Dc-Dc Converter

EE3501E – Power Electronics
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
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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.
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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