UL file for PTC fuse

Power Power Supplies (off-board)
1

Hello

I am in the process of getting a custom PCB approved by our local Canadian CSA inspector.
I am using this 6V resettable use SMD1206B200TF to protect this 2725-K7803-2000R3L-ND DC-DC convert (12V in, 5V out) from the 5V side to draw more current than the 12V input side could handle to give.
The inspector is asking me to provide the UL file for this fuse. Do you have access to this?
Also do you have any certification document for the DC-DC converter?

Thank you very much.

2

Hello Stephan,

Welcome to the forum. I am checking with our Product Manager for the UL file number.

Best regards,

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3

Hello Troy.

When can I expect an update on this?

Thanks.

4

Greetings,

Please see attached pdf’s.

UL PTC E482628.pdf (729.9 KB)
TUV PTC—B 096 048 00122.pdf (400.6 KB)

Best regards,

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5

Thank you. This is the UL document for

Do you have either a UL or CE certificate for the 2nd item I inquired about?
2725-K7803-2000R3L-ND DC-DC

Thank you,

6

Hi stephan,

While you wait for that, is there a reason you are placing the fuse after the regulator rather than before it? If you did so, you would need to switch to the 750mA SMD1206B075TF/13.2, but there would be some real advantages. Placing the fuse before the regulator would have the following advantages:

  1. It would protect the regulator as well as all of the downstream components.

  2. It would dissipate much less power, since the current passing through the fuse would be only about 32% of that on the output side, and the power dissipated is a function of the square of the current (P = I² x R, where R is the resistance of the fuse). At the maximum load of 2A, and maximum non-tripped resistance of the fuse of 0.08Ω, the fuse will have to dissipate 0.32W ({2A}² x 0.08Ω = 0.32W). Alternatively, under the same conditions of maximum load and the regulator working at 85% efficiency, the regulator would only draw 0.647A (6.6W/85% = 7.765W input power, 7.765W/12V = 0.647A input current). Then, with the max resistance of the SMD1206B075TF/13.2 at 0.35Ω, the power dissipated in the fuse would only be 0.147W ({0.647A}² x 0.35Ω = 0.147W), which is less than half the power it would dissipate if placed on the output side.

  3. It would not affect the output voltage. With the fuse placed at the output of the regulator, the voltage after the fuse will drop as a function of I x R losses due to the resistance of the fuse, and that voltage drop will vary as the current varies. Alternately, placing the fuse before the regulator would mean a fixed voltage at the load of 3.3V ±4% (the output specification of the K7803-2000R3L itself), regardless of the load current.

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7

Hi Stephan,

Regarding part number K7803-2000R3L. Unfortunately this item does not have UL certification. This product has CE certification. You may find the certification document on their website by clicking on the link Markings : CE MORNSUN DC/DCConverter-Switching Regulator_Regulated Output (0.5-3A)_K7803-2000R3L

Best regards,

8

Hello Troy.

Thank you for all the documents. And also thank you for your good suggestions about fuse placement.
I might do that in the future.

Right now placing the fuse after the converter was done because our CSA certification inspector is worried about the converter not being a class 2 device and so maybe catching fire under certain circumstances. By placing the fuse between the converter and the part of my device that consumes most current I thought I would remove the inspectors worries.

Designing devices seems to be all about the “what if something shorts out” worry :wink:
I already had a long conversation about this issue here: Is this DC DC CONVERTER a SCR device? if you are interested.

If you have any more suggestions I am happy to hear them.

9

Hi stephan,

I understand your concern. However, if the regulator is current protected at the input side by the fuse, then by definition, it is current limited on the output side, since you can’t draw more current if the fuse feeding the regulator is open. It’s like closing a dam upstream – once the dam is closed, water will quit flowing down stream.

Furthermore, if you place the fuse on the output side of the regulator, and if the built-in short-circuit protection within the regulator fails for some reason, and if a short were to occur between the regulator and the fuse, then the regulator would be more likely to catch on fire. Granted, if you place the fuse very close to the output of the regulator, the risk of this is minimized.

The point is, I can’t think of any scenario where placing the fuse after the regulator improves safety compared to placing it before the regulator.

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10

Thank you David for the electrical advice, it’s really appreciated.
Thank you Troy for those file.

You both were a great help.

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