After the second occurrence, we tested each of the external circuit components. We verified them individually, including the catch and boost diodes. We determined they were within spec and working correctly. The layout was verified, and so was the PCB itself for any new or previously undiscovered problems.
Confounded by the problem, I again started looking at the datasheet of the switcher. I also looked at the application notes on Linear's Website for any source of inspiration. I came across this article. It talks about how ceramic capacitors on the input of the switcher can cause overvoltage transients due to their extremely low ESR. That could potentially push it over the tolerance limits of the device and damage it.
My design had ceramic caps on the input. Based on the design constraints, we figured that an electrolytic/tantalum capacitor was unnecessary on the input side. Besides, the datasheet commented on how the device is stable with just ceramic capacitors. In fact, it even recommended their use.
When I probed the input supply pin of the switcher and power-cycled the board a few times, sure enough, I saw huge voltage transients. With the bench top power supply supplying +24V DC, I saw transients routinely at over +38V DC and a few times even as high as +52V DC. The absolute maximum rating of the device, at its input, was +36V DC. There was the smoking gun!
On replacing the switcher for the third time, and upon replacing one of the ceramic caps at the input with an aluminum electrolytic with a large enough ESR to slow down the ramp up of the input and thereby preventing the sharp transients, I was able to get the switcher working again and was able to consistently reduce the transients to under +30V DC at a +24V DC supply.
We put the modified interface back into the complete test setup, and it worked perfectly for the rest of the scheduled testing.
For future reference, you should not overlook the extremely low ESR of ceramic capacitors. In most situations, its a desirable feature with no potential threat at all. Yet all input stages of switchers should have mechanisms in place to take this fact into account.
This entry was submitted by Girish Ramachandran and edited by Rob Spiegel.
Girish Ramachandran has a master's degree in electrical and computer engineering from the University of Florida and has worked in electronics design for UAV applications, robotics, and medical equipment. He currently works as lead manufacturing engineer for Prioria Robotics in Gainesville, Fla.
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