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LT3799
OPERATION
V SUPPLY
GATE
3799 F03
Figure 3. Clamp
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 Q g MOSFET is
recommended to maximize efficiency. In most applications,
the R DS(ON) should be chosen to limit the temperature rise
of the MOSFET. The drain of the MOSFET is stressed to
V OUT ? N PS + V IN 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
Table 1. Predesigned Transformers—Typical Specifications , Unless Otherwise Noted
TRANSFORMER
PART NUMBER
JA4429
7508110210
750813002
750811330
750813144
750813134
750811291
750813390
750811290
X-11181-002
SIZE
(L × W × H)
21.1mm × 21.1mm × 17.3mm
15.75mm × 15mm × 18.5mm
15.75mm × 15mm × 18.5mm
43.2mm × 39.6mm × 30.5mm
16.5mm × 18mm × 18mm
16.5mm × 18mm × 18mm
31mm × 31mm × 25mm
43.18mm × 39.6mm ×
30.48mm
31mm × 31mm × 25mm
23.5mm × 21.4mm × 9.5mm
L PRI
(μH)
400
2000
2000
300
600
600
400
100
460
500
N PSA
(N P :N S :N A )
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
R PRI
(mΩ)
252
5100
6100
150
2400
1850
550
150
600
1000
R SEC
(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
Würth Elektronik
Premo
TARGET
APPLICATION
(V OUT /I OUT )
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
3799fa
14
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