The design of a small planar transformer / inductor is introduced in detail.

The design of a small planar transformer / inductor is introduced in detail.

1 Introduction

With the rapid development of electronic information technology, a variety of electronic equipment has entered the era of SMT (mounting technology Surface), electronic devices are increasingly required light, thin, small. Power electronic transformer and inductor in the traditional tube, although discrete transistor era played an important role, and today the modular electronic equipment, because the volume is too large to use, how to develop a small plane electronic transformer and inductor is a hotspot of design personnel attention. This paper describes the use of multilayer printed circuit board manufacturing technology, CNC machining technology, surface coating technology and the use of high frequency low loss ferrite core design and small planar transformer and 20A and produced 230kHz, up to 120W 10 H high current inductor filter.

2 circuit form and transformer, inductor technical indicators

Figure 1 is the main circuit of the active clamp / reset single ended forward converter. The circuit has zero voltage switching function, which is beneficial to improve the efficiency and reduce the EMI/RFI.

The circuit is composed of VQ2, VD2 and Ccl clamp circuit, leakage inductance and inductance for L1 Lm storage transfer provides a low resistance pathway, VQ2 conduction after Ccl continue to be charged, the clamping circuit to reduce the resonant current. Due to the rectifier tube VD1 cut-off, L1 and Lm were in series connection, the resonant frequency of L1, Lm and Ccl decided, so the transformer primary have a certain amount of inductance.

In addition, the circuit of VQ1 after the closing of the transformer winding polarity reversal voltage, Ca is charged, the charging process, the magnetization current decreases, by selecting appropriate parameters, to achieve the opening of VQ2 in the magnetizing current zero, provides the possibility to change the direction of the magnetization current, the anti back flow of magnetizing current, primary winding clamp voltage Ucl to reverse transformer, drive transformer B-H work area extends to the second and third quadrants. At the same time, the Ccl capacitor storage can be transferred to L1 and Lm storage. The working point of B-H is from the third quadrant, the normal working area is symmetrical with the B-H axis origin, and the symmetrical region is shown as: B-H one way change is consistent with the traditional single ended forward converter. In order to maintain the normal regulation of output, applying the same volt second product to the transformer, the core loss compared to the single ended forward converter is the same. In actual work, should choose the maximum flux density (Bm), the transformer can work in Bm ~ +Bm, the B=2Bm, as shown in figure 2.

T1 we need to design the circuit of the transformer, the working frequency of f=230KHz, the input voltage is Vin=230V, the primary inductance Lm=117 H + 10%, maximum 0.45, output voltage Vo=5V, output current Io=20A, Lo Lo=10 H, filter inductor, the working environment temperature of -45 to 50 DEG C, the temperature below 50 DEG C the test voltage, 2KV, transformer, inductor height less than 12mm, length and width were about 40mm.

3 planar transformer, inductor core and structure

3.1 magnetic core

At the present stage, there are many kinds of magnetic materials, such as: slope mullite alloy, amorphous alloy, ultra micro crystal alloy, ferrite and so on. Selection of ferrite materials for core, to make full use of effective space, must choose the core column thicker and wider bandwidth, which is conducive to reducing the number of turns and reduce the current density. In view of the overall height of the restrictions, it is necessary to carry out the necessary processing.

3.2 winding

The traditional winding coil is wound on the skeleton, and the wire is circular, and work in high frequency, high frequency alternating current flows through the wire, it is also affected by the skin effect in the penetration depth limit, the calculation formula is as follows

In the Delta for penetration depth (mm), Omega is the angular frequency, Omega PI =2 f (RAD). The conductivity of the wire (H/m), and the conductivity of the conductor (S/m).

The relative permeability of copper is equal to 1, that is, the vacuum permeability

The plug type can be simplified to

Type f=230KHz is available in the wire diameter of 2 =0.275mm. It is generally in the case of large current transformer windings are used more than the wire winding system, which will make the core window hole utilization rate is greatly reduced. We decided to small current primary and auxiliary windings respectively with multilayer printed circuit board and double panel manufacturing, up to 20A of the secondary winding inductance and winding by folding the strip with a rectangular cross section of manufacturing, to make the most effective use of the window.

4 transformer design

4.1 to determine the size of the magnetic core by the power transfer capability

The power transfer capability of the transformer is determined by the product of the Ap value of the area of the core column and the area of the window.

In the formula: Pt is the transformer primary and secondary power, and the efficiency of the transformer is 2 times higher than that of the output power. Kj for the magnetic core structure constant, its value is between 365 ~ 632, we take 450. The B for incremental induction circuit, according to the B=2Bm, Bm 0.1T, =0.2T is B. F for the working frequency of 230 KHz. Ku for window utilization, between 0.3 to 0.4. Kf for the waveform coefficient, rectangular wave to take 4, 4.44 of the sine wave.

The above data into the calculation

Between AP=0.6855 and 1.1638.

After repeated comparisons and calculations, we select the PQ40 type magnetic core and make the size we need. As shown in Figure 3, the AP value is only 0.69.

4.2 winding

(1) calculation of primary turns

Type

Up1 230V is the minimum input voltage amplitude transformer, Delta B for incremental induction of 0.2T, a maximum ratio of 0.45, Sc as the core cross-sectional area 1.74mm2.

The above data into the calculation