Powerful bipolar stabilized power supply 2x44 volts 4 amps per channel. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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In amateur radio literature, the opinion has repeatedly been expressed about the need to power the UMZCH from a stabilized power source to ensure its more natural sound. Indeed, at the maximum output power of the amplifier, the voltage ripple of an unregulated source can reach several volts. In this case, the supply voltage can be significantly reduced due to the discharge of the filter capacitors. This is imperceptible at peak values ​​of the output voltage at higher audio frequencies, due to the sufficient capacitance of the filter capacitors, but it affects the amplification of high-level low-frequency components, since they have a long duration in the music signal. As a result, the filter capacitors have time to discharge, the supply voltage decreases, and hence the maximum output power of the amplifier. If a decrease in the supply voltage leads to a decrease in the quiescent current of the output stage of the amplifier, then this can also lead to the appearance of additional non-linear distortions.

On the other hand, the use of a stabilized power supply built according to the usual scheme of a parametric stabilizer increases the power consumed by it from the network and requires the use of a network transformer of greater mass and dimensions. In addition, there is a need to "removal of heat dissipated by the output transistors of the stabilizer. And often the power dissipated by the output transistors UMZCH is equal to the power dissipated by the output transistors of the stabilizer, i.e. half of the power is wasted. Switching voltage regulators have a high efficiency, but are quite difficult to manufacture, have a high level of high-frequency interference and are not always reliable.

If there are no strict requirements for voltage stability and ripple level to the power supply, then a conventional bipolar power supply can be used as a power source, the circuit diagram of which is shown in Fig.1.

Powerful composite transistors VT7 and VT8, connected according to the emitter follower circuit, provide fairly good filtering of the supply voltage ripple with the mains frequency and stabilization of the output voltage thanks to the zener diodes VD5 - VD10 installed in the base circuit of the transistors. Elements LI, L2, R16, R17, C11, C12 eliminate the possibility of high-frequency generation, the tendency to which is explained by the large current gain of composite transistors. The value of the AC voltage coming from the network transformer is chosen so that at the maximum output power of the UMZCH (which corresponds to a load current of 4A), the voltage across the filter capacitors C1 - C8 drops to approximately 46 ... 45 V. In this case, the voltage drop across the transistors VT7, VT8 will not exceed 4 V, and the power dissipated by the transistors will be 16 W. With a decrease in the power consumed from the power source, the voltage drop across the transistors VT7, VT8 increases, but the power dissipated by them remains constant due to a decrease in the current consumed. The power supply works as a voltage stabilizer at low and medium load currents, and at maximum current - as a transistor filter. In this mode, its output voltage can drop to 42...41 V, the output ripple level reaches 200 mV, and the efficiency is 90%.

As the breadboarding showed, fuses cannot protect the amplifier and power supply from overcurrent due to their inertia. For this reason, a high-speed protection device against short circuit and exceeding the permissible load current, assembled on transistors VT1-VT6, was used. Moreover, the protection functions for overloads of positive polarity are performed by transistors VT1, VT2, VT5, resistors Rl, R3, R5. R7 - R9, R13 and capacitor C9, and negative - transistors VT4, VT3, VT6, resistors R2, R4, R6, R10-R12, R14 and capacitor C10.

Consider the operation of the device with overloads of positive polarity. In the initial state at rated load, all transistors of the protection device are closed. With an increase in the load current, the voltage drop across the resistor R7 begins to grow, and if it exceeds the permissible value, the transistor VT1 begins to open, followed by the transistors VT2 and VT5. The latter reduce the voltage on the basis of the regulating transistor VT7, and hence the voltage at the output of the power supply. At the same time, due to the positive feedback provided by the resistor R13, a decrease in the voltage at the output of the power supply leads to an acceleration of the further opening of the transistors VT1, VT2, VT5 and the rapid closing of the transistor VT7. If the resistance of the positive feedback resistor R13 is small, then after the protection device is triggered, the voltage at the output of the power supply is not restored even after the load is turned off. In this mode, it would be necessary to provide a start button that turns off, for example, resistor R13 for a short time after the protection is triggered and at the moment the power supply is turned on. However, if the resistance of the resistor R13 is chosen so that the current is not equal to zero during a short circuit of the load, then the voltage at the output of the power supply will be restored after the protection device trips when the load current decreases to a safe value. In practice, the resistance of the resistor R13 is chosen such a value that ensures reliable switching on of the power supply when the short-circuit current is limited to 0,1 ... 0,5 A. The operating current of the protection device determines the resistor R7. The power supply protection device operates similarly during negative polarity overloads.

Construction and details

All parts of the power supply are located on one board. The exception is transistors VT7, VT8 of the citation unit, placed on separate heat sinks with a dissipating surface area of ​​300 cm² every. Coils LI, L2 of the power supply (Fig. 3) contain 30-40 turns of PEV-1 1,0 wire wound on the body of the C5-5 or MLT-2 resistor. Resistors R7, R12 of the power supply are a piece of copper wire PEL, PEV-1 or PELSHO with a diameter of 0,33 and a length of 150 mm, wound on the body of the MLT-1 resistor. The power transformer is made on a toroidal magnetic core made of E320 electrical steel, 0,35 mm thick, tape width 40 mm, inner diameter of the magnetic core 80, outer diameter 130 mm. The network winding contains 700 turns of PELSHO 0,47 wire, the secondary - 2x130 turns of PELSHO 1,2 wire.

Each of the KT825G transistors can be replaced by composite transistors KT814G, KT818G, and KT827A - by composite transistors KT815G, KT819G. Instead of KS515A zener diodes, you can use D814A (B, C, G, D) and KS512A zener diodes connected in series.

Checking the health of the power supply

To do this, replacing the resistors R7, R12 of the power supply with higher-resistance ones (approximately 0,2 ... 0,3 Ohms), they check the operability of the power supply of the protection device. It should operate at a load current of 1 ... 2 A. After making sure that the power supply and UMZCH are operating normally, resistors R7, R12 are installed with a nominal resistance indicated on the circuit diagram, controlling the absence of operation of the protection device.

Literature

1. Lexins Valentin and Victor. On the visibility of non-linear distortions of the power amplifier. - Radio, 1984, No. 2, p. 33-35.
2. Solntsev Yu. What kind of Kg is acceptable? - Radio, 1985, No. 2, p. 26-28.
3. Solntsev Yu. High-quality power amplifier.- Radio, 1984, No. 5, pp. 29-34.
4. Gumelya E. Quality and circuitry UMZCH. - Radio, 1985, No. 9, p. 31-34.

Publication: cxem.net

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