Switching power supply (SMPS) is currently the most widely used and is successfully used in all modern electronic devices.
Figure 3 shows a block diagram of a switching power supply made according to the traditional scheme. Secondary rectifiers are made according to a half-wave scheme. The names of these nodes reveal their purpose and do not need explanation. The main nodes of the primary circuit are: the input filter, the mains voltage rectifier and the RF Converter of the rectified supply voltage with a transformer.
Mains rectifier filter
Transformer
RF converter
Secondary rectifiers
Input filter
Figure 3 - Structural diagram of a pulsed power supply
The basic principle underlying the operation of the SMPS is to convert the mains alternating voltage of 220 volts and a frequency of 50 Hz into a rectangular high-frequency alternating voltage, which is transformed to the required values, rectified and filtered.
The conversion is carried out using a powerful transistor operating in the key mode and a pulse transformer, together forming an RF converter circuit. As for the circuit design, there are two options for converters: the first one is performed according to the pulsed self-oscillator circuit (for example, this was used in the UPS of TVs) and the second with external control (used in most modern electronic devices).
Since the converter frequency is usually selected from 18 to 50 kHz, the dimensions of the pulse transformer, and, consequently, the entire power supply, are quite compact, which is an important parameter for modern equipment. A simplified diagram of an externally controlled pulse converter is shown in Figure 4.
Figure 4 - Schematic diagram of a pulsed power supply unit with a VU.
The converter is made on a transistor VT1 and a transformer T1. The mains voltage through the network filter (SF) is supplied to the mains rectifier (CB), where it is rectified, filtered by the filter capacitor (CF) and through the winding W1 of the transformer T1 is fed to the collector of the transistor VT1. When a rectangular pulse is applied to the base circuit of the transistor, the transistor opens and an increasing current flows through it I j. The same current will also flow through the winding W1 of the transformer T1, which will lead to the fact that the magnetic flux increases in the core of the transformer, while the self-induction EMF is induced in the secondary winding W2 of the transformer. Eventually, a positive voltage will appear at the output of the VD diode. Moreover, if we increase the duration of the pulse applied to the base of the transistor VT1, the voltage will increase in the secondary circuit, because. more energy will be given away, and if you reduce the duration, the voltage will decrease accordingly. Thus, by changing the pulse duration in the base circuit of the transistor, we can change the output voltages of the secondary winding T1, and therefore stabilize the output voltages of the PSU. The only thing that is needed for this is a circuit that will generate trigger pulses and control their duration (width). A PWM controller is used as such a circuit. PWM - pulse width modulation.
To stabilize the output voltages of the UPS, the PWM controller circuit "must know" the value of the output voltages. For these purposes, a tracking circuit (or feedback circuit) is used, made on the optocoupler U1 and resistor R2. An increase in voltage in the secondary circuit of the transformer T1 will lead to an increase in the intensity of the LED radiation, and consequently a decrease in the resistance of the transition of the phototransistor (which are part of the optocoupler U1). Which in turn will lead to an increase in the voltage drop across the resistor R2, which is connected in series with the phototransistor and a decrease in the voltage at pin 1 of the PWM controller. Reducing the voltage causes the logic circuit, which is part of the PWM controller, to increase the pulse duration until the voltage at the 1st output matches the specified parameters. When the voltage decreases, the process is reversed.
The UPS uses two principles for implementing tracking circuits - "direct" and "indirect". The method described above is called "direct", since the feedback voltage is taken directly from the secondary rectifier. With "indirect" tracking, the feedback voltage is removed from the additional winding of the pulse transformer (Figure 5).
Figure 5 - Schematic diagram of a pulsed power supply unit with a VU.
A decrease or increase in the voltage on the winding W2 will also lead to a change in the voltage on the winding W3, which is also applied to pin 1 of the PWM controller through resistor R2.
SMPS short circuit protection.
Short circuit (short circuit) in the UPS load. In this case, all the energy given to the secondary circuit of the UPS will be lost and the output voltage will be almost zero. Accordingly, the PWM controller circuit will try to increase the pulse duration in order to raise the level of this voltage to the appropriate value. As a result, the transistor VT1 will be longer and longer in the open state, and the current flowing through it will increase. In the end, this will lead to the failure of this transistor. The UPS is designed to protect the inverter transistor from overcurrent in such abnormal situations. It is based on a resistor Rprotect, connected in series to the circuit through which the collector current Ik flows. An increase in the current Ik flowing through the transistor VT1 will increase the voltage drop across this resistor, and, consequently, the voltage supplied to pin 2 of the PWM controller will also decrease. When this voltage drops to a certain level, which corresponds to the maximum allowable current of the transistor, the PWM controller logic circuit will stop generating pulses at pin 3 and the power supply will go into protection mode or, in other words, turn off.
In conclusion, it is necessary to elaborate on the advantages of the UPS. As already mentioned, the frequency of the pulse converter is quite high, and therefore, the overall dimensions of the pulse transformer are reduced, which means, paradoxically, the cost of a UPS is less than a traditional power supply unit. less metal consumption for the magnetic circuit and copper for the windings, even though the number of parts in the UPS is increasing. Another advantage of the UPS is the small capacitance of the filter capacitor of the secondary rectifier compared to a conventional power supply. The reduction in capacitance was made possible by increasing the frequency. And finally, the efficiency of the switching power supply reaches 80%. This is due to the fact that the UPS consumes the energy of the electrical network only during the open transistor of the converter; when it is closed, energy is transferred to the load due to the discharge of the filter capacitor of the secondary circuit.
The disadvantages include the complication of the UPS circuit and the increase in impulse noise emitted by the UPS. The increase in noise is due to the fact that the converter transistor operates in the key mode. In this mode, the transistor is a source of impulse noise that occurs at the moments of transient processes of the transistor. This is a disadvantage of any transistor operating in the key mode. But if the transistor operates with low voltages (for example, transistor logic with a voltage of 5V), this is not scary, in our case, the voltage applied to the collector of the transistor is approximately 315 V. To combat this interference, the UPS uses more complex network circuits filters than in a conventional PSU.
The scope of switching power supplies in everyday life is constantly expanding. Such sources are used to power all modern household and computer equipment, to implement uninterruptible power supplies, chargers for batteries for various purposes, to implement low-voltage lighting systems, and for other needs.
In some cases, buying a ready-made power supply is not very acceptable from an economic or technical point of view, and assembling a switching power supply with your own hands is the best way out of this situation. Simplifies this option and the wide availability of modern element base at low prices.
The most popular in everyday life are switching sources powered by a standard AC network and a powerful low-voltage output. The block diagram of such a source is shown in the figure.
The mains rectifier CB converts the alternating voltage of the supply network into a constant one and smoothes out the ripples of the rectified voltage at the output. The high-frequency VChP converter converts the rectified voltage into an alternating or unipolar one, having the form of rectangular pulses of the required amplitude.
In the future, such a voltage either directly or after rectification (HV) is supplied to a smoothing filter, to the output of which a load is connected. The VChP is controlled by a control system that receives a feedback signal from the load rectifier.
Such a structure of the device can be criticized due to the presence of several conversion links, which reduces the efficiency of the source. However, with the right choice of semiconductor elements and high-quality calculation and manufacture of coil units, the level of power losses in the circuit is small, which makes it possible to obtain real efficiency values above 90%.
Schematic diagrams of switching power supplies
Structural block solutions include not only the rationale for choosing circuit implementation options, but also practical recommendations for choosing the main elements.
To rectify the mains single-phase voltage, one of the three classic schemes shown in the figure is used:
- half-wave;
- zero (two-half-wave with a midpoint);
- two-half-wave bridge.
Each of them has advantages and disadvantages that determine the scope.
Half wave circuit characterized by ease of implementation and a minimum number of semiconductor components. The main disadvantages of such a rectifier are a significant amount of output voltage ripple (in the rectified one there is only one half-wave of the mains voltage) and a low rectification factor.
Rectification ratio Kv determined by the ratio of the average value of the voltage at the output of the rectifier Udk effective value of the phase mains voltage Uph.
For a half-wave circuit, Kv \u003d 0.45.
To smooth out the ripple at the output of such a rectifier, powerful filters are required.
Zero, or full-wave circuit with a midpoint, although it requires a double number of rectifier diodes, however, this disadvantage is largely offset by a lower level of rectified voltage ripple and an increase in the rectification factor to 0.9.
The main disadvantage of such a scheme for use in domestic conditions is the need to organize the midpoint of the mains voltage, which implies the presence of a mains transformer. Its dimensions and weight turn out to be incompatible with the idea of a small-sized self-made pulsed source.
full wave bridge rectification has the same indicators in terms of ripple level and rectification factor as the zero circuit, but does not require a network. This compensates for the main drawback - twice the number of rectifier diodes, both in terms of efficiency and cost.
To smooth out the ripple of the rectified voltage, the best solution is to use a capacitive filter. Its use allows you to raise the value of the rectified voltage to the amplitude value of the mains (at Uph=220V Ufm=314V). The disadvantages of such a filter are considered to be large values of the pulsed currents of the rectifier elements, but this disadvantage is not critical.
The choice of rectifier diodes is carried out according to the average forward current Ia and the maximum reverse voltage U BM.
Taking the value of the output voltage ripple coefficient Kp=10%, we obtain the average value of the rectified voltage Ud=300V. Taking into account the load power and the efficiency of the RF converter (80% is taken for calculation, but in practice it will turn out higher, this will allow you to get some margin).
Ia is the average current of the rectifier diode, Рн is the load power, η is the efficiency of the RF converter.
The maximum reverse voltage of the rectifier element does not exceed the amplitude value of the mains voltage (314V), which allows the use of components with a value of U BM =400V with a significant margin. You can use both discrete diodes and ready-made rectifier bridges from various manufacturers.
To ensure a given (10%) ripple at the rectifier output, the capacitance of the filter capacitors is taken at the rate of 1 μF per 1 W of output power. Electrolytic capacitors with a maximum voltage of at least 350V are used. Filter capacities for various capacities are shown in the table.
High frequency converter: its functions and circuits
The high-frequency converter is a single-cycle or two-cycle key converter (inverter) with a pulse transformer. Variants of circuits of RF converters are shown in the figure.
Single cycle circuit. With a minimum number of power elements and ease of implementation, it has several disadvantages.
- The transformer in the circuit operates on a private hysteresis loop, which requires an increase in its size and overall power;
- To provide output power, it is necessary to obtain a significant amplitude of the pulsed current flowing through the semiconductor switch.
The scheme has found the greatest application in low-power devices, where the influence of these disadvantages is not so significant.
To change or install a new meter yourself, no special skills are required. Choosing the right one will ensure that the current consumed is correctly accounted for and will increase the safety of the home electrical network.
In modern lighting conditions, both indoors and outdoors, motion sensors are increasingly being used. This gives not only comfort and convenience to our homes, but also allows you to save a lot. You can find out practical tips on choosing an installation site, connection diagrams.
Push-Pull Circuit with Transformer Midpoint (Push-Pull). It got its second name from the English version (push-pull) of the job description. The circuit is free from the shortcomings of the single-cycle version, but has its own - a complicated design of the transformer (it is required to manufacture identical sections of the primary winding) and increased requirements for the maximum voltage of the switches. Otherwise, the solution deserves attention and is widely used in do-it-yourself switching power supplies and not only.
Push-Pull Half-Bridge. In terms of parameters, the circuit is similar to the circuit with a midpoint, but does not require a complex configuration of the transformer windings. The inherent disadvantage of the circuit is the need to organize the middle point of the rectifier filter, which entails a fourfold increase in the number of capacitors.
Due to the ease of implementation, the circuit is most widely used in switching power supplies up to 3 kW. At high powers, the cost of the filter capacitors becomes unacceptably high compared to the semiconductor switches of the inverter, and the bridge circuit turns out to be the most profitable.
Push-Pull Bridge. Similar in parameters to other push-pull circuits, but without the need to create artificial "midpoints". The price for this is a doubled number of power switches, which is beneficial from an economic and technical point of view for building powerful pulsed sources.
The choice of inverter keys is carried out according to the amplitude of the collector (drain) current I KMAX and the maximum collector-emitter voltage U KEMAC. For the calculation, the load power and the transformation ratio of the pulse transformer are used.
However, first you need to calculate the transformer itself. The pulse transformer is made on a core made of ferrite, permalloy or transformer iron twisted into a ring. For powers up to units of kW, ferrite cores of an annular or W-shaped type are quite suitable. The calculation of the transformer is based on the required power and conversion frequency. To exclude the appearance of acoustic noise, it is desirable to move the conversion frequency outside the audio range (make it higher than 20 kHz).
At the same time, it must be remembered that at frequencies close to 100 kHz, losses in ferrite magnetic circuits increase significantly. The calculation of the transformer itself is not difficult and can be easily found in the literature. Some results for various power sources and magnetic cores are shown in the table below.
The calculation was made for a conversion frequency of 50 kHz. It is worth noting that when operating at a high frequency, the effect of current displacement to the surface of the conductor takes place, which leads to a decrease in the effective winding area. To prevent this kind of trouble and reduce losses in conductors, it is necessary to wind from several cores of a smaller cross section. At a frequency of 50 kHz, the permissible diameter of the winding wire does not exceed 0.85 mm.
Knowing the load power and the transformation ratio, it is possible to calculate the current in the primary winding of the transformer and the maximum collector current of the power switch. The voltage on the transistor in the closed state is selected higher than the rectified voltage supplied to the input of the RF converter with a certain margin (U KEMAH>=400V). Based on this data, keys are selected. Currently, the best option is to use IGBT or MOSFET power transistors.
For rectifier diodes on the secondary side, one rule must be observed - their maximum operating frequency must exceed the conversion frequency. Otherwise, the efficiency of the output rectifier and the converter as a whole will be significantly reduced.
Video on the manufacture of the simplest switching power supply
Switching power supply- this is an inverter system in which the input alternating voltage is rectified, and then the received direct voltage is converted into high-frequency pulses and a set duty cycle, which, as a rule, are fed 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 change.
Negative feedback can be implemented using one of the additional windings in the 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 makes it possible to implement galvanic isolation from the AC 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 of work;
- 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 principle of operation of the converter circuit. To partially eliminate this shortcoming, screening of the circuit is used. Also, due to this drawback, in some devices, the use of this type of power supply is impossible.
Switching power supplies have become in fact an indispensable attribute of any modern household appliances that consume more than 100 watts of power from the network. This category includes computers, televisions, monitors.
To create switching power supplies, examples of a specific embodiment of which will be given below, special circuit solutions are used.
So, to exclude through currents through the output transistors of some switching power supplies, a special pulse shape is used, namely, bipolar rectangular pulses that have a time gap between them.
The duration of this gap must be longer than the dissipation time of minority carriers in the base of the output transistors, otherwise these transistors will be damaged. The width of the control pulses in order to stabilize the output voltage can be changed using feedback.
Usually, to ensure reliability in switching power supplies, high-voltage transistors are used, which, due to technological features, do not differ for the better (they have low switching frequencies, low current transfer coefficients, significant leakage currents, large voltage drops at the collector junction in the open state).
This is especially true for now obsolete models of domestic transistors such as KT809, KT812, KT826, KT828 and many others. It is worth saying that in recent years a worthy replacement for bipolar transistors, traditionally used in the output stages of switching power supplies, has appeared.
These are special high-voltage field-effect transistors of domestic and, mainly, foreign production. In addition, there are numerous chips for switching power supplies.
Variable Width Pulse Generator Circuit
Bipolar symmetrical pulses of adjustable width make it possible to obtain a pulse generator according to the scheme in Fig.1. The device can be used in circuits for automatic control 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 the resistor R1. The operating frequency of the generator (44 kHz), if necessary, can be changed by selecting the capacitance of capacitor C1.
Rice. 1. Scheme of the shaper of bipolar symmetrical pulses of adjustable duration.
Voltage comparators are assembled on the elements DA1.1, DA1.3 (K561KTZ); on DA1.2, DA1.4 - output keys. Rectangular pulses are fed to the inputs of the comparators-keys DA1.1, DA1.3 in antiphase through the forming RC diode chains (R3, C2, VD2 and R6, C3, VD5).
The charge of capacitors C2, C3 occurs according to an exponential law through R3 and R5, respectively; discharge - almost instantly through the diodes VD2 and VD5. When the voltage on the capacitor C2 or C3 reaches the threshold of operation of the comparator-keys DA1.1 or DA1.3, respectively, they are turned on, and the 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 keys are turned on in antiphase, such switching occurs strictly alternately, with a pause between pulses, which excludes the possibility of through current flowing through the DA1.2 and DA1.4 keys and the converter transistors controlled by them, if a bipolar pulse generator is used in a switching power supply circuit.
Smooth regulation of the pulse width is carried out by simultaneously applying the starting (initial) voltage to the inputs of the comparators (capacitors C2, C3) from the potentiometer R5 through the diode-resistive chains VD3, R7 and VD4, R8. The limit level of the control voltage (the maximum width of the output pulses) is set by selecting the resistor R4.
The load resistance can be connected in a bridge circuit - between the connection point of the elements DA1.2, DA1.4 and the capacitors Ca, Cb. Pulses from the generator can also be applied to a transistor power amplifier.
When using a bipolar pulse generator in a switching power supply circuit, the resistive divider R4, R5 should include a control element - a field effect transistor, an optocoupler photodiode, etc., which allows you to automatically adjust the width of the generated pulse when the load current decreases / increases, thereby controlling the output converter power.
As an example of the practical implementation of switching power supplies, we present descriptions and diagrams of some of them.
Scheme of switching power supply
Switching power supply(Fig. 2) consists of mains voltage rectifiers, a master oscillator, a shaper of rectangular pulses of adjustable duration, a two-stage power amplifier, output rectifiers and an output voltage stabilization circuit.
The master oscillator is made on a K555LAZ chip (elements DDI .1, DDI .2) and generates rectangular pulses with a frequency of 150 kHz. On the elements DD1.3, DD1.4, an RS flip-flop is assembled, at the output of which the frequency is half as much - 75 kHz. The unit for controlling the duration of switching pulses is implemented on a K555LI1 microcircuit (elements DD2.1, DD2.2), and the duration is adjusted using an optocoupler U1.
The output stage of the switching pulse shaper is assembled on the elements DD2.3, DD2.4. The maximum output power of the pulse shaper reaches 40 mW. The preliminary power amplifier is made on transistors VT1, VT2 of the KT645A type, and the final one is on transistors VT3, VT4 of the KT828 type or more modern. The output power of the cascades is 2 and 60 ... 65 W, respectively.
On transistors VT5, VT6 and optocoupler U1, an output voltage stabilization circuit is assembled. If the voltage at the output of the power supply is below normal (12 V), the zener diodes VD19, VD20 (KS182 + KS139) are closed, the transistor VT5 is closed, the transistor VT6 is open, a current limited by the resistance R14 flows through the LED (U1.2) of the optocoupler; the resistance of the photodiode (U1.1) of the optocoupler is minimal.
The signal taken from the output of the DD2.1 element and supplied to the inputs of the DD2.2 coincidence circuit directly and through the adjustable delay element (R3 - R5, C4, VD2, U1.1), due to its low time constant, arrives almost simultaneously at the circuit inputs matches (item DD2.2).
At the output of this element, wide control pulses are formed. On the primary winding of the transformer T1 (outputs of the elements DD2.3, DD2.4) bipolar pulses of adjustable duration are formed.
Rice. 2. Scheme of a switching power supply.
If, for any reason, the voltage at the output of the power supply increases beyond the norm, current will begin to flow through the zener diodes VD19, VD20, the transistor VT5 will open a little, VT6 will close, reducing the current through the LED of the optocoupler U1.2.
This increases the resistance of the photodiode of the optocoupler U1.1. 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 minimum. The SB1 button 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 a long time for dissolving excess carriers at the base junction. The output transistors are mounted on ribbed heat sinks with an area of at least 200 cm^2. It is desirable to install resistances of 10 ... 51 Ohms in the base circuits of these transistors.
The power amplification stages and the bipolar pulse formation circuit 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 the transformer T1 contains 165 turns of PELSHO 0.12 wire, the secondary - 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 wire 0.15 mm, the secondary - 2 × 40 turns of the same wire. The primary winding of the TZ transformer contains 31 turns of MGShV wire threaded into 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 the windings of transformers, it is necessary to phase them correctly. The beginnings of the windings are shown in the figure with asterisks.
The power supply is operable in the mains voltage range of 130 ... 250 V. The maximum output power with a symmetrical load reaches 60 ... 65 W (stabilized voltage of positive and negative polarity 12 S and stabilized AC voltage with a frequency of 75 kHz, taken from the secondary winding of the T3 transformer) . The ripple voltage at the output of the power supply does not exceed 0.6 V.
When establishing a power source, mains voltage is supplied to it through an isolating transformer or a ferroresonant stabilizer with an output isolated from the mains. All soldering in the source is permissible only when the device is completely disconnected from the network.
It is recommended to turn on a 60 W 220 V incandescent lamp in series with the output stage while the device is being set up. 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
The network switching power supply (Fig. 3) is designed for telephones with automatic caller identification or for other devices with a power consumption of 3 ... 5W, powered by a voltage of 5 ... 24V.
The power supply is short-circuit protected at the output. 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.
The controlled pulse generator provides a signal with a frequency of 25 ... 30 kHz based on the transistor VT3.
Inductors L1, L2 and L3 are wound on K10x6x3 type magnetic cores made of presspermalloy MP140. The inductor windings L1, L2 contain 20 turns of 0.35 mm PETV wire and are each located on their own half of the ring with a gap between the windings of at least 1 mm.
Choke L3 is wound with PETV wire 0.63 mm turn to turn in one layer along the inner perimeter of the ring. Transformer T1 is made on the magnetic circuit B22 of M2000NM1 ferrite.
Rice. 3. Scheme of a network switching power supply.
Its windings are wound on a collapsible frame turn to turn with PETV wire and impregnated with glue. Winding I is first wound in several layers, containing 260 turns of 0.12 mm wire. A shielding winding with one lead is wound with the same wire (shown in dotted line in Fig. 3), then BF-2 glue is applied and wrapped with one layer of Lakot-kani.
Winding III is wound with a wire of 0.56 mm. 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 DC power supply
To create a high voltage (30 ... 35 kV at a load current of up to 1 mA) to power an electro-fluvial chandelier (chandelier by A. L. Chizhevsky), a DC power source is designed based on a specialized microcircuit of the type K1182GGZ.
The power supply consists of a mains voltage rectifier on a VD1 diode bridge, a filter capacitor C1 and a high-voltage half-bridge self-oscillator on a DA1 chip of the K1182GGZ type. The DA1 chip, together with the transformer T1, converts the direct rectified mains voltage into a high-frequency (30 ... 50 kHz) pulsed voltage.
The rectified mains voltage is supplied to the DA1 microcircuit, and the starting chain R2, C2 starts the microcircuit oscillator. Chains R3, C3 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 the winding L4 of the transformer and is fed to a voltage multiplier on diodes VD2 - VD7 and capacitors C7 - C12. The rectified voltage is applied to the load through the limiting resistor R5.
The 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 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 a voltage of 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. Scheme 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 10 MΩ MLT-2 resistors.
A television horizontal transformer is used as a transformer, for example, TVS-110LA. The high-voltage winding is left, the rest are removed and new windings are placed in their place. The windings L1, L3 each contain 7 turns of PEL wire 0.2 mm, and the winding L2 contains 90 turns of the same wire.
The chain of resistors R5, which limit the short circuit current, is recommended to be included in the "negative" wire, which is connected to the chandelier. This wire should have Vyuoko-Volt 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 at 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. Scheme of the power factor corrector on the TOP202YA3 chip.
The maximum input voltage is 265 V. The average frequency of the converter is 100 kHz. Corrector efficiency - 0.95.
Switching power supply with a microcircuit
The power supply circuit with a microcircuit of the same Power Integration company is shown in fig. 6. The device is applied semiconductor voltage limiter- 1.5KE250A.
The converter provides galvanic isolation of the output voltage from the mains voltage. With the ratings and elements indicated on 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%. The conversion frequency is 100 Hz. The device is protected against short circuits in the load.
Rice. 6. Scheme of a 24V switching power supply on a Power Integration chip.
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 microcircuits of the TOP221Y - TOP227Y series.
Simple and highly efficient voltage converter
On the basis of one of the TOP200/204/214 chips from Power Integration, a simple and high efficiency voltage converter(Fig. 7) with output power up to 100 W.
Rice. 7. Scheme of a pulsed Buck-Boost converter on a TOR200/204/214 chip.
The converter contains a mains filter (C1, L1, L2), a bridge rectifier (VD1 - VD4), the U1 converter itself, 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. Transformer T1 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 M2000 ferrite cores can be used.
Winding I has 4 × 90 turns of PEV-2 0.15 mm; II - 3 × 6 of the same wire; III - 2 × 21 turns of PEV-2 0.35 mm. All windings are wound turn to turn. Reliable insulation must be provided between layers.
Unlike traditional linear power supplies, which assume the damping of excessive unstabilized voltage on a linear through element, pulsed power supplies use other methods and physical phenomena to generate a stabilized voltage, namely: the effect of energy accumulation in inductors, as well as the possibility of high-frequency transformation and the conversion of accumulated energy into constant pressure. There are three typical schemes for constructing pulsed IP: step-up (output voltage higher than input) fig. one,
Rice. 1. Step-up 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 of connecting the inductance, otherwise, the principle of operation remains unchanged, namely.
A key element (usually bipolar or MOS transistors are used), operating at a frequency of about 20-100 kHz, periodically for a short time (no more than 50% of the time) applies the full input unstabilized voltage to the inductor. impulse current. flowing through the coil, ensures the accumulation of energy in its magnetic field 1/2LI^2 on each pulse. - the energy stored in this way from the coil will be transferred to the load (either directly, using a rectifying diode, or through the secondary winding with subsequent rectification), the output smoothing filter capacitor ensures that the output voltage and current are constant. Stabilization of the output voltage is provided by automatic adjustment of the width or frequency of the pulses on the key element (the feedback circuit is designed to monitor the output voltage).
This, although rather complex, scheme can significantly increase the efficiency of the entire device. The fact is that, in this case, in addition to the load itself, there are no power elements in the circuit that dissipate significant power. Key transistors operate in a saturated key mode (i.e., the voltage drop across them is small) and dissipate power only in fairly short time intervals (the time of the pulse). In addition, by increasing the conversion frequency, it is possible to significantly increase the power and improve the weight and size characteristics.
An important technological advantage of pulsed IP is the possibility of building on their basis small-sized network IP with galvanic isolation from the network to power a wide variety of equipment. Such IPs are built without the use of a bulky low-frequency power transformer according to the high-frequency converter circuit. This is, in fact, a typical circuit of a pulsed power supply with a voltage reduction, where a rectified mains voltage is used as an input voltage, and a high-frequency transformer (small-sized and with high efficiency) is used as a storage element, from the secondary winding of which the output stabilized voltage is removed (this transformer also provides galvanic isolation from the network).
The disadvantages of pulsed power supplies include: the presence of a high level of impulse noise at the output, high complexity and low reliability (especially in handicraft production), the need to use expensive high-voltage high-frequency components, which, in the event of the slightest malfunction, easily fail “en masse” (with this, as a rule, one can observe impressive pyrotechnic effects). Those who like to delve into the insides of devices with a screwdriver and a soldering iron will have to be extremely careful when designing a network pulsed IP, since many elements of such circuits are under high voltage.
SWITCHED POWER SUPPLY
It is known that power supplies are an integral part of radio engineering devices, which are subject to a number of requirements; they are a complex of elements, devices and devices that generate electrical energy and convert it to the form necessary to ensure the required operating conditions for radio devices.
Power sources are divided into two groups: primary and secondary power sources: Primary sources are devices that convert various types of energy into electrical energy (electrical generators, electrochemical current sources, photoelectric and thermionic converters, etc.).
Secondary power devices are converters of one type of electrical energy into another. These include: AC to DC voltage converters (rectifier); AC voltage value converters (transformers); DC to AC voltage converters (inverters).
The share of power supply sources currently accounts for 30 to 70% of the total mass and volume of REA equipment. Therefore, the problem of creating a miniature, lightweight and reliable power supply device with good technical and economic indicators is important and relevant. This work is devoted to the development of a secondary power source (SSE) with minimal weight and size and high technical characteristics.
A prerequisite for the design of secondary power sources is a clear knowledge of the requirements for them. These requirements are very diverse and are determined by the characteristics of the operation of those REA complexes that are powered by a given PSE. The main requirements are: to the design - reliability, maintainability, size and mass restrictions, thermal conditions; to the technical and economic characteristics - the cost and manufacturability.
The main directions for improving the weight and size and technical and economic indicators of IP: the use of the latest electrical materials; application of the element base using the integral-hybrid technology; increasing the frequency of electrical energy conversion; search for new effective circuit solutions. To select the PVE scheme, an analysis was made of the efficiency of using switching power supplies (SMPS) in comparison with power PSs made using traditional technology.
The main disadvantages of power IP are high weight and size characteristics, as well as a significant effect on other REE devices of a strong magnetic field of power transformers. The problem of SMPS is the creation of high-frequency interference by them, and, as a result of this, electromagnetic incompatibility with some types of electronic equipment. The analysis showed that IIP meet the requirements most fully, which is confirmed by their wide use in REA.
The paper considers an SMPS with a power of 800 W, which differs from other SMPSs by using field-effect transistors and a transformer with a primary winding with an average output in the converter. The FETs provide higher efficiency and reduced high-frequency noise, while the mid-terminal transformer delivers half the current through the key transistors and eliminates the need for an isolation transformer in their gate circuits.
On the basis of the selected circuit diagram, a design was developed and a prototype of the SMPS was manufactured. The whole structure is presented as a module installed in an aluminum case. After initial tests, a number of shortcomings were identified: a noticeable heating of the radiators of key transistors, the difficulty of removing heat from powerful domestic resistors, and large dimensions.
The design has been improved: the design of the control board has been changed using surface-mounted components on a double-sided board, its perpendicular installation on the main board; the use of a radiator with a built-in fan from a computer; all heat-stressed elements of the circuit were specially located on one side of the case along the blowing direction of the main fan for the most effective cooling. As a result of refinement, the dimensions of the IPP decreased three times and the shortcomings identified during the initial tests were eliminated. The modified sample has the following characteristics: supply voltage Upit=~180-240 V, frequency fwork=90 kHz, output power Pp=800 W, efficiency=85%, weight=2.1 kg, overall dimensions 145X145X80 mm.
This work is devoted to the design of a switching power supply designed to power an audio frequency power amplifier, which is part of a high-power home sound reproducing system. The creation of a home sound reproducing system began with the choice of a UMZCH circuit design. For this, an analysis of the circuit design of sound-reproducing devices was carried out. The choice was stopped on the UMZCH high fidelity scheme.
This amplifier has very high characteristics, it contains overload and short circuit protection devices, devices for maintaining the zero potential of a constant voltage at the output and a device for compensating the resistance of the wires connecting the amplifier to the acoustics. Despite the fact that the UMZCH circuit has been published for a long time, radio amateurs to this day repeat its design, references to which can be found in almost any literature relating to the assembly of devices for high-quality music reproduction. Based on this article, it was decided to assemble a four-channel UMZCH, the total power consumption of which was 800 watts. Therefore, the next stage in the assembly of the UMZCH was the development and assembly of a power supply design that provides an output power of at least 800 W, small dimensions and weight, reliability in operation and protection against overload and short circuits.
Power supplies are built mainly according to two schemes: traditional classical and according to the scheme of switching voltage converters. Therefore, it was decided to assemble and refine the design of a switching power supply.
Research of sources of secondary power supply. Power supplies are divided into two groups: primary and secondary power supplies.
Primary sources are devices that convert various types of energy into electrical energy (electric machine generators, electrochemical current sources, photoelectric and thermionic converters, etc.).
Secondary power devices are converters of one type of electrical energy into another. These include:
- * AC to DC voltage converters (rectifiers);
- * AC voltage transducers (transformers);
- * DC-to-AC converters (inverters).
Secondary power supplies are built mainly according to two schemes: traditional classical and according to the scheme of pulse voltage converters. The main drawback of power MTs, made according to the traditional classical scheme, is their large weight and size characteristics, as well as a significant effect on other REE devices of a strong magnetic field of power transformers. The problem of SMPS is the creation of high-frequency interference by them, and as a result of this - electromagnetic incompatibility with some types of REA. The analysis showed that IIP meet the requirements most fully, which is confirmed by their wide use in REA.
Transformers of switching power supplies differ from traditional ones in the following way: - supply with rectangular voltage; complicated form of windings (midpoint leads) and operation at higher frequencies (up to several tens of kHz). In addition, the parameters of the transformer have a significant impact on the operation of semiconductor devices and the characteristics of the converter. So, the magnetizing inductance of the transformer increases the switching time of the transistors; leakage inductance (with a rapidly changing current) is the cause of overvoltages on transistors, which can lead to their breakdown; no-load current reduces the efficiency of the converter and worsens the thermal regime of transistors. The noted features are taken into account when calculating and designing SMPS transformers.
In this paper, a switching power supply with a power of 800 W is considered. It differs from those described earlier by the use of field-effect transistors and a transformer with a primary winding with an average output in the converter. The first provides a higher efficiency and a reduced level of high-frequency noise, and the second - half the current through the key transistors and eliminates the need for an isolation transformer in their gate circuits.
The disadvantage of such a circuit design is the high voltage on the halves of the primary winding, which requires the use of transistors with the appropriate allowable voltage. True, unlike a bridge converter, in this case two transistors are enough instead of four, which simplifies the design and increases the efficiency of the device.
Switching power supplies (UPS) use one- and two-stroke high-frequency converters. The efficiency of the former is lower than the latter, so it is not advisable to design single-cycle UPSs with a power of more than 40 ... 60 W. Push-pull converters allow you to get much more output power with high efficiency. They are divided into several groups, characterized by the method of excitation of the output key transistors and the circuit for including them in the circuit of the primary winding of the converter transformer. If we talk about the method of excitation, then two groups can be distinguished: with self-excitation and external excitation.
The former are less popular due to difficulties in establishing. When designing powerful (more than 200 W) UPSs, the complexity of their manufacture unreasonably increases, so they are of little use for such power supplies. Externally excited converters are well suited for high power UPS applications and sometimes require little or no maintenance. As for connecting the key transistors to the transformer, there are three schemes here: the so-called half-bridge (Fig. 1, a), bridge (Fig. 1, b). To date, the most widely used half-bridge converter.
It requires two transistors with a relatively low voltage Ukemax. As can be seen from Fig. 1a, capacitors C1 and C2 form a voltage divider, to which the primary (I) winding of transformer T2 is connected. When opening the key transistor, the amplitude of the voltage pulse on the winding reaches the value Upit / 2 - Uke nac. The bridge converter is similar to the half-bridge one, but in it the capacitors are replaced by transistors VT3 and VT4 (Fig. 1b), which open diagonally in pairs. This converter has a slightly higher efficiency due to an increase in the voltage supplied to the primary winding of the transformer, and therefore, a decrease in the current flowing through the transistors VT1-VT4. The voltage amplitude on the primary winding of the transformer in this case reaches the value Upit - 2Uke us.
Of particular note is the converter according to the scheme in Fig. 1c, which is distinguished by the highest efficiency. This is achieved by reducing the primary winding current and, as a result, reducing the power dissipation in key transistors, which is extremely important for powerful UPSs. The amplitude of the voltage pulses in half of the primary winding increases to the value Upit - Uke us.
It should also be noted that, unlike other converters, it does not require an input isolation transformer. In the device according to the scheme in Fig. 1c, it is necessary to use transistors with a high Uke max value. Since the end of the upper (according to the scheme) half of the primary winding is connected to the beginning of the lower one, when current flows in the first of them (VT1 is open), a voltage is created in the second that is equal (in absolute value) to the amplitude of the voltage on the first, but opposite in sign relative to Upit. In other words, the voltage at the collector of the closed transistor VT2 reaches 2Upit. therefore, its Uke max must be greater than 2Upit. In the proposed UPS, a push-pull converter with a transformer is used, the primary winding of which has an average output. It has high efficiency, low ripple and weakly radiates interference into the surrounding space.
