5 V, 7.2 A and 36 W, or a short guide to choosing the right power supply.

A few times in the comments, and later in private messages, people asked me to review power supplies with a specific voltage. I replied that I’d try to get hold of some for review and testing.
Today I’m reviewing a 5 V power supply.
But simply reviewing it would be rather boring, so this time I’ll try to explain what the different components in a power supply do and what to look for when choosing one.

There’ll be plenty of text and not too many photos in this review. I’ll do my best to keep things easy to understand, but I might slip into using technical jargon like “common-mode,” “saturation,” “leakage,” and so on. If anything isn’t clear, just ask and I’ll explain :)

Originally, I planned to order two power supplies with different power ratings, 18 and 36 W, but then decided that 18 W wasn’t very interesting and ordered only the 36 W version. That’s the one we’ll be looking at.

As always, I’ll start the review with the packaging, since that’s what makes the first impression.
The power supply arrived in a small brown cardboard box marked to indicate that it’s a 5 V, 7.2 A power supply.


Judging by the markings, power supplies in this enclosure are made with different power ratings and output voltages. I’ve come across a 12 V power supply in the same enclosure before.
The specifications printed on the label are:
Input voltage: 100–240 V
Mains frequency: 50/60 Hz.
Output voltage: 5 V
Maximum output current: 7.2 A
Maximum power: 36 W. It says “total,” though it’s not entirely clear what that means in this case.


The power supply is relatively small. Its height is about the same as a matchbox, at 37 mm.
It weighs just 133 g (in general, the higher this figure, the better, although it’s only an indirect indicator).
It’s 85 mm long and 58 mm wide.


The input, output, and ground connections are all brought out to a single terminal block.
The terminal block has a cover, but it doesn’t open all the way—it falls just short. Next to it are a trimmer for adjusting the output voltage and an LED indicating that the power supply is on.


There’s nothing particularly interesting on the outside of the power supply, apart from the shiny perforated cover that protects against electric shock and interference. So let’s see what’s inside and how it all works.
Undo a couple of screws and we can get inside.
No complaints about the appearance. The first thing that says a lot about manufacturing quality is the assembly. If the components are neatly aligned, there are no empty spots on the circuit board, and large components are secured with glue (or sealant), those are usually signs of a good power supply rather than a bad one.
Everything here is neatly installed, though there are still a few empty spots.


The external inspection is complete; now we can take a closer look.
First, the design: this power supply uses passive cooling for its components.
Some of the heat is transferred to the aluminum case, which acts as a heatsink. This is a fairly classic cooling method for power supplies like this.
By the way, you can improve cooling efficiency by attaching the power supply to something that dissipates heat. Avoid mounting a power supply like this on an insulating surface, or do so only if you reduce the load.


Heat is transferred to the case from two components: the high-voltage transistor and the output diode. I’ll tell you more about them later. Thermal paste was applied between the components and the case, and the components themselves are held in place by a steel plate.


Now let’s look at the individual parts of a typical power supply, and I’ll try to explain what each one does.
1. Terminal block. This one’s self-explanatory: it connects the input and output wires. At high currents, multiple terminals of the same polarity are used—for example, two positive terminals and two negative ones. They’ve cut costs a little here, since the output current is up to 7.2 A, but there’s only one terminal per polarity. I wouldn’t call that critical, but it’s better when you can distribute the load.
2. Input filter.
3. Bridge rectifier, which rectifies the mains voltage. It’s sometimes mounted on a heatsink if it’s a separate component, but that isn’t necessary in low-power supplies.
4. Input rectifier capacitor
5. High-voltage transistor
6. Transformer
7. Output rectifier diode.
8. Output power filter
9. Output voltage regulation and adjustment circuit.


Next, I’ll take a closer look at the components mentioned above.
Input power filter. Its main purpose is to filter interference that gets from the power supply back into the mains. If your radio picks up interference when you switch on a switching power supply, first check whether it has this kind of filter.
A full version includes a two-winding inductor, two X capacitors (yellow in the photo), and two Y capacitors (usually small and blue). The interference filter also includes a capacitor connecting the primary and secondary sides of the power supply, and one connecting the negative output terminal to the case, but these mainly help suppress noise at the output.
Because of these Y1 capacitors, an unearthed power supply usually gives you a little shock.
The inductor and X capacitors are straightforward: the higher the inductance and capacitance, the better. Sometimes two-stage filters are used, with two inductors.
In some cases, the filter is simplified to just an inductor, one X capacitor, and one or two Y1 capacitors (between the primary and secondary sides of the power supply, and between the power supply negative and the case). This is also a perfectly reasonable solution. But sometimes the inductor is replaced with “specially trained jumpers,” or the filter is removed altogether. Don’t do that—the interference is guaranteed.
This is an “economy version,” but it works perfectly well and could have been left as is. However, instead of proper Y1 capacitors, the manufacturer installed ordinary high-voltage capacitors (2.2 nF, 2 kV). This is unsafe: if one of these capacitors fails, the power supply output will be connected to the input and could give you an electric shock. It can fail due to a voltage spike caused, for example, by a powerful lightning strike near a power line.
In short, the filter is perfectly usable, but for safe operation it’s best to replace the blue capacitors marked CY on the circuit board with proper Y1 capacitors, or ground the power supply case.
Unfortunately, this is probably a common failing in 90% of inexpensive power supplies.
A special thermistor is also installed before the input filter in switching power supplies to limit the inrush current when switched on. There isn’t one here—or rather, the inductor partly serves that purpose. That’s not ideal, but acceptable in this case. For a high-power power supply (and therefore one with high-capacitance capacitors), a thermistor is essential. In particularly demanding cases, there’s even a special circuit that shorts it out after startup.
Here’s how it works: while the thermistor is cold, its resistance is high, so it limits the current. After the power supply is switched on, it heats up and its resistance drops, so it doesn’t cause significant losses. But if you switch the power supply off and then back on before the thermistor has cooled down, the inrush current will be barely limited.


After the input filter comes a bridge rectifier, which converts AC to DC. The DC then goes to the electrolytic capacitor.
Bridge rectifiers come in different forms: they can be made from individual diodes or packaged as a single component, and are sometimes mounted on a heatsink. This one uses four individual diodes. They’re standard 1N4007 diodes, which are perfectly adequate for this power supply. Cheap power supplies sometimes use just one diode, which is very bad because the input capacitor is then used inefficiently.
The input electrolytic capacitor. This one’s simple: within reason, the higher the capacitance, the better.
For a power supply designed for 230 V (±10%) only, the capacitor’s capacitance should be roughly equal to the power supply’s wattage. In other words, a 90 W power supply would use a 100 µF capacitor.
For power supplies designed for the wider 100–240 V range, this capacitor’s capacitance should be 2–3 times higher.
This one uses a 47 µF capacitor rated for 450 V (which is very good; 400 V capacitors are more common). Its capacitance is more than adequate for a 230 V input (with a 36 W power supply), but it’s too small for operation at 100–150 V.
The capacitor’s capacitance affects the following characteristics.
1. The input voltage range over which the power supply operates normally.
2. The capacitor’s service life. Higher ripple current makes a lower-capacitance capacitor age faster; the higher the capacitance, the longer it will last.
3. Increasing the capacitance improves the power supply’s efficiency, although only slightly.


A high-voltage transistor. There isn't much to say about this one.
Except that the rule “the bigger, the better” doesn't apply here. The transistor's parameters should be optimal for the PWM controller IC being used.
The maximum voltage rating can also matter. This transistor is rated for 600 V, which is perfectly fine for this circuit. I've occasionally seen 800 V parts, but they're very rare.
The package type matters too. Some have an all-plastic package, while others have a metal tab, in which case the transistor is attached to the heatsink/case through an insulating pad. Personally, I prefer the fully insulated package.


The power transformer.
To put it very simply, the rule here is: the bigger, the better.
This power supply uses a flyback converter topology: first the transistor turns on and “charges up” the transformer (strictly speaking, it isn't quite the transformer itself, but that doesn't matter here); then the transistor turns off and the energy from the transformer is “transferred” to the load through the output diode.
I said “to put it simply” because the transformer's size depends not only on the power, but also on the power supply's operating frequency. The higher the frequency, the smaller the transformer can be. But most mass-market power supplies operate in the 60–130 kHz range, so the rule still applies.
Higher-frequency controllers are available, but high frequencies require very high-quality transformer materials, so a power supply like that will cost considerably more.
I've seen transformers about half the size of a matchbox in cheap 250–300 W ATX power supplies, but that wasn't because they operated at a very high frequency—it was just extreme cost-cutting :(
People sometimes ask whether you can modify a power supply from 5 V to 9 V, or from 19 V to 12 V.
Usually you can't, because the transformer has a fixed turns ratio between its primary and secondary windings, and the modified power supply would operate outside its optimal range—or might not work at all. The transformer also has another winding that powers the PWM controller IC, and the voltage on that winding depends on the voltages on the other windings.
The transformer in this power supply is a good match for its rated power.


The output rectifier diode.
The reliability of the power supply depends quite a lot on this diode. One rule of thumb is that the diode should be rated for a current 2.5–3 times higher than the power supply's maximum output current. In our case, that's 7.2 × 3 = 21.6.
This power supply uses a dual diode assembly. According to the datasheet, the diode is rated for 20 A (2 × 10) and 100 V.
Its current rating meets the requirements, while its voltage rating is considerably higher than necessary.
For a 5 V power supply, a diode rated for 45–60 V is usually enough; for 12 V, 100 V; and for 24 V, you need 150 V.
But in reality, too much of a good thing can be bad. Here's why.
Schottky diodes are great: they have a low forward voltage drop and switch quickly, which improves the power supply's efficiency and reduces heating.
But unlike ordinary diodes, their forward voltage drop varies more noticeably with the diode's maximum voltage rating. In other words, a 45 V diode can easily have a voltage drop 1.5 times lower than a 100 V diode. So this power supply would be better off with a 30–40 A, 60 V diode: efficiency would be higher, and the price would be about the same.
So, in practice, this power supply has a good diode with plenty of voltage headroom. That's reliable; I think that if anything burns out, it'll be one of the last parts to go. It's just not quite optimal.


Output filter and regulation stage.
There are some rules of thumb here too. For example, the total capacitance should ideally be 1000 µF per 1 A of output current. In practice, though, the power supply works perfectly well with half that capacitance. The capacitors' maximum voltage rating and type are just as important.
The capacitor voltage rating should generally be:
For a 5 V power supply: 16 V, or 10 V at the absolute minimum—never 6.3 V.
For 12 V: 25 V, or 16 V at the absolute minimum.
For 24 V: 35 V, never 25 V.
The capacitors should have low equivalent series resistance (LowESR) and be rated for 105 °C. That way, they'll last a long time.
The capacitors in this power supply are rated at 1000 µF each, for a total of 2000 µF. Based on that, a continuous current above 4–5 A isn't recommended. You can draw more for short periods, but it will shorten the capacitors' service life.
By the way, there's room in this power supply for proper 10 mm-diameter capacitors, even though the ones installed now are small, just 7 mm in diameter.
As for the output inductor, more is definitely better. But size isn't the only thing that matters: you also need to consider the current rating. If the inductor is wound with thin wire, it'll get hot. And if the ferrite core it's wound on overheats, its characteristics deteriorate sharply once it exceeds a certain temperature. Induction soldering irons work on roughly the same principle, but there they've turned a drawback into an advantage. That's a topic for another review, though.
The inductor used here isn't particularly powerful. We'll come back to it during testing.
The output voltage regulation circuit. I'll write about it a little later, since it's on the underside of the circuit board. On top, there's only a trimmer for fine-tuning the output voltage and an LED indicating that the power supply is on and working (which aren't always the same thing :).


We've gradually moved on to more delicate electronics. In this power supply, most of the components are mounted on the underside, on the trace side, because it uses surface-mount (SMD) components. A power supply can use conventional through-hole parts too; it doesn't make much difference, so there's no need to pay much attention to that.
The board assembly, on the other hand, is worth checking. The board should be well made, with the leads soldered and trimmed—not sticking out in every direction. Ideally, the flux should be washed off, at least most of it.
I have no major complaints about this power supply. A well-deserved 4 points. I wouldn't call it perfect, but it's decent.
I usually coat the board with varnish after assembly and cleaning, but you only see that from top-tier brands, and even then mostly in industrial equipment.
I was a little disappointed that there's no safety slot beneath the optocoupler separating the high-voltage and low-voltage sections. Ideally, there should be slots between conductors on opposite sides of the power supply where they run close together. This improves safety.


I traced out the schematic from the circuit board. Basically, I took the schematic of one of the power supplies reviewed earlier and made the necessary additions and corrections, since most of these power supplies use similar (if not identical) circuit designs.


A closer look at the primary side of the power supply.
The PWM controller and its surrounding components are clearly visible, along with a shunt made up of several SMD resistors and the resistors that form part of the snubber.
Speaking of the snubber, this circuit suppresses parasitic voltage spikes on the transformer’s high-voltage winding. It comes in several variations:
1. Diode + resistor + capacitor (as used in this power supply); these are R3, C3, and DB1 in the schematic.
2. Diode + TVS diode (an equivalent of a very powerful Zener diode—a voltage limiter).
3. A combination of options 1 and 2, usually used at higher power levels.
4. Chinese know-how: leave it out altogether. This is usually found in the cheapest power supplies, such as chargers for e-cigarettes and mobile phones that sell for next to nothing.
This circuit affects the reliability of the power supply.

The shunt, made up of several SMD resistors numbered 9, 19, 21, 22, and 23, measures the current through the high-voltage transistor. This is needed to protect the power supply from overloads and short circuits. When the power supply fails, the shunt usually goes to the great beyond along with the high-voltage transistor, the PWM controller, and the resistor between the transistor and the controller.
The soldering is neat, and the components are even glued down. That’s one of the signs of a reasonably decent power supply.


This power supply uses a PWM controller of unknown origin, but its pinout is identical to that of the 63D39 controller, which is itself an equivalent of FAN6862.
Small power supplies typically use one of three circuit designs:
1. A PWM controller IC + a high-voltage MOSFET.
2. A powerful PWM controller IC with the MOSFET and shunt built in (sometimes, instead of using a shunt, it measures the voltage drop across the MOSFET when it is on).
Examples include TOP from Power Integrations, Viper, and so on.
3. A self-oscillating circuit with no ICs, and sometimes no overcurrent protection either.
The first two types are essentially equivalent; the third is much worse. If you see a small IC, there’s a 99% chance you have the first type of power supply. If the board has a high-voltage transistor with another 1–2 smaller transistors next to it, then there’s a 99% chance it’s a self-oscillating circuit.
This one uses the right design; no complaints.


The secondary side is responsible for rectifying and regulating the output voltage.
Some people mistakenly think the primary side is responsible for keeping the output voltage stable (although some power supplies are designed that way). It’s actually the secondary side that regulates the output voltage, since it monitors the primary side’s behavior.
A small IC called the TL431 handles the regulation. In this photo, it’s in a very small three-pin package marked V3. This IC is an adjustable Zener diode: when the power supply’s output voltage is applied to it, it controls the optocoupler (visible at the top of the board, between the transformer and the transistor). The optocoupler sends a signal to the PWM controller, which then adjusts the power supply’s output so the voltage stays stable.
The voltage is fed to the IC through a divider, sometimes just two resistors, and sometimes with an added trimmer resistor that lets you adjust the output voltage within a small range.
There’s another misconception that when a power supply fails, whatever is connected to it usually gets damaged too. Here’s the thing: it’s theoretically possible, but in practice it happens VERY rarely. The secondary side is also the least likely to be damaged when a power supply fails; most problems occur on the primary (high-voltage) side.
Some manufacturers don’t regulate the output voltage using a dedicated IC and optocoupler, which isn’t ideal. What’s more, I’ve even reviewed a power supply that had an optocoupler, but it wasn’t connected to anything.
Even the way the traces that measure the output voltage are routed can make a difference. This is critical, especially at high currents.
In short, if there’s an optocoupler and a small three-pin IC near the power supply’s output, it’s probably using proper voltage regulation.


To make it easier to understand what the primary (also known as the “hot”) side and secondary (the “cold”) side are, I marked them in different colors on the schematic. Components that belong to both sides are shown in black.


First, I switched it on for the first time (it had to be turned on eventually). Everything worked, and nothing burned out :).
When switched on, the power supply showed an output voltage of 5.12 V.
I checked the adjustment range: it was 4.98–5.19 V, which is perfectly fine.
After that, I set the output to the specified 5 V.


To test the power supply, I use the “test setup” familiar to my readers, consisting of:
An electronic load
A multimeter
An oscilloscope
A non-contact thermometer.
A pen and a sheet of paper

As before, I run stepped tests, 20 minutes each, increasing the load current after each successful test. The oscilloscope probe is set to 1:1.


The first test is with no load: 5 V, with almost no ripple.
2. Load: 2 A; voltage: 5 V; ripple: 30–40 mV. Excellent.


1. Load: 4 A; voltage: 5 V; ripple: about 40 mV. Excellent.
2. Load: 6 A; the voltage dipped slightly to 4.99 V, while ripple remained practically unchanged at about 40 mV. Excellent.


1. Load current: 7.2 A; voltage: 4.99 V, but ripple has increased significantly. That's bad.
The increase in ripple is caused not only by the load current, but more likely by the inductor heating up (or rather, overheating). As I mentioned above, the inductor (and transformer) core changes its characteristics when heated above a certain temperature. In this case, the inductor starts acting like a simple piece of wire, filtering almost nothing. If the transformer overheats like this, I'll be off to buy another power supply. I use temperature measurements to determine how the power supply is operating and what its maximum power is.
The inductor in this power supply is wound with thin wire, so it has high resistance and gets very hot.
As an experiment, I cooled the inductor and measured the ripple under load again. Just in case, I took a photo of the oscilloscope screen “in real time,” rather than in peak-hold mode.
2. Load current: 7.2 A; the inductor cooled to 88 degrees (although I inadvertently cooled the whole power supply a little too, I mainly cooled the inductor); ripple is no more than 50 mV.


Based on the test results, I put together a small table showing the temperatures of the main components in this power supply.
A few words about temperatures.
Don't be alarmed by transistor and diode temperatures approaching 100 degrees; they can handle those temperatures just fine.
The transformer and inductor temperatures are much more critical, as are those of the electrolytic capacitors. In this power supply, after 1 hour 40 minutes of testing (the last column, plus 20 minutes at maximum current), the output capacitors reached 104.2 degrees. That's very bad, but judging by the inductor's temperature of 142 degrees, I think it was the main contributor to this result. Replacing it should significantly reduce the capacitor temperatures.
Diodes and transistors can generally operate normally at 130–140 degrees, but I consider that too hot. Our old reference books used to say that components must not be operated beyond any one of their specified limits. I try not to exceed any limits at all.
The output inductor is the hottest component in this power supply. Even at maximum current and after a long warm-up, the other components stay within safe temperature limits. I was even surprised by how little the diode heated up.
When measuring temperatures, I measured the components themselves, not the heatsinks they were mounted on. This gives a more accurate picture of what's going on.


Summary.
Pros
The power supply holds its output voltage exceptionally well. So far, this is the best result among the power supplies I've tested.
The ripple level would be very good if the inductor didn't overheat at maximum current, causing the ripple to increase.
Overall heating of the power supply stays within acceptable limits.
The overall build quality is pretty good.
450 V input capacitor

Cons
The inductor is undersized for the power supply's output current and overheats.
The output capacitors have less capacitance than they should.
The wrong type of Y capacitors are used: ordinary high-voltage ones.

My take: This power supply can be used quite safely with a load current of up to 5–6 A. But if you replace the output inductor and capacitors, it should be fine for extended use at 7 A as well. During testing, I briefly loaded it to 7.5 A, and it worked without any problems. In other words, this power supply has some power headroom.
It's a shame they cut costs again on the capacitors connecting the primary and secondary sides of the power supply, using ordinary high-voltage ones instead. But based on my experience taking apart inexpensive power supplies, this is very common :(
I was very pleased with the output voltage regulation accuracy. As the load current changed from no load to 7.5 A, the output voltage dropped by just 10 mV. That's excellent; honestly, I didn't expect it.
All in all, it's a bit of a DIY power supply with good potential, but one that's practically begging for some modifications.

That's all for now. I hope I've helped a little with choosing a power supply. This review partly answers many of the questions I get in private messages and comments, but I plan to continue it (or rather, add to it) with another, noticeably more powerful power supply. I've also ordered a second one for a review at readers' request, and I hope it's already on its way to me.

As always, I look forward to your questions and suggestions in the comments :)


The product was provided by the store for review.