Lecture 2
Text
Power Electronics ‐ A First Course Author: Ned Mohan ‐ / 2012 John Wiley and Sons
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Lecture 1 ‐ Review
• • • • • • What is Power Electronics? Applications of power electronics. Linear vs Switched power conversion. Switching‐power pole Inductors and capacitors‐basic properties. Pulse‐Width‐Modulation (PWM)
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Linear vs Switch mode conversion
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Switching Power‐Pole
+ Vin q
A
qA 1
vv A
+ vA 
A
Vin
0 0
t
• Bi‐Positional Switch • Voltage Port – where a capacitor is connected in parallel to Vin so that it cannot change instantaneously • Current Port – where an inductor is connected in series through which current cannot change instantaneously 4
Control by Pulse‐Width Modulation (PWM)
qA + 1
idA
d A Ts +
iA
0 Tup Ts vA 0 Vin
dA
t
Vin

vA

qA 1or 0
vA
t
• The power‐pole chops the input voltage into high‐frequency voltage pulses • Output is synthesized as the switching‐cycle‐average of these pulses • Within each switching cycle, the average value is controlled by the duty‐ratio
dA
Tup Ts 0 dA 1
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v A d AVin ,
Switching Power‐Pole in a Buck DC‐DC Converter: An Example qA iin 0 d ATs Ts Vin 1 t
Vin
iL vA
vA 0 Vo iL 0 iin
vA
t
qA
t
(a) 0 (b) t
Vo v A d AVin
0 Vo Vin
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Problems
P.1.23
In a Buck converter, the input voltage Vin 12V . The output voltage Vo is required to be 9V . The switching frequency f s 400kHz. Assume ideal switchign power pole, calculate the pulse width Tup of the switching signal and the duty  ratio d A of the power pole. 7
Problems
P.1.24
In a Buck converter in Problem P  1.23, Assuming the current through the inductor to be ripple  free with average value of 15A, draw the waveforms of voltage v A and the input current iin .
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Problems
P.1.25
Using the same specifications given in Problem 1.23, calculate the maximum Energy  Efficiency expected of a Linear Regulator where the excess input voltage is dropped across a transistor, placed in series between the intput and the output.
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Design of Switching Power‐Poles
• Power Semiconductor Devices
– Diodes – Transistors
• Losses in Switching Power‐Poles
– Switching Losses – Conduction Losses
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SELECTION OF POWER TRANSISTORS AND POWER DIODES
• Voltage Ratings
– Maximum instantaneous voltage that device can block when in OFF‐state
• Current Ratings
– Maximum current (instantaneous, average and/or rms) that device can carry in ON‐state
• Switching Speeds
– Speed of transition from ON to OFF state or vice‐versa
• On‐State Voltage Drop
– Voltage drop across the device when it in ON
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Choice of Power Transistors
Power (VA)
Thyristor
108 106 104 102
IGCT IGBT
IGCT (a)
IGBT
MOSFET
MOSFET
101 102 103 104 Switching Frequency (Hz)
• MOSFET (Metal‐Oxide‐Semiconductor Field‐Effect Transistor) • IGBT (Insulated‐Gate Bipolar Transistors) • IGCT (Insulated‐Gate‐Controlled Thyristors) • GTO (Gate‐Turn‐Off Thyristors) • Others (BJT, Thyristors) 12
MOSFET Characteristics
2.5 RDS ( on ) VDSSto 2.7 VDSS blocking voltage rating D iD G VGS VDS iD R DS ( on ) 1/slope VGS 11V 9V 7V 5V VGS VGS (th ) VDS iD Io
S
0
0
VGS ( th) VGS ( I o )
(a) n‐channel MOSFET
(b) i‐v characteristics of MOSFET
(c)
VGS
• MOSFETs have low on‐state loss at low voltages, and fast switching speeds • Suitable for voltages low 200V and switching frequencies in excess of 100kHz • MOSFETs cannot block negative‐polarity voltage due to the 13 intrinsic anti‐parallel diode.
IGBT Characteristics
C iC G VGE E VCE iC VGE
VCE
(a)
(b)
• Ease of control with low on‐state losses even at fairly high voltage ratings • Switching frequency up to 30kHz • Ratings up to 3.3kV and 1200A 14
Power Diodes
iAK
A
K
0
(b)
v AK
•...