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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Bipolar voltage regulator with water cooling, 220/±41 volts 4 amps. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies Compensatory voltage stabilizers of continuous operation of the serial type have a low efficiency, but a large stabilization coefficient and low output impedance. Therefore, they are still widely used. However, they are characterized by low reliability in case of overload or short circuit in the load. This is especially dangerous for transistor devices, so it is necessary to introduce complex protection units with current sensors into stabilizers. In the powerful bipolar voltage regulator discussed in this article, the output current is limited. The device is not afraid of overloads and can work on high-capacity filter capacitors. An analysis of UMZCH circuits allows us to conclude that continuous voltage stabilizers are rarely used to power their output stages. The reasons for this are the high cost of such stabilizers, large energy losses during their use, and most importantly, "it will do," because it works without a stabilizer. When there is no stabilizer, the amplifier supply voltage varies depending on the load over a wide range (in the Pioneer-714 AV receiver - 30 ... 50 V). The fact is that the average output voltage of a rectifier with a capacitive filter is highly dependent on the load outflow. Moreover, the filter capacitors are charged with pulses in each half-cycle of the mains voltage. The process may take several half-cycles, and this is partially transferred to the UMZCH load. In amateur radio literature, the opinion has repeatedly been expressed about the need to power the UMZCH from a stabilized source to ensure a more natural sound. Indeed, at the maximum output power of the amplifier, the range of voltage ripples of an unstabilized source reaches several tens of volts. This is imperceptible at the peak values of the high-frequency components of the audio signals, but affects the amplification of their low-frequency components of a large level, the peaks of which have a long duration. As a result, the filter capacitors have time to discharge, the supply voltage decreases, and hence the peak output power of the amplifier. If the decrease in the supply voltage is such that it leads to a decrease in the quiescent current of the output transistors of the amplifier, this can cause additional non-linear distortion. The cardinal way to suppress ripples and instability of the supply voltage is its stabilization. The stabilizer reduces the voltage ripple on the power lines by one or two orders of magnitude, which makes it easy to obtain the maximum amplitude of the amplifier output signal. In addition to reducing the background level with a frequency of 50 (100) Hz, non-linear distortion and the likelihood of signal clipping at loudness peaks are also reduced. The margin for the maximum permissible parameters of the transistors of the output stage of the amplifier is increased. Reduces the likelihood of mains interference entering the amplifier output. In addition, the use of a stabilizer allows you to simplify the amplifier, which has a beneficial effect on the sound. Another plus - the function of protecting the output stage of the amplifier from overload can also be entrusted to the stabilizer. Of the minuses - the implementation of a powerful and reliable voltage stabilizer of continuous operation becomes a significant financial problem and a technically difficult task. In addition, it becomes necessary to remove a large amount of heat from the power transistors of the stabilizer. The overall efficiency and power dissipation of the amplifier with a stabilizer is much worse than without it. To improve the quality of the power supply, it is desirable to use a network transformer with reduced induction. As you know, the starting current of conventional transformers reaches values that are much higher than the operating current. Reducing the induction amplitude in the magnetic circuit by half significantly increases reliability, reduces the leakage flux of the transformer and reduces its starting current to a value not exceeding the rated no-load current. However, lower induction leads to an increase in the required number of turns of the windings and, as a result, to a deterioration in the weight and size of the transformer, its cost and an increase in energy losses on the active resistance of the windings. But we are talking about really high-quality sound reproduction, right? And the sound of an amplifier powered by a stabilized voltage is significantly better compared to the sound of the same amplifier without a stabilizer. A bipolar voltage regulator, the circuit of which is shown in the figure, is designed to power the UMZCH.
Main technical parameters
It consists of two independent voltage regulators of positive and negative polarity relative to the common wire. The upper part of the circuit refers to the positive polarity stabilizer, and the lower part refers to the negative polarity. The negative polarity regulator circuit is essentially a mirror image of the positive polarity regulator circuit. Therefore, we will consider in detail only the voltage regulator of positive polarity. The alternating voltage taken from the winding II of the transformer T1 rectifies the full-wave rectifier on dual Schottky diodes VD3 and VD4 SR30100P, which have an insulated housing, so it is convenient to mount them on a common heat sink. Through the noise suppression inductor L1, the rectified voltage is supplied to the smoothing and noise suppression capacitors C8-C16 and then to the equalizing emitter currents of parallel-connected transistors VT1-VT9 resistors R3-R11. These resistors have a fairly high resistance, which contributes to the effective "isolation" of the collector circuits of transistors VT1 -VT9 from network interference. Together with the VT20 transistor, the VT1-VT9 transistors form a powerful composite transistor with a high current amplification factor. The base current of the transistor VT20 flows into the collector of the transistor VT22. Transistor VT22 controls the voltage from the output of the op-amp DA3.1. Zener diodes VD13, VD14 connected in series are connected to the output of the stabilizer, the total stabilization voltage of which serves as an exemplary for the considered stabilizer. Instead of zener diodes, you can install a resistor of such resistance that, together with resistor R29, it provides zero potential at the point of their connection at the rated output voltage of the stabilizer. But compared to zener diodes, this is a less efficient option. The potential shifted by zener diodes or a resistor in the stabilization system is a mismatch signal and is fed to the inverting input of the DA3.1 op-amp, the non-inverting input of which is connected to the "0" wire. Keep in mind that the wires "O" and "Comm." must be connected to each other and to the common wire of the device (amplifier) powered by the stabilizer on the board of the latter. This significantly reduces the level of interference and noise in the stabilized voltage. Resistor R21 ensures the performance of the stabilizer when no amplifier is connected to it. During operation, the op-amp continuously compares the potential at its inverting input with the zero potential at the non-inverting input. Further, he controls the transistor VT22 in such a way, and with it the composite transistor VT20, VT1-VT9, so that the specified voltage is maintained at the output of the stabilizer. Suppose the voltage at the output of the stabilizer has decreased due to an increase in load current. The potential at the inverting input of the op-amp DA3.1 will become negative relative to the non-inverting one, and the voltage at the output of the op-amp will increase. This will increase the collector current of the VT22 transistor, and with it the base and emitter current of the VT20 transistor. As a result, the total collector current of transistors VT1-VT9 will increase, compensating for the increase in load current. The output voltage will return to its previous value. The soft start device on the transistor VT19 and relay K1 provide a smooth increase in voltage on the bank of capacitors C28-C30, C34-C63 when the stabilizer (primary winding of transformer T1) is connected to the network. At this moment, a current begins to flow through the resistor R2, charging the capacitor C27. When, after 30 ... 35 s, the voltage applied to the Zener diode VD9 reaches 36 V, it opens. This leads to the opening of the transistor VT19 and the operation of the relay K1, which switches the resistors that limit the output current of the stabilizer. While the relay has not worked, this current is limited by the resistor R32 to 450 ... 650 mA, which eliminates the inrush of the charging current of the battery of capacitors C28-C3O, C34-C63 with a total capacity of more than 100000 uF. The triggered relay connects resistor R32 in parallel with resistor R35. From this point on, the stabilizer can supply current up to 4 A to the load. If the stabilizer output is accidentally closed with a common wire, the current will also not exceed 4 A, but the power dissipated by the Vt1-VT9 transistors will sharply increase. However, it will not exceed 25 watts per transistor. It follows from this that the voltage regulator is reliable and is not afraid of short circuits in the load. To accurately set the current limiting levels, it is necessary to temporarily replace the resistor R32 with a variable resistor of about 500 kΩ, and the resistor R35 is not installed. Move the variable resistor slider to the maximum resistance position. Having closed the output of the stabilizer with an ammeter, turn on the stabilizer and gradually reduce the resistance of the variable resistor, observing the readings of the ammeter. When the required safe starting current is reached, turn off the regulator, measure the input resistance of the variable resistor and replace it with a fixed resistor of the same resistance. Then, instead of resistor R35, connect a variable resistor with a resistance of 100 kOhm, and the maximum load to the output of the stabilizer through an ammeter. Turn on the stabilizer and wait for the relay to operate. After that, begin to gradually reduce the resistance of the variable resistor. When the rated stabilization voltage and the specified maximum load current are reached, turn off the stabilizer, measure the input resistance of the variable resistor and replace it with a constant one. The same procedure must be performed with a negative voltage stabilizer. You cannot simply install resistors R33 and R36 of the same resistance as R32 and R35, respectively. The fact is that the current transfer coefficients of the transistors used in both stabilizers differ significantly. For example, for 2SA1943 transistors it is about 140, and for 2SC5200 it is only 85. Transformers T1 and T2 are custom-made with reduced induction and secondary windings for 2x54 V (with medium leads) at a load current of 5 A. Each transformer is installed on its own side in the lowest part of the heat exchanger (aquablock) of the stabilizer water cooling system. The aquablock serves as a kind of chassis on which all the nodes of the device are located. Before installing the transformers, they are molded with epoxy into perfectly flat landing pads. Then, with M12 threaded studs, the transformers are pressed against the aquablock. In idle mode, the voltage at the outputs of the rectifiers (inputs of the stabilizers themselves) is 76 V. When connected to the output of a load stabilizer with a resistance of 10 ohms, it drops to 64 V. If more load current is needed, for example 10 A, then the values of the resistors R3-R20 should be reduced up to 10 ohm. Suppressor diodes VD1 and VD2 are designed to dampen surges during transients that accompany the inclusion of the stabilizer in the network. With proper installation and assembly, the stabilizer starts working without any problems. With a continuous load of 4 A, the transistors VT1-VT9 dissipate about 60 W of power (6 W for each transistor). On each of the resistors R3-R11 - 4 watts. Together, the positive and negative voltage regulators dissipate about 180 watts. Two pairs of stabilizers for powering the amplifiers of the left and right stereo channels, mounted on a common aquablock, dissipate 360 watts. The aquablock consists of two pieces of duralumin rail with a section of 100x10 mm and a length of 1000 mm, fastened with screws along the perimeter. An automotive sealant was used to seal the joint between the tires. On the inner surface of each tire, two parallel grooves measuring 960x15x4 mm are milled, through which cooling water flows. The total cross section of the water supply channel is 15x8 mm, its total length is 1920 mm, the water flow rate is 0,75 l/min, the water temperature at the aquablock inlet is 24 °C, and at the outlet - 29 °C. Water comes from the water supply through a single-stage filter. Four years of experience in operating such an open water cooling system showed the stability of its thermal parameters. But the system can also be made closed with distilled water circulating through the aquablock and an external car radiator. Transistors VT1-VT18 are mounted on a printed circuit board with an aluminum substrate pressed against the aquablock using heat-conducting paste. The board surface temperature is about 34 °C. Transistors 2SA1943 and 2SC5200 heat up to a temperature of about 50 °C. Tests showed that this temperature remained unchanged during three hours of operation. The described cooling system is compact, efficient and absolutely silent. It allows you to divert about a kilowatt of thermal power. As a signaling device for the emergency lack of running water in the system, a pressure sensor DRD-40 is installed in its supply pipeline. It is ideal for standard plumbing. In the event of an emergency shutdown of water, the contacts of this sensor open and disconnect the stabilizer from the electrical network. In addition, it is necessary to install temperature sensors on one or more 2SA1943 transistors, which, as practice has shown, heat up more than 2SC5200 transistors. The same sensors are recommended to be installed on transformers. Author: V. Fedosov
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