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參數(shù)資料
| 型號： | AN-23 |
| 英文描述： | TinySwitch Flyback Design Methodology |
| 中文描述： | 據(jù)稱，TinySwitch反激式設計方法 |
| 文件頁數(shù)： | 5/12頁 |
| 文件大?。?/td> | 401K |
| 代理商： | AN-23 |
A
7/99
AN-23
5
Diode Type
Schottky
Ultrafast-PN
Fast-PN
V
D
(V)
0.5
1.0
1.0
Efficiency Loss
(0.5/V
O
)
×
100%
(1.0/V
O
)
×
100%
(1.0/V
O
)
×
100%
Table 2. Diode forward voltage (V
D
) and efficiency loss.
Step by Step Design Procedure
Symbols and parameters used in this design procedure are
defined in Application Note
“
TOPSwitch
Flyback Design
Methodology
”
(AN-16)
.
Step 1.
Determine system requirements: V
ACMIN
, V
ACMAX
, f
L
,
V
O
, P
O
,
η
Determine input voltage range from Table 1.
Step 2.
Select output diode. Estimate associated efficiency loss.
The output diode can be selected based on expected
power supply efficiency and cost (see Table 2).
- Use a Schottky diode for highest efficiency for
output voltages up to 7.5 V.
- For output voltages beyond 7.5 V use an Ultra Fast PN-
diode.
- If efficiency is not a concern (or cost is
paramount), use a Fast PN-diode.
- The Schottky and Ultrafast may be used with
continuous mode of operation. The Fast PN-diode should
be used only with discontinuous mode of operation.
- Choose output diode type. Table 2 shows approximate
forward voltage (V
D
) for types of output diode discussed
above.
Output diode efficiency loss is the power supply efficiency
reduction (in percentage) caused by the diode.
Input (VAC)
100/115
230
Universal
V
ACMIN
(VAC)
85
195
85
V
ACMAX
(VAC)
132
265
265
Table 1. Input Voltage Range.
The estimated efficiency loss due to the output diode is also
shown in Table 2.
Table 3 shows some commonly used output diodes. V
is the
diode reverse voltage rating. I
D
is the diode DC current
rating.
The final diode current rating is to be determined in Step 20
to accommodate continuous short circuit current I
OS
.
Table 3. Output diodes.
Output Diode
V
R
(V) I
D
(A)
Manufacturer
1N5819
1N5822
40
40
45
45
45
100
100
200
200
200
100
200
100
200
100
200
200
200
1.0
3.0
7.5
10.0
16.0
1.0
1.0
1.0
1.0
2.0
3.0
3.0
4.0
4.0
8.0
8.0
8.0
20.0
Motorola
Motorola
Motorola
Motorola
Motorola
GI
Motorola
Motorola
GI
Philips, GI
GI
GI
Motorola
Motorola
Motorola
Motorola
Philips, GI
Philips
Schottky MBR745
MBR1045
MBR1645
UF4002
MUR110
MUR120
UF4003
BYV27-200
UF5401
UFR
UF5402
MUR410
MUR420
MUR810
MUR820
BYW29-200
BYV32-200
Step 3.
Select clamp/snubber circuit and determine associated
efficiency loss.
Clamp/snubber circuit is required at DRAIN to keep DRAIN
voltage below rated BV:
- A snubber alone may be used at low power (< 3 W
with Universal input) and will provide lower video noise
and superior EMI performance.
- An RCD clamp may be used for power levels < 3 W for
higher efficiency and is required at power levels > 3 W
with Universal input.
Table 4 shows the approximate efficiency loss due to clamp/
snubber circuits.
Step 4.
Estimate power supply efficiency
η
.
Total efficiency loss is the sum of the output diode efficiency
loss (from Step 2) and the clamp/snubber efficiency loss
(from Step 3).
Clamp/Snubber
RC Snubber
RCD clamp
P
O
0 to 3 W
> 3 W
Efficiency Loss
20%
15%
Table 4. Clamp/Snubber efficiency loss.
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