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:
- The board with the problem
- A multimeter to measure DC voltage and resistance
- A microscope or eye loop
- The datasheet of the part you are troubleshooting
- The schematic of the board
- DC power supply with wires
- A soldering station
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:
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:
- INPUT – you must have the voltage at this pin for the device to work at all. In this case, you need 5-15 VDC and the voltage must be at least 1.5 VDC higher than you’re trying to get at the output.
- OUTPUT – This is the voltage you are trying to generate. It should be the same as the voltage at the Vsw pin.
- FB – This is the pin that is used to set the output voltage. The way it does this is the maintain a particular voltage at the pin (in this case 1.21 VDC) and use the R1/R2 voltage divider network to set the output voltage.
As a reminder on how voltage dividers work:
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.
- All the other capacitors and diodes are used for filtering and protection.
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)
- With the PCB not powered on, check for shorts at Vin to GND. If its shorted, your Vin will be 0 VDC and the electronic device will not work.
- Still with the PCB not powered on, check for shorts at Vout. If this is shorted, the device will not be able to handle the current and it will be 0 VDC. If this is the case, fix the short. Once you do, if it still does not work, the chip may have been damaged because of the short and will need to be replaced.
- If both Vin and Vout are not shorted, power on the PCB and check Vin. If it is not in the expected range, troubleshoot why.
- If all the above looks okay, and you are still getting 0 VDC at Vout, you probably have a defective device OR unsoldered or shorted pins on the device or nearby components. Inspect the pins and if everything looks okay, replace the device.
- Make sure the protection diode is not installed backwards, which would cause 0 VDC.
Voltage exists, but is low
This is probably due to one of two causes:
- Too much current is being drawn on the circuit. This could be a defective chip that the circuit is driving or a short somewhere on the output of the circuit. If the board is being powered by a power supply, look at the current being drawn and compare it to a good board. If it is too high, this is likely the problem. NOTE: this could also be caused by setting the current limit on a power supply too low. Raise it to no more than 2x what a good board draws and see if the problem goes away. If you have the soldering skills, you can also inject voltage by adding wires onto any capacitor on the output power rail and power it with a power supply with the current limit set to no more than 2x the expected current to measure it.
- The R1/R2 resistor network has wrong components or one of them is broken.
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
- If the SW pin is not properly connected to the Vout or R1/R2 was wrong, it is possible on some devices that they were trying to drive the circuit too hard and may have been damaged.
- Similarly, if the Vout has issues on the downstream circuit, the device may be damaged.
- If the voltages look okay but noisy, check the capacitors and inductor.
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.
Engineering Program Manager