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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Powerful power supply, 220/32 volts 1000 watts. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies In recent years, voltage conversion at a frequency of several tens of kilohertz has been increasingly used to reduce the size and weight of network power supplies. Such a source contains a mains voltage rectifier, a ripple filter with a doubled mains frequency, a voltage converter, a step-down transformer, a rectifier, and a ripple filter with a doubled conversion frequency. The converter is usually performed according to the scheme of a bridge or half-bridge inverter, in which the transistors open and close alternately after half a switching period. The disadvantage of such a converter is the presence of a through collector current at the moments of closing the transistors. Because of this, a large instantaneous electric power is allocated to them, the permissible value of which limits the power of such devices. The permissible instantaneous power of silicon transistors commonly used in voltage converters, for example, the KT812 series, does not exceed several hundred watts. To a certain extent, this limitation can be removed by using a bridge inverter loaded with a series resonant circuit. The transistors of such a device close in the absence of collector currents, the maximum collector voltage (relative to the emitter) and the maximum collector current act on the transistor at different times, so the instantaneous electrical power released on it turns out to be small. The possibilities of a bridge inverter with a series resonant circuit are illustrated by the mains power supply described below. It is intended to be used as a 27-volt equivalent vehicle electrical system (resistive or inductive-active load). The schematic diagram of the device is shown in the figure. Its main components are the C1L1C2 filter, which prevents interference from the frequency converter from penetrating into the network; mains voltage rectifier on diodes VD1-VD4 with filter C3-C5L2C6-C8; bridge inverter on transistors VT1 - VT4 with a resonant circuit L3C10C11, step-down transformer 74, increased frequency voltage rectifier on diodes VD13-VD18 with a filter C12-C15L4C16C17; inverter control unit on DD1-DD4 microcircuits and transistors VT5, VT6 and two sources feeding it: unstabilized (VD19) and stabilized (VD20 DA1). LED HL1 - indicator of the inclusion of the unit in the network.
The bridge inverter control unit consists of a clock generator made on single vibrators of the DD1 microcircuit, a pulse distributor on the DD2.2 trigger and elements of the DD4 microcircuit, two amplifiers (DD3.3; VT5 and DD3.4, VT6) and an overload protection device ( VD21, DD2.1) with synchronizer (DD3.1, DD3.2). LED HL2 signals the operation of the protection device. When the unit is connected to the network, the toggle switch Q1 supplies voltage to the control unit, and positive pulses with a duration of 1.2 μs with a constant repetition rate of about 17 kHz appear at the inverse output of the DD40 single vibrator. Trigger DD2.2 logical 1 signals arising on its direct and inverse outputs, alternately "opens" the elements DD4.1, DD4.2. and the pulses are fed to the input of one amplifier (DD3.3, VT5), then another (DD3.4, VT6). As a result, pulses of opening polarity are fed to the emitter junction of transistors VT1, VT4, then VT2, VT3. Some time after the appearance of the pulses of the clock generator (the delay is due to the rather large time constant of the filter C3-C5L2C6-C8), a smoothly increasing rectified voltage appears on the capacitor C9 and the inverter converts it into an alternating voltage with a frequency of 20 kHz applied to the winding I of the transformer T4. The voltage taken from its winding I is rectified by the VD13-VD18 diodes and fed through the filter C12-C15L4C16C17 to the load. Resistor R13 reduces the output voltage of an unloaded rectifier. The inverter can be divided into four phases. In the first, with a duration of 17 μs, transistors VT1, VT4 open, and capacitors C10, C11 are charged through them, the primary winding of the transformer T4 and the inductor L3. The current in this circuit first increases from zero to a maximum value, and then, as the capacitors charge, decreases to zero. The shape of the current resembles a half cycle of a sinusoid. In the second phase, lasting 8 μs, the closing polarity voltage is applied to the bases of transistors VT1, VT4, and they close. In the third phase (like the first, with a duration of 17 μs), transistors VT2, VT3 open, and almost all of the voltage rectified by the VD1-VD4 diodes is applied to each of the closed transistors VT1, VT4 (with a load of 1 Ohm - about 260 V). The recharging current of capacitors C10, C11 to the maximum voltage of opposite polarity, as well as in the first phase, flows through the series circuit formed by the capacitors, the inductor L3 and the primary winding of the transformer T4. The voltage to which they are recharged depends on the load resistance: the smaller it is, the greater this voltage (with a load of 1 ohm - about 200 V). At the moment when the collector current of transistors VT2, VT3 decreases to zero, the fourth phase of the inverter operation begins, lasting, like the second, 8 μs: closing voltage is applied to the bases of transistors from the windings of transformers T2 and T5. Transistors VT1, VT4 all this time continue to remain closed. A pause is necessary so that the transistors VT2, VT3 are completely closed and when the transistors VT1, VT4 are opened, there is no through current pulse through the transistors of adjacent arms. Due to the fact that the switching voltage is supplied to the emitter junctions at times when there is no collector current, the instantaneous electric power at the collector junction does not exceed a few watts in the worst case. Node overload protection block works as follows. After the supply voltage is applied, the trigger DD2.7 is set to a single state (at the inverse output - the voltage of logical 0), and at the output of the element DD3.2 (pin 11) a logic 1 voltage appears, creating conditions for the passage of clock generator pulses through the elements DD4.1 and DD4.2 In this state, the trigger remains all the time while the power delivered to the load is less than 1 kW. When the power limit is reached, the amplitude of the first pulse received at the counting input of the trigger DD2.1 from the secondary winding of the current transformer T3 through the bridge VD21 is sufficient to put the trigger in the zero state (at the inverted output - logic voltage 1). Changing the low logic level to a high one at the top input of the DD3.2 element according to the circuit leads to the fact that with the arrival of the next clock pulse, a logic 0 voltage is set at its output, and the passage of pulses through the elements DD4.1, DD4.2 stops. Thanks to the RS-trigger on the elements DD3.1, DD3.2, the inhibit signal appears only at the moment the pause between pulses begins, which prevents the inverter transistors from failing (closing in the presence of a collector current would lead to their failure due to an excessive increase in instantaneous electric power). The node protects the transistors of the inverter in the event of a short circuit of the load. To return the power supply to its original state after the protection has been triggered, it must be turned off and turned on again with the Q1 toggle switch. When the unit is turned off, the filter capacitors C3 - C8 are discharged through resistors R1 and R2. This is necessary so that during the increase in the amplitude of the base current pulses of the transistors VT1 - VT4 after switching on again, when they do not open completely (i.e., do not enter saturation mode), their collectors do not immediately have a large voltage that can lead to failure. Capacitors (C10, C11) K71-4 for a rated voltage of 250 V are used in the resonant circuit of the converter. Filter capacitors C12-C15 - K73-16 for a rated voltage of 63 V. Resistor R13 - PEV-10. The remaining resistors and capacitors are of any type. Switch Q1 - TV1-2. A unified transformer ТН13 127/220-50 is used in the power supply of the control unit. All other transformers and chokes of the device are homemade. Winding data are shown in the table. The L3 inductor and both windings of the T4 transformer are wound with wires twisted into a bundle. To reduce the leakage inductance of this transformer, winding II is wound with two bundles folded together. The tap is obtained by connecting the output of the beginning of one of the half-windings with the output of the end of the other. The magnetic circuits of all chokes are assembled with a non-magnetic gap of 0,5 mm. The inverter control unit and its power source are mounted on a printed circuit board made of foil fiberglass 2 mm thick. Most of the other parts of the block are hinged mounted on three boards measuring 220x85 mm made of textolite 3 mm thick: on one of them diodes VD1-VD4 and parts of filters C1L1C2 and C3-C5L2C6-C9 are fixed, on the other - transformers T2, T3, T5 and parts inverter, on the third - inductor L3 and filter parts C12-C15L4C16C17. Transistors VT1 - VT4 are mounted on duralumin heat sinks in the form of plates with dimensions of 70x60x8 mm (with sides of 60x8 mm they are attached to the circuit board), diodes VD1-VD4 - on U-shaped heat sinks bent from aluminum plates with dimensions of 100x25x1,5 mm, diodes VD13 ... VD18 and T4 transformer - on a ribbed duralumin heat sink with a cooling surface area of about 1000 cm2, fixed in the rear of the unit case. Setting up the device starts without fuse FU1. Turning on the power of the control unit, using an oscilloscope, they make sure that there are pulses of positive polarity with a duration of 1 μs at the emitter junctions of the transistors VT4-VT17 with a repetition rate of about 20 kHz (the oscillation period is approximately 50 μs). When connecting any output of the secondary winding of the current transformer 73 with the positive output of the power supply of the microcircuits of the control unit, these pulses should disappear. Then the output of the inductor L3 is disconnected from the primary winding of the transformer T4, the fuse FU1 is replaced, and instead of contacts 7 and 8 of the power switch Q1, a milliammeter is turned on. The current drawn by the inverter without load should be less than 15mA. After making sure of this, the terminals of the inductor L3 and the primary winding of the transformer T4 are connected with an additional resistor with a resistance of approximately 0,5 Ohm, the network terminals of the rectifier bridge VD1 - VD4 are soldered from the inductor L1 and an alternating voltage of 20 is applied to them from an adjustable autotransformer (for example, LATR) .. .30 V. A load equivalent is connected to the output of the block - a resistor with a resistance of 1 Ohm with a dissipation power of 700 ... 800 W. By controlling the voltage shape on the additional resistor with an oscilloscope, a non-magnetic gap in the magnetic circuit of the inductor L3 is selected so that the pulses (both positive and negative polarity) on the screen become as similar as possible to half-waves of a sinusoid. Further, observing the shape of the pulses, increase the voltage at the input of the bridge VD1 - VD4 to 220 V. The output power at the equivalent load increases to 650 ... 700 W, but the shape of the pulses should remain practically unchanged. If, at such a power, they sharpen, then this indicates saturation of the magnetic circuit of the inductor L3 or transformer T4 and it must be replaced with a more massive one (with a larger cross section). Finally, having excluded an additional resistor from the circuit, the resistor R18 is selected so that the overload protection unit operates at an output power of 1 kW (it is obtained by reducing the resistance of the equivalent load). During the adjustment, safety precautions should be observed, since many power supply circuits, in particular those subject to monitoring by an oscilloscope, are under high voltage. A load with a power of up to 700 W can be connected directly to the output of the unit and the power can be switched using a toggle switch. With higher power, it is desirable to provide an additional switch in the load circuit and first connect the unit to the network, and then the load to its output Author: S.Tsvetaev
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