參數(shù)資料
型號(hào)： LT1105IN8
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS
中文描述： 1.5 A SWITCHING CONTROLLER, 200 kHz SWITCHING FREQ-MAX, PDIP8
封裝： 0.300 INCH, PLASTIC, DIP-8
文件頁數(shù)： 25/32頁
文件大?。?/td> 452K
代理商： LT1105IN8
25
LT1103/LT1105
where N is the transformer turns ratio of secondary to
primary and DC is the duty cycle. This formula can be
rewritten in terms of duty cycle as:
DC=
V
V
N V
OUT
+
OUT
IN
(
)
It is important to define the full range of input voltage, the
range of output loading conditions and the regulation
requirements for a design. Duty cycle should be calculated
for both minimum and maximum input voltage.
In many applications, N can vary over a wide range without
degrading performance. If maximum output power is
desired, N can be optimized:
N
=
V
Vf
– V
V – V
OPT
OUT
IN MAX
SNUB
(
)
)
+
(
)
where
Vf = Forward voltage of the output diode
V
M
= Maximum switch voltage
V
SNUB
= Snubber clamp level – primary flyback
voltage.
In the isolated flyback mode, the LT1103/LT1105 sense
and regulate the transformer primary voltage V
PRI
during
“switch off” time. The secondary output voltage will be
regulated if V
PRI
is regulated. V
PRI
is related to V
OUT
by:
V
Vf
N
This allows duty cycle for an isolated flyback converter to
be rewritten as:
V
V
PRI
An important transformer parameter to be determined is
the primary inductance L
PRI
. The value of this inductance
is a trade-off between core size, regulation requirements,
leakage inductance effects and magnetizing current
I.
Magnetizing current is the difference between the primary
current at the start of “switch on” time and the current at
V
=
PRI
OUT
+
(
)
DC=Duty Cycle=
V
PRI
+
IN
(
)
APPLICATIO
S I
FOR
ATIO
U
W
U
U
the end of “switch on” time. If maximum output power is
needed, a reasonable starting value is found by assigning
I a value of 20% of the peak switch current (2A for the
LT1103 and set by the external FET rating used with the
LT1105). With this design approach, L
PRI
is defined as:
L
V
( I)(f) 1
V
V
PRI
IN
IN
PRI
=
+
If maximum output power is not required, then
I can be
increased which results in lower primary inductance and
smaller magnetics. Maximum output power with an isolated
flyback converter is defined by the primary flyback voltage
and the peak allowed switch current and is limited to:
(
+
(
where
R = Total “switch” on resistance
I
P
= Maximum switch current
E = Overall efficiency
≈
75%
Peak primary current is used to determine core size for the
transformer and is found from:
=
(
P
V
V
V
V
I
2
– Ip R E
OUT(MAX)
PRI
PRI
IN
IN P
2
=
)
)
I
V
I
V
V
V
E V
I
2
PRI
OUT
OUT
PRI
IN
PRI
IN
)(
)
+
(
)(
)
)
+
A second consideration on primary inductance is the
transition point from continuous mode to discontinuous
mode. At light loads, the flyback pulse across the primary
will drop to zero before the end of “switch off” time. The
load current at which this starts to occur can be calculated
from:
I
V
V
V
V
2V
)( )(
OUT(TRANSITION)
PRI
2
) (
IN
2
PRI
IN
OUT
PRI
=
(
)
+
(
)
The forward converter as shown below is another
transformer-based topology that converts one voltage to
either a higher or a lower voltage.
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