Low-power power transformer design and its examples - Database & Sql Blog Articles

July 29, 2022

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In this article, we are not going to deduce the relevant application formulas, but we hope that readers will achieve the same effect from this design example.

1. Calculate secondary output power (P2)

The maximum output voltage in "Li" is 24V, assuming a rated output current of 1A, the voltage drop of the K790 tube of the adjustment tube is 3V, and the power consumption of the voltage doubler rectifier circuit is negligible, then:

P2=(24+3)x1=27W

2. Calculate primary power (P1)

Assuming transformer efficiency η = 0.75, then P1 = 27W / 0.75 = 36W

Note: The efficiency of the transformer varies slightly depending on the output power. Generally, the transformer η with a capacity below 100W is between 0.7-0.8, below 100W to 1000W, between 0.8-0.9. In actual operation, the output power The lower one takes a small value.

3. Calculate the wire diameter of the primary and secondary coils (d)

Where, I - winding operating current, J - current density (usually J takes 3-3.5A/mm2)

Primary winding current I1=36/220=0.164(A)

3.1 primary winding wire diameter

3.2 secondary winding wire diameter:

Secondary winding current I2=1x1.17=1.17A

1.17 is the rectification coefficient of the transformer secondary AC current.

Since there is no 0.67mm in the enameled wire specification, it is 0.7mm.

Usually we take the current density of the secondary coil to a small value to obtain a small power supply internal resistance and reduce the temperature rise.

4. Calculate the cross-sectional area of the core

We use the following formula to calculate the core cross-sectional area (personally think it is a simple empirical formula)

Where S - core cross-sectional area K - coefficient P2 - secondary power

The value of K is related to the output power of the transformer. For K below 100W, take 1.25-1.1 (the power is larger), 100W-1000W can be taken directly, and in this case, 1.15, then:

Theoretically speaking, in the case of the core cross-sectional area constant, the tongue width and stack thickness of the transformer core can be taken at any ratio, but in the actual design, the manufacturing process of the coil, the shape uniformity, the leakage reactance and other factors must be considered. Generally, the width of the tongue is about 1.5 to 2 times, and in this example, the EI piece with a width of 66 mm is selected, and the stack thickness is 2.7 cm.

5. Primary and secondary winding turns

5.1 Calculating the number of turns per volt (W0)

Where f - alternating current frequency (Hz) B - magnetic flux density (T) S - core cross-sectional area (CM2)

The B value will vary depending on the core material. The general silicon steel sheet material and its value are shown in the table below:

In this example, H23 is selected, and B is 1.42.

5.2 primary number of turns (W1)

W1=220W0=1160(T)

5.2 secondary turns (W2)

In the formula, 1.2 is the coefficient of the rectified and filtered capacitor, and 1.12 is to compensate the secondary voltage drop caused by the winding impedance after the load. Generally, the secondary turns should be increased by 5% to 25% (the output power is small). .

At this point, the design work of the transformer is basically completed.

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