DER-737 / LNK3294

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The choke used in the design example is a Bourns RLB9012-152KL. In the data sheet, no maximum voltage rating is given. Do you have more information which not is found in the data sheet? I think the voltage rating is an issue here.


Can you elaborate more on why you think there is an issue? And by issue do you mean with the DER or the choke? Also, by datasheet, are you referring to the datasheet of the choke? Could you send me your reference file?

I'll look into the DER and the component in question as well. Thank you.


In the data sheet of the choke there is no maximum rating given for the operating voltage. The choke will see 440V * sqrt(2) = 622Vpeak across it. We had a few months ago a similar application with a comparable choke, and no manufacturer we contacted would guarantee us a safe operation under these conditions. Finally we had to use a costly custom component.
So I have to assume the maxiumum operating voltage is indeed an issue here.

RLB9012.pdf252.26 KB

Hi hpvoelpel,
What do you mean by 'safe operation'? Buck topology is non-isolated therefore the output should not be accessible to end-users for safe operation.
If withstand voltage is the issue, this should depend on the magnet wire used by the manufacturer. Magnet wires have different withstand voltage ratings depending on its 'insulation grade' which primarily depend on the insulation material and thickness. Also, the 662Vpeak is equally divided across the number of turns. The worst case voltage differential should happen between adjacent layers. Usually, due to the high inductance, these chokes have multiple turns and number of layers that the voltage experienced across adjacent windings should be relatively low compared to the insulation rating of the magnet wire. Barring insulation scratches, arcing should not be an issue.
It would also be great if the manufacturer decides to share their magnetic specs so that the magnet wire withstand voltage can be determined. The construction can also be analyzed and the worst case differential voltage across the adjacent layers determined. This information should help when deciding if the choke is suitable for the application.
Otherwise we are left with the option of using custom chokes with safety-rated double/triple insulated wires. This should be considerably more costly but would guarantee insulation integrity even at extreme conditions. This, however, should not be necessary.

"Safe operation" means here to not go up in smoke in the long run.
All of your considerations are correct in my opinion. These are exactly the reasons why I posted my question here. So we are back to the start. Are you with Power Integrations? Then maybe someone in your company knows more, perhaps the author(s) of the DER?

Users with 'PI' in their usernames are with Power Integrations. I also personally know the author of the DER.
Either way, I believe the best course of action would be to conduct 'stress tests' to check PSU robustness using several different chokes of your choice. These tests should be outlined on your company's quality procedures and could include elevated ambient temperatures, humidity levels, soak times, etc. for a specific number of test units depending on your application. The results should help in deciding the course of action.
Also the LNK-TN2 product family has a significant number of safety mechanisms built into the IC so the chances of the PSU going up in smoke in the long run should be minimized.

to put it short, we will have to stay with our custom build chokes. I was hoping you had some more information on the Bourns choke, which apparently you don't.
Maybe PI could get in touch with Bourns (or another manufacturer) , so that they can address the problem.