Potential Flaw with a Voltage Regulator

In the updated version of my Ultimate Lora Backplane I was considering on changing the linear voltage regulator used to a switching one, this would mean while adding £3 to the BOM that it would be possible to use a 12V Power Supply instead without producing a lot of heat. As during some testing with the linear I measured 3W at 7.5V and 6W at 12V of which almost all of the 6W would be getting wasted as heat.

I did the initial testing with the switching voltage regulator and measured around 3W at 7.5, 9 and 12v, excellent! I then touched the regulator module of which then suddenly smoke started coming from the board. I quickly unplugged it all from the power supplies and found that the NanoPi DUO had a component fried on it.

The first thing I did was check my RAK831 still worked which so far has been fine.

A bit later on I then got the oscilioscope out to do some testing and figured out why.

The regulator output the full 12V instead of 5V
It seems that if you touch one side of the IC either on purpose or by accident, even for a second it causes the output voltage to be that of the input. As you can see the smooth 5V rail then spikes to 10-11V which is just 1V below the voltage I put in.

DS1Z_QuickPrint2

DS1Z_QuickPrint4

Now while obviously electronics is sensitive to touch, usually ESD prone components are marked in the datasheet however in none of them does it state that its ESD Sensitive. At the same time it's a very bad design as if a user was to accidentally touch the module while working on another part of a project you literally fry your project.

Update - 10th of June 2018,

Just a quick update on this. I got sent a replacement module by the supplier and the fault was still there. There's been a bit of debate as to weather it was a fault or not as of this being an apparent factor of using a switching module.

I've instead designed a different switching regulator directly onto V2 of my board and it works no issue and can even be touched without frying it.

I'll be writing a blogpost if you're interested in V2 of the board, however details can be found at https://www.thethingsnetwork.org/forum/t/ultimate-lora-backplane-board/14928 and if you're interested in one you can sign up to the waiting list at https://www.indiegogo.com/projects/ultimate-lora-gateway-backplane/coming_soon

Second Update
The voltage regulator is a Murata oki-78sr with the fixed 5V model designed as a drop in replacement for a LM7805 and costs around £3.80 from most retailers. https://power.murata.com/data/power/oki-78sr.pdf

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