12 V, 20 A, 240 W Power Supply with Passive Cooling

There’s not much point in going into detail about why I like tinkering with power supplies, but I’ll explain why I’m focusing on 12 V.
That’s just how things have worked out: 12 V power supplies are among the most popular, alongside 5 V and 19 V models.
5 V is used to power small devices, but its popularity got a further boost from the fact that USB ports provide the same voltage, so these power supplies started popping up everywhere.
19 V is used in laptops, and hobbyists also use these power supplies for various soldering stations and amplifiers, mainly because they offer a decent amount of power in a compact package.
As for 12 V, it’s a safe voltage to start with, while still allowing you to deliver quite a lot of power. Of course, in my opinion, you can often use 24 V instead (and sometimes you should), but that voltage is more commonly used in industrial equipment.
At home, 12 V can power the LED strips that have become popular for decorative lighting and illumination, as well as video surveillance systems, sometimes small computers, and various rotary tools, 3D printers, and so on.

I’m planning to review several similar power supplies with different power ratings, and today I’ve got a 240 W model with passive cooling on my bench.
Most fanless power supplies currently available are rated for up to 240–300 W. The 300 W models are much less common, and I’d say 240 W is already close to the limit.

That’s enough for a brief introduction; let’s move on to the subject of the review.
It’s a power supply in the familiar metal case. I’m sure many of you have seen similar models for sale.
It came in a plain white box. I didn’t get a photo of it, and there wasn’t much to see anyway.


The input and output connections are brought out to a single large terminal block. There’s a label on top showing what each terminal is for, but it’s stuck on crooked, which could confuse an inexperienced user.


The terminal block has a protective cover that opens to 90 degrees. It’s a small plus, but a plus nonetheless, since some versions have covers that don’t open all the way.


To the right of the terminal block are a trimmer and an LED power indicator.
The stated specs are 12 V, 20 A. The actual manufacturer is unknown; the model number is the standard one used on many inexpensive power supplies: S-240-12.
There’s a 110/200 V input voltage selector on the side. Before switching it on for the first time, it’s best to check that it’s set correctly.
The date of manufacture is late 2016, so the power supply is fairly recent.


First, let’s measure the output voltage setting.
It’s set to 12.3 V, with an adjustment range of 10–14.5 V. After checking it, I set it to something close to 12 V.


There’s nothing else to inspect on the outside, so let’s remove the top cover and see what’s inside.


Inside, this power supply is no different from other inexpensive models of this kind.
At first glance, it reminded me of the 48 V 240 W power supply. I’d even say they’re identical.
Actually, that’s not quite right: it’s essentially the same power supply, just with a different output voltage. That’s why I said at the beginning that the actual manufacturer is unknown.


A classic look at the internals.
1. There’s an input filter, though it’s incomplete: the capacitor after the inductor and the varistor are missing. Unfortunately, this is true of the vast majority of Chinese power supplies.
2. The interference-suppression capacitors are Y1 in the hazardous circuit and a regular high-voltage type in the less hazardous one. That’s acceptable.
3. The input bridge rectifier is generously rated at 8 A, 1000 V, but there’s no heatsink. The previous version had a 20 A bridge rectifier.
Two thermistors connected in parallel are also visible nearby.
4. The input capacitors are Rubicong—a Rubicon knockoff. It would be nice if their specs matched the stated values, but more on that later.
5. A pair of high-voltage transistors are clamped to the aluminum case, which acts as a heatsink.
6. The power transformer is clearly marked 240 W, 12 V. It looks pretty good, and you can see traces of varnish impregnation.


Chinese manufacturers keep churning out power supplies based on classic components. I’m not saying that’s a bad thing, but the better-known manufacturers are much less likely to make power supplies based on the TL494 these days.
This does have its advantages: these power supplies are fairly easy to repair, the components are readily available, and there’s plenty of documentation.


As with the 48 V version, this one also uses a reinforced heatsink. The output diode assembly is clamped to a finned heatsink, which then transfers some of the heat to the case. If this wasn’t particularly necessary in the 48 V version, at 20 A it’s definitely a welcome feature.


1. The output inductor is wound with just two wires, despite its perfectly reasonable size, and the wire gauge is comparable to what was used in the 48 V power supply.
2. The output capacitors are rated at 2200 µF. The manufacturer is also unknown, though I wasn’t expecting to find Nichicon or even Samwha capacitors here.
3,4. I checked the clamping of the power components separately, since last time I had serious complaints about how the diode assembly was secured. In this case, it’s basically fine. You could nitpick the transistor clamping a little (on the left), but in practice everything turned out to be okay.


Let’s take the board out of the case and check the soldering quality, looking for any manufacturing “oopsies.”


The high-voltage transistors are generously rated, so there’s no need to worry. Their TO247 package also improves heat transfer to the heatsink.
The MBR30200 output diode assembly consists of two high-voltage Schottky diodes. I’m a little skeptical about using high-voltage Schottky diodes, since they no longer have an advantage over conventional diodes in terms of forward voltage drop. They do, however, switch faster, which means lower switching losses.


Overall view of the underside of the circuit board.


The soldering looks perfectly fine. Everything is okay in this part of the power supply, and it’s even clean.


The power traces have an extra layer of solder to increase their cross-section. No major complaints here either, although in some places there seems to be a bit too little solder.


But I did find one unpleasant issue. One of the power contacts isn’t soldered very well. You could say there are three contacts per pole, but it could happen that this one ends up carrying the load. That’s why I always recommend checking how a power supply is assembled before buying it. Actually, more accurately: always check the build quality when buying an inexpensive power supply.

There’s some marking on the board that I don’t quite understand. It looks like the board is designed for power supplies up to 365 W, but that would probably require active cooling (there’s a spot for a fan connector on the board, but the connector and the necessary components are missing).


While I was at it, I measured the capacitor values.
The input capacitors have a combined capacitance of 166 µF (two 330 µF capacitors in series), although they’re marked 470 µF (which would give 235 µF combined). That’s a bit low for 240 W.
The output capacitors add up to about 6600 µF, as expected from the 2200 × 3 marking. No issues there; that’s normal for power supplies with similar specs, even branded ones. Though branded power supplies do use higher-quality capacitors.


Since the power supply circuit is practically identical to the 48 V model, I just made the necessary changes instead of drawing it from scratch. I can’t guarantee a 100% match, but I think it’s 99% there :)


Now we can run some tests.
The test setup consisted of:
1. Electronic load
2. Multimeter
3. Oscilloscope
4. Thermal imager
5. Thermometer
6. A pen and paper. No link to the paper.


1. No-load condition.
2. 5 A load, ripple around 50 mV


1. 10 A load, the voltage dropped only slightly, and the ripple stayed at the same level
2. 15 A load, practically no change


Since running the big battery test, I’ve modified the load to increase its maximum current to 30 A. But something didn’t quite go as planned, and the maximum current is limited to 16383 mA (14 bits). So, to continue the test, I had to use some ordinary Soviet 10 Ω resistors. At 12 V, they provide a load current of about 3.6 A.


1. At 20 A, the voltage dropped by just 70 mV. The ripple level was practically the same as in the previous tests, at 60 mV.
2. As an additional heat test, I decided to raise the output voltage to 12.55 V and run the power supply for another 15 minutes. Its output power was about 250 W.
As you can see in the photo, this had practically no effect on the results.


In my previous review, I was so impressed by the power supply's performance that I even tested it at 150% overload. I drew 360 W from a 240 W power supply, and only then did I start to get genuinely concerned about overheating.
But things are a little less encouraging in this case. First, here's a thermal camera photo taken right at the end of the test at 250 W.
The hottest component is the output inductor. The same thing happened during the test of the 48 V power supply. But as I wrote then, the material this inductor is made of can actually withstand these temperatures; the limiting factor is the temperature rating of the wire insulation.

For comparison, I photographed the load resistors, which were dissipating only about 50 W. The electronic load was handling about 200 W, and its heatsinks were at 61°C.


As before, I put all the data into one table.
The test was conducted at room temperature, with the power supply lying horizontally on the table, which somewhat worsened its thermal conditions. It would have cooled better in a vertical position.
Each stage lasted 20 minutes, followed by a temperature measurement and an increase in current by one step.
The final stage was an additional test and lasted 15 minutes, bringing the total to 20+20+20+20+15 = 1 h 35 min.

The results are noticeably better than those for the 48 V power supply, but I'd say they're quite acceptable. The most delicate component, the power transformer, doesn't overheat.


The low efficiency of power supplies like these came up in the comments once, and I got genuinely curious about what their efficiency actually is.
Of course, I can't claim high accuracy, since several measuring instruments are involved and each has its own measurement error, but I did my best to measure as accurately as possible.
So, I measured the power supply's input power with no load and at 33%, 66%, and 100% load. Here are the results:
Input - Output - Efficiency.
4.2 - 0 - 0
96.2 - 79 - 82%
189.3 - 159 - 84%
290.4 - 238 - 82%

People said that the efficiency of power supplies like these is around 60–70%. Honestly, I didn't believe it. But until now I'd been judging by how much heat they produce, because it's hard not to notice an extra 100 W of heat. So I decided to run this test, and I think it was worth it.


Of course, people may start asking in the comments: “What about Mean Well? Why not Mean Well?” I think very highly of this company's power supplies and use them quite often, so I decided to compare the power supply under review with a Mean Well unit out of curiosity. It's worth noting that I compared it with a power supply from the RS series, specifically the RS-150-12, rated at 12 V and 150 W. This power supply currently costs about $36 — link.
Power supplies in this series are excellent, and their reliability really is top-notch. The unit you see here ran in a video surveillance system for about 3 years at close to 90% load, and was replaced with a new one as part of scheduled maintenance.

The manufacturer claims:


But this applies specifically to the RS series. The regular MenWell S-xxx-xx power supplies are a little simpler, though they also cost less.



The input filter is more complete than the one in the unit under review, but there is still no input varistor.


1. The thermistor is covered with heat-shrink tubing. Interestingly, while reviewing the photos after taking the unit apart, I noticed there are actually two thermistors; the second one is bare and sits to the right of the switch.
2. The input capacitors are Rubicon, not RubiconG. Total capacitance is 165 µF at an output power of 150 W.
3. The high-voltage transistor has additional insulation. A different PWM controller is used, so the area next to it is completely empty.
4. There are two output diode assemblies, and both have ferrite beads on their leads—something you almost never see in inexpensive Chinese power supplies. Some of the capacitors have these beads too.
5. The output inductor, however, is made in the finest Chinese tradition :) The winding is crooked, and it's been slathered with some kind of glue.
6. The output capacitors are branded, with a capacitance of 3 × 1000 µF and a voltage rating of 35 V, which is quite appropriate. The unit under review has 25 V capacitors, but that's fine in a push-pull circuit (computer power supplies use 16 V capacitors).


I won't list the pros and cons today; I'll just share my impressions of the power supply.
In my opinion, this is a typical “average” Chinese power supply. Temperatures stay within acceptable limits, the build quality is average, but ripple is low and the output voltage doesn't “drift” as the unit warms up (which is quite important). The manufacturer doesn't seem too concerned about the components, judging by the questionable input capacitors. Their capacitance is adequate according to the markings, but low when measured. In a situation like this, I simply added a 100 µF, 400 V capacitor desoldered from a monitor board.
The components most critical to this power supply's service life are the output capacitors.
Otherwise, it's a perfectly decent power supply. It passed all the tests without any problems, but unfortunately I couldn't get results like I did with its 48 V version. In my opinion, it's an average power supply at a perfectly reasonable price.

I hope you found the review useful. I tried to include as much information as possible.