Datasheet

LT3748
13
3748fa
APPLICATIONS INFORMATION
Table 1. Pre-Designed Transformers—Typical Specifications Unless Otherwise Noted
TRANSFORMER
PART NUMBER Size (W x L x H) mm
L
PRI
(μH)
L
LEAK
(nH)
N
PS
(N
P
:N
S
)
I
SAT
(A)
R
PRI
(mΩ)
R
SEC
(mΩ) MANUFACTURER
TARGET APPLICATION
†
INPUT (V) OUTPUT
750311424 17.7 × 14.0 × 12.7 100 844 3:1 3 180 29 Würth Electronics 40 to 75 12V/1A
750311456* 17.7 × 14.0 × 12.7 100 900 3:1 2.4 225 31 Würth Electronics 40 to 75 12V/1A
750311439 17.7 × 14.0 × 12.7 37 750 2:1 2.8 89 28 Würth Electronics 30 to 75 12V/1A
750311423 17.7 × 14.0 × 12.7 50 570 4:1 4 90 12 Würth Electronics 30 to 75 5V/3A
750311457 17.7 × 14.0 × 12.7 50 600 4:1 3.7 115 12 Würth Electronics 30 to 75 5V/3A
750311689 17.7 × 14.0 × 12.7 50 600 4:1 3.7 115 12 Würth Electronics 30 to 75 5V/3A
750311458* 17.7 × 14.0 × 12.7 15 175 3:1 5 35 6 Würth Electronics 10 to 40 5V/2.5A
750311564 17.7 × 14.0 × 12.7 9 120 3:1 8 36 7 Würth Electronics 10 to 40 5V/3A
750311624 17.7 × 14.0 × 12.7 9 150 1.5:1 8 34 21 Würth Electronics 10 to 40 15V/1A
750311604 29.08 × 23.11 × 11.43 8 300 1:1 9.5 30 12 Würth Electronics 10 to 40 24V/1.3A
750311599 29.08 × 23.11 × 11.43 8 500 1.5:1 12 30 12 Würth Electronics 10 to 40 15V/2A
750311600 29.08 × 23.11 × 11.43 12 500 3:1 11 30 40 Würth Electronics 20 to 75 15V/2A
750311608 29.08 × 23.11 × 11.43 12 500 1.5:1 9 30 20 Würth Electronics 20 to 75 24V/1.3A
750311607 29.08 × 23.11 × 11.43 14 500 2.5:1 9.5 40 10 Würth Electronics 20 to 75 12V/2.5A
750311590 32.31 × 27.03 × 13.69 8 200 2:1 18 15 8 Würth Electronics 10 to 40 12V/3.8A
750311591 32.31 × 27.03 × 13.69 8 200 1.5:1 20 15 12 Würth Electronics 10 to 40 15V/3A
750311592 32.31 × 27.03 × 13.69 8 200 1:1 18 15 20 Würth Electronics 10 to 40 24V/1.9A
750311594 32.31 × 27.03 × 13.69 15 400 2.33:1 18 35 15 Würth Electronics 20 to 75 12V/3.8A
750311595 32.31 × 27.03 × 13.69 12 200 3:1 18 15 12 Würth Electronics 20 to 70 15V/3A
750311596 32.31 × 27.03 × 13.69 12 200 1.5:1 16 30 30 Würth Electronics 20 to 70 24V/1.9A
PA2367NL 17.7 × 14.0 × 12.7 85 750 2.7:1 1.7 325 26 Pulse Engineering 20 to 75 12V/1A
PA1276NL 17.7 × 14.0 × 12.7 77.4 800 1.47:1 1.6 100 75 Pulse Engineering 20 to 75 12V/1A
PA2467NL 17.7 × 14.0 × 12.7 37 750 2:1 2.9 89 28 Pulse Engineering 20 to 75 12V/1A
PA1260NL 17.7 × 14.0 × 12.7 77.4 800 3.67:1 1.5 220 18 Pulse Engineering 20 to 75 5V/2A
PA3177NL 29.21 × 21.84 × 11.43 8.3 100 2:1 8.6 10 7 Pulse Engineering 10 to 40 10V/2.5A
*2.5k isolation, others are rated for 1.5kV isolation.
†
TARGET APPLICATION, NOT GUARANTEED.
Turns Ratio and RMS Diode Current
Note that when using an R
FB
/R
REF
resistor ratio to set
output voltage, the user has relative freedom in selecting
a transformer turns ratio to suit a given application. In
contrast, simpler ratios of small integers (e.g., 1:1, 2:1,
3:2, etc.) can be employed to provide more freedom in
setting total turns and mutual inductance.
While the turns ratio can be selected to maximize output
power for a given current limit, minimizing the turns
ratio and increasing the current limit will often increase
efficiency and better utilize the saturation current of a
given transformer. Figure 3 shows the maximum output
power using three transformers with different windings
ratios that have the same output inductance and peak
output current, illustrating that increasing current while
decreasing turns ratio can deliver more power.
There are two significant constraints on the turns ratio.
First, as described in the previous section on limitations
to output power, the drain of the MOSFET switch will
see a voltage equal to the maximum input supply plus