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We are looking for a replacement for the NFL18ST307X1C3D component, this one.
It is a T-filter in a 0603 housing with 3 connections, also for signal lines. The cutoff frequency is not that critical for our application – we primarily want to filter EMI in an industrial device that operates in the RF range with a frequency up to 3 GHz.
The operating voltage on the circuit board is up to 15 V DC. Therefore, we need a component that is specified for continuous operation at 15 V or at least sufficiently qualified. The footprint (0603, 3 connections) should ideally remain the same to avoid layout changes. I have already found the NFL18ZT307H1A3D component, which is a very good electrical fit. However, the rated voltage is specified at 10 V, while the withstand voltage is 30 V. Is it still permissible to operate this component at 15 V, e.g., with appropriate derating considerations?
Alternatively, we have also considered the feed-through capacitor SF0603C101SBNBT. This raises the question whether this component offers a comparable filter effect to a complete T/Pi-filter. Can you recommend such a replacement solution? As the manufacturer has not yet responded to our support request, I hope to find a suitable alternative with your assistance.
Welcome to the DigiKey TechForum.
I was unable to find a suitable alternative for this filter.
There are some rated for the 10V, as you found.
For normal operation, you should stick to the recommended voltage of 10V. The withstanding voltage is what it can handle for short periods, without destroying the device.
I do not know what effects it will have if you normally run it at a higher voltage that what is recommended.
Maybe one of the Engineers can comment on using the capacitor.
It’s unclear what one might contemplate de-rating in an effort to compensate for encroaching on a part’s dielectric breakdown limits. Ceramic devices by nature and of necessity tend to have a fairly generous margin of safety built in on this front, but using a device beyond spec comes with a laundry list of risks and liabilities. To misappropriate a well-known author, “everything is permissible, but not everything is beneficial”
A simple feed-through capacitor will not behave the same as a T-filter, just as an apple does not behave like an orange. Whether one is a tolerable alternative to the other however is subjective; the behavior of a circuit incorporating the device will vary depending on source and load impedance characteristics, and it’s not obvious at all what sort of behavior the application requires in order to meet the design objective. At one end the part might be completely unnecessary to meeting EMI requirements, at the other it might be the key to letting the device just barely scrape by.
Unfortunately, it’s up to you to make that call for your particular application.
Hi David,
Yes, the problem is that my PCB is not a new developed one, but an old one, which uses 15V for some OP-Amps.
Considering the rated voltage, that’s what I was thinking. There is no guarantee that it will work or break during operation.
You’re absolutely right – it’s impossible to predict with certainty how long a component operated near or beyond its rated dielectric strength will remain reliable, if at all. While a simple capacitor might be a step in the right direction, it’s clear that a proper filter would provide more predictable behavior. Unfortunately, such integrated EMI filters are hard to find for the frequency range in question.
From what I can tell, the ESL and ESR characteristics of the capacitor will be critical in determining its effectiveness across the relevant frequency spectrum. The impedance plots I’ve reviewed look promising so far, but with EMI there’s always uncertainty, and simulation or theoretical analysis can only go so far.
Given the constraints – especially limited PCB space – I’m leaning toward a redesign that would potentially allow for a discrete filter implementation. Ultimately, though, this will come down to actual measurement to verify whether the setup performs adequately in terms of EMI compliance.
What would your plan of action be in this situation?
Thanks again for your detailed and thoughtful input.
I’d start by trying to articulate the need/goal more specifically; it’s tough to win a race without knowing where the finish line is.
A discrete implementation will likely be a lot more future-proof than something integrated. Approximating the values mentioned in the datasheet could be a good starting point. And depending on your test costs, trying out the feedthrough cap as a quick fix might be worthwhile.
