Welcome to the combined product training module of Linear ...

Welcome to the combined product training module of Linear Technology and Wurth
Electronics about their efficient design solution for DC/DC Flyback Converters. As
leading manufacturers in their industries, Linear Technologies’ and Wurth Electronics’
goal is it to deliver complete and easy to design and source solutions to the engineering
market.
The Product Module contains information about the high input voltage isolated flyback
controller LT3748, as well as Wurth Electronics standard flyback transformers trimmed to
work with Controllers of Linear Technolgogy. In addition this Product Training Module
also introduces the Evaluation Kits with demoboards from LT with a pre-selection of
transformers for flexible voltage outputs.
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The LT3748 is a simple to use high input voltage isolated flyback controller that does not
need an optocoupler and shunt regulator/opto-driver to complete the circuit. These
features combined with extra pin spacing for high voltage operation, simplify the design,
allow for a smaller solution size and require only one component crossing the isolation
barrier. The competition requires an optocoupler and a shunt regulator/opto-driver to
complete the circuit, and does not have extra IC pin spacing for high voltage operation.
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The LT3748 is a high input voltage isolated flyback DC/DC controller that significantly
simplifies the design of an isolated DC/DC converter. No opto-isolator, third winding or
signal transformer is required for feedback since the output voltage is sensed from the
primary-side flyback signal during the off time of the power switch. The LT3748 operates
over a 5V to 100V input voltage range, drives an external N-channel power MOSFET
and can deliver up to about 60 watts.
The LT3748 operates in boundary mode, which is a variable frequency current mode
control switching scheme, resulting in +/-5% typical regulation over the full line, load and
temperature range. Boundary mode also permits the use of a smaller transformer
compared to equivalent continuous conduction mode designs. The output voltage is
easily set by two external resistors and the transformer turns ratio. Twenty five off-theshelf
transformers are identified in the data sheet for ease of use.
Additional features include an onboard low drop regulator for IC power, programmable
soft start, undervoltage lockout, adjustable current limit and output voltage temperature
compensation. The LT3748 is available in a small MSOP-16 package with 4 pins
removed for additional high voltage pin spacing and has an operating junction
temperature range at -40°C to 125°C.
3

This PR Photo shows the basic parts for the circuit and that the LT3748 can accept up to
100V on the IC.
4

This schematic shows all the parts required to complete a LT3748 circuit. This app
circuit converts a nominal 48V (36V to 72V) input to an isolated 12V @ 2A.
5

The LT3748 maintains a tight +/- 1% load regulation from 0.1A to full load.
6

The LT3748 is available in a MSOP-16 package with 4 pins removed for high voltage
spacing requirements.
7

All transformers of the WE-FB series are optimized for a very low leakage inductance in
order to guarantee a good efficiency of the device. High leakage inductances cause high
voltage spices on the switch and therefore need to be reduced by a snubber network.
The losses generated in the snubber are related to the height of the voltage spike. This
means lower leakage inductance causes a lower voltage peak and therefore lower
losses.
The low leakage inductance is achieved by full flat layers and split windings. Wire is
selected to fill the layers. When needed, multiple strands are used to fully fill the layer.
As an example, Picture 1 shows the winding build of 750311675. You can clearly see
that Primary and Secondary is build by multiple strands (bifilar resp. qudfilar). In addition
to this, they are split parallel. So the winding order is. Secondary 1 => Primary 1 =>
Auxiliary => Secondary 2 => Primary 2. Secondary 1 and 2 resp. Primary 1 and 2 are
connected in parallel.
With this structure, the typical leakage inductance of 760311675 is 130nH at 100kHz.
9

Wurth Electronics has released over 60 standard transformers for the use with Linear
Technologies LT3748/LT3573/LT3574/LT3575. The clear advantage is the ability to find
the right transformer for the DC/DC flyback design with out going through the process of
a sourcing a custom transformer. The transformers are available from

Linear Technology and Wurth Electronics released for the shown DC/DC flyback
converter solution two evaluation kits for 30W and 12W output power. The kit 750108
contains the Linear Tech eval board DC1694A with 3 pre-selected transformers for
applications with up to 30W. The transformers have output voltages of 12V, 15V and
24V. In addition to those suggested parts, other versions with several output voltages
and currents are available from stock. To make this evaluation kit a real solution for
every design engineer, Linear and Wurth added all the documentation and specs as well
as design tips for the assortment.
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The LT3748 can be used in a broad range of isolated requirements in industrial, 48V
Telecom and datacom requirements. Isolation can also be required to keep noise from
permeating through on a bus voltage in a noise sensitive application, such as GPS
systems, programmable logic controllers, displays and medical systems.
The LT3748’s high level of integration results in a simple, clean application solution to
the traditionally tough problem of isolated power delivery.
All products shown are in stock at www.digikey.com
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