how to choose the Shape of High Frequency Magnetic Core in Switching Power Supply

High frequency transformer is a device for converting AC voltage, current and impedance. When AC current is passed through the primary coil, AC magnetic flux will be generated in the iron core (or magnetic core), which will induce voltage (or current) in the secondary coil. The transformer is composed of an iron core (or magnetic core) and a coil. The coil has two or more windings. The winding connected to the power supply is called the primary coil, and the other windings are called the secondary coil. The magnetic cores of the transformer include tank type core, RM type core, E type core, EC, ETD and EER type core, PQ type core, EP type core, EP type core, annular core and other magnetic cores. How do these magnetic cores affect the work of the transformer? Now let's see the specific analysis.

1,Pot cores 
The skeleton and winding are almost completely wrapped by the magnetic core, which makes it very effective for EMI shielding; The size of the tank type magnetic core conforms to the IEC standard, and the interchangeability is very good during manufacturing; It can provide simple skeleton (without pin) and PCB mounting skeleton (with pin); Due to the design of tank shape, the cost is higher than other types of magnetic cores with the same size; Because its shape is not conducive to heat dissipation, it is not suitable for high-power transformer inductors.

2.RM ferrite core
Compared with the Pot type cores , the two symmetrical sides of the pot type are cut off, which is more conducive to heat dissipation and lead out of large size; Compared with the tank shape, it saves about 40% of the installation space; Whether the skeleton has needle type and pin type; A pair of clips can be used for installation; The RM type magnetic core can be made into a flat shape (suitable for the current planar transformer or the magnetic core can be directly assembled to the printed circuit board with designed windings); Although the shielding effect is not as good as the pot type, it is still good.

4. E-core

Compared with the tank type magnetic core, the cost of the E-type magnetic core is much lower. In addition, the winding and assembly are relatively simple. This magnetic core shape is now the most widely used, but its disadvantage is that it cannot provide self shielding; The E-type magnetic core can be installed in different directions, or several pairs of magnetic cores can be stacked to apply more power; The magnetic core can be made into a flat shape (which is a very popular magnetic core shape for planar transformers at present); Needle free and pin type skeletons can also be provided; Because its heat dissipation is very good and it can be used together, generally high-power inductors and transformers use magnetic cores of this shape.

5. EC, ETD and EER cores

These types of magnetic core structures are between the E-type and the tank type. Like the E-type magnetic core, they can provide enough space for large cross-section leads to lead out (suitable for the current trend of low-voltage switching power supply); These shaped cores also have excellent heat dissipation; Since the central column is cylindrical, compared with the cuboid with the same section, the length of the single turn winding is shortened by 11%, which reduces the copper loss by 11%, and at the same time, the magnetic core can provide a higher output power; At the same time, the central column is cylindrical, which, compared with the rectangular central column, also avoids the hidden danger of damaging the winding wire insulation due to the rectangular corners during winding.

6. PQ type magnetic core

PQ type magnetic core is specially designed for inductor and transformer used in switching power supply. The design of PQ shape optimizes the ratio of core volume, surface area and winding area; This design makes it possible to use the smallest magnetic core to provide the maximum inductance and the maximum winding area; With this design, the maximum output power can be obtained with the minimum transformer volume and weight, and the minimum PCB installation space can be occupied; A pair of clips can be used for installation and fixation; This effective design also makes the magnetic circuit section area of the magnetic core more uniform, so this core structure also makes it possible to have fewer hot spots than other core structure designs.

7. EP type magnetic core

The circular central column solid structure of EP type magnetic core completely wraps the winding except for the end contacting with PCB board, and the shielding is very good; This unique shape minimizes the influence of the air gap formed by the contact surface during the assembly of two magnetic cores, and provides a larger volume and total space utilization ratio.

8. Toroidal core

For manufacturers, the annular magnetic core is the most economical, and its cost is the lowest among the comparable magnetic cores; Due to the use of the skeleton, the additional and assembly costs are equal to zero; When appropriate, the winding machine can be used for winding; Its shielding is also very good.

9. Summary

The above is a summary of the analysis of how the shape of the high-frequency transformer core affects the work of the transformer. I hope it can help you

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