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Transformer Design Details

Transformer Design Details

(Summary description)The design process of the transformer includes five steps: 1. Determine the turns ratio of the primary and secondary sides; 2. Determine the number of turns of the primary and secondary sides; 3. Determine the wire diameter of the winding; 4. Determine the number of wire strands of the winding; 5. Calculate the window area.

Transformer Design Details

(Summary description)The design process of the transformer includes five steps: 1. Determine the turns ratio of the primary and secondary sides; 2. Determine the number of turns of the primary and secondary sides; 3. Determine the wire diameter of the winding; 4. Determine the number of wire strands of the winding; 5. Calculate the window area.

Information

The design process of the transformer includes five steps: 1. Determine the turns ratio of the primary and secondary sides; 2. Determine the number of turns of the primary and secondary sides; 3. Determine the wire diameter of the winding; 4. Determine the number of wire strands of the winding; 5. Calculate the window area.
(1) Primary and secondary side transformation ratio
In order to improve the utilization rate of the high-frequency transformer, reduce the current of the switch tube, reduce the reverse voltage of the output rectifier diode, reduce the loss and reduce the cost, the transformation ratio of the primary and secondary sides of the high-frequency transformer should be as large as possible.
In order to obtain the required voltage at any input voltage, the transformation ratio of the transformer should be selected according to the lowest input voltage. Select the maximum duty cycle of the secondary side as , then the minimum value of the secondary side voltage can be calculated as: , where is the maximum output voltage, is the on-state voltage drop of the output rectifier diode, and is the DC voltage drop on the filter inductor. The transformation ratio of the primary and secondary sides is: .
(2) Determine the number of turns of the primary side and the secondary side
First select the magnetic core. In order to reduce the iron loss, according to the switching frequency, refer to the magnetic core material manual to determine the maximum working magnetic density, the effective magnetic permeability cross-sectional area of ​​the magnetic core, and the window area. Then the number of turns on the secondary side of the transformer is: . According to the number of turns on the secondary side and the transformation ratio, the number of turns on the primary side can be calculated as: .
(3) Determine the wire diameter of the winding
When selecting the wire diameter, the skin effect of the wire should be considered. The so-called skin effect means that when alternating current flows through the wire, the current distribution on the cross section of the wire is uneven, the current density in the middle part is small, and the current density in the edge part is large, which reduces the effective conductive area of ​​the wire and increases the resistance. Under the condition of power frequency, the skin effect has less influence, but it has more influence at high frequency. The reduction in the effective conductive area of ​​a wire is generally expressed in terms of penetration depth. The so-called penetration depth refers to the radial depth when the current density drops to 0.368 of the current density on the surface of the wire (ie: ). , in the formula, , is the magnetic permeability of the wire, and the relative permeability of copper is , that is: the magnetic permeability of copper is the magnetic permeability in a vacuum, is the conductivity of the wire, and the conductivity of copper is .
In order to effectively use the wire and reduce the influence of the skin effect, the wire diameter of the wire is generally required to be less than twice the penetration depth, ie . If the wire diameter of the winding is required to be larger than the maximum wire diameter determined by the penetration depth, a small wire diameter wire can be used for multi-strand winding or a flat and wide copper skin can be used for winding, and the thickness of the copper skin should be less than twice. penetration depth.
(4) Determine the number of wire strands of the winding
The number of wire strands in the winding is determined by the maximum RMS current flowing in the winding and the wire diameter of the wire. After considering the skin effect to determine the wire diameter of the wire, let's calculate the maximum rms current flowing in the winding.
The number of wire strands of the primary winding: the maximum value of the RMS current of the primary side of the transformer, then the number of wire strands of the primary winding (in the formula, J is the current density of the wire, generally J=3~5, is the conductivity of each wire area.).
The number of conductive strands of the secondary winding: ①Full-bridge mode: The transformer has only one secondary winding. According to the current relationship between the primary and secondary sides of the transformer, the maximum value of the RMS current on the secondary side is: ; ②Half-wave mode: The transformer has two secondary sides winding, each load winding provides half-cycle load current, so its effective value is ( is the maximum output current). Therefore, the number of strands in the secondary winding is
(5) Accounting window area
After calculating the number of turns of the primary and secondary sides of the transformer, the wire diameter and the number of strands, it is necessary to calculate whether the window area of ​​the magnetic core can be wound down or whether the window is too large. If the window area is too small, the magnetic core is too small, and a larger magnetic core should be selected; if the window area is too large, the magnetic core is too large, and a smaller magnetic core should be selected. After re-selecting the magnetic core, recalculate until the selected magnetic core is basically suitable.

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