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The production process of switching power adapter (2)

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.03.09
The design and production of switch mode power adapters should start from the main circuit, where the power conversion circuit is the core of the switch mode power adapter. The structure of a power conversion circuit, also known as a switching power supply topology, has multiple types. The topology also determines the corresponding PWM controller and output rectifier/filter circuit. The previous article explained the circuit structure and control circuit, while this article will introduce the auxiliary circuit of the switch power adapter, circuit schematic organization, and high-frequency transformer production.




1. Determine the auxiliary circuit

Switching power supply adapters usually consist of input anti electromagnetic interference filters, rectification filtering circuits, power conversion circuits, PWM controller circuits, output rectification filtering circuits, etc. The power conversion circuit is the main circuit of the switching power supply adapter, which plays a crucial role in the performance of the switching power supply adapter. According to different topology structures, switch mode power adapters also require some auxiliary circuits to function properly. Some auxiliary circuits may be included in the main circuit components. The common auxiliary circuits for switch mode power adapters are as follows:

a) Voltage feedback circuit
b) Peak voltage absorption circuit
c) Input filtering circuit
d) Rectification and filtering circuit
e) Output overvoltage protection circuit
f) Output overcurrent protection circuit
g) Peak current suppression circuit

The voltage feedback circuit is an auxiliary circuit that various types of switching power adapters have. The peak voltage absorption circuit is an essential auxiliary circuit for the flyback switching power supply adapter. Input filtering circuits usually only appear in AC/DC converters. The rectification filtering circuit includes power frequency rectification filtering and high-frequency rectification filtering. The self-excited oscillation type itself has the characteristic of output overcurrent protection. Sometimes it is necessary for the switch power adapter to have lightning protection circuits, input overvoltage and undervoltage protection circuits, etc. Design and development personnel can make appropriate selections based on design requirements.

2. Organize circuit schematic diagrams

After determining the topology, control circuit, and auxiliary circuit of the switch power adapter, the circuit schematic can be organized and drawn. In order to determine the model, parameters, shape, size, and quantity of all components, and complete the electrical connection between the pins of each component. The circuit schematic should be divided into different areas according to the signal flow and function, striving for clear and tidy wiring, reasonable density distribution, and clear signal flow. Then determine the packaging of all components for component layout and wiring during circuit board design.

3. Making high-frequency transformers


The design of high-frequency transformers is the technical key to making switch mode power adapters. In half bridge, full bridge, and push-pull switching power supplies, high-frequency transformers pass through alternating currents and there is no DC magnetization problem. The design method is basically the same as that of power frequency transformers, except that different magnetic core materials are used, making the design relatively simple. The high-frequency transformer of the forward switching power supply has similarities with the full bridge type, but there is a DC magnetization problem, which makes the design more complex. Therefore, sometimes demagnetization windings are added to high-frequency transformers to reduce design difficulty. Flyback switching power supplies are most commonly used in low-power switching power supplies, but the design of their high-frequency transformers is also the most complex.

The high-frequency transformer of the flyback switching power supply is equivalent to an energy storage inductor. At a fixed switching frequency, the amount of energy stored directly affects the output power of the switching power supply. When designing a high-frequency transformer for a flyback switching power supply, the following steps are required:

a) Calculate the inductance Lp once
b) Select magnetic core and skeleton
c) Calculate the number of turns Np of the primary winding
d) Calculate the number of turns Ns in the secondary winding
e) Calculate the length of the air gap
f) Verify magnetic flux density Bm

Firstly, the inductance Lp should be calculated based on the peak current Ip of the primary winding and the output power Po of the switching power supply. Then, select the magnetic core and skeleton and determine the relevant parameters. Next, calculate the number of turns Np based on the selected magnetic core cross-sectional area and magnetic circuit length parameters. Calculate the number of turns Ns in the secondary winding based on the ratio of primary and secondary transformations. To prevent magnetic saturation in high-frequency transformers, air gaps are usually added to the magnetic core, and the length of the air gap needs to be calculated based on the primary inductance Lp and the selected magnetic core parameters. We also need to calculate the magnetic flux density Bm based on the peak current Ip, the number of turns of the primary winding Np, and the magnetic core parameters to verify whether it meets the requirements of the magnetic core material. When some conditions cannot be met, the magnetic core and skeleton should be re selected for calculation and inspection until the design requirements are met.
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