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參數(shù)資料
| 型號： | AND8112 |
| 廠商： | ON SEMICONDUCTOR |
| 英文描述： | A Quasi-Resonant SPICE Model Eases Feedback Loop Designs |
| 中文描述： | 阿準(zhǔn)諧振SPICE模型反饋環(huán)的設(shè)計(jì)更易 |
| 文件頁數(shù)： | 1/12頁 |
| 文件大?。?/td> | 205K |
| 代理商： | AND8112 |
Semiconductor Components Industries, LLC, 2003
October, 2003 Rev. 1
1
Publication Order Number:
AND8112/D
AND8112/D
A QuasiResonant SPICE
Model Eases Feedback
Loop Designs
Prepared by: Christophe Basso
Prepared by:
Joel Turchi
ON Semiconductor
Within the wide family of Switch Mode Power Supplies
(SMPS), the Flyback converters represent the structure of
choice for use in small and medium power applications. For
compact
designs
and
applications, e.g. TV sets or settop boxes, Quasi Resonant
power supplies start to take a significant market share over
the traditional fixed frequency topology. However, if the
feedback loop control is well understood with this latter, for
instance via a comprehensive literature and SPICE models,
the situation differs for selfoscillating variable switching
frequency structures where no model still exists. This article
will show how a simple largesignal averaged SPICE model
can be derived and used to ease the design work during
stability analysis.
radiofrequency
sensitive
QuasiResonant Operation
It is difficult to abruptly dig into the analytical analysis
without giving a basic idea of the operation of a converter
working in QuasiResonance (QR). Figure 1 depicts a
typical FLYBACK converter drainsource waveform as you
probably have already observed. When the switch is closed,
the drainsource voltage V
DS
is near 0 V and the input
voltage V
g
appears across the primary inductance L
P
: the
current inside L
P
ramps up with a slope of
SON
Vg
LP
(eq. 1)
When the controller instructs the switch opening, the
drainsource quickly rises and the energy transfer between
primary and secondary takes place: the secondary diode
conducts and the output voltage flies back on the primary
side, over L
P
. This “Flyback” plateau is equal to V
g
+ (V +
V
f
) / N, where N is the secondary to primary turn ratio, V the
output voltage and V
f
the diode forward voltage drop.
During this time, the primary current decreases with a slope
now imposed by the reflected voltage
SOFF
(V
N
Vf)
LP
(eq. 2)
Figure 2 zooms on the simulated primary current (actually
circulating in the magnetizing inductor), showing how it
moves over one switching cycle.
ON
OFF
Figure 1. A Typical FLYBACK DrainSource
Waveform
Core is Reset
Valleys
V
in
Plateau:
(V
out
+ V
f
)/N
Leakage Inductance
OFF
Figure 2. The Primary Current Ramps Up and Down
to Zero in DCM
ON
0
I
peak
S
on
= V
g
/L
P
I
P
= 0, Reset
S
off
= (V + V
f
) / (L
P
x N)
When the primary current reaches zero, the transformer
core is fully demagnetized: we are in Discontinuous
Conduction Mode (DCM). The primary inductance L
P
together with all the surrounding capacitive elements C
tot
create a LC filter. When the secondary diode stops conducting
at I
P
= 0, the drain branch is left floating since the MOSFET
is already open. As a result, a natural oscillation occurs,
exhibiting the following frequency value:
Fring
1
LP
2
Ctot
(eq. 3)
http://onsemi.com
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