Switching power supply is a seemingly simple but extremely complex electronic system. There are many interesting questions in it. I put them here to make myself think:
1. Generally, the output voltage and input voltage of Buck system are calculated according to the duty cycle of switch. For example, if the input voltage is 24 V and the output voltage is 12 V, the duty cycle should be 50%. In most cases, this theoretical deduction is not followed. Because this is derived under the full scale of switching power supply. In fact, switching power supply generally works less than the maximum range output. There is a premise that the input energy will be offset by the output energy in a cycle. What if it can't offset? Then the switching power supply will enter the burping mode (light jump mode), that is, there will be switching waveform output in several consecutive working cycles, and then there will be no switching waveform in several consecutive cycles. Ensure that the input energy is equal to the output energy. Theoretically speaking, it can be considered as a switch waveform with lower frequency and lower duty cycle. It can also be understood that the accuracy of switch control in the topology of switching power supply is limited, and the resolution of duty cycle can only reach 0.5% in one cycle. To achieve the accuracy of 0.01%, more cycles can only be used to approximate. For example, four consecutive switching cycles of 5% 4% 4% 4% combine into a 4.25% duty cycle.
This reminds me that DLP projectors and rear projection color TV sets are actually the same principle. The core chip of DLP can only make one point bright or dim. The exposure light is projected on DLP through the color wheel, and the reflected light of DLP is emitted through the lens. If you use a high-speed camera DLP picture, you should be able to see monochrome pictures of different colors. Through the transient effect of human eyes, space synthesis is carried out.
The same is true in software, volume and speed, software can use space for time, can use time for space.
2. Three kinds of switching power supply systems, buck, boost, buck boost, start from 0 state to work state when cold start. The capacitor used for filtering is also charged from 0 to the specified voltage. In fact, if not limited, a large inrush current can be generated. Irreversible damage to the system. In the same way, in rush current will appear when the load changes and the output power changes. The reason for this phenomenon is the control method of control circuit. Generally, the output of switching power supply is a constant voltage source, which only tracks the voltage. Voltage is the target of control, the switch tube flow in the past is not voltage, but energy. Energy should be controlled and limited, not bare voltage.
3. Invert is a very complex power supply, especially the three-phase high-frequency PWM rectifier. There are various control methods. But we often ignore some of the most basic understanding of the circuit. Why can invert power supply achieve bidirectional energy flow? That is because when AC > DC, the external AC power supply, inductance and switch tube form a boost to input energy to the HVDC bus; When DC > AC, DC bus is used as power supply, and a buck is formed by inductor and switch tube to supply power to AC power supply. Because the AC source of commercial power can be equivalent to a huge load, and boost and buck are power sources with variable internal resistance (appropriate control method). Even if the AC line voltage turns back and forth, it will not affect the operation of the circuit. The most basic knowledge makes me understand that no matter how complex things are, they are made up of simple things. The foundation is often more important than specific technology.
