Inductances and chokes in chargers

　　The charger manufacturer introduced the following winding components (inductance and current holding ring)

　　1. Simple inductance (no DC current through)

　　2. Common-mode line filter inductance (especially dual-winding inductance with large symmetrical power frequency current)

　　3. Series line filter inductor (carrying large and asymmetrical power frequency current inductance)

　　4. Flow coil (inductance wrapped around a ferrite core with an air gap and carrying a large DC bias current)

　　5. Rod-shaped flow ring (a flow-throwing ring wound on a ferrite core or a rod-shaped iron powder core)

　　In order to facilitate the discussion, the "inductance" we are talking about here refers to the winding components without DC current, and the "choke coil" refers to the winding components with large bias current and relatively small AC ripple.

　　The design and material selection of winding components must fully consider the place of application. In addition, the design must be repeated many times, and the relationship between some interconnected but opposite variables must be coordinated.

　　If the engineer can fully understand and grasp the theoretical and actual specifications required for the optimal design of various winding components in the charger, then the design skills he possesses will be precious and unique.

　　The design method used here is mainly based on its scope of application, focusing on the three aspects of cost, size and loss. The final design can only be a compromise solution. Since these three main aspects are contradictory, it can only be used A compromise solution. The task of the designer is to obtain the best compromise.

　　In charger applications, inductors without DC bias are generally limited to low-pass filters used in the charger circuit. Here, their main function is to prevent high noise from being transmitted back to the charger circuit. For this type of application, we should choose a core material with a high conductivity.

　　Chokes (inductors carrying large bias DC currents) are used in high-frequency power output filters and continuous buck-boost converter "transformers". In these applications, low permeability and high frequency should be preferred Magnetic core material with low magnetic loss.

　　In order to reduce the number of turns and reduce the copper loss, the most ideal core material should have high permeability and small magnetic loss. Unfortunately, in the design of the current-carrying circle, the existence of large DC components and the actual use The limited saturation magnetic flux density of magnetic materials makes us have to choose low-conductivity materials or introduce air into the core. However, due to the too low effective permeability, more windings are needed to achieve the required inductance. value. Therefore, in the choke coil design, in order to pass a larger DC current, both low copper loss and high efficiency must be taken into account.

　　Simple inductor

　　In the application field of charger, pure inductance (cannot carry DC current component or forced AC high current component) is rare. Unlike the common-mode filter inductors that will be introduced below, since no air gap is required, the value of this inductance can be obtained directly from the given core inductance A value, so the design is relatively simple, and this will not be described. But it must be remembered that the size of this type of inductance is proportional to the square of the number of turns. Therefore, A1 must be given for 1 turn (as shown in the following formula), or A1 for multiple turns must be given. At this time, the value of A should be reduced to the value of 1 turn by dividing by the square of the number of turns.

　　L=N2AL

　　Common mode line filter inductance

　　The figure shows a typical balanced line filter, which is used in offline chargers to limit conducted RFI noise. It can be seen from the figure that two independent inductors L1(a) and L1(b) are wound on the same magnetic core to form a dual-winding common-mode line filter. It can also be seen from the figure that the inductor L2 is one Single winding series inductor.

　　The figure shows two typical dual-winding common-mode filter inductors.

　　The common mode filter inductor has two independent windings with the same number of turns. The two windings are connected to the circuit in an inverted manner, so that the series-connected power frequency current can pass through. Therefore, the magnetic field generated by the normal series AC (or even DC) charger current will cancel to zero.

　　When the two windings are connected in anti-phase and pass a series current, the inductance presented is only the leakage inductance between them. Therefore, the low-frequency line current will not be able to saturate the magnetic core, so materials with high permeability can be used without introducing an air gap in the magnetic core, so that a larger inductance can be obtained with a smaller number of winding turns.

　　However, for common mode noise (noise current or voltage to ground generated at both ends of the line at the same time), the two windings are parallel and in phase, and exhibit a high inductance under common mode current. Therefore, in order to prevent any effective common mode interference current from being conducted to the input charger circuit, common mode noise current can be introduced from the bypass circuit to ground through capacitors C1 and C2.

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