Switching power supply product quality design lecture: Overcoming voltage surge and current surge difficulties

　　In order to prevent the distributed inductance and capacitance of the high-speed switching circuit in the switching power supply system from generating surge voltage and noise under the influence of the diode accumulated charge, the paper adopts RC or LC absorption circuit to absorb the surge voltage generated by the diode accumulated charge using amorphous core and rectangular core, thus solving the generation and suppression of surge current in the switching power supply.

　　introduction

　　Most of the main components of switching power supplies have parasitic inductance and capacitance. The parasitic capacitance Cp is usually connected in parallel with the switching element or diode, while the parasitic inductance L is usually connected in series with it. Due to the effects of these parasitic capacitances and inductances, the switching element often generates large voltage surges and current surges when it is turned on and off.

　　The on and off of the switch and the reverse recovery of the diode both generate large current surges and voltage surges. The most effective way to suppress the current surge when the switch is turned on is to use a zero voltage switching circuit. On the other hand, the voltage surge when the switch is turned off and the voltage surge when the diode is reverse recovered may damage semiconductor components and also cause noise. For this reason, an absorption circuit is required when the switch is turned off. When the diode reverse recovers, the voltage surge generation mechanism is the same as when the switch is turned off, so this absorption circuit is also applicable to the diode circuit. This article introduces absorption circuits such as RC, RCD, and LC. The basic working principle of these absorption circuits is to provide a bypass for the switch when the switch is turned off to absorb the energy accumulated in the parasitic inductance and clamp the switch voltage, thereby suppressing the surge current.

　　RC snubber circuit

　　Figure 1 RC absorption network circuit

　　Figure 1 shows a circuit diagram of an RC absorption network. It is a circuit in which a resistor Rs and a capacitor Cs are connected in series, and are connected in parallel with a switch. If the switch is disconnected, the energy accumulated in the parasitic inductance will charge the parasitic capacitance of the switch, and will also charge the absorption capacitor through the absorption resistor. In this way, due to the effect of the absorption resistor, its impedance will become larger, and then the absorption capacitor will equivalently increase the capacity of the parallel capacitance of the switch, thereby suppressing the voltage surge when the switch is disconnected. When the switch is turned on, the absorption capacitor is discharged through the switch, and at this time, its discharge current will be limited by the absorption resistor.

　　RCD absorption circuit

　　The RCD absorption circuit given in this article is shown in Figure 2. It consists of a resistor Rs, a capacitor Cs and a diode VDs, where the resistor Rs can also be connected in parallel with the diode VDs. If the switch is disconnected, the energy accumulated in the parasitic inductance will be charged through the parasitic capacitance of the switch, and the switch voltage will rise. When its voltage rises to the voltage of the absorption capacitor, the absorption diode is turned on, so that the switch voltage is clamped by the absorption diode (about 1 V), and the energy accumulated in the parasitic inductance also charges the absorption capacitor. During the switch on period, the absorption capacitor is discharged through the resistor.

　　Figure 2 RCD absorption network

　　The transformer can also be demagnetized by using RC and RCD absorption circuits without the need to set up a demagnetization circuit consisting of transformer windings and diodes. The transformer's excitation energy will be consumed in the absorption resistor. RC and RCD absorption circuits can not only consume the energy accumulated in the transformer leakage inductance, but also consume the transformer's excitation energy. Therefore, this method also reduces the conversion efficiency of the converter.

　　Since the RCD absorption circuit clamps the switching voltage through a diode, the effect is better than that of RC. At the same time, it can also use a larger resistor, but the energy loss is also smaller than that of RC.

　　LC absorption circuit

　　An LC circuit is a circuit composed of components such as capacitors, inductors, resistors and electronic devices that can generate oscillating current or have a filtering effect. The LC circuit formed by connecting an inductor coil L and a capacitor C is the simplest LC circuit.

　　Figure 3 LC absorption network

　　The LC absorption circuit is shown in Figure 3, which consists of Ls, Cs, VDs1 and VDs2. If the switch is turned off, the energy accumulated in the inductance such as leakage or excitation can be discharged through VDs1 via capacitor Cs, so that the voltage of the absorption capacitor Cs is reversed, so that the transformer is demagnetized by the capacitor voltage. During this period, the polarity of the voltage added to the switch by the input voltage and the voltage of the absorption capacitor is reversed again. Under normal circumstances, the LC absorption circuit does not consume energy.

　　Conclusion

　　To increase the switching frequency and improve the quality of switching power supply products, the voltage surge and current surge issues must be considered. This article proposes this effective suppression measure based on the analysis of the interference generation mechanism and a lot of practice. Therefore, to solve the surge problem, it is also necessary to combine the actual design, analyze the mechanism of surge generation, and design the surge absorption circuit based on the actual situation to minimize the surge interference of the switching power supply.

　 　The tide of green power design is sweeping in, and "reducing losses and improving efficiency" has become an urgent issue in the industry. In this technical seminar, we noticed that in any kind of power design, excellent power protection is necessary to work more safely and reliably. Circuit protection is crucial for every power engineer. In power design, electromagnetic radiation needs ESD for protection, but in another field, the use of electromagnetic radiation has become the technical basis of a new market, that is, wireless charging technology. The current challenge of wireless charging is charging efficiency, and digital power is undoubtedly a very important means to improve power management efficiency. With the increasing energy efficiency requirements of various systems, digital power is becoming more and more popular. High-voltage AC transmission can effectively increase the cost of energy transmission and use. Will it be the same for high-voltage DC? In power design, stable and reliable power supply testing is an indispensable step. Do you have the most reliable and effective power supply testing method?