Design principle of high frequency power transformer core

1 Introduction

With the rapid development of electronic information industry, new requirements for high frequency switching power supply have been put forward. According to reports, the world's switching power supply market size has exceeded $10 billion [1]. Communications, computers and consumer electronics products are the three main switching power market. The large switching power supply market is mainly composed of two parts: AC/DC and DC/DC switching power supply. It is predicted that the global sales of AC/DC switching power supply will increase from $9 billion 100 million in 1999 to $12 billion 200 million in 2004, an average annual growth rate of 5.9%. Low power (0 ~ 300W) AC/DC will face the growth of consumer electronics and consumer electronics market; high power (750 ~ AC/DC) 1500W power supply will be oriented to strong growth in the telecommunications market. DC/DC power supply about 30% of the whole switch power supply market, but the rapid integration of computer and communication technology, led to the rapid growth of the DC/DC module power supply. Expected in the next few years, DC/DC power module growth rate will exceed AC/DC power, it is estimated that China in the next five years, DC/DC power module market growth will reach 15%, growth is mainly in the field of telecommunications. The development trend of switching power supply technology is high density, high efficiency, low noise, and surface mount technology. Either AC/DC or DC/DC power supply, in addition to the power transistor, main transformer, made of soft ferrite choke inductors and other (such as anti noise filter) is a very important element, the magnetic properties and the size is directly related to the power conversion efficiency and power density etc.. In the design of the transformer, mainly including the design of the winding and the design of the core. This paper intends to focus on the design of the main transformer core should be considered through the power, performance factor, thermal resistance and other parameters, and to reduce the total loss of the magnetic core material microscopic design should be considered.

2 power transformer magnetic core performance requirements and materials classification

In order to meet the requirement of switching power supply to improve efficiency and reduce the size and weight, a high flux density and high frequency and low loss transformer core is required. Although the amorphous soft magnetic alloy with high performance competition, but from the view of performance to price, soft ferrite material is still the best choice; especially in the high frequency field 100kHz to 1MHz, the new low loss high-frequency power ferrite material has its unique advantages. In order to maximize the use of magnetic core, for the larger power operating conditions of soft ferrite materials, in the high temperature range of work (such as 100 ~ 80 degrees C), should have the following the most important magnetic properties:

1) high saturation magnetic flux density or high amplitude permeability. Thus, the magnetic core of the transformer is provided with a large magnetic flux deviation under the specified frequency, and the result can be reduced to turn; this is also beneficial to the high frequency application of ferrite, because the cut-off frequency is proportional to the saturation magnetic flux density.

2) the total loss of the magnetic core in the operating frequency range. Under a given temperature rise, low core loss will allow a high pass power.

With the request is also a high Curie point, high resistivity, good mechanical strength, etc..

Newly released "soft ferrite materials classification" industry standards (equivalent to IEC61332:1995), the high magnetic flux density applications power ferrite materials are divided into five categories, see table 1. In addition to the requirements for the amplitude permeability and power loss of each type of ferrite material, the "performance factor" parameters (this parameter will be described below) are also presented. From PW1 to PW5 categories, the operating frequency is gradually improved, such as PW1 material for the frequency is 15 ~ 100kHz, mainly used in the flyback transformer core; PW2 material for the frequency is 25 ~ 200kHz, mainly used in switching power transformer core; PW3 material, application frequency is 100 ~ 300kHz; PW4 application of frequency is 300kHz ~ 1MHz; PW5 material for the frequency is 1 ~ 3MHz. Now the country has been able to produce the equivalent of PW1 ~ PW3 materials, PW4 materials can only be a small amount of trial production, PW5 material is yet to be developed.

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3 transmission power of transformer

As everyone knows, the transformer can transmit power Pth is proportional to the frequency of F, the maximum magnetic flux density Bmax (or allowable Delta B and magnetic flux offset) cross-sectional area of Ae, and is expressed as

Pth = CfBmaxAeWd (1)

In the formula, C and switching power supply circuit working type related coefficient (such as push-pull forward converter C=1; C=0.71; C=0.61; Wd Reverse Converter) for winding design parameters (including current density S, duty factor fCu, winding cross-sectional area of AN).

Here, we focus on the (fBmaxAe) parameters (not to discuss the design parameters of the winding Wd). Increasing the size of the magnetic core (Ae) can increase the power of the transformer, but the goal of the current switching power supply is to reduce the size and weight of the power supply. The assumed fixed temperature rise, for a given size of the magnetic core, is proportional to the frequency through the power approximation. Fig. 1 shows the relationship between the transmission power Pth and the frequency f of the transformer. To improve the switching frequency in addition to the application of fast transistor, but also by other circuits affected by restrictions, such as the rapid change of voltage and current, generate expanded harmonic lines in a switch circuit, caused by radio frequency interference, radiation power. To improve the working frequency of the transformer core, it is required to improve the loss of the high frequency magnetic core. Figure 1 N67 (SIEMENS) has the power loss is lower than N27 material, allowing the magnetic flux density offset Delta B bigger, so the transformer power transmission more. Figure 2 shows the relationship between the magnetic core loss and frequency. The relationship between the total loss of the magnetic core PL and the working frequency f and the working magnetic flux density B is expressed by the following formula:

PL=KfmBnVe (2)

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In the formula, n is the Steinmetz index, and the typical value is 2.5 for the power ferrite, and the exponent is m=1 ~ 1.3.