = Voltage Conversion Ratio 1 ) 3 2 = V out × 1 = L p 2 ⋅ f 2 Pout switch N s + V out = + ( ) N p V in _ max = + ( ) _ = + 2 = = ( ) 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Flyback Converter ° DCM − The primary current drops to zero. − Primary Inductance value is relatively smaller. Capacitance may need to be larger. − Three states per switching cycle: • The ON state: Transistor is ON and Diode is OFF • The OFF state: Transistor is OFF and Diode is ON • The IDLE state: Both Transistor and Diode are OFF − MOSFET turn on at zero current − Worst case at low Vin 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Flyback Converter (DCM) ° Switch closed (Transistor ON): − Primary winding current rises from zero to Ip_max in t on = DT _ = = % 2 5 ! 6 7 4 = ! IL V in A sec/ Lp Imax ∆I DT D2T D3T T 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Flyback Converter (DCM) ° Switch opened (Transistor OFF): − Secondary winding current falls from I s_max to zero in t off = D 2T. _ = _ % 8 2 5 6 7 42 = = IL − Vout Imax A sec/ Ls ∆I DT D2T D3T T 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Flyback Converter (DCM) ° Idle State (Transistor & Diode OFF): – Only Capacitor delivers energy to load during D 3T 43 = 1)D)D2 IL Imax ∆I DT D2T D3T T 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Flyback Converter (DCM) ° Inductance value (L DCM ): – Maximum inductance to obtain a discontinuous conduction mode at maximum output power 2 1 L < V out × DCM 2P ⋅ f 2 out _ max switch N s + V out N p V in _ min 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Comparison Discontinuous / Continuous Flyback Continuous Flyback Discontinuous Flyback Still current on diode when „switching“ off => Current on diode already Zero when needs a ultrafast switching diode „switching“ off No ringing on MOSFET Ringing at MOSFET when current becomes Zero Duty cycle is well defined Duty cycle is not defined because of dead time Peak currents are variable Fixed peak current Secondary triangle is fix Low peak current High peak current Low ripple current High ripple current High inductance Low inductance 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Pick a Core – AreaProduct – AP Product Making the conversion of current density from A/m 2 to Ac cmils/A and units into CGS for convenience we get 506 .7 • P • J AP = in cmils / A cm 4 K • f • ∆B Lpath AP = WaAc Wa Lw Typical value for K is 0.3-0.4 and for J cmils/A is 300-600 AP EFD15= 0,019 Wa = Window Area in cm 2 AP EF16= 0,043 Ac = Core Area in cm 2 AP EFD20= 0,087 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Safety requirements 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Safety requirements ° standards: e.g. EN (IEC)60950, (IT-equipment) EN (IEC)61558 Part 2-16 (transformer general) ° standards define Clearance distance Creepage distance (depends on pollution degree) Distance through insulation ° standards define electrical breakdown voltage ° There are differences between the standards! 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Safety requirements ° Defining the Insulation Level requires: − Grade of Insulation (Functional, Basic, Supplementary, Reinforced) − Working Voltage − Pollution Degree ° This information is application dependent. ° It is not driven by the transformer. ° It’s impact on the transformer construction & price can be significant. 2018 AL | Technical Academy | Public | WE & ST seminar : Flyback converter © 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 Safety requirements ° Working Voltage Highest voltage to which the insulation or the component under consideration is, or can be, subjected to when the equipment is operating under conditions of normal use.
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