Power Supply very hot and not functioning

Hardware
1

I’m hoping someone here can set me on the right track to fix a schematic that went wrong somewhere.

I made a power supply sub-system (image below) for my PCB about a month ago. My project is powered by a 3.6V 13Ah LiSOCl2 battery. Battery-VIN is connected to D1. The purpose of this power supply system is to ensure that I would always have a steady 3.6V output even during high current draws (up to 800mA) and cold-weather conditions (down to -20C). Previous tests without this power supply system showed that under these conditions the battery-voltage could drop as low as 2.8V which would create frequent brownouts on my microcontroller (Particle Electron).


I based my schematic + parts on this datasheet (page 1).

Parts:
D1: SBR05U20LP-7
L1: MLP2012H2R2MT0S1
U1: TPS610995YFFR
C1/C2/C3: 0402ZD106MAT2A

Note that since VIN is 2.8-3.6V, and VOUT should be 3.6V, I went with the 610995 model with FB connected to GND. W1+W2 are two wires with a JST-PH2 connector that can be inserted into my microcontroller to power it.


My DRC clearance is set to 6mils, and object clearance is set to 10mils (as per manufacturer’s recommendation).

My problem:
Yesterday, I finally received this PCB (10 copies). I tested it and while it worked for 3 copies, for the remaining 7 copies some parts (particularly U1 and L1) became extremely hot to the touch and VOUT was 0. Clearly, something went terribly wrong. I looked at my parts again, and here’s my thoughts:

(1) The maximum rating for D1 is 500mA, which is too little for my project. Nonetheless, just by powering up my PCB these components should draw nowhere near that amount.
(2) The saturation current for L1 is 1A, which ideally should be higher. But again, simply powering up my PCB should draw nowhere near that amount.
(3) C1+C2 = 20uF, but the maximum here can be 100uF. To better mitigate current drops, stronger capacitors should be used.

Basically, after seeing that my PCBs didn’t work, I looked into the parts and came up with several things that can be improved, but nothing so far that could explain why my system entirely didn’t work.

I’m really hoping someone here can give me a push in the right direction as I’m not sure what else to look at.

2

If your seeing 0V on the Output and things are getting really HOT then it sounds like a short circuit condition.

How sure are you that the BGA pads on that Regulator Chip are not shorting out? It’s a super small part that would be very easy to get solder between the 6 pads considering the BGA clearances.

It would be best to switch over to the WSON chip option to make Surface Mounting via Reflow Oven a more reliable process.

1 Like
3

I’m not really sure how to check for this though, but you’re right, considering it has the smallest clearances + most pads of all parts, it’s the most likely culprit.

So replacing this part with the TPS610995DRVT, which uses WSON-6 package, is a good choice?

4

Yes, that would be a good move to make it less likely that you end up with shorted pins.

The diode is getting hot because your pulling so much power through it due to the short circuit condition.

1 Like
5

That makes total sense and it explains why I’m only encountering this issue on some of my boards, but not all.

I’ll replace my part with the larger-package one, and hope for the best :smiley: Thanks!

6

Are you using a stainless solder paste stencil?

How are you re-flowing the board?

7

I don’t. The manufacturer both produces the boards and assembles them. I don’t even have these tools right now. So far I never had too many issues with my PCBs, but I guess I got a bad batch delivered.

8

Dam, that sucks!

A decent fab house should have the ability to properly place these BGA packages without issues.

Still the bigger part will be better for everybody.

9

It’s a pretty common package with reasonable clearance (6mils) on the pads. Then again, I really went for a low-budget fab house so perhaps I shouldn’t feel too surprised :joy:

To be safe, I’ll just up my DRC clearance to 8mils for the PCB design to avoid this issue with other parts.

10

Did they place the chip on Backwards?

11

You know…I just realized that due to the small size of the component + the designator that is positioned close to it, the marking (pin 1) is actually very hard to see. You may actually be on to something; perhaps they did put it on backwards!