The Basic Design Principles of High Frequency Transformer That Electronics Enthusiasts Muct Know !

Label:Magnetic Core, Monolithic Switching Power Supply, Leakage Inductance, Design Principles

Mar 11, 20219420

The Basic Design Principles of High Frequency Transformer That Electronics Enthusiasts Muct Know !

The expanding demand for electronic equipment is fueling the need for more high frequency transformers.

At JRPanel, we focus on high-end products for 20+ years, and provide high-quality products and services to customers who have high requirements for quality. 

Today, JRPanel would like to share some basic principles that help to design a high frequency transformer for all the electronics enthusiasts.

The design of a high frequency transformer should first start with the Magnetic Core.

Switching power supply transformer cores are mostly soft magnetic materials used in low magnetic fields. They have high permeability, low coercivity, and high resistivity.

A magnetic core with high magnetic permeability can withstand a higher applied voltage through a small excitation current when the number of turns of the coil is constant. Therefore, when a certain output power is required, the volume of the magnetic core can be appropriately reduced.

The coercivity of the magnetic core is low, and if the hysteresis area is small, the iron loss is also small.

If the resistivity of the magnetic core is high, the eddy current is small and the iron loss is small. The ferrite material is a composite oxide sintered body with high resistivity, suitable for high frequency use, but the Bs value is relatively small, and it is often used in switching power supplies.

The high-frequency link inverter technology replaces the bulky power-frequency transformer in the traditional inverter with a high-frequency transformer, which greatly reduces the size and weight of the inverter. In the hardware circuit design of the high-frequency chain, the high-frequency transformer is an important part.

The design of high frequency transformer usually adopts two methods:

1. First we may obtain the product AP of the core window area AW and the core effective cross-sectional area Ae (AP=AW×Ae, called the core area product), and according to the AP value, we find the number of the required magnetic material.

2. The second is to find out the geometric parameters first, then we look up the table to find out the core number, after which we do the design.

After understanding the design principles of the magnetic core, then we will introduce the respective design points according to the type of transformer. Due to limited space, this episode will first talk about the Monolithic Switching Power Supply high-frequency transformer.

The high-frequency transformer is the core component of the monolithic switching power supply. In view of the particularity of the design of this high-frequency transformer, the method of reducing its loss and suppressing audio noise is specifically explained for the reference of high-frequency transformer designers.

The monolithic switching power supply integrated circuit has the advantages of high integration, high cost performance, simplest peripheral circuit, best performance index, etc., and can form a high-efficiency isolated switching power supply without a power frequency transformer.

From 1994 to 2001, various series of monolithic switching power supply products such as TOtch, TOtch-Ⅱ, TOtch-FX, TOtch-GX, Tintch, Tintch-Ⅱ, etc. have been successively launched internationally, and they have now become the the optimal integrated circuit for medium and small power switches, precision switching power supply.

The high-frequency transformer is an important part of energy storage and transmission in the switching power supply. The performance of the high-frequency transformer in the monolithic switching power supply not only has a greater impact on the efficiency of the power supply, but also directly relates to other technical indicators and electromagnetic compatibility of the power supply. (EMC).

For this reason, a high-efficiency high-frequency transformer should have low DC loss and AC loss, low leakage inductance, small distributed capacitance of the winding itself and small coupling capacitance between each winding.

The DC loss of the high frequency transformer is caused by the copper loss of the coil. In order to improve efficiency, thicker wires should be selected as much as possible, and the current density J=4~10A/mm2.

The AC loss of a high-frequency transformer is caused by the skin effect of the high-frequency current and the loss of the magnetic core.

When high-frequency current passes through the wire, it always tends to flow from the surface, which reduces the effective flow area of the wire and makes the AC equivalent impedance of the wire much higher than the copper resistance. The penetration ability of high-frequency current to the conductor is inversely proportional to the square root of the switching frequency. In order to reduce the AC copper impedance, the radius of the wire must not exceed 2 times the depth of the high-frequency current.

Leakage inductance must be minimized when designing high-frequency transformers. Because the greater the leakage inductance, the higher the amplitude of the peak voltage generated, the greater the loss of the drain clamp circuit, which will inevitably lead to a decrease in power supply efficiency.

For a high-frequency transformer that meets insulation and safety standards, its leakage inductance should be 1% to 3% of the primary inductance when the secondary is open. To achieve the index below 1%, it will be difficult to achieve in the manufacturing process. The following measures can be taken to reduce leakage inductance:

Reduce the number of turns NP of the primary winding;

Increase the width of the winding (for example, select EE core to increase the width of the skeleton);

Increase the height and width ratio of the winding;

Reduce the insulation layer between each winding;

Increase the degree of coupling between the windings.

Now, we believe you have learned much design knowledgement from this episode. In next episode, JRPanel is going to share more design principles of different high frequency transformers. If something still confuses you, Click To Contact Us.