Datasheet

LT3799
14
3799fa
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
Figure 3. Clamp
operaTion
Table 1. Predesigned Transformers—Typical Specifications, Unless Otherwise Noted
TRANSFORMER
PART NUMBER
SIZE
(L × W × H)
L
PRI
(µH)
N
PSA
(N
P
:N
S
:N
A
)
R
PRI
(mΩ)
R
SEC
(mΩ) MANUFACTURER
TARGET
APPLICATION
(V
OUT
/I
OUT
)
JA4429 21.1mm × 21.1mm × 17.3mm 400 1:0.24:0.24 252 126 Coilcraft 22V/1A
7508110210 15.75mm × 15mm × 18.5mm 2000 6.67:1:1.67 5100 165 Würth Elektronik 10V/0.4A
750813002 15.75mm × 15mm × 18.5mm 2000 20:1.0:5.0 6100 25 Würth Elektronik 3.8V/1.1A
750811330 43.2mm × 39.6mm × 30.5mm 300 6:1.0:1.0 150 25 Würth Elektronik 18V/5A
750813144 16.5mm × 18mm × 18mm 600 4:1:0.71 2400 420 Würth Elektronik 28V/0.5A
750813134 16.5mm × 18mm × 18mm 600 8:1:1.28 1850 105 Würth Elektronik 14V/1A
750811291 31mm × 31mm × 25mm 400 1:1:0.24 550 1230 Würth Elektronik 85V/0.4A
750813390 43.18mm × 39.6mm ×
30.48mm
100 1:1:0.22 150 688 Würth Elektronik 90V/1A
750811290 31mm × 31mm × 25mm 460 1:1:0.17 600 560 Würth Elektronik 125V/0.32A
X-11181-002 23.5mm × 21.4mm × 9.5mm 500 72:16:10 1000 80 Premo 30V/0.5A
3799 F03
GATE
V
SUPPLY