Advantages and disadvantages of external and built-in LLC resonant inductors

The LLC power supply has a resonant inductor and a resonant capacitor, but in many cases actual PCB you see has only an LLC transformer and a resonant capacitor, and no resonant coil. integrated into transformer, so this inductor is actually a resonant inductor, consisting of leakage inductance of primary and secondary sides. We don't want leakage inductance in a hard switch. We hope that smaller better, because energy stored in leakage inductance needs to be produced by a special circuit, but leakage inductance in resonant converter is beneficial and can be used as a resonant inductor, so that you do not need to make another inductor, to make a resonant inductor This is a transformer with built-in resonant inductor. And some power supplies have independent resonant inductors and transformers. This is an external resonant coil circuit. Next, we will discuss advantages and disadvantages of these two inductor methods.

1. Advantages of external resonant inductor:

The external resonant inductor is a separate inductor that is separate from main transformer. This design is very flexible. Although inductance is achieved through a special design, a particular inductor can be made from different materials. , to optimize inductance loss, and power density can be made high, and there are many options for main transformer core frame. External resonant inductance does not affect adjustment of our transformer in any way, because. in debugging process, number of turns of our primary winding can be adjusted, and adjusting primary winding will affect leakage inductance, because this is a discrete transformer, total leakage. The inductance is controlled very low, which is very small compared to external resonant inductance, so fine tuning number of turns on primary side has little effect on resonant inductance and can be ignored.

2. Disadvantages of an external resonant inductor:

The cost of an external resonant inductor is higher than built-in one, this is an inductor made separately, and material will have extra magnetic circuits and frames, copper wires around inductor, etc., and then cost of man-hours. One is cost of manufacturing an inductor, and second cost of insertion in manufacture of power supply. The working current of resonant inductor is variable, so intensity of magnetic induction works in first quadrant and third quadrant. In order to reduce loss of magnetic core, Bmax value is usually relatively low, resulting in an average ferrite core. The air in hole of column is relatively large, which can easily lead to relatively large eddy current losses of resonant inductor, which will lead to a relatively high temperature.

Third, advantages of built-in resonant inductors:

Integrated resonant inductors can save some cost in terms of materials and labor, but integrated inductors require special windings. 20% of main inductance, i.e. leakage inductance should reach 20% of primary winding. inductance, so connection between primary and secondary does not have to be very good.

In order to increase leakage inductance, we usually need to use some professional frameworks. The advantage of this type of framework is that leakage inductance is large, and parasitic capacitance of primary and secondary sides becomes smaller, which is beneficial for our LLC. This should be good. Generally, integrated frame we use is a slotted frame that separates primary side from secondary, so that safety rules between primary and secondary sides are much better than separateYou can use enameled wire for both sides. primary side and secondary side. The safety regulations can mostly be ignored and whole winding is much easier.

Fourth: disadvantages of built-in resonant inductors:

The disadvantage of this built-in transformer is also very obvious. The volume of entire transformer is much larger than that of ordinary ones. Since it is wound in separate grooves, winding space usually needs to be relatively large, and leakage inductance must be relatively large. Such skeletons usually need to be tuned , which increases cost. Of course, if quantity is relatively large, additional cost can be largely ignored. Because resonant inductance is leakage inductance of primary side, it has a great influence on size of leakage inductance when primary side of winding is turned. After resonant leakage inductance is fixed, number of turns is not easy to adjust, so built-in resonant inductor is generally made. The power is not very large, and power of thousands of watts mainly uses discrete inductors as resonant inductors.

In general, question of whether we use built-in resonant inductors in design or discrete ones still needs to be considered depending on situation in each company. If number is relatively large and power is not very large, integrated resonant inductor There are great advantages, because power is small, usually resonant inductance is relatively large, and excitation inductance is also relatively large. If an external inductance is used, ferrite is used as resonant inductor. The number of coils winding inductance is large, winding resistance is large. Copper loss is relatively large, and there is no such problem with integrated type, but eddy current loss of integrated type magnetic core is relatively large at high power, so external type is used.