One article to understand design of DC power supply circuit modules.
A constant current source is a power source that can provide constant current to a load, so scope of a constant current source is very wide, and in many cases it is necessary. For example, when using a conventional charger to charge a battery, as voltage at battery terminals gradually increases, charging current will decrease accordingly. To ensure constant current charging, output voltage of charger must be increased at any time, but after charging with a constant current source, there is no need to adjust output voltage, thereby reducing labor intensity and improving production efficiency. DC sources are also widely used in measurement circuits, such as resistor resistance measurement and classification, cable resistance measurement, etc., and more stable current, more accurate measurement.
1. Linear DC power supply, consisting of an integrated operational amplifier
The linear DC power supply circuit, consisting of an integrated operational amplifier, is composed as shown in figure. Two operational amplifiers (one of 324) make up comparison and amplification link, transistors BG1 and BG2 make up adjustment link, RL adjustment link. load resistance and RS is sampling resistance, RW provides voltage reference for circuit. How it works: If Uin decreases due to fluctuations in supply voltage, so that load current decreases, sampling voltage US must decrease, so that difference between sampling voltage and reference voltage (US-Uref) must decrease. Since MUA is an inverting amplifier, its output voltage Ub=(R5/R4)×Ua must be increased in order for US to rise back to its original stable value through control link, ensuring stability of US voltage so that current is stable. When Uin increases, principle is same as before, and circuit causes US to drop to its original stable value through a closed loop feedback system so that current remains constant. Adjust RW, then change Uref, and current value can be continuously adjusted from 0 to 4A.
Second, switching DC power supply with switching power supply
Scheme of switching DC power supply using a switching power supply is shown in Figure 2.3.2. BG1 is switching tube, BG2 is drive tube, RL is terminating resistor, RS is selection resistor, SG35 24 is PWM controller, L1, E2, E3 and E4 are energy storage components, and RW provides voltage reference Uref. Working principle of switching DC power supply using switching power supply: Reducing conduction loss and switching loss of switching device is key to improve efficiency of circuit. For this reason, a switching transistor with a low saturation voltage drop and good frequency characteristics and a freewheeling Schottky diode are selected in device.
The auxiliary coil is wound on magnetic core of choke coil L1, and saturation voltage drop of switching transistor is reduced by electromagnetic feedback, and a reasonable structural design is adopted to effectively control distribution parameters of circuit. When power supply voltage decreases or load resistance RL decreases, voltage across selective resistance RS will also decrease, and duty cycle of square wave at pins 12 and 13 of SG3524 will increase, thereby increasing conduction time of BG1 lengthens, and voltage U0 returns to its original stable value. . After BG1 is turned off, energy storage elements L1, E2, E3 and E4 ensure that voltage on load remains unchanged. When input voltage of power supply increases or value of load resistance increases, resulting in an increase in U0, principle is similar to previous one. The circuit causes U0 to fall to its original stable value through a closed-loop feedback system in order to achieve goal of stabilizing load current IL.
3. Switching DC power supply, consisting of a built-in voltage regulator
Scheme of a pulsed DC power supply, consisting of a built-in voltage regulator, is shown in figure. The MC7805 is a 3-terminal fixed integrated voltage regulator, RL is a terminating resistor and RW is an adjustable resistor. How it works: The fixed built-in voltage regulator operates in suspended state, and RW potentiometer is connected between output terminal 2 and common terminal 3, forming a fixed constant current source. Adjusting RW can change size of current, output current is: IL=(Uout/RW) +Iq where Iq is MC7805 quiescent current which is less than 10mA. When RW is small, that is, output current is large, Iq can be ignored. When load resistance RL changes, MC7805 maintains current through load by changing its own voltage difference.
Defining RW: RW value can be defined as RW=Uout/IL. Since Uout = 5V, IL = 0.5~2A, so detection range is 2.5~10Ω. Determining output voltage range and load variation: According to design requirements, output voltage in this example is U0=10V. Since adjustable output current range of DC power supply is 0.5-2A, corresponding load range is 5-20Ω. The above direct current sources have a simple structure, high reliability and convenient setting, and have found application in scientific research. Among them, linear constant current supply is suitable for DC battery discharge, switch constant current supply is suitable for DC battery charging, and constant current supply consisting of a built-in voltage regulator is suitable for measurement resistance and so on.
Voltage Controlled DC Power Supply Circuit Four
Voltage Controlled DC Supply Diagram The voltage controlled DC supply is an important part of system. Its function is to use voltage to control change in current. Since system has relatively high requirements for output current and accuracy, it is best to choose a voltage regulated DC power supply circuit is especially important. The following scheme has been adopted: schematic diagram of circuit is shown in Figure 2.4.3. The DC power supply circuit is composed of an op amp, high power FET Q1, selection resistor R2, load resistor RL, etc. 1. Hardware circuit.
The circuit control tube uses a powerful IRF640 field tube. The use of field lamps facilitates implementation of current-to-voltage linear control, which can not only meet maximum output current requirement of 2A, but also better provide approximately linear current-to-voltage control. Because when FET is in saturation, leakage current Id is approximately equal to current controlled by voltage Ugs. That is, when Ud is constant, it satisfies condition: Id=f(Ugs), while Ugs remains unchanged, Id remains unchanged. In this circuit, resistor R2 is a selective resistor, which is made of constantan wire (resistance value changes slightly with temperature), and resistance value is 0.35 ohm. The op amp uses OP-07 as a voltage follower, UI=Up=Un, FET Id=Is (gate current is relatively small and negligible), so Io=Is= Un/R2= UI/R2. Just because Io = UI/R2, input voltage of UI circuit controls current Io, that is, Io does not change when RL changes, in order to realize a voltage controlled constant current. At same time, it can be seen from design requirements: since output voltage range is U<=10V, Iomax=2A, we can conclude that load resistance RLmax=5 Ohm.
5. Power Supply Diagram
This system places higher demands on power supply. When designing power supply, it is necessary to ensure high stability of power supply and ensure that power supply can supply more than 2A current. Therefore, this system uses a 1264 triode to expand current, and should fully consider efficiency of power supply when using power supply. The power supply circuit is shown in figure. This power supply circuit uses LM317 and LM337. Its output voltage is continuously adjustable. +15V power supply load is not lower than 2A, so three-stage 1264 is used to expand current. In addition, use LM7805 to generate +5V voltage for Sunplus SPCE061A microcontroller.
Linear DC power supply, switchable DC power supply, high reliability, easy adjustment have been applied in scientific research. Among them, linear constant current supply is suitable for DC battery discharge, switch constant current supply is suitable for DC battery charging, and constant current supply consisting of a built-in voltage regulator is suitable for measurement resistance and so on. The single-chip microcomputer SPCE061A is used as central controller. This system has characteristics of reliable operation, reliable operation, small size and simple circuit. The minimum step of this system is 1mA, and accuracy is relatively high. The output current range is wide. , thermal stability of selection resistor in hardware is better, main module in hardware is a voltage controlled constant current supply, and its main component uses a field effect tube, its performance and stability are higher than triode
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