When I worked as the electronics design engineer on a medical device prototype, I designed an interface board as part of a larger project. The goal was to enable signal breakout from a Cyclone 3 FPGA development board to other development boards.
In addition to signal breakout, each board had different power supply requirements. To accommodate all of the requirements, I included a DC-DC switching regulator in the design to generate 5V DC, 3.3V DC, and 1.8V DC. I chose the LT3507 based on its small form factor and its highly integrated monolithic multiple switchers. Thereby, only a small number of external components were required.
The switcher was initially tested and characterized using a 12V DC power supply input to the interface board. I tested it multiple times. I also did support testing on the other development boards and the rest of the hardware.
After about two weeks, we decided to integrate all the hardware with the software and test the system. Given that the input supply required for another power control board in the system was +24V DC, and the LT3507 can accept a maximum input supply of +36V DC, we decided to split the input +24V DC to the interface board, as well. That consolidated the input power to a single source. So far, so good -- everything was falling in place.
Upon moving the interface board from one test setup to another, we had multiple occasions of power cycle. During one of the test setups, when some signals were being prepped to be probed, the 3.3V DC rail was shorted to ground due to a stray wire, and there was excessive current draw for a few moments. On further power cycles, the switcher worked intermittently, then finally stopped working altogether.
On further investigation, we determined that the switcher had indeed blown during the signal probing, and we replaced it. We tested the second switcher intensively for an hour or so at a supply voltage of 12V DC from a linear bench top supply. Once it was characterized satisfactorily, we moved back to the complete test setup, where we powered it with the +24V DC input from the linear bench top supply.
On the first power cycle, the switcher didn't seem to generate any of the rails, in spite of the fact that it had previously worked on the primary test setup for an hour or so. On further investigation, we determined that the second switcher was blown, as well. That was just hours after the first one was replaced.
It's amazing how the difference in materials in a capacitor can determine failure or success. This took quite an effort of perseverance and deduction to determine the source of the problem.
This is also one of the more technical Sherlok Ohms postings we're seen. I'd like to hear from commenters about whether you prefer tech problems like this or the more everyday problems like how to figure out why your car is stalling.
This case is interesting in that it illustrates how what sometimes should work doesn't. Components themselves can be very finicky, and none more so than capacitors. For example, we've seen numerous cases over the past decade and a half of what can happen when cheap capacitors are used on computer motherboards. Here, the ceramic versus electrolytic is not a quality issue per se, but it certainly relates to functionality.
Many years ago, I added a ferrite to the input power of a switching regulator. After production started experiencing testing failures (and even worse customer field failures), we found out the ferrite and ceramic input caps formed a series tuned circuit that caused ringing that exceeded the input voltage of the regulator. The solution was to use a smaller ferrite with more capacitance.
We even now occasionally place series resistors in front of larger value ceramics to reduce the Q of the resulting circuit to stop transient ringing effects.
I like the mix of tech and mundane problems. This was a good one as a lessons learned that could help others.
I'm in layout now on a board with an LT switcher. I'm not sure where I read about ceramics on the switcher input, but I made sure to have some tant on the input as well. Reading this made me go back and double check.
These new (at least to me) 10uF and 22uF or larger, low ESR, relatively high voltage ceramics (25V) are exotic creatures (compared to what we had only 10 years ago). Couple that with low inductance PWB layouts and you can blow away (pardon the pun) a lot of assumptions about how circuits behave.
#1) Rob, Personally I currently enjoy the mix of technical issues with the more mundane (appliance nightmares, etc.) However, IF there has to be a tilt, I'd prefer reading about the technical issues more. It seems that they are more "entertaining."
#2) Regarding the power supplies. For decades, ever since the homely 78xx series has been used, I've always including a few additonal components. On the input side, I always place a 10 ohm, 1 W resitor. Additionally, I put a .01uf disc as close as possible to the input terminal to ground. Close by, I use a 10uf electrolytic. On the output side, I always use at least a 50uf electrolytic. Finally, I install a 1N400x (reverse) bridging the output of the 78xx to the high side of the 10 ohm R. While this may sound supurfluous or overkill, in all my designs whether static logic circuits or rf circuits of low to high power, I've NEVER had an issue of corruption, parasitic oscillation, etc. with the p/s rail. Many years ago, there was a caution about the 78xx series, suggesting that some parts could be subject to oscillation under certain operating conditions IF there was no bypass C as close as possible to the input & ground terminal of the TO-220 device.
I'm interested in your story as a possible posting for Sherlock Ohms. If you're willing to draft it, showing the problem and the process of finding a solution, it would be a great contribution to Sherlock. If you're interested, we would also need a short bio.
If you decide yes, you can send it to: rob.spiegel@ubm.com
I've been using similar practices for many years for both linears and switchers, but I also combine yours with one suggested by an older post. Especially when using those new extreme-K low ESR chips (10uF and up), I ALWAYS put in a series R so I can guarantee a minimum ESR for the combination. This includes both input and output caps. It's a bit of a balancing act between noise control and stability, but having the footprint in the layout allows optimizing. If it turns out to be unneeded, you simply use a zero-ohm R! Even in the most cost-sensitive applications, the cost of a chip R is a tiny fraction of a cent in high volumes now; typically, the cost of placing the part is higher, so the part itself is basically free. There are other components that also can benefit from this (like high-power audio amp ICs). This also helps a lot when designing "hot-plug" circuits.
Most of the circuits I've done couldn't tolerate a 10 ohm resistor input to ground for multiple reasons (efficiency, "off" state current draw, etc.). In the original post, at 24VDC input, this would draw 2.4ADC! That's 57.6 W, and the resistor would fail because you let the smoke out. Did you mean 10K???
P.S. I may be an OLDER curmudgeon, going back to TUBE days. Worked for the old HP New Jersey division (formerly Harrison Labs) for a few months after I graduated, designing one of the very first digitally-controlled high power lab supplies.
I remember HARRISON LABS also. Didn't work there, but knew of their work. My first engineering position was for a radio communications company. My specialty was designing linear power amplifiers in the range of 400W (output) to 5 KW, in the fregquency range from 2.0 MHz to 400 MHz. Some were auto-tune, some not, but the projects were varied & interesting. My favorite days were those when I get update inserts for the giganticly thick SPRAGUE ELECTRONICS & EIMAC catalogs. It was always a treat to see a new tube introduced...... How can I use this one? The last time I saw an 8660 catalogued (4CX1500B) was about 5 or 6 years ago in the ALLIED catalog. They were selling them for about $1100 / ea. Of course they weren't EIMAC brand anymore. If my memory serves me correctly, I think we paid about $75/ ea. in production quantity, but then again, that was about 40+ years ago!!!
That $75 sounds about right. I never could afford to use those hefty boys in my homebrew, used 807s, 6146s, and (for VHF) 2E26s. First Xmtr was a 6AQ5 10W CW one. Never even owned a linear; I was QRP for economic reasons! Still licensed, not active, no gear. The vast majority of the tubes you can get today are from Russia. My younger son at one time worked for the US master distributor for Sovtek, the largest Russian manufacturer.
BTW, re-reading your post, I now realize that the 10 ohm 1W you mentioned was IN LINE with the input, not to ground! BIG difference. It actually acted as a fuse, and also helped protect aginst both OV transients and reduced the power dissipation for steady-state voltages near the max. Used that myself on occasion. Sorry for the error.
Ironic part of it was that I never had a Class license. My next door neighbor was an active ham, and we discussed his shack many times, but I never got the urge to paddle. Left work in the office. Was into a lot of other things, but not amateur radio.
Also used 3-400Z, 3-500Z, 8877 (3CX1500), 4CX2500, 5CX5000 & 6146B as drivers (3 in parallel, 150W out PEP). And, used an Amperex glass (tetrode) "bulb", but can't remember the #, as well as some RCA ceramic (tetrode) tubes for the higher frequencies. And, other EIMACs as well. One disappointment though ..... all were air-cooled tubes. Would have liked to design a XMTR with some of EIMAC's water-cooled tubes, just for the fun of it.
You're re-read is correct. The input V goes into the 10 ohm. The "low" side into the input pin of the 78xx. Tied to this pin are a .01uf ceramic to gnd AND a 10uf to 22uf alum electrolytic to gnd. The output side of the 78xx usually fitted w/ the 100uf @ Vout + S.F. The 1N400x is wired anode to the output, cathode to the high side of the 10 ohm. This is especially useful when the load circuit has considerable Xl. Helps drain off that inductive kickback. Of course, I agree w/ you about the incidental cost increment for these extra components, but my projects were (and, still are) pricey enough that a few extra components to ensure long operational life are not relevant.
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