Transcript
TSTE19 Power Electronics Lecture 6 Tomas Jonsson ISY/EKS
TSTE19/Tomas Jonsson
Outline § DC power supplies § DC-DC Converter § Step-down (buck) § Step-up (boost)
§ Other converter topologies (overview)
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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
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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
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Linear power supply § Bulky transformer § Low frequency
§ Poor efficiency § 30 – 60 %
§ Low EMI
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Switch-mode dc power supply schematic § Small size § High efficiency
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Multiple voltages § Linear control may be applied if multiple controlled voltages are required § High efficiency (70 – 90 %)
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Step down converter principle § Vd > Vo § Ts constant, ton toff changing § Large ripple on Vd
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DC/DC-converter control •
Pulse width modulation, PWM, to control switching
•
Switching frequency fs
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PWM waveform, duty cycle § Switch duty cycle (duty ratio) =
0<
=
ˆ
<1
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Step-down (buck) converter § Add filter to reduce ripple voltage compared to principle § Diode added to protect switch § VL -> infinity if no diode and instantaneous switching! § Parasitic capacitances Cx would be charged by the inductor current
Cx
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Step-down converter waveforms § Ts = ton + toff § Average output voltage =
=
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Input voltage before low-pass filter § Voi = Vd when switch on § When switch off § Voi = 0 if iL > 0 § Voi = Vo if iL = 0
§ LP filter BW(fc) << fs
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Current Conduction modes § Average iL equals io § Two current conduction modes (iL) § Continuous current conduction § Non-continuous current conduction
§ Converter characteristics different depending on mode § Both modes can be supported by a converter § Mode selected depending on load
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Continuous Conduction mode § Switch on (diode off)
§ Switch off (diode on)
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Continuous conduction mode, cont. § iL never zero § Steady state => A = B
−
=
−
=>
=
§ Average voi output voltage, average vl zero in steady state +0
=
§ No power loss in converter §
=
⇒
=
=
= 1
§ DC transformer with turns ratio equal to D
§ id still slanted square wave
=
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Discontinuous/Continuous Conduction mode boundary
§ iL reach zero at end of period § Average Io § §
=
1 2
,
=
=
2
2
=
2
−
−
1−
§ For fixed input voltage Vd § ILB,max at 50% duty cycle
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Discontinuous conduction mode − ,
=
2
=
= ,
=
+Δ
Δ
4
+
1 4
=0
Δ
+Δ 2
+Δ Δ =
Δ =
=
+ −
,
/
,
2
Δ
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Constant Vd step-down characteristic § Very low load result in increased output voltage!
,
=
8
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Discontinuous conduction mode with constant Vo §
,
=
2
§ Control ratio for constant Vo
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Output voltage ripple § Assuming: § ripple current in C § Average current in R Δ
∆
Δ
=
Δ
=
=
1 8
1 1Δ = 2 2 2
π = 1− 2
1−
1−
=
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Step-down (buck) converter summary •
Output vs input =
• •
=
1
High ripple current =2
Practical for D not lower than 0.2
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Lecture 6 Exercises, buck-converter
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7-1 •
In a step-down converter, consider all components to be ideal. Let vo » Vo 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: Vd is 10-40 V , Po ³5 W, and fs = 50 kHz.
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7-2 •
Consider all components to be ideal. Assume Vo = 5 V, fs = 20 kHz, L = 1 mH, and C = 470 µF.
•
Calculate DVo (peak-peak) if Vd = 12.6 V, and I0 = 200 mA.
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Step-up (boost) converter § Output must be larger than input voltage § Otherwise is Vd driving Vo directly => Vo = Vd
§ Load energy into inductor, then output energy into load while still consuming energy from source § C large enough to give vo(t) » Vo
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Step-up converter waveform, continuous conduction mode § Switch on, diode off
§ Switch off, diode on
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Step-up converter, continuous mode
/
+
=
−
Lossless circuit: ⇒
=
= 1−
1 1− =
=0
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Boundary between continuous and discontinuous mode =
1 2
=
=
=
2
,
2
2
=
1 2
1−
1−
1−
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Boundary between continuous and discontinuous mode max when D = 0.5 ,
,
=
8 max when D = 1/3 2 = = 0.074 27 = 4D 1 − , 27 = 1− , 4
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Step-up, discontinuous mode +
= 4 = 27
2
− Δ Δ + = Δ Δ = Δ +
=0
+Δ
=
Δ 2 Controller −1
,
/
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Step up converter characteristics, Vo constant ,
= 0.074
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Effects of parasitics § Losses in L, diode, switch, C § Limited also by acceptable D ratio
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Output voltage ripple Δ
Δ
τ=
=
=
Δ
=
=
τ
where time constant
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Lecture 6 Exercises on boost converter
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7-7 • In a step-up converter, consider all components to be ideal. Let Vd be 8-16 V, Vo = 24 V (regulated), fs = 20 kHz, and C = 470 µF. • Calculate Lmin that will keep the converter operating in a continuous-conduction mode if Po ³ 5 W.
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7-8 • In a step-up converter, Vd =12 V, Vo = 24 V, I0 = 0.5 A, L = 150 µH, C = 470 µF, and fs =20 kHz. • Calculate DVo (peak-peak).
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Lecture 6 Other converter topologies
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Flyback converter § Derived from buck-boost structure § Second winding gives electric isolation § Only flux flow in one direction § Never negative currents in the transformer
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Flyback converter circuit states § Switch on and switch off § Continuous conduction mode § Incomplete demagnetization
§ Lm size important § Ideal transformer have inifinite Lm
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Flyback converter waveforms § Same control equation as for buck-boost converter
=
=
1−
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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
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Alternative flyback converter topologies § Paralleling flyback converter § Same frequency of switching § Phase-shifting switches π
§ Allow higher power § Redundancy § Increased effective switching frequency
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Forward converter § Derived from step-down converter § Ideal transformer assumed § Transformer magnetizing current not included § Converter failure if not taken care of
=
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Practical forward converter § Feed magnetic current back to source § Requires a third winding
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Practical forward converter waveforms § To guarantee demagnitized transformer
1−
=1− =
=
1+
1
/
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Other forward converter topologies § Two-switch forward converter § Commonly used § Voltage rating half of single transistor case § No snubbers necessary
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Other forward converter topologies § Parallelled forward converter § Same advantages as parallelled flyback converter
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Push-pull converter § Derived from step-down converter § Diodes due to leakage inductances § PWM control =2
0<
< 0.5
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Half-bridge converter § Derived from step-down converter § Additional diodes for switch protection
0<
=
< 0.5
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Full-bridge converter § Derived from step-down converter § Switches carry half the current compared to the half bridge converter
=2
0<
< 0.5
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Current-source dc-dc converter § Ld and D > 0.5 gives current source input § One or both switches always on
§ Operates like a step-up converter
=
1 2 1−
> 0.5
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