How a Power Supply Works, Part 5

As the simplest circuit, I'll show a version with one diode and one capacitor. This circuit is used in flyback power supplies, which make up the vast majority of power supplies today.


In a finished power supply, it looks like this photo shows.
These power supplies most often come with inexpensive equipment.


Next is a full-wave rectifier. I used this circuit quite often in the past, but recently it has been replaced by another one, which I'll show later.
This topology is most often found in high-power supplies. It's especially convenient in unregulated supplies based on the IR2151-2153 driver, which I discussed in the previous part.


As I said then, it's well suited to building primary power supplies that aren't regulated, but have good efficiency and can power other devices—for example, this power supply for a bench power supply.


A particular advantage of this circuit is that it's very easy to convert it to a split-rail supply for powering power amplifiers. In this version, you only need to add a couple of diodes and a capacitor.


When a flyback topology can't provide enough power, its forward-converter counterpart is used. In this circuit, energy is first stored in the inductor during one switching cycle, then delivered to the load through the lower diode. This topology is very similar to a classic step-down converter.


It's easy to tell that a power supply uses this topology: there will be a large inductor on the board. Inductors of this size are rarely used for filtering, so it's hard to mistake one for another.


But there's an alternative version of this circuit. It's most often used in computer power supplies and dates back to the first AT-format power supplies.


This circuit has an energy-storage inductor, and the primary winding of the power transformer is coupled to one of the windings of the control transformer. If you remove the inductor from this circuit, the power supply will fail when the load exceeds a certain level.
The same applies to the previous circuit.


It's easy to tell apart the last two types of power supplies. On the left is a supply built on the same principle as an AT-format power supply; the transformer next to the transistors is immediately noticeable. On the right is a single-switch forward converter, with no transformer here.
The inductors are different sizes, but that's due to differences in operating frequency and, sometimes, the manufacturer's cost-cutting. The smaller inductor will most likely overheat in operation, and the circuit may not work reliably at maximum power.


Schottky diodes are most often used as the output diodes in switching power supplies. They have two important advantages over regular diodes:
1. Their forward voltage drop is 1.5–2 times lower.
2. They're faster than regular diodes, so switching losses are lower.

Power supplies designed for high output voltages most often use regular diodes, since the forward voltage drop of high-voltage regular and Schottky diodes is about the same. But because Schottky diodes are faster, they can reduce snubber losses, so I recommend using them here too.


Since there will be noticeable ripple on the capacitor after rectification, an LC filter is added after it—or, in plain English, an inductor and a capacitor.


Here's an example of a “people's” power supply, with both the inductor and two capacitors clearly visible.


The inductor doesn't have to be large; it can be tiny. It won't work as well, of course, but it's better than nothing.


Sometimes the inductor is omitted altogether, even though there's room for it. This is penny-pinching, plain and simple. I always recommend installing an inductor there.


Here's a comparison of the ripple level with and without the inductor. Keep in mind, though, that after installing the inductor, ripple on the first capacitor will increase because it will carry the “surge” current. It's usually the first component to fail.


You can improve things by connecting ceramic capacitors in parallel with the electrolytic capacitors. This can significantly ease the load on the electrolytics. However, keep in mind that these are only effective in power supplies with relatively low power—or, more precisely, relatively low current. Of course, you can install a lot of these capacitors, but that gets expensive and takes up space.


When modifying the unit, you can solder the capacitors directly to the leads of the electrolytic capacitors.
I use capacitors with a capacitance of 0.1–0.47 µF.


To improve performance a little further, pay closer attention to the PCB layout. If you route the board along the lines I showed in the diagram, the ripple may decrease a little more—and it costs nothing.


And the final step: installing a common-mode choke at the power supply output. This is most often used in branded power supplies that need to pass certification for radiated emissions. You almost never see it in cheap units.


Now, about the output capacitors.
If you use cheap power supplies, you’ll most likely find either completely unbranded models at the output.


Or counterfeits of branded ones. For example, the popular power supply uses counterfeits labeled Sanyo or Nichicon. They’re easy to check by the markings. Most likely, you either won’t find capacitors from that series at all, or that series won’t include a capacitor with your capacitance and dimensions, or the color will be different, as in this case.
These counterfeits aren’t actually the worst option, but it’s better to use genuine parts.
By the way, capacitors in push-pull power supplies usually last longer, and the quality requirements are less stringent than for single-ended flyback supplies.


Still, it’s better to use genuine capacitors rather than substitutes bearing their name. The photo shows a Mean Well power supply.


One way to reduce the strain on capacitors is to replace one or two high-capacitance capacitors with a larger number of lower-capacitance ones. This distributes the load more evenly, helping the capacitors last longer.


Regulation circuit.
The simplest option is to regulate the voltage using feedback from an auxiliary transformer winding. However, this is also the worst option for stability, since it is affected by the magnetic coupling between the windings and their resistance. The upside is that it’s cheap.


The next option is more complex and uses a Zener diode as the threshold element. In this configuration, the power supply’s output voltage equals the voltage drop across the Zener diode plus the voltage across the optocoupler’s LED. The circuit’s performance is nothing special, but it’s perfectly acceptable for non-critical loads.


Here’s an example of a power supply with this type of regulation. There’s nothing on the top side near the optocoupler.


The Zener diode and several resistors are on the underside.


But the circuit using an adjustable TL431 shunt regulator performs much better. It offers much better performance and regulation accuracy, and maintains its parameters better when the temperature changes.


On the board, it usually looks like this, as shown in the photo.


It looks much like an ordinary transistor in a TO-92 package; the only difference is the marking. This is the most common version. Another version you may come across is the SMD SOT-23 package.


Pinouts for the different package options.


For example, the “popular” power supply uses the SMD package. The photo shows the feedback divider resistors and auxiliary resistors, such as pull-up resistors to the supply rail that provide the minimum operating current for the Zener diode.


A couple more photos: there’s nothing on the top side of the board, and the TL431 Zener diode is on the underside.



Sometimes a trimmer resistor is placed in the feedback circuit. But first, a few words about how the voltage divider is calculated.
If a standard two-resistor divider is used, its resistor values are chosen so that the voltage at their junction is 2.5 V at the required output voltage. The TL431 is designed for exactly this reference voltage. However, there is also a lower-voltage version of this IC, rated at 1.25 V, although it is much less common.
Now, about the trimmer resistor. For convenience, a trimmer may be fitted on the board to adjust the output voltage within a small range, usually +/- 10-20%. A wider range is not recommended, as the power supply may become unstable.
The trimmer should always be connected in series with the lower resistor in the divider. That way, if it fails, the power supply output will drop to its minimum voltage rather than rise to the maximum, as it would if the trimmer were at the top of the divider.
Also, trimmers are often not very reliable. If you don't need this feature, it's better to replace the trimmer with a fixed resistor, choosing the right value first.


Here is what the whole circuit looks like on the board.


A few words about the output load resistor.
A switching power supply doesn't work well without a load, so a load resistor is usually connected across the output to provide the minimum load needed for stable operation.
This solution has a downside: the resistor usually gets hot, sometimes quite noticeably. It can also heat up nearby capacitors, as in this photo.


Sometimes they get so hot that you can see signs of overheating on the board. This heat can also affect the power supply's stability if it warms the feedback-divider resistors and they are standard types rather than precision or temperature-stable ones.
As the resistors heat up, their values start to drift, changing the power supply's output voltage. So it's best to position the divider resistors where they won't be exposed to heat. It's also better to use precision resistors, which are much less affected by temperature.


Sometimes manufacturers choose the wrong value for the load resistor, causing it to run hotter than it should. For example, this was the case with the 24 V version of the "people's" power supply. I later had to replace it with a resistor twice the value.


For your power supplies to work reliably, choose the components carefully.
Choose diodes with a safety margin of 2x for a push-pull topology and 3x for a single-ended topology. For example, for a 5-7 A power supply, use a 15-20 A diode.
The diode's voltage rating should be at least four times the power supply's output voltage: for a 12 V power supply, use a 60 V diode; for 24 V, use a 100 V diode.
All these parameters are listed in the diode datasheet.


They are also marked on the diodes themselves.


Choose low-impedance or LowESR capacitors; this is usually stated in the component datasheet as well.
Choose the capacitance based on 0.5-1 thousand µF per 1 A of output current. For a push-pull topology, the voltage rating should be 1.5-2 times the output voltage; for a single-ended flyback topology, it should be at least 2x the output voltage.


As for manufacturers, look for well-known brands. I covered this in the article about the input filter, and the recommendations here are similar.






Choosing the output inductor is much simpler: its rated current should be at least as high as the power supply's maximum output current. It's better to use an inductor rated for a higher current, as it will run much cooler. Inductance should be 4.7-22 µH, depending on the output current. An inductor rated for a high current will be quite large.


Inductors are usually either drum-core types or shielded types; the latter are more often intended for surface mounting.



That's about it. And, of course, here's the video version of this article. As always, I'll be glad to hear your questions and suggestions.