Operating principle of switching power supplies. Switching power supplies. Types and work. Features and Application

Types of switching power supplies

Switching or switching power supplies are currently no less widespread than linear voltage stabilizers. Their main advantages are: high efficiency, small dimensions and weight, high power density. This became possible thanks to the use of a key operating mode of power elements. In the switching mode, the operating point most of the time is in the saturation region or the cutoff region of the current-voltage characteristic, and the active (linear) mode zone passes at high speed in a very short switching time. In the saturation state, the voltage across the transistor is close to zero, and in the cutoff mode there is no current, due to which the losses in the transistor are quite small. Therefore, the average power dissipated in the switching transistor over the switching period is much less than in linear regulators. Small losses in power switches lead to a reduction or complete elimination of radiators.

The improvement in the weight and size characteristics of power supplies is due, first of all, to the fact that a power transformer operating at a frequency of 50 Hz is excluded from the power supply circuit. Instead, a high-frequency transformer or inductor is introduced into the circuit, the dimensions and weight of which are much smaller than the low-frequency power transformer.

The disadvantages of switching power supplies include: circuit complexity, the presence of high-frequency noise and interference, increased output voltage ripple, big time exit to operating mode. Comparative characteristics of conventional (i.e., with a low-frequency power transformer) and switching power supplies are given in Table 2.1.

A comparison of these characteristics shows that the efficiency of switching power supplies increases compared to conventional (linear) ones in the ratio of 1:2, and the power density in the ratio of 1:4. When the conversion frequency increases from 20 kHz to 200 kHz, the power density increases in the ratio 1:8, i.e. almost twice. Switching power supplies also have longer time hold output voltage at sudden shutdown networks.

This is due to the fact that the network rectifier of the pulse source uses high-capacity capacitors with high operating voltage (up to 400 V). In this case, the size of the capacitors increases in proportion to the product CU, and the energy of the capacitor is proportional to CU 2. This capacitor energy is enough to maintain the power supply in working condition for approximately 30 ms, which is very important for saving information in computers during a sudden power outage.

Table 2.1 – Comparison of pulsed and linear sources

At the same time, the output voltage ripple in switching power supplies is greater than in linear ones, which is due to the difficulty of suppressing short pulses when operating a pulse converter. Other characteristics of these sources are almost the same.

Structure of construction of IVEP. With all the variety of structural diagrams in Figures 2.1...2.8, the presence of a power cascade is mandatory,

carrying out the transformation DC voltage into another constant, we will conventionally assume that pulse converters implement the function of electrical isolation (galvanic isolation) of input and output circuits, while pulse stabilizers do not. The functional purpose of the power stages of converters and stabilizers is the same.

Compensating type IVEPs, made with feedback, are widely used. Figure 2.1, Power cascade 3, to the control input of which a sequence of pulses with certain time parameters is supplied, carries out pulsed conversion of direct current voltage from the primary source Ep into output voltage Un (thick lines show power circuits IVEP).

In the general case, one PVEP can have several output circuits with voltages Un. Pulse amplifier 2 can perform not only the function of amplifying control pulses in power for transistors 3, but also the functions of forming pulses: it performs temporary separation of pulses, for example, for push-pull voltage converters, it generates short control pulses for circuits 3 with current transformers or special types of power transistors and etc..

Figure 2.1 - Block diagram of a pulsed compensation IVEP

Pulses synchronizing the operation of the IVEP are generated by modulator 1. The output DC voltage Un is supplied to the input of comparison circuit 4, where it is compared with the reference voltage Uop. The mismatch (error) signal is supplied to the input of the modulator, which sets the timing parameters of the synchronizing pulses. An increase or decrease in voltage Un leads to a change in the mismatch signal at output 4 and the timing parameters of synchronizing pulses at input 1, which causes the restoration of the previous value of voltage Un, i.e. its stabilization. Thus, the IVEP, made according to the circuit of Figure 2.1, is a stabilizing pulse voltage converter of the compensation type, maintaining the output voltage unchanged when changing the output current In, input voltage Ep, ambient temperature and the influence of other destabilizing factors.

Let's consider an IVEP with invariant (sometimes called parametric) stabilization of the output voltage in Figure 2.2.

The essence of this stabilization method is that when exposed to any factor that can cause the voltage value Un to deviate from the specified value, the timing parameters of the control pulses change, leading to Un to remain unchanged. However, unlike compensating stabilizers, the change in the timing characteristics of the control pulses in this case depends on the magnitude of the deviation of the destabilizing effect itself.

Figure 2.2 - Block diagram of a pulsed parametric IVEP

In Figure 2.2 there is a generator providing such functional dependence, designated 1. Here the dashed line shows the connection between Ep and the control input of the generator to ensure the law of invariance of Un from Ep.

Secondary power supplies without output voltage stabilization are made according to the circuit shown in Figure 2.3. Pulse generator 1 produces pulses with constant time parameters. Obviously, for the voltage Un to remain constant, it is necessary to have a stable voltage Ep.

Figure 2.3 - Block diagram of an unstabilized IVEP

The IVEP presented in Figure 2.4 carries out double conversion of direct current energy. The first power stage 1, as a rule, a pulse stabilizer converts the voltage Ep into a stabilized voltage Ep1. The second power stage 2 provides galvanic isolation of voltage and, if necessary, additional stabilization of Un. In the general case, compensation and invariant stabilization can be carried out not only in 1, but also in both cascades, which is shown by the dashed lines of the negative circuits feedback. Power cascades 1 and 2 can be different versions of the power cascades of any of the IVEP.

Figure 2.4 - Block diagram of double conversion IVEP

The block diagram of a block PVEP with a stepwise increase in power is shown in Figure 2.5. To increase the output power, parallel connection of stages 3...5 was used.

Figure 2.5 - Block diagram of the modular IVEP

Since the parallel inclusion of traditional PVEPs without the use of special measures to equalize the power of each of them is impossible, then in this case The principle of multiphase construction of the IEVP was used. It lies in the fact that the MF modulator-shaper not only converts the CC mismatch signal into the corresponding pulse sequence, but also performs the function of phase distribution of pulse signals over several power stages. As a result of such work of the IEP, the time stages of the open and closed states of the power switches of the transistors of various power cascades turn out to be separated in time.

All considered IVEP schemes can be compared according to various parameters - stability of output voltages, weight and size characteristics, energy indicators, manufacturability and cost, as well as the possibility of unification. At the same time, the same scheme, depending on the specified requirements, may turn out to be suboptimal in terms of a set of indicators. It is impossible in advance to select a specific scheme as the most effective, so it is advisable to consider the most general properties of the given schemes. We will assume that the reliability, energy and weight-size indicators of the power stages are the same and equally depend on the power, output voltage and conversion frequency.

The highest stability of the output voltage is possessed by the IVEP, implemented according to the circuit in Figure 2.1, since the feedback affecting the timing parameters of the control pulses is taken directly from the output of the IEVP. The IVEP circuit shown in Figure 2.4 also has high stability of the output voltage, if the feedback to the CC is taken from the output - Un. IVEP, made according to the scheme of Figure 2.2, has slightly worse stability, but a more simple control circuit. However, this does not take into account the change in voltage drop across the inductive and active elements 3 when the load current In changes. Destabilizing changes in voltage Ep can be compensated by introducing an additional, direct connection (dashed line). There are IVEPs with invariant stabilization of not only the disturbing influence of voltage Ep, but also the disturbing influence of load current In, ambient temperature, etc., but they are not widely used. IVEPs made according to the scheme in Figure 2.3 have the worst stability due to the absence of any feedback when exposed to destabilizing factors. The IVEP circuit in Figure 2.4, as indicated above, can in principle have high stability of the output voltage, however, in the absence of invariant or compensatory regulation channels, its performance is identical to the circuit in Figure 2.3.

The use of IVEP circuits in Figure 2.2 is preferable at relatively high voltages Un, many times greater than the voltage drop across power switches 3, since obtaining the required function 1, which takes into account changes in the voltage drop across these switches with fluctuations in load current and ambient temperature, is difficult.

Thus, in cases where the output voltage of the PVEP is small (does not exceed a few volts) and there are significant changes in the load current, ambient temperature and voltage Ep, it is necessary to use PVEP, made according to block diagrams (see Figures 2.2,2.4,2.5) with a compensatory regulation principle.

The circuit in Figure 2.2 can also be used to satisfy the compromise requirements for stability of the output voltage and simplicity of the control circuit of the IVEP. If the primary voltage is stable and changes in the voltage drop on the internal elements of the SC do not noticeably affect the accuracy of maintaining the voltage Un, simpler PVEPs are used (Figures 2.3 and 2.5).

The above IVEP circuits can be used in a wide range of primary voltages - from unity to hundreds of volts. However, for high primary voltages, the IVEP circuit of Figure 2.4 may be appropriate, in which double conversion of electrical energy makes it possible to reduce the high primary DC voltage Ep to Ep1 using the pulse stabilizer SKI and use it as the primary for the pulse converter SK2. In this case, the SK2 converter, as a more complex device compared to SCI, operates in lightweight electrical modes, which can reduce the number of elements, increase operational reliability and improve the energy performance of the converter.

Large-sized, most material-intensive and difficult to microminiaturize elements are chokes and transformers. In IVEP schemes it is necessary to strive to minimize their number. In the IVEP circuit of Figure 2.4, double energy conversion requires two power stages with the fundamentally necessary inductive elements.

A block increase in output power is required for the construction of various power supply systems, which must be carried out on the basis of the same type, unified PVEP. In this case, the development and production of IVEPs that supply electronic equipment, it is advisable when using blocks of the same type with the possibility parallel connection to obtain the required total output power. As a result, it is possible to obtain an economic effect. In this case, one of the main goals of developing an IVEP is the selection of a discrete power value of a single unit, which must satisfy all the technical and economic requirements of the existing power supply systems. Another advantage of block (multiphase) converters is a reduction in the total capacitance of the output filter capacitors, which is explained by the time distribution of energy transfer processes to the output of individual power stages. In addition, multiphase converters allow various options to be implemented complex systems power supplies, consisting of identical unified blocks.

Figure 2.6 shows a diagram of an IVEP containing an unregulated mains rectifier 1 and a rectified mains voltage converter. The converter consists of an adjustable inverter 2 operating at a higher frequency (usually 20...100 kHz), a transformer rectifier unit 3 and a high-frequency filter 4. A control circuit 5 is used to stabilize the output voltage.

Figure 2.6 - Block diagram of a pulsed power source with a controlled inverter

The control circuit compares the output voltage Un and the voltage of the reference source 6. The difference between these voltages, called an error signal, is used to adjust the frequency of the adjustable inverter (f = var) or the duty cycle of pulses at a constant frequency (g = var). A converter made on the basis of a single-cycle transformer inverter is called a transformer single-cycle converter - CURRENT. A converter made on the basis of a push-pull transformer inverter is called a transformer push-pull converter - TDK.

Figure 2.7 shows a diagram of an IVEP with a regulated mains rectifier 1 and an unregulated inverter 2. The remaining nodes of this diagram have the same purpose as the previous diagrams. A distinctive feature of this block diagram is the use of an unregulated inverter (NI). Stabilization of the output voltage in this circuit is ensured by regulating the voltage at the input of the converter using 1, which is usually performed on phase-controlled thyristors.

Figure 2.7 - Block diagram of a pulsed power source with an adjustable network rectifier

For the circuit shown in Figure 2.6, it is characteristic that the inverter must be designed to operate from the rectified mains voltage, which has maximum value about 311V for single-phase network and about 530 V for three-phase network. In addition, changing the frequency or duty cycle of the pulses of inverter 2 leads to worse filtering of the output voltage. As a result, the weight and size parameters of filter 4 deteriorate, since its parameters are calculated based on the minimum duty cycle of the pulses g min, provided that the current in the load is continuous.

The positive properties of the circuit in Figure 2.7 is the combination of the function of voltage conversion and stabilization of the output voltage Un. This makes it possible to simplify the control scheme 5, since the number of managed keys is reduced. In addition, the presence of a pause allows you to eliminate through currents in the inverter switches. The advantage of the circuit is also the ability to ensure the operation of the inverter at a reduced input voltage (usually it is reduced by 1.5...2 times, that is, up to 130...200V). This greatly facilitates the operation of the transistor inverter switches. Another advantage of this circuit is that the inverter operates with a maximum duty cycle g max of pulses, which greatly simplifies filtering the output voltage. A study of the efficiency and power density of both circuits showed that these indicators differ slightly.

Schemes of multi-channel IVEP with unregulated rectifier 1 are shown in Figures 2.8 and 2.9. In the circuit in Figure 2.8, an unregulated inverter 2 and individual stabilizers 5...7 are used, in separate channels. This block diagram can be used with a small number of output channels. As the number of output channels increases, the circuit becomes uneconomical.

Figure 2.8 - Block diagram of a multi-channel IVEP with individual stabilization

The circuit shown in Figure 2.9 operates on the principle of group stabilization of the output voltage. To do this, it uses an adjustable inverter, which is controlled by the voltage of the most powerful channel. In this case, the stabilization of output voltages in other channels worsens, since they are not covered by negative feedback. To improve voltage stabilization in other channels, you can use additional individual stabilizers, just as in the circuit of Figure 2.8.

Figure 2.9 - Block diagram of IVEP with group stabilization

PULSE POWER SUPPLIES

Unlike traditional linear power supplies, which involve extinguishing excess unstabilized voltage on a pass-through linear element, pulse 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 (the output voltage is higher than the input voltage), step-down (the output voltage is lower than the input voltage) and inverting (the 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 fact is that, in this case, in addition to the load itself, there are no power elements in the circuit that dissipate significant power. Key transistors operate in 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 high level 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” (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. Schematic diagram such a stabilizer is shown in Fig. 3.4-2. The stabilizer is assembled according to a standard voltage reduction circuit (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 inductive reactance 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 when maximum load and increased pulsation levels. To suppress high-frequency pulsations, filter L2, C5 is included at the output of the stabilizer.

The scheme is quite simple and most effective for this level 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 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 battery power, 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 of a 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 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 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(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, the number of turns is 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-high-speed diodes, field-effect transistors with an insulated gate, integrated circuits for controlling 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 ready-to-control switching power supplies. 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.

Introduction

Switching power supplies are now confidently replacing outdated linear ones. Reason - inherent in these power sources high performance, compactness and improved stabilization performance.

With the rapid changes that the power supply principles of electronic equipment have undergone over the past lately, information about the calculation, construction and use of switching power supplies is becoming more and more relevant.

Recently, switching power supplies have gained particular popularity among specialists in the field of electronics and radio engineering, as well as in industrial production. There has been a tendency to abandon standard bulky transformer units 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 further development specified area of ​​power supplies.

Operating principle of switching power supplies

Switching power supply is inverter system. In switching power supplies, the AC input voltage is first rectified. The resulting DC voltage is converted to square pulses increased frequency and a certain duty cycle, either supplied to a transformer (in the case of pulse power supplies with galvanic isolation from the supply network) or directly to the output low-pass filter (in pulse 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 to transmit equivalent power decrease. In most cases, such a core can be made of ferromagnetic materials, in contrast to the cores of low-frequency transformers, for which electrical steel is used.

Figure 1 - Block diagram of a switching power supply

The mains voltage is supplied to the rectifier, after which it is smoothed 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 switch. The control device ensures periodic switching on and off of the transistor. To reliably start the power supply, a master oscillator made on a microcircuit is used. The pulses are supplied to the base of the key transistor and cause the start of the autogenerator operating cycle. The control device is responsible for monitoring the output voltage level, generating an error signal and, often, directly controlling the key. The master oscillator microcircuit is powered by a chain of resistors directly from the input of the storage capacitor, 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 sooner the power transistor will turn off and the less energy will accumulate in the transformer, which will stop the increase in voltage at the output of the source. The operating mode of the power supply has arrived, where an important role is played by the optocoupler, as a regulator and manager of the output voltages.

The specification of an industrial power supply is more stringent than that of a regular household power supply. This is expressed not only in the fact that there is a high voltage at the input of the power source three phase voltage, but also that industrial power supplies must remain operational even 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 more phases.

Figure 2 - Schematic diagram of a switching power supply.

The scheme works as follows. Three-phase input can be made via three-wire, four-wire circuit 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 tripping makes the use of separate fuse links unnecessary. 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 the U1 chip opens, the source potential of Q1 decreases, the gate current is provided by resistors R6, R7 and R8, respectively, the capacitance of the transitions VR1 ... VR3 unlocks Q1. Zener diode VR4 limits the source-gate voltage applied to Q1. When MOSFET U1 turns off, the drain voltage is limited to 450 volts by the limiter 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 across Q1 and U1.

The absorption circuit VR5, D9, R10 absorbs the excess voltage on the primary winding resulting from the induction leakage of the transformer during the reverse stroke.

Output rectification is carried out by diode D1. C2 - output filter. L2 and C3 form the 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 the current through the optocoupler transistor U2. When this current exceeds the threshold at the FB pin of U1, the next duty cycle is skipped. The specified level of output voltage is maintained by regulating the number of missed and completed 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 provides bias to VR6.

This circuit is protected from feedback loop breakage, output short circuit, and overload thanks to the functions built into U1 (LNK304). Since the microcircuit is powered directly from its drain pin, a separate power winding is not required.

In switching power supplies, voltage stabilization is ensured 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 size. Feedback can be organized in different ways. In the case of pulsed sources with galvanic isolation from the supply network, the most common methods are to use 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, then, as a rule, a simple resistive voltage divider is used. Thus, the power supply maintains a stable output voltage.

The principle of realizing secondary power through the use of additional devices that provide energy to circuits has been used for quite a long time in most electrical appliances. These devices are power supplies. They serve to convert voltage to the required level. PSUs can be either built-in or separate elements. There are two principles for converting electricity. The first is based on the use of analog transformers, and the second is based on the use of switching power supplies. The difference between these principles is quite big, but, unfortunately, not everyone understands it. In this article we will figure out how a switching power supply works and how it differs so much from an analog one. Let's get started. Let's go!

Transformer power supplies were the first to appear. Their operating principle is that they change the voltage structure using a power transformer, which is connected to a 220 V network. There, the amplitude of the sinusoidal harmonic is reduced, which is sent further to the rectifier device. Then the voltage is smoothed by a parallel connected capacitor, which is selected according to the permissible power. Voltage regulation at the output terminals is ensured by changing the position of trimming resistors.

Now let's move on to pulse power supplies. They appeared a little later, however, they immediately gained considerable popularity due to a number of positive features, namely:

• Availability of packaging;
• Reliability;
• Possibility to expand the operating range for output voltages.

All devices that incorporate the principle switching power supply, are practically no different from each other.

The elements of a pulse power supply are:

• Linear power supply;
• Standby power supply;
• Generator (ZPI, control);
• Key transistor;
• Optocoupler;
• Control circuits.

To select a power supply with a specific set of parameters, use the ChipHunt website.

Let's finally figure out how a switching power supply works. It uses the principles of interaction between the elements of the inverter circuit and it is thanks to this that a stabilized voltage is achieved.

First, the rectifier receives a normal voltage of 220 V, then the amplitude is smoothed using capacitive filter capacitors. After this, the passing sinusoids are rectified by the output diode bridge. Then the sinusoids are converted into high-frequency pulses. The conversion can be performed either with galvanic separation of the power supply network from the output circuits, or without such isolation.

If the power supply is galvanically isolated, then the high-frequency signals are sent to a transformer, which performs galvanic isolation. To increase the efficiency of the transformer, the frequency is increased.

The operation of a pulse power supply is based on the interaction of three chains:

• PWM controller (controls pulse width modulation conversion);
• A cascade of power switches (consists of transistors that are turned on according to one of three schemes: bridge, half-bridge, with a midpoint);
• Pulse transformer (has primary and secondary windings, which are mounted around the magnetic core).

If the power supply is without decoupling, then the high-frequency isolation transformer is not used, and the signal is fed directly to the low-pass filter.

Comparing switching power supplies with analog ones, you can see the obvious advantages of the former. UPSs have less weight, while their efficiency is significantly higher. They have a wider supply voltage range and built-in protection. The cost of such power supplies is usually lower.

Disadvantages include the presence of high-frequency interference and power limitations (both at high and low loads).

You can check the UPS using a regular incandescent lamp. Please note that you should not connect the lamp into the gap of the remote transistor, since the primary winding is not designed to pass D.C., therefore, under no circumstances should it be allowed to pass through.

If the lamp lights up, then the power supply is working normally, but if it doesn’t light up, then the power supply is not working. A short flash indicates that the UPS is locked immediately after startup. Very bright glow indicates a lack of output voltage stabilization.

Now you will know what the operating principle of switching and conventional analog power supplies is based on. Each of them has its own structural and operating features that should be understood. You can also check the performance of the UPS using a regular incandescent lamp. Write in the comments whether this article was useful to you and ask any questions you have about the topic discussed.

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 of a strong magnetic field on other electronic devices. 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. FETs provide higher efficiency and reduced high-frequency noise, and the mid-tap transformer provides half the current through the switching 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; 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 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 f operating = 90 kHz, output power P = 800 W, efficiency = 85%, weight = 2.1 kg, overall dimensions 145Х145Х80 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 protection devices against overload 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. Despite the fact that the UMZCH circuit was 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. Therefore, the next stage in the assembly of the UMZCH was the development and assembly of a power supply design that would provide 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: - voltage supply rectangular shape; complicated shape of the windings (midpoint terminals) and work on 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 such a circuit solution is 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 provide significantly higher output power with 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, 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 the power dissipation in key transistors, which is extremely important for powerful UPSs. 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.