LT3799EMSE 查看數據表（PDF） - Linear Technology
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LT3799EMSE Datasheet PDF : 20 Pages
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LT3799
OPERATION
Transformer Design Considerations
Transformer specification and design is a critical part of
successfully applying the LT3799. 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 LT3799. Table 1 shows the details of several of these
transformers.
Loop Compensation
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.
MOSFET and Diode Selection
With a strong 1.9A gate driver, the LT3799 can effectively
drive most high voltage MOSFETs. A low Qg MOSFET is
recommended to maximize efficiency. In most applications,
the RDS(ON) should be chosen to limit the temperature rise
of the MOSFET. The drain of the MOSFET is stressed to
VOUT • NPS + VIN during the time the MOSFET is off and
the secondary diode is conducting current. But in most
applications, the leakage inductance voltage spike exceeds
this voltage. The voltage of this stress is determined
by the switch voltage clamp. Always check the switch
waveform with an oscilloscope to make sure the leakage
inductance voltage spike is below the breakdown voltage
of the MOSFET. A transient voltage suppressor and diode
are slower than the leakage inductance voltage spike,
therefore causing a higher voltage than calculated.
Table 1. Predesigned Transformers—Typical Specifications, Unless Otherwise Noted
TRANSFORMER SIZE
PART NUMBER (L × W × H)
JA4429
21.1mm × 21.1mm × 17.3mm
7508110210 15.75mm × 15mm × 18.5mm
750813002
15.75mm × 15mm × 18.5mm
750811330
43.2mm × 39.6mm × 30.5mm
750813144
16.5mm × 18mm × 18mm
750813134
16.5mm × 18mm × 18mm
750811291
31mm × 31mm × 25mm
750813390
43.18mm × 39.6mm ×
30.48mm
750811290
31mm × 31mm × 25mm
X-11181-002 23.5mm × 21.4mm × 9.5mm
LPRI
NPSA
RPRI
(µH)
(NP:NS:NA)
(mΩ)
400
1:0.24:0.24
252
2000
6.67:1:1.67
5100
2000
20:1.0:5.0
6100
300
6:1.0:1.0
150
600
4:1:0.71
2400
600
8:1:1.28
1850
400
1:1:0.24
550
100
1:1:0.22
150
460
1:1:0.17
600
500
72:16:10
1000
RSEC
(mΩ)
126
165
25
25
420
105
1230
688
560
80
MANUFACTURER
Coilcraft
Würth Elektronik
Würth Elektronik
Würth Elektronik
Würth Elektronik
Würth Elektronik
Würth Elektronik
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
Würth Elektronik
Premo
125V/0.32A
30V/0.5A
3799p
15
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