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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING We defend ourselves ... in nutrition. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network, uninterruptible power supplies When operating the equipment from the AC mains, there are many situations when the failure of the power supply unit "orders a long life" for the rest of the equipment. Let us turn to the power supply (PSU) circuit shown in fig. one.
Alternating current with a voltage of 220 V flows in the primary circuit of the transformer T1 through the closed contacts of the SA1 mains switch and the fuse FU1, which protects the PSU from complete destruction in the event of a failure of the transformer T1. The power filter C5-L1-L2-C6 does not allow interference from the network into the equipment, and vice versa, interference that occurs during the operation of powered radio equipment into the network. A rectifier and a capacitive filter are connected to the secondary winding T1, the capacitors in which, at high operating currents, have a large capacitance (C9 -100000 uF). When they are charged at the moment of switching on, a very large current pulse occurs, which can not only burn the FU1 fuse, but also break through the rectifier diodes (VD2, VD3), which will lead to the flow of alternating current through them to the filter capacitors, heating the latter and explosion. To protect against this, the starting current of the PSU should be limited by connecting resistor R1 in series to the primary winding T7, which after a few seconds is short-circuited using relay contacts K1.1, rated (for reliability) for a current of 5 ... 10 A. The PSU turn-on delay time is determined by the resistance R11 and capacitance C11. Immediately after switching on, C11 shunts the winding of relay K1, preventing it from working. As C11 charges, the voltage across it increases, and when it reaches the actuation voltage of relay K1, the latter turns on and shorts R1.1 with contacts K7, providing a working current in the primary winding of transformer T1. Diode VD7 is designed to suppress voltage surges on the relay winding when it is triggered. It is very convenient to use diode bridges in AC rectifiers, especially since they are available in block design and are easy to install. However, with an increase in the current given by the PSU to the load, the question of "drawdown * of the supply voltage under load, which in the bridge circuit increases due to two diodes connected in series (the total voltage drop across them is up to 1.4 V for silicon diodes or up to 0,8, XNUMX V for germanium and Schottky diodes). By changing the rectifier from a bridge to a mid-point circuit, we get a voltage drop of about 0,7 V for silicon diodes and 0,3 ... 0,4 V for germanium and Schottky diodes. The use of Schottky diodes is also justified because less power is dissipated on them, and this reduces the size of the radiators on which the diodes are installed at high rectified currents. It becomes more convenient to wind the secondary winding of a power transformer, since the diameter of the winding wire decreases (the current flowing in each half of the winding is half of the total current at the output of the rectifier). True, you will have to wind twice as many turns, but for a low output voltage this is not too difficult, since there are not many turns. In high-voltage rectifiers, it is more expedient to use rectifier bridges. A capacitor (C7, C8) is connected in parallel to each rectifier diode. These capacitors protect the PSU from the so-called "multiplicative" background, when the rectifier diodes react to RF interference from the network like antennas. For the operation of the regulating transistor of a series linear stabilizer following the filter, a certain minimum collector-emitter voltage difference for bipolar transistors (BT) or drain-source for field-effect transistors (FET) is required, at which they still work. In the case of powerful BTs, this is 3 ... 5 V, and for powerful FETs - 0,5 ... 3 V. It follows that with a maximum load current of 30 A and a stabilizer output voltage of 13,8 V, the voltage at the source of the transistor VT2 should not fall below 13,8 + 0,5 = 14,3 (B). Thus, it is possible to select the minimum required capacitance C9 in the finished PSU by loading its output with a maximum current (for example, 30 A) and measuring the voltage drop across the regulating transistor. The supply of this voltage, of course, will not hurt in the sense of compensating for a decrease in the mains voltage, but it is fraught with an increase in the power dissipated on the VT2 transistor, which will lead to the need to increase the size of the radiator on which this transistor is installed. Indeed, at a current of 30 A and a voltage drop of 0,5 V, 2-0,5 \u30d 15 (W) is dissipated on VT3, and at the same current, but a drop of 3 V - 30 90 \uXNUMXd XNUMX (W). The difference is very significant! The scheme of the described stabilizer (without protection) is borrowed from [1] (additional details continue the reference designations from the original). The high quality characteristics of the given stabilizer are due to the use of a powerful p-channel field-effect transistor IRL2505. To increase the stabilization coefficient in the PSU, an "adjustable zener diode" is used - the TL431 microcircuit (the domestic analogue is KR142EN19). This microcircuit is produced in the TO-92 package (Fig. 2). The internal structure of the IC is shown in fig. 3, and the maximum permissible parameters are given in the table. The control characteristics of TL431 are given by the graphs in fig. 4.
Transistor VT1 in the power supply (Fig. 1) is a matching, zener diode VD1 stabilizes the voltage in its base circuit. The output voltage of the stabilizer can be calculated by the formula: Uout=2.5(1+R5/R6) The stabilizer works as follows. Suppose, when the load is connected, the output voltage of the stabilizer has decreased. Then the voltage will also decrease at the midpoint of the divider R5-R6. Chip DA1. as a parallel stabilizer, it will consume less current, and the voltage drop on its load (resistor R2) will decrease. This resistor is in the emitter target of the transistor VT1, therefore, with a stabilized voltage based on VT1, the transistor will close, providing an increase in the voltage at the gate of the regulating transistor VT2, which will open more and compensate for the voltage drop at the PSU output. Resistor R6 sets the output voltage. Zener diode VD6, connected between the source and gate VT2. serves to protect the FET from exceeding the permissible gate-source voltage and is an indispensable element in stabilizers with increased input voltage (from 15 V and above). The stabilizer is good for everyone, but what happens if the load current exceeds the limit value for the regulating transistor (a short circuit occurs)? Obeying the algorithm of its work, VT2 will fully open, and then fail due to channel overheating. To limit the maximum current through the FET, you can choose the operating mode of the transistor VT1. but it is still more reliable to apply special protection. For example, on an optocoupler, as described in [2]. This protection is presented in a slightly modified form in the proposed BP. The parametric stabilizer on the VD4 zener diode provides a voltage of 6,2 8. For greater stability of this voltage, using the load resistor R8, the VD4 operating point is brought closer to the middle of its characteristic (IVD410 mA). The noise of the zener diode is blocked by the capacitor SU. The output voltage of the stabilizer is compared with the obtained reference voltage through the chain: LED of the optocoupler VU 1 - diode VD5-limiting resistor R10. While the output voltage of the stabilizer is higher (more negative) than the reference, the VD5 diode is locked, no current flows through the LED. If the output terminals are short-circuited on the right (according to the diagram) output of the resistor R10, the negative voltage will disappear, the reference will open the VD5 diode, the optocoupler LED will light up, the optocoupler phototriac will work, which will close the gate VT2 with a source, and the transistor will close. The output current of the stabilizer will stop. To bring the PSU into operation, turn it off using the SA1 power switch. remove the short circuit and turn it on again. The protection returns to its original state. The use of such stabilizers on the FET makes unnecessary the protection circuit against overvoltage resulting from the breakdown of the regulating transistor, since here this voltage will increase by only 0.5 ... 1 V. For more critical equipment, we can offer a "hard" limiter circuit, called in the West " crow bar". The principle of protection when the set threshold voltage at the output of the stabilizer is exceeded is to burn out the fuse connected in series with the load using a powerful thyristor. If desired, such protection can be introduced into other stabilizers. The stabilizer is placed on a printed circuit board measuring 52x55 mm. The drawing of the board is shown in fig. 5, and the location of the elements is in fig. 6. In fig. 1, this node is circled by a dotted line. The board is made of double-sided foil fiberglass with a thickness of 1...1.5 mm. The foil on the underside of the board is connected to the negative rail of the stabilizer. Free conclusions of the optocoupler VU1 can not be soldered. Additional protection parts can be mounted by surface mounting, using as racks, for example, patches made of foil fiberglass glued to the VT2 radiator. As K1 in the PSU, you can use the RES9 relay with a 12 V winding, connecting its contact groups in parallel. The mains filter consists of two capacitors with a capacity of 0,01 microfarads for an operating voltage of 630 V and two coils connected between them. The coils are wound with a flat power cord on a ferrite rod with a diameter of 8 ... 10 mm and a length of 140 .... 160 mm from the magnetic antenna of the radio receiver. The same simultaneous winding of coils on a ferrite ring with a permeability of 2000 ... 10000 and a diameter of 32 ... 60 mm is possible before filling. The transformer for such a PSU must have an overall power Rg of the order of 500 watts. In fact, let's count. The output voltage of the stabilizer is 13.8 V, the maximum current is 30 A. The voltage drop across the control transistor, diodes and connecting wires will total about 1 V. The power on the secondary winding of the transformer T1 P will be: P \u13.8d (1 + 30) 444 \u1d 10 ( W) We take into account the losses for the remagnetization of the core T44,4 - 444%. or 44.4 watts. Then Pg=488,4+500=1 (W). The rest /P, up to 1 W, we will leave in reserve for the PSU's own consumption. The core cross section S, for example, for the W-shaped core T2, will be: S=(P)22,4/2=500 (cm220). The current in the primary winding will be 2.27/1=0.8 (A). Primary wire diameter: d1=2(I)0.8/1,5= 1,2-30= 15 (mm). Similarly, we consider the diameter of the wire of the secondary winding, given that in the rectification circuit with a midpoint, the current in the secondary half-windings is half as much (not 16, but 2 A). Let's take a small margin, including for the "own needs" of the PSU. and we will assume that a current of 0.8 A "walks" in the secondary windings. Hence, the wire diameter: d16 = 1(2)3.2/1 = 1(MM). The use of wires of a smaller cross section will lead to an increase in the "drawdown" of the voltage at the input of the stabilizer, which will not allow you to get the maximum current from the PSU. for which it is designed. The calculation of the number of turns of the transformer for our case is also not difficult. The number of turns in the windings T1 per XNUMX V - wXNUMX: w1 = 50/S = 50/22,36 = 2.24. Number of winding turns I -W1: W1=w1Ui= 2.24-220= 493 (turn), windings 2 (secondary identical windings - two) - W2: W2 \u1d w2U2,24 \u14,8d 33-XNUMX \uXNUMXd XNUMX (turn). To improve the parameters of the PSU after winding the secondary windings, it is imperative to balance the output voltages T1 so that both halves of the secondary winding give exactly the same voltage. Before assembling the PSU, be sure to check the ratings of all parts and their serviceability. In parallel with all oxide capacitors, non-polar capacitors with a capacity of 0,1 ... 0,22 μF should be soldered directly to their terminals. When using the PSU as a laboratory, it is more convenient to display the R6 axis on the front panel of the device, and also to equip the PSU with measuring heads for measuring voltage and current. The appearance of my block is shown in Fig. 7. When working with radio transmitting equipment, interference with parts of the stabilizer and wires should be excluded. At the PSU output terminals, it is recommended to turn on a filter similar to a network filter (Fig. 1), with the only difference being that the coils must be wound on a ferrite ring or ferrite tube, used in old monitors and foreign-made TVs, and contain only 2-3 a turn of insulated wire of large cross section, and capacitors are designed for a lower operating voltage. Information sources
Author: V.Besedin, UA9LAQ, Tyumen
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