Design of voltage-stabilized power supply based on program-controlled switch

　　A switching power supply is a power supply that uses modern power electronics technology to control the time ratio of the switch tube to be turned on and off to maintain a stable output voltage. Due to its high efficiency and high power density, the use of switching power supplies in modern electronic systems is becoming more and more popular. High frequency, modularization and intelligence are the development directions of switching power supplies. Among them, step-adjustable and real-time display are one of the research directions of intelligent switching power supplies. A switching power supply is now designed with the following technical indicators: adjustable output voltage from 30V to 36V, maximum output current of 2A, overcurrent protection function, keyboard setting and step adjustment of output voltage, step value of 1V, and real-time display of output voltage and current of the switching regulated power supply.

　　1 Overall design plan

　　The AT89S52 single-chip microcomputer is used as the control core to optimize the design of the ordinary switching power supply control part, and the intelligent control of the switching power supply is realized through software programming. The design uses an isolation transformer to transform the mains power and then send it to the DC-DC boost converter through rectification and filtering. After a series of control integration circuits, the design requirements can be achieved. The overall block diagram of the system is shown in Figure 1.1.

 

　　1.1 DC-DC main circuit topology

　　The DC-DC conversion circuit is composed of UC3842 and MAX4080. UC3842 is a typical single-ended current-mode PWM control integrated circuit with complete functions, which includes error amplifier, current detection comparator, PWM latch, oscillator, internal reference power supply and undervoltage lockout units. The current-controlled boost DC-DC conversion circuit has few external components, flexible control, low cost, and the output power can easily reach more than 100W. Of course, the DC-DC conversion circuit can also use finished modules. If the DPA-Switch produced by PI is used to design the switching power supply, it has high integration, simple peripheral circuits, low heat generation, and excellent performance indicators.

　　The DC-DC boost circuit designed by UC3842 directly uses the error signal to control the inductor peak current and indirectly controls the PWM pulse width to achieve the purpose of controlling the output voltage. The switch tube opens and closes at the frequency cycle set by UC3842, allowing the inductor L to store and release energy. When the switch tube is turned on, the inductor is charged and the energy is stored in L. When the switch is turned off, L generates a reverse induced voltage, and the stored electrical energy is released to the output capacitor through the diode. The output voltage is controlled by the amount of energy transferred, and the amount of energy transferred is controlled by the peak value of the current passing through the inductor. The specific design circuit is shown in Figure 1.2.

 

　　1.2 Protection circuit

　　The chip is easily damaged in the case of high current, so circuit protection is required for high current. The design uses a single-chip microcomputer to control the on and off of the relay to control the current in the circuit, sample the output circuit current, compare the sampled value with the rated value, and the feedback comparison circuit is shown in Figure 1.3. When the current is greater than 2.5A, a signal is generated to make the single-chip microcomputer enter the interrupt processing program, start the relay, and realize the power-off of the DC-DC circuit, thereby achieving the function of protecting the circuit. The single-chip microcomputer control circuit is shown in Figure 1.4. In this scheme, the single-chip microcomputer controls the relay with a short pull-in time and is easy to implement.

 

　　1.3 Digital setting and display circuit

　　The AT89S52 single-chip computer and the integrated chip CD4051 are used to realize program control and stepping. The single-chip computer controls the keyboard to realize the initial setting of the output voltage, which can realize the voltage step of 1V and step of 1V. The output voltage and current are displayed using a liquid crystal display, and the conversion switch can be toggled to select the display voltage/current mode.

 

 

　　1.4 Programming

　　On the basis of designing related circuits, the switching power supply is intelligently controlled by the single-chip microcomputer through programming. The system is controlled by the single-chip microcomputer AT89S52. The power supply system has "+‰" and "-" stepping functions, and the stepping amplitude is 1V. At the same time, AT89S52 combines relays and other circuits to realize the circuit overcurrent protection function, and can display the output voltage and current of the switching power supply in real time. The overall program flow chart and interrupt flow chart are shown in Figure 1 (5, 6). [page]

 

　　2. Improve efficiency

　　How to improve the efficiency of the switching power supply is particularly important. The following measures have been taken to improve the efficiency of the switching power supply.

　　2.1 The inductance in the DC-DC conversion circuit greatly affects the efficiency of the system. It is difficult to buy an inductor that meets the requirements on the market. The requirements for the inductor core and enameled wire during winding are very high, and the output voltage ripple should be minimized.

　　2.2 The switch tube in the DC-DC conversion circuit uses a MOS tube to replace the bipolar transistor. The series gate resistor will attenuate the high-frequency parasitic oscillation generated by the MOS input capacitance and the gate-source circuit lead inductance. It can effectively improve the conversion efficiency. If several MOS tubes IRF530 are selected in parallel, the efficiency can be further improved.

　　2.3 The freewheeling diode should be a Schottky diode, which has a short turn-on time and a small tube voltage drop, allowing the inductor to store large energy, which is beneficial to improving the power conversion efficiency.

　　2.4 The gates of diodes, inductors and MOS tubes should be welded as close as possible to reduce losses and improve system efficiency.

　　3 Test Data and Analysis

　　3.1 Voltage Regulation SU

　　Voltage regulation SU refers to the rate of change of output voltage U0 when U2 changes within a specified range. Use an auto-regulator to adjust U2 from 15V to 21V, and measure the output voltage when the output current is 2A to obtain the voltage regulation SU.

 

　　3.2 Load Regulation SI

　　Load regulation SI refers to the rate of change of output voltage U0 when I0 changes within a specified range. When the load resistance is changed so that the output current changes within 0 to 2A, the load regulation data is as follows.

 

　　3.3 DC-DC Converter Efficiency

　　Efficiency η=P0/PIN, where P0=U0I0, PIN=UINIIN. The converter efficiency can be obtained by directly reading the input and output voltage and current values ​​at the DC-DC module port using a millivoltmeter.

 

　　3.4 Ripple Current

　　In the design of the switching power supply, the source of the MOS tube is connected to a 1kΩ resistor, and a non-polar capacitor is added to the power supply filter to filter out the high-frequency ripple. The actual measured data of the current ripple is as follows.

 

　　The switching voltage-stabilized power supply based on AT89S52 has good intelligent control and stepping functions. The test data shows that the power supply system has a high voltage regulation rate and load regulation rate, and has a high efficiency. The power supply can work continuously and safely for a long enough time at the maximum output power. Of course, the power supply performance can be further improved by selecting MOS tubes and related components, optimizing circuit design, or selecting DC-DC finished modules.