[Analog Circuit] Practical DC Stabilized Power Supply Design

1 Project introduction

        In the process of learning analog circuits, we will learn the design of linear regulated power supplies. A good power supply is especially important in the daily learning process. In addition to performance, portability also needs to be considered. Based on the course learning content, we designed a DC stabilized power supply case that integrates practicality and functionality.

Application scenarios

• Analog circuit simulation teaching
• Circuit schematic and PCB design teaching
• Electronic component identification and welding course
• Used as a power supply module in daily study and experiments

Circuit characteristics

• Input voltage is DC9-30V
• Output fixed voltage 5V, 3.3V and adjustable voltage value
• When the input and output voltage difference does not exceed 10V, ensure that the operating current is less than 600mA. When the voltage difference exceeds 10V, please keep the output current less than 400mA.
• Working temperature range: 0-100℃ (To prevent over-temperature damage, you can increase the heat sink or other heat dissipation methods by yourself)

2 Overall design plan

        Before designing the circuit, let's first think about the basic requirements for this power supply. Since commonly used microcontrollers and chips can operate at 5V and 3.3V, a stable 5V and 3.3V power output is required. In some special occasions, 9V or other voltage power supplies may also be used. At this time, we want to have an output that can adjust the output power supply voltage. The rough circuit block diagram is as follows:

Figure 1 Block diagram of DC regulated power supply system

        Combining the advantages of linear power supply with simple structure, small output ripple, small high-frequency interference and easy maintenance, this circuit is designed using a three-terminal voltage regulator. According to the input characteristics of the three-terminal voltage regulator, it is divided into two types: positive polarity and negative polarity, which output positive and negative power supplies respectively. Here we take the positive power input voltage stabilization as an example. Commonly used positive polarity three-terminal voltage regulators with fixed output include the LM78XX series and LM1117-XX, as well as related chips such as adjustable LM317 and LM1117-ADJ. After selection, STMicroelectronics' LM317T, LM7805 and LD1117V33 were finally selected for design.

        The circuit design process should be divided into the following stages. The project can be simulated and verified first before designing the schematic diagram and PCB.

Figure 2 Circuit design process description

3 Principle analysis

        Before designing the circuit, be sure to check the data sheet provided with the device, which can help us better understand the characteristics of the selected chip. Note that it is best to download the data sheet from the official website of the selected device. Data sheets provided by other manufacturers may not be consistent with the selected device. Some devices may have the same name but different pin arrangements and different parameters.

At the beginning of the LM317 data sheet, we can learn that the chip can achieve an adjustable output from 1.2V to 37V, and can provide an output current of more than 1.5A; the linear and load adjustment rates are only 0.1%, and it also supports high-voltage floating protection, Current limit and internal voltage short circuit protection features.

Figure 3 Screenshot of LM317 data sheet

        The chip pin definitions and reference circuits on the data sheet must be viewed for our circuit design. LM317 has three pins, namely INPUT (input pin), OUTPUT (output pin) and ADJUST (adjustment pin). When designing the circuit, pay attention to the pin arrangement to avoid misconnection that may cause a short circuit in the power supply. Combining the reference circuit and design experience provided in the manual, we designed the following circuit:

Figure 4 LM317 application circuit

        In this circuit, C1 and C2 are input filter capacitors, and C3 and C4 are output filter capacitors. The large capacitor is used to filter out the low-frequency components in the signal, and the small capacitor is used to filter out high-frequency noise. Two diodes are used in the circuit. The D1 diode prevents the output pin voltage from being higher than the input power supply due to the capacitor energy storage during power outage. The D2 diode prevents the ADJ pin voltage from being higher than the output pin, both to prevent chip damage. Play a protective role. R1 and R2 work together, and adjusting the resistance of the R2 potentiometer can change the output voltage. R3 resistor and LED2 play the role of output power indication.

        Refer to the corresponding data sheets to design the circuits of LM7805 and LD1117V33 as follows:

Figure 5 LM7805 and LD1117V33 application circuit

4 Simulation diagram design

4.1 Simulation diagram creation

        Switch the EasyEDA standard version mode to the simulation mode, create a project folder, create a new simulation drawing and save it to the project folder.

        Find the voltage source_DC source (DC) under the power supply category in the simulation base library as the power input, and set the voltage to 12V; find the two devices LM317 and 7805 in the voltage stabilizing device category; click the simulation library, search for LM1117_3V3, and Select LM1117_3V3 in the system library and place it on the canvas; then find the multimeter in the instrument list in the basic library and place four of them, connect them in parallel to the circuit to measure the voltage value; find the light-emitting diodes in the diode category, place four of them, click on the diode, and The diode settings on the right can be set to red, blue, green and yellow to represent different outputs; resistors, variable resistors, capacitors and electrolytic capacitors found in general devices should be placed and processed according to the parameters in the following simulation diagram Wired.

Figure 6 Experimental DC regulated power supply simulation diagram

4.2 Simulation verification

        After clicking the simulation, the 5V and 3.3V output voltages are displayed directly. Change the scaling factor of the sliding rheostat in the LM317 circuit to 80% (the resistance connected to the circuit is 1KΩ), and check the multimeter reading. When the scaling factor is adjusted to 20%, then Are the simulation results consistent with the theoretical calculations? If not, please think about the reasons.

5 Schematic design

5.1 Add schematic diagram

      After passing the simulation verification, I believe everyone can’t wait to start designing the schematic diagram. First save the simulation diagram, switch the simulation mode to the standard mode, and start preparing the design of the schematic diagram. Since the devices we need to eventually participate in the PCB design in a project come from the schematic diagram, the devices in the simulation diagram should not be involved in the PCB, so we need to open the simulation diagram first and set the devices inside to Do not transfer to PCB and Not to PCB. Adding BOM is very important! Select the created project folder, right-click, create a new schematic, and start schematic design.

5.2 Power input circuit

      Power input can be provided in many ways. Here we have chosen two commonly used methods: DC socket and terminal block for power supply. You can choose one of them according to the actual situation.

Figure 7 DC power socket and terminal block diagram

        Select DC005-T20 in the power supply category in the basic library; select CONN-TH_2P-5.00 terminal block in the connector category; then place a resistor in the resistor category, and select the option R_AXIAL-0.4_EU or R_AXIAL-0.4_US in the package pull-down; Select light-emitting diode in the diode category, and select LED-TH-3mm_R as the package type. Place a VCC identifier and GND identifier in the electrical floating window. The circuit connection is shown in the figure below:

Figure 8 Power input circuit

5.3 Three-terminal voltage regulator circuit

        Draw the three-terminal voltage regulator circuit according to Figure 10. Taking into account the convenience of welding and the size of the power, the plug-in package is used. Each output is led out with a terminal block and three pin headers.

Figure 9 Three-terminal voltage regulator circuit

5.4 Component selection instructions

        There are many options for symbols for each component. In actual application, practicality and maintainability need to be considered when selecting. As an electronic engineer, we should make selections when designing the schematic diagram. The devices provided by different manufacturers are different, so the best idea for device selection is to search for the device we need in the component library, select the required package and manufacturer, and check the price, brand, inventory and other related information of the selected device. Here we take LM317 as an example. Search for LM317 in the component library of Easy EDA Standard Edition. The search results are as shown below:

Figure 10 Device search and selection

        You can see that a large number of related devices have been searched in the library column. It is recommended to choose the Lichuang Mall library and Jiali Tiechuang chip library to find a chip that meets our requirements. Considering the inventory, price and packaging, you can choose the package. It is the TO-220-3 LM317T-DG chip from ST Company. You can search in the same way when selecting other chips and RC devices.

6 PCB design

        After completing the schematic design, first check whether the circuit is connected correctly and whether the network is missing or not connected. After everything is checked, click Design - Convert Schematic to PCB in the menu bar at the top of the schematic to start PCB design.

6.1 Appearance design

        After generating the PCB, you need to set a PCB shape. The shape needs to be designed according to the number of components and the shell or personal wishes. Adhering to the principle of appropriate size and beauty, this project can set a rectangle with a length of 80mm and a width of 60mm. As the size of the PCB board. Note that when designing the PCB shape, try not to exceed 10cm*10cm, otherwise the price will be slightly higher.

6.2 Component layout

        After the components in the schematic diagram are transferred to the PCB, the component layout is relatively messy. In the second step of designing the PCB, the components need to be classified and laid out. The classification is based on placing the components of each circuit module together, which is provided by Easy EDA. The layout transfer function can quickly layout each circuit module. Note that the interface devices should be placed on the edge of the board to facilitate wiring and operation.

Figure 11 Layout reference diagram

6.3 PCB routing

        At this step, look back and see that you have completed the simulation diagram, schematic diagram and PCB component layout of the DC stabilized power supply, but there is only one last step left: PCB routing.

         PCB traces are divided into top traces and bottom traces when designing a double-layer circuit board. The top traces are red lines by default and the bottom traces are blue lines. The traces are connected to the copper wires in the circuit board. Just select the layer in Layers and Elements, and then connect the two pads of the same net. It seems like a simple Lianliankan, but in fact it requires our patience to make adjustments. The placement and layout of components will also affect the difficulty of wiring. The following reference suggestions are provided in the wiring of this project:

(1) The power line is set to 35mil and the signal line is set to 25mil width.

(2) Use bottom-layer wiring to make it easier to make your own PCB in school

(3) No wiring is required for GND, just use the bottom layer of copper.

(4) During the wiring process, give priority to straight lines, and use obtuse angles or arc turns where corners are required.

(5) After completing the wiring, add appropriate silk screen marks to indicate the purpose and interface functions of the PCB board.

Figure 12 Routing reference diagram (copper pouring is hidden) and 3D preview

7 Debugging Precautions

        After the PCB design is completed, export the Gerber file to the factory for PCB proofing. After purchasing the relevant components, prepare for welding and debugging. There are the following points to note during the welding debugging process:

(1) Pay attention to electrical safety during welding, and do not touch the soldering iron tip with your hands to avoid burns.

(2) During the welding process, first weld the components from low to high.

(3) The chip and the heat sink are first fixed together with screws and then welded so that they are close together.

(4) Use DC power for input during the test. You can use a DC power socket or directly connect it to the P2 terminal, turn on the switch for testing, and use a multimeter to test whether each output voltage meets the simulation results.

        At this point, the DC regulated power supply design process ends. This circuit can also be used to power other circuits in daily electronic learning, which is simple and convenient. If you need a negative power output, you are welcome to design it yourself and put the positive and negative power supplies on the same circuit board. I believe that smart people can design their own DC stabilized power supply circuit board for experiments.

        Click to view [Teach you step by step] DC stabilized power supply teaching video, which will help you learn from scratch from simulation diagram design, schematic design, PCB design and welding teaching!

Video link: https://www.bilibili.com/video/BV15R4y1n7q1