In most modern electronic devices, analog (transformer) power supplies are practically not used; they have been replaced by pulse voltage converters. To understand why this happened, it is necessary to consider the design features, as well as the strengths and weaknesses of these devices. We will also talk about the purpose of the main components of pulsed sources, we will give a simple implementation example that can be assembled by hand.

Design features and principle of operation

Of the several ways to convert voltage to power electronic components, two of the most widely used can be distinguished:

1. Analog, the main element of which is a step-down transformer, in addition to the main function, it also provides galvanic isolation.
2. impulse principle.

Let's take a look at the difference between these two options.

PSU based on power transformer

Consider a simplified block diagram of this device. As can be seen from the figure, a step-down transformer is installed at the input, with its help the amplitude of the supply voltage is converted, for example, from 220 V we get 15 V. The next block is a rectifier, its task is to convert the sinusoidal current into a pulsed one (the harmonic is shown above the symbolic image). For this purpose, rectifier semiconductor elements (diodes) connected in a bridge circuit are used. Their principle of operation can be found on our website.

The next block performs two functions: it smoothes the voltage (a capacitor of the appropriate capacity is used for this purpose) and stabilizes it. The latter is necessary so that the voltage does not “fall through” with increasing load.

The given block diagram is greatly simplified, as a rule, this type of source has an input filter and protective circuits, but this is not essential for explaining the operation of the device.

All the disadvantages of the above option are directly or indirectly related to the main structural element - the transformer. First, its weight and dimensions limit miniaturization. In order not to be unfounded, we give as an example a 220/12 V step-down transformer with a rated power of 250 W. The weight of such a unit is about 4 kilograms, dimensions are 125x124x89 mm. You can imagine how much a laptop charger based on it would weigh.

Secondly, the price of such devices sometimes many times exceeds the total cost of other components.

Impulse devices

As can be seen from the block diagram shown in Figure 3, the principle of operation of these devices differs significantly from analog converters, first of all, by the absence of an input step-down transformer.

Figure 3. Structural diagram of a switching power supply

Consider the algorithm of such a source:

• Power is supplied to the surge protector, its task is to minimize network interference, both incoming and outgoing, resulting from operation.
• Next, a unit for converting a sinusoidal voltage into a pulsed constant and a smoothing filter come into operation.
• At the next stage, an inverter is connected to the process, its task is to form rectangular high-frequency signals. Feedback to the inverter is carried out through the control unit.
• The next block is IT, it is necessary for automatic generator mode, supply voltage to the circuits, protection, controller control, as well as the load. In addition, the task of IT is to provide galvanic isolation between high and low voltage circuits.

Unlike a step-down transformer, the core of this device is made of ferrimagnetic materials, this contributes to the reliable transmission of RF signals, which can be in the range of 20-100 kHz. A characteristic feature of IT is that when it is connected, it is critical to turn on the beginning and end of the windings. The small dimensions of this device make it possible to manufacture devices of miniature size, as an example, the electronic piping (ballast) of an LED or energy-saving lamp can be given.

• Next, the output rectifier comes into operation, since it operates with a high-frequency voltage, the process requires high-speed semiconductor elements, therefore, Schottky diodes are used for this purpose.
• At the final phase, smoothing is performed on an advantageous filter, after which the voltage is applied to the load.

Now, as promised, we will consider the principle of operation of the main element of this device - the inverter.

How does an inverter work?

RF modulation can be done in three ways:

• frequency-pulse;
• phase-pulse;
• pulse width.

In practice, the latter option is used. This is due both to the simplicity of execution and the fact that PWM has a constant communication frequency, unlike the other two modulation methods. A block diagram describing the operation of the controller is shown below.

The device operation algorithm is as follows:

The master frequency generator generates a series of rectangular signals, the frequency of which corresponds to the reference one. Based on this signal, U P of a sawtooth shape is formed, which is fed to the input of the comparator K PWM. The second input of this device is supplied with the signal U US coming from the control amplifier. The signal generated by this amplifier corresponds to the proportional difference between U P (reference voltage) and U PC (control signal from the feedback circuit). That is, the control signal U US, in fact, is a mismatch voltage with a level that depends both on the current on the load and on the voltage on it (U OUT).

This implementation method allows you to organize a closed circuit that allows you to control the output voltage, that is, in fact, we are talking about a linear-discrete functional unit. At its output, pulses are formed, with a duration depending on the difference between the reference and control signal. Based on it, a voltage is created to control the key transistor of the inverter.

The process of stabilizing the output voltage is carried out by monitoring its level, when it changes, the voltage of the regulating signal U PC changes proportionally, which leads to an increase or decrease in the duration between pulses.

As a result, there is a change in the power of the secondary circuits, which ensures the stabilization of the output voltage.

To ensure safety, galvanic isolation between the supply network and the feedback is required. As a rule, optocouplers are used for this purpose.

Strengths and weaknesses of impulse sources

If we compare analog and pulse devices of the same power, then the latter will have the following advantages:

• Small size and weight, due to the absence of a low-frequency step-down transformer and control elements that require heat dissipation using large radiators. Through the use of high-frequency signal conversion technology, it is possible to reduce the capacitance of the capacitors used in the filters, which allows the installation of smaller elements.
• Higher efficiency, since the main losses are caused only by transients, while in analog circuits a lot of energy is constantly lost during electromagnetic conversion. The result speaks for itself, an increase in efficiency up to 95-98%.
• Lower cost due to the use of less powerful semiconductor elements.
• Wider input voltage range. This type of equipment is not demanding on frequency and amplitude, therefore, connection to networks of various standards is allowed.
• Availability of reliable protection against short circuit, overload and other emergency situations.

The disadvantages of impulse technology include:

The presence of RF interference, this is a consequence of the operation of the high-frequency converter. Such a factor requires the installation of a filter that suppresses interference. Unfortunately, its operation is not always efficient, which imposes some restrictions on the use of devices of this type in high-precision equipment.

Special requirements for the load, it should not be reduced or increased. As soon as the current level exceeds the upper or lower threshold, the output voltage characteristics will begin to differ significantly from the standard ones. As a rule, manufacturers (recently even Chinese) provide for such situations and install appropriate protection in their products.

Scope of application

Almost all modern electronics is powered by blocks of this type, as an example we can give:

We assemble a pulsed power supply unit with our own hands

Consider a simple power supply circuit, where the above principle of operation is applied.

Designations:

• Resistors: R1 - 100 Ohm, R2 - from 150 kOhm to 300 kOhm (selected), R3 - 1 kOhm.
• Capacitances: C1 and C2 - 0.01 uF x 630 V, C3 -22 uF x 450 V, C4 - 0.22 uF x 400 V, C5 - 6800 -15000 pF (selected), 012 uF, C6 - 10 uF x 50 V, C7 - 220 uF x 25 V, C8 - 22 uF x 25 V.
• Diodes: VD1-4 - KD258V, VD5 and VD7 - KD510A, VD6 - KS156A, VD8-11 - KD258A.
• Transistor VT1 - KT872A.
• The voltage regulator D1 is a KR142 chip with the index EH5 - EH8 (depending on the required output voltage).
• Transformer T1 - a w-shaped ferrite core with dimensions of 5x5 is used. The primary winding is wound with 600 turns of wire Ø 0.1 mm, the secondary (terminals 3-4) contains 44 turns Ø 0.25 mm, and the last - 5 turns Ø 0.1 mm.
• Fuse FU1 - 0.25A.

The setting is reduced to the selection of R2 and C5 ratings, which provide excitation of the generator at an input voltage of 185-240 V.

Introduction

Switching power supplies are now confidently replacing outdated linear ones. The reason is the high performance inherent in these power supplies, compactness and improved stabilization performance.

With those rapid changes that have undergone the principles of powering electronic equipment in recent years, information on the calculation, construction and use of switching power supplies is becoming more and more relevant.

Recently, among specialists in the field of electronics and radio engineering, as well as in industrial production, switching power supplies have gained particular popularity. There has been a tendency to abandon typical bulky transformers and switch to small-sized designs of switching power supplies, voltage converters, converters, and inverters.

In general, the topic of switching power supplies is quite relevant and interesting, and is one of the most important areas of power electronics. This area of ​​electronics is promising and rapidly developing. And its main goal is to develop powerful power devices that meet modern requirements for reliability, quality, durability, minimization of weight, size, energy and material consumption. It should be noted that almost all modern electronics, including all kinds of computers, audio, video equipment and other modern devices, are powered by compact switching power supplies, which once again confirms the relevance of further development of this area of ​​power supplies.

The principle of operation of switching power supplies

The switching power supply is an inverter system. In switching power supplies, the AC input voltage is first rectified. The resulting DC voltage is converted into rectangular pulses of increased frequency and a certain duty cycle, either supplied to the transformer (in the case of pulsed power supplies with galvanic isolation from the mains) or directly to the output low-pass filter (in pulsed power supplies without galvanic isolation). In pulse power supplies, small-sized transformers can be used - this is explained by the fact that with increasing frequency, the efficiency of the transformer increases and the requirements for the dimensions (section) of the core required for transferring equivalent power decrease. In most cases, such a core can be made of ferromagnetic materials, in contrast to the cores of low-frequency transformers, which use electrical steel.

Figure 1 - Structural diagram of a switching power supply

The mains voltage is supplied to the rectifier, after which it is smoothed out by a capacitive filter. From the filter capacitor, the voltage of which increases, the rectified voltage through the transformer winding is supplied to the collector of the transistor, which acts as a key. The control device provides periodic switching on and off of the transistor. To reliably start the PSU, a master oscillator made on a microcircuit is used. The pulses are fed to the base of the key transistor and cause the start of the oscillator operation cycle. The control device is responsible for monitoring the output voltage level, generating an error signal and, often, direct control of the key. The master oscillator microcircuit is powered by a chain of resistors directly from the storage capacitance input, stabilizing the voltage with the reference capacitance. The master oscillator and the key transistor of the secondary circuit are responsible for the operation of the optocoupler. The more open the transistors responsible for the operation of the optocoupler, the smaller the amplitude of the feedback pulses, the earlier the power transistor will turn off and the less energy will accumulate in the transformer, which will cause the voltage to rise at the output of the source to stop. The operating mode of the power supply has come, where the optocoupler plays an important role, as the regulator and manager of the output voltages.

The specification of an industrial power supply is more stringent than that of a conventional household power supply. This is expressed not only in the fact that a high three-phase voltage acts at the input of the power supply, but also in the fact that industrial power supplies must remain operational with a significant deviation of the input voltage from the nominal value, including voltage dips and surges, as well as the loss of one or multiple phases.

Figure 2 - Schematic diagram of a switching power supply.

The scheme works as follows. The three-phase input can be three-wire, four-wire, or even single-phase. The three-phase rectifier consists of diodes D1 - D8.

Resistors R1 - R4 provide surge protection. The use of protective resistors with overload opening makes it unnecessary to use separate fuse links. The input rectified voltage is filtered by a U-shaped filter consisting of C5, C6, C7, C8 and L1.

Resistors R13 and R15 equalize the voltage across the input filter capacitors.

When the MOSFET of U1 opens, the source potential of Q1 drops, the gate current is provided by resistors R6, R7 and R8, respectively, the capacitance of the junctions VR1 ... VR3 opens Q1. Zener diode VR4 limits the source-gate voltage applied to Q1. When MOSFET U1 closes, the drain voltage is limited to 450 volts by the clamp circuit VR1, VR2, VR3. Any additional voltage at the end of the winding will be dissipated by Q1. This connection effectively distributes the total rectified voltage to Q1 and U1.

Absorption circuit VR5, D9, R10 absorbs excess voltage on the primary winding due to leakage induction of the transformer during reverse operation.

The output rectification is carried out by the diode D1. C2 - output filter. L2 and C3 form a second filter stage to reduce output voltage instability.

VR6 begins to conduct when the output voltage exceeds the drop across VR6 and the optocoupler. A change in the output voltage causes a change in the current flowing through the optocoupler diode U2, which in turn causes a change in current through the optocoupler transistor U2. When this current exceeds the threshold on the FB pin of U1, the next duty cycle is skipped. The specified output voltage level is maintained by adjusting the number of skipped and perfect work cycles. Once the duty cycle has begun, it will end when the current through U1 reaches the set internal limit. R11 limits the current through the optocoupler and sets the feedback gain. Resistor R12 supplies a bias to VR6.

This circuit is protected from open loop, output short circuit, overload due to the functions built into U1 (LNK304). Since the chip is powered directly from its drain pin, no separate power winding is required.

In switching power supplies, voltage stabilization is provided through negative feedback. Feedback allows you to maintain the output voltage at a relatively constant level, regardless of fluctuations in the input voltage and load. Feedback can be organized in a variety of ways. In the case of impulse sources with galvanic isolation from the supply mains, the most common methods are the use of communication through one of the output windings of the transformer or using an optocoupler. Depending on the magnitude of the feedback signal (depending on the output voltage), the duty cycle of the pulses at the output of the PWM controller changes. If decoupling is not required, a simple resistive voltage divider is usually used. Thus, the power supply maintains a stable output voltage.

Unlike traditional linear power supplies, which assume the damping of excessive unstabilized voltage on a linear through 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 the conversion of accumulated energy into constant pressure. There are three typical schemes for constructing pulsed power supplies (see Fig. 3.4-1): step-up (output voltage is higher than input), step-down (output voltage is lower than input) 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 of connecting the inductance, otherwise, the principle of operation remains unchanged, namely.

A key element (usually bipolar or MOS transistors are used), operating at a frequency of the order of 20-100 kHz, periodically for a short time (no more than 50% of the time) is applied

gives the inductor the full input unregulated voltage. impulse current. flowing through the coil, ensures the accumulation of energy in its magnetic field 1/2LI^2 on 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 and then rectified), the output smoothing filter capacitor ensures that the output voltage and current are constant. Stabilization of the output voltage is provided by automatic adjustment of the width or frequency of the pulses on the key element (the feedback circuit is designed to monitor the output voltage).

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

An important technological advantage of pulsed IP is the possibility of building on their basis small-sized network IP with galvanic isolation from the network to power a wide variety of equipment. Such IPs are built without the use of a bulky low-frequency power transformer according to the high-frequency converter circuit. This is, in fact, a typical circuit of a pulsed power supply with a voltage reduction, where a rectified mains voltage is used as an 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 impulse 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 this, as a rule, one can observe impressive pyrotechnic effects). 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 a network pulsed IP, since many elements of such circuits are under high voltage.

3.4.1 Efficient Low Sophistication Switching Regulator

On the element base, similar to that used in the linear stabilizer described above (Fig. 3.3-3), you can build a switching voltage regulator. 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 a typical scheme with a voltage drop (Fig. 3.4-1a).

When first turned on, when the capacitor C4 is discharged and a sufficiently powerful load is connected to the output, the 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 resistance L1 is large and a sufficiently large current flows through the transistor. The voltage drop across R5 opens the main key element - transistor VT2. Current. growing in L1, charges C4, while writing through the feedback on R8

before the stabilizer and the 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 at a frequency of 20-30 kHz.

Chain R3. R4, C2 will set the output voltage level. It can be smoothly adjusted within a small range, from Uct DA1 to Uin. However, if Vout is raised close to Vin, there is some instability at maximum load and an increased level of ripple. To suppress high-frequency ripples, a 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 supplied with small radiators. The input voltage must not exceed 30 V, which is the maximum for KR142EN8 stabilizers. Rectifier diodes should be used for a current of at least 3 A.

3.4.2 Uninterruptible power supply device based on switching regulator

On fig. 3.4-3, a device for uninterruptible power supply of security and video surveillance systems based on a switching stabilizer combined with a charger is proposed for consideration. The stabilizer includes protection systems against overload, overheating, output surges, short circuits.

The stabilizer has the following parameters:

Input voltage, Vvx - 20-30 V:

Output stabilized voltage, Uvyx-12V:

Rated load current, Iload rated -5A;

Current of operation of system of protection against an overload, Izasch - 7A;.

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

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

Ripple level. Uppulse - 100 mV

Temperature of operation of system of protection against an overheat, Тzasch - 120 With;

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

The principle of operation of the switching stabilizer in the described device is the same as that of the stabilizer presented above.

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

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

The voltage sensor is assembled on a divider R13, R15. resistance R15 sets the exact level of operation of overvoltage protection (13 V). When the voltage at the output of the stabilizer is exceeded (in the event of the failure of the last one), relay S1 disconnects the load from the stabilizer and connects it to the battery. In the event of a power failure, 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 circuit, designed for the maximum current consumption.

3.4.3 Power supplies based on a 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 way out is to use a power supply based on high-voltage high-frequency pulse converters. which are connected to the ~220 V network without the use of an overall low-frequency step-down transformer and can provide high power with small dimensions and heat dissipation.

The block diagram of a typical pulse converter powered by an industrial network is shown in Figure 34-4.

The input filter is designed to prevent the penetration of impulse noise into the network. Power switches ensure the supply of high voltage pulses to the primary winding of a high-frequency transformer (single and

duplex circuits). The frequency and duration of the pulses are set by a controlled generator (usually pulse width control is used, less often - frequency). Unlike low-frequency sine-wave transformers, pulsed power supplies use wideband devices to provide efficient power transfer 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 feature of the output rectifier is the use of not ordinary power diodes, but high-speed Schottky diodes, which is due to the high frequency of the rectified voltage. The output filter smoothes the output voltage ripple. The feedback voltage is compared with the reference voltage and then controls the generator. Pay attention to the presence of galvanic isolation in the feedback circuit, which is necessary if we want to provide isolation of the output voltage from the network.

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

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

On fig. 3.4-5 shows a schematic diagram of a classic IP based on a high-frequency converter. The filter, consisting of capacitors C1, C2, C3 and chokes L1, L2, serves to protect the power supply from high-frequency interference from the converter. The generator is built according to a self-oscillating circuit and is combined with a key stage. The key transistors VT1 and VT2 operate in antiphase, opening and closing in turn. Starting the generator and reliable operation is ensured by the VT3 transistor, which operates in the avalanche breakdown mode. When the voltage on C6 rises 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. Choke and transformer data: L1-1. L2 is wound on rings made of ferrite 2000NM K12x8x3 in two wires with a PELSHO 0.25 wire: 20 turns. TP1 - on two rings put together, ferrite 2000NN KZ 1x18.5x7;

winding 1 - 82 turns with wire PEV-2 0.5: winding II - 25 + 25 turns with wire PEV-2 1.0: winding III - 2 turns with wire PEV-2 0.3. 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 around the entire 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, the number of turns is 30. KT809A can be used as key transistors VT1, VT2. KT812, KT841.

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

The described circuit has significant drawbacks due to the desire to minimize the number of components used. This is a low "level of output voltage stabilization, and unstable unreliable operation, and low output current. However, it is quite suitable for powering simple structures of different power (when using appropriate components), such such as: calculators, callers, lighting fixtures, etc.

Another IP circuit based on a high-frequency pulse converter is shown in fig. 3.4-6. The main difference between this circuit and the standard structure shown in Fig. 3.4-4 is the lack of a feedback loop. In this regard, the voltage stability at the output windings of the RF transformer Tr2 is quite low and the use of secondary stabilizers is required (the circuit uses universal integrated stabilizers on the KR142 series ICs).

3.4.4 Switching regulator with a key MIS transistor with current sensing.

Miniaturization and increase in efficiency in the development and design of switching power supplies is promoted 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 an insulated gate, integrated circuits for controlling key elements. All these elements are available on the domestic market and can be used in the design of high-efficiency power supplies, converters, ignition systems for internal combustion engines (ICE), fluorescent lamp start systems (LDS). Of great interest to developers can also be a class of power devices called HEXSense - MIS transistors with current sensing. They are ideal switching elements for ready-to-operate switching power supplies. The ability to read the current of the switching transistor can be used in pulsed power supplies for the current feedback required by the PWM controller. This achieves a simplification of the design of the power supply - the exclusion of current resistors and transformers from it.

On fig. 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 VDC:

Load current: 4.8 A:

Switching frequency: 110 kHz:

Efficiency at full load : 78%;

Efficiency at 1/3 load: 83%.

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

The key transistor control signal 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 DA1 is provided by the chain VD5, C5, C6, R6. Resistor R6 is designed to supply voltage during the start of the generator; subsequently, voltage feedback is activated through LI, VD5. This feedback is obtained from an additional winding in the output choke, which operates in flyback mode. In addition to powering the generator, the feedback voltage through the chain VD4, Cl, Rl, R2 is fed to the voltage feedback input DA1 (pin 2). Through R3 and C2 a compensation is provided which guarantees the stability of the feedback loop.

On the basis of this scheme, it is possible to build switching stabilizers with other output parameters.

SWITCHED POWER SUPPLY

It is known that power supplies are an integral part of radio engineering devices, which are subject to a number of requirements; they are a complex of elements, devices and devices that generate electrical energy and convert it to the form necessary to ensure the required operating conditions for 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 (electrical 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 (rectifier); AC voltage value converters (transformers); DC to AC voltage converters (inverters).

The share of power supply sources currently accounts for 30 to 70% of the total mass and volume of REA equipment. Therefore, the problem of creating a miniature, lightweight and reliable power supply device with good technical and economic performance is important and relevant. This work is devoted to the development of a secondary power source (SSE) with minimal weight and size and high technical characteristics.

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

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

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

The paper considers an SMPS with a power of 800 W, which differs from other SMPSs by using field-effect transistors and a transformer with a primary winding with an average output in the converter. The FETs provide higher efficiency and reduced high-frequency noise, while the mid-terminal transformer delivers half the current through the key transistors and eliminates the need for an isolation transformer in their gate circuits.

On the basis of the selected circuit diagram, a design was developed and a prototype of the SMPS was manufactured. The whole structure is presented as a module installed in an aluminum case. After initial tests, a number of shortcomings were revealed: a noticeable heating of the radiators of key transistors, the difficulty of removing heat from powerful domestic resistors, and large dimensions.

The design has been improved: 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; the use of 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 efficient cooling. As a result of refinement, the dimensions of the IPP decreased three times and the shortcomings identified during the initial tests were eliminated. The modified sample has the following characteristics: supply voltage Upit=~180-240 V, frequency fwork=90 kHz, output power Pp=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 frequency power amplifier, which is part of a high-power home sound reproducing system. The creation of a home sound reproducing system began with the choice of a UMZCH circuit design. For this, an analysis of the circuit design of sound-reproducing devices was carried out. The choice was stopped on the UMZCH high fidelity scheme.

This amplifier has very high performance, contains overload and short circuit protection devices, devices for maintaining the zero potential of a constant voltage at the output, and a device for compensating the resistance of the wires connecting the amplifier to the acoustics. Despite the fact that the UMZCH circuit has been published for a long time, radio amateurs to this day repeat its design, references to which can be found in almost any literature relating to 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 watts. Therefore, the next stage in the assembly of the UMZCH was the development and assembly of a power supply design that provides an output power of at least 800 W, small dimensions and weight, reliability in operation and protection against overload and short circuits.

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

Research of sources of secondary power supply. Power supplies are divided into two groups: primary and secondary power supplies.

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 transducers (transformers);
• * DC-to-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 drawback of power MTs, made according to the traditional classical scheme, is their large weight and size characteristics, as well as a significant effect on other REE devices of a strong magnetic field of power transformers. The problem of SMPS is the creation of high-frequency interference by them, and as a result of this - electromagnetic incompatibility with some types of REA. The analysis showed that IIP meet the requirements most fully, which is confirmed by their wide use in REA.

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

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

The disadvantage of such a circuit design 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-stroke high-frequency converters. The efficiency of the former is lower than the latter, so it is not advisable to design single-cycle UPSs with a power of more than 40 ... 60 W. Push-pull converters allow you to get much more output power with high efficiency. They are divided into several groups, characterized by the method of excitation of the output key transistors and the circuit for including them in the circuit of the primary winding of the converter transformer. If we talk about the method of excitation, then two groups can be distinguished: 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 unreasonably increases, so they are of little use for such power supplies. Externally excited converters are well suited for high power UPS applications and sometimes require little or no maintenance. As for connecting the key transistors to the transformer, there are three schemes here: the so-called half-bridge (Fig. 1, a), bridge (Fig. 1, b). To date, the most widely used half-bridge converter.

It requires two transistors with a relatively low 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 opening the key transistor, the amplitude of the voltage pulse on the winding reaches the value Upit / 2 - Uke nac. The bridge converter is similar to the half-bridge one, but in it the capacitors are replaced by transistors VT3 and VT4 (Fig. 1b), which open diagonally in pairs. 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 the transistors VT1-VT4. The voltage amplitude on the primary winding of the transformer in this case reaches the value Upit - 2Uke us.

Of particular note is the converter according to the scheme in Fig. 1c, which is distinguished by the highest efficiency. This is achieved by reducing the primary winding current and, as a result, reducing the power dissipation in key transistors, which is extremely important for powerful UPSs. The amplitude of the voltage 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 scheme in Fig. 1c, it is necessary to use transistors with a high Uke max value. Since the end of the upper (according to the scheme) half of the primary winding is connected to the beginning of the lower one, when current flows in the first of them (VT1 is open), a voltage is created in the second that is equal (in absolute value) to the amplitude of the voltage 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 must be greater than 2Upit. In the proposed UPS, a push-pull converter with a transformer is used, the primary winding of which has an average output. It has high efficiency, low ripple and weakly radiates interference into the surrounding space.

STABILIZATION OF OUTPUT VOLTAGES
PULSE POWER SUPPLY

THE ARTICLE IS PREPARED ON THE BASIS OF THE BOOK BY A. V. GOLOVKOV and V. B LYUBITSKY "POWER SUPPLIES FOR SYSTEM MODULES OF THE TYPE IBM PC-XT/AT" PUBLISHING HOUSE "LAD i N"

The output voltage stabilization circuit in the UPS class under consideration is a closed automatic control loop (Fig. 31). This loop includes:
control scheme 8;
matching preamp stage 9;
control transformer DT;
power stage 2;
power pulse transformer RT;
rectifier block 3;
choke interchannel communication 4;
filter unit 5;
feedback voltage divider 6;
reference voltage divider 7.
The control circuit 8 includes the following functional units:
error signal amplifier 8.1 with correction circuit Zk;
PWM comparator (modulator) 8.2;
sawtooth voltage generator (oscillator) 8.3;
reference stabilized voltage source Uref 8.4.
During operation, the error signal amplifier 8.1 compares the output signal of the voltage divider b with the reference voltage of the divider 7. The amplified error signal is fed to the pulse-width modulator 8.2, which controls the pre-terminal stage of the power amplifier 9, which, in turn, supplies a modulated control signal to the power stage converter 2 via control transformer DT. The power stage is powered by a transformerless circuit. The alternating voltage of the supply network is rectified by the mains rectifier 1 and fed to the power stage, where it is smoothed 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. Pulse-width modulator 8.2 converts the analog control signal into a pulse-width modulated signal with a variable pulse duty cycle. In the considered class of UPS, the modulator circuit compares the signal coming from the output of the error signal amplifier with the sawtooth voltage, which is obtained from a special generator 8.3.

Figure 31. The control circuit of a typical switching power supply based on the TL494 control chip.

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

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

Figure 34. Appis UPS output voltage level adjustment.

Figure 35. GT-200W UPS output voltage level adjustment.

However, the most common case is when there is no adjustment that allows you to influence the output voltages of the unit. In this case, the voltage at any of the inputs 1 or 2 is chosen arbitrarily in the range from +2.5 to +5 V, and the voltage at the remaining input is selected using a high-ohm shunt resistor so that the unit produces the output voltages specified in the passport in the nominal load mode. Rice. 35 illustrates the case of selection of the reference voltage level, fig. 34 - shows the case of selecting the level of the feedback signal. It was previously noted that the value of 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 the DA3 amplifier).
However, the maximum value of the gain DA3 is limited by the condition of stability. Since both the UPS and the load contain reactive elements (inductance or capacitance) that accumulate energy, in transient conditions, energy is redistributed between these elements. This circumstance can lead to the fact that, under 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 overshoot in the transient mode will reach unacceptable values.

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

On fig. 36 shows the transients of the output voltage during an abrupt change in the load current and input voltage. The UPS operates stably if the output voltage again assumes a steady value after the cessation of the disturbance that brought it out of its original state (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. The transient process, depending on the parameters of the UPS, is oscillatory or aperiodic, while the output voltage of the UPS has a certain overshoot value and the transient 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 inertia control scheme will continue its influence 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 reverses the output voltage again, and so on. In order to ensure the stability of the UPS output voltage control loop with a minimum duration of the transient process, the amplitude-frequency characteristic of the error amplifier DA3 is corrected. This is done using RC circuits, included as a negative feedback circuit, covering the DA3 amplifier. Examples of such corrective chains are shown in fig. 38.

Figure 38. Configuration examples of corrective RC circuits for voltage error amplifier DA3.

To reduce the level of interference, aperiodic RC circuits are installed on the secondary side of the switching power supply. Let us dwell in more detail on the principle of their action.
The transient process of current through the rectifier diodes at the moments of switching occurs in the form of shock excitation (Fig. 39, a).

Figure 39. Reverse recovery diode voltage timing diagrams:
a) - without RC chain; b) - in the presence of an RC chain.

MAIN PARAMETERS OF SWITCH POWER SUPPLY FOR IBM		The main parameters of switching power supplies are considered, the pinout of the connector is given, the principle of operation from a mains voltage of 110 and 220 volts,
		The TL494 microcircuit, the switching circuit and the use cases for controlling the power switches of switching power supplies are described in detail.
CONTROL OF THE POWER KEYS OF THE SLEEPING POWER SUPPLY WITH THE HELP OF TL494		The main methods of controlling the basic circuits of power transistors of switching power supplies, options for constructing secondary power rectifiers are described.
STABILIZATION OF OUTPUT VOLTAGES OF PULSE POWER SUPPLY		The options for using error amplifiers TL494 for stabilizing output voltages are described, the principle of operation of the group stabilization choke is described.
PROTECTION SCHEMES		Several options for constructing systems for protecting impulse power supplies from overload are described.
"SLOW START" SCHEME		The principles of soft start formation and POWER GOOD voltage generation are described.
EXAMPLE OF CONSTRUCTION OF ONE OF THE PULSED POWER SUPPLY		A complete description of the circuit diagram and its operation of a switching power supply