Design of DC regulated power supply
Second, the design purpose
1. Learn the preliminary experience of comprehensive application of basic theory in practice, master the basic methods and design steps of analog circuit design, and cultivate the ability of comprehensive design and debugging.
2. Learn the design method and performance index test method of DC regulated power supply.
3. Cultivate practical skills and improve the ability to analyze and solve practical problems.
Three. Design tasks and requirements
1. Design and manufacture continuously adjustable DC regulated power supply. The main technical requirements are as follows:
① adjustable output voltage: UO =+6V ~+ 13V
② Maximum output current: Iomax =1a.
③ output voltage change: δ uo ≤ 15mv.
④ Voltage stability coefficient: SV≤0.003.
2. Design the circuit structure, select the circuit components, calculate and determine the component parameters, and draw the practical principle circuit diagram.
3. Draw up the experimental methods, steps and data sheets, put forward the specifications and quantities of instruments and components needed for the test, and submit them to the instructor for review.
4. After approval, enter the laboratory for assembly and debugging, and test its main performance parameters.
Fourth, the design steps
1. Circuit diagram design
(1) Determine the goal: design the whole system composed of these modules, and signal transmission between modules, and draw the block diagram of DC regulated power supply.
(2) System analysis: According to the system function, select the circuit form used by each module.
(3) Parameter selection: according to the system index requirements, determine the parameters of components in each module circuit.
(4) General circuit diagram: connecting the circuits of each module.
2. Circuit installation and debugging
(1) In order to improve students' practical ability, students design and solder printed circuit boards by themselves.
(2) Add a signal to the input of each module circuit, test the output signal, and verify whether each module can reach the specified index.
(3) Focus on testing the voltage stabilizing coefficient of the voltage stabilizing circuit.
(4) Connect the circuits of each module, debug the whole machine and measure the system indicators.
The overall design idea of verb (abbreviation of verb)
Design idea of 1.DC regulated power supply
(1) The power supply voltage of the power grid is 220V (effective value) 50Hz. In order to obtain low-voltage DC output, power transformer must be used to reduce the grid voltage to obtain the required AC voltage.
(2) The stepped-down AC voltage becomes unidirectional DC through the rectifier circuit, but its amplitude changes greatly (that is, the pulse is large).
(3) The DC voltage of a large pulse must be changed into a smooth DC voltage of a small pulse through a filter circuit, that is, the AC component is filtered and its DC component is retained.
(4) After the filtered DC voltage is stabilized by the voltage stabilizing circuit, a stable DC voltage output basically unaffected by the outside world can be obtained and provided to the load RL.
2. Principle of 2.DC regulated power supply
DC regulated power supply is a device that converts 220V power frequency alternating current into regulated output DC voltage. It needs four steps: voltage transformation, rectification, filtering and voltage stabilization, as shown in figure 1.
Figure 1 DC block diagram of regulated power supply
These include:
(1) power transformer: it is a step-down transformer, which converts the 220V AC voltage into the required AC voltage and sends it to the rectifier circuit. The transformation ratio of the transformer is determined by the secondary voltage of the transformer.
(2) Rectifier circuit: 50Hz sinusoidal alternating current is converted into pulsating direct current by unidirectional conductive elements.
(3) Filter circuit: Most AC components in the output voltage of the rectifier circuit can be filtered out, thus obtaining a relatively smooth DC voltage.
(4) Voltage stabilizing circuit: The function of the voltage stabilizing circuit is to stabilize the output DC voltage, which does not change with the change of AC grid voltage and load.
The rectifier circuit often adopts diode single-phase full-wave rectifier circuit, as shown in Figure 2. During the positive half cycle of u2, diodes D 1 and D2 are turned on, and D3 and D4 are turned off; In the negative half cycle of u2, D3 and D4 are turned on, and D 1 and D2 are turned off. There is a load resistor RL, and the current flows in the positive and negative half cycles in the same direction. The output waveform of this circuit is shown in Figure 3.
In the bridge rectifier circuit, each diode conducts electricity only in half a period, so the average current flowing through each diode is equal to half of the average output current, that is. The maximum reverse voltage of each diode in the circuit is (U2 is the effective value of the transformer secondary voltage).
In the design, the voltage at both ends of the capacitor and the current flowing through the inductor can not be abruptly changed, and the capacitor and the load capacitor are connected in parallel or the capacitor and the load resistor are connected in series, so as to basically smooth the output waveform. After selecting the capacitor filter circuit, the DC output voltage is uo1= (1.1~1.2) U2, and the DC output current is (I2 is the effective value of the secondary current of the transformer. ), and the voltage stabilizing circuit can optionally be integrated with the three-terminal voltage stabilizing circuit.
The overall principle circuit is shown in Figure 4.
3. Introduction of design method
(1) According to the performance index required by the design, select the integrated three-terminal regulator.
Because the output voltage is required to be adjustable, a three-terminal adjustable integrated regulator is selected. Adjustable integrated voltage regulators, mainly CW3 17 and CW337. 3 17 series voltage regulators output continuously adjustable positive voltage, 337 series voltage regulators output adjustable negative voltage, the adjustable range is 6V~ 13V, and the maximum output current is1.5a. The voltage regulators have built-in overcurrent and overheating protection circuits, which are safe, reliable, excellent in performance, not easy to be damaged and convenient to use. Its voltage regulation rate and current regulation rate are better than the adjustable regulated power supply composed of fixed integrated regulated power supply. The pin functions of the circuit series are the same, and the pin diagram and typical circuit are shown in Figure 5.
Figure 5 Typical circuit
The output voltage expression is:
In the formula, 1.25 is the inherent reference voltage between the output terminal and the regulating terminal of the integrated regulator block. When this voltage is applied across a given resistor, the potentiometer will generate a constant current by adjusting the output voltage, and this resistor is usually a value. Generally, a precision potentiometer is used, and the capacitor C connected in parallel with it can further reduce the ripple of the output voltage. Diode D is added in the figure to prevent short circuit at the output. The three-terminal voltage regulator is damaged when the large capacitor discharges and flows backwards.
Adjustable range of output voltage:1.2v ~ 37v.
Output load current:1.5a.
Input-output working pressure difference δ u = UI-UO: 3 ~ 40V
It can meet the design requirements, so the voltage stabilizing circuit is selected.
(2) Select the power transformer
1) Determine the secondary voltage U2:
According to the performance requirements: Uomin=3V Uomax=9V.
∫UI-UO maximum ≥ (UI-UO) minimum ≤ (UI-UO) maximum.
Where: (Ui-Uoin) minimum value =3V, and (Ui-Uo) maximum value =40V.
∴ 12V≤Ui≤43V
Optional within this range: Ui= 14V=Uo 1.
According to uo1= (1.1~1.2) U2.
Available secondary voltage of transformer:
2) Determine the transformer secondary current I2.
∫Io 1 = Io
Secondary current I2 = (1.5 ~ 2) iO 1, I = iOMAX = 800 Ma.
Then I2 =1.5 * 0.8a =1.2a.
3) Select the power of the transformer
Transformer output power: po & gti2u2 =14.4w.
(3) Select the diode in the rectifier circuit.
Secondary voltage of transformer U2 = 12V
∴ The highest reverse voltage borne by diode in bridge rectifier circuit is:
The highest average current borne by diode in bridge rectifier circuit is:
Check the manual to select the rectifier diode IN400 1, and its parameters are: reverse breakdown voltage UBR = 50V> 17V.
Maximum rectified current if =1a >; 0.4A
(4) Selection of filter capacitor in filter circuit
The size of the filter capacitor can be obtained by formula.
1) Find δ ui:
According to the definition of voltage stabilizing coefficient of voltage stabilizing circuit:
Design requirements δ UO ≤ 15mV, SV≤0.003.
Uo=+3V~+9V
Ui= 14V
Substituting the above formula, you can get Δ ui.
2) filter capacitor c
Set Io=Iomax=0.8A, t = 0.01s.
Then you can get C.
The highest voltage of the filter capacitor in the circuit is 0, so the withstand voltage of the selected capacitor should be greater than 17V.
Note: Because the large-capacity electrolytic capacitor has a certain winding inductance distribution inductance, it is easy to cause self-excited oscillation and form high-frequency interference, so the input and output terminals of the voltage regulator are often combined with the small-capacity capacitor of the ceramic dielectric to cancel the inductance effect and suppress high-frequency interference.
Six, experimental equipment and components
1 .multimeter 2. oscilloscope
3. AC millivolt meter 4. Three-terminal adjustable regulator
Seven, test requirements
1. Test and record the output waveforms of each link in the circuit.
2. Measure the regulation range and maximum output current of the regulated power supply.
3. Measure the output resistance Ro.
4. Measure the voltage stabilizing coefficient.
By changing the input AC voltage, simulating the change of Ui and measuring the corresponding change of output DC voltage, the voltage stabilizing coefficient SV can be calculated. (Note: 220v AC is changed by voltage regulator 10%. I.e. Ui = 44V)
5. The AC ripple voltage in the output DC voltage can be measured by millivoltmeter, and its waveform can be observed and recorded by oscilloscope.
6. Analyze the measurement results, discuss and put forward suggestions for improvement.
Eight, design report requirements
1. Design purpose.
2. Design indicators.
3. Overall design block diagram, and explain the functions of each module.
4. Function module, which can have multiple schemes, and demonstrate and compare the schemes with detailed principle explanation.
5. General circuit diagram design and principle description.
6. Implement instruments and tools.
7. Analyze the measurement results, discuss and put forward suggestions for improvement.
8. Summary: Problems encountered and solutions, experiences, opinions and suggestions.
Nine, matters needing attention
1. When welding, each functional module circuit should be tested separately, and some temporary circuits can be designed for debugging if necessary.
2. When testing the circuit, it is necessary to ensure correct welding before turning on the power supply to prevent the components from burning out.
4. When welding according to the schematic diagram, it is necessary to ensure reliable grounding.
X. Error analysis of this circuit
Through comprehensive analysis, we can know that the possible error factors in the circuit testing process are:
① Error caused by tiny resistance between contacts when measuring output current;
(2) Error caused by internal resistance of ammeter in series circuit;
③ Error caused by oscilloscope when measuring ripple voltage;
(4) System error caused by the precision of oscilloscope and multimeter;
The circuit can be improved by the following methods:
(1) Reduce the tiny resistance of the contact point;
② The measurement result can be corrected according to the internal resistance of ammeter;
③ When measuring ripple, the oscilloscope adopts manual synchronization;
(4) Use more accurate instruments to detect;
XI。 Comprehensive summary
Through this design, we can further understand the working principle, requirements and performance indicators of DC regulated power supply, and also recognize the problems existing in this design circuit. Through continuous efforts to solve these problems, I have gained something from solving design problems. This DC stabilized power supply circuit we designed this time; Voltage regulator (uA723) and regulator (2SC3280) are adopted to realize voltage regulation. The control of the circuit is also realized by single chip microcomputer (89C5 1), and multi-function is realized. The current stabilizing part can be realized by an adjustable three-terminal regulated power supply tube.
Twelve. refer to
◆& lt； & lt Fundamentals of Electronic Circuits >:> Department of Physics, East China Normal University, edited by Wan Jiaruo and Yun, Higher Education Press.
◆& lt； < Fundamentals of Electronic Technology > > Department of Electronics, Huazhong Institute of Technology, edited by Kang, Higher Education Press.
◆& lt； & lt Electronic Circuit Design >:> (Second Edition) edited by Xie, Huazhong University of Science and Technology, published by Huazhong University of Science and Technology Press.