In the electronics manufacturing world, we occasionally encounter printed circuit board (PCB) failures that need to be troubleshot. With our nearly two decades of experience with electronics manufacturing, we thought an article about how to fix an electrical issue would helpful.
This post is going to be a more on the technical side, but hopefully will be easy to follow. What it will cover is how to troubleshoot why a power rail is just not right on your PCB assembly. It will focus on the DC/DC converter chip, since that is a common power supply component on a printed circuit board. One of the first things you should check while troubleshooting a board is to see if a chip is getting the voltage (usually designated Vcc) that it is supposed to. If it’s not, find the chip that is supposed to be creating that voltage and follow these steps.
What you will (or might) need:
I will use the older Linear Technology LT1578 adjustable step=down switching regulator as an example, but this same technique applies to many devices. From its datasheet, here is a typical application:
Basic Operation
The datasheet of the device you are using will give you lots of information about designing the circuit, but for most troubleshooting, the basics are the following:
As a reminder on how voltage dividers work:
Or
Since we know that Vfb is maintained (for this chip) at Vfb = 1.21 VDC, we can calculate what this actual Vout will be based on this example:
Personally, I NEVER calculate this directly. I always do an internet search for “voltage divider calculator” and plug in the numbers. It is a good idea to run this check to make sure the circuit is properly designed.
Troubleshooting
There are only a few typical symptoms that you will see. Here’s what they are and what to look for. Once you find something that is wrong, you can focus on that area using basic troubleshooting techniques to solve that problem.
No voltage is being generated (Vout = 0 VDC)
Voltage exists, but is low
This is probably due to one of two causes:
Voltage exists, but is high
This is the easiest to fix but is also the least likely. R1 or R2 is the wrong part which is causing Vout to adjust too high.
A few other notes
Hopefully this has helped troubleshoot your DC/DC regulator issue or at least gave a better basic understanding of how they work. At ACDi, we build, inspect and test to ensure a quality product. Strong troubleshooting skills enable us to repair electronic issues which minimizes scrap and helps control costs.
Bob DiDonato
Engineering Program Manager
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