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參數(shù)資料
| 型號： | LTC1871 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Quadruple 2-Input Positive-AND Gate 14-TVSOP -40 to 85 |
| 中文描述： | 寬輸入范圍，無檢測電阻電流模式升壓，反激和SEPIC控制器 |
| 文件頁數(shù)： | 26/36頁 |
| 文件大?。?/td> | 411K |
| 代理商： | LTC1871 |
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26
LTC1871
MOSFET to ensure the V
DS
remains below the maximum
rating for the device.
During the MOSFET’s on-time, the control circuit limits the
maximum voltage drop across the power MOSFET to
about 150mV (at low duty cycle). The peak inductor
current is therefore limited to 150mV/R
DS(ON)
. The rela-
tionship between the maximum load current, duty cycle
and the R
DS(ON)
of the power MOSFET is:
R
V
I
V
V
V
DS ON
(
SENSE MAX
O MAX
(
T
O
IN MIN
(
D
)
)
(
)
)
≤
+
+
+
1
1
2
1
1
χ
ρ
The V
SENSE(MAX)
term is typically 150mV at low duty cycle
and is reduced to about 100mV at a duty cycle of 92% due
to slope compensation, as shown in Figure 8. The constant
‘
χ
’ in the denominator represents the ripple current in the
inductors relative to their maximum current. For example,
if 30% ripple current is chosen, then
χ
= 0.30. The
ρ
T
term
accounts for the temperature coefficient of the R
DS(ON)
of
the MOSFET, which is typically 0.4%/
°
C. Figure 9 illus-
trates the variation of normalized R
DS(ON)
over tempera-
ture for a typical power MOSFET.
Another method of choosing which power MOSFET to use
is to check what the maximum output current is for a given
R
DS(ON)
since MOSFET on-resistances are available in
discrete values.
I
V
R
V
V
V
O MAX
(
SENSE MAX
DS ON
(
T
O
IN MIN
(
D
)
)
(
)
)
≤
+
+
+
1
1
2
1
1
χ
ρ
Calculating Power MOSFET Switching and Conduction
Losses and Junction Temperatures
In order to calculate the junction temperature of the power
MOSFET, the power dissipated by the device must be
known. This power dissipation is a function of the duty
cycle, the load current and the junction temperature itself.
As a result, some iterative calculation is normally required
to determine a reasonably accurate value. Since the con-
troller is using the MOSFET as both a switching and a
sensing element, care should be taken to ensure that the
converter is capable of delivering the required load current
over all operating conditions (load, line and temperature)
and for the worst-case specifications for V
SENSE(MAX)
and
the R
DS(ON)
of the MOSFET listed in the manufacturer’s
data sheet.
The power dissipated by the MOSFET in a SEPIC converter
is:
The first term in the equation above represents the I
2
R
losses in the device and the second term, the switching
losses. The constant k = 1.7 is an empirical factor inversely
related to the gate drive current and has the dimension of
1/current.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
T
J
= T
A
+ P
FET
R
TH(JA)
The R
TH(JA)
to be used in this equation normally includes
the R
TH(JC)
for the device plus the thermal resistance from
the board to the ambient temperature in the enclosure.
This value of T
J
can then be used to check the original
assumption for the junction temperature in the iterative
calculation process.
P
I
D
–
D
R
D
k
V
V
I
D
–
D
C
f
FET
O MAX
(
MAX
MAX
.
1 85
DS ON
(
MAX
T
IN MIN
(
O
O MAX
(
MAX
MAX
RSS
=
+
+
(
)
)
)
)
)
1
1
2
ρ
SEPIC Converter: Output Diode Selection
To maximize efficiency, a fast-switching diode with low
forward drop and low reverse leakage is desired. The
output diode in a SEPIC converter conducts current during
the switch off-time. The peak reverse voltage that the
diode must withstand is equal to V
IN(MAX)
+ V
O
. The
average forward current in normal operation is equal to the
output current, and the peak current is equal to:
I
I
V
V
V
D PEAK
(
O MAX
(
O
IN MIN
(
D
)
)
)
=
+
+
+
1
2
1
χ
The power dissipated by the diode is:
P
D
= I
O(MAX)
V
D
APPLICATIOU
W
U
U
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