I. Objectives and requirements
1. experimental purpose
Through the design, installation and debugging of integrated DC regulated power supply, it is necessary to understand:
(1) Choose transformer, rectifier diode, filter capacitor and integrated voltage regulator to design DC regulated power supply;
(2) Master the debugging of DC voltage stabilizing circuit and the testing method of main technical indexes.
2. Design tasks
Design waveform DC regulated power supply to meet the following requirements:
(1) When the input voltage is 220v10%, the output voltage is 3- 12v, and the output current is greater than1a; ;
(2) The output ripple voltage is less than 5mV, the voltage stabilizing coefficient is less than 5× 10-3, and the output internal resistance is less than 0. 1ω.
3. Design requirements
(1) The power transformer is only a theoretical design;
(2) Reasonable selection of integrated voltage regulator;
(3) Complete the theoretical design, computer-aided analysis and simulation, installation and debugging of the whole circuit, draw the circuit diagram and make the printed board;
(4) Write design report, debugging summary report and instructions.
Second, the instruments and equipment
Automatic voltage regulator, double-trace oscilloscope, multimeter (analog or digital), AC millivoltmeter, and a set of self-made circuit board tools.
Several components.
Third, the principle and analysis
Basic principle of 1.DC regulated power supply
DC stabilized power supply is generally composed of power transformer T, rectifier filter circuit and stabilized voltage circuit. The basic block diagram is as follows. The role of each part:
(1) The function of the power transformer T is to convert the AC voltage of 220V in the power grid into the AC voltage Ui required by the rectifier and filter circuit. The power ratio of the secondary side to the primary side of the transformer is P2/ P 1=η, where η is the efficiency of the transformer.
(2) Rectifier and filter circuit: The rectifier circuit converts AC voltage Ui into pulsating DC voltage. Then, the components with large ripple are filtered by the filter circuit, and the DC voltage with small ripple is output. The commonly used rectifier and filter circuits are full-wave rectifier and bridge rectifier and filter.
Each filter capacitor c satisfies RL-c = (3 ~ 5) t/2, or where t is the period of the input AC signal and RL is the equivalent load resistance of the rectifier filter circuit.
(3) Three-terminal integrated voltage regulator: Common integrated voltage regulators include fixed three-terminal voltage regulator and adjustable three-terminal voltage regulator. Commonly used adjustable positive pressure integrated voltage regulators are CW3 17(LM3 17) series, whose output voltage can be adjusted in the range of 1.25 V-37 V, and the simplest circuit external components only need a fixed resistor and a potentiometer. The chip is protected by transition, overheating and safe working area, and the maximum output current is1.5a. Its typical circuit is shown in Figure 2, in which the resistor R 1 and the potentiometer R2 form an output voltage regulator, and the expression of the output voltage Uo is: uo =1.25 (1+R2/r/.
Where R 1 is generally taken as 120-240 ohms, and the voltage difference between the output terminal and the regulating terminal is the reference voltage of the regulator (typical value is 1.25V).
2. Performance index and test method of stable current
There are two technical indexes of regulated power supply: one is characteristic index, including allowable input voltage, output voltage, output current and output voltage adjustment range; The other is the quality index, which is used to measure the stability of the output DC voltage, including voltage regulation coefficient (or voltage regulation rate), output resistance (or current regulation rate), ripple voltage (ripple coefficient) and temperature coefficient. The test circuit is shown in Figure 3.
(1) ripple voltage: AC voltage component superimposed on the output voltage. The peak value observed by oscilloscope is generally in the order of millivolts. The effective value can also be measured by AC millivoltmeter, but there is some error because the ripple is not sine wave.
(2) Voltage stabilization coefficient: Under the condition that the load current and the ambient temperature are unchanged, the relative change of input voltage causes the relative change of output voltage, namely:
(3) Voltage regulation rate: the relative change of output voltage when the relative change of input voltage is 10%. Both the voltage stabilization coefficient and the voltage regulation rate indicate the influence of the input voltage change on the output voltage, so only one of them needs to be tested.
(4) Output resistance and current regulation rate
The output resistance is the same as the amplifier, and its value is the absolute value of the ratio of output voltage change to output current change when the input voltage is constant. Current regulation rate: the relative change value of output voltage when the output current changes from 0 to the maximum. Both output resistance and current regulation rate indicate the influence of load current change on output voltage, and we only need to test one of them to supplement Hao.