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

Tue Jul 27 09:33:07 EDT 2021

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. 

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.

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)

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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