Laboratory power supplies based on the tl494 chip. TL494CN: connection diagram, description in Russian, converter diagram. Scope of application specified by the manufacturer
Semiconductor devices were used in radio engineering even before the invention vacuum tubes. The inventor of radio, A. S. Popov, first used a coherer (a glass tube with metal filings) and then contact of a steel needle with a carbon electrode to detect electromagnetic waves.
This was the first semiconductor diode— detector. Later, detectors were created using natural and artificial crystalline semiconductors (galena, zincite, chalcopyrite, etc.).
Such a detector consisted of a semiconductor crystal soldered into a holder cup and a steel or tungsten spring with a pointed end (Fig. 1). The position of the tip on the crystal was found experimentally, achieving the highest volume of the radio station transmission.
Rice. 1. Semiconductor diode - detector.
In 1922, O. V. Losev, an employee of the Nizhny Novgorod Radio Laboratory, discovered a remarkable phenomenon: a crystal detector, it turns out, can generate and amplify electrical oscillations.
It was a real sensation, but insufficient scientific knowledge, the lack of the necessary experimental equipment did not allow at that time to deeply explore the essence of the processes occurring in a semiconductor and to create semiconductor devices capable of competing with an electron tube.
Semiconductor diode
Semiconductor diodes denoted by a symbol preserved in general outline since the time of the first radio receivers (Fig. 2.6).
Rice. 2. Designation and structure of a semiconductor diode.
The top of the triangle in this symbol indicates the direction of greatest conductivity (the triangle symbolizes the anode of the diode, and the short line perpendicular to the lead lines is its cathode).
The same symbol denotes semiconductor rectifiers, consisting, for example, of several diodes connected in series, parallel or mixed (rectifier columns, etc.).
Diode bridges
Bridge rectifiers are often used to power radio equipment. The outline of the same diode connection diagram (a square, the sides of which are formed by diode symbols) has long become generally accepted, therefore, to designate such rectifiers, a simplified symbol began to be used - a square with the symbol of one diode inside (Fig. 3).
Rice. 3. Designation of the diode bridge.
Depending on the value of the rectified voltage, each arm of the bridge can consist of one, two or more diodes. The polarity of the rectified voltage is not indicated on the diagrams since it is clearly determined by the diode symbol inside the square.
Bridges are structurally combined in one housing and are depicted separately, showing that they belong to one product in a positional designation. Next to the positional designation of diodes, like all other semiconductor devices, their type is usually indicated.
Built on the basis of the diode symbol symbols semiconductor diodes with special properties. To get the desired symbol use special signs, depicted either on the base symbol itself or in close proximity to it, and in order to focus attention on some of them, the base symbol is placed in a circle - a symbol for the body of a semiconductor device.
Tunnel diodes
A sign resembling a straight bracket denotes the cathode of tunnel diodes (Fig. 4a). They are made from semiconductor materials with a very high impurity content, as a result of which the semiconductor turns into a semimetal. Due to the unusual shape of the current-voltage characteristic (it has a section of negative resistance), tunnel diodes are used to amplify and generate electrical signals and in switching devices. An important advantage of these diodes is that they can operate at very high frequencies.
Rice. 4. Tunnel diode and its designation.
A type of tunnel diodes are reverse diodes, which at low voltage r-n transition e conductivity in reverse direction more than directly.
Such diodes are used in reverse connection. In the symbol for a reversed diode, the cathode dash is depicted with two dashes touching it with their middle (Fig. 4.6).
Zener diodes
Semiconductor zener diodes, which also operate on the reverse branch of the current-voltage characteristic, have won a strong place in power supplies, especially low-voltage ones.
These are planar silicon diodes made using special technology. When you turn them on in the opposite direction and at a certain voltage -at the crossing the latter “breaks through”, and subsequently, despite the increase in current through the junction, the voltage across it remained almost unchanged.
Rice. 5. Zener diode and its designation on the diagrams.
Thanks to this property, zener diodes are widely used as independent stabilizing elements, as well as sources of reference voltages in transistor stabilizers.
To obtain small reference voltages, the zener diodes are switched on in the forward direction, with the stabilization voltage of one zener diode equal to 0.7... 0.8 V. The same results are obtained when conventional silicon diodes are switched on in the forward direction.
To stabilize low voltages, special semiconductor diodes - stabistors - have been developed and are widely used. Their difference from zener diodes is that they operate on the direct branch of the current-voltage characteristic, i.e. when switched on in the forward (conducting) direction.
To show a zener diode in the diagram, the cathode dash of the basic symbol is supplemented with a short dash directed towards the anode symbol (Fig. 5a). It should be noted that the location of the stroke relative to the anode symbol should be unchanged regardless of the position of the zener diode symbol on the diagram.
This fully applies to the symbol of a two-anode (double-sided) zener diode (Fig. 5.6), which can be included in electrical circuit in any direction (essentially, these are two identical zener diodes connected back to back).
Varicaps
Electron-hole transition to which is applied reverse voltage, has the properties of a capacitor. In this case, the role of the dielectric is played by the pn junction itself, in which there are few free charge carriers, and the role of the plates is played by the adjacent layers of the semiconductor with electric charges different sign- electrons and holes. By changing the voltage applied to the pn junction, you can change its thickness, and therefore the capacitance between the layers of the semiconductor.
Rice. 6. Varicaps and their designation on circuit diagrams.
This phenomenon is used in special semiconductor devices - varicapah[from English words vari(able) - variable and cap(acitor) - capacitor]. Varicaps are widely used for tuning oscillatory circuits, in automatic frequency control devices, and also as frequency modulators in various generators.
The conventional graphic designation of a varicap (see Fig. 6, a) clearly reflects their essence: parallel lines at the bottom are perceived as a symbol of a capacitor. Kick and variable capacitors, varicaps are often made in the form of blocks (they are called matrices) with a common cathode and separate anodes. For example in Fig. 6.6 shows the designation of a matrix of two varicaps, and Fig. 6,c - out of three.
Thyristors
Based on the basic symbol of the diode, conditional thyristor designations(from Greek Thyra– door and English (resi) stor- resistor). These are diodes, which are alternating layers of silicon with electrical conductivity types p and n. There are four such layers in the thyristor, i.e. it has three pn junctions (pnpp structure).
Thyristors have found wide application in various alternating voltage regulators, relaxation generators, switching devices, etc.
Rice. 7. Thyristor and its designation on circuit diagrams.
Thyristors with leads only from the outer layers of the structure are called dynistorimn and are designated by a diode symbol crossed out by a line segment parallel to the cathode line (Figure 7, a). The same technique was used in constructing the designation of a symmetrical dinistor (Fig. 7, b), conducting current (after switching on) in both directions.
Thyristors with an additional (third) output (from one of the internal layers of the structure) are called thyristors. Control along the cathode in the designation of these devices is shown by a broken line attached to the cathode symbol (Fig. 7, c), along the anode - by a line extending one of the sides of the triangle symbolizing the anode (Fig. 7, d).
The symbol for a symmetrical (bidirectional) triistor is obtained from the symbol for a symmetrical dinistor by adding a third terminal (Fig. 7, (5).
Photodiodes
Main part photodiode is a junction operating under reverse bias. Its body has a window through which the semiconductor crystal is illuminated. In the absence of light, the current through the pn junction is very small - it does not exceed the reverse current of a conventional diode.
Rice. 8. Photodiodes and their representation on diagrams.
When the crystal is illuminated, the reverse resistance of the junction drops sharply, and the current through it increases. To show such a semiconductor diode in a diagram, the basic symbol of the diode is placed in a circle, and next to it (top left, regardless of the position of the symbol) the sign of the photoelectric effect is depicted - two oblique parallel arrows directed towards the symbol (Fig. 8a).
In a similar way, it is not difficult to construct a symbol for any other semiconductor device that changes its properties under the influence of optical radiation. As an example in Fig. 8.6 shows the designation of the photodinistor.
LEDs and LED indicators
Semiconductor diodes, emitting light when current passes through р-n junction, are called LEDs. Such diodes are turned on in the forward direction. The conventional graphic symbol of an LED is similar to the symbol of a photodiode and differs from it in that the arrows indicating optical radiation are placed to the right of the circle and directed towards the opposite side(Fig. 9).
Rice. 9. LEDs and their representation on diagrams.
To display numbers, letters and other characters in low-voltage equipment, LED character indicators are often used, which are sets of light-emitting crystals arranged in a certain way and filled with transparent plastic.
ESKD standards do not provide symbols for such products, but in practice they often use symbols similar to those shown in Fig. 10 (seven-segment indicator symbol for displaying numbers and a comma).
Rice. 10. Designation of LED segment indicators.
As you can see, such a graphic designation clearly reflects the actual location of the light-emitting elements (segments) in the indicator, although it is not without a drawback: it does not carry information about the polarity of the inclusion of the indicator terminals in the electrical circuit (indicators are produced both with an anode terminal common to all segments and and with a common cathode terminal).
However, this usually does not cause any particular difficulties, since the connection of the common output of the indicator (as well as the microcircuits) is specified in the diagram.
Optocouplers
Light-emitting crystals are widely used in optocouplers - special devices used for communications. individual parts electronic devices in cases where their galvanic isolation is necessary. In the diagrams, optocouplers are depicted as shown in Fig. eleven.
The optical connection of the light emitter (LED) with the photodetector is shown by two parallel arrows perpendicular to the lead lines of the optocoupler. The photodetector in an optocoupler can be not only a photodiode (Fig. 11,a), but also a photoresistor (Fig. 11,6), photodinistor (Fig. 11,c), etc. The mutual orientation of the symbols of the emitter and photodetector is not regulated.
Rice. 11. Designation of optocouplers (optocouplers).
If necessary, the components of the optocoupler can be depicted separately, but in this case, the optical connection sign should be replaced with the signs of optical radiation and photoelectric effect, and the belonging of the parts to the optocoupler should be shown in the positional designation (Fig. 11, d).
Literature: V.V. Frolov, Language of radio circuits, Moscow, 1998.
Hi all!
In this article we will analyze the operating principle of such a semiconductor device as a DIODE.
Let's start in order.
So, diode- this is which, simply put, one side (in the forward direction) passes current well, and the other (i.e. in the opposite direction) poorly. Diode has two terminals: positive - anode and negative - cathode.
I’ll say right away that in the design of almost everyone (99.99%) electronic device there is a diode or diodes present.
This semiconductor device is used as a rectifier. For example, using a diode bridge, which consists of four diodes, you can rectify alternating current and it will become constant. If you use six diodes, you can turn a three-phase voltage into a single-phase one. Diodes are used in power supplies, in various audio and video devices, telephones and many other places.
If you connect a diode to a power source, the output voltage will differ from the original one downward by 0.5...0.7 V. For a smaller voltage drop, Schottky diodes are used, in this case the voltage drop will be approximately 0.1 V.
Diode device shown in the figure below:
1 – crystal, 2 – conductors (leads), 3 – electrodes, 4 – plane p-ntransition.
Diode crystals are made mainly from silicon or germanium. One region of the crystal has p-type electrical conductivity (hole, has an artificially created lack of electrons), the other contains an excess of electrons and has n-type electrical conductivity. The border between regions is called p-n junction. IN Latin The word “positive” begins with the letter p, and the word “negative” begins with the letter n. Direct connection of a diode is called connecting a positive voltage to the anode and a negative voltage to the cathode. In this connection the diode is open. If you connect it the other way around, then the diode will be closed and no current will pass through. This connection is called “reverse”. The diode's reverse resistance is very high; in circuits it is used as an insulator (or dielectric).
Watch diode operation you can do this.
You need to take a power source, an incandescent lamp and, in fact, a diode. Let's put together a simple diagram:
We connect the “plus” of the power source to the anode of the diode, the “minus” to one terminal of the lamp. We connect the cathode of the diode to the second terminal of the lamp. In such a “direct” connection the lamp will glow. Now let's turn the diode over, i.e. Let's make a “reverse” connection. In such a connection, the lamp will not light up, since the transition is closed.
How to make a full-fledged power supply yourself with an adjustable voltage range of 2.5-24 volts is very simple; anyone can repeat it without any amateur radio experience.
We'll make it out of old computer unit power supply, TX or ATX, it doesn’t matter, fortunately, over the years of the PC Era, every home has already accumulated a sufficient amount of old computer hardware and a power supply unit is probably also there, so the cost homemade products will be insignificant, and for some masters it will be equal to zero rubles.
I got this AT block for modification.
The more powerful you use the power supply, the better result, my donor is only 250W with 10 amperes on the +12v bus, but in fact, with a load of only 4 A, it can no longer cope, the output voltage drops completely.
Look what is written on the case.
Therefore, see for yourself what kind of current you plan to receive from your regulated power supply, this potential of the donor and lay it in right away.
There are many options for modifying a standard computer power supply, but they are all based on a change in the wiring of the IC chip - TL494CN (its analogues DBL494, KA7500, IR3M02, A494, MV3759, M1114EU, MPC494C, etc.).
Fig No. 0 Pinout of the TL494CN microcircuit and analogues.
Let's look at several options execution of computer power supply circuits, perhaps one of them will be yours and dealing with the wiring will become much easier.
Scheme No. 1.
Let's get to work.
First you need to disassemble the power supply housing, unscrew the four bolts, remove the cover and look inside.
We are looking for a chip on the board from the list above, if there is none, then you can look for a modification option on the Internet for your IC.
In my case, a KA7500 chip was found on the board, which means we can begin to study the wiring and the location of unnecessary parts that need to be removed.
For ease of operation, first completely unscrew the entire board and remove it from the case.
In the photo the power connector is 220v.
Let's disconnect the power and fan, solder or cut out the output wires so that they don't interfere with our understanding of the circuit, leave only the necessary ones, one yellow (+12v), black (common) and green* (start ON) if there is one.
My AT unit does not have a green wire, so it starts immediately when plugged into the outlet. If the unit is ATX, then it must have a green wire, it must be soldered to the “common” one, and if you want to make a separate power button on the case, then just put a switch in the gap of this wire.
Now you need to look at how many volts the large output capacitors cost, if they say less than 30v, then you need to replace them with similar ones, only with an operating voltage of at least 30 volts.
In the photo there are black capacitors as a replacement option for the blue one.
This is done because our modified unit will produce not +12 volts, but up to +24 volts, and without replacement, the capacitors will simply explode during the first test at 24v, after a few minutes of operation. When selecting a new electrolyte, it is not advisable to reduce the capacity; increasing it is always recommended.
The most important part of the job.
We will remove all unnecessary parts in the IC494 harness and solder other nominal parts so that the result is a harness like this (Fig. No. 1).
Rice. No. 1 Change in the wiring of the IC 494 microcircuit (revision scheme).
We will only need these legs of the microcircuit No. 1, 2, 3, 4, 15 and 16, do not pay attention to the rest.
Rice. No. 2 Option for improvement based on the example of scheme No. 1
Explanation of symbols.
You should do something like this, we find leg No. 1 (where the dot is on the body) of the microcircuit and study what is connected to it, all circuits must be removed and disconnected. Depending on how the tracks will be arranged and the parts soldered in your specific modification of the board, you select best option modifications, this could be desoldering and lifting one leg of the part (breaking the chain) or it would be easier to cut the track with a knife. Having decided on the action plan, we begin the remodeling process according to the revision scheme.
The photo shows replacing resistors with the required value.
In the photo - by lifting the legs of unnecessary parts, we break the chains.
Some resistors that are already soldered into the wiring diagram can be suitable without replacing them, for example, we need to put a resistor at R=2.7k connected to the “common”, but there is already R=3k connected to the “common”, this suits us quite well and we leave it there unchanged (example in Fig. No. 2, green resistors do not change).
On the picture- cut tracks and added new jumpers, write down the old values with a marker, you may need to restore everything back.
Thus, we review and redo all the circuits on the six legs of the microcircuit.
This was the most difficult point in the rework.
We make voltage and current regulators.
We take variable resistors of 22k (voltage regulator) and 330Ohm (current regulator), solder two 15cm wires to them, solder the other ends to the board according to the diagram (Fig. No. 1). Install on the front panel.
Voltage and current control.
To control we need a voltmeter (0-30v) and an ammeter (0-6A).
These devices can be purchased in Chinese online stores at the best price; my voltmeter cost me only 60 rubles with delivery. (Voltmeter: )
I used my own ammeter, from old USSR stocks.
IMPORTANT- inside the device there is a Current resistor (Current sensor), which we need according to the diagram (Fig. No. 1), therefore, if you use an ammeter, then you do not need to install an additional Current resistor; you need to install it without an ammeter. Usually a homemade RC is made, a wire D = 0.5-0.6 mm is wound around a 2-watt MLT resistance, turn to turn for the entire length, solder the ends to the resistance terminals, that's all.
Everyone will make the body of the device for themselves.
You can leave it completely metal by cutting holes for regulators and control devices. I used laminate scraps, they are easier to drill and cut.
SWITCH POWER SUPPLY FOR TL494 AND IR2110
Most automotive and network voltage converters are based on a specialized TL494 controller, and since it is the main one, it would be unfair not to briefly talk about the principle of its operation.
The TL494 controller is a plastic DIP16 package (there are also options in a planar package, but it is not used in these designs). The functional diagram of the controller is shown in Fig. 1.
Picture 1 - Structural scheme TL494 chips.
As can be seen from the figure, the TL494 microcircuit has very developed control circuits, which makes it possible to build converters on its basis to suit almost any requirements, but first a few words about the functional units of the controller.
ION circuits and protection against undervoltage.
The circuit turns on when the power reaches the threshold of 5.5..7.0 V (typical value 6.4V). Until this moment, the internal control buses prohibit the operation of the generator and the logical part of the circuit. The no-load current at supply voltage +15V (output transistors are disabled) is no more than 10 mA. ION +5V (+4.75..+5.25 V, output stabilization no worse than +/- 25mV) provides a flowing current of up to 10 mA. The ION can only be boosted using an NPN emitter follower (see TI pp. 19-20), but the voltage at the output of such a “stabilizer” will greatly depend on the load current.
Generator generates on the timing capacitor Ct (pin 5) sawtooth voltage 0..+3.0V (the amplitude is set by the ION) for TL494 Texas Instruments and 0...+2.8V for TL494 Motorola (what can we expect from others?), respectively, for TI F=1.0/(RtCt), for Motorola F= 1.1/(RtCt).
Allowable operating frequencies
from 1 to 300 kHz, with the recommended range Rt = 1...500 kOhm, Ct = 470pF...10 μF. In this case, the typical temperature drift of frequency is (naturally, without taking into account the drift of attached components) +/-3%, and the frequency drift depending on the supply voltage is within 0.1% over the entire permissible range.
For remote shutdown
generator, you can use an external key to short-circuit the Rt input (6) to the ION output, or short-circuit Ct to ground. Of course, the leakage resistance of the open switch must be taken into account when selecting Rt, Ct.
Rest phase control input
(duty factor) through the rest phase comparator sets the required minimum pause between pulses in the arms of the circuit. This is necessary both to prevent through current in the power stages outside the IC, and for stable operation of the trigger - the switching time of the digital part of the TL494 is 200 ns. The output signal is enabled when the saw exceeds the voltage at control input 4 (DT) by Ct. At clock frequencies up to 150 kHz with zero control voltage, the resting phase = 3% of the period (equivalent bias of the control signal 100..120 mV), at high frequencies the built-in correction expands the resting phase to 200..300 ns.
Using the DT input circuit, it is possible to set a fixed resting phase ( R-R divider), soft start mode (R-C), remote shutdown (key), and also use DT as a linear control input. The input circuit is assembled using PNP transistors, so the input current (up to 1.0 μA) flows out of the IC rather than into it. The current is quite large, so high-resistance resistors (no more than 100 kOhm) should be avoided. See TI, page 23 for an example of surge protection using a TL430 (431) 3-lead zener diode.
Error Amplifiers
- in fact, operational amplifiers with Ku = 70..95 dB constant voltage(60 dB for early episodes), Ku=1 at 350 kHz. The input circuits are assembled using PNP transistors, so the input current (up to 1.0 μA) flows out of the IC rather than into it. The current is quite large for the op-amp, the bias voltage is also high (up to 10 mV), so high-resistance resistors in the control circuits (no more than 100 kOhm) should be avoided. But thanks to the use of pnp inputs, the input voltage range is from -0.3V to Vsupply-2V
When using an RC frequency-dependent OS, you should remember that the output of the amplifiers is actually single-ended (series diode!), so it will charge the capacitance (upward) and will take a long time to discharge downward. The voltage at this output is within 0..+3.5V (slightly more than the generator swing), then the voltage coefficient drops sharply and at approximately 4.5V at the output the amplifiers are saturated. Likewise, low-resistance resistors in the amplifier output circuit (feedback loop) should be avoided.
Amplifiers are not designed to operate within one clock cycle of the operating frequency. With a signal propagation delay inside the amplifier of 400 ns, they are too slow for this, and the trigger control logic does not allow it (side pulses would appear at the output). In real PN circuits, the cutoff frequency of the OS circuit is selected on the order of 200-10000 Hz.
Trigger and output control logic
- With a supply voltage of at least 7V, if the saw voltage at the generator is greater than at the DT control input, and if the saw voltage is greater than at any of the error amplifiers (taking into account the built-in thresholds and offsets) - the circuit output is allowed. When the generator is reset from maximum to zero, the outputs are switched off. A trigger with paraphase output divides the frequency in half. With logical 0 at input 13 (output mode), the trigger phases are combined by OR and supplied simultaneously to both outputs; with logical 1, they are supplied in phase to each output separately.
Output transistors
- npn Darlingtons with built-in thermal protection (but without current protection). Thus, the minimum voltage drop between the collector (usually closed to the positive bus) and the emitter (at the load) is 1.5 V (typical at 200 mA), and in a circuit with a common emitter it is a little better, 1.1 V typical. The maximum output current (with one open transistor) is limited to 500 mA, the maximum power for the entire chip is 1 W.
Switching power supplies are gradually replacing their traditional relatives in audio engineering, since they look noticeably more attractive both economically and in size. The same factor that switching power supplies contribute significantly to the distortion of the amplifier, namely the appearance of additional overtones, is no longer relevant mainly for two reasons - the modern element base makes it possible to design converters with a conversion frequency significantly higher than 40 kHz, therefore the power modulation introduced by the power supply will already be in ultrasound. In addition, more high frequency It is much easier to filter the power supply and the use of two L-shaped LC filters along the power supply circuits already sufficiently smoothes out the ripples at these frequencies.
Of course, there is a fly in the ointment in this barrel of honey - the difference in price between a typical power supply for a power amplifier and a pulsed one becomes more noticeable as the power of this unit increases, i.e. The more powerful the power supply, the more profitable it is in relation to its standard counterpart.
And that is not all. When using switching power supplies, it is necessary to adhere to the rules for installing high-frequency devices, namely the use of additional screens, feeding the power part of the common wire to the heat sinks, as well as correct ground wiring and connection of shielding braids and conductors.
After a little lyrical digression about the features of switching power supplies for power amplifiers, the actual circuit diagram of a 400W power supply:
Picture 1. Schematic diagram switching power supply for power amplifiers up to 400 W
ENLARGE IN GOOD QUALITY
The control controller in this power supply is TL494. Of course, there are more modern chips to perform this task, but we use this particular controller for two reasons - it is VERY easy to purchase. For quite a long time, TL494 from Texas Instruments was used in the manufactured power supplies; no quality problems were found. The error amplifier is covered by OOS, which makes it possible to achieve a fairly large coefficient. stabilization (ratio of resistors R4 and R6).
After the TL494 controller there is an IR2110 half-bridge driver, which actually controls the gates of the power transistors. The use of the driver made it possible to abandon the matching transformer, which is widely used in computer power supplies. The IR2110 driver is loaded onto the gates through the R24-VD4 and R25-VD5 chains that accelerate the closing of the field gates.
Power switches VT2 and VT3 operate on the primary winding of the power transformer. The midpoint required to obtain alternating voltage in the primary winding of the transformer is formed by elements R30-C26 and R31-C27.
A few words about the operating algorithm of the switching power supply on the TL494:
At the time of submission mains voltage The 220 V capacitances of the primary power supply filters C15 and C16 are charged through resistors R8 and R11, which prevents the VD diol bridge from being overloaded by a short circuit current of completely discharged C15 and C16. At the same time, capacitors C1, C3, C6, C19 are charged through a line of resistors R16, R18, R20 and R22, stabilizer 7815 and resistor R21.
As soon as the voltage on capacitor C6 reaches 12 V, the zener diode VD1 “breaks through” and current begins to flow through it, charging capacitor C18, and as soon as the positive terminal of this capacitor reaches a value sufficient to open thyristor VS2, it will open. This will turn on relay K1, which with its contacts will bypass current-limiting resistors R8 and R11. In addition, the opened thyristor VS2 will open transistor VT1 to both the TL494 controller and the IR2110 half-bridge driver. The controller will begin a soft start mode, the duration of which depends on the ratings of R7 and C13.
During a soft start, the duration of the pulses that open the power transistors increases gradually, thereby gradually charging the secondary power capacitors and limiting the current through the rectifier diodes. The duration increases until the secondary supply is sufficient to open the LED of optocoupler IC1. As soon as the brightness of the optocoupler LED becomes sufficient to open the transistor, the pulse duration will stop increasing (Figure 2).
Figure 2. Soft start mode.
It should be noted here that the duration of the soft start is limited, since the current passing through resistors R16, R18, R20, R22 is not enough to power the TL494 controller, the IR2110 driver and the switched-on relay winding - the supply voltage of these microcircuits will begin to decrease and will soon decrease to a value at which TL494 will stop generating control pulses. And it is up to this moment that the soft start mode must be completed and the converter must return to normal operating mode, since the TL494 controller and IR2110 driver receive their main power from a power transformer (VD9, VD10 - midpoint rectifier, R23-C1-C3 - RC filter, IC3 - 15 V stabilizer) and that is why capacitors C1, C3, C6, C19 have such large ratings - they must maintain the controller’s power supply until it returns to normal operation.
The TL494 stabilizes the output voltage by changing the duration of control pulses of power transistors at a constant frequency - Pulse-Width Modulation - PWM. This is only possible if the value of the secondary voltage of the power transformer is higher than that required at the output of the stabilizer by at least 30%, but not more than 60%.
Figure 3. Operating principle of a PWM stabilizer.
As the load increases, the output voltage begins to decrease, the optocoupler LED IC1 begins to glow less, the optocoupler transistor closes, reducing the voltage on the error amplifier and thereby increasing the duration of the control pulses until the effective voltage reaches the stabilization value (Figure 3). As the load decreases, the voltage will begin to increase, the LED of optocoupler IC1 will begin to glow brighter, thereby opening the transistor and reducing the duration of the control pulses until the effective value of the output voltage decreases to a stabilized value. The magnitude of the stabilized voltage is regulated by trimming resistor R26.
It should be noted that the TL494 controller does not regulate the duration of each pulse depending on the output voltage, but only the average value, i.e. the measuring part has some inertia. However, even with capacitors installed in the secondary power supply with a capacity of 2200 μF, power failures at peak short-term loads do not exceed 5%, which is quite acceptable for HI-FI class equipment. We usually install capacitors in the secondary power supply of 4700 μF, which gives a confident margin for peak values, and the use of a group stabilization choke allows you to control all 4 output power voltages.
The pulse block The power supply is equipped with overload protection, the measuring element of which is the current transformer TV1. As soon as the current reaches a critical value, thyristor VS1 opens and bypasses the power supply to the final stage of the controller. The control pulses disappear and the power supply goes into standby mode, which it can remain in for quite a long time, since the thyristor VS2 continues to remain open - the current flowing through resistors R16, R18, R20 and R22 is enough to keep it in the open state. How to calculate a current transformer.
To exit the power supply from standby mode, you must press the SA3 button, which will bypass the thyristor VS2 with its contacts, the current will stop flowing through it and it will close. As soon as the contacts SA3 open, the transistor VT1 closes itself, removing power from the controller and driver. Thus, the control circuit will switch to minimum consumption mode - thyristor VS2 is closed, therefore relay K1 is turned off, transistor VT1 is closed, therefore the controller and driver are de-energized. Capacitors C1, C3, C6 and C19 begin to charge and as soon as the voltage reaches 12 V, the thyristor VS2 opens and the switching power supply starts.
If you need to put the power supply into standby mode, you can use the SA2 button, when pressed, the base and emitter of transistor VT1 will be connected. The transistor will close and de-energize the controller and driver. The control pulses will disappear, and the secondary voltages will disappear. However, the power will not be removed from relay K1 and the converter will not restart.
This circuit design allows you to assemble power supplies from 300-400 W to 2000 W, of course, some circuit elements will have to be replaced, since their parameters simply cannot withstand heavy loads.
When assembling more powerful options, you should pay attention to the capacitors of the primary power supply smoothing filters C15 and C16. The total capacitance of these capacitors must be proportional to the power of the power supply and correspond to the proportion 1 W of the output power of the voltage converter corresponds to 1 µF of the capacitance of the primary power filter capacitor. In other words, if the power of the power supply is 400 W, then 2 capacitors of 220 μF should be used, if the power is 1000 W, then 2 capacitors of 470 μF or two of 680 μF must be installed.
This requirement has two purposes. Firstly, the ripple of the primary supply voltage is reduced, which makes it easier to stabilize the output voltage. Secondly, using two capacitors instead of one facilitates the operation of the capacitor itself, since electrolytic capacitors The TK series are much easier to obtain, and they are not entirely intended for use in high-frequency power supplies - the internal resistance is too high and these capacitors will heat up at high frequencies. Using two pieces, the internal resistance is reduced, and the resulting heating is divided between two capacitors.
When used as power transistors IRF740, IRF840, STP10NK60 and similar ones (for more information about the transistors most commonly used in network converters, see the table at the bottom of the page), diodes VD4 and VD5 can be abandoned altogether, and the values of resistors R24 and R25 can be reduced to 22 Ohms - power The IR2110 driver is quite enough to control these transistors. If a more powerful switching power supply is being assembled, then more powerful transistors will be required. Attention should be paid to both the maximum current of the transistor and its dissipation power - switching stabilized power supplies are very sensitive to the correct installation of the snubber and without it, the power transistors heat up more because currents formed due to self-induction begin to flow through the diodes installed in the transistors. Read more about choosing a snubber.
Also, the closing time that increases without a snubber makes a significant contribution to heating - the transistor stays in linear mode longer.
Quite often they forget about one more feature field effect transistors- with increasing temperature, their maximum current decreases, and quite strongly. Based on this, when choosing power transistors for switching power supplies, you should have at least a two-fold maximum current reserve for power amplifier power supplies and a three-fold reserve for devices operating on a large, unchanging load, for example, an induction smelter or decorative lighting, powering low-voltage power tools.
The output voltage is stabilized using the group stabilization choke L1 (GLS). You should pay attention to the direction of the windings of this inductor. The number of turns must be proportional to the output voltages. Of course, there are formulas for calculating this winding unit, but experience has shown that the overall power of the core for a DGS should be 20-25% of the overall power of the power transformer. You can wind until the window is filled by about 2/3, not forgetting that if the output voltages are different, then the winding with more high voltage must be proportionally larger, for example, you need two bipolar voltages, one at ±35 V, and the second to power the subwoofer with a voltage of ±50 V.
We wind the DGS into four wires at once until 2/3 of the window is filled, counting the turns. The diameter is calculated based on a current intensity of 3-4 A/mm2. Let's say we got 22 turns, let's make up the proportion:
22 turns / 35 V = X turns / 50 V.
X turns = 22 × 50 / 35 = 31.4 ≈ 31 turns
Next, I’ll cut two wires for ±35 V and wind up another 9 turns for a voltage of ±50.
ATTENTION! Remember that the quality of stabilization directly depends on how quickly the voltage changes to which the optocoupler diode is connected. To improve the stabilization coefficient, it makes sense to connect an additional load to each voltage in the form of 2 W resistors with a resistance of 3.3 kOhm. The load resistor connected to the voltage controlled by the optocoupler should be 1.7...2.2 times less.
The circuit data for network switching power supplies on ferrite rings with a permeability of 2000 Nm are summarized in Table 1.
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However, it is not always possible to recognize the brand of ferrite, especially if it is ferrite from horizontal transformers of televisions. You can get out of the situation by finding out the number of turns experimentally. More details about this in the video:
Using the above circuitry of a switching power supply, several submodifications were developed and tested, designed to solve a particular problem at various powers. The printed circuit board drawings for these power supplies are shown below.
Printed circuit board for a switching stabilized power supply with a power of up to 1200...1500 W. Board size 269x130 mm. In fact, this is a more improved version of the previous one. printed circuit board. It is distinguished by the presence of a group stabilization choke, which allows you to control the magnitude of all power voltages, as well as an additional LC filter. Has fan control and overload protection. The output voltages consist of two bipolar power sources and one bipolar low-current source, designed to power the preliminary stages.
Appearance PCB power supply up to 1500 W. DOWNLOAD IN LAY FORMAT
A stabilized switching network power supply with a power of up to 1500...1800 W can be made on a printed circuit board measuring 272x100 mm. The power supply is designed for a power transformer made on K45 rings and located horizontally. It has two bipolar power sources, which can be combined into one source to power an amplifier with two-level power supply and one bipolar low-current source for preliminary stages.
Printed circuit board of a switching power supply up to 1800 W. DOWNLOAD IN LAY FORMAT
This power supply can be used to power high-power automotive equipment, such as powerful car amplifiers and car air conditioners. Board dimensions 188x123. The Schottky rectifier diodes used are parallelized by jumpers and the output current can reach 120 A at a voltage of 14 V. In addition, the power supply can produce bipolar voltage with a load capacity of up to 1 A (installed integrated voltage stabilizers no longer allow). The power transformer is made on K45 rings, the filtering power voltage choke is made on two K40x25x11 rings. Built-in overload protection.
External view of the printed circuit board of the power supply for automotive equipment DOWNLOAD IN LAY FORMAT
The power supply up to 2000 W is made on two boards measuring 275x99, located one above the other. The voltage is controlled by one voltage. Has overload protection. The file contains several options for the “second floor” for two bipolar voltages, for two unipolar voltages, for the voltages required for two and three level voltages. The power transformer is located horizontally and is made on K45 rings.
Appearance of a “two-story” power supply DOWNLOAD IN LAY FORMAT
A power supply with two bipolar voltages or one for a two-level amplifier is made on a board measuring 277x154. Has a group stabilization choke and overload protection. The power transformer is on K45 rings and is located horizontally. Power up to 2000 W.
External view of the printed circuit board DOWNLOAD IN LAY FORMAT
Almost the same power supply as above, but has one bipolar output voltage.
External view of the printed circuit board DOWNLOAD IN LAY FORMAT
The switching power supply has two power bipolar stabilized voltages and one bipolar low current. Equipped with fan control and overload protection. It has a group stabilization choke and additional LC filters. Power up to 2000...2400 W. The board has dimensions 278x146 mm
External view of the printed circuit board DOWNLOAD IN LAY FORMAT
The printed circuit board of a switching power supply for a power amplifier with two-level power supplies, measuring 284x184 mm, has a group stabilization choke and additional LC filters, overload protection and fan control. Distinctive feature is the use of discrete transistors to speed up the turn-off of power transistors. Power up to 2500...2800 W.
with two-level power supply DOWNLOAD IN LAY FORMAT
A slightly modified version of the previous PCB with two bipolar voltages. Size 285x172. Power up to 3000 W.
External view of the printed circuit board of the power supply for the amplifier DOWNLOAD IN LAY FORMAT
Bridged network switching power supply with a power of up to 4000...4500 W is made on a printed circuit board measuring 269x198 mm. It has two bipolar power voltages, fan control and overload protection. Uses group stabilization choke. It is advisable to use remote additional secondary power supply filters.
External view of the printed circuit board of the power supply for the amplifier DOWNLOAD IN LAY FORMAT
There is much more space for ferrites on boards than there could be. The fact is that it is not always necessary to go beyond the sound range. Therefore, additional areas are provided on the boards. Just in case, a small selection of reference data on power transistors and links to where I would buy them. By the way, I have ordered both TL494 and IR2110 more than once, and of course power transistors. It’s true that I didn’t take the entire assortment, but so far I haven’t come across any defects.
POPULAR TRANSISTORS FOR PULSE POWER SUPPLY |
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