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參數(shù)資料
| 型號： | AN-35 |
| 英文描述： | LinkSwitch Design Guide |
| 中文描述： | 使用LinkSwitch設(shè)計(jì)指南 |
| 文件頁數(shù)： | 2/16頁 |
| 文件大?。?/td> | 504K |
| 代理商： | AN-35 |
AN-35
2
B
4/03
During CC operation, duty cycle is controlled by the peak drain
current limit (I
). The device current limit is designed to be a
function of reflected voltage such that the load current remains
approximately constant as the load impedance is reduced. When
the output voltage falls to approximately 30% of nominal value
(normally associated with a failed battery),
LinkSwitch
enters the
auto-restart mode of operation to safely limit average fault current
(typically 8% of I
O
).
With discontinuous mode design, maximum output power is
independent of input voltage and is a simple function of primary
inductance, peak primary current squared and switching
frequency (Equation 6).
LinkSwitch
controls and cancels out
variations normally associated with frequency and peak current
by specifying a device I
2
f term. This allows users to easily
design for a specific corner point where CV mode transitions to
CC mode.
Scope
This application note is for engineers designing an AC-DC
power supply using the
LinkSwitch
LNK500 or LNK501 devices
in a discontinuous mode flyback converter. Since
LinkSwitch
is
designed to replace linear transformer based power supplies,
the output characteristic provides an approximate CV
characteristic, offering much better line and load regulation
than an equivalent linear transformer. The very simple nature of
the
LinkSwitch
circuit allows an initial paper design to be
completed quickly using simple design equations. It is then
recommended that the circuit performance be tuned with a
prototype power supply to finalize external component choices.
This document therefore highlights the key design parameters
and provides expressions to calculate the transformer turns
ratio, primary inductance and clamp/feedback component values.
This enables designers to build an operating prototype and
iterate to reach the final design.
For readers who want to generate a design as quickly as
possible, the Quick Start section provides enough information
to generate an initial prototype.
This document does not address transformer construction.
Please see
LinkSwitch
DAK Engineering Prototype Report for
examples showing typical transformer construction techniques.
Further details of support tools and updates to this document
can be found at
www.powerint.com
.
CV/CC Circuit Design
The
LinkSwitch
circuit shown in Figure 3 serves as a CV/CC
charger example to illustrate design techniques. Nominal output
voltage is 5.5 V and nominal CC output current is 500 mA.
LinkSwitch
design methodology is very simple. Transformer
turns ratios and bias component values are selected at the
Figure 1 shows the key parameters and components
needed to generate an initial
LinkSwitch
design.
Where initial estimates can be used, they are shown
below the parameter they refer to.
1)
Let V
OR
equal 50 V.
2)
Define the transformer turns ratio according to
Equation 5. If no better estimates or measure-
ments are available, then let V
equal 0.7 V
for a Schottky or 1.1 V for a PN diode, R
equal
0.3
, R
equal 0.15
, I
equal 2
x I
, and
I
equal 4 x I
, where I
is the desired CC
output current and V
is the desired output
voltage at the CV/CC transition point.
3)
Calculate P
according to Equation 13. As an
initial estimate for P
CORE
use 0.1 W.
4)
Calculate L
according to Equation 14 and
other transformer parameters from Equations
15, 16, 17, 18 and 19.
5)
Calculate value for feedback resistor R
accord-
ing to Equations 20, 21, 22, 23 and 24.
This should be a 1/4 W, 1% part.
6)
Set clamp capacitor C
as a 0.1
μ
F, 100 V
metalized plastic film type.
7)
Set clamp resistor R
LF
as 100
, 1/4 W.
8)
Set CONTROL pin capacitor C
to be 0.22
μ
F,
10 V for battery loads or 1
μ
F, 10 V for resistive
loads.
9)
Select input and output components. See
Figure 3 and relevant sections.
10) Construct prototype.
11) Iterate design (see Hints and Tips section).
QUICK START
nominal peak power point output voltage V
,
while transformer
primary inductance is calculated from the total output power.
Few components require computations, while the balance are
selected from the included recommendations.
Design and selection criteria for each component are described
starting with the transformer. Once set, transformer parameters
and behavior are used to design clamp, bias and feedback
components for proper supply operation. Output capacitors and
the input circuitry can then be determined.
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