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tny290 faulting well below current limit

Posted by: JoMo on 10/11/2021


I am using a tny290 as shown in the diagram. I am also using a 749118205 but when I apply load that would equate to a draw of 300mA or more I am having the circuit go into fault mode. I am currently using a protoboard and not a custom pcb. I am able to get higher current before fault mode with 750871110. I am unsure why tho.


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5 Volt USB_JL_0930.pdf 542.65 KB


Submitted by PI-BERJ on 10/12/2021

Hi JoMo,

Wurth's 749118205 has a lower inductance than 750871110, i.e. 0.9mH vs 1.6mH. Given how TNY290 works, this allows the higher inductance transformer to store more energy before a maximum frequency limit is achieved thereby allowing it to be able to transfer more power to the load.

I would recommend checking the datasheet to see how TNY290 works and the corresponding design examples (DER's) and (RDK's) under the resources tab in the link shown below to gain insights especially on the transformer design, and the critical considerations concerning proper layout.

You can also check Power Integrations PI Expert Online tool. You just need to plug in the input and output parameters and the tool will generate a design for you. Link is also shown below.

The parameters generated by the tool should help you choose the proper transformer from a myriad of options available online. You may choose from these the best fit for your design or you can opt to design the transformer yourself.