Flyback Power Supply Magnetic Core Calculation Method

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Flyback Power Supply Magnetic Core Calculation Method [Copy link]

 

Transformer primary peak current Ipk

Number of primary turns Np Number of secondary turns Ns

Ip_rms is the effective current of the primary winding

Is_rms is the effective current of the secondary winding

J Winding current density 6A/mm^2

Vor reflected voltage

Kw is the utilization rate of the core winding window 0.4

The window area occupied by the primary winding is:

Because there will be retaining walls, insulating tapes, gaps between wires, etc. during the winding process of the transformer, the window utilization rate of our core cannot reach 100%. According to the experience of some engineers, the window utilization rate of the core is generally around 0.4. Here we choose Kw=0.4.

Then the actual Aw1=Aw÷0.4

From the above formula, we can conclude that the higher the power, the larger the core selection, and the higher the frequency, the smaller the core selection. After calculating Ap according to the above formula, we can choose the corresponding core.

After the core is determined, the number of turns of the transformer can be calculated based on the cross-sectional area Ae value of the core.

Design transformer for calculation (CCM mode)

According to the input/output, switching frequency and selected core parameters, we can set the efficiency, maximum duty cycle, magnetic induction intensity change and other parameters, and then proceed to the next step of calculation to obtain the power, average and peak voltage and current, number of turns and inductance. The input peak current here is about 4 times the input average current. This is a rule of thumb, and can also be deduced based on the following figure:

Relationship between peak current and average current After confirming the number of turns, the thickness of the enameled wire can be directly determined without complicated calculations.

The wire diameter is the same as that of conventional resistors, and there are fixed values. Just remember several commonly used fixed wire diameters. Here, the primary current is relatively small, so you can directly choose φ0.25 per strand. The auxiliary winding is φ0.25 per strand. The main output winding is φ0.4 or 0.5 three strands. Don't choose a thicker one, otherwise the hardness of the enameled wire will make it difficult for the operator to wind it.

Many people will do a return calculation to verify the window area of the transformer after "calculating" in this step. I personally think that return verification is redundant, because if the winding fails, the transformer factory will also give you feedback, and what you have verified may not necessarily pass in practice; after all, it is still closely related to the proficiency and sparseness of the actual winding process.

The next step is to determine the size of the input and output capacitors so that you can proceed with layout and board layout.

Disclaimer: This article is compiled from the Internet and is for communication and learning purposes only. For more power supply information, search [森木磊石] on WeChat.