參數(shù)資料
型號(hào)： AN-16
英文描述： TOPSwitch Flyback Design Methodology
中文描述： TOPSwitch的反激式設(shè)計(jì)方法
文件頁(yè)數(shù)： 18/32頁(yè)
文件大小： 247K
代理商： AN-16
AN-16
A
6/96
18
I
I
K
D
P
AVG
RP
2
MAX
=
1
I
I
K
R
P
RP
=
×
I
I
D
K
K
RMS
P
MAX
RP
3
RP
=
×
×
+
2
1
Step 8. Select TOPSwitch based on TOPSwitch data
sheet minimum I
specification and required I
P
(from Step 7) such that:
0 9
×
≥
minimum
I
I
LIMIT
P
The minimum value of current limit I
in
TOPSwitch
data
sheet is specified at room temperature. To accommodate the
slight reduction of this parameter at high temperature, the room
temperature limit should be derated by 10%. This can be
accomplished by dividing the I
by 0.9 and comparing this value
to the minimum I
in the datasheet. The smallest
TOPSwitch
that has an I
higher than this value should be selected as the
first choice for the lowest cost.
Step 9 to Step 10. Check thermal limitation - Use
bigger TOPSwitch if necessary to reduce power loss
Calculate
TOPSwitch
conduction loss at low line:
P
I
R
C
IR
RMS
DS ON
(
=
×
°
2
100
)
(
)
Calculate
TOPSwitch
switching loss at low line:
P
C
V
V
f
CXT
XT
MAX
OR
S
×
×
+
×
1
2
2
(
)
where C
XT
is the external capacitance at the drain node.
Calculate junction temperature Tj of
TOPSwitch
as a
function of total loss
T
C
P
P
J
IR
CXT
JA
=
° +
+
×
25
(
)
θ
If Tj > 100
o
C, choose bigger
TOPSwitch
.
For non-critical applications, refer to AN-14 Table 2
for
TOPSwitch
recommendations with practical
heatsinking.
TOPSwitch
thermal environment can vary significantly from
application to application. Fully enclosed lap top adapters with
no ventilation pose significant limitations on the power that can
be dissipated inside the box without exceeding acceptable
surface temperatures on the outside of the box. Heat sinks in
this application only help to distribute the heat across the
surface of the box. The actual power capability at a given
surface temperature is determined largely by the surface area of
the box. In contrast, a PC power supply has a fan which
provides forced air cooling. Here a larger heat sink could be the
answer to higher power dissapation.
It is therefore important to first estimate the losses in the
TOPSwitch
to see whether it is acceptable in a given application.
The conduction losses (P
) at low line tend to be the dominant
loss factor and can be calculated using the I
and the R
at 100
°
C from the output characteristic curve in the
TOPSwitch
data sheet. If the losses are unacceptable, a larger
TOPSwitch
with a lower R
DS(ON)
could be chosen to lower the power
dissipation.
Switching losses at low line due to internal drain capacitance
are negligible and can be ignored. If significant external
capacitance C
is present, the switching losses (P
) should
also be estimated. Even though low line is usually the worst case
for
TOPSwitch
losses, it is prudent to verify this by calculating the
conduction and switching losses at high line, especially if there is
significant external capacitance on the drain.
Once the worst case loss in the
TOPSwitch
is known, the
maximum die temperature at worst case ambient (internal
ambient should be used for enclosed supplies) can be estimated
using the thermal impedance from die to tab/heat sink of the
PI-1902-61096
IP
IR
IP
}
IR
}
KRP =
P
KRP = 1.0
DRAIN CURRENT WAVEFORM SHAPES
Continuous Mode
(a)
Discontinuous Mode
(b)
Figure 12. Primary Current Waveform.
相關(guān)PDF資料
PDF描述
AN-17 Flyback Transformer Design for TOPSwitch Power Supplies
AN-18 TOPSwitch Flyback Transformer Construction Guide
AN-19 TOPSwitch Flyback Power Supply Efficiency
AN-20 Transient Suppression Techniques for TOPSwitch Power Supplies
AN-22 OBSOLETE when inventory is depleted. 10% tolerance no l
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