# Dc-Dc Converter

Topics: Diode, Transistor, Power semiconductor device Pages: 14 (1381 words) Published: April 7, 2013
EE3501E – Power Electronics
Lecture 2
Text
Power Electronics ‐ A First Course Author: Ned Mohan ‐ / 2012 John Wiley and Sons

1

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)

2

Linear vs Switch mode conversion

3

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
5

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
6

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 .

8

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.

9

Design of Switching Power‐Poles
• Power Semiconductor Devices
– Diodes – Transistors

• Losses in Switching Power‐Poles
– Switching Losses – Conduction Losses

10

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
11

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

•...