LT3798IMSE-PBF Просмотр технического описания (PDF) - Linear Technology
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LT3798IMSE-PBF Datasheet PDF : 20 Pages
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LT3798
OPERATION
voltage and the MOSFET switching frequency to determine
the expected power dissipation:
PSNUBBER = fSW • CSNUBBER • VDRAIN2/2
Decreasing the value of the capacitor will reduce the dis-
sipated power in the snubber at the expense of increased
peak voltage on the MOSFET drain, while increasing the
value of the capacitance will decrease the overshoot.
Transformer Design Considerations
Transformer specification and design is a critical part of
successfully applying the LT3798. In addition to the usual
list of caveats dealing with high frequency isolated power
supply transformer design, the following information
should be carefully considered. Since the current on the
secondary side of the transformer is inferred by the current
sampled on the primary, the transformer turns ratio must
be tightly controlled to ensure a consistent output current.
A tolerance of ±5% in turns ratio from transformer to
transformer could result in a variation of more than ±5% in
output regulation. Fortunately, most magnetic component
manufacturers are capable of guaranteeing a turns ratio
tolerance of 1% or better. Linear Technology has worked
with several leading magnetic component manufacturers
to produce predesigned flyback transformers for use with
the LT3798. Table 1 shows the details of several of these
transformers.
Loop Compensation
The voltage feedback loop is a traditional GM error ampli-
fier. The loop cross-over frequency is set much lower than
twice the line frequency for PFC to work properly.
The current output feedback loop is an integrator con-
figuration with the compensation capacitor between the
negative input and output of the operational amplifier.
This is a one-pole system therefore a zero is not needed
in the compensation. For offline applications with PFC,
the crossover should be set an order of magnitude lower
than the line frequency of 120Hz or 100Hz. In a typical
application, the compensation capacitor is 0.1μF.
In non-PFC applications, the crossover frequency may be
increased to improve transient performance. The desired
crossover frequency needs to be set an order of magnitude
below the switching frequency for optimal performance.
Table 1. Predesigned Transformers—Typical Specifications, Unless Otherwise Noted
TRANSFORMER SIZE
LPRI
PART NUMBER (L × W × H)
(µH)
JA4429
21.1mm × 21.1mm × 17.3mm
400
7508110210 15.75mm × 15mm × 18.5mm
2000
750813002 15.75mm × 15mm × 18.5mm
2000
750811330 43.2mm × 39.6mm × 30.5mm
300
750813144 16.5mm × 18mm × 18mm
600
750813134 16.5mm × 18mm × 18mm
600
750811291 31mm × 31mm × 25mm
400
750813390
43.18mm × 39.6mm × 30.48mm
100
750811290
31mm × 31mm × 25mm
460
X-11181-002 23.5mm × 21.4mm × 9.5mm
500
750811248 31mm × 31mm × 25mm
300
RLLT-1001
25mm × 22.2mm × 16mm
820
750312872 43.2mm × 39.6mm × 30.5mm
14
NPSA
(NP:NS:NA)
1:0.24:0.24
6.67:1:1.67
20:1.0:5.0
6:1.0:1.0
4:1:0.71
8:1:1.28
1:1:0.24
1:1:0.22
1:1:0.17
72:16:10
4:1.0:1.0
16:1.0:4.0
1:1:0.8
RPRI
(mΩ)
252
5100
6100
150
2400
1850
550
150
600
1000
280
1150
11
RSEC
(mΩ)
126
165
25
25
420
105
1230
688
560
80
25
10
11
MANUFACTURER
Coilcraft
Würth Elektronik
Würth Elektronik
Würth Elektronik
Würth Elektronik
Würth Elektronik
Würth Elektronik
Würth Elektronik
Würth Elektronik
Premo
Würth Elektronik
Renco
Würth Elektronik
TARGET
APPLICATION
(VOUT / IOUT)
22V/1A
10V/0.4A
3.8V/1.1A
18V/5A
28V/0.5A
14V/1A
85V/0.4A
90V/1A
125V/0.32A
30V/0.5A
24V/2A
5V/4A
28V/4A
3798fa
14
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