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參數(shù)資料
| 型號： | AN-16 |
| 英文描述： | TOPSwitch Flyback Design Methodology |
| 中文描述： | TOPSwitch的反激式設(shè)計方法 |
| 文件頁數(shù)： | 21/32頁 |
| 文件大?。?/td> | 247K |
| 代理商： | AN-16 |
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A
6/96
AN-16
21
Step 23. Determine
secondary parameters I
SP,
I
SRMS
,
I
RIPPLE
, DIA
S,
OD
S
The secondary peak current I
can be derived from the primary
peak current I
P
and the turns ratio between primary and secondary
N
P
/N
S
I
I
N
N
SP
P
P
S
=
×
The K
of the secondary is always identical to that of the
primary, since it is only a reflected version of the primary
current with duty cycle (1-D). Therefore, the secondary RMS
current I
can be expressed in a manner similar to the primary
RMS current, only with D
MAX
replaced by (1-D
MAX
).
I
I
D
K
K
SRMS
SP
MAX
RP
3
RP
=
×
×
+
(
)
1
1
2
I
is the RMS ripple current of the output capacitor. Because
of current conservation, it is found that:
I
I
I
RIPPLE
SRMS
O
=
2
2
I
is the power supply output current which can be calculated,
if not already specified, as
I
P
V
O
O
O
=
With the secondary RMS current I
available, the minimum
secondary wire diameter DIA
S
(in mm), can be calculated as
follows:
DIA
CMA
1 27
.
I
S
SRMS
=
×
×
×
×
4
25 4
1000
.
π
Note that in the AN-17 spreadsheet, a practical wire size,
AWG
, is derived from primary current capacity CMA and
secondary RMS current I
using an empirical equation.
DIA
S
is then determined from AWG
S
.
If the required secondary wire diameter turns out to be larger
than that of the 26 AWG wire which corresponds to twice the
skin depth at 100 KHz, a parallel configuration of windings
using a gauge equal to or smaller than 26 AWG should be used
to provide the same effective cross sectional area. The parallel
windings must have identical number of turns equal to N
. For
example, if the equation above indicates a 23 AWG wire, a
winding consisting of N
turns of two parallel strands of
26 AWG will be a good choice.
Note that if triple insulated wire is to be used for secondary, the
insulated wire diameter is actually larger than DIA
by twice the
thickness of the insulator. Therefore, the maximum outside
diameter OD
S
(in mm) must be calculated:
OD
BW
M
N
S
S
=
×
(
)
2
A triple insulated wire should be specified with a conductor
diameter equal to or greater than DIA
S
and an insulated outside
diameter equal to or less than OD
S
.
Step 24. Determine maximum peak inverse voltages
PIV
S
, PIV
B
for secondary and bias windings.
The peak inverse voltage across the secondary rectifier diode is
given by:
PIV
V
V
N
N
S
O
MAX
S
P
=
+
×
Similarly, the peak inverse voltage across the bias rectifier
diode is given by:
PIV
V
V
N
N
B
B
MAX
B
P
=
+
×
Step 25. Select clamp Zener and blocking diode for
primary clamping based on input voltage and V
CLO
(see Step 4)
Step 26. Select output rectifier
The peak inverse voltage across the secondary diode PIV
is
calculated in Step 24. The diodes should be chosen with a
reverse voltage rating V
equal to or greater than 1.25 X PIV
S
to keep the PIV
S
at no more than 80% of the diode V
R
rating.
The rule of thumb on the diode current rating is to choose one
with rated DC current of at least three times the maximum
output DC current.
Schottky diodes are recommended for V
less than 45V which
would correspond to low output voltages such as 5V or 3.3V.
For V
requirements that are higher than 45V, ultra fast recovery
PN diodes should be used for the lowest cost. (See Table 8 for
recommended diodes.)
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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