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. 1
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. 2
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
Wurth Electronics WE-FB series is designed for several Linear Technology IC s. Besides the LT3748, these components are also trimmed to work for LT s LT3575, LT3754 or LT3573. Wurth Electronics WE-FB series contains over 60 standard parts which are available from stock at digikey.com with several different output configurations. The components are deisgned with standard materials which make them a cost effective and easy to source transformer solution. Due to Wurth Electronics strength in winding technology of magnetics, the WE-FB transformers are trimmed to a low leakage inductance to increase the efficiency of the DC/DC converter. In addition, Wurth Electronics transformers are equipped with an outstanding EMI behavior. 8
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 <1W of up to 50W max. power and are designed to 1.5kV isolation voltage. Selected values are designed to even meet 2.5kV or 4kV isolation. All shown parts are available from stock at digikey.com. 10
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. 11
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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 13