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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Automotive UMZCH with a power supply. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Automotive power amplifiers The voltage of the on-board network limits the power of the automotive UMZCH, and this circumstance can be overcome by using a switching supply voltage converter. The article describes the design of a powerful two-channel UMZCH with a built-in powerful converter based on the KR1114EU4 chip. Currently, there are many different models of car radios on the market for car audio equipment. Modern radio tape recorders usually have a four-channel amplifier, and the output power declared by the manufacturers is in the tens of watts. But do the inscriptions on the front panel, indicating the output power, for example, 4x40, 4x50 W, correspond to the generally accepted parameter? More often, its peak output power is indicated (as a rule, with a supply voltage of 14,4 V at a load of 4 ohms). In practice, the nominal output power of the car radio is usually no more than 10 ... 12 W per channel [1]. For a real increase in power, UMZCH is used in a bridge connection. For a more powerful load, the car radio is supplemented with a power amplifier. Based on the fact that almost all car speaker systems and most speakers of wide use have an electrical resistance of 4 ohms, the voltage of the vehicle's on-board network is insufficient, therefore, secondary power supplies must be used for UMZCH. The automobile two-channel power amplifier described here is combined with a switching power supply. The equipment is distinguished by a simple circuit design and the availability of manufacturing by radio amateurs. The rated output power of the UMZCH with a non-linear distortion factor of 0,5% in the "Stereo" mode is approximately 2x70 W (2x4 Ohms), in the "Mono" mode - about 150 W (8 Ohms). It requires almost no tuning. Amplifier. The amplifier is made on two chips DA1, DA2. The TDA7294 integrated circuit is a high performance power amplifier and is relatively cheap. The final and pre-terminal stages of the TDA7294 are built on field-effect transistors, they have protection against overheating and short circuit at the output. When the crystal temperature reaches 145°C, the protection unit switches the microcircuit to the "MUTE" mode, and when it reaches 150°C - to the "STAND-BY" mode. Due to the wide range of supply voltages, the TDA7294 chip can be used in conjunction with a load with a resistance of more than 8 ohms without a significant loss in output power. When using two microcircuits connected in a bridge circuit, the upper limit of the resistance rises to 16 ohms. With the optimal choice of supply voltage, its maximum output power at a low-resistance load (4 ohms and below) is limited only by the maximum allowable current of the final stage, equal to 10 A, and reaches 100 W. With a harmonic distortion factor of 0,5%, the microcircuit delivers power up to 70 watts to the load. More detailed information about the microcircuit can be obtained from [2] or on the website of ST Microelectronics. A schematic diagram of the UMZCH without a power supply is shown in fig. one.
In the proposed scheme, the "STAND-BY" and "MUTE" functions are not used, since the amplifier is turned on in the power supply. Resistors R1, R4 set the input impedance of the UMZCH. Pairs of elements R1, C1 and R4, C4 form a high-pass filter at the inputs of both channels, limiting the bandwidth of the amplifier from below. Similarly, the elements R2, C2 and R5, C5 in the OOS chain define the lower limit of the bandwidth. The resistance ratios R3/R2, R6/R5 set the UMZCH gain. With the specified ratings of the elements R2, R3, R5, R6, the voltage gain is 30 dB. The SA1 switch selects the UMZCH "Stereo / Mono" operating mode. In the "Stereo" mode, the DA1 and DA2 microcircuits work as two independent non-inverting amplifiers; in the "Mono" mode, the DA2 amplifier turns from a non-inverting amplifier with a gain of Kj = R6 / R5 + 1 into an inverting amplifier with unity gain. Position SA1 on the diagram corresponds to the "Stereo" mode. When using UMZCH in bridge mode, the output "+" AC is connected to the output DA1, and the output "-" to the output DA2. Amplifier power supply converter (Fig. 2) is built mainly on the KR1114EU4 chip - an imported analogue of the TL494CN from Texas Instruments.
A detailed description of the microcircuit can be found in [3], its block diagram is shown in Fig. 3. It includes a pulse-width modulator (PWM) and its control circuit. The microcircuit provides ample opportunities to control the duration of the output pulses. Since the TDA7294 microcircuits have their own protection nodes, there is no need to use them in the power supply itself.
The KR1114EU4 microcircuit can work both in push-pull and single-cycle converters; the operating mode is set by the OTS input (pin 13). In this power supply, pin 13 is connected to a +5 V reference voltage source and the converter operates in push-pull mode. The duty cycle of the pulses can vary over a wide range. The outputs of the microcircuit can be connected directly through resistors R16, R17 to the bases of powerful bipolar transistors VT1 and VT2 of the converter due to the high limit value of the output current (up to 200 mA). Since the converter chip has collector and emitter outputs for output transistors (pins 8-11), it is possible to turn them on according to a common emitter or common collector circuit, depending on the structure of transistors VT1 and VT2. In the described block with transistors of the npn structure, the second option is used. When using field-effect transistors (n-channel FETs) as keys, remove resistors R18 and R19. The KR1114EU4 chip has its own sawtooth pulse generator. Elements R8, C8 are timing, and the generation frequency can be determined by the formula f = 1/(R8C8). When operating in push-pull mode, the frequency of the oscillator of the microcircuit must be twice as high as the frequency at the output of the converter. For the values of the timing circuit indicated in the diagram, the generator frequency is about 160 kHz, and the output pulse frequency is about 80 kHz. The stability of the converter in a wide range of supply voltages is provided by the built-in reference voltage source (pin 14) +5 V. The R9C7 circuit provides a smooth increase in the output pulse width of the unit and power in the load after power is turned on. Diode VD1 prevents the failure of the unit when the polarity of the supply voltage is reversed; in this case, only fuse FU1 will blow. The power supply has voltage stabilization at the load due to feedback. It is carried out through resistors R10 - R15 from each rectifier arm. These resistors form two voltage dividers, through which part of the voltage from the output of the power supply goes to the error amplifiers (pins 1, 15). As a voltage standard, with which the output voltages of the power supply are compared, a reference voltage source (ION) is used. The outputs of the error amplifiers inside DA1 are connected together via diodes. Pin 3 is for local feedback limiting the gain of the amplifiers. In this block, pin 3 is used to drive the converter, and the amplifiers act as comparators. From the pulse transformer T1, the voltage is rectified by diodes VD2-VD5 and smoothed out by capacitors C11-C14. To reduce the power dissipation on the UMZCH DA1 and DA2 microcircuits and increase the maximum output power of the amplifier, you need to correctly select the output voltage of the converter, based on the load resistance. This UMZCH is designed to work in conjunction with a load of 4 ohms in the "Stereo" mode and with a load of 8 ohms in the bridge mode. The value of the supply voltage DA1, DA2 recommended by the manufacturer for a given load resistance is ±25. ..27 V, the pulse converter is designed for this voltage. In the one shown in Fig. 2 power supply circuit requires a sufficiently powerful switch to turn it on. Often this method of inclusion is inconvenient or unacceptable. On fig. 4 shows a diagram of the device for automatic control of the start of the converter. It ensures that the UMZCH is turned on when a constant voltage of more than 20 V is applied to the resistor R1 or when an audio signal is applied to the capacitor C15 with an effective voltage value of at least 0,6 V.
The first option can be used if the car radio has an output for controlling external devices, for example, an electric retractable antenna. Another option is also suitable if a subwoofer is installed in the car. Then the capacitor C15 is connected to one of the outputs of the UMZCH car radio, and now the amplifier will automatically turn on when the output power of the car radio is more than 0,15 ... 0,2 W and turn off at less. It is unacceptable to connect two inputs to the radio at the same time, as this can disable it. Capacitor C16 simultaneously smooths out AC voltage ripples and delays turning off the amplifier after the input signal disappears (with a delay of about 30 s). Diodes VD7, VD8 prevent the influence of the switching circuit on the operation of the SHI modulator. They also set the voltage threshold on the VT3 collector, above which the duration of the pulses at the DA3 output will begin to gradually decrease and, when it reaches 4 ... 4,5 V, the power supply will turn off. If this amplifier is used only for a subwoofer, you will need a node, the diagram of which is shown in fig. 5. This is a second-order low-pass filter with a cutoff frequency of 80 Hz; it is turned on before entering the UMZCH. In the diagram, in parentheses, the conclusions of the op-amp of the second channel are indicated. Integrated voltage regulators DA2, DA3 are installed in the power circuit. If the amplifier is planned to be used only in bridged mode, a single op amp can be used instead of dual op amps.
Details and construction. All parts of the amplifier and power supply, except for the amplifier switch SA1, fuse FU1 and input and output connectors (not shown in the diagram), are mounted on a printed circuit board made of fiberglass 2 mm thick foiled on one side. The drawing of the board and the location of the elements on it are shown in fig. 6.
As VD1, you can use diodes of the KD2997, KD2999 series with any letter index. Diodes KD2997B (VD2 - VD5) can be replaced with KD2997A, KD2999A, KD2999B. Instead of transistors KT898A (VT1, VT2), it is permissible to use others: KT890 with any letter index, KT896A, KT896B, KT898B, KP958A - KP958V, KP954A - KP954V. You can use imported field-effect transistors IRFZ48, IRFZ44, IRF540, IRF640, IRF530, BUZ11 A, BUZ22 or their analogues by removing resistors R18, R19. Powerful PSU transistors VT1, VT2 and amplifier microcircuits DA1, DA2 are installed on separate heat sinks. It is permissible to install microcircuits on one heat sink without insulation, but at the same time isolate it from the amplifier case, since the metal substrate of the microcircuits has a voltage of -Upit relative to the common wire. It is unacceptable to install transistors on one heat sink without insulation. Mica can be used as an insulating material. When mounting power elements on heat sinks, it is desirable to use heat-conducting paste KPT-8, which will greatly facilitate the thermal operation of these elements. Diodes VD1 - VD5 are installed perpendicular to the board. The magnetic circuit of the pulse transformer T1 is made up of three rings of size K40x25x11 glued together from M2000NM1 ferrite. Windings I, II are wound in 4 turns with a bundle of five wires PEV-2 1,2 mm. Windings III, IV are wound in 10 turns with a bundle of four wires PEV-2 0,8 mm. Windings I, II and III, IV must be symmetrical. Before winding, the sharp edges of the glued ring must be rounded off with a needle file. Three or four layers of fluoroplastic tape insulation are laid between the windings. The transformer is installed in the center of the printed circuit board using a rectangular or round plate pressing on top with a hole in the center and an M5 or Mb screw with a nut. In the converter start control circuit, any low-power silicon diodes are suitable as VD1 - VD3, KT3102A (VT1) is replaced by a transistor with any letter index from this series or KT315. In the low-pass filter (see Fig. 5), it is permissible to install the OU KR574UD2, KR140UD20, KR544UD4. Instead of stabilizers DA2, DA3, you can use any integrated 15 V positive and negative voltage stabilizers. We must try to connect the power wires of the amplifier as close as possible to the car battery (on the fuse box) in order to exclude the influence of other current consumers. Since the peak current drawn by the amplifier can be up to 15 A, large gauge wires (3...5 mm2) should be used in the power circuit. If there is a device that is critical to high-frequency voltage ripples in the on-board network, it is necessary to increase the capacitance C9, and if this does not bring the desired effect, then turn on a high-frequency filter in the power supply circuit of the converter. Establishment. With serviceable elements, the amplifier starts working immediately. Only the power supply needs to be configured. Therefore, it is advisable to carry out installation and configuration in two stages as follows. Only power supply elements are installed on the printed circuit board (amplifier parts are not soldered). Next, the R14 resistor is soldered and a load equivalent is connected between the common wire and the positive output of the power supply - a wire resistor with a resistance of 6 ... 7 Ohms with a power of at least 100 watts. After turning on the power, measure the voltage across this resistor, it should be in the range of 26 ... 28 V. Further, the load resistance is increased to 50 ohms. By rotating the engine of the tuned resistor R13, the same output voltage of the power supply is achieved as with a 100-watt load. Then R14 is soldered in, and R12 is soldered out. Setting the second stabilization circuit is similar. At the end of the adjustment, solder the resistor R12. Then, the UM34 parts are mounted and the operability of the assembled device is checked for load equivalents from the audio frequency generator. The device for automatically turning on the amplifier (see Fig. 4) does not need to be configured, but if the converter starts even in the absence of input signals, then the resistance R21 is reduced to a value at which the voltage on the VT1 collector is in the range of 6 ... 6,5 V . Author: A.Kolganov, Kaluga
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Comments on the article: Jury Tell me, please, what form (in the sense of the oscillogram) should be the voltage on the secondary winding of the transformer T1, before the diode bridge in the circuit shown in Fig. 2? Regards, Yuri. Alexander beat off the developer of this transformer circuit [down]
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