Well, that was an interesting failure mode…

I have a bunch of eBay-sourced DC-DC converters that I use for a bunch of purposes around the house. Most are ordinary “LM2596” (in scare quotes, as most seem to be clones: they’re marked as LM2596 and generally work well, but have different switching frequencies. Supposedly this is an issue with such things.) buck converters configured as adjustable, constant voltage power supplies where the output voltage is set by a multi-turn potentiometer. Very handy.

Others can be used in either constant voltage mode or constant current mode. For the latter, a serpentine strip of PCB trace acts as a low-value sense resistor. An LM358 dual op-amp integrates the difference between the voltage across the trace and a voltage set by a potentiometer, with the output connected to the regulator’s feedback pin via an LED so you can tell when the regulator is in constant current mode. Another potentiometer sets when the “charging” LED lights up; this is purely cosmetic, and the LED turns off when the current through the regulator drops below the setpoint set by the potentiometer.

Caleb Engineering has an excellent teardown of such a regulator here.

Here’s a few pictures of mine:

The overall regulator module. The input is on the left and the output on the right. The three potentiometers control, from left-to-right, the constant-voltage setpoint, the “charging” LED setpoint, and the constant current setpoint. Sorry for the bad lighting, but you can see the main regulator chip on the left, the op-amp on the right, and the linear regulator supplying the op-amp in the center.
The serpentine PCB trace used as a sense resistor. I typically measure around a 9mV drop from the “OUT-” pin in the top-right (to which the load’s 0V/ground is connected) and a test point partially seen at the extreme left of the picture when a current of 500mA is flowing.

Today, I wanted to use one of these modules to charge some supercapacitors in a controlled way, so I grabbed one of the buck modules, set the voltage limit to 2.6V (to stay within the 2.7V maximum limit of the supercapacitor) and the current limit to 500 mA. For testing, I connected the input to a 12V supply and everything worked fine.

I then connected the input to a 5V supply, which is more convenient for most things I do, only to watch the regulator go into current-limiting mode and push out 3.5A (!!). The current limiting potentiometer did nothing, even when turned all the way down to zero. The capacitors and the LM2596 started getting toasty warm (uh-oh), so I unplugged things to investigate.

It turns out I forgot a crucial detail: the op-amp is powered by a 78L05 5V linear regulator connected to the input voltage. Although the LM2596 switching regulator used to power the load has a dropout voltage of less than a volt (and the 2.4V difference between the 5V input and 2.6V output is perfectly suitable in any operating condition), the 78L05 regulator for the op-amp requires at least 7V input to stay in regulation. Supplying it with only 5V input meant the output voltage was less than the regulator needed, and so the feedback loop was broken and the LM2596 tried its hardest to pull the voltage up to 2.6V, maxing out its output current.

The culprit.

As soon as I connected the input of the module to a 9V or 12V supply, it worked great, since the 78L05 had a sufficient voltage difference to stay in regulation.

It’s worth being aware of this issue, particularly if your input power supply doesn’t have a lot of “oomph” behind it: if the input voltage ever drops below 7V (such as when supplying a heavy load) the 78L05 will drop out of regulation and the LM2596 will draw even more current, thus holding down the input voltage and preventing the system from recovering. Fuses are your friend in such conditions.

To prevent such issues, you might consider using some of the buck-boost modules (which are also available in constant voltage only, or CV/CC variants). They use a boost converter to first step up the voltage to a higher voltage (I have several different ones, some with LM2575 boost converters, while others have XL6009 chips, both boost to around 28V), which the LM2596 then bucks down to the desired output voltage. The 78L05 can handle input voltages up to 30V and the op-amp currents are low, so it works fine. There’s some loss of efficiency when using two converters instead of one and the maximum output voltage is slightly lower, but I haven’t found any edge conditions in the buck-boost configuration that cause bizarre failures like with the buck-only converters — one such buck-boost constant current supply has been driving the IR LEDs in my DIY babycam for more than a year from a 5V input without any hitches.

Edit: Although the constant current buck-boost modules commonly found on eBay will work fine with lower input voltages because the linear regulator gets its input from the boosted voltage from the first stage, it seems they cannot start up properly if they’re connected to a dead short when they’re first connected to input power. The switching regulators go into current-limiting mode and the linear regulator doesn’t get enough voltage to properly start the op-amp for constant current mode. I blew a bunch of fuses testing this (better than blowing up components!). Once the switching regulators have started up and the linear regulator is in regulation, the constant current regulation works as expected.

This issue could have been avoided by adding two resistors and a small capacitor to the ON/OFF pin on the LM2596 buck regulator for a delayed startup. This would keep the buck regulator offline for enough time that the boost and linear regulator, as well as the op-amp, would start up and be ready. Alas, due to the layout of the boards from the eBay suppliers, modifying the existing board isn’t really feasible.

In short: the buck-boost regulators with constant current regulation are better in general since they have fewer failure modes once they’re running, but the output current needs to be limited for a few moments when they’re first connected while the constant current regulation circuitry comes online otherwise they just max out their current. Not what you want. Adding some inrush limiting circuitry (e.g. an NTC thermistor and, optionally, a bypass MOSFET for higher efficiency) would work great.

Here’s an example of the buck-boost boards that I like, even with the above-mentioned limitation.

Investigating Fake MAX3232 TTL-to-RS-232 Chips

A while back I needed to interface a GPS timing receiver that only has an RS-232 serial connection with one of my Raspberry Pis. The Pi only supports TTL-level serial and only tolerates voltages between 0-3.3V its the UART pins.

Enter the MAX3232, a chip from Maxim Integrated that converts between RS-232 and TTL serial with supply voltages from 3.0 to 5.5V. It produces “true” RS-232-level voltages (both positive and negative) using built-in charge pumps and some small external capacitors. Just the ticket for what I needed.

Alas, the MAX3232 isn’t really something one can run down to the local electronics shop (and there isn’t any such shop where I live in Switzerland, as far as I know) and pick up. Typically it’s purchased by manufacturers in quantity from major suppliers. Hobbyists like myself need to turn to the internet where such things are available in abundance for cheap from China, though one must be wary of counterfeits. Of course, I could order from legitimate Swiss distributors, but small-quantity pricing and shipping are extremely high (>$10 USD per chip!) compared to major US distributors like DigiKey and Mouser.

In my case, I ended up buying a few boards like this one from an online vendor in China. The listing specifically states it had a MAX3232 chip. My thought was that if it was a legitimate chip, cool. If not, it’d be an interesting experiment and I’d get some cheap DE-9 connectors out of the deal.

Fake MAX3232 Board
Fake “MAX3232” Board

To the naked eye, everything seemed to be reasonable. The chip did have markings identifying it as a MAX3232 (falsely, as I later discovered; read on!). The board worked and the chip functioned within the specs in the Maxim datasheet and it even broke out the chip’s second RS-232-to-TTL channel on the header pins.

However, the first board failed after a few weeks, drew significant current, and dramatically overheated. By “overheated” I mean “blister-raising burn on my fingertip”-level-hot. Also, the data-transfer LEDs were glowing faintly all the time rather than flickering on and off when data was flowing.

Figuring this was just some bad luck on my part (this is before I got the anti-static mat on my desk), I swapped it out for another board. I was particularly careful with anti-static precautions, and put the board over a small glass container just in case it overheated and caught fire. Although it didn’t catch fire (thankfully!), it did fail after a few weeks and overheated just like the first one.

Ok, so something’s going on, but what? Since the chips had obviously failed, I figured I couldn’t harm them with some ham-fisted SMD desoldering, so I took them off their respective boards.

Here’s the results of my handiwork, as viewed under a petrographic microscope in the lab (I used a handheld camera aimed through the microscope eyepiece for all the microscope photos, hence the weird vignetting at the edges):

Fake MAX3232 Package
Fake MAX3232 #1 Package
Fake MAX3232 Package
Fake MAX3232 #2 Package

All right, quit laughing! De-soldering SOIC chips using a handheld soldering iron is no fun.

Anyway, the markings are inconsistent and seem pretty low-quality. Definitely not something I’d expect from Maxim. For comparison, I had ordered a free MAX3232 sample directly from Maxim (thanks, guys!) and it arrived a few days later. Here’s the legit chip:

Genuine MAX3232 Package
Genuine MAX3232 Package

According to the date code, I ended up killing this chip in the name of science about 2-4 weeks after it was made. Sorry, little guy. Anyway, you can see the markings of the real chip are distinctly different from the fake chips. The differences are striking even with a handheld magnifying glass: the real chip has distinct laser-etched markings with a textured surface. The fake chips have much “weaker” etching with a much flatter, duller surface.

Next, I decapsulated all three chips (two fake and one genuine) by dissolving them in hot nitric acid followed by an acetone wash and a few minutes in the ultrasonic cleaner. Don’t try this at home (or at least not in my home!): I did it in a controlled environment with a fume hood, proper ventilation, protective gear, etc. Zeptobars and the CCC have some interesting guides on how to do this if you’re interested. Be careful.

Alas, I wasn’t able to do the preferred method of just dissolving the package over the silicon die itself. Instead, I decided to dissolve the entire chip package, legs and all. Interestingly, the genuine one dissolved much more rapidly (but at a consistent rate) than the fakes; the fakes resisted the acid for nearly an hour, then quickly dissolved in a few minutes. The package seemed to be the same blackish epoxy that most chips are encased in, so I have no idea why they would behave so differently in acid. Any materials-type people have any ideas?

Here’s what the real MAX3232 die looks like:

Genuine MAX3232 Die
Genuine MAX3232 Die

Note the gold bond wires are still attached and didn’t dissolve in the acid bath. I’m not a semiconductor expert, so I can’t tell you what the actual regions of the chip actually are, but it’s a pretty picture and serves as a good reference for comparing the others. Also, the genuine die is about 50% longer than the fakes so it couldn’t fit entirely in the field of view of the microscope.

Here’s a closeup of the “Maxim” marking on the genuine chip as well as a date code. Presumably it refers to when the chip design was finalized.

Genuine Maxim Mark on MAX3232
Maxim Mark on Genuine MAX3232

Now, let’s look at the fakes. Both fakes were, unsurprisingly, identical.

Without magnification, the dies are pretty small and it’s hard to make out any detail.

The fake MAX3232 die is quite small. The real one is only a millimeter or so longer.
The fake MAX3232 die is quite small. The real one is only a millimeter or so longer.

Under the microscope, we can see the die quite well.

Fake MAX3232 Die
Fake MAX3232 Die

The above photo was taken at the same magnification as the genuine MAX3232 die photo: you can see the fake die is much smaller, has a much different appearance, and didn’t use gold bond wires. Whatever metal was used for the wire dissolved away in the acid.

Both chips had the same markings: they appear to have been designed in November 2009, and the marking on the second line appears to be “WWW01” (though I’m not sure if it’s the number zero or the letter “O”). I’ve had no luck figuring out what that means.

Fake MAX3232 Date Code & Manufacturer(??) Mark

As I mentioned above, I’m not an expert on low-level chip design or failure analysis and I was unable to find any obvious-to-the-layman failure in the chips that would have resulted in them passing significant current and overheating. Any ideas?