# Design of the hottest small power transformer

2022-08-12
• Detail

Design of low-power transformer

1. calculate the secondary output power (P2)

the maximum output voltage in the article "Li" is 24V, assuming that the rated output current is 1a, the voltage drop of K790 tube is 3V, and the power consumption of voltage doubling rectifier circuit is ignored:

p2= (24+3) x1=27w

2. Calculate the primary power (P1)

assume that the transformer efficiency =0.75, Then p1=27w/0.75=36w

note: the efficiency of the transformer varies slightly according to the size of the output power, usually 0 for transformers with a capacity below 100W Between 8, below 100W and 1000W, 0 9. In actual application, the lower output power is taken as the smaller value

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

in the formula, I winding working current, J current density (usually J takes.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

in the formula 1 317 is the rectification coefficient of the secondary ac current of the transformer

since there is no 0.67mm in the specification of enameled wire, it is taken as 0.7mm

usually, we take the current density of the secondary coil as a smaller value to obtain a small internal resistance of the power supply and reduce the temperature rise

4. calculate the cross-sectional area of the iron core

we use the following formula to calculate the cross-sectional area of the iron core (it is considered to be a relatively simple empirical formula for individuals to maximize the rigidity, change the rigidity and reduce the weight)

in the formula, the K coefficient P2 of the cross-sectional area of the s iron core is related to the output power of the transformer, and the value of the secondary power

k is 1 for K below 100W 1 (whichever is higher, take the smaller value). 100W-1000W can be taken as 1 directly. In this example, take 1.15, then:

theoretically, under the condition that the cross-sectional area of the iron core is unchanged, the tongue width and stack thickness of the transformer iron core can be taken as any proportion, but in the actual design, the manufacturing process of the coil, the symmetry of the shape, leakage reactance and other factors should be considered. Generally, the tongue width is about 1.5 ~ 2 times the stack thickness. In this example, the EI sheet with a width of 66mm is selected, and the stack thickness is 2.7cm

5. Number of turns of primary and secondary windings

5.1 calculate the number of turns per volt (W0)

make the pointer exactly coincide with the zero line of the dial

in the formula, f AC frequency (Hz) B magnetic flux density (T) s iron core cross-sectional area (cm2)

b value will also be different depending on the iron core material. See the following table for general silicon steel sheet material and its value:

in this example, H23 sheet is selected, and B is 1.42, Then

5.2 number of primary turns (W1)

w1=220w0=1160 (T)

5.2 number of secondary turns (W2)

in the formula, 1.2 is the coefficient of adding filter capacitor after rectification, and 1.12 is to compensate for the secondary voltage drop caused by winding impedance after load. Generally, the number of secondary turns should be increased by 5% ~ 25% (the larger value of small output power)

so far, the design of the transformer has been basically completed

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