Most modern electronic devices practically do not use analog (transformer) power supplies; they are replaced by pulsed voltage converters. To understand why this happened, it is necessary to consider the design features, as well as the strengths and weaknesses of these devices. We will also talk about the purpose of the main components of pulsed sources and provide a simple example of an implementation that can be assembled with your own hands.
Of the several methods of converting voltage to power electronic components, two that are most widespread can be identified:
Let's look at how these two options differ.
Let's consider a simplified block diagram of this device. As can be seen from the figure, a step-down transformer is installed at the input, with its help the amplitude of the supply voltage is converted, for example, from 220 V we get 15 V. The next block is a rectifier, its task is to convert the sinusoidal current into a pulsed one (the harmonic is shown above the symbolic image). For this purpose, rectifying semiconductor elements (diodes) connected via a bridge circuit are used. Their operating principle can be found on our website.
The next block performs two functions: it smoothes the voltage (a capacitor of appropriate capacity is used for this purpose) and stabilizes it. The latter is necessary so that the voltage does not “drop” when the load increases.
The given block diagram is greatly simplified; as a rule, a source of this type has an input filter and protective circuits, but this is not important for explaining the operation of the device.
All the disadvantages of the above option are directly or indirectly related to the main design element - the transformer. Firstly, its weight and dimensions limit miniaturization. In order not to be unfounded, we will use as an example a step-down transformer 220/12 V with a rated power of 250 W. The weight of such a unit is about 4 kilograms, dimensions 125x124x89 mm. You can imagine how much a laptop charger based on it would weigh.
Secondly, the price of such devices is sometimes many times higher than the total cost of the other components.
As can be seen from the block diagram shown in Figure 3, the operating principle of these devices differs significantly from analog converters, primarily in the absence of an input step-down transformer.
Figure 3. Block diagram of a switching power supply Let's consider the operating algorithm of such a source:
Unlike a step-down transformer, the core of this device is made of ferrimagnetic materials, this contributes to the reliable transmission of RF signals, which can be in the range of 20-100 kHz. A characteristic feature of IT is that when connecting it, the inclusion of the beginning and end of the windings is critical. The small dimensions of this device make it possible to produce miniature devices; an example is the electronic harness (ballast) of an LED or energy-saving lamp.
Now, as promised, let’s look at the operating principle of the main element of this device – the inverter.
RF modulation can be done in three ways:
In practice, the last option is used. This is due both to the simplicity of implementation and to the fact that PWM has a constant communication frequency, unlike the other two modulation methods. A block diagram describing the operation of the controller is shown below.
The device operation algorithm is as follows:
The reference frequency generator generates a series of rectangular signals, the frequency of which corresponds to the reference one. Based on this signal, a sawtooth U P is formed, which is supplied to the input of the comparator K PWM. The UUS signal coming from the control amplifier is supplied to the second input of this device. The signal generated by this amplifier corresponds to the proportional difference between U P (reference voltage) and U RS (control signal from the feedback circuit). That is, the control signal UUS is, in fact, a mismatch voltage with a level that depends on both the current on the load and the voltage on it (U OUT).
This implementation method allows you to organize a closed circuit that allows you to control the output voltage, that is, in fact, we are talking about a linear-discrete functional unit. Pulses are generated at its output, with a duration depending on the difference between the reference and control signals. Based on it, a voltage is created to control the key transistor of the inverter.
The process of stabilizing the output voltage is carried out by monitoring its level; when it changes, the voltage of the control signal U PC changes proportionally, which leads to an increase or decrease in the duration between pulses.
As a result, the power of the secondary circuits changes, which ensures stabilization of the output voltage.
To ensure safety, galvanic isolation between the power supply and feedback is necessary. As a rule, optocouplers are used for this purpose.
If we compare analog and pulse devices of the same power, the latter will have the following advantages:
The disadvantages of pulse technology include:
The presence of RF interference is a consequence of the operation of the high-frequency converter. This factor requires the installation of a filter that suppresses interference. Unfortunately, its operation is not always effective, which imposes some restrictions on the use of devices of this type in high-precision equipment.
Special requirements for the load, it should not be reduced or increased. As soon as the current level exceeds the upper or lower threshold, the output voltage characteristics will begin to differ significantly from the standard ones. As a rule, manufacturers (even recently Chinese ones) provide for such situations and install appropriate protection in their products.
Almost all modern electronics are powered from blocks of this type; examples include:
Let's consider the circuit of a simple power supply, where the above-described principle of operation is applied.
Designations:
The setup comes down to selecting the values of R2 and C5, which ensure excitation of the generator at an input voltage of 185-240 V.
Sooner or later, a radio amateur faces the problem of making a universal power supply unit (PSU) that would be useful for “all occasions.” That is, it had sufficient power, reliability, and a widely adjustable output voltage; moreover, it protected the load from “excessive consumption” of current during testing and was not afraid of short circuits.
It is proposed, in the author's opinion, that the power supply that best satisfies these conditions is simple enough to repeat, providing a stabilized voltage of 1.5-24 V with an output current of up to ZA. In addition, it can operate in current source mode with the ability to smoothly adjust the stabilization current within 10-100 mA or with fixed current values of 0.1 A, 1 A, 3 A.
Let's look at the diagram power supply(see figure). Its basis is a traditional voltage stabilizer circuit, the “heart” is the KR142EN12 microcircuit, which is currently available to a wide range of radio amateurs. A fairly powerful unified incandescent transformer TN-56 was chosen as the power transformer, which has four secondary windings with a permissible current of 3.4 A and a voltage of each 6.3 V. Depending on the required output voltage, switch SA2 connects two, three or four series-connected windings This is necessary to reduce the power dissipated on the control element, and, consequently, increase the efficiency of the device and facilitate the temperature regime. Indeed, in the most unfavorable mode, with the maximum difference between the input and output voltages (of course, if the output voltage corresponds to the range specified by switch SA2) and the maximum current FOR, the power dissipated on the control element will be: Ppacc.max = (Uвx.max-2Uvd- Uout.min)*Imax (1) Rdis.max = (12.6-2*0.7-1.5)*3 = 29.1 W, where Uin.max is the maximum input effective voltage of this range; Uout.min - minimum output voltage of this range; Uvd is the voltage drop across the rectifier bridge diode. It is easy to check that without dividing the output voltage into ranges, the power dissipated by the control element reaches 70 W.
The alternating voltage is rectified by the diode bridge VD1-VD4 and smoothed by capacitor C5. Fuse FU2 protects the transformer when the rectifier diodes fail. Transistors VT1, VT2 serve to increase the output current of the power supply unit and facilitate the operation of the integrated stabilizer DA1. Resistor R1 sets the current through DA1, opening VT2:
IDA1 = Ubevt2/R1 = 0.7/51 = 0.014 A, (2)
where Ubevt2 is the opening voltage of the emitter-base of transistor VT2. At a current of 14 mA, the DA1 chip can operate without a heatsink. To increase the stability of the output voltage, the control voltage is removed from the line of resistors R2-R4 connected to the output of the microcircuit and supplied to the “control” pin 01 DA1 through the decoupling diode VD6. The output voltage is adjusted by resistors: R4 - “COARSE” and R3 - “FINE”. The current stabilizer is made of DA1, current-setting resistors R5-R9 and decoupling diode VD7. The selection of the required discrete stabilization current is carried out by switch SA3. In addition, at the “10-100 mA” limit, it is possible to smoothly regulate the current using resistor R9. If necessary, you can change the stabilization current by changing the values of the setting resistors using the formula:
R = 1.35/Istab, (3)
where R is the resistance of the current-setting resistor, Ohm; Istab - stabilization current, A. The power of current-setting resistors is determined by the formula:
P = I*I*R, (4)
where I is the range stabilization current; R is the resistance of the resistor. In reality, the power of the current-setting resistors has been deliberately increased for reasons of reliability. So resistor R8 type C5-16V is selected with a power of 10 W. In current stabilization mode (switch SA3 in the “FOR” position), the power dissipated by the resistor is 3.8 W. And even if you install a five-watt resistor, its power load will be 72% of the maximum permissible. Similarly, R7 type C5-16V has a power of 5 W, but MLT-2 can also be used. Resistor R6 is type MLT-2, but you can use MLT-1. R9 is a wire-wound variable resistor of type PPZ-43 with a power of 3 W. R5 type MLT-1. These resistors must be positioned so that they are cooled in the best possible way and do not, if possible, heat other elements of the circuit, as well as each other. To make the adjustment (set current) clearer, mark 10, 20, 50, 75 and 100 mA on the resistor R9 dial using an external milliammeter (tester) and connecting it directly to the power supply sockets.
Additional convenience when working with a power supply is provided by a pV voltmeter, which is an M95 microammeter with a total deviation current of 0.15 mA.
The resistance of resistor R11 is selected so that the final scale value corresponds to a voltage of 30 V. You can also use any other measuring head with a total deviation current of up to 1.5 mA by selecting a current-limiting resistor R11.
As switches SA2, SA3, biscuits are used - type 11P3NMP. To increase the permissible switching current, the equivalent terminals of the three biscuits are paralleled. The lock is installed depending on the number of positions.
Capacitor C5 is prefabricated and consists of five parallel-connected capacitors of type K50-12 with a capacity of 2000 μF x 50 V.
Transistor VT1 is installed externally on a radiator with an area of 400 cm2. It can be replaced with KT803A, KT808A, VT2 can be replaced with KT816G. A pair of transistors VT1, VT2 can be replaced with one KT827A, B, V or D. Any diodes VD6, VD7, preferably germanium with a lower forward voltage drop and a reverse voltage drop of at least 30 V. Diodes VD1 -VD4 type KD206A, KD202A, B, V or similar installed on radiators.
When making your own transformer TV1, you can follow the methodology described in. The overall power of the transformer must be at least 100 W, preferably 120 W. In this case, it will be possible to wind up another winding with a voltage of 6.3 V. In this case, another range of 24 - 30 V will be added, which will provide an output voltage regulation range of 1.5-30 V at a load current of 3 A.
Setting up the power supply It is carried out according to a well-known method and has no special features. A correctly assembled power supply starts working immediately. When working with a power supply, first select the required output voltage range using the SA2 switch, and use the “RUB” and “FINE” resistors to set the required output voltage, based on the readings of the built-in voltmeter. Switch SA3 selects the current limit limit and connects the load. It should be noted that despite the simplicity of the circuit, this power supply combines two devices: a voltage stabilizer plus a current stabilizer. The power supply is not afraid of short circuits and can even protect the elements of an electronic device connected to it, which is very important when conducting various tests in amateur radio practice.
LITERATURE
1. Nefedov A.V., Aksenov A.I., Circuit elements of household radio equipment, microcircuits: Reference book.-M: Radio communication, 1993.
2. Akimov N.N., Resistors, capacitors, transformers, chokes, switching devices REA: Directory. - Minsk: Belarus, 1994.
3. Semiconductor receiving and amplifying devices: Amateur Radio Handbook / R.M. Tereshchuk, K.M. Tereshchuk. - Kyiv: Naukova Dumka, 1988.
Radiohobby 05-1999
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| DA1 | Linear regulator | LM317 | 1 | KR142EN12 | To notepad | |
| VT1 | Bipolar transistor | KT819GM | 1 | To notepad | ||
| VT2 | Bipolar transistor | KT814G | 1 | To notepad | ||
| VD1-VD4 | Diode | KD206A | 4 | To notepad | ||
| VD5 | Diode | KD212A | 1 | To notepad | ||
| VD6, VD7 | Diode | D9E | 2 | To notepad | ||
| C1-C4, C7 | Capacitor | 2.2 nF 63 V | 5 | To notepad | ||
| C5 | 10000 µF 50 V | 1 | To notepad | |||
| C6 | Electrolytic capacitor | 220 µF 63 V | 1 | To notepad | ||
| R1 | Resistor | 51 Ohm | 1 | To notepad | ||
| R2 | Resistor | 1.2 kOhm | 1 | To notepad | ||
| R3 | Variable resistor | 3.3 kOhm | 1 | To notepad | ||
| R4 | Variable resistor | 22 kOhm | 1 | To notepad | ||
| R5 | Resistor | 13 ohm | 1 | 1 W | To notepad | |
| R6 | Resistor | 4.3 Ohm | 1 | 2 W | To notepad | |
| R7 | Resistor | 1.2 Ohm | 1 | 5 W | To notepad | |
| R8 | Resistor | 0.43 Ohm | 1 | 10 W | To notepad | |
| R9 | Variable resistor | 100 Ohm | 1 | 3 W | To notepad | |
| R10 | Resistor |
Good day, forum members and site guests. Radio circuits! Wanting to put together a decent, but not too expensive and cool power supply, so that it has everything and it doesn’t cost anything. In the end, I chose the best, in my opinion, circuit with current and voltage regulation, which consists of only five transistors, not counting a couple of dozen resistors and capacitors. Nevertheless, it works reliably and is highly repeatable. This scheme has already been reviewed on the site, but with the help of colleagues we managed to improve it somewhat.
I assembled this circuit in its original form and encountered one unpleasant problem. When adjusting the current, I can’t set it to 0.1 A - at least 1.5 A at R6 0.22 Ohm. When I increased the resistance of R6 to 1.2 Ohms, the current during a short circuit turned out to be at least 0.5 A. But now R6 began to heat up quickly and strongly. Then I used a small modification and got a much wider current regulation. Approximately 16 mA to maximum. You can also make it from 120 mA if you transfer the end of the resistor R8 to the T4 base. The bottom line is that before the resistor voltage drops, a drop in the B-E junction is added and this additional voltage allows you to open T5 earlier, and as a result, limit the current earlier.
Based on this proposal, I conducted successful tests and eventually received a simple laboratory power supply. I am posting a photo of my laboratory power supply with three outputs, where:
Also, in addition to the output voltage regulation board, the device was supplemented with a power filter board with a fuse block. What happened in the end - see below.
For two voltages (+5 and +12 V) is shown in Fig. 1:
The stabilizer provides two output voltages: 5 V, at a current of 0.75 A; 12 V at a current of about 200 mA. The main voltage generated by the switching stabilizer is +5 volts. The second voltage is obtained due to the autotransformer winding II of transformer T1.
The article "Laboratory power supply" was published in the magazine No. 11 in 1980. According to the original source, in the 80s a functioning power supply unit was manufactured, which is still working today.
The main advantages of laboratory nutrition are:
Wide range of output voltages (0... ±40 V);
Possibility of smooth adjustment of tension in the shoulders, both separately and symmetrically;
The boost circuit can be implemented on the MC33063A/MC34063A pulse converter controller, or their Russian analogue KR1156EU5R/KF1156EU5T. MC33063A/MC34063A microcircuits differ from each other only in the type of housing, i.e. DIP-8 or SO8 respectively. Input voltage from 3 to 40 volts.
In this circuit, the output of the converter produces 28 volts, with an input voltage of 12 volts, the load current will be 175 milliamps.
Another voltage value at the boost output can be obtained by changing the ratio R1/R2 according to the formula:
V out=1.25 x(1+R2/R1).
For implementation except
Many of us have accumulated various power supplies from laptops, printers or monitors with voltages of +12, +19, +22. These are excellent power supplies that are protected against both short circuits and overheating. Whereas in home, amateur radio practice, an adjustable, stabilized source is constantly required. If it is not advisable to make changes to the circuit of existing power supplies, then a very simple attachment to such a unit will come to the rescue.
