Transformers for LT3573/3574/3575 and LT3748
Linear Technology launched a family of ICs for isolated flyback converters which allow a very simple. In close cooperation with Würth Elektronik, there are more than 60 standard transformers developed which comply with the requirements of these ICs. By selecting the correct transformers and simply changing two resistors on the board, users can select the output voltage required. All transformers are optimized for a very low leakage inductance in order to maximize efficiency of the circuit. High leakage inductances cause high voltage spikes on the switch, which 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 Figure 1 shows the winding build of transformer 750311675750311675. You can clearly see that primary and secondary are constructed from multiple strands (bifilar resp. quadfilar). In addition, they are split in 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 the 750311675750311675 transformer is 130nH inductance at 100kHz. As shown, Würth Elektronik offers a wide range of standard transformers which are suited for the LT357x and LT3748 IC families. These are optimized for a very low leakage inductance. Würth Elektronik provides LTspice
models for all standard transformers. The demo boards feature simple design and small solution size. Würth Elektronik offers a wide variety of demo kits, each of which includes a demo board. This board can be adjusted to input and output requirements by changing only two resistors and the transformer, thus offering a quick and easy solution for an isolated switch mode supply.
For more go to Design Kit Demoboard
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