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LT3748E

100V Isolated Flyback Controller

Linear Technology 

LT3748

Applications Information

The current limitation on output power delivery is gen-

erally constrained by transformer saturation current in

higher power applications, although the MOSFET switch

and output diode will need to be rated for the desired

currents, as well. Increasing the peak current on the pri-

mary side of the flyback by reducing the RSENSE resistor

is the primary way to increase output power, and power

delivered increases fairly linearly with current limit as

shown in Figure 2, until parasitic losses begin to dominate.

However, once the saturation current of the transformer

is exceeded the energy coupling between the primary and

the secondary will be reduced and incremental power will

not be delivered to the output. In addition, the primary

inductance will drop, the SENSE pin overcurrent threshold

may trip due to a corresponding rapid rise in current, and

the transformer will have to absorb the energy that is not

transferred through the saturated core, leading to heating.

Some manufacturers may not specify the rated saturation

current but it is a necessary specification when trying to

minimize transformer size and maximize output power

and efficiency. Also necessary for proper design is data

on saturation current over temperature­—the saturation

of typical power ferrites may reduce by over 20% from

25°C to 100°C.

The thermal limitation in flyback applications for lower

output voltages will be dominated by losses in the output

diode, with resistive and leakage losses in the transformer

50

ILIM = 3A

40

ILIM = 2A

30

20

ILIM = 1A

10

0

0

20

40

60

80 100

INPUT VOLTAGE (V)

3748 F02

Figure 2. Maximum Output Power at 12VOUT

with 150V VDS(MAX) and ILIM = 1A, 2A, 3A

increasing as a percentage basis of loss as the output

voltage is increased. As power levels increase the output

diode and transformer may exceed their rated temperature

specifications. Minimizing RMS output diode current,

selecting a diode with minimal forward drop at expected

currents and minimizing parasitic resistances and leakage

inductance in the transformer will keep those components

below their maximum temperatures while maximizing

efficiency. The following section discussing transformer

selection will further help focus on how to minimize losses

in the output diode.

While quiescent current in the LT3748 itself is low (ap-

proximately 300µA from VIN and 1mA from INTVCC), the

current required to drive the external MOSFET (fSW • QG),

if drawn from VIN through the LT3748 INTVCC LDO, dis-

sipates (VIN – INTVCC) • fSW • QG. If that power is high

enough to cause significant heating of the LT3748 the

current may need to be drawn from a third winding. Doing

so will push all thermal limitations outside of the LT3748.

Selecting a Transformer

Transformer specification and design is perhaps the most

critical part of successfully applying the LT3748. In addi-

tion to the usual list of caveats dealing with high frequency

isolated power supply transformer design, the following

information should be carefully considered.

First and most importantly, since the voltage on the sec-

ondary side of the transformer is inferred by the voltage

sampled on the primary, the transformer turns ratio must

be tightly controlled to ensure a consistent output volt-

age. 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 mag-

netic component manufacturers to produce predesigned

flyback transformers for use with the LT3748. Table 1

shows the details of several of these transformers.

12

For more information www.linear.com/LT3748

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