The Design of a Small Planar Transformer / Inductor Is Introduced in Detail

With the rapid development of electronic information technology, various electronic devices have entered the SMT (surface mounting technology) era, and electronic devices are increasingly required to be light, thin and miniaturized. Although the traditional power electronic transformers and inductors have played an important role in the era of electronic tubes and discrete transistors, they can not be used in today's modular electronic equipment because of their large volume. How to develop small planar electronic transformers and inductors is the focus of designers. This paper describes the design and manufacture of 230khz, 120W small planar transformer and 20a, 10 with multilayer printed board manufacturing technology, NC machine tool processing technology, surface coating technology and high frequency and low loss ferrite core H high current filter inductor.

2. Circuit form and technical indexes of transformer and inductor

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

The circuit consists of vq2, VD2 and CCL, which provides a low resistance working path for the energy storage transfer of leakage inductance L1 and excitation inductance LM. After vq2 is turned on, CCL continues to be charged, and the current of the clamping circuit decreases in a resonant manner. Since the rectifier VD1 is cut off, L1 and LM are connected in series, and the resonant frequency is determined by L1, LM and Cl, there are certain inductance requirements for the primary of the transformer.

In addition, after the circuit VQ1 is cut off, the voltage polarity of the transformer winding is reversed and Ca is charged. During the charging process, the magnetization current gradually decreases. By properly selecting parameters, vq2 is turned on before the zero crossing of the magnetization current, which makes it possible to change the direction of the magnetization current. The magnetization current is reversed and the clamping voltage UCL is reversed to the primary winding of the transformer, The working area of the drive transformer B-H extends to the second quadrant and the third quadrant. At the same time, the discharge of CCL capacitor energy storage is transferred to L1 and LM storage. After VQ1 is turned on, the B-H working point starts from the third quadrant, and the normal working area is basically symmetrical with the origin of the B-H axis. In this symmetrical area, the unidirectional variation value of B-H is consistent with that of the traditional single ended forward converter. In order to maintain normal output regulation, the same volt second product is applied to the transformer, and the resulting core loss is consistent with that of the single ended forward converter. During actual operation, the maximum working magnetic flux density (BM) shall be selected. The transformer can work in - BM BM, so B = 2bm, as shown in Figure 2.

T1 in the circuit is the transformer we need to design, with working frequency f = 230khz, input voltage Vin = 230V and primary inductance LM = 117 H 10%, maximum working ratio 0.45, output voltage Vo = 5V, output current IO = 20a, Lo is filter inductance, Lo = 10 H. The working environment temperature is - 45 50 , the temperature rise is 50 , the test voltage is 2KV, the height of transformer and inductor is 12mm, and the length and width are about 40mm.

3. Plane transformer, inductor core and structure

3.1 magnetic core

At present, the magnetic materials used in power switching transformers include permalloy, amorphous alloy, ultramicrocrystalline alloy, ferrite and other materials. Ferrite material is selected to make magnetic core. In order to make full use of effective space, magnetic core with thicker core column and wider window width must be selected, which is conducive to reducing turns and current density. In view of the limitation of the overall height, necessary processing is also required.

3.2 winding

In the traditional winding, the coil is wound on the skeleton, and the conductor is of circular section. In addition, when the conductor flows through the high-frequency alternating current, it is also limited by the penetration depth of the skin effect. The calculation formula is

Where is the penetration depth (mm), Is the angular frequency, = 2f(rad) Is the conductor permeability (H / M), Is the conductor conductivity (s / M).

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

Substituting this into the above formula can be simplified to

Where f = 230khz, the available conductor diameter is 2 = 0.275mm. Therefore, under the condition of high current, the transformer winding adopts multi strand winding, which will greatly reduce the utilization rate of magnetic core window. We decide that the primary winding and auxiliary winding with low current are made of multilayer printed board and double panel respectively, and the secondary winding and filter inductance winding up to 20A are made of folded copper strip with rectangular section, so as to make the most effective use of window holes.

4. Transformer design

4.1 the core size is determined by the power transmission capacity

The power transmission capacity of a transformer depends on the product of the area of the magnetic core column and the area of the window hole, AP

Where: Pt is the sum of the primary and secondary power of the transformer. When the efficiency of the transformer is high, twice the output power can be taken. KJ is the structure constant of the magnetic core, which is between 365 and 632, and we take 450. B is the incremental magnetic induction intensity. According to the circuit B = 2bm, BM is taken as 0.1t, then B = 0.2T. F is the operating frequency 230 kHz. Ku is the window utilization, between 0.3 and 0.4. KF is the waveform coefficient, 4 for rectangular wave and 4.44 for sine wave.

Substitute the above data into the calculation

After repeated comparison and calculation, we selected PQ40 magnetic core and ground it into the size we need. As shown in Figure 3, the AP value is only 0.69.

4.2 winding

(1) Primary turns calculation

Where

Up1 is the minimum amplitude of transformer input voltage 230V, B is the incremental magnetic induction intensity 0.2T, Is the maximum working ratio of 0.45, and SC is the cross-sectional area of the magnetic core of 1.74mm2.

Substitute the above data into the calculation

W1 = 12.93 turns.

(2) Secondary turns calculation

Where: up2 is the voltage amplitude of secondary winding,

Uo is the output voltage 5V. U2 is the voltage drop of rectifier tube and line. If 1.7V is taken, up2 = 14.889v, W2 = 0.837 turns.

After adjusting the number of turns to an integer

W1 = 15 turns

W2 = 1 turn

4.3 winding current

Ignoring the exciting current, the effective values of the primary and secondary currents are calculated according to the unidirectional pulse square wave,

4.4 winding form and temperature rise

Since the effective value of the primary winding current is 0.895a, we use 8 layers of printed boards, each layer has 7.5 turns, 4 layers are connected in parallel, and then 7.5 turns are connected in series to form 15 turns of primary winding, as shown in Figure 4.

When the primary winding works at full load, the loss is 1.07w.

The effective value of the secondary winding current is 13.42a. Considering the limitation of skin effect penetration rate, we use two copper strips with a thickness of 0.3mm, which are processed into the shape shown in Figure 5 by CNC machine tool. When the secondary winding works at full load, the loss is 0.709w. One turn of auxiliary winding and one turn of feedback winding are made of double-sided plate, and the shape is shown in Fig. 6. Since the current is very small, the loss is negligible.

According to the data, the iron loss calculated from the working frequency, BM value and working temperature is 1.296w.

The appearance of the transformer after assembly is shown in Figure 7, and its heat dissipation area is s = 42.88cm2. Dissipated power per unit area q = 0.0524w/cm2. According to Fig. 8, the temperature rise is 42 . The measured temperature rise at full load is 34 .

5. Filter inductance design

In the design of filter inductor, we use pq32 magnetic core and grind it into the shape and size shown in Figure 9.

5.1 determine the number of turns of filter inductance W

Where l is the inductance required by the technical index. The leakage inductance in the case of atmospheric gap accounts for 20%. The core inductance only needs to be 0.8L. LG is the air gap length. Considering that the inductance requires good linearity, LG is taken as 1.8mm. AG is the equivalent section at the air gap, Ag is 1.2 times the magnetic core section, Ag = 1.267mm2.

Substitute the above data to obtain

W = 9.52 turns, rounded to 10 turns.

5.2 determination of winding form and temperature rise calculation

Considering that the inductance current reaches 20a, only the ripple frequency is 230khz, and the main component is still DC current, a copper strip with thickness of 0.45mm and width of 4.5mm is used as the winding.

Processed by CNC machine tool, the folded shape is shown in FIG. 10 and expanded as shown in FIG. 11.

After calculation, the winding section is s = 2.025mm2, the winding length is L = 0.612m, and the winding loss PM = 2.7992w.

Since B is very low during operation, the iron loss is ignored.

The outline of filter inductor after assembly is shown in Figure 12. According to the overall dimensions, the heat dissipation area is s = 27.04cm2, and the dissipated power per unit area is q = 0.10352w/cm2. According to Fig. 8, the temperature rise is 65 . The measured temperature rise at full load is 48 .

6. Analysis that the temperature rise of transformer (filter inductor) assembled on aluminum substrate with radiator can be greatly reduced

Because the contact area between the mounting bracket and the base plate of the traditional transformer is less than 1% of the overall area, and no measures are taken, the base plate is not included in the scope of helping heat dissipation. The good contact area between the plane transformer and the base plate can reach about 25%, which greatly improves the heat dissipation conditions.

Heat conduction refers to the study of the way and effect of energy transfer of each part of the object in direct contact.

We will discuss the effect of transformer assembly on aluminum substrate with radiator. The following conditions must be known:

The theoretical temperature rise of transformer is 42 .

After the transformer works at full load for several hours, the actual temperature rise of the bottom surface in contact with the aluminum substrate is 29 . The surface temperature rise of the radiator in contact with the cold plate is 27 .

According to the conversion of 1 kW · H = 859.8 kcal, the total loss of transformer is 2.051 w · H = 1.763 kcal.

Heat Q derived from heat conduction engineering calculation under stable working condition of multi-layer flat wall:

Where: t1-t5 is the temperature difference of multi-layer surface wall of 2 . Rr1... RR4 is the total thermal resistance of multilayer flat wall (· H / kcal). Is the thickness of flat wall of each layer (m). Thermal conductive adhesive 0.0001, copper foil 0.00015, medium 0.00015, aluminum substrate 0.002. Is the thermal conductivity of each layer of flat wall (kcal / (m · h ·). Thermal conductive adhesive 0.194, copper foil 330, medium 0.26, aluminum substrate 204. A is the contact area between the bottom of the transformer and the flat wall 0.00104m2.

Substitute the above data to obtain

Q=0.3873(kcal)

That is, the aluminum substrate with radiator transfers 21.96% of the total loss of 2.051w of the transformer, so it is reasonable to reduce the actual temperature rise by about 20%.

In the same way, the effect of aluminum substrate with radiator on filter inductance can be calculated, which will not be repeated here.

7. Conclusion

The transformer and filter inductor designed above have passed the electrical performance test, high and low temperature cycle test and high and low temperature storage test, and their performance meets the requirements.

Through the transformer and filter inductance designed in this scheme, the following conclusions can be drawn:

The folded copper strip processed by NC machine tool not only meets the high frequency limited by the penetration rate of skin effect, but also has a rectangular section. In addition, the copper strip surface is insulated with paint, which greatly improves the utilization rate of window holes. Compared with manual winding, the winding of folded copper strip, multilayer printed board and double-sided board has good consistency of distribution parameters and is convenient for circuit debugging.

For switching transformers and filter inductors with operating frequencies above 200kHz, they can be designed into small planarization, and their height can be reduced to the same order of magnitude as components such as integrated circuits and capacitors. They can also be assembled on aluminum substrates with radiators. With the help of radiators, the temperature rise can be reduced by more than 20% under the condition of the same dissipated power.

In case of mass production, the magnetic core shall be redesigned as required to increase its bottom area and make the window more reasonable, and the temperature rise and size of transformer and filter inductor will be further reduced.