Linear and switching power supplies
Let's start with the basics. The power supply in the computer performs three functions. First, alternating current from the household power supply must be converted to direct current. The second task of the PSU is to lower the voltage of 110-230 V, which is redundant for computer electronics, to the standard values required by the power converters for individual PC components - 12 V, 5 V and 3.3 V (as well as negative voltages, which we will talk about a little later) . Finally, the PSU plays the role of a voltage stabilizer.
There are two main types of power supplies that perform these functions - linear and switching. The simplest linear PSU is based on a transformer, on which the AC voltage is reduced to the required value, and then the current is rectified by a diode bridge.
However, the PSU is also required to stabilize the output voltage, which is due to both the instability of the voltage in the household network and the voltage drop in response to an increase in current in the load.
To compensate for the voltage drop, in a linear power supply, the transformer is dimensioned to provide excess power. Then, at a high current in the load, the required voltage will be observed. However, the overvoltage that will occur without any means of compensation at low current in the payload is also unacceptable. Excessive voltage is eliminated by including a non-useful load in the circuit. In the simplest case, this is a resistor or transistor connected via a Zener diode. In a more advanced one, the transistor is controlled by a microcircuit with a comparator. Be that as it may, excess power is simply dissipated in the form of heat, which negatively affects the efficiency of the device.
In the switching power supply circuit, another variable appears, on which the output voltage depends, in addition to the two already available: the input voltage and the load resistance. In series with the load there is a key (which in the case of interest to us is a transistor), controlled by a microcontroller in pulse-width modulation (PWM) mode. The higher the duration of the open states of the transistor in relation to their period (this parameter is called the duty cycle, in Russian terminology the inverse value is used - the duty cycle), the higher the output voltage. Due to the presence of a key, a switching power supply is also called Switched-Mode Power Supply (SMPS).
No current flows through a closed transistor, and the resistance of an open transistor is ideally negligible. In reality, an open transistor has resistance and dissipates some of the power in the form of heat. Also, the transition between transistor states is not perfectly discrete. And yet, the efficiency of a pulsed current source can exceed 90%, while the efficiency of a linear PSU with a stabilizer reaches 50% at best.
Another advantage of switching power supplies is a radical reduction in the size and weight of the transformer compared to linear power supplies of the same power. It is known that the higher the frequency of alternating current in the primary winding of the transformer, the smaller the required core size and the number of turns of the winding. Therefore, the key transistor in the circuit is placed not after, but before the transformer and, in addition to voltage stabilization, it is used to obtain high-frequency alternating current (for computer PSUs, this is from 30 to 100 kHz and higher, and as a rule - about 60 kHz). A transformer operating at a mains frequency of 50-60 Hz, for the power required by a standard computer, would be ten times more massive.
Linear PSUs today are used mainly in the case of low power devices, when the relatively complex electronics required for a switching power supply is a more sensitive cost item compared to a transformer. These are, for example, 9 V power supplies, which are used for guitar effects pedals, and once - for game consoles, etc. But chargers for smartphones are already completely pulsed - here the costs are justified. Due to the significantly lower amplitude of the voltage ripple at the output, linear power supplies are also used in areas where this quality is in demand.
⇡ The general scheme of the ATX standard power supply
A desktop computer power supply unit is a switching power supply, the input of which is supplied with household mains voltage with parameters of 110/230 V, 50-60 Hz, and at the output there are a number of DC lines, the main of which are rated 12, 5 and 3.3 V In addition, the PSU provides the -12V and, at one time, the -5V required for the ISA bus. But the latter was at some point excluded from the ATX standard due to the termination of support for ISA itself.
In the simplified diagram of a standard switching power supply presented above, four main stages can be distinguished. In the same order, we consider the components of power supplies in the reviews, namely:
- EMI filter - electromagnetic interference (RFI filter);
- primary circuit - input rectifier (rectifier), key transistors (switcher) that create high-frequency alternating current on the primary winding of the transformer;
- main transformer;
- secondary circuit - current rectifiers from the secondary winding of the transformer (rectifiers), smoothing filters at the output (filtering).
⇡ EMI filter
The filter at the PSU input serves to suppress two types of electromagnetic interference: differential (differential-mode) - when the interference current flows in different directions in the power lines, and common-mode (common-mode) - when the current flows in one direction.
Differential noise is suppressed by a CX capacitor (large yellow film capacitor in the photo above) connected in parallel with the load. Sometimes a choke is additionally hung on each wire, which performs the same function (not in the diagram).
The common mode filter is formed by CY capacitors (blue teardrop-shaped ceramic capacitors in the photo), at a common point connecting the power lines to ground, and the so-called. Common mode choke (common-mode choke, LF1 in the diagram), the current in the two windings of which flows in the same direction, which creates resistance to common mode noise.
In cheap models, a minimum set of filter parts is installed; in more expensive, the described schemes form repeating (in whole or in part) links. In the past, it was not uncommon to see PSUs without an EMI filter at all. Now this is rather a curious exception, although when buying a very cheap PSU, you can still run into such a surprise. As a result, not only and not so much the computer itself will suffer, but other equipment included in the household network - pulsed power supplies are a powerful source of interference.
In the area of \u200b\u200bthe filter of a good PSU, you can find several details that protect the device itself or its owner from damage. There is almost always a simple fuse for short circuit protection (F1 in the diagram). Note that when the fuse blows, the protected object is no longer the power supply. If a short circuit has occurred, then it means that the key transistors have already broken through, and it is important to at least prevent the ignition of the electrical wiring. If a fuse suddenly blows in the PSU, then it is most likely pointless to change it to a new one.
Separately, protection against short-term voltage surges using a varistor (MOV - Metal Oxide Varistor). But there are no means of protection against a prolonged increase in voltage in computer power supplies. This function is performed by external stabilizers with their own transformer inside.
The capacitor in the PFC circuit after the rectifier can retain a significant charge after being disconnected from the power supply. So that a careless person who puts his finger into the power connector is not shocked, a high-value discharge resistor (bleeder resistor) is installed between the wires. In a more sophisticated version - along with a control circuit that prevents the charge from leaking when the device is in operation.
By the way, the presence of a filter in the PC power supply (and it is also in the PSU of a monitor and almost any computer equipment) means that buying a separate “surge filter” instead of a conventional extension cord is, in general, useless. He has the same inside. The only condition in any case is normal three-pin wiring with grounding. Otherwise, the CY capacitors connected to ground will simply not be able to perform their function.
⇡ Input rectifier
After the filter, the alternating current is converted to direct current using a diode bridge - usually in the form of an assembly in a common housing. A separate radiator for cooling the bridge is strongly welcomed. A bridge assembled from four discrete diodes is an attribute of cheap power supplies. You can also ask what current the bridge is designed to determine if it matches the power of the PSU itself. Although this parameter, as a rule, there is a good margin.
⇡ Active PFC block
In an AC circuit with a linear load (such as an incandescent lamp or electric stove), the flowing current follows the same sinusoid as the voltage. But this is not the case with devices that have an input rectifier, such as switching power supplies. The power supply passes current in short pulses, approximately coinciding in time with the peaks of the voltage sine wave (i.e., the maximum instantaneous voltage), when the rectifier smoothing capacitor is recharged.
The distorted current signal is decomposed into several harmonic oscillations in total with a sinusoid of a given amplitude (an ideal signal that would occur with a linear load).
The power used to perform useful work (which, in fact, is the heating of PC components) is indicated in the characteristics of the PSU and is called active. The rest of the power generated by harmonic current oscillations is called reactive power. It does no useful work, but heats up wires and puts a strain on transformers and other power equipment.
The vector sum of reactive and active power is called apparent power. And the ratio of active power to full power is called the power factor (power factor) - not to be confused with efficiency!
A switching PSU has a rather low power factor initially - about 0.7. For a private consumer, reactive power is not a problem (fortunately, it is not taken into account by electricity meters), unless he uses a UPS. The uninterruptible power supply just bears the full power of the load. On the scale of an office or a city network, the excess reactive power generated by switching power supplies already significantly reduces the quality of power supply and causes costs, so it is being actively combated.
In particular, the vast majority of computer PSUs are equipped with active power factor correction (Active PFC) circuits. The unit with active PFC is easily identified by the single large capacitor and inductor installed after the rectifier. In essence, Active PFC is another switching converter that maintains a constant charge of about 400 V on the capacitor. In this case, the current from the mains is consumed by short pulses, the width of which is chosen so that the signal is approximated by a sinusoid - which is required to simulate a linear load . To synchronize the current demand signal with the voltage sine wave, the PFC controller has special logic.
The active PFC circuit contains one or two key transistors and a powerful diode, which are placed on the same radiator with the key transistors of the main power supply converter. As a rule, the PWM controller of the main converter key and the Active PFC key are one chip (PWM/PFC Combo).
The power factor of switching power supplies with active PFC reaches 0.95 and higher. In addition, they have one additional advantage - they do not require a 110/230 V mains switch and a corresponding voltage doubler inside the PSU. Most PFC circuits digest voltages from 85 to 265 V. In addition, the PSU's sensitivity to short-term voltage dips is reduced.
By the way, in addition to active PFC correction, there is also a passive one, which involves installing a high inductance inductor in series with the load. Its effectiveness is low, and you are unlikely to find this in a modern PSU.
⇡ Main transducer
The general principle of operation for all pulsed power supplies of an isolated topology (with a transformer) is the same: the key transistor (or transistors) creates an alternating current on the primary winding of the transformer, and the PWM controller controls the duty cycle of their switching. Specific circuits, however, differ both in the number of key transistors and other elements, and in qualitative characteristics: efficiency, signal shape, interference, etc. But here too much depends on the specific implementation to be worth focusing on. For those interested, we present a set of diagrams and a table that will allow them to be identified in specific devices by the composition of parts.
| transistors | Diodes | Capacitors | Legs of the primary winding of the transformer | |
| Single Transistor Forward | 1 | 1 | 1 | 4 |
| 2 | 2 | 0 | 2 | |
| 2 | 0 | 2 | 2 | |
| 4 | 0 | 0 | 2 | |
| 2 | 0 | 0 | 3 |
In addition to the above topologies, in expensive PSUs there are resonant (resonant) versions of Half Bridge, which are easy to identify by an additional large inductor (or two) and a capacitor forming an oscillatory circuit.
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Single Transistor Forward |
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⇡ Secondary circuit
The secondary circuit is everything that is after the secondary winding of the transformer. In most modern power supplies, the transformer has two windings: 12 V is removed from one of them, and 5 V is removed from the other. The current is first rectified using an assembly of two Schottky diodes - one or more per bus (on the most heavily loaded bus - 12 V - there are four assemblies in powerful power supplies). More efficient in terms of efficiency are synchronous rectifiers, which use field-effect transistors instead of diodes. But this is the prerogative of truly advanced and expensive PSUs that claim the 80 PLUS Platinum certificate.
The 3.3V rail is typically derived from the same winding as the 5V rail, only the voltage is stepped down with a saturable choke (Mag Amp). A special winding on a 3.3 V transformer is an exotic option. Of the negative voltages in the current ATX standard, only -12 V remains, which is removed from the secondary winding under the 12 V bus through separate low-current diodes.
PWM key control of the converter changes the voltage on the primary winding of the transformer, and therefore on all the secondary windings at once. At the same time, the current consumption by the computer is by no means evenly distributed between the PSU buses. In modern hardware, the most loaded bus is 12-V.
Additional measures are required for separate voltage stabilization on different buses. The classic method involves the use of a group stabilization choke. Three main tires are passed through its windings, and as a result, if the current increases on one bus, then the voltage drops on the others. Suppose the current increased on the 12 V bus, and in order to prevent a voltage drop, the PWM controller reduced the duty cycle of the key transistors. As a result, the voltage on the 5 V bus could go beyond the permissible limits, but was suppressed by the group stabilization inductor.
The 3.3V rail voltage is additionally regulated by another saturable choke.
In a more advanced version, separate stabilization of the 5 and 12 V buses is provided due to saturable chokes, but now this design in expensive high-quality PSUs has given way to DC-DC converters. In the latter case, the transformer has a single secondary winding with a voltage of 12 V, and the voltages of 5 V and 3.3 V are obtained thanks to DC converters. This method is most favorable for voltage stability.
Output filter
The final stage on each bus is a filter that smooths out the voltage ripple caused by the key transistors. In addition, pulsations of the input rectifier, whose frequency is equal to twice the frequency of the mains, break through to the secondary circuit of the PSU to one degree or another.
The ripple filter includes a choke and large capacitors. High-quality power supplies are characterized by a capacitance of at least 2,000 microfarads, but manufacturers of cheap models have a reserve for savings when they install capacitors, for example, of half the value, which inevitably affects the ripple amplitude.
⇡ Standby power supply +5VSB
A description of the components of the power supply would be incomplete without mentioning the standby voltage of 5 V, which makes it possible to sleep the PC and ensures the operation of all devices that must be turned on all the time. "Duty room" is powered by a separate pulse converter with a low-power transformer. In some power supplies, there is also a third transformer used in the feedback circuit to isolate the PWM controller from the primary circuit of the main converter. In other cases, this function is performed by optocouplers (LED and phototransistor in one package).
⇡ Power supply testing methodology
One of the main parameters of the PSU is voltage stability, which is reflected in the so-called. cross-load characteristic. KNKH is a diagram in which the current or power on the 12 V bus is plotted on one axis, and the total current or power on the 3.3 and 5 V buses is plotted on the other. At the intersection points, for different values of both variables, the voltage deviation from the nominal by one tire or another. Accordingly, we publish two different KNX - for the 12 V bus and for the 5 / 3.3 V bus.
The color of the dot means the deviation percentage:
- green: ≤ 1%;
- light green: ≤ 2%;
- yellow: ≤ 3%;
- orange: ≤ 4%;
- red: ≤ 5%.
- white: > 5% (not allowed by the ATX standard).
To obtain CNC, a custom-made power supply test bench is used, which creates a load due to heat dissipation on powerful field-effect transistors.
Another equally important test is to determine the range of ripples at the PSU output. The ATX standard allows ripples within 120 mV for a 12 V bus and 50 mV for a 5 V bus. There are high-frequency ripples (at twice the frequency of the main converter key) and low-frequency ripples (at twice the mains frequency).
We measure this parameter using the Hantek DSO-6022BE USB oscilloscope at the maximum load on the power supply unit specified by the specifications. In the oscillogram below, the green graph corresponds to a 12 V bus, yellow - 5 V. It can be seen that the ripples are within normal limits, and even with a margin.
For comparison, we present a picture of ripples at the output of the PSU of an old computer. This block wasn't great initially, but clearly hasn't gotten any better over time. Judging by the range of low-frequency ripples (note that the voltage base division is increased to 50 mV to fit the oscillations on the screen), the smoothing capacitor at the input has already become unusable. High-frequency ripple on the 5 V bus is on the verge of an acceptable 50 mV.
The following test determines the efficiency of the unit at a load of 10 to 100% of the rated power (by comparing the output power with the input power measured with a household wattmeter). For comparison, the graph shows the criteria for different categories of 80 PLUS. However, it does not arouse much interest these days. The graph shows the results of the top Corsair PSU in comparison with the very cheap Antec, and the difference is not that very big.
A more pressing issue for the user is the noise from the built-in fan. It is impossible to directly measure it near the roaring power supply test stand, so we measure the speed of rotation of the impeller with a laser tachometer - also at power from 10 to 100%. You can see in the graph below that at low load on this PSU, the 135mm fan keeps low RPMs and is hardly audible at all. At maximum load, the noise can already be distinguished, but the level is still quite acceptable.
Switching power supply or linear. Background
It's probably not a secret for anyone that most specialists, radio amateurs and simply technically literate buyers of power supplies are wary of switching power supplies, preferring linear ones.
The reason is simple and clear. The reputation of switching power supplies was seriously undermined back in the 80s, during the time of massive failures of domestic color TVs, low-quality imported video equipment equipped with the first switching power supplies.
What do we have today? Almost all modern TVs, video equipment, household appliances, computers use impulseblocksnutrition. There are fewer and fewer areas of application of linear (analogue, parametric) sources. You will hardly find a linear power supply in household equipment today. But the stereotype remained. And this is not conservatism, despite the rapid progress of electronics, overcoming stereotypes is very slow.
Let's try to objectively look at the current situation and try to change the opinion of experts. Consider the "stereotypical" and inherent switching power supplies disadvantages: complexity, unreliability, interference.
Impulse power block. Stereotype "difficulty"
Yes, switching power supplies complex, more precisely more difficult than analog, but much simpler than a computer or TV. You do not need to understand their circuitry, as well as the circuitry of a color TV. Leave it to the professionals. For professionals there is nothing complicated.
Impulse power block. Stereotype "insecure"
The element base of the switching power supply does not stand still. Modern equipment used in switching power supplies today allows us to say with confidence: unreliability is a myth. Basically, the reliability of a switching power supply, like any other equipment, depends on the quality of the element base used. The more expensive the switching power supply, the more expensive the element base in it. High integration allows you to implement a large number of built-in protections that are sometimes not available in linear sources.
Impulse power block. Stereotype "interference"
And what are the advantages of a switching power supply?
Impulse power block. High efficiency
The high efficiency (up to 98%) of the switching power supply is associated with a feature of the circuitry. The main losses in the analog source are the network transformer and the analog stabilizer (regulator). In a switching power supply, there is neither one nor the other. Instead of a network transformer, a high-frequency one is used, and instead of a stabilizer, a key element is used. Since the key elements are either on or off most of the time, energy loss in the switching power supply is minimal. The efficiency of an analog source can be about 50%, that is, half of its energy (and your money) is spent on heating the surrounding air, in other words, they fly away into the wind.
Impulse power block. light weight
The switching power supply has less weight due to the fact that with increasing frequency, smaller transformers can be used for the same transmitted power. The mass of a switching power supply is several times less than an analog one.
Impulse power block. lower cost
Demand creates supply. Thanks to the mass production of a unified element base and the development of key high-power transistors, today we have low prices for the power base of switching power supplies. The higher the output power, the cheaper the source is compared to the cost of a similar linear source. In addition, the main components of an analog source (copper, transformer iron, aluminum heatsinks) are constantly becoming more expensive.
Impulse power block. Reliability
You heard right, reliability. At the moment, switching power supplies are more reliable than linear ones due to the presence in modern power supplies of built-in protection circuits from various unforeseen situations, for example, from short circuits, overloads, power surges, reverse polarity of output circuits. High efficiency causes less heat loss, which in turn causes less overheating of the element base of the switching power supply, which is also an indicator of reliability.
Impulse power block. Mains voltage requirements
What is going on in the domestic power grids, you probably know firsthand. 220 volts in the outlet is more of a rarity than the norm. And switching power supplies allow the widest range of supply voltage, unattainable for linear. The typical lower threshold of the mains voltage for a switching power supply is 90 ... 110 V, any analog source at this voltage will “ripple” at best or simply turn off.
So, pulse or linear? In any case, the choice is yours, we just wanted to help you take an objective look at switching power supplies and make the right choice. Just do not forget that a high-quality source is a source made professionally, based on high-quality components. And quality is always a price. Free cheese only in a mousetrap. However, the last phrase applies equally to any source, both pulsed and analog.
Secondary power sources are an integral part of the design of any electronic device. They are designed to convert alternating or direct voltage from the mains or battery into direct or alternating voltage required for the operation of the device, these are power supplies.
Kinds
Power supplies are not only included in the circuit of any device, but can also be performed as a separate unit and even occupy entire power supply workshops.
There are several requirements for power supplies. Among them: high efficiency, high quality of the output voltage, the presence of protection, network compatibility, small size and weight, etc.
Among the tasks of the power supply can be listed:
- Transmission of electrical power with a minimum of losses;
- Transformation of one type of voltage into another;
- Formation of a frequency different from the source current frequency;
- Change in voltage value;
- Stabilization. The power supply must output a stable current and voltage. These parameters must not exceed or fall below a certain limit;
- Protection against short circuit and other faults in the power supply, which can lead to damage to the device that provides the power supply;
- Galvanic isolation. Method of protection against the flow of leveling and other currents. Such currents can damage equipment and injure people.
But often, power supplies in household appliances have only two tasks - to convert alternating electrical voltage to direct current and to convert the frequency of the mains current.
There are two types of power supplies that are most common. They differ in design. These are linear (transformer) and switching power supplies.
Linear power supplies
Initially, power supplies were made only in this form. The voltage in them is converted by a power transformer. lowers the amplitude of the sinusoidal harmonic, which is then rectified by a diode bridge (there are circuits with a single diode). convert the current to pulsating. And then the pulsating current is smoothed out using a filter on the capacitor. At the end, the current is stabilized with .
Just to understand what's happening, imagine a sine wave - this is what the shape of the voltage entering our power supply looks like. The transformer, as it were, flattens this sinusoid. The diode bridge horizontally cuts it in half and flips the lower part of the sinusoid up. Already a constant, but still pulsating voltage is obtained. The capacitor filter finishes the job and “presses” this sinusoid to such an extent that it turns out to be an almost straight line, and this is direct current. Approximately so, perhaps too simply and roughly, you can describe the operation of a linear power supply.
Pros and cons of linear power supplies
The advantages include the simplicity of the device, its reliability and the absence of high-frequency interference, unlike pulse analogues.
The disadvantages include a large weight and size, increasing in proportion to the power of the device. Also, triodes that go at the end of the circuit and stabilize the voltage reduce the efficiency of the device. The more stable the voltage, the greater its losses will be at the output.
Switching power supplies
Switching power supplies of this design appeared in the 60s of the last century. They work on the principle of an inverter. That is, they not only convert a constant voltage into an alternating one, but also change its value. The voltage from the mains entering the device is rectified by the input rectifier. Then the amplitude is smoothed by the input capacitors. High-frequency rectangular pulses with a certain repetition and pulse duration are obtained.
The further path of the pulses depends on the design of the power supply:
- In units with galvanic isolation, the pulse enters the transformer.
- In a PSU without decoupling, the pulse goes directly to the output filter, which cuts off the low frequencies.
Pulse PSU with galvanic isolation
High-frequency pulses from the capacitors enter the transformer, which separates one electrical circuit from another. This is the point. Due to the high frequency of the signal, the efficiency of the transformer is increased. This allows you to reduce the weight of the transformer and its dimensions in pulse power supplies, and, consequently, the entire device. Ferromagnetic compounds are used as the core. This also reduces the size of the device.
This type of design involves the conversion of current in three stages:
- Pulse width modulator;
- Transistor cascade;
- Pulse transformer.
What is a pulse width modulator
In another way, this converter is called a PWM controller. Its task is to change the time during which a rectangular pulse will be applied. changes the time the pulse stays on. It changes the time at which the impulse is not given. But the feed frequency remains the same.
How voltage stabilizes in switching power supplies
In all switching power supplies, a type of feedback is implemented, in which, with the help of a part of the output voltage, the influence of the input voltage on the system is compensated. This allows you to stabilize random input and output voltage changes.
In systems with galvanic isolation, they are used to create negative feedback. In a PSU without decoupling, feedback is implemented by a voltage divider.
Pros and cons of switching power supplies
Of the pluses, one can single out a smaller mass and dimensions. High efficiency, by reducing losses associated with transition processes in electrical circuits. Lower price compared to linear PSUs. The ability to use the same PSU in different countries of the world, where the parameters of the power grid differ from each other. Presence of protection against short circuit.
The disadvantages of switching power supplies are their inability to work at too high or too low loads. Not suitable for certain types of precision devices, because they create radio interference.
Application
Linear power supplies are being actively replaced by their switching counterparts. Now linear power supplies can be found in washing machines, microwave ovens, and heating systems.
Switching power supplies are used almost everywhere: in computer technology and televisions, in medical equipment, in most household appliances, in office equipment.
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If you are looking for a simple and reliable linear power supply circuit, then this article is for you. Here you will find complete assembly instructions, as well as setting up this power supply. The author of this homemade product is Roman (YouTube channel "Open Frime TV").
To start, a little background. More recently, the author was redoing his workplace and wanted to install a linear unit as the third power supply, since sometimes he has to assemble circuits that cannot tolerate voltage ripples. And as we know, at the output of a linear block, the voltage ripple is almost completely absent.
Up to this point, the author was not very interested in linear blocks, and somehow he didn’t really delve into this topic. When the idea to build such a block came up, Roman immediately opened the beloved and widely known YouTube video hosting. As a result, after a long search, the author was able to identify 2 schemes for himself. The author of the first one is AKA KASYAN (the author of the YouTube channel of the same name), and the second circuit is based on opamps.
But since opamps can operate at voltages up to 32V, the output voltage, respectively, could not exceed this limit, which means this circuit is no longer needed.
Okay, you can assemble a circuit from Kasyan, but even here we were disappointed. This scheme is afraid of statics. This was manifested by the explosion of transistors if you take on the output contacts.
This happened several times. And then the author decided to leave this scheme alone. You will say that the Internet is full of linear power supply circuits.
Yes, of course it is, but only these two schemes mentioned above had normally set seals, which could be simply downloaded. Everything else, either without seals, or assembled by hanging. And we (radio amateurs) are used to the fact that everything is served on a silver platter.
The author decided to breed a normal signet. The board turned out to be quite compact. After testing this scheme, surprisingly, it proved to be excellent.
With such simplicity, the author liked it so much that he even decided to make a kit-set from this board. To do this, it is necessary to convert the signet into a Gerber file (a file with the .gbr extension, which is a printed circuit board design for subsequent production of photomasks on various equipment). Then you need to send the boards for manufacturing.
And now, a couple of weeks after the order, we receive our long-awaited boards. Having opened the package and examined the boards closer, we can make sure that everything turned out very high quality and beautifully.
So, let's already solder this board and check it in operation. There are not so many components for installation, it takes about 20 minutes to solder, no more.
Done with soldering. We make the first inclusion. And here we are in for a little disappointment. This board was not without jambs. They manifested themselves in the fact that when the potentiometer knob is rotated to the left, the voltage and current increase, and when the potentiometer knob is rotated to the right, a decrease occurs.
This happened because the author put the resistors for this board on the wires (for subsequent installation on the case) and there it was possible to change the direction of rotation without any problems simply by changing the side contacts. Okay, but everything else works as expected.
But still, the author corrected the signet, now there is an increase in voltage when the potentiometer is rotated to the right, everything is as it should be. So you can safely download and repeat this design (the archive with this printed circuit board is in the description under the author's original video, you must follow the SOURCE link at the end of the article).
And now let's move on to a detailed examination of the circuit and the board itself. You can see the scheme on your screens.
This power supply is equipped with a voltage and current regulator, as well as a short circuit protection system, which is simply necessary in such blocks.
Imagine for a moment what happens during a short circuit when the input voltage is 36V. It turns out that all the voltage is dissipated on the power transistor, which, of course, is unlikely to withstand such mockery.
Protection can be configured here. With the help of this trimming resistor, we set any trip current.
A 12V protection relay is installed here, and the input voltage can reach 40V. Therefore, it was necessary to obtain a voltage of 12V.
This can be implemented using a parametric stabilizer on a transistor and a zener diode. A 13V zener diode, as there is a voltage drop across the collector-emitter junctions of two transistors.
So, now you can start testing this linear power supply. We supply a voltage of 40V from a laboratory power supply. On the load we hang a light bulb designed for a voltage of 36V, with a power of 100W.
Then we begin to slowly rotate the variable resistor.
As you can see, the voltage regulation works fine. Now let's try to regulate the current.
As you can see, when the second resistor rotates, the current decreases, which means that the circuit is operating normally.
Since this is a linear unit and all the "extra" voltage is converted into heat, it needs a rather large radiator. For these purposes, heatsinks from a computer processor have proven to be excellent. Such radiators have a large dissipation area, and if they are also equipped with a fan, then you can, in principle, completely forget about overheating of the transistor.
Which power supply to choose: switching or linear?
Of course, the choice is yours, but we want to share interesting and useful information with you!
Most technical specialists and buyers with experience will be wary of switching power supplies, back in the 80s, their reputation was seriously undermined, it started from mass failures of work, domestic color TVs and imported video equipment equipped with a switching power supply.
And what do we end up with? Almost all household appliances, video equipment, televisions, computers are equipped with switching power supplies and that's it, you can see the use of linear power supplies less. Let's define the advantages, reliability, disadvantages of switching power supplies.
What is supposedly the complexity of switching power supplies? The fact that they are more complicated than analogues, but simpler than a computer and a TV. And of course, I think you don’t need to understand this, let the professionals do it.
Let's determine the reliability of impulse blocks? Constant modernization of the element base of the switching power supply and modern equipment is not reliable. And it would be more correct to say that the reliability of a switching power supply lies and depends on the correct use of the element base. Integration allows you to implement a large number of built-in protections that are not available in linear sources.
Switching power supplies, of course, are more reliable than linear ones due to the presence in modern power supplies of built-in protection circuits from various situations, for example, from overload, power surges, short circuits, reverse polarity of output circuits. And high efficiency guarantees less heat loss, which in turn gives less overheating of the element base of the switching power supply, which is an indicator of reliability.
Efficiency of a switching power supply. Efficiency is a coefficient of efficiency, the designation of this parameter determines how efficiently the power supply can convert energy for components. The measurement is in percentage, and the higher it is to 100%, the higher the efficiency. The efficiency in switching power supplies is high up to 98%. In an analog power supply, the main losses are a transformer and an analog stabilizer, which is not in switching power supplies, a high-frequency transformer is used instead of a mains transformer, and a key element is used instead of a stabilizer. And as long as most of the time the key elements are on or off, then the energy loss in the pulse unit is minimal. The efficiency of an analog power supply of about 50% is simply spent on heating the surrounding air, in general, you lose them.
The mass of the switching power supply is much less than the analog one. And the small weight of the switching power supply due to the fact that with increasing frequency, smaller transformers can be used with the same transmitted power.
And thanks to the mass production of a unified element base and the development of key high-power transistors, today we have low prices for the power base of switching power supplies. The higher the output power, the cheaper the switching power supply is compared to a similar linear power supply.
What are the mains voltage requirements for switching power supplies? For us, as you know, 220 volts in the outlet, this is most likely a rarity than the norm. In switching power supplies, a large range of supply voltage is allowed, which cannot be said about linear blocks.
And so, what to stop at when choosing a power supply? I think you will make the right choice and we hope that the article was useful and interesting for you. Trust professionals and choose a high-quality power supply, which is made on the basis of high-quality components!
You can get acquainted with the power supplies of well-known manufacturers