Basic components of a switching power supply
1 Introduction to switching power supply
This document is an intensive training after Zhang Zhansong’s advanced switching power supply design. Based on the plan, after Shen Gong explained the transformer design, this article briefly introduces the principle of transformer design, focusing on the circuit working principle and the calculation of key components in the design process. and selection.
The working process of a switching power supply is quite easy to understand, and it has three distinct features:
Switches: Power electronics operate in a switching state rather than a linear state
High frequency: power electronic devices work at high frequency instead of low frequency close to industrial frequency
DC: The switching power supply outputs DC instead of AC, and can also output high-frequency AC such as electronic transformers
Switching power supply classification:
Switching power supplies are divided into many types according to the topology: buck boost forward flyback half-bridge full-bridge LLC, etc., but in essence, switching power supply has only two working modes: forward: it transmits energy when the switch tube is turned on, and flyback: Energy is transferred when the switch is off.
Current mode PWM single-ended flyback power supply
Advantages of this type of flyback power supply: simple structure and low price, suitable for low-power power supply.
Disadvantages of this type of flyback power supply: small power, generally below 150w, large ripple, low voltage load regulation, generally greater than 5%.
The difficulty in the design of this type of flyback power supply is mainly the design of the transformer, especially the transformer with wide input voltage and multiple outputs.
Example to explain the design process
In order to understand the detailed calculation process in the design more clearly, we will take 220VAC-380VAC input, +5V±3% (5A), ±15±5% (0.5A) three-way common-ground output flyback power supply as an example to explain the design process.
Putting forward the above requirements, the selection ideas are as follows:
The total output power of the power supply P=5*5W+15*0.5*2=40W, the power is small, you can choose the flyback switching power supply.
The power of the flyback power supply is only 40W and it belongs to multiple outputs, +5V±3%, ripple ±150mV, ±15±5%. 5V requires high precision, so 5v is used as the main feedback of the power supply. Considering the cross control ability of 5V to ±15V, the switching power supply adopts discontinuous mode (DCM).
The chip power supply coil uses 15V output, but its power is very small, so it is ignored in the calculation process.
The power supply is small, and the input voltage variation range is only ±30%, so no PFC circuit is needed.
