Design of a feedback control circuit for a small DC switching power supply

At present, in various electronic devices and modern communication equipment, feedback control circuits have been widely used to meet certain requirements or achieve certain technical indicators under various working conditions. As an automatic adjustment circuit in electronic equipment and systems, the main function of the feedback control circuit is that when the electronic system is subjected to a certain disturbance, the system can correct certain system parameters through the adjustment of its own feedback control circuit, so that the various indicators of the system still reach the predetermined accuracy. The feedback control circuit usually consists of a negative feedback closed loop composed of a comparator, a control signal generator, a controllable device and a feedback network, as shown in Figure 1.

Figure 1 Schematic diagram of feedback control circuit composition

Based on the design concept of miniaturization, low power and high efficiency, the main technical requirements of the DC switching power supply corresponding to the feedback control circuit designed in this paper are as follows:

Input AC voltage: VACMIN=85V;VACMAX=265V;Input voltage frequency: fL=50Hz;Output voltage: VO=36V;Output power: PO=72W;Power efficiency: η=80%;Loss factor Z: Z=0.6 (Z represents the ratio of secondary loss to total loss).

The corresponding DC switching power supply composition is shown in Figure 2.

Figure 2 Schematic diagram of the DC switching power supply corresponding to the feedback control circuit.

1. Feedback control circuit design process

The feedback control circuit in the switching power supply is used to ensure the stability of the output voltage and current when the load changes. The DC switching power supply corresponding to the feedback control circuit designed in this paper uses PWM pulse width modulation to maintain the stability of the output voltage. Among them, PWM modulation is divided into current control mode and voltage control mode. Compared with the latter, the former has better voltage regulation and load regulation. While reducing the number of components, reducing costs, and increasing the power of the switching power supply, it can further ensure the stability of the system and significantly improve the dynamic characteristics of the system. It is especially important for the miniaturization, modularization, and efficiency of the system.

In addition, the feedback usually used in DC switching power supplies is negative feedback. In the feedback, the commonly used feedback is the lowest cost of primary feedback (only suitable for low-power applications); the cost of using optocoupler/voltage regulator feedback is low and the output accuracy is good; in addition, the output accuracy is best when using optocoupler/TL431 feedback. Considering the principle of low power and high efficiency reflected in the design of this article, it is decided to use the PWM current regulation control method with three-terminal shunt voltage regulator TL431 and optocoupler PC817, respectively for reference, sampling, isolation, and amplification, thereby forming a negative feedback loop.

1.1 Feedback control circuit principle and design

The feedback control circuit designed in this paper is shown in Figure 3. Its basic control principle is: when the output voltage is divided by R11 and R12, the sampling voltage can be obtained, and then the sampling voltage is compared with the 2.5V reference voltage provided by TL431. When the output voltage is normal, the sampling voltage is basically equal to the reference voltage 2.5V of TL431, so the cathode potential of TL431 remains unchanged, and the current flowing through the light-emitting diode in the optocoupler remains unchanged, so that the voltage of the control pin C of the TOP247Y chip is stable, and the control drive duty cycle remains unchanged, and the output voltage remains stable. When the output voltage is lower than the expected voltage, the voltage value obtained after the voltage division resistors R11 and R12 is lower than 2.5V, the cathode potential of TL431 increases, and the current flowing through the light-emitting diode in the optocoupler decreases, then the current flowing through the CE pole of the optocoupler also decreases, and the potential of the control pin C of TOP247 increases, which increases the duty cycle, thereby increasing the output voltage, so as to keep the output stable. When the output voltage is higher than the expected voltage, the voltage divided by the voltage-dividing resistors R11 and R12 is higher than 2.5V, the cathode potential of TL431 decreases, the current flowing through the light-emitting diode in the optocoupler increases, and the current flowing through the CE pole of the optocoupler also increases. The potential of the control pin C of TOP247 decreases, which reduces the duty cycle and reduces the output voltage, thereby stabilizing the output.

Fig. 3 Schematic diagram of feedback control circuit.

1.2 Parameter setting and analysis of TL431 and resistor divider

TL431 is an adjustable three-terminal voltage regulator. Any reference voltage value in the range of 2.5V-36V can be set by using an external resistor divider. TL431 has low dynamic impedance, with a typical value of 0.2 ohms. As shown in Figure 3, the voltage is obtained through the resistor divider R11 and R12, and compared with the reference voltage 2.5V of TL431 to form an error amplifier, and then the current change of PC817 is used to further control the change of the output duty cycle of TOP247Y. From the technical parameters of TL431, it can be seen that the allowable range of cathode operating voltage is 2.5V-36V, and the cathode operating current varies in the range of 1~100mA. Generally, the cathode current is selected to be 20mA, which can not only work stably but also provide a part of dead resistance.

Assume that the current flowing through the bridge voltage divider is 250uA. Since the reference voltage of TL431 is 2.5V, then:

And because the output voltage UO:

So we can get:

1.3 Feedback compensation circuit analysis and design

When capacitor CZERO is not added, the feedback loop transfer function is:

In Figure 3, it is not difficult to find that the LED is connected before the secondary LC filter, which avoids gain in the high-frequency region when the LC network begins to resonate. Of course, high-frequency noise can also be reduced by the LC filter. The resonant frequency of the filter should be selected to be more than 10 times the selected crossover frequency to avoid mutual interference.

In addition, after adding capacitor Czero, a pole is introduced at the origin, and the complete feedback loop transfer function is:

It is easy to find that there is a pole fpo at the origin and a pole fz introduced by the fast track structure. Since the amplifier type 2 is used in the design of this article, a pole fp is required elsewhere.

In this way, we can add a capacitor between the output node and the ground, and get the final control formula:

In this way, the pole and zero locations can be found:

Therefore, the K factor method can be applied to design the required amplifier type2:

Crossover frequency = 1kHz; required phase margin = 70°; gain reduction at crossover frequency Gfc = -20dB; phase at crossover frequency = -55°, K factor calculated as: k = 4.5; fz = 222kHz; fp = 4.5kHz; G = 10; CTR = 0.8.

According to the formulas obtained above, we can get:

So far, the entire feedback network design process is completed.

2. Experimental results

According to the specific design of the feedback control circuit and the above data, HSpice is used for simulation, and the simulation results are shown in Figure 4. After careful observation, it is not difficult to find from the system waveform that the system has obvious stability and reliability.

Figure 4. Bode plot of current mode operation in DCM or CCM.

3. Conclusion

This paper designs the feedback control circuit of the DC switching power supply by using the PWM current regulation method combining the optocoupler 817 and the three-terminal shunt regulator TL431. The design results better reflect the characteristics of miniaturization, low power and high efficiency. The experimental results show that the system has good stability and reliability.

With the gradual acceleration of the modularization process of switching power supplies, the peripheral components of switching power supplies are becoming fewer and fewer. Therefore, how to better ensure the compactness, intelligence, and efficiency of switching power supplies, as well as the stability, safety, and good heat dissipation performance of the corresponding circuit system will be the author's main research direction in the next step.