TSTE25 Power Electronics. Lecture 6 Tomas Jonsson ISY/EKS

Transcription

1 TSTE25 Power Electronics Lecture 6 Tomas Jonsson ISY/EKS

2 Outline DC power supplies DC-DC Converter Step-down (buck) Step-up (boost) Other converter topologies (overview) Exercises 7-1, 7-2, 7-7, 7-8

3 Basic use of DC-DC converter Unregulated DC input, controlled DC output Regulated DC may be larger or smaller than the unregulated DC voltage Input to DC-DC converter may vary a lot

4 DC Power supplies Regulated output Defined tolerance of output voltages Isolation No direct electric connection to supply voltage Multiple outputs Both positive and negative possible Various current and voltage ratings

5 Linear power supply Bulky transformer Low frequency Poor efficiency % Low EMI

6 Switch-mode dc power supply schematic Small size High efficiency EMI protection

7 Multiple voltages Linear control may be applied if multiple controlled voltages are required High efficiency (70 90 %)

8 Step down converter principle V d > V o T s constant, t on and t off changing Large ripple on V d

9 DC/DC-converter control Pulse width modulation, PWM, to control switching Switching frequency f s

10 PWM waveform, duty cycle Switch duty cycle (duty ratio) D = t on = v control T s V st 0 < D < 1

11 Step-down (buck) converter Add filter to reduce ripple voltage Diode added to protect switch V L -> infinity if no diode and instantaneous switching! Parasitic capacitances C x would be charged by the inductor current C x

12 Step-down converter waveforms T s = t on + t off Average output voltage V o = t on T s V d = D V d

13 Input voltage before low-pass filter V oi = V d when switch on When switch off V oi = 0 if i L > 0 V oi = V o if i L = 0 LP filter BW(f c ) << f s

14 Current Conduction modes Average i L equals i o Two current conduction modes (i L ) Continuous current conduction Non-continuous current conduction Converter characteristics different depending on mode Both modes can be supported by a converter Mode applicable is depending on load

15 Continuous Conduction mode Switch on (diode off) Switch off (diode on)

16 Continuous conduction mode, cont. i L never zero Steady state => A = B Average v oi output voltage, average v L zero in steady state V d t on + 0 t off T s t on V d V o = V o Ideal conditions: No power loss in converter = V o T s t on => V o V d = t on T s = D P d = P o V d I d = V o I o I o = V d = 1 I d V o D DC transformer with turns ratio equal to D i d still slanted square wave

17 i L reach zero at end of period Average I o I LB = 1 2 i L,peak = t on 2L V d Vo Discontinuous/Continuous Conduction mode boundary I LB = DT s 2L V d V o I LB = DT s 2L V d 1 D < T sv d 8L For fixed input voltage V d I LB,max at 50% duty cycle I LB,max = T sv d 8L

18 Discontinuous conduction mode Equal area: V d V o DT s + V o Δ 1 T s = 0 V o D = V d D + Δ 1 i L,peak = V o L Δ 1T s I o = i L,peak D + Δ 1 2 I o = V ot s 2L D + Δ 1 Δ 1 = V dt s 2L DΔ 1 I o Δ 1 = 4I LB,max D V o V d = D 2 D I o/i LB,max v oi v L,i L v d v o

19 Constant V d step-down characteristic Very low load result in increased output voltage! I LB,max = T sv d 8L

20 Discontinuous conduction mode with constant V o I LB,max = T sv o 2L Control ratio for constant V o

21 Output voltage ripple Assuming: ripple current in C Average current in R ΔV o = ΔQ C = 1 C 1 ΔI L T s I L = V o L 1 D T s ΔV o = 1 T 2 s 1 D = V o 8 LC = π2 2 1 D f c f s 2

22 Step-down (buck) converter summary Output vs input V o V d = D I o I d = 1 D High ripple current I Lpeak = 2I o Practical for D not lower than 0.2

23 Lecture 6 Exercises, buck-converter

24 In a step-down converter, consider all components to be ideal. Let v o V o be held constant at 5 V by controlling the switch duty ratio D. Calculate the minimum inductance L required to keep the converter operation in a continuous-conduction mode under all conditions if: V d is V, P o 5 W, and f s = 50 khz.

25 Consider all components to be ideal. Assume V o = 5 V, f s = 20 khz, L = 1 mh, and C = 470 µf. Calculate V o (peak-peak) if V d = 12.6 V, and I 0 = 200 ma.

26 Lecture 6 Step-up, boost-converter

27 Step-up (boost) converter Output must be larger than input voltage Otherwise is V d driving V o directly => V o = V d Load energy into inductor, then output energy into load while still consuming energy from source C large enough to give low ripple, v o (t) V o

28 Step-up converter waveform, continuous conduction mode Switch on, diode off Switch off, diode on

29 Step-up converter, continuous mode V d t on + V d V o t off = 0 V o /V d = T s = 1 t off 1 D Lossless circuit: V d I d = V o I o I o I d = 1 D

30 Boundary between continuous and discontinuous mode I LB = 1 2 i L,peak = 1 2 V d L t on I LB = T sv o 2L I ob = T sv o 2L D 1 D D 1 D 2 L = T sv o 2I ob D 1 D 2

31 Boundary between continuous and discontinuous mode V o constant I LB max when D = 0.5 I LB,max = T sv o 8L I ob max when D = 1/3 I ob,max = 2 T s V o 27 L = T sv o L I LB = 4D 1 D I LB,max I ob = 27 4 D 1 D 2 I ob,max

32 Step-up, discontinuous mode V d DT s + V d V o Δ 1 T s = 0 V o = Δ 1 + D V d Δ 1 I o = Δ 1 I d Δ 1 + D D = 4 V o 27 V d I d = V d 2L DT s D + Δ 1 I o = T sv d 2L DΔ 1 Controller V o V d 1 I o I ob,max 1/2

33 Step up converter characteristics, V o constant I ob,max = T sv o L

34 Effects of parasitics Losses in L, diode, switch, C Limited also by acceptable D ratio

35 Output voltage ripple ΔV o = ΔQ C = I odt s C ΔV o = DT s V o RC = D T s τ where τ = RCtime constant

36 Lecture 6 Exercises on boost converter

37 In a step-up converter, consider all components to be ideal. Let V d be 8-16 V, V o = 24 V (regulated), f s = 20 khz, and C = 470 µf. Calculate L min that will keep the converter operating in a continuous-conduction mode if Po 5 W.

38 In a step-up converter, V d =12 V, V o = 24 V, I 0 = 0.5 A, L = 150 µh, C = 470 µf, and f s =20 khz. Calculate V o (peak-peak).

39 Equivalent Circuits in DC-DC Converters a) Buck, b) Boost, c) Buck-Boost, d) Cúk

40 Lecture 6 Other converter topologies

41 Flyback converter Derived from buck-boost structure Second winding gives electric isolation Only flux flow in one direction Never negative currents in the transformer

42 Flyback converter circuit states Switch on and switch off Continuous conduction mode Incomplete demagnetization L m size important Ideal transformer have inifinite L m

43 Flyback converter waveforms Same control equation as for buck-boost converter D = t on T s V o = N 2 D V d N 1 1 D

44 Alternative flyback converter topologies Two transistor flyback Both turn on and off simultaneously Voltage rating half compared to single transistor No snubber necessary because of diodes

45 Alternative flyback converter topologies Paralleling flyback converter Same frequency of switching Phase-shifting switches π Allow higher power Redundancy Increased effective switching frequency

46 Forward converter Derived from step-down converter Ideal transformer assumed Transformer magnetizing current not included Converter failure if not taken care of V o V d = N 2 N 1 D

47 Practical forward converter Feed magnetic current back to source Requires a third winding

48 Practical forward converter waveforms To guarantee demagnitized transformer max t m T s = 1 D 1 D max = N 3 N 1 D max D max = N 3 /N 1

49 Other forward converter topologies Two-switch forward converter Commonly used Voltage rating half of single transistor case No snubbers necessary

50 Other forward converter topologies Parallelled forward converter Same advantages as parallelled flyback converter

51 Push-pull converter Derived from step-down converter Diodes due to leakage inductances PWM control V o V d = 2 N 2 N 1 D 0 < D < 0.5

52 Half-bridge converter Derived from step-down converter Additional diodes for switch protection V o V d = N 2 N 1 D 0 < D < 0.5

53 Full-bridge converter Derived from step-down converter Switches carry half the current compared to the half bridge converter V o V d = 2 N 2 N 1 D 0 < D < 0.5

54 Current-source dc-dc converter L d and D > 0.5 gives current source input One or both switches always on Operates like a step-up converter V o V d = N 2 N D D > 0.5

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