[TangerineSDR] [HamSCI] RE: Notes from PSWS / TangerineSDR call of 07-26-2021

Wed Jul 28 12:38:28 EDT 2021

Hi Tom,

  Isn't it also the case that conducted EMI from the DC-DC converter (not
radiated) can be a large problem without proper bypass?  There is more than
one way to corrupt the RF you want to measure.

73
Phil W1PJE

On Wed, Jul 28, 2021 at 10:55 AM Tom McDermott <tom.n5eg at gmail.com> wrote:

> Hi Jonathan - a concern is that the DC-DC converter input stage would
> become unstable and oscillate
> with the cable inductance.  If that happens then there could be high
> frequency AC voltages present at the
> input of the converter.  Would those over-voltage the input?  Possibly.
> Would it radiate RFI from the cable?  Likely.
>
> Good design dictates sufficiently low source impedance to the DC-DC
> converter.  Almost all linear regulators
> require it.  An electrolytic and ceramic bypass pair right at the DC-DC
> input would be considered good design by many.
>
> -- Tom, N5EG
>
>
>
>
>
>
> On Wed, Jul 28, 2021 at 7:30 AM Jonathan <emuman100 at gmail.com> wrote:
>
>> John,
>>
>> Thanks for the suggestion! I think I’ll start with the overvoltage age
>> spike of 50V for 100ms or less based on the datasheet spec of the DC-DC
>> converter. I was thinking of some sort of fusable circuit protection now
>> that I’ll be using a TVS diode to mitigate stress from the follow current.
>>
>>
>> Tom,
>>
>> The datasheet wasn’t clear on the need for input filtering. On the output
>> of both the IP2415S, I have 0.1uF ceramic capacitors. In the datasheet, it
>> only mentions a spec of “Input Reflected Rated Current” with a series
>> inductor and shunt capacitor. I don’t really understand exactly what this
>> is, but it lists “20mAp-p through a 12uH inductor and 47uF capacitor”.
>> Nothing else is mentioned about input protection or filtering.
>>
>> In both situations that the DC-DC failed, the cable length was only 4’ of
>> cat 5. This circuit that was proved and tested by Paul, including the
>> receiver and schematic I attached, did not have such an issue, but the only
>> difference is that the voltage was lower than 30V. That’s why I was
>> thinking it was a turn-on/off spike. I think that in a lot of typical
>> applications, the input voltage isn’t that high, so spikes don’t normally
>> present a problem. The DC-DC that feeds the receiver DC-DC has never failed
>> and it was always fed with 18V or less.
>>
>>
>> Jules,
>>
>> You recommend the Schottky diode in parallel with the TVS, then a series
>> PTC, so during a spike, the TVS will conduct and the PTC would fuse, and
>> during a reverse polarity condition, the Schottky diode will conduct with
>> limited current due to the PTC fusing? I can see the benefit that the
>> series PTC in normal a normal circuit condition would drop much less
>> voltage that the Schottky diode.
>>
>>
>> All in all, I agree with adding some capacitance to filter voltage spikes
>> and reduce or eliminate them. I’ll need to be sure not to exceed the DC-DC
>> max load capacitance, which is 47uF in this case.
>>
>> But, to me, the most plausible cause for the failures are turn on/off
>> voltage spikes, even with a short length of cable. Adding electrolytics is
>> a bit difficult due to the tight space constraint as seen in my previous
>> email, so I’ll try out the TVS and a PTC. Other than voltage spikes, does
>> anything else come to mind?
>>
>>
>> Thanks guys.
>>
>> Jonathan
>>
>> KC3EEY
>>
>> On Tue, Jul 27, 2021 at 9:34 AM Jonathan <emuman100 at gmail.com> wrote:
>>
>>> Hi Everyone,
>>>
>>> Before the meeting ended last night, I wanted to discuss the issues I’ve
>>> experienced with my VLF active antenna. As I mentioned, the DC-DC converter
>>> (XP Power IP2415S) failed after I applied power. I turned the volume up
>>> because I wasn’t hearing sferics, and shortly after, I heard the squeal of
>>> the DC-DC converter. When I pulled out the smaller foam core partially, I
>>> heard a squeal coming from the DC-DC converter itself.
>>> [image: image3.jpeg]
>>>
>>> I had a failure of the DC-DC converter when I first built the VLF
>>> preamp. It was being powered by the Raspberry Pi box (same power circuit as
>>> my text box). I had a power failure and when the power was restored, the
>>> DC-DC converter failed.  In both situations, it was being supplied with
>>> 30VDC, with the limit is 36V. The reason for using 30V is to overcome the
>>> voltage drop from long cable lengths.
>>>
>>> After both failures, there was a low impedance across the input of the
>>> DC-DC converter causing the supply to be pulled down. Because the input and
>>> output are isolated, I figured the fault had to have been caused by a
>>> voltage spike on the input due to power being turned on and off. The
>>> datasheet mentions that it can withstand 50V spikes at 100ms, but I can’t
>>> think of anything else that could cause the DC-DC to fail, especially that
>>> 30V is used to power it and any voltage spike would ride on top of that
>>> 30V. I contacted XP Power about the issue.
>>>
>>> Here is the schematic of the power path. The blocks are the IP2415S
>>> DC-DC converter.
>>> [image: image1.jpeg]
>>> I’m powering the DC-DC converter with 30V from another DC-DC converter
>>> in the test box and Raspberry Pi box (Raspberry Pi box not shown, but same
>>> circuit). Keep in mind, the length is still relatively short in my
>>> testing, so cable inductance isn’t the primary reason for the spike. Plus,
>>> at long lengths, the voltage would be much lower due to voltage drop across
>>> the length, and I think this was why Paul might not have seen this issue
>>> when he originally tried and built this design.
>>>
>>> To mitigate this problem with the supply voltage at 30V, I decided to
>>> use a TVS diode across the input of the DC-DC converter. Here is the
>>> schematic: (my apologies for the TVS diode being backwards)
>>> [image: image2.jpeg]
>>> I chose a unidirectional TVS so there is also protection if the voltage
>>> spikes have any negative components. I picked a TVS with a working voltage
>>> of 33V, a Littel Fuse P4KE39A. According to the datasheet, it looks like
>>> this would be adequate. TVS diodes have a PN junction that is more rugged,
>>> with a larger surface area for greater current density. One thing I
>>> overlooked with this type of circuit protection is that once the TVS
>>> conducts, current from the power supply will be shunted too, known as the
>>> follow current, and in some situations, this may cause issues requiring a
>>> fuse. In this case, since the spikes are probably short duration, and since
>>> the DC-DC converter in the text/Pi box has short circuit protection, this
>>> should not be an issue.
>>>
>>> So the questions I have are: do you also think it could be voltage
>>> spikes that caused the failures, and if so, was my choice of TVS diode
>>> adequate. Also, one important question, if I connect the anode of the TVS
>>> diode to the receiver side ground, will I lose isolation? I know that
>>> during spike current conduction, I will. I’m sure I can still get adequate
>>> protection with the TVS diode anode connected to the negative of the
>>> isolated power loop too, but I just wanted to be sure.
>>>
>>> Lastly, since the magnetometer board will undergo another revision, I
>>> feel it may be worth it to add some sort or adjustable regulator or use of
>>> an external power source (not connected to the 5V pin of the Pi header)
>>> because even with paralleling wires or pairs, some installations will
>>> require very long lengths (especially since interference-free locations are
>>> often far away from the shack) and the voltage may drop to where the LDO
>>> cant regulate. Using a higher voltage at the Pi end (like I do with the VLF
>>> preamp) may be required for some installations. I think this should be
>>> considered if it already wasn’t addressed.
>>>
>>> Jonathan
>>> KC3EEY
>>>
>>>
>>>
>>> On Jul 27, 2021, at 7:21 AM, Dr. Nathaniel A. Frissell Ph.D. <
>>> nathaniel.frissell at scranton.edu> wrote:
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> Thank you, Tom.
>>>
>>>
>>>
>>>
>>>
>>> The recording will be available later today at
>>>
>>> https://youtu.be/AammohuygMw and hamsci.org/telecons.
>>>
>>>
>>>
>>>
>>>
>>> 73 de Nathaniel W2NAF
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> *From:* TangerineSDR <tangerinesdr-bounces at lists.tapr.org>
>>>
>>> *On Behalf Of *Tom McDermott via TangerineSDR
>>>
>>>
>>> *Sent:* Monday, July 26, 2021 10:02 PM
>>>
>>>
>>> *To:* TAPR TangerineSDR Modular Software Defined Radio <
>>> tangerinesdr at lists.tapr.org>
>>>
>>>
>>> *Cc:* Tom McDermott <tom.n5eg at gmail.com>
>>>
>>>
>>> *Subject:* [TangerineSDR] Notes from PSWS / TangerineSDR call of
>>> 07-26-2021
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> Notes from PSWS / TangerineSDR call of 07-26-2021
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> 1. Bill is using chart.js for magnetometer charting. He is setting up a
>>> database using Django web and database framework for Python.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> 2. Scotty is looking at the Intel (Altera) Arria 10 GX FPGA 10GX270 for
>>> the version 2 Data Engine (supporting 10GE). These FPGAs appear to be more
>>> available than the MAX10 FPGAs. The intention is to develop DE Ver 1 and DE
>>> Ver 2 in parallel
>>>
>>> while awaiting FPGA component availability. The 10 GX development boards
>>> are pretty expensive.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> -- Tom, N5EG
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> --
>>>
>>>
>>> Please follow the HamSCI Community Participation Guidelines at
>>> http://hamsci.org/hamsci-community-participation-guidelines.
>>>
>>>
>>> ---
>>>
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>

-- 
----
Phil Erickson
phil.erickson at gmail.com
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