Switching power supply - a selection of circuits for self-production

In everyday life, a powerful power source with a fixed voltage is often needed. It can be used as a charger, to power audio equipment (amplifiers), etc. It is advisable to make such power supplies using a pulse circuit. This circuitry allows you to create lightweight and powerful DC voltage sources. The complexity of the circuit begins to fade into the background compared to its advantages already at load currents of more than 2A. You can make a switching power supply with your own hands if you have the equipment and certain qualifications.

Types and principle of operation of switching power supplies

The basic operating principle of a switching power supply (SMPS) is that direct voltage (rectified mains voltage or from a third-party source) is converted into pulsed voltage with a frequency of up to hundreds of kilohertz. Due to this, the winding parts (transformers, chokes) are light and compact.

Fundamentally, IIPs are divided into two categories:

• with pulse transformer;
• with storage inductance (it can also have secondary windings)

The former are similar to conventional transformer mains power supplies; their output voltage is regulated by changing the average current through the transformer winding. The latter work on a different principle - they are regulated by changing the amount of accumulated energy.

Based on other characteristics, IIP can be divided into unstabilized and stabilized, unipolar and bipolar, etc. These features are not of such a fundamental nature.

⇡#“Gaming PCs do not need 1 kW units” - commentators under articles on 3DNews.ru

We often see comments like this when it comes to gaming PCs. In the vast majority of cases - and we have found this out in practice - this is so. However, in 2022 there is a system that can amaze with its energy consumption.

We are, of course, talking about an extreme build in its, so to speak, maximum combat form. Not long ago, an article was published on our website “What the fastest gaming PC of 2022 can do. Testing a system with two GeForce RTX 2080 Ti in 8K resolution” - in it we talked in detail about the performance of a pair of the fastest GeForce video cards in 4K and 8K resolution. The system is fast, but the components are selected in such a way that it is very easy to make it even faster. In addition, it turned out that overclocking the Core i9-9900K to 5.2 GHz is completely useful in the case of the GeForce RTX 2080 Ti SLI array and Ultra HD games. Only at its peak, as we see, such an overclocked configuration consumes more than 800 W. Therefore, for such a system under such conditions, a kilowatt power supply will definitely not be superfluous.

Structural and schematic diagram of the main parts of the block

Generalized block diagram of a pulse power supply.

A surge filter is installed at the input of the power supply. In principle, it does not affect the operation of a homemade or industrial switching power supply - everything will function without it. But you cannot abandon the filtering circuit - due to the extremely nonlinear shape of the consumed current, pulsed sources intensively “sprinkle” interference into a 220-volt household network. For this reason, devices running on the same network on microprocessors and microcontrollers - from electronic watches to computers - will malfunction.

Network filter circuit.

The purpose of the input device is to protect against two types of interference:

• common-mode (asymmetrical) – occurs between any wire and the ground (case) of the power supply unit;
• differential (symmetrical) – between the power wires (poles).

The filter, like the entire power supply, is protected at the input by fuse F (fuseable or self-recovering). After the fuse there is a varistor - a resistor whose resistance depends on the applied voltage. As long as the input voltage is normal, the resistance of the varistor is high and it has no effect on the operation of the circuit. If the voltage increases, the resistance of the varistor drops sharply, which causes an increase in current and the fuse burns.

Capacitors Cx block differential noise at the input and output of the filter in the range up to 30 MHz. At a frequency of 50 Hz their resistance is high, so they do not affect the mains voltage. Their capacitance can be selected from 10 to 330 nF. Resistor Rd is installed for safety - the capacitors are discharged through it after the power is turned off.

Common-mode interference is suppressed by a filter on Cy and L. Their values ​​for the cutoff frequency f are related by the Thompson formula:

f=1/(2*π*√L*C) , where:

• f – cutoff frequency in kHz (the conversion frequency of the pulse generator is taken);
• L – inductance of the inductor, μH;
• C – capacitance Cy, µF.

The common mode choke is wound on a ferrite ring. The windings are identical, winding on opposite sides.

Common mode choke design.

Unlike the output filter, the rated current of the power supply unit does not affect the calculation of the noise protection filter elements, with the exception of the wire with which the inductor is wound.

After the filter, the mains voltage is rectified. In most cases, a standard full-wave bridge rectifier is used.

⇡#Conclusions

If you carefully read the article, you have identified several main points that you need to keep in mind when choosing a power supply. Let's list them all again:

• Unfortunately, it is impossible to rely on the TDP indicators declared by the manufacturer of the video card or processor;
• the energy consumption of computer equipment does not change much from year to year and is within certain limits - therefore, a high-quality power supply purchased now will serve for a long time and will definitely come in handy during the assembly of the next system;
• the needs for cable management of the system unit also influence the choice of a power supply of a certain power;
• Not all power connectors on the motherboard need to be used;
• a power supply with a lower power is not always more profitable (in terms of price) than a more powerful model;
• when choosing a power supply, you need to look, among other things, at how many watts the device produces via the 12-volt line;
• support for a certain 80 PLUS standard indirectly indicates the quality of the element base of the power supply;
• There is absolutely no shame in using a power supply whose honest power is twice (or even more) the maximum power consumption of the computer.

Quite often you can hear the phrase: “More is not less.” This very laconic aphorism perfectly describes the situation when choosing a power supply. For your new PC, take a model with a good power reserve - it definitely won’t be worse, and in most cases it will only be better. Even for an inexpensive gaming system unit, which consumes about 220-250 W at maximum load, it still makes sense to take a good model with an honest 600-650 W. Because this block:

• it will work more quietly, and in the case of some models - absolutely silently;
• it will be colder;
• will be more effective;
• will allow you to easily overclock the system, increasing the performance of the central processor, video card and RAM;
• will allow you to easily upgrade the main components of the system;
• will survive several upgrades, and also (if the power supply is really good) will live in the second or third system unit;
• It will also allow you to save money during the subsequent assembly of the system unit.

I think few readers will refuse a good power supply. It is clear that it is not always possible to immediately buy a high-quality device with a large reserve for the future. Sometimes, when buying a new system unit and having a limited budget, you want to get a more powerful processor, faster video cards, and a higher-capacity SSD - all this is understandable. But if you have the opportunity to buy a good power supply with a reserve, there is no need to save on it.

We would like to express our gratitude to the companies ASUS and Corsair, as well as the Regard computer store for the equipment provided for testing.

Inverter circuits

The resulting rectified voltage is supplied to the converter (inverter). It is performed on bipolar or field-effect transistors, as well as on IGBT elements that combine the properties of field-effect and bipolar ones. In recent years, high-power, low-cost insulated gate field-effect transistors (MOSFETs) have become widespread. It is convenient to build key inverter circuits on such elements. Switching power supply circuits use various options for connecting MOSFETs, but push-pull circuits are mainly used due to their simplicity and the ability to increase power without significant modifications.

Push-pull scheme

Push-pull converter circuit.

A push-pull inverter (push - push, pull - pull) is an example of a push-pull converter. Transistor switches operate on the primary winding of the transformer, consisting of two half-windings I and II. The transistors open alternately for a given period of time. When the top transistor in the circuit is open, current flows through half-winding I (red arrow), when the second one flows through half-winding II (green). To avoid a situation where both switches are open (due to the finite speed of the transistors), the control circuit creates a pause called Dead time.

Transistor control taking into account Dead time.

This circuit works well at low supply voltage (up to +12 volts). The downside is the presence of surges with an amplitude equal to twice the supply voltage. This entails the use of transistors designed for twice the voltage.

Bridge circuit

The push-pull bridge is free from the main drawback of the previous scheme.

Push-pull bridge inverter circuit.

Here, a pair of transistors T1 and T4 , then T2 and T3 (the key control signal is generated taking into account Dead time). In this case, the primary winding is connected to the power source first on one side, then on the other. The pulse amplitude is equal to the full supply voltage, and there are no voltage surges. The disadvantages include the use of four transistors instead of two. In addition to increasing the size of the power supply, this leads to doubled voltage losses.

Half bridge circuit

In practice, a half-bridge inverter circuit is often used - to a certain extent, a compromise between the previous two circuits.

Half-bridge circuit.

In this case, one side of the winding is switched by alternately opening transistors T1 and T2, and the other is connected to the midpoint of the capacitive divider C1, C2. Advantages of the scheme:

• unlike push-pull, there are no voltage surges;
• unlike bridge, only two transistors are used.

On the other side of the scale, the transformer winding is powered by only half the supply voltage.

Single-ended circuits

In the circuitry of converters, single-cycle circuits are also used - forward and flyback. Their fundamental difference from push-pull ones is that the transformer (more precisely, its primary winding) simultaneously serves as a storage inductance. In flyback circuits, energy is accumulated in the primary winding during the open state of the transistor, and is transferred to the load through the secondary winding during the closed state. In direct thrusters, energy accumulation and release to the consumer occur simultaneously.

Two phases of operation of a flyback single-cycle inverter.

Power transformer

The power transformer operates at high frequencies (up to several tens of kilohertz), so it can be made on a core made of ferrite rather than transformer iron. Also, due to the increased frequency, its dimensions will be smaller than that of a network one, designed for conversion at a frequency of 50 Hz. The calculation of a pulse transformer is quite extensive. You can understand it for general development, but for practical purposes it is better to use some program, including online services.

Interface of the Lite-CalcIT program.

The Lite-CalcIT program is popular. She can calculate a transformer for an existing core, or she can select the optimal one based on the entered data.

Snubber

To compensate for current and voltage surges that inevitably arise when switching the primary winding of a transformer, damper circuits are used, called snubbers in English technical literature. Such circuits can be installed along the power supply (parallel to the primary winding of the transformer) or separately on each switch. The design of snubbers can be different, but the most widespread are dampers in the form of a sequential RC chain (diagram b in the figure).

Various damper schemes.

There is no substantiated method for calculating a snubber. To do this, it is necessary to take into account all parasitic inductances (windings, tracks, capacitors) at many frequencies and for unknown wave impedances. Therefore, all existing calculations are empirical in nature.

The main (and only) active element of the damper is the capacitor. It “absorbs” impulse emissions. The resistor only worsens the damping properties, but limits the current through the capacitor, which can reach significant values, albeit for a short time. This scheme is more relevant in thyristor converters.

You can find out what a snubber or damper is by watching the video.

The RCD snubber circuits (c and d in the figure) contain diodes. They can be useful for limiting reverse polarity pulses in thyristor and bipolar transistor circuits. If the switches are assembled on field-effect or IGBT transistors, then there is no point in installing valves - they duplicate the diodes present inside these transistors.

The capacitance of the capacitor is selected in the range of 0.1–0.33 μF. In 90+ percent of cases this is enough. An increase or decrease in value is used for keys operating under non-standard conditions (increased conversion frequency, etc.)

Auxiliary nodes

The design can include auxiliary components, for example, indicators or voltage switches. The main thing is not to overdo it and make the device in accordance with all standards and recommendations.

Indicator LEDs

The design can include LED indicators, which are used in factory units and chargers. LEDs serve as an indicator that useful operation of the transformer is being performed and the voltage corresponds to the required value.

Ammeter and voltmeter

To make calculations and select elements, as well as to correctly assemble the power supply, you must use an ammeter and a voltmeter.

Rectifier

The voltage of the secondary winding must be rectified. For levels up to 12 volts, it is advisable to use a full-wave, midpoint circuit.

Diagram of a rectifier with a midpoint and the passage of current through it.

The advantage of this circuit is that the current flows in each direction through only one diode, and the voltage drop across the valves, in contrast to the classic bridge circuit, is half as much. This can significantly reduce the required number of turns of the secondary winding. The same purpose is served by the use of Schottky diodes and assemblies of them.

Bridge rectifier circuit and current flow through it.

If the output voltage of the power supply is higher than +12 volts, then saving 0.6 volts becomes insignificant, and you can make a rectifier according to the standard circuit and use a transformer without a tap.

If the output of a switching power supply must be bipolar, tapping from the midpoint again becomes rational. In this case, 4 diodes and radiators for them are saved at once - the gain in size can be significant.

Bipolar rectifier with midpoint.

How much current does a screwdriver consume?

Before choosing a suitable power supply, you need to understand what current consumption you need to count on. Unfortunately, cordless screwdriver manufacturers do not indicate the current consumed by the motor. The capacity of the battery itself in ampere-hours, which is necessarily indicated on the battery, does not allow us to understand how much current the screwdriver consumes in operating mode

. The maximum that the manufacturer can indicate is the power in watts, but this is very rare, usually the power is indicated directly in torque.

If the power in watts is still indicated, we can have an idea of ​​the current consumption and select an appropriate power supply with a small current/power margin. To calculate the current, it is enough to divide the power in watts by the operating voltage of the screwdriver, in this case it is 12 volts. So, if the manufacturer indicated a power of, for example, 200 watts - 200:12 = 16.6 A - this is the current consumed by the screwdriver in operating mode.

However, the indicated power is very rare and there is no universal figure that characterizes all 12-volt screwdrivers. You need to understand that when the motor shaft is fully braked, the currents can significantly exceed the rated ones and calculating this value is not very easy. At the same time, an analysis of various forums and our own experience has shown that a current of 10 A is often sufficient to operate a screwdriver; this is enough to perform many screwing and drilling functions. It is known that current surges during complete braking of the shaft can exceed 30 A.

Well, what conclusion can be drawn from all this? A 12 V power supply providing 10 A current is suitable for a screwdriver; if it is possible to use a 20-30 A unit, this is even better. These are average figures that apply to most screwdrivers.

Filter

The output voltage must be filtered - it contains a large number of conversion products. Since the inverter operates at a fairly high frequency, filters containing not only capacitors, but also small-sized chokes with relatively low inductance become effective.

L- and U-shaped LC filters.

To calculate the filter elements, it is necessary to specify the pulsation coefficient Kp. It is selected from the expected load:

• sensitive equipment for radio reception, preliminary stages of audio equipment, microphone amplifiers - Kp = 10-5..10-4 ;
• audio frequency amplifiers – Кп=10-4..10-3 ;
• middle and low class receiving and sound reproducing equipment – ​​Kp=10-2..10-3 .

For an L-shaped filter installed after a full-wave rectifier, the following relations apply:

• L*C=25000/(f2+Kp);
• L/C=1000/R2н.

In these formulas:

• L – inductance of the inductor in µH;
• C is the capacitance of the capacitor in microfarads;
• f – conversion frequency in Hz;
• Rн – load resistance in Ohms.

For U-shaped filter:

• С1=С2=С;
• L/C=1176/R2н.

The dimension of the values ​​is the same as for the previous filter.

Circuits and manufacturing of switching power supplies

Switching power supplies are assembled on various element bases. Typically, specialized microcircuits specially designed for creating such devices are used to build SMPS. Except for the simplest blocks.

Powerful pulse unit on ir2153

Simple power supplies can be built on the IR2153 chip. It is a powerful integrated driver with a timer similar to the NE555. The generation frequency is set by external elements. The microcircuit does not have inputs for organizing feedback, so current and voltage stabilization cannot be obtained using the PWM method.

Pin layout of the IR2153 chip.

The assignment of the pins is given in the table.

Internal circuit of IR2153.

To better understand the operation and pin assignments, it is best to study the internal circuitry. The main point that you need to pay attention to is that the output switches are assembled using a half-bridge circuit.

Using this chip you can assemble a simple power supply.

Scheme of a simple power supply based on IR2153.

The IR2153 is powered by 220 volts through a quenching resistor R1, a diode rectifier VD3, and a filter on C4. The generation frequency is set by elements C5, R2 (with the values ​​​​indicated in the diagram, it turns out to be about 47 kHz). A transformer can be considered a program. The author's version used a power transformer from a computer power supply. The standard windings have been removed, the primary is wound into two cores with enamel insulated wire with a diameter of 0.6 mm.

Device layout

After all the components have been selected, or there is a clear idea of ​​what they will be, you can begin to assemble the device. It is also important to understand what the future body of the device will be like. You can choose a ready-made one, or you can make it yourself if you have the materials and skills.

There are no special rules for the layout of components inside the housing. But it is advisable to arrange the nodes so that they are connected by conductors in series, as in the diagram, and over the shortest distance. It is better to place the output terminals on the side opposite the network cable. It is better to mount the power switch and fuse on the back wall of the device. To rationally use the inter-body space, some of the units can be installed vertically, but it is better to fix the diode bridge horizontally. When mounted vertically, convection currents of hot air from the lower diodes will flow around the upper elements and additionally heat them.

For those who don’t understand, watch the video: Simple DIY power supply.

Assembling a fixed supply DC power supply is not difficult. An average craftsman can do this; only basic knowledge of electrical engineering and minimal installation skills are needed.