Electronic Power Conversion

Transcription

1 Electronic Power Conersion Switch Moe DC-DC Conerters with Isolation Challenge the future

2 Switch moe c-c conerters Frequent requirements: regulate output isolation (safety) multiple outputs Solutions: Linear power supplies Switch moe power supplies Drawbacks of linear power supplies 50Hz transformer require transistor absorbs oltage è loss, heat 2

3 Switch-moe power supplies Major aantages: transistor operates as switch no 50Hz transformer neee è reuce weight an costs howeer: more complex measures to preent (conucte) EMI neee resonant type c power supplies are not consiere here (Ch9) 3

4 Multiple outputs one output is regulate by PWM if neee other output can be post-regulate by linear regulators 4

5 Transformer Ieal transformer Faraay s law φ φ = 2 = 2 t t = 2 2 Ampere s law i ii= φ R i i 2 2 i=,2 Core reluctance l R= µa = φ R 0 i = i 2 2 5

6 Transformer Magnetising inuctance R= l µa 0 on-zero core reluctance i i = φ R 2 2 φ = t = ( i i ) = Lm im t t 2 2 R m 2 L m = R 2 = 2 i i i 6

7 Transformer Leakage flux ϕ l,2 leakage flux φ = φ + φ l φ2 = φ + φ l 2 R,2 ohmic resistances of the winings φ φ l φ = Ri + = Ri + t + t t φ2 φ l 2 φ 2= Ri = Ri t t t 7

8 Equialent Transformer Circuit φ φ l φ = Ri + = Ri + L = φ / i t + t t φ2 φ l 2 φ 2= Ri = Ri t t t i = i + i 2 m 2 i =Ri + Ll + e t i = Ri L + e t l 2 2 i im t t i i t t =Ri + Ll + Lm 2 2 2= Ri 2 2 Ll2 + Lm m Basic Electrical an Magnetic Circuit Concepts 8

9 Transformer representation t slope L m i m / = / 2 2 an i = i 2 2 L l an L l2 are often neglecte because they are intentionally small an hae a minor effect on the oltage transfer function (they are important for switch selection an snubber esign) L m is taken into account because it affects circuit operation 9

10 To unerstan operation of transformer-isolate conerters: replace transformer by equialent circuit moel containing magnetizing inuctance analyze conerter as usual, treating magnetizing inuctance as any other inuctor apply olt-secon balance to all conerter inuctors, incluing magnetizing inuctance 0

11 DC-DC conerters with electrical isolation Uniirectional core excitation flyback conerter forwar conerter Biirectional core excitation full-brige conerter half-brige conerter push-pull conerter

12 Flyback conerter Derie from buck-boost Magnetic component: energy storage (couple inuctor) transformation isolation 2

13 Flyback conerter I Here: I + 2 I 2 = 0 Steay state: L, a = 0 t - ( T - t ) = 0 on 0 s on 2 so: 0 2 D = - D 3

14 Approach in book: up-flux - own flux ˆ φ(t) = φ(0) + t on im() t = i m(0) + t L m è gies same result. Inuctor current φ ˆ 0 ( T s ) = φ - ( Ts - ton ) 2 = φ 0 + t - T - t 0 ( ) on ( s on ) 2 ˆ ( / ) i t = i - t - t 0 2 m() m ( on) Lm Switch utilisation ˆ T = + 2 o = D Example: =350; D=0.5 è T,max =700 ˆ I = i (0) + t L T m on m 4 4

15 CCM Larger transformer Smaller peak currents DCM Smaller transformer Larger peak currents BCM (bounary conition moe) Design of low power power supplies course Q3 (control, EMI, magnetics esign, loss calculation) Flyback conerter Low part count (cheap) High transistor oltage stress 5

16 Other flyback conerter topologies Two-switch flyback conerter T an T2 switch simultaneously switch oltage hale ˆ 2 T = + x Paralleling flyback conerters phase shifte operation common output cap C filter ouble ripple current frequency at input cap an output cap è smaller passies 6

17 Forwar conerter 0? = 0 La, è switch on, D on L= L 2 = - La, 0 - switch off, D 2 on 0 (0 < t < t on ) ( t on < t < T s ) = 0 2 è 0 t - on 0 ( T - t s on) = 0 Demagnetisation wining function: remoe energy store in L m è 0 2 = D 7

18 i + i 3 3= i 2 2 switch off: Lm, a = 0 or: è t = t = t m 3 = D T s on m 3 () with t < T ( D) (2) m s D From ()&(2): D < + / 3 Mostly = 3 so D<0.5 Switch oltage: or = + ˆ = 2 ˆT T 3 8

19 Other forwar conerter topologies Two-switch forwar conerter T an T2 switch simultaneously switch oltage hale no emagnetisation wining neee Paralleling forwar conerters phase shifte operation single LC filter ouble ripple current frequency at input cap an LC filter è smaller passies 9

20 0? Full-brige conerter L = L 2 = (0 < t < DTs) (DTs < t < T s /2) La, = 0 è 0 2 = 2 D Switch oltage: = ˆT DT s T s /2 T s /2+DT s T s 20

21 Half-brige conerter 0? L = L 2 2 = 0 La, = 0-0 è La, = 0 (0 < t < t on ) ( t on < t < T s ) 0 2 = D alue of i T in full brige is lower than in half brige with same power rating: ( I T) HB= 2( I T) FB 2

22 Push-pull conerter 0? = L 2-0 La, = 0 (0 < t < t on ) = L 0 ( t on < t < T s ) La, = 0 è 0 2 = 2 D (0 < D < 0.5) Switch oltage: ˆ T = 2 risk of transformer saturation 22

23 DC-DC conerters with electrical isolation Uniirectional core excitation (ΔB p_p =B m -B r ) flyback conerter forwar conerter Biirectional core excitation (ΔB p_p =2B m ) push-pull conerter half-brige conerter full-brige conerter 23

24 Transformer core selection Core loss ensity = k f a s [ ( ΔB ) max ] b ( ΔB ) max = (at D = 0.5) 4 A f c s 24

25 PWM-control of c-c conerters with isolation 25

26 3. A forwar conerter shown below is operating in a continuous conuction moe. The emagnetising wining is chosen to be 3=. All components can be assume to be ieal, except for the presence of transformer magnetising inuctance. The conerter specifications are: =48±0% o=5 fs=00khz Ploa=5-50W a) (0) Calculate the transformer turns ratio 2/ is this turns ratio is esire to be as small as possible. b) (0) Calculate the minimum alue of the filter inuctance. c) (5) Calculate the oltage rating of the switch in terms of the input oltage. 26

27 Design a flyback conerter operating in the iscontinuous conuction moe with the following specifications: Input oltage (nominal alue 400) Output power 0 P o 50W (nominal alue 50W) Output oltage 20 o 30W (nominal alue 27) Switching frequency f s =50kHz Peak to-peak oltage ripple Δ 0p_p =20m. a) (5) Select the transformer turn ratio 2 /, the magnetising inuctance L m an the output capacitance C. You may assume that the maximum uty cycle alue is 0.5. The oltage rop across the ioe when it is in on-state is. b) Sketch the waeforms an calculate RMS an DC alues of the transistor an output ioe current at the nominal operating point. c) Sketch the waeforms an calculate the maximum oltages across the transistor an output ioe. 27

28 In the figure below two parallel forwar conerters are shown. Draw the input current i an i L waeforms if each conerter is operating at a uty ratio of 0.3 in a continuous-conuction moe. Compare these two waeforms with those if a single forwar conerter (with twice the power rating but with the same alue of the output filter inuctance as in ouble forwar) is use. 28

29 Image creits All uncreite iagrams are from the book Power Electronics: Conerters, Applications, an Design by. Mohan, T.M. Unelan an W.P. Robbins. All other uncreite images are from research one at the EWI faculty. 29