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AN021 Schematic ( PDF Datasheet ) - AIC

Teilenummer AN021
Beschreibung A Handy Method to Obtain Satisfactory Response of Buck Converter
Hersteller AIC
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Gesamt 7 Seiten
AN021 Datasheet, Funktion
AN021
A Handy Method to Obtain Satisfactory Response of
Buck Converter
Introduction
The focus of this application note is to help users to
select a good set of components of the compensation
section of the Buck converter. Traditionally, engineers
first concern about the power circuit for a circuit design.
After meeting the power requirement, what follows is
control loop design. The first priority of control design is
the stability. And after that, the choice of components
can substantially effect the transient response. Here
shows a simple method to obtain a satisfactory
transient response with an acceptable steady state
error.
Analysis:
Fig.1 shows the unity feedback system for Buck
converter. We first identify transfer functions for each
of the corresponding block.
As to the modeling of the low-frequency behavior of
power switches in square-wave power converters,
please refer to appendix [1]. The circuit of Buck
converter is shown in Fig.2 and the model of its power
switches is shown in Fig.3. Please note that the circuit
in Fig.3 is linear. Fig.4 shows the circuit for small signal
analysis.
+
dVOUT(s)=0
_
Gc(s)
PWM
Gp(s) dVOUT(s)
dVc(s)
dD(s)
Fig.1 The Unity Feedback Control Loop for Buck Converter
R1
VIN
R2
VOUT
+
Fig. 2 Buck Converter
October, 2001
1






AN021 Datasheet, Funktion
AN021
R2C3=
1
250k
. C3=7.14 8p
Fig.12 The Transient Response
(Ch2 is current curve, 1A/div)
idea to verify the steady state error by the load
regulation. From Fig.12, it shows that the steady state
error of two settings is small, and the two curves are
almost identical.
5.1
5.05
5
4.95
C2=820p R2=560k
C3=8p
C2=6800p R2=75k
C3=56p
4.9
0
2
4 Iou6t(A) 8
10 12
Fig.14 The Comparison of Load Regulation of
Different Components (VIN=8V, VOUT=5V)
From the results of transient response and the steady
state error, the second set of components can achieve
a satisfactory response.
Fig.13
The Corresponding Simulation of
Frequency Response
Referring to Fig. 13, the crossover frequency of open
loop transfer function is raised. And we could also
expect the bandwidth of the closed loop transfer
function being increased. Indeed, in Fig 12, the set of
control components, which we just used, shows high
speed of response and less overshoot.
Although the pole at origin (according to equation (5))
means the infinitive gain at DC (ω=0), it is still a good
October, 2001
Summary
By taking the advantage of a high gain at low
frequency with a low gain at high frequency, the
best performance of the Buck Converter can be
achieved. The objectives are to improve the steady
state error at low frequency and reduce noise
disturbance at high frequency.
The key points of the compensation are:
1) Locate the zero around the LC resonant
frequency for the issue of stability.
2) Locate the pole around
1
5
fSwitching
for
the
noise
reduction at high frequency.
3) The combination of the above 2 procedures can
make the crossover frequency at the -20dB/
decade situation. Thus a satisfactory phase
6

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