Switching power supply. Switching power supplies

The switching power supply is used to convert the input voltage to the value required by the internal elements of the device. Another name pulse sources, which have become widespread, are inverters.

What is it?

An inverter is a secondary power source that uses double conversion of the input AC voltage. The magnitude of the output parameters is adjusted by changing the duration (width) of the pulses and, in some cases, their repetition rate. This type of modulation is called pulse width modulation.

Operating principle of a switching power supply

The operation of the inverter is based on the rectification of the primary voltage and its further conversion into a sequence of high-frequency pulses. This is how it differs from a conventional transformer. The output voltage of the block serves to generate a negative signal feedback, which allows you to adjust the pulse parameters. By controlling the pulse width, it is easy to organize stabilization and adjustment of output parameters, voltage or current. That is, it can be either a voltage stabilizer or a current stabilizer.

The number and polarity of output values ​​can be very different depending on how the switching power supply operates.

Types of power supplies

Several types of inverters, which differ in their construction scheme, have found application:

• transformerless;
• transformer

The first ones differ in that the pulse sequence goes directly to the output rectifier and smoothing filter of the device. This scheme has a minimum of components. A simple inverter includes a specialized integrated circuit - a pulse width generator.

The main disadvantage of transformerless devices is that they do not have galvanic isolation from the supply network and can pose a risk of electric shock. They also usually have low power and only produce 1 output voltage.

More common are transformer devices in which a high-frequency pulse sequence is supplied to the primary winding of the transformer. There can be as many secondary windings as desired, which makes it possible to generate several output voltages. Each secondary winding is loaded with its own rectifier and smoothing filter.

A powerful switching power supply for any computer is built according to a circuit that has high reliability and safety. For the feedback signal, a voltage of 5 or 12 Volts is used here, since these values ​​​​require the most accurate stabilization.

The use of transformers to convert high-frequency voltage (tens of kilohertz instead of 50 Hz) made it possible to significantly reduce their dimensions and weight and use ferromagnetic materials with high coercive force as the core material (magnetic core) rather than electrical iron.

DC-DC converters are also built based on pulse-width modulation. Without the use of inverter circuits, conversion was very difficult.

Power supply circuit

The most common configuration of a pulse converter includes:

• network noise suppression filter;
• rectifier;
• smoothing filter;
• pulse width converter;
• key transistors;
• high frequency output transformer;
• output rectifiers;
• output individual and group filters.

The purpose of the noise suppression filter is to delay interference from the operation of the device into the power supply network. Switching of powerful semiconductor elements can be accompanied by the creation of short-term pulses in a wide range of frequencies. Therefore, here it is necessary to use elements designed specifically for this purpose as pass-through capacitors of the filter units.

The rectifier is used to convert the input alternating voltage into direct voltage, and a smoothing filter installed next eliminates the ripple of the rectified voltage.

In the case when is used, the rectifier and filter become unnecessary, and input signal, having passed through the noise suppression filter circuits, it is supplied directly to the pulse-width converter (modulator), abbreviated PWM.

PWM is the most complex part of the switching power supply circuit. His tasks include:

• generation of high-frequency pulses;
• control of the block's output parameters and correction of the pulse sequence in accordance with the feedback signal;
• control and protection against overloads.

The PWM signal is supplied to the control terminals of powerful key transistors connected in a bridge or half-bridge circuit. The power terminals of the transistors are loaded onto the primary winding of the high-frequency output transformer. Instead of traditional ones, IGBT or MOSFET transistors are used, which are characterized by a low voltage drop across transitions and high speed. Improved transistor parameters help reduce power dissipation with the same dimensions and technical parameters designs.

The output pulse transformer uses the same conversion principle as the classic one. The exception is operation at higher frequencies. As a result, high-frequency transformers with the same transmitted powers have smaller dimensions.

The voltage from the secondary winding (there may be several of them) is supplied to the output rectifiers. Unlike the input rectifier, the diodes of the secondary circuit rectifier must have an increased operating frequency. Schottky diodes work best in this section of the circuit. Their advantages over conventional ones:

• high operating frequency;
• reduced p-n junction capacitance;
• low voltage drop.

The purpose of the output filter of a switching power supply is to reduce to the required minimum ripple of the rectified output voltage. Since the ripple frequency is much higher than that of the mains voltage, there is no need for large values ​​of capacitor capacitance and inductance in the coils.

Scope of application of a switching power supply

Pulse voltage converters are used in most cases instead of traditional transformers with semiconductor stabilizers. With the same power, inverters are distinguished by smaller overall dimensions and weight, high reliability, and most importantly, higher efficiency and the ability to operate in a wide input voltage range. And with comparable dimensions, the maximum power of the inverter is several times higher.

In such an area as DC voltage conversion, pulsed sources have practically no alternative replacement and are capable of working not only to reduce voltage, but also to generate increased voltage and organize a change in polarity. The high conversion frequency greatly facilitates filtering and stabilization of output parameters.

Small-sized inverters on specialized integrated circuits are used as chargers for all kinds of gadgets, and their reliability is such that the service life charging block may exceed uptime mobile device several times.

12 Volt power drivers for turning on LED lighting sources are also built using a pulse circuit.

How to make a switching power supply with your own hands

Inverters, especially powerful ones, have complex circuitry and can only be replicated by experienced radio amateurs. For self-assembly of network power supplies, we can recommend simple low-power circuits using specialized PWM controller chips. Such ICs have a small number of wiring elements and have proven standard switching circuits that practically do not require adjustment and configuration.

When working with homemade structures or repairs industrial devices It must be remembered that part of the circuit will always be at network potential, so safety precautions must be observed.

PULSE POWER SUPPLIES

Unlike traditional linear power supplies, which involve extinguishing excessive unstabilized voltage at the pass-through linear element, pulsed power supplies use other methods and physical phenomena to generate a stabilized voltage, namely: the effect of energy accumulation in inductors, as well as the possibility of high-frequency transformation and conversion of accumulated energy into direct voltage. There are three typical circuits for constructing pulsed power supplies (see Fig. 3.4-1): step-up (output voltage is higher than input voltage), step-down (output voltage is lower than input voltage) and inverting (output voltage has the opposite polarity with respect to the input). As can be seen from the figure, they differ only in the way they connect the inductance; otherwise, the principle of operation remains unchanged, namely.

Key element (usually bipolar or MOS transistors), operating with a frequency of the order of 20-100 kHz, periodically for a short time (no more than 50% of the time)

gives the full input unstabilized voltage to the inductor. Pulse current. flowing through the coil ensures the accumulation of energy reserves in its magnetic field of 1/2LI^2 at each pulse. The energy stored in this way from the coil is transferred to the load (either directly, using a rectifying diode, or through the secondary winding with subsequent rectification), the output smoothing filter capacitor ensures a constant output voltage and current. Output voltage stabilization is ensured automatic adjustment the width or repetition rate of the pulses on the key element (a feedback circuit is designed to monitor the output voltage).

This, although quite complex, scheme can significantly increase the efficiency of the entire device. The point is that, in in this case, except for the load itself, there are no power elements in the circuit that dissipate significant power. Key transistors operate in the saturated switch mode (i.e., the voltage drop across them is small) and dissipate power only in fairly short time intervals (pulse time). In addition, by increasing the conversion frequency, it is possible to significantly increase power and improve weight and size characteristics.

An important technological advantage of pulse power supplies is the ability to build on their basis small-sized network power supplies with galvanic isolation from the network to power a wide variety of equipment. Such power supplies are built without the use of a bulky low-frequency power transformer using a high-frequency converter circuit. This is, in fact, a typical switching power supply circuit with voltage reduction, where rectified mains voltage is used as the input voltage, and a high-frequency transformer (small-sized and with high efficiency) is used as a storage element, from the secondary winding of which the output stabilized voltage is removed (this transformer also provides galvanic isolation from the network).

The disadvantages of pulsed power supplies include: the presence of a high level of pulsed noise at the output, high complexity and low reliability (especially in handicraft production), the need to use expensive high-voltage high-frequency components, which in the event of the slightest malfunction easily fail “en masse” (with In this case, as a rule, impressive pyrotechnic effects can be observed). Those who like to delve into the insides of devices with a screwdriver and a soldering iron will have to be extremely careful when designing network switching power supplies, since many elements of such circuits are under high voltage.

Rice. 3.4-1 Typical block diagrams of switching power supplies

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2. Effective low-complexity pulse stabilizer.

Efficient low-complexity switching stabilizer

On an element base similar to that used in the linear stabilizer described above (Fig. 3.3-3), it is possible to build a pulse voltage stabilizer. With the same characteristics, it will have significantly smaller dimensions and better thermal conditions. A schematic diagram of such a stabilizer is shown in Fig. 3.4-2. The stabilizer is assembled according to standard scheme with a decrease in voltage (Fig. 3.4-1a).

When first turned on, when capacitor C4 is discharged and a sufficiently powerful load is connected to the output, current flows through the linear regulator IC DA1. The voltage drop across R1 caused by this current unlocks the key transistor VT1, which immediately enters saturation mode, since the inductive reactance of L1 is large and enough flows through the transistor high current. The voltage drop across R5 opens the main key element - transistor VT2. Current. increasing in L1, charges C4, while through feedback on R8 the recording occurs

Damage to the stabilizer and key transistor. The energy stored in the coil powers the load. When the voltage at C4 drops below the stabilization voltage, DA1 and the key transistor open. The cycle is repeated with a frequency of 20-30 kHz.

Circuit R3. R4, C2 will set the output voltage level. It can be smoothly adjusted within small limits, from Uct DA1 to Uin. However, if Uout is raised close to Uin, some instability appears at maximum load and an increased level of ripple. To suppress high-frequency ripples, filter L2, C5 is included at the output of the stabilizer.

The scheme is quite simple and most effective for this level of complexity. All power elements VT1, VT2, VD1, DA1 are equipped with small radiators. The input voltage must not exceed 30 V, which is the maximum for KR142EN8 stabilizers. Rectifier diodes apply a current of at least 3 A.

Rice. 3.4-2 Scheme of an effective pulse stabilizer based on a simple element base

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3. Device uninterruptible power supply based on a high-frequency pulse converter.

Uninterruptible power supply device based on a switching stabilizer

In Fig. 3.4-3 we propose for consideration a device for uninterruptible power supply of security and video surveillance systems based on a pulse stabilizer combined with a charger. The stabilizer includes protection systems against overload, overheating, output voltage surges, and short circuits.

The stabilizer has the following parameters:

Input voltage, Uvx - 20-30 V:

Output stabilized voltage, Uvyx-12V:

Rated load current, Iload nom -5A;

Trip current of the overload protection system, Iprotect - 7A;.

Operation voltage of the overvoltage protection system, Uout protection - 13 V;

Maximum battery charging current, Icharge battery max - 0.7 A;

Ripple level. Upulse - 100 mV,

Temperature of operation of the overheating protection system, Tzasch - 120 C;

Switching speed to power from the battery, tswitch - 10ms (relay RES-b RFO.452.112).

The operating principle of the pulse stabilizer in the described device is the same as that of the stabilizer presented above.

The device is supplemented with a charger made on elements DA2, R7, R8, R9, R10, VD2, C7. Voltage stabilizer IC DA2 with current divider on R7. R8 limits the maximum initial charge current, the divider R9, R10 sets the output charge voltage, diode VD2 protects the battery from self-discharge in the absence of supply voltage.

Overheat protection uses thermistor R16 as a temperature sensor. When the protection is triggered, the sound alarm, assembled on the DD 1 IC, turns on and, at the same time, the load is disconnected from the stabilizer, switching to power from the battery. The thermistor is mounted on the radiator of transistor VT1. Fine adjustment of the temperature protection response level is carried out by resistance R18.

The voltage sensor is assembled on the divider R13, R15. resistance R15 sets the exact level of overvoltage protection (13 V). If the voltage at the output of the stabilizer exceeds (if the latter fails), relay S1 disconnects the load from the stabilizer and connects it to the battery. If the supply voltage is turned off, relay S1 goes into the “default” state - i.e. connects the load to the battery.

The circuit shown here does not have electronic short circuit protection for the battery. This role is performed by a fuse in the load power supply circuit, designed for the maximum current consumption.

Rice. 3.4-3 Diagram of a 12V 5A uninterruptible power supply device with a multifunctional protection system

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4. Power supplies based on a high-frequency pulse converter.

Power supplies based on high-frequency pulse converter

Quite often, when designing devices, there are strict requirements for the size of the power source. In this case, the only solution is to use a power supply based on high-voltage, high-frequency pulse converters. which are connected to a ~220 V network without the use of a large low-frequency step-down transformer and can provide high power with small size and heat dissipation.

Block diagram typical pulse converter powered by industrial network presented in Figure 34-4.

The input filter is designed to prevent impulse noise from entering the network. Power switches provide high-voltage pulses to the primary winding of a high-frequency transformer (single- and

push-pull circuits). The frequency and duration of the pulses are set by a controlled generator (control of the pulse width is usually used, less often - frequency). Unlike low-frequency sinusoidal signal transformers, pulsed power supplies use broadband devices that provide efficient transfer power on signals with fast edges. This imposes significant requirements on the type of magnetic circuit used and the design of the transformer. On the other hand, with increasing frequency, the required dimensions of the transformer (while maintaining the transmitted power) decrease (modern materials make it possible to build powerful transformers with acceptable efficiency at frequencies up to 100-400 kHz). A special feature of the output rectifier is the use of high-speed Schottky diodes rather than conventional power diodes, which is due to the high frequency of the rectified voltage. The output filter smoothes out output voltage ripple. The feedback voltage is compared with the reference voltage and then drives the oscillator. Please note the presence of galvanic isolation in the feedback circuit, which is necessary if we want to ensure isolation of the output voltage from the network.

In the manufacture of such IP, serious requirements arise for the components used (which increases their cost compared to traditional ones). Firstly, this concerns the operating voltage of the rectifier diodes, filter capacitors and key transistors, which should not be less than 350 V to avoid breakdowns. Secondly, high-frequency key transistors (operating frequency 20-100 kHz) and special ceramic capacitors should be used (conventional oxide electrolytes will overheat at high frequencies due to their high inductance

activity). And thirdly, the saturation frequency of the high-frequency transformer, determined by the type of magnetic core used (as a rule, toroidal cores are used) must be significantly higher than the operating frequency of the converter.

In Fig. 3.4-5 shows a schematic diagram of a classic power supply based on a high-frequency converter. The filter, consisting of capacitors C1, C2, SZ and chokes L1, L2, serves to protect the supply network from high-frequency interference from the converter. The generator is built according to a self-oscillating circuit and combined with a key stage. Key transistors VT1 and VT2 operate in antiphase, opening and closing in turn. Starting the generator and reliable operation is ensured by transistor VT3, operating in avalanche breakdown mode. When the voltage on C6 increases through R3, the transistor opens and the capacitor is discharged to the base of VT2, starting the generator. The feedback voltage is removed from the additional (III) winding of the power transformer Tpl.

Transistors VT1. VT2 is installed on plate radiators of at least 100 cm^2. Diodes VD2-VD5 with a Schottky barrier are placed on a small radiator 5 cm^2. Data of chokes and transformers: L1-1. L2 is wound on ferrite rings 2000NM K12x8x3 into two wires using PELSHO wire 0.25: 20 turns. TP1 - on two rings folded together, ferrite 2000NN KZ 1x18.5x7;

winding 1 - 82 turns with PEV-2 0.5 wire: winding II - 25+25 turns with PEV-2 1.0 wire: winding III - 2 turns with PEV-2 0.3 wire. TP2 is wound on a ferrite ring 2000NN K10x6x5. all windings are made with PEV-2 0.3 wire: winding 1 - 10 turns:

windings II and III - 6 turns each, both windings (II and III) are wound so that they occupy 50% of the area on the ring without touching or overlapping each other, winding I is wound evenly throughout the ring and insulated with a layer of varnished cloth. Rectifier filter coils L3, L4 are wound on ferrite 2000NM K 12x8x3 with PEV-2 1.0 wire, number of turns - 30. KT809A can be used as key transistors VT1, VT2. KT812, KT841.

The element ratings and winding data of the transformers are given for an output voltage of 35 V. In the case when other operating parameter values ​​are required, the number of turns in winding 2 Tr1 should be changed accordingly.

The described circuit has significant drawbacks due to the desire to extremely reduce the number of components used. These include a low level of output voltage stabilization, unstable unreliable operation, and low output current. However, it is quite suitable for powering the simplest designs different power(when using appropriate components), such as: calculators. Caller IDs. lighting fixtures, etc.

Another power supply circuit based on a high-frequency pulse converter is shown in Fig. 3.4-6. The main difference between this scheme and the standard structure shown in Fig. 3 .4-4 is the absence of a feedback circuit. In this regard, the voltage stability on the output windings of the HF transformer Tr2 is quite low and the use of secondary stabilizers is required (the circuit uses universal integrated stabilizers based on the KR142 series IC).

Rice. 3.4-4 Block diagram of a typical high-frequency pulse converter powered from an industrial network

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Miniaturization and increased efficiency in the development and construction of switching power supplies is facilitated by the use of a new class of semiconductor inverters - MOS transistors, as well as: high-power diodes with fast reverse recovery, Schottky diodes, ultra-fast diodes, field-effect transistors with insulated gate, integrated circuits management key elements. All these items are available on domestic market and can be used in the design of highly efficient power supplies, converters, ignition systems for internal combustion engines (ICE), and starting systems for fluorescent lamps (FLL). A class of power devices called HEXSense - MOS transistors with current sensing - may also be of great interest to developers. They are ideal switching elements for switching power supplies with ready-made control. The ability to read switch transistor current can be used in switching power supplies to provide the current feedback required by a pulse width modulation controller. This achieves simplification of the design of the power source - the exclusion of current resistors and transformers from it.

In Fig. Figure 3.4-7 shows a diagram of a 230 W switching power supply. Its main performance characteristics are as follows:

Input voltage: -110V 60Hz:

Output voltage: 48 V DC:

Load current: 4.8 A:

Switching frequency: 110 kHz:

Efficiency at full load : 78%;

Efficiency at 1/3 load: 83%.

The circuit is built on the basis of a pulse-width modulator (PWM) with a high-frequency converter at the output. The operating principle is as follows.

The control signal for the key transistor comes from output 6 of the PWM controller DA1, the duty cycle is limited to 50% by resistor R4, R4 and SZ are the timing elements of the generator. Power supply for DA1 is provided by the chain VD5, C5, C6, R6. Resistor R6 is designed to supply supply voltage during generator startup; subsequently, voltage feedback through LI, VD5 is activated. This feedback is obtained from the additional winding of the output choke, which operates in reverse mode. In addition to powering the generator, the feedback voltage through the chain VD4, Cl, Rl, R2 is supplied to the voltage feedback input DA1 (pin 2). Through R3 and C2 compensation is provided, which guarantees the stability of the feedback loop.

Based on this circuit, it is possible to build pulse stabilizers with other output parameters.

PULSE POWER SUPPLIES

It is known that power supplies are an integral part of radio engineering devices, which are subject to a number of requirements; they represent a complex of elements, instruments and apparatus that generate electrical energy and convert it to the form necessary to ensure the required operating conditions of radio devices.

Power sources are divided into two groups: primary and secondary power sources: Primary sources are devices that convert various types of energy into electrical energy (electric machine generators, electrochemical current sources, photoelectric and thermionic converters, etc.).

Secondary power devices are converters of one type of electrical energy into another. These include: AC-DC voltage converters (rectifier); AC voltage converters (transformers); DC-AC converters (inverters).

Power supplies currently account for 30 to 70% of the total mass and volume of electronic equipment. Therefore, the problem of creating a miniature, lightweight and reliable power supply device with good technical and economic indicators is important and relevant. This work is devoted to the development of a secondary power source (SPS) with minimal weight and size and high technical characteristics.

A prerequisite for designing secondary power sources is a clear knowledge of the requirements for them. These requirements are very diverse and are determined by the operating features of those REA complexes that are powered by a given RES. The main requirements are: for the design - reliability, maintainability, size and weight restrictions, thermal conditions; to technical and economic characteristics - cost and manufacturability.

The main directions for improving the weight, size and technical and economic indicators of IP: the use of the latest electrical materials; application of element base using integral-hybrid technology; increasing the frequency of electrical energy conversion; searches for new effective circuit solutions. To select a power supply circuit, an analysis was made of the efficiency of using switching power supplies (PSS) in comparison with power PSs made using traditional technology.

The main disadvantages of power transformers are their high weight and size characteristics, as well as the significant impact on other electronic devices of a strong magnetic field power transformers. The problem with SMPS is their creation of high-frequency interference, and, as a consequence of this, electromagnetic incompatibility with certain types of electronic equipment. The analysis showed that SMPS most fully meet the requirements, which is confirmed by their widespread use in REA.

The work examines an 800 W SMPS, which differs from other SMPS by using field-effect transistors and a transformer with a primary winding having a middle terminal in the converter. Field effect transistors provide more high efficiency and a reduced level of high-frequency interference, and a transformer with a middle terminal - half the current through the key transistors and eliminates the need for an isolation transformer in their gate circuits.

Based on the chosen principle electrical diagram a design was developed and a prototype of the SMPS was manufactured. The entire structure is presented in the form of a module installed in an aluminum case. After initial tests, a number of shortcomings were identified: noticeable heating of the radiators of key transistors, the difficulty of removing heat from powerful domestic resistors and large dimensions.

The design has been modified: the design of the control board has been changed using surface-mounted components on a double-sided board, its perpendicular installation on the main board; using a radiator with a built-in fan from a computer; all heat-stressed elements of the circuit were specially located on one side of the case along the blowing direction of the main fan for the most effective cooling. As a result of the modification, the dimensions of the IPP were reduced by three times and the shortcomings identified during the initial tests were eliminated. The modified sample has the following characteristics: supply voltage Up = ~ 180-240 V, frequency fr = 90 kHz, output power P = 800 W, efficiency = 85%, weight = 2.1 kg, overall dimensions 145x145x80 mm.

This work is devoted to the design of a switching power supply designed to power an audio power amplifier, which is part of a home audio reproduction system. high power. The creation of a home sound reproduction system began with the choice of a circuit design for the UMZCH. For this purpose, an analysis of the circuit design of sound-reproducing devices was carried out. The choice was made on the high-fidelity UMZCH circuit.

This amplifier has very high characteristics, contains overload protection devices and short circuits, devices for maintaining zero potential of constant voltage at the output and a device for compensating the resistance of the wires connecting the amplifier to the acoustics. Even though UMZCH diagram published a long time ago, radio amateurs to this day repeat its design, references to which can be found in almost any literature concerning the assembly of devices for high-quality music playback. Based on this article, it was decided to assemble a four-channel UMZCH, the total power consumption of which was 800 W. That's why next step The assembly of the UMZCH involved the development and assembly of a power supply design that provides an output power of at least 800 W, small dimensions and weight, operational reliability and protection against overload and short circuits.

Power supplies are built mainly according to two schemes: traditional classical and according to the scheme of pulsed voltage converters. Therefore, it was decided to assemble and refine the design of a switching power supply.

Study of secondary power sources. Power sources are divided into two groups: primary and secondary power sources.

Primary sources are devices that convert various types of energy into electrical energy (electric machine generators, electrochemical current sources, photoelectric and thermionic converters, etc.).

Secondary power devices are converters of one type of electrical energy into another. These include:

• * AC to DC voltage converters (rectifiers);
• * AC voltage converters (transformers);
• * DC-AC converters (inverters).

Secondary power supplies are built mainly according to two schemes: traditional classical and according to the scheme of pulse voltage converters. The main disadvantage of power transformers made according to the traditional classical design is their large weight and size characteristics, as well as the significant influence of the strong magnetic field of power transformers on other electronic devices. The problem with SMPS is their creation of high-frequency interference, and as a consequence of this, electromagnetic incompatibility with certain types of electronic equipment. The analysis showed that SMPS most fully meet the requirements, which is confirmed by their widespread use in REA.

Transformers of switching power supplies differ from traditional ones in the following: - rectangular voltage supply; complicated shape of the windings (midpoint terminals) and operation at higher frequencies (up to several tens of kHz). In addition, the transformer parameters have a significant impact on the operating mode of semiconductor devices and the characteristics of the converter. Thus, the magnetizing inductance of the transformer increases the switching time of the transistors; leakage inductance (with a rapidly changing current) causes overvoltages to occur on transistors, which can lead to their breakdown; The no-load current reduces the efficiency of the converter and worsens the thermal conditions of the transistors. The noted features are taken into account when calculating and designing SMPS transformers.

This paper examines an 800 W switching power supply. It differs from those described earlier by the use of field-effect transistors and a transformer with a primary winding with a middle terminal in the converter. The first provides higher efficiency and a reduced level of high-frequency interference, and the second provides half the current through the key transistors and eliminates the need for an isolation transformer in their gate circuits.

The disadvantage of this circuit solution is the high voltage on the halves of the primary winding, which requires the use of transistors with the appropriate allowable voltage. True, unlike a bridge converter, in this case two transistors are enough instead of four, which simplifies the design and increases the efficiency of the device.

Switching power supplies (UPS) use one- and two-cycle high-frequency converters. The efficiency of the former is lower than that of the latter, so it is not practical to design single-cycle UPSs with a power of more than 40...60 W. Push-pull converters make it possible to obtain significantly higher output power at high efficiency. They are divided into several groups, characterized by the method of exciting the output key transistors and the circuit for connecting them to the circuit of the primary winding of the converter transformer. If we talk about the method of excitation, then we can distinguish two groups: with self-excitation and external excitation.

The former are less popular due to difficulties in establishing. When designing powerful (more than 200 W) UPSs, the complexity of their manufacture increases unjustifiably, so they are of little use for such power supplies. Converters with external excitation are well suited for creating high-power UPSs and sometimes require almost no setup. As for connecting key transistors to a transformer, there are three circuits: the so-called half-bridge (Fig. 1, a), bridge (Fig. 1, b). Today, the half-bridge converter is most widely used.

It requires two transistors with relatively little high value voltage Ukemax. As can be seen from Fig. 1a, capacitors C1 and C2 form a voltage divider, to which the primary (I) winding of transformer T2 is connected. When the key transistor opens, the amplitude of the voltage pulse on the winding reaches the value Upit/2 - Uke nas. The bridge converter is similar to the half-bridge converter, but in it the capacitors are replaced by transistors VT3 and VT4 (Fig. 1b), which open in pairs diagonally. This converter has a slightly higher efficiency due to an increase in the voltage supplied to the primary winding of the transformer, and therefore a decrease in the current flowing through transistors VT1-VT4. The voltage amplitude on the primary winding of the transformer in this case reaches the value Upit - 2Uke us.

Particularly worth noting is the converter according to the circuit in Fig. 1c, which is characterized by the highest efficiency. This is achieved by reducing the primary winding current and, as a result, reducing power dissipation in key transistors, which is extremely important for powerful UPS. The voltage amplitude of the pulses in half of the primary winding increases to the value Upit - Uke us.

It should also be noted that, unlike other converters, it does not require an input isolation transformer. In the device according to the circuit in Fig. 1c, it is necessary to use transistors with a high Uke max value. Since the end of the upper (according to the diagram) half of the primary winding is connected to the beginning of the lower, when current flows in the first of them (VT1 is open), a voltage is created in the second, equal (in absolute value) to the voltage amplitude on the first, but opposite in sign relative to Upit. In other words, the voltage at the collector of the closed transistor VT2 reaches 2Upit. therefore, its Uke max should be greater than 2Upit. The proposed UPS uses a push-pull converter with a transformer, the primary winding of which has a middle terminal. It has high efficiency, low level pulsations and weakly emits interference into the surrounding space.

OUTPUT VOLTAGE STABILIZATION
SWITCH POWER SUPPLY

THE ARTICLE WAS PREPARED BASED ON THE BOOK BY A. V. GOLOVKOV and V. B LYUBITSKY "POWER SUPPLY FOR SYSTEM MODULES OF THE IBM PC-XT/AT TYPE" BY THE PUBLISHING HOUSE "LAD&N"

The output voltage stabilization circuit in the class of UPS under consideration is a closed automatic control loop (Fig. 31). This loop includes:
control circuit 8;
matching preamplifier stage 9;
control transformer DT;
power stage 2;
power pulse transformer RT;
rectifier block 3;
interchannel communication choke 4;
filter block 5;
feedback voltage divider 6;
reference voltage divider 7.
Control circuit 8 contains the following functional units:
mismatch signal amplifier 8.1 with Zk correction circuit;
PWM comparator (modulator) 8.2;
sawtooth voltage generator (oscillator) 8.3;
source of reference stabilized voltage Uref 8.4.
During operation, the error signal amplifier 8.1 compares the output signal of voltage divider b with the reference voltage of divider 7. Boosted signal mismatch is supplied to the pulse-width modulator 8.2, which controls the pre-final stage of the power amplifier 9, which, in turn, supplies a modulated control signal to the power stage of the converter 2 through the control transformer DT. The power cascade is powered using a transformerless circuit. The alternating voltage of the supply network is rectified by mains rectifier 1 and supplied to the power stage, where it is smoothed out by the capacitors of the capacitive rack. Part of the output voltage of the stabilizer is compared with a constant reference voltage and then the resulting difference (mismatch signal) is amplified with the introduction of appropriate compensation. The 8.2 pulse width modulator converts analog signal control into a width-modulated signal with a variable pulse duty cycle. In the class of UPS under consideration, the modulator circuit compares the signal coming from the output of the error signal amplifier with a sawtooth voltage, which is obtained from a special generator 8.3.

Figure 31. Regulatory loop of a typical switching power supply based on the TL494 control chip.

Figure 32. Adjusting the output voltage level of the PS-200B UPS.

Figure 33. Adjusting the output voltage level of the LPS-02-150XT UPS.

Figure 34. Adjusting the output voltage level of the Appis UPS.

Figure 35. Adjusting the output voltage level of the GT-200W UPS.

However, the most common case is when there is no adjustment to influence the output voltages of the unit. In this case, the voltage at any of inputs 1 or 2 is selected arbitrarily in the range from +2.5 to +5 V, and the voltage at the remaining input is selected using a high-impedance shunt resistor so that the unit produces the output voltages specified in the passport at the nominal load mode. Rice. 35 illustrates the case of selecting the reference voltage level, Fig. 34 - shows the case of selecting the level of the feedback signal. It was previously noted that the instability of the output voltage under the influence of any destabilizing factors (changes in load current, supply voltage and ambient temperature) could be reduced by increasing the gain of the feedback circuit (gain of amplifier DA3).
However maximum value the gain DA3 is limited by the stability condition. Since both the UPS and the load contain reactive elements (inductance or capacitance) that accumulate energy, energy is redistributed between these elements in transient modes. This circumstance may lead to the fact that, with certain parameters of the elements, the transient process of establishing the output voltages of the UPS will take on the character of undamped oscillations, or the amount of overregulation in the transient mode will reach unacceptable values.

Figure 36. Transient processes (oscillatory and aperiodic) of the UPS output voltage with an abrupt change in the load current (a) and input voltage (b).

In Fig. Figure 36 shows the transient processes of the output voltage with an abrupt change in the load current and input voltage. The UPS operates stably if the output voltage returns to its steady value after the disturbance that brought it out of operation ceases. original condition(Fig. 37, a).

Figure 37. UPS output voltage transients in stable (a) and unstable (b) systems.

If this condition is not met, then the system is unstable (Fig. 37.6). Ensuring the stability of the switching power supply is a necessary condition for its normal functioning. Transition process depending on the parameters of the UPS, it is oscillatory or aperiodic in nature, while the output voltage of the UPS has a certain overshoot value and transient process time. The deviation of the output voltage from the nominal value is detected in the measuring element of the feedback circuit (in the UPS under consideration, a resistive divider connected to the +5V output voltage bus is used as a measuring element). Due to the inertia of the control loop, the nominal value of the output voltage is set with a certain delay. In this case, the control circuit by inertia will continue to act in the same direction for some time. As a result, overshoot occurs, i.e. deviation of the output voltage from its nominal value in the direction opposite to the original deviation. The control circuit again changes the output voltage in the opposite direction, etc. In order to ensure the stability of the UPS output voltage regulation loop with a minimum duration of the transient process, the amplitude-frequency characteristic of the error amplifier DA3 is subject to correction. This is done using RC circuits connected as negative feedback circuits surrounding the DA3 amplifier. Examples of such corrective chains are shown in Fig. 38.

Figure 38. Examples of configuration of correcting RC chains for the voltage error amplifier DA3.

To reduce the level of noise generation, aperiodic RC circuits are installed on the secondary side of the switching power supply. Let's take a closer look at the principle of their operation.
The transient process of the current through the rectifier diodes at the moments of switching occurs in the form of shock excitation (Fig. 39, a).

Figure 39. Voltage timing diagrams on the reverse resistance recovery diode:
a) - without RC chain; b) - in the presence of an RC chain.

Switching power supply – electronic circuit, where the input voltage is rectified, filtered, and cut into bursts of high-frequency pulses for transmission through a small-sized transformer. The block becomes manageable, with flexibly adjustable parameters. The mass of the heaviest part of the source, the transformer, is reduced. In English-language literature such devices are called Switching-Mode Power Supply(SMPS).

SMPS (Switching Mode Power Supply) device

The emergence of switching power supplies

The dimensions of transformers also worried Tesla. The scientist, repeating experiment after experiment, established: high frequencies current are safe for humans and cause large losses in transformer cores. The result of the debate was the adoption of a frequency of 60 Hz for the construction of the Niagara Hydroelectric Power Station. We started with Nikola Tesla, because he was the first person who realized that rapid oscillations mechanically you won't get it. Therefore, one has to use oscillatory circuits. This is how the Tesla transformer appeared (September 22, 1896), with the help of which the scientist decided to transmit messages and energy over a distance.

The essence of the invention is described in the section about, we present brief information. The transformer is formed by two parts connected in series. The primary winding of the first was connected to an alternating voltage source of relatively low frequency. Due to the low transformation ratio, the capacitor connected to the secondary winding was charged to a high potential. The voltage reached the threshold, the spark gap connected in parallel with the capacitor broke through. The oscillatory process of discharge began through the primary winding of the second transformer into the external circuit. Tesla received radio voltages with an amplitude of millions of volts.

The first step in creating switching power supplies, where relatively low-frequency voltage is converted into pulses. A similar design was created in 1910 by Charles Kettering, equipping car ignition systems. Switching power supplies appeared in the 60s. The idea of ​​minimizing the size of transformers (after Nikola Tesla) was put forward by General Electric in 1959 in the person of Joseph Murphy and Francis Starchetz (U.S. Patent 3,040,271). The idea did not immediately find a warm response (there was no suitable element base), in 1970 Tectronics released a line of oscilloscopes with a new power supply.

Two years later, inverters are used in electronics (Patent US3697854 A), the main thing is that the first domestic models appear! Patents refer to each other, it is impossible to understand who first proposed using the idea in personal computers. In the USSR, development began in 1970, associated with the appearance on sale of high-frequency powerful germanium transistor 2T809A. As stated in the literature, the first to achieve success in 1972 was a Muscovite, candidate of technical sciences L. N. Sharov. Later, a 400 W switching power supply appeared, authored by A. I. Ginzburg, S. A. Eranosyan. EC computers were equipped with a new product in 1976 by a team led by Zh. A. Mkrtchyan.

The first switching power supplies, known to domestic consumers by digital TVs and VCRs often broke down, modern products have no drawback - they work continuously for years. The moment of the early 90s provides the following information:

1. Specific power: 35 - 120 W per cubic decimeter.
2. Inverter operating frequency: 30 - 150 kHz.
3. Efficiency: 75 - 85%.
4. MTBF: 50 - 200 thousand hours (6250 working days).

Advantages of switching power supplies

Linear power supplies are bulky, and efficiency is poor. Efficiency rarely exceeds 30%. For switching power supplies, the average figures are in the range of 70 - 80%; there are products that stand out greatly from the range. For the better, of course. The following information is provided: The efficiency of the switching power supply reaches 98%. At the same time, the required filtration of capacitor capacitance is reduced. The energy stored per period decreases greatly with increasing frequency. Depends directly proportionally on the capacitance of the capacitor, quadratically on the voltage amplitude.

An increase to a frequency of 20 kHz (compared to 50/60) reduces the linear dimensions of the elements by 4 times. Flowers compared to expectations in the radio range. Explains the reason for equipping receivers with small capacitors.

Switching power supply design

The input voltage is rectified. The process is carried out by a diode bridge, or less often by a single diode. Then the voltage is cut into pulses; here the literature quickly moves on to a description of the transformer. Readers are probably wondering how a chopper (a device that generates pulses) works. Based on a microcircuit powered directly by a mains voltage of 230 volts. Less often, a zener diode (parallel type stabilizer) is specially installed.

The microcircuit generates pulses (20 - 200 kHz), of relatively small amplitude, that control a thyristor or other semiconductor power switch. The thyristor cuts high voltage in pulses, flexible program, generated by the generator microcircuit. Since at the input there is high voltage, need protection. The generator is protected by a varistor, the resistance of which drops sharply when the threshold is exceeded, shorting the harmful surge to ground. From the power switch, packets of pulses are sent to a small-sized high-frequency transformer. Linear dimensions are relatively low. For a 500W computer power supply, it fits in a child's palm.

The resulting voltage is rectified again. Schottky diodes are used due to the low voltage drop of the metal-semiconductor junction. The rectified voltage is filtered and supplied to consumers. Due to the presence of many secondary windings, it is quite easy to obtain values ​​of different polarity and amplitude. The story is incomplete without mentioning the feedback loop. The output voltages are compared with a standard (for example, a zener diode), and the pulse generator mode is adjusted: the transmitted power (amplitude) depends on the frequency and duty cycle. The products are considered relatively unpretentious and can operate in a wide range of supply voltages.

Case power supply

The technology is called inverter and is used by welders, microwave ovens, induction hobs, adapters cell phones, iPad. A computer power supply works in a similar way.

Circuitry of switching power supplies

Nature provided 14 basic topologies implementation of switching power supplies. With inherent advantages and unique characteristics. Some are suitable for creating low-power power supplies (below 200 W), while others show best qualities when powered by a mains voltage of 230 volts (50/60 Hz). And to choose the right topology, be able to imagine the properties of each. Historically, the first three are called:

• Buck - buck, deer, dollar.
• Boost – acceleration.
• Polarity inverter – polarity inverter.

Three topologies are related to linear regulators. The type of device is considered the predecessor of switching power supplies, without including advantages. The voltage is supplied through a transformer, straightened, and cut into a power switch. The operation of the regulator is controlled by feedback, the task of which is to generate an error signal. This type of device accounted for a multi-billion-dollar turnover in the 60s; it could only reduce the voltage, and the common wire of the consumer was connected to the power supply network.

Buck topology

This is how the “deer” appeared. Initially intended for constant voltage, the input signal was cut into pulses, then the packets were straightened and filtered to obtain medium power. The feedback controlled the duty cycle and frequency (pulse width modulation). A similar thing is being done today computer blocks nutrition. Almost immediately, power densities of 1 - 4 W per cubic inch were achieved (later up to 50 W per cubic inch). It’s great that it’s now possible to get multiple output voltages decoupled from the input.

The disadvantage is the loss at the moment the transistor switches; the voltage changes polarity and remains below zero until the next pulse. The specified part of the signal, bypassing the diode, is shorted to ground, not reaching the filter. The existence of optimal switching frequencies at which costs are minimized has been discovered. Range 25 - 50 kHz.

Boost topology

The topology is called a ring choke and is placed in front of the switch. It is possible to increase the input voltage to the desired rating. The scheme works as follows:

1. At the initial moment of time, the transistor is open, the inductor stores the energy of the voltage source through the collector, emitter p-n junctions, and ground.
2. Then the key is locked and the capacitor charging process begins. The throttle releases energy.
3. At some point the feedback amplifier operates and the load begins to be powered. The capacitor is unable to transfer energy towards the power switch; the diode interferes. The charge is taken by the payload.
4. The voltage drop will cause the feedback circuit to fire again, and the inductor will begin to accumulate energy.

Polarity Inverter topology

The topology of the polar inverter is similar to the previous circuit; the inductor is located behind the switch. It works like this:

In this case, we observe parallelism in the processes of storing/expending energy. All three schemes considered demonstrate the following disadvantages:

1. There is a connection via DC between input and output. In other words, there is no galvanic isolation.
2. It is not possible to obtain multiple voltage values ​​from one circuit.

The disadvantages are eliminated by push-pull and latter topologies. Both use a chopper with forward technology. In the first case, a differential pair of transistors is used. It becomes possible to use one key for half the period. To control, you need a special forming circuit that alternately swings this swing, improving the conditions for heat removal. The chopped voltage is bipolar, powers the primary winding of the transformer, there are many secondary windings - in accordance with the requirements of consumers.

In the retarded topology, one transistor is replaced by a diode. The circuit is often operated by low-power power supplies (up to 200 W) with a constant output voltage of 60 - 200 V.