Design and debugging of a switching power supply suitable for teaching

0 Introduction

The linear voltage regulator circuit has the advantages of simple structure, convenient adjustment, and small output voltage pulsation, but the disadvantages are low efficiency, generally only 20% to 40%, and relatively bulky. The switching voltage regulator circuit can overcome the shortcomings of the linear voltage regulator, with high efficiency, generally reaching 65% to 90%, small size, light weight, and low requirements for grid voltage, so it is widely used in real life. It is precisely because of its wide application that students of corresponding majors should master it more deeply and skillfully. Based on the design of pulse width modulation switching circuit (PWM), the debugging process of the system is explained in detail.

1 System Design Principles

The overall block diagram of the PWM type switching power supply is shown in Figure 1. The transformer, rectifier, and filter modules are relatively simple to handle. They can be realized by using the corresponding transformer, single-phase full-wave rectifier, and capacitive filter. No more space is needed to introduce them here. The core module of this system is the closed (negative feedback) module in the block diagram. If the Boost type DC-DC booster is directly used, it is simple to implement, but the output/input voltage ratio is too large, the duty cycle is also large, and the output voltage range will become smaller, making it difficult to achieve higher indicators, and it is open-loop control. For this, the dedicated switch chip TL494 chip produced by Maxim is used. It uses switch pulse width modulation (PWM), has high efficiency, and the peripheral circuit is relatively simple, which can easily realize closed-loop control.

1.1 TL494 Working Principle

The internal structure of TL494 is shown in Figure 2. It is a control circuit with a fixed frequency that can be set by itself and uses pulse modulation, where the oscillation frequency fosc=1.1/(RTCT). Specifically, due to the unequal values ​​of the error amplifier input ports 1, 2 (or 3, 4), a deviation is generated. The deviation is sent to the PWM comparator and compared with the sawtooth wave (the frequency of the sawtooth wave is determined by the oscillation frequency, and the amplitude is a fixed value). When the deviation is greater than the sawtooth wave range, port 9 (or port 10) outputs a low level, and when the deviation is less than the sawtooth wave range, port 9 (or port 10) outputs a high level. The larger the deviation value, the smaller the interval of the TL494 outputting a high level. It can be seen that the duty cycle can be changed by adjusting the deviation of the error amplifier input port.

1.2 Working Principle of Boost Converter

As shown in Figure 3, the output voltage of the boost converter can be controlled by controlling the conduction ratio of the switch tube Q1. Its working principle is: suppose the switch tube Q1 is controlled by the signal VG. When VG is at a high level, Q1 is turned on, otherwise, Q1 is turned off. When Q1 is turned on, the voltage across the inductor VL=Vi, the inductor energy storage increases, and the load is powered by the capacitor. When Q1 is turned off, because the current on the inductor L cannot change suddenly, the inductor current iL supplies power to the capacitor and the load, and the energy stored in the inductor is transferred to the capacitor and the load side. At this time, iL decreases, and the induced electromotive force VL on L<0, so Vo>Vi. Therefore, when Q1 is turned on for a longer time (that is, the larger the duty cycle), the more energy is stored in the inductor, and the larger Vo is.

2 Overall system design

Based on the previous analysis, the designed system wiring diagram is shown in Figure 3.

The inverting port 2 of the error amplifier inputs a given value (which can be implemented by a single-chip microcomputer, but is not introduced due to space limitations) to control the output voltage; the inverting port 1 inputs the feedback voltage of the output voltage to form a closed-loop control. When the output voltage is higher than the expected value, the voltage of the feedback input port 1 increases, the output of the error amplifier increases, and the duty cycle decreases; when the output voltage decreases, it can basically be equal to the expected value, thereby maintaining the stability of the output voltage. If you want to increase the output, you can increase the voltage of port 2. The control process is as follows: the original system is stable, and when the voltage of port 2 is increased, the voltage of port 1 remains unchanged, the output of the error amplifier decreases, the duty cycle increases, and the voltage increases. If you want to reduce the output, you can reduce the voltage of port 2.

3 System Debugging

After determining the above overall design, the circuit debugging is carried out using a modular debugging method.

3.1 TL494 performance test

Connect as shown in Figure 4, test the input voltage of port 2 (the inverting end of the error amplifier port 2 uses the reference voltage input), change the input voltage of port 1, and observe the output waveforms of ports 9 and 3. From the experiment, we can get: the reference voltage of TL494 is 3.5 V; the output waveform is PWM wave; the error amplifier works in the nonlinear region, and PWM is adjustable only when the deviation of the input (1, 2) ports is between zero and tens of millivolts; changing the voltage of port 1 can change the duty cycle of PWM.

3.2 Boost Converter Performance Test

Connect according to Figure 5, and add a square wave signal to port 1 to saturate the switch tube:

(1) Change the duty cycle and frequency of the square wave signal;

(2) Add a load to the output terminal.

From the experiment, we can get that changing the duty cycle can change the output voltage; when adding a load, the voltage decreases, but by adjusting the duty cycle, the voltage can be increased; the greater the frequency of the square wave signal, the smaller the range of adjusting the output voltage by changing the duty cycle.

3.3 Joint debugging

After the above two steps can obtain accurate information, the two modules are jointly debugged, as shown in Figure 4. If there is no error, the output end can achieve a stable voltage output, and the output voltage can be changed within a certain range (boost) by changing the given value of port 2. The specific range is related to the selected inductance, capacitance and system operating frequency, which is not introduced here due to space limitations.

3.4 Add MOSFET (IRF640) driver

After completing the above circuit, the next step is to consider the system performance indicators. In addition to the above parameters of capacitance, inductance, and operating frequency, the superiority of performance indicators is also related to MOSFET. For this reason, the drive circuit IR2111 is added between the 9th port of TL494 and the IRF9540 switch tube, as shown in Figure 6.

4 Conclusion

According to the above steps, the system design is not only simple, but also can help you understand the working principle of TL494, the working principle of switching power supply, the working principle of negative feedback, etc., and it is also convenient to find circuit errors. For the performance index test of this circuit, due to the different parameters of components, the indicators are slightly different, but basically the indicators of each parameter are high, such as the efficiency of DC-DC converter can reach more than 85%.