Engineer sharing: Detailed design steps of flyback transformer inductor (Part 2)

Calculate the air gap width (Al_value) and the number of primary winding turns and wire diameter (Basic design steps - transformer structure design)

The air gap width, Al_value, number of primary winding turns, and wire diameter are determined after repeatedly performing the following steps: "temporarily determine the air gap width" → "calculate Al_value" → "calculate the number of primary winding turns" → "calculate the magnetic flux density" → "calculate the copper loss of the primary winding". There are two methods for this operation: using the core characteristic data provided by the core manufacturer and calculating everything.

When obtaining from characteristic data, use the graph shown below. First, provisionally set the core air gap width, and then obtain Al_value from the graph on the right side of Figure 1-7.

When calculating Al_value from the air gap width using a calculation formula, use the following formula.

Next, use the following formula to calculate the number of primary side winding wire turns.

接下来，从图1-7左边的图表中，检证磁束密度。图表中的NI[AT是指，NI[AT]=一次卷线卷数N1[Turn]×一次卷线电流峰值IN1P[A]。算出的结果在图表中的20%的线的左下侧，磁束密度将为变低。正确的，用以下公式算出磁束密度。此结果，将会比1.1.7項中初期设定的最大磁束密度要小。

接下来，从1.3.1項中算出的一次卷线断面积SN1[mm2]与1.3.3項中决定的变压器构造、变压器骨架的卷框宽及本项中所算出的一次卷线卷数N1中，用以下公式，算出一次卷线的线径。

The number of turns of the wire at one time, including the number of turns of the folded-back portion, should be one more than the necessary number of turns. N1+1.

The balance between the wire diameter and the number of parallel connections is determined by the line area ratio and the winding copper loss. As long as the line area ratio does not exceed 100%, it is better to make it as high as possible, and the winding copper loss is better to keep it as small as possible.

Key points of design: current density and copper loss

电流密度始终作为设计的标准来考虑，最终根据卷线铜损来决定卷线线径。电流密度的参数中，没有考虑电线的长度，即使是暂定同样的电流密度，也会因并联数与线径的组合使卷线铜损出现2倍左右的差。铜损的目标是，所有的卷线的铜损在1.3.3.3項中所算出的容许磁芯损失2/ 3以下。但是，根据冷却条件、卷线构造的不同而各有所异，因此最终以实测数据为基准进行调整。

Calculating the core loss (Basic design steps - transformer structure design)

磁芯的损失，用如产品目录上所记载的如例图5所示的磁束密度-频率―磁芯损耗图表来算出。为了算出磁芯损耗的磁束密度，要提前算出连续最低输入电压、连续最大输出电流时。具体的，将在1.3.3.2项的磁束密度算出式中的一次卷线电流峰值IN1P[A]切换成为了求出1.3.1項中一次卷线实效电流算出的a的值，因此可以知道连续最低输入电压时的最大磁束密度。另外，可根据此磁束密度和图1-8知道磁芯损耗。

Key points of design: Correction of chart reading

・The vertical axis represents core loss per unit volume, so the value obtained from this graph must be added with the effective volume of the core actually used.

・The core loss obtained from this graph is based on the assumption that the magnetic flux changes in a sinusoidal waveform. In the case of a flyback transformer where the magnetic flux density changes in a triangular waveform, the loss will be greater than the data obtained from the graph.

・The core loss obtained from this graph is based on the assumption that the magnetic flux density changes periodically in both positive and negative directions. If the direction of the magnetic flux is unidirectional and the ratio of increase and decrease is small, the loss will be smaller than the data obtained from this graph.

Key points of design: core loss standards

・The core loss standard is 2 to 3% of the rated output power. However, due to cooling conditions, there is no problem even if it exceeds this range. In addition, when the output power is large, it is necessary to reduce the ratio further than this standard.

Next, as shown in Figure 1-9, use the temperature rise-total loss characteristic graph provided in the data sheet of the core manufacturer to calculate the maximum allowable loss. Use this value to determine whether the core loss calculated above is correct. For example, in Figure 1-9, under the condition of an ambient temperature of 50°C, if you want to use the core temperature below 90°C, the maximum allowable loss is 1.4W or less.

Calculate the number of secondary winding turns and wire diameter (Basic design steps - transformer structure design)

以在1.2.2項中算出的卷线比为基准，从在1.3.3.2项中算出的一次卷线卷数算出二次卷线卷数。

The secondary winding wire diameter is the same as the primary winding wire diameter calculated in Section 1.3.3.2 and is calculated using the following formula. For other precautions, refer to Section 1.3.3.2.

Calculating the withstand voltage of semiconductor components (basic design steps)

Determine the withstand voltage of the switching component Q1 and the rectifying component D1 in Figure 1-10.