What is a switching power supply and how does it differ from a regular analog one? Switching power supply

OUTPUT VOLTAGE STABILIZATION
SWITCH POWER SUPPLY

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Unlike traditional linear power supplies, which involve extinguishing excessive unstabilized voltage at the pass-through linear element, pulsed power supplies use other methods and physical phenomena to generate a stabilized voltage, namely: the effect of energy accumulation in inductors, as well as the possibility of high-frequency transformation and conversion of accumulated energy into constant voltage. There are three typical circuits for constructing pulsed power supplies: step-up (output voltage higher than input) Fig. 1,

Rice. 1. Boost switching power supply (Uout>Uin).

step-down (output voltage lower than input)

Rice. 2. Step-down switching power supply (Uout

Step-down switching power supply (Uout

Rice. 3. Inverting switching power supply (Uout

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 at short time(no more than 50% of the time) applies 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 the constancy of the output voltage and current. Output voltage stabilization is ensured automatic adjustment width or pulse repetition rate per key element(a feedback circuit is designed to monitor the output voltage).

This, although quite complex, scheme can significantly increase the efficiency of the entire device. The point is that, in in this case, except for the load itself, there are no power elements in the circuit that dissipate significant power. Key transistors operate in 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 IP are built without the use of bulky low-frequency power transformer according to the high-frequency converter circuit. This is, in fact, typical diagram pulse power supply with voltage reduction, where the rectified mains voltage is used as the input voltage, and a high-frequency transformer (small-sized and with high efficiency), 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.

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 constant tension. There are three typical circuits for constructing pulsed power supplies (see Fig. 3.4-1): step-up (output voltage is higher than input voltage), step-down (output voltage is lower than input voltage) and inverting (output voltage has the opposite polarity with respect to the input). As can be seen from the figure, they differ only in the way they connect the inductance; otherwise, the principle of operation remains unchanged, namely.

The key element (usually bipolar or MOS transistors are used), operating with a frequency of the order of 20-100 kHz, is periodically applied 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. Stabilization of the output voltage is ensured by automatic adjustment of the pulse width or frequency 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 of a high level of pulsed noise at the output, high complexity and low reliability (especially in handicraft production), the need to use expensive high-voltage high-frequency components, which in the event of the slightest malfunction easily fail “en masse” (with In this case, as a rule, impressive pyrotechnic effects can be observed). Those who like to delve into the insides of devices with a screwdriver and a soldering iron will have to be extremely careful when designing network pulse power supplies, since many elements of such circuits are under high voltage.

3.4.1 Efficient low-complexity switching regulator

On an element base similar to that used in the linear stabilizer described above (Fig. 3.3-3), it is possible to build a pulse voltage stabilizer. With the same characteristics, it will have significantly smaller dimensions and better thermal conditions. A schematic diagram of such a stabilizer is shown in Fig. 3.4-2. The stabilizer is assembled according to 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 the inductive reactance of L1 is large and enough flows through the transistor high current. The voltage drop across R5 opens the main key element - transistor VT2. Current. increasing in L1, charges C4, while through feedback on R8 the recording occurs

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

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

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

3.4.2 Uninterruptible power supply device based on a switching stabilizer

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

The stabilizer has the following parameters:

Input voltage, Uvx - 20-30 V:

Output stabilized voltage, Uvyx-12V:

Rated load current, Iload nom -5A;

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

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

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

Ripple level. Upulse - 100 mV,

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

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

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

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

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

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

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

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

In the manufacture of such IP, serious requirements arise for the components used (which increases their cost compared to traditional ones). Firstly, this concerns the operating voltage of the rectifier diodes, filter capacitors and key transistors, which should not be less than 350 V to avoid breakdowns. Secondly, high-frequency key transistors (operating frequency 20-100 kHz) and special ceramic capacitors(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 is given circuit diagram 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 over 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 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 of different power (if appropriate components are used), such like: calculators. Caller IDs. lighting devices, 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).

3.4.4 Switching stabilizer with a key MOS transistor with current reading.

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 elements are available on the domestic market and can be used in the design of highly efficient power supplies, converters, ignition systems for internal combustion engines (ICE), and lamp starting systems daylight(LDS). 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.

Switching power supply- This inverter system, in which the input alternating voltage is rectified, and then the resulting DC voltage is converted into pulses high frequency and set duty cycle, which are usually supplied to a pulse transformer.

Pulse transformers are manufactured according to the same principle as low-frequency transformers, only the core is not steel (steel plates), but ferromagnetic materials - ferrite cores.

Rice. How does a switching power supply work?

Switching Power Supply Output Voltage stabilized, this is done through negative feedback, which allows you to keep the output voltage at the same level even when the input voltage and load power at the output of the unit changes.

Reverse negative connection can be implemented using one of the additional windings in a pulse transformer, or using an optocoupler that is connected to the output circuits of the power source. The use of an optocoupler or one of the transformer windings allows for galvanic isolation from the alternating voltage network.

The main advantages of switching power supplies (SMPS):

• low weight of the structure;
• small sizes;
• high power;
• high efficiency;
• low cost;
• high stability;
• wide range of supply voltages;
• many ready-made component solutions.

The disadvantages of SMPS include the fact that such power supplies are sources of interference, this is due to the operating principle of the converter circuit. To partially eliminate this drawback, shielding of the circuit is used. Also, due to this drawback, in some devices the use of this type power supplies is not possible.

Pulse sources nutrition has become virtually an indispensable attribute of any modern household appliances consuming power from the network in excess of 100 W. Computers, televisions, and monitors fall into this category.

To create switching power supplies, examples of specific implementations of which will be given below, special circuit solutions are used.

So, to eliminate through currents through the output transistors of some switching power supplies, they use special form pulses, namely bipolar pulses rectangular shape, having a time interval between them.

The duration of this interval must be greater than the time of resorption of minority carriers in the base of the output transistors, otherwise these transistors will be damaged. The width of control pulses can be changed using feedback to stabilize the output voltage.

Usually, to ensure reliability, high-voltage transistors are used in switching power supplies, which, due to technological features, do not differ for the better (they have low frequencies switching, low current transfer coefficients, significant leakage currents, large voltage drops at the collector junction in the open state).

This is especially true for now outdated models of domestic transistors such as KT809, KT812, KT826, KT828 and many others. It is worth saying that in recent years a worthy replacement has appeared bipolar transistors, traditionally used in the output stages of switching power supplies.

These are special high-voltage field-effect transistors of domestic and, mainly, foreign production. In addition, there are numerous microcircuits for switching power supplies.

Adjustable width pulse generator circuit

Bipolar symmetrical pulses of adjustable width can be obtained using a pulse generator according to the circuit in Fig. 1. The device can be used in circuits for automatic regulation of the output power of switching power supplies. On the DD1 chip (K561LE5/K561 LAT) a rectangular pulse generator with a duty cycle of 2 is assembled.

The symmetry of the generated pulses is achieved by adjusting resistor R1. The operating frequency of the generator (44 kHz), if necessary, can be changed by selecting the capacitance of capacitor C1.

Rice. 1. Circuit of a shaper of bipolar symmetrical pulses of adjustable duration.

Voltage comparators are assembled on elements DA1.1, DA1.3 (K561KTZ); on DA1.2, DA1.4 - output keys. The inputs of comparator switches DA1.1, DA1.3 are supplied in antiphase through forming RC diode chains (R3, C2, VD2 and R6, SZ, VD5). square pulses.

The charging of capacitors C2, SZ occurs according to an exponential law through R3 and R5, respectively; discharge - almost instantly through diodes VD2 and VD5. When the voltage on capacitor C2 or SZ reaches the operating threshold of the comparator switches DA1.1 or DA1.3, respectively, they are turned on, and resistors R9 and R10, as well as the control inputs of the keys DA1.2 and DA1.4, are connected to the positive pole of the source nutrition.

Since the switches are switched on in antiphase, such switching occurs strictly one by one, with a pause between pulses, which eliminates the possibility of through current flowing through switches DA1.2 and DA1.4 and the converter transistors controlled by them, if a bipolar pulse generator is used in a switching power supply circuit.

Smooth control of the pulse width is carried out by simultaneously applying a starting (initial) voltage to the inputs of the comparators (capacitors C2, SZ) from potentiometer R5 through diode-resistive chains VD3, R7 and VD4, R8. The maximum control voltage level (maximum output pulse width) is set by selecting resistor R4.

The load resistance can be connected using a bridge circuit - between the connection point of elements DA1.2, DA1.4 and capacitors Ca, Cb. Pulses from the generator can also be applied to transistor amplifier power.

When using a bipolar pulse generator in a switching power supply circuit, a regulating element should be included in the resistive divider R4, R5 - field effect transistor, optocoupler photodiode, etc., which allows, when the load current decreases/increases, to automatically adjust the width of the generated pulse, thereby controlling the output power of the converter.

As an example of the practical implementation of switching power supplies, we provide descriptions and diagrams of some of them.

Switching power supply circuit

Switching power supply(Fig. 2) consists of mains voltage rectifiers, a master oscillator, a rectangular pulse shaper of adjustable duration, a two-stage power amplifier, output rectifiers and an output voltage stabilization circuit.

The master oscillator is made on a K555LAZ type microcircuit (elements DDI .1, DDI .2) and produces rectangular pulses with a frequency of 150 kHz. An RS trigger is assembled on elements DD1.3, DD1.4, the output frequency of which is half as low - 75 kHz. The switching pulse duration control unit is implemented on a K555LI1 type microcircuit (elements DD2.1, DD2.2), and the duration is adjusted using optocoupler U1.

The output stage of the switching pulse shaper is assembled on elements DD2.3, DD2.4. Maximum power at the output of the pulse shaper reaches 40 mW. Pre-amplifier power is made on transistors VT1, VT2 type KT645A, and the final one - on transistors VT3, VT4 type KT828 or more modern. The output power of the cascades is 2 and 60...65 W, respectively.

A circuit for stabilizing the output voltage is assembled using transistors VT5, VT6 and optocoupler U1. If the voltage at the output of the power supply is below normal (12 V), the zener diodes VD19, VD20 (KS182+KS139) are closed, transistor VT5 is closed, transistor VT6 is open, a current flows through the LED (U1.2) of the optocoupler, limited by resistance R14; The resistance of the photodiode (U1.1) of the optocoupler is minimal.

The signal taken from the output of element DD2.1 and supplied to the inputs of the coincidence circuit DD2.2 directly and through an adjustable delay element (R3 - R5, C4, VD2, U1.1), due to its small time constant, arrives almost simultaneously at the inputs of the circuit matches (element DD2.2).

At the output of this element, wide control pulses are formed. Bipolar pulses of adjustable duration are formed on the primary winding of transformer T1 (outputs of elements DD2.3, DD2.4).

Rice. 2. Switching power supply circuit.

If for any reason the voltage at the output of the power supply increases above normal, current will begin to flow through the zener diodes VD19, VD20, transistor VT5 will open slightly, VT6 will close, reducing the current through the LED of the optocoupler U1.2.

In this case, the resistance of the photodiode of the optocoupler U1.1 increases. The duration of the control pulses decreases, and the output voltage (power) decreases. When the load is short-circuited, the optocoupler LED goes out, the resistance of the optocoupler photodiode is maximum, and the duration of the control pulses is minimal. Button SB1 is designed to start the circuit.

At the maximum duration, positive and negative control pulses do not overlap in time, since there is a time gap between them due to the presence of resistor R3 in the forming circuit.

This reduces the likelihood of through currents flowing through the output relatively low-frequency transistors of the final power amplification stage, which have big time resorption of excess carriers at the base transition. The output transistors are installed on finned heat sinks with an area of ​​at least 200 cm^2. It is advisable to install resistances of 10...51 Ohms in the base circuits of these transistors.

The power amplification stages and the circuit for generating bipolar pulses are powered by rectifiers made on diodes VD5 - VD12 and elements R9 - R11, C6 - C9, C12, VD3, VD4.

Transformers T1, T2 are made on ferrite rings K10x6x4.5 ZOOONM; TZ - K28x16x9 ZOOONM. The primary winding of transformer T1 contains 165 turns of PELSHO 0.12 wire, the secondary winding contains 2×65 turns of PEL-2 0.45 (winding in two wires).

The primary winding of the T2 transformer contains 165 turns of PEV-2 0.15 mm wire, the secondary windings contain 2x40 turns of the same wire. The primary winding of the TZ transformer contains 31 turns of MGShV wire, threaded into a cambric and having a cross-section of 0.35 mm^2, the secondary winding has 3 × 6 turns of PEV-2 wire 1.28 mm ( parallel connection). When connecting transformer windings, it is necessary to phase them correctly. The beginnings of the windings are shown in the figure with asterisks.

The power supply is operational within the mains voltage range of 130…250 V. Maximum output power with a symmetrical load reaches 60...65 W (stabilized voltage of positive and negative polarity 12 S and stabilized voltage AC frequency 75 kHz, removed from the secondary winding of transformer T3). The ripple voltage at the output of the power supply does not exceed 0.6 V.

When setting up a power source, the mains voltage is supplied to it through an isolation transformer or a ferroresonant stabilizer with an output isolated from the mains. All resoldering in the source is permissible only when complete shutdown devices from the network.

It is recommended to turn on a 60 W 220 V incandescent lamp in series with the output stage while setting up the device. This lamp will protect the output transistors in case of installation errors. Optocoupler U1 must have an insulation breakdown voltage of at least 400 V. Operation of the device without load is not allowed.

Network switching power supply

Network switching power supply (Fig. 3) is designed for telephone sets with automatic caller ID or for other devices with a power consumption of 3...5W, powered by a voltage of 5...24V.

The power supply is protected against output short circuit. The instability of the output voltage does not exceed 5% when the supply voltage changes from 150 to 240 V and the load current is within 20... 100% of the nominal value.

A controlled pulse generator provides a signal with a frequency of 25...30 kHz based on the VT3 transistor.

Chokes L1, L2 and L3 are wound on magnetic cores of type K10x6x3 from pressed permalloy MP140. The inductor windings L1, L2 contain 20 turns of 0.35 mm PETV wire and are each located on its own half of the ring with a gap between the windings of at least 1 mm.

Choke L3 is wound with 0.63 mm PETV wire turn to turn in one layer along the inner perimeter of the ring. Transformer T1 is made on a magnetic core B22 made of M2000NM1 ferrite.

Rice. 3. Diagram of a network switching power supply.

Its windings are wound on a collapsible frame turn to turn with PETV wire and impregnated with glue. The first winding I is wound in several layers, containing 260 turns of 0.12 mm wire. A shielding winding with one terminal is wound with the same wire (Fig. 3 shows dotted line), then apply BF-2 glue and wrap it with one layer of Lakot-Kani.

Winding III is wound with 0.56 mm wire. For an output voltage of 5V, it contains 13 turns. Winding II is wound last. It contains 22 turns of wire 0.15...0.18 mm. A non-magnetic gap is provided between the cups.

High voltage constant voltage source

To create high voltage(30…35 kV at a load current of up to 1 mA) a power source is designed to power an electroeffluvial chandelier (A.L. Chizhevsky’s chandelier) DC based on specialized chip type K1182GGZ.

The power supply consists of a mains voltage rectifier on a diode bridge VD1, a filter capacitor C1 and a high-voltage half-bridge oscillator on a DA1 chip of the K1182GGZ type. The DA1 chip, together with transformer T1, converts direct rectified mains voltage into high-frequency (30...50 kHz) pulsed voltage.

The rectified mains voltage is supplied to the DA1 microcircuit, and the starting circuit R2, C2 starts the microcircuit's self-oscillator. Chains R3, SZ and R4, C4 set the frequency of the generator. Resistors R3 and R4 stabilize the duration of the half-cycles of the generated pulses. The output voltage is increased by winding L4 of the transformer and supplied to a voltage multiplier using diodes VD2 - VD7 and capacitors C7 - C12. The rectified voltage is supplied to the load through limiting resistor R5.

Line filter capacitor C1 is designed for an operating voltage of 450 V (K50-29), C2 - of any type for a voltage of 30 V. Capacitors C5, C6 are selected within the range of 0.022 ... 0.22 μF for a voltage of at least 250 V (K71-7, K73 -17). Multiplier capacitors C7 - C12 type KVI-3 for voltage 10 kV. It is possible to replace it with capacitors of types K15-4, K73-4, POV and others with an operating voltage of 10 kV or higher.

Rice. 4. Circuit diagram of a high voltage DC power supply.

High-voltage diodes VD2 - VD7 type KTs106G (KTs105D). Limiting resistor R5 type KEV-1. It can be replaced with three MLT-2 type resistors of 10 MOhm each.

A television line transformer, for example, TVS-110LA, is used as a transformer. The high-voltage winding is left, the rest are removed and new windings are placed in their place. Windings L1, L3 each contain 7 turns of 0.2 mm PEL wire, and winding L2 contains 90 turns of the same wire.

It is recommended to include a chain of resistors R5, which limits the short circuit current, in the “negative” wire, which is connected to the chandelier. This wire must have high-voltage insulation.

Power factor corrector

The device, called a power factor corrector (Fig. 5), is assembled on the basis of a specialized TOP202YA3 microcircuit (Power Integration) and provides a power factor of at least 0.95 with a load power of 65 W. The corrector brings the shape of the current consumed by the load closer to a sinusoidal one.

Rice. 5. Power factor corrector circuit based on the TOP202YA3 microcircuit.

The maximum input voltage is 265 V. The average frequency of the converter is 100 kHz. The efficiency of the corrector is 0.95.

Switching power supply with microcircuit

The diagram of a power supply with a microcircuit from the same company Power Integration is shown in Fig. 6. The device uses semiconductor voltage limiter- 1.5KE250A.

The converter provides galvanic isolation of the output voltage from the mains voltage. With the ratings and elements indicated in the diagram, the device allows you to connect a load that consumes 20 W at a voltage of 24 V. The efficiency of the converter approaches 90%. Conversion frequency - 100 Hz. The device is protected from short circuits under load.

Rice. 6. Circuit diagram of a 24V switching power supply on a microcircuit from Power Integration.

The output power of the converter is determined by the type of microcircuit used, the main characteristics of which are given in Table 1.

Table 1. Characteristics of TOP221Y - TOP227Y series microcircuits.

Simple and highly efficient voltage converter

Based on one of the TOP200/204/214 microcircuits from Power Integration, a simple and high efficiency voltage converter(Fig. 7) with output power up to 100 W.

Rice. 7. Circuit of a pulse Buck-Boost converter based on the TOP200/204/214 microcircuit.

The converter contains surge protector(C1, L1, L2), a bridge rectifier (VD1 - VD4), the converter itself U1, an output voltage stabilization circuit, rectifiers and an output LC filter.

The input filter L1, L2 is wound in two wires on an M2000 ferrite ring (2×8 turns). The inductance of the resulting coil is 18...40 mH. The T1 transformer is made on a ferrite core with a standard ETD34 frame from Siemens or Matsushita, although other imported cores such as EP, EC, EF or domestic W-shaped ferrite cores M2000 can be used.

Winding I has 4×90 turns PEV-2 0.15 mm; II - 3x6 of the same wire; III - 2×21 turns PEV-2 0.35 mm. All windings are wound turn to turn. Reliable insulation must be provided between layers.

Many electrical devices have long used the principle of realizing secondary power through the use of additional devices, which are entrusted with the functions of providing electricity to circuits that require power from certain types of voltage, frequency, current...

For this purpose, additional elements are created: converting voltage of one type to another. They may be:

built inside the consumer case, as on many microprocessor devices;

or manufactured in separate modules with connecting wires similar to a conventional charger at the mobile phone.

In modern electrical engineering, two principles of energy conversion for electrical consumers, based on:

1. using analog transformer devices to transfer power to the secondary circuit;

2. switching power supplies.

They have fundamental differences in their design, they work using different technologies.

Transformer power supplies

Initially, only such designs were created. They change the voltage structure due to the operation of a power transformer, powered from a 220-volt household network, in which the amplitude of the sinusoidal harmonic decreases, which is then sent to a rectifier device consisting of power diodes, usually connected in a bridge circuit.

After this, the pulsating voltage is smoothed out by a parallel-connected capacitance, selected according to the permissible power, and stabilized by a semiconductor circuit with power transistors.

By changing the position of the trimming resistors in the stabilization circuit, it is possible to regulate the voltage at the output terminals.

Switching power supplies (UPS)

Similar design developments appeared en masse several decades ago and became increasingly popular in electrical devices due to:

availability of common components;

reliability in execution;

possibilities to expand the operating range of output voltages.

Almost all sources switching power supply They differ slightly in design and operate according to the same scheme, typical for other devices.

The main parts of power supplies include:

a network rectifier assembled from: input chokes, an electromechanical filter that provides noise rejection and static isolation from capacitors, a network fuse and a diode bridge;

storage filter tank;

key power transistor;

master oscillator;

feedback circuit made using transistors;

optocoupler;

a switching power supply, from the secondary winding of which voltage emanates to be converted into a power circuit;

rectifier diodes of the output circuit;

output voltage control circuits, for example, 12 volts with adjustment made using an optocoupler and transistors;

filter capacitors;

power chokes that perform the role of voltage correction and diagnostics in the network;

output connectors.

An example of an electronic board of such a switching power supply with a short designation element base shown in the picture.

How does a switching power supply work?

Pulse block The power supply produces a stabilized supply voltage through the use of the principles of interaction between the elements of the inverter circuit.

The 220 volt network voltage is supplied through the connected wires to the rectifier. Its amplitude is smoothed by a capacitive filter through the use of capacitors that can withstand peaks of about 300 volts, and is separated by a noise filter.