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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)： | 26/32頁 |
| 文件大?。?/td> | 247K |
| 代理商： | AN-16 |
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AN-16
A
6/96
26
switching power supply. Peak voltage or current stress levels
are higher than quasi square wave power converters, depending
on whether a zero-voltage switching or a zero-current switching
topology is used. The most effective resonant converters use
both high side and low side switches, adding to circuit
complexity. Resonant converters are not cost effective at low
output power levels.
Flyback Theory
Basic Flyback Operation
A basic flyback power supply circuit utilizing
TOPSwitch
is
shown in Figure 1. Transformer T1 is used both for energy
storage, output isolation, and output voltage transformation.
When the
TOPSwitch
is on, secondary diode D2 is reverse
biased, and current ramps up in the transformer primary winding
according to the equation
I
I
V
V
t
L
PRI
I
IN
DS ON
(
ON
P
=
+
×
(
)
)
(1)
I
is the primary current in amperes, I
is the initial value of the
primary current in amperes, V
is the DC input voltage after the
bridge, V
is the drain to source voltage drop across the
TOPSwitch
output MOSFET, t
is the on time of the
TOPSwitch
,
and L
is the transformer primary inductance in Henries. Since
the transformer is isolated from the output load circuit by the
reverse biased D2, energy is supplied to R
L
from the output
capacitor C1 during the
TOPSwitch
on time.
When the
TOPSwitch
turns off, the magnetic flux in the
transformer core starts to decay, and the polarity of the secondary
winding is reversed. D2 turns on, and the energy stored in the
transformer during the on time of the
TOPSwitch
is discharged
into the load circuit, supplying current to the load R
and
replenishing the charge depleted from C1 during the on time.
The initial value of the secondary current at the instant the
TOPSwitch
turns off will be equal to I
x N
/N
, where I
P
is the
peak value of I
at the end of
TOPSwitch
on time and N
is the
number of primary turns and N
is the number of secondary
turns. The secondary current decays from its initial value
according to Equation (2).
I
I
N
N
V
V
t
L
N
N
SEC
P
P
S
O
D
OFF
P
S
)
P
=
×
+
×
×
×
(
)
2
2
2
(
I
SEC
≥
0
(2)
V
is the output voltage of the supply, V
is the forward voltage
drop of D2, and t
is the
TOPSwitch
off time. If the secondary
current decays to zero during the off time of the primary switch,
the output current is then supplied by the output capacitor C1.
There are two distinct modes of flyback supply operation,
depending on the value of I
at the end of the
TOPSwitch
off
time. If I
decays to zero at or before the end of the
TOPSwitch
off time, the supply is running in the discontinuous mode. If I
is greater than zero at the end of the off time, the supply is
running in the continuous mode of operation.
Ideal Model (Discontinuous Mode)
There are three distinct intervals of circuit operation for flyback
power supplies operating in the discontinuous mode as shown
in Figure 6.
The first interval (1) of operation occurs when the
TOPSwitch
is on. Current I
ramps up linearly in the transformer primary
winding, causing a magnetic field to build in the transformer
0
PI-1616-021496
VIN
D2
C1
RL
VO
-
+
+
-
+
-
IPRI
ISEC
VIN
V
DRAIN
1
Interval
I
PRI
2
3
I
I
SEC
VIN+VOR
VDS(ON)
DRAIN
SOURCE
CONTROL
P
VOR
Figure 6. Ideal Flyback Converter Waveforms - Discontinuous Mode.
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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