A704
The operation of this regulator topology has two distinct time periods. The first one occurs when the series
switch is “on”, the input voltage is connected to the input of the inductor. The output of the inductor is the output
voltage, and the rectifier (or catch diode) is reverse biased. During this period, since there is a constant voltage source
connected across the inductor, the inductor current begins to linearly ramp upwards, as described by the following
equation:
(V −V )×t
on
in
out
I
=
L(on)
L
During this “on” period, energy is stored within the core material in the form of magnetic flux. If the inductor is
properly designed, there is sufficient energy stored to carry the requirements of the load during the “off” period.
The next period is the “off” period of the power switch. When the power switch turns off, the voltage across the
inductor reverses its polarity and is clamped at one diode voltage drop below ground by the catch diode. The current
now flows through the catch diode thus maintaining the load current loop. This removes the stored energy from the
inductor. The inductor current during this time is:
Vout × (Ts − ton
)
IL(off )
=
L
This period ends when the power switch is once again turned on. Regulation of the converter is accomplished by
varying the duty cycle of the power switch. It is possible to describe the duty cycle as follows:
t
on
D =
T
s
Where, T is period of switching, and t
is ” turn on” time of Q1.
on
s
For the buck converter with ideal components, the duty cycle can also be described as: Figure 3 shows the buck
converter, idealized waveforms of the catch diode voltage and the inductor current.
Vd - Vout
Time
-Vout
IPK
ILoad(AV)
Time
t
T
0
on
s
Fig 3. Basic Buck converter
Contack Info:Samsun Zhang 13556850583 IC9898@163.com
Copyright © 2008 ADDtek Corp.
7
A704_V0.6 -- AUGUST 2008
ADDTEK [ ADDTEK CORP ]
