Three-terminal voltage regulator power supply circuit designed with LM317T

The working process of the three-terminal voltage regulator LM317T chip and the calculation formula of its output voltage are introduced. Based on the consideration of the power supply of the circuit board chip, a power supply circuit using a three-terminal voltage regulator of LM317T is designed to power the control board chip. When designing the circuit, the circuit design method is analyzed in detail, and the calculation and experimental waveform of the main parameters of this circuit are given. Finally, this power supply circuit is made into a printed circuit board, the voltage signals of each important test point are measured, and the power supply circuit is simulated with Protel. The simulation results are consistent with the actual test calculation values, which proves that the designed power supply circuit is reliable and has certain practical value.

Keywords: power supply design; three-terminal voltage regulator LM317T; rectifier bridge; simulation

Introduction

When designing a small single-chip control system, the integrated chip on the control board needs an external DC power supply, and in order to improve the stability of the chip and the entire system, the power quality of the DC power supply is also required to be high. There are generally two ways to add a DC power supply: external and internal. The external type is to place the power supply required by the chip outside, usually composed of a power module, which directly generates the DC voltage required by the chip. Built-in means making the DC voltage power supply required by the chip inside the control board. External power supply can make board layout more convenient, but the cost is higher; while built-in power supply has lower cost and is more troublesome to layout. The method commonly used in China is to directly use external power supply to facilitate board layout.

LM317T is a three-port voltage regulator produced by National Semiconductor Corporation in 2001. Its output voltage can be adjusted to a certain extent by adjusting the resistor. The output voltage amplitude is between 1.2 and 27V, which can basically meet the voltage amplitude required by most integrated chips. Based on economic considerations, the author designed a built-in power supply circuit. After board making, experimental tests and software simulations proved that this power supply is feasible and reliable.

Power supply design ideas

In terms of power supply stability, most electrolytic capacitors are used in the design. On the one hand, they play a filtering role, and on the other hand, they stabilize the reference voltage (the working voltage of the chip). See the schematic diagram given below. For input and output capacitors, the general requirement is that the input capacitor should be as large as possible. Relative to the capacity requirement, the ESR requirement can be lowered, because the input capacitor is mainly for withstand voltage. If a switching power supply is used, it can also absorb the MOSFET switching pulse. Without causing vibration of the switching circuit, the withstand voltage and capacity of the output capacitor can be lower, and the ESR requirement should be higher, because sufficient current flow should be guaranteed.

Referring to the power supply part of the schematic diagram, it can be seen that this power supply is similar to the ordinary power supply circuit, which is also AC rectification, filtering, and voltage stabilization. In order to prevent the intrusion of high-frequency signals, a group of filtering capacitors are added to the output port. The process is shown in Figure 1.

Figure 1 The main conversion process of power supply voltage

LM317T is used in the welding control board

Circuit structure and working principle
The power supply circuit principle is shown in Figure 2.

Figure 2 Power supply circuit schematic Voltage stabilizer LM317T In the above schematic circuit, a three-terminal voltage stabilizer LM317T is mainly used, and its main function is to stabilize the voltage signal in order to improve the stability and reliability of the system. LM317T is such a device: after the working voltage is provided by the Vin terminal, it can keep the voltage of its +Vout terminal (pin 2) 1125V higher than that of its ADJ terminal (pin 1). Therefore, only a very small current is needed to adjust the voltage at the ADJ terminal to obtain a relatively large current output at the Vout terminal, and the voltage is a constant 1125V higher than the ADJ terminal. The output voltage can also be changed by adjusting the resistance value of the ADJ terminal (terminal 1) (LM317T will ensure that the voltage on the part of the resistor connected to the ADJ terminal and +Vout terminal is 1125V). Therefore, when the resistance value of the ADJ terminal (terminal 1) increases, the output voltage will increase. The output voltage of the LM317T can be continuously adjusted from 1125V to 37V, and its output voltage value can be calculated by formula (1): It is worth noting that the LM317T has a minimum load current problem, that is, it can only play a role in voltage regulation when the load current exceeds a certain value. This current varies from 3 to 8mA depending on the manufacturer of the device, which can be solved by connecting a suitable resistor to the load port. Experimental indicators and main waveforms In order to ensure the correctness of the design, the experimental data were tested after the board was designed and manufactured, and the data are shown in Tables 1 to 3. Calculation process of the output voltage value of the voltage regulator: RG2 output value: RG1 output value: In the simulation process, there are many softwares to choose from. However, if the simulation software can directly carry the model of the device chip used, the whole simulation process will be simpler. The author chose the popular board making software Protel, which has the chip model of LM317T in its internal simulation library. Figure 3 and Figure 4 are the voltage waveforms for generating 5V and 15V DC voltages respectively. Due to the difference in simulation models, the simulation values ​​are somewhat different from the theoretical values. Figure 3 5V DC voltage signal waveform Figure 4 15V DC voltage signal waveform Conclusion The output voltage value of the power module composed of LMT317 devices is closely related to the external resistor. Therefore, if other output voltage values ​​are required, the resistance value of the external resistor can be changed. Therefore, it is more flexible in designing the output voltage value of the power supply.