Single-ended converters with high efficiency, 12/220 volts
Some common household electrical appliances, such as a fluorescent lamp, a photo flash and a number of others, are sometimes convenient to use in a car.
Since most devices are designed to be powered from a network with an operating voltage of 220 V, a step-up converter is needed. An electric razor or a small fluorescent lamp consumes no more than 6...25 W of power. Moreover, such a converter often does not require an alternating voltage at the output. The above household electrical appliances operate normally when powered by direct or unipolar pulsating current.
The first version of a single-cycle (flyback) pulse DC voltage converter 12 V/220 V is made on an imported UC3845N PWM controller chip and a powerful N-channel field-effect transistor BUZ11 (Fig. 4.10). These elements are more affordable than their domestic counterparts, and make it possible to achieve high efficiency from the device, including due to the low source-drain voltage drop across an open field-effect transistor (the efficiency of the converter also depends on the ratio of the width of the pulses transmitting energy to the transformer to the pause).
The specified microcircuit is specially designed for single-cycle converters and has all the necessary components inside, which allows reducing the number of external elements. It has a high-current quasi-complementary output stage specifically designed for direct power control. M-channel field-effect transistor with insulated gate. The operating pulse frequency at the output of the microcircuit can reach 500 kHz. The frequency is determined by the ratings of elements R4-C4 and in the above circuit is about 33 kHz (T = 50 μs).
Rice. 4.10. Circuit of a single-cycle pulse converter that increases voltage
The chip also contains a protection circuit to shut down the converter when the supply voltage drops below 7.6 V, which is useful when powering devices from a battery.
Let's take a closer look at the operation of the converter. In Fig. Figure 4.11 shows voltage diagrams that explain the ongoing processes. When positive pulses appear at the gate of the field-effect transistor (Fig. 4.11, a), it opens and resistors R7-R8 will receive the pulses shown in Fig. 4.11, c.
The slope of the top of the pulse depends on the inductance of the transformer winding, and if at the top there is a sharp increase in the voltage amplitude, as shown by the dotted line, this indicates saturation of the magnetic circuit. At the same time, conversion losses increase sharply, which leads to heating of the elements and deteriorates the operation of the device. To eliminate saturation, you will need to reduce the pulse width or increase the gap in the center of the magnetic circuit. Usually a gap of 0.1...0.5 mm is sufficient.
When the power transistor is turned off, the inductance of the transformer windings causes voltage surges to appear, as shown in the figures.
Rice. 4.11. Voltage diagrams at circuit control points
With proper manufacturing of transformer T1 (sectioning of the secondary winding) and low-voltage power supply, the surge amplitude does not reach a value dangerous for the transistor and therefore, in this circuit, special measures in the form of damping circuits in the primary winding of T1 are not used. And in order to suppress surges in the current feedback signal coming to the input of the DA1.3 microcircuit, a simple RC filter from elements R6-C5 is installed.
The voltage at the converter input, depending on the condition of the battery, can vary from 9 to 15 V (which is 40%). To limit the change in output voltage, input feedback is removed from the divider of resistors R1-R2. In this case, the output voltage at the load will be maintained in the range of 210...230 V (Rload = 2200 Ohm), see table. 4.2, i.e. it changes by no more than 10%, which is quite acceptable.
Table 4.2. Circuit parameters when changing supply voltage
Stabilization of the output voltage is carried out by automatically changing the width of the pulse that opens transistor VT1 from 20 μs at Upit = 9 V to 15 μs (Upit = 15 V).
All elements of the circuit, except for capacitor C6, are placed on a single-sided printed circuit board made of fiberglass with dimensions of 90x55 mm (Fig. 4.12).
Rice. 4.12. PCB topology and arrangement of elements
Transformer T1 is mounted on the board using an M4x30 screw through a rubber gasket, as shown in Fig. 4.13.
Rice. 4.13 Mounting type of transformer T1
Transistor VT1 is installed on the radiator. Plug design. XP1 must prevent erroneous supply of voltage to the circuit.
The T1 pulse transformer is made using the widely used BZO armor cups from the M2000NM1 magnetic core. At the same time, in the central part they should have a gap of 0.1...0.5 mm.
The magnetic core can be purchased with an existing gap or it can be made using coarse sandpaper. It is better to select the gap size experimentally when tuning so that the magnetic circuit does not enter the saturation mode - this is convenient to control by the shape of the voltage at the source VT1 (see Fig. 4.11, c).
For transformer T1, winding 1-2 contains 9 turns of wire with a diameter of 0.5-0.6 mm, windings 3-4 and 5-6 each contain 180 turns of wire with a diameter of 0.15...0.23 mm (wire type PEL or PEV). In this case, the primary winding (1-2) is located between two secondary windings, i.e. First, winding 3-4 is wound, and then 1-2 and 5-6.
When connecting the transformer windings, it is important to observe the phasing shown in the diagram. Incorrect phasing will not damage the circuit, but it will not work as intended.
The following parts were used during assembly: adjusted resistor R2 - SPZ-19a, fixed resistors R7 and R8 type S5-16M for 1 W, the rest can be of any type; electrolytic capacitors C1 - K50-35 for 25 V, C2 - K53-1A for 16 V, C6 - K50-29V for 450 V, and the rest are of the K10-17 type. Transistor VT1 is installed on a small (by the size of the board) radiator made of duralumin profile. Setting up the circuit consists of checking the correct phrasing of connecting the secondary winding using an oscilloscope, as well as setting resistor R4 to the desired frequency. Resistor R2 sets the output voltage at the XS1 sockets when the load is on.
The given converter circuit is designed to work with a previously known load power (6...30 W - permanently connected). At idle, the voltage at the circuit output can reach 400 V, which is not acceptable for all devices, as it can lead to damage due to insulation breakdown.
If the converter is intended to be used in operation with a load of different power, which is also turned on during operation of the converter, then it is necessary to remove the voltage feedback signal from the output. A variant of such a scheme is shown in Fig. 4.14. This not only allows you to limit the output voltage of the circuit in idle mode to 245 V, but also reduces the power consumption in this mode by about 10 times (Ipot=0.19 A; P=2.28 W; Uh=245 V).
Rice. 4.14. Single-cycle converter circuit with maximum no-load voltage limitation
Transformer T1 has the same magnetic circuit and winding data as in the circuit (Fig. 4.10), but contains an additional winding (7-4) - 14 turns of PELSHO wire with a diameter of 0.12.0.18 mm (it is wound last). The remaining windings are made in the same way as in the transformer described above.
To manufacture a pulse transformer, you can also use square cores of the series. KV12 made of M2500NM ferrite - the number of turns in the windings in this case will not change. To replace armor magnetic cores (B) with more modern square ones (KB), you can use the table. 4.3.
The voltage feedback signal from winding 7-8 is supplied through a diode to the input (2) of the microcircuit, which makes it possible to more accurately maintain the output voltage in a given range, as well as provide galvanic isolation between the primary and output circuits. The parameters of such a converter, depending on the supply voltage, are given in table. 4.4.
Table 4.4. Circuit parameters when changing supply voltage
The efficiency of the described converters can be increased a little more if the pulse transformers are secured to the board with a dielectric screw or heat-resistant glue. A variant of the printed circuit board topology for assembling the circuit is shown in Fig. 4.15.
Rice. 4.15. PCB topology and arrangement of elements
Using such a converter, you can power electric shavers "Agidel", "Kharkov" and a number of other devices from the vehicle's on-board network.
The Beetle is assembled according to the Hartley circuit with a non-standard inclusion of feedback, due to which its efficiency is 10-20% higher than similar circuits. This circuit is similar to that used in the simplest telephone bug. It has been circulating on the Internet for a long time, and site owners continue to copy it from each other, not noticing the gross error in the scheme. This error has been fixed here.
|
R1=R3=R4 - 9.1 k, |
As a rule, the circuit starts working immediately after assembly. If a squeak is heard in the receiver, you should bypass the circuit with a capacitor with a capacity of at least 1 µF. It is better to connect the antenna through a conductor with a capacity of 1-2 pf. With an antenna length of 20cm, my range was 140m.
Photos of the finished device in the version powered by 2 lithium tablets CR-1220 (6v). (work for a very long time):
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| VT1 | Bipolar transistor | KT315A | 1 | To notepad | ||
| VT2 | Transistor | KT325VM | 1 | To notepad | ||
| C1 | Capacitor | 0.1 µF | 1 | To notepad | ||
| C2 | Capacitor | 56 pF | 1 | To notepad | ||
| C3 | Capacitor | 24 pF | 1 | To notepad | ||
| Capacitor | 1-2 pF | 1 | To connect the antenna | To notepad | ||
| Capacitor | Not less than 1 µF | 1 | To bypass the circuit | To notepad | ||
| R1, R3, R4 | Resistor | 9.1 kOhm | 3 | To notepad | ||
| R2 | Resistor | 300 kOhm | 1 | To notepad | ||
| L1 | Inductor | 1 |
The modern automotive industry has reached a level of development at which, without fundamental scientific research, it is almost impossible to achieve fundamental improvements in the design of traditional internal combustion engines. This situation forces designers to pay attention to alternative power plant designs. Some engineering centers have focused their efforts on creating and adapting hybrid and electric models for serial production, while other automakers are investing in the development of engines using fuel from renewable sources (for example, biodiesel using rapeseed oil). There are other powertrain projects that could eventually become the new standard propulsion system for vehicles.
Among the possible sources of mechanical energy for future cars is the external combustion engine, which was invented in the mid-19th century by the Scot Robert Stirling as a thermal expansion engine.
Scheme of work
The Stirling engine converts thermal energy supplied from outside into useful mechanical work by changes in working fluid temperature(gas or liquid) circulating in a closed volume.
In general, the operating diagram of the device is as follows: in the lower part of the engine, the working substance (for example, air) heats up and, increasing in volume, pushes the piston upward. Hot air enters the upper part of the engine, where it is cooled by a radiator. The pressure of the working fluid decreases, the piston is lowered for the next cycle. In this case, the system is sealed and the working substance is not consumed, but only moves inside the cylinder.
There are several design options for power units using the Stirling principle.
Stirling modification "Alpha"
The engine consists of two separate power pistons (hot and cold), each of which is located in its own cylinder. Heat is supplied to the cylinder with the hot piston, and the cold cylinder is located in a cooling heat exchanger.
Stirling modification "Beta"
The cylinder containing the piston is heated at one end and cooled at the opposite end. A power piston and a displacer move in the cylinder, designed to change the volume of the working gas. The regenerator carries out the return movement of the cooled working substance into the hot cavity of the engine.
Stirling modification "Gamma"
The design consists of two cylinders. The first is completely cold, in which the power piston moves, and the second, hot on one side and cold on the other, serves to move the displacer. A regenerator for circulating cold gas can be common to both cylinders or be part of the displacer design.
Advantages of the Stirling engine
Like most external combustion engines, Stirling is characterized multi-fuel: the engine operates due to temperature changes, regardless of the reasons that caused it.
Interesting fact! An installation was once demonstrated that operated on twenty fuel options. Without stopping the engine, gasoline, diesel fuel, methane, crude oil and vegetable oil were supplied to the external combustion chamber - the power unit continued to operate stably.
The engine has simplicity of design and does not require additional systems and attachments (timing belt, starter, gearbox).
The features of the device guarantee a long service life: more than one hundred thousand hours of continuous operation.
The Stirling engine is silent, since detonation does not occur in the cylinders and there is no need to remove exhaust gases. The “Beta” modification, equipped with a rhombic crank mechanism, is a perfectly balanced system that has no vibrations during operation.
There are no processes occurring in the engine cylinders that could have a negative impact on the environment. By choosing a suitable heat source (eg solar energy), Stirling can be absolutely environmentally friendly power unit.
Disadvantages of the Stirling design
Despite all the positive properties, immediate mass use of Stirling engines is impossible for the following reasons:
The main problem is the material consumption of the structure. Cooling the working fluid requires large-volume radiators, which significantly increases the size and metal consumption of the installation.
The current technological level will allow the Stirling engine to compare in performance with modern gasoline engines only through the use of complex types of working fluid (helium or hydrogen) under pressure of more than one hundred atmospheres. This fact raises serious questions both in the field of materials science and in ensuring user safety.
An important operational problem is related to issues of thermal conductivity and temperature resistance of metals. Heat is supplied to the working volume through heat exchangers, which leads to inevitable losses. In addition, the heat exchanger must be made of heat-resistant metals that can withstand high pressure. Suitable materials are very expensive and difficult to process.
The principles of changing the modes of the Stirling engine are also fundamentally different from traditional ones, which requires the development of special control devices. Thus, to change power it is necessary to change the pressure in the cylinders, the phase angle between the displacer and the power piston, or influence the capacity of the cavity with the working fluid.
One way to control the shaft rotation speed on a Stirling engine model can be seen in the following video:
Efficiency
In theoretical calculations, the efficiency of the Stirling engine depends on the temperature difference of the working fluid and can reach 70% or more in accordance with the Carnot cycle.
However, the first samples realized in metal had extremely low efficiency for the following reasons:
- ineffective coolant (working fluid) options that limit the maximum heating temperature;
- energy losses due to friction of parts and thermal conductivity of the engine housing;
- lack of construction materials resistant to high pressure.
Engineering solutions constantly improved the design of the power unit. Thus, in the second half of the 20th century, a four-cylinder automobile The Stirling engine with a rhombic drive showed an efficiency of 35% in tests on a water coolant with a temperature of 55 ° C. Careful design development, the use of new materials and fine-tuning of working units ensured the efficiency of the experimental samples was 39%.
Note! Modern gasoline engines of similar power have an efficiency of 28-30%, and turbocharged diesel engines within 32-35%.
Modern examples of the Stirling engine, such as that created by the American company Mechanical Technology Inc, demonstrate efficiency of up to 43.5%. And with the development of the production of heat-resistant ceramics and similar innovative materials, it will be possible to significantly increase the temperature of the working environment and achieve an efficiency of 60%.
Examples of successful implementation of automobile Stirlings
Despite all the difficulties, there are many known efficient Stirling engine models that are applicable to the automotive industry.
Interest in Stirling, suitable for installation in a car, appeared in the 50s of the 20th century. Work in this direction was carried out by such concerns as Ford Motor Company, Volkswagen Group and others.
The UNITED STIRLING company (Sweden) developed Stirling, which made maximum use of serial components and assemblies produced by automakers (crankshaft, connecting rods). The resulting four-cylinder V-engine had a specific weight of 2.4 kg/kW, which is comparable to the characteristics of a compact diesel engine. This unit was successfully tested as a power plant for a seven-ton cargo van.
One of the successful samples is a four-cylinder Stirling engine made in the Netherlands, model “Philips 4-125DA”, intended for installation in a passenger car. The engine had a working power of 173 hp. With. in dimensions similar to a classic gasoline unit.
General Motors engineers achieved significant results by building an eight-cylinder (4 working and 4 compression cylinders) V-shaped Stirling engine with a standard crank mechanism in the 70s.
A similar power plant in 1972 equipped with a limited series of Ford Torino cars, whose fuel consumption has decreased by 25% compared to the classic gasoline V-shaped eight.
Currently, more than fifty foreign companies are working to improve the design of the Stirling engine in order to adapt it to mass production for the needs of the automotive industry. And if it is possible to eliminate the disadvantages of this type of engine, while at the same time maintaining its advantages, then it will be Stirling, and not turbines and electric motors, that will replace gasoline internal combustion engines.
The described device provides exceptionally high conversion efficiency, allows regulation of the output voltage and its stabilization, and operates stably when the load power varies. This type of converter is interesting and undeservedly little widespread - quasi-resonant, which is largely free from the disadvantages of other popular circuits. The idea of creating such a converter is not new, but practical implementation became feasible relatively recently, after the advent of powerful high-voltage transistors that allow significant pulse collector current at a saturation voltage of about 1.5 V. The main distinctive feature and main advantage of this type of power source is the high efficiency of the voltage converter , reaching 97...98% without taking into account losses on the secondary circuit rectifier, which are mainly determined by the load current.
The quasi-resonant converter differs from a conventional pulse converter, in which by the moment the switching transistors are closed, the current flowing through them is maximum, the quasi-resonant one differs in that by the moment the transistors are closed, their collector current is close to zero. Moreover, the reduction in current at the moment of closing is ensured by the reactive elements of the device. It differs from resonant in that the conversion frequency is not determined by the resonant frequency of the collector load. Thanks to this, it is possible to regulate the output voltage by changing the conversion frequency and realize stabilization of this voltage. Since by the time the transistor closes, the reactive elements reduce the collector current to a minimum, the base current will also be minimal and, therefore, the closing time of the transistor is reduced to the value of its opening time. Thus, the problem of through current that occurs during switching is completely eliminated. In Fig. Figure 4.22 shows a schematic diagram of a self-oscillating unstabilized power supply.
Main technical characteristics:
Overall efficiency of the unit, %................................................... ....................92;
Output voltage, V, with a load resistance of 8 Ohms....... 18;
Operating frequency of the converter, kHz....................................20;
Maximum output power, W...................................................55;
Maximum amplitude of output voltage ripple with operating frequency, V
The main share of power losses in the unit falls on the heating of the rectifier diodes of the secondary circuit, and the efficiency of the converter itself is such that there is no need for heat sinks for transistors. The power loss on each of them does not exceed 0.4 W. Special selection of transistors according to any parameters also not required. When the output is shorted or the maximum output power is exceeded, generation is interrupted, protecting the transistors from overheating and breakdown.
The filter, consisting of capacitors C1...SZ and inductor LI, L2, is designed to protect the supply network from high-frequency interference from the converter. The autogenerator is started by circuit R4, C6 and capacitor C5. The generation of oscillations occurs as a result of the action of positive feedback through transformer T1, and their frequency is determined by the inductance of the primary winding of this transformer and the resistance of resistor R3 (as the resistance increases, the frequency increases).
Chokes LI, L2 and transformer T1 are wound on identical ring magnetic cores K12x8x3 made of 2000NM ferrite. The inductor windings are performed simultaneously, “in two wires,” using PELSHO-0.25 wire; number of turns - 20. Winding I of the TI transformer contains 200 turns of PEV-2-0.1 wire, wound in bulk, evenly around the entire ring. Windings II and III are wound “in two wires” - 4 turns of PELSHO-0.25 wire; winding IV is a turn of the same wire. For the T2 transformer, a K28x16x9 ring magnetic core made of 3000NN ferrite was used. Winding I contains 130 turns of PELI10-0.25 wire, laid turn to turn. Windings II and III - 25 turns of PELSHO-0.56 wire each; winding - “in two wires”, evenly around the ring.
Choke L3 contains 20 turns of PELI10-0.25 wire, wound on two folded together ring magnetic cores K12x8x3 made of 2000NM ferrite. Diodes VD7, VD8 must be installed on heat sinks with a dissipation area of at least 2 cm2 each.
The described device was designed for use in conjunction with analog stabilizers for various voltage values, so there was no need for deep ripple suppression at the output of the unit. Ripple can be reduced to the required level by using LC filters that are common in such cases, such as, for example, in another version of this converter with the following basic technical characteristics:
Rated output voltage, V................................................... 5,
Maximum output current, A................................................... ......... 2;
Maximum pulsation amplitude, mV............................................50;
Change in output voltage, mV, no more, when the load current changes
from 0.5 to 2 A and mains voltage from 190 to 250 V........................150;
Maximum conversion frequency, kHz.................................... 20.
The circuit of a stabilized power supply based on a quasi-resonant converter is shown in Fig. 4.23.
The output voltage is stabilized by a corresponding change in the operating frequency of the converter. As in the previous block, powerful transistors VT1 and VT2 do not need heat sinks. Symmetrical control of these transistors is implemented using a separate master pulse generator assembled on a DDI chip. Trigger DD1.1 operates in the generator itself.
The pulses have a constant duration specified by the circuit R7, C12. The period is changed by the OS circuit, which includes optocoupler U1, so that the voltage at the output of the unit is maintained constant. The minimum period is set by circuit R8, C13. Trigger DDI.2 divides the repetition frequency of these pulses by two, and the square wave voltage is supplied from the direct output to the transistor current amplifier VT4, VT5. Next, the current-amplified control pulses are differentiated by the circuit R2, C7, and then, already shortened to a duration of approximately 1 μs, they enter through the transformer T1 into the base circuit of transistors VT1, VT2 of the converter. These short pulses serve only to switch transistors - closing one of them and opening the other.
In addition, the main power from the excitation generator is consumed only when switching powerful transistors, so the average current consumed by it is small and does not exceed 3 mA, taking into account the current of the zener diode VD5. This allows it to be powered directly from the primary network through the quenching resistor R1. Transistor VT3 is a control signal voltage amplifier, as in a compensation stabilizer. The stabilization coefficient of the block's output voltage is directly proportional to the static current transfer coefficient of this transistor.
The use of transistor optocoupler U1 ensures reliable galvanic isolation of the secondary circuit from the network and high noise immunity at the control input of the master oscillator. After the next switching of transistors VT1, VT2, the capacitor SY begins to recharge and the voltage at the base of the transistor VT3 begins to increase, the collector current also increases. As a result, the optocoupler transistor opens, maintaining the master oscillator capacitor C13 in a discharged state. After the rectifier diodes VD8, VD9 are closed, the capacitor SY begins to discharge to the load and the voltage across it drops. Transistor VT3 closes, as a result of which capacitor C13 begins charging through resistor R8. As soon as the capacitor is charged to the switching voltage of the trigger DD1.1, a high voltage level will be established at its direct output. At this moment, the next switching of transistors VT1, VT2 occurs, as well as the discharge of the SI capacitor through the opened optocoupler transistor.
The next process of recharging the capacitor SY begins, and the trigger DD1.1 after 3...4 μs will return to the zero state again due to the small time constant of the circuit R7, C12, after which the entire control cycle is repeated, regardless of which of the transistors is VT1 or VT2 - open during the current half-term. When the source is turned on, at the initial moment, when the capacitor SY is completely discharged, there is no current through the optocoupler LED, the generation frequency is maximum and is determined mainly by the time constant of the circuit R8, C13 (the time constant of the circuit R7, C12 is several times smaller). With the ratings of these elements indicated in the diagram, this frequency will be about 40 kHz, and after it is divided by the DDI.2 trigger - 20 kHz. After charging the capacitor SY to the operating voltage, the OS stabilizing loop on the elements VD10, VT3, U1 comes into operation, after which the conversion frequency will already depend on the input voltage and load current. Voltage fluctuations on the capacitor SY are smoothed out by filter L4, C9. Chokes LI, L2 and L3 are the same as in the previous block.
Transformer T1 is made on two ring magnetic cores K12x8x3 folded together from 2000NM ferrite. The primary winding is wound in bulk evenly throughout the entire ring and contains 320 turns of PEV-2-0.08 wire. Windings II and III each contain 40 turns of wire PEL1110-0.15; they are wound “in two wires”. Winding IV consists of 8 turns of PELSHO-0.25 wire. Transformer T2 is made on a ring magnetic core K28x16x9 made of 3000NN ferrite. Winding I - 120 turns of PELSHO-0.15 wire, and II and III - 6 turns of PEL1110-0.56 wire, wound “in two wires”. Instead of PELSHO wire, you can use PEV-2 wire of the appropriate diameter, but in this case it is necessary to lay two or three layers of varnished cloth between the windings.
Choke L4 contains 25 turns of wire PEV-2-0.56, wound on a ring magnetic core K12x6x4.5 made of 100NNH1 ferrite. Any ready-made inductor with an inductance of 30...60 μH for a saturation current of at least 3 A and an operating frequency of 20 kHz is also suitable. All fixed resistors are MJIT. Resistor R4 - adjusted, of any type. Capacitors C1...C4, C8 - K73-17, C5, C6, C9, SY - K50-24, the rest - KM-6. The KS212K zener diode can be replaced with KS212Zh or KS512A. Diodes VD8, VD9 must be installed on radiators with a dissipation area of at least 20 cm2 each. The efficiency of both blocks can be increased if, instead of KD213A diodes, Schottky diodes are used, for example, any of the KD2997 series. In this case, heat sinks for diodes will not be required.
