Analysis and design of high frequency transformer in high frequency link
Abstract: the high frequency link inverter technology using high frequency transformer instead of the traditional inverter cumbersome power frequency transformer, greatly reducing the volume and weight of the inverter. In the design of hardware circuit of high frequency link, the high frequency transformer is an important part. The design process of high frequency transformer is described. The experimental results show that the design meets the requirements.
Key words: high frequency chain; high frequency transformer; inverter
0 Introduction
MESPELAGE proposed a new concept of high frequency link inverter technology in 1977. High frequency link inverter technology and conventional inverter technology is the biggest difference is that the use of high frequency transformer to achieve the input and output of the electrical isolation, reducing the size and weight of the transformer. In recent years, high frequency link technology has attracted more and more people's interest.
1 Overview
Figure 1 is the traditional inverter block diagram. The disadvantage is that the bulky power frequency transformer and filter inductor are used, which lead to low efficiency, high noise and poor reliability. In addition, the harmonic content is large, the waveform distortion is serious, and the demand of the high quality sine wave far.
Fig. 1 traditional inverter
Figure 2 shows the block diagram of the voltage source high frequency link inverter, the program is the most advanced research program, is also used in this paper. The use of this program has a series of advantages, such as small high-frequency transformer instead of power frequency transformer; only two power conversion; sine wave quality is high; control flexibility and so on. The high frequency transformer is the core component of the high frequency link, which is responsible for the isolation and transmission power. The performance of the high frequency transformer is directly determined by the performance of the inverter. Unqualified transformer Wen Shenggao, low efficiency, serious leakage, the output waveform distortion, a direct impact on the stability and reliability of the circuit, and even damage to the switch devices, leading to the failure of the experiment.
Fig. 2 voltage source high frequency link inverter
2 design of high frequency transformer
Design high frequency transformer should start from the magnetic core. Switching power supply transformer magnetic core is used in low magnetic field of soft magnetic material, it has a high permeability, low coercivity, high resistivity. High magnetic permeability, a certain number of turns, through the small magnetizing current can withstand high voltage, therefore, a certain power in the output request, can reduce the volume of the magnetic core. The magnetic core coercivity is low, the hysteresis area is small, then the iron loss is also low. High resistivity, the eddy is small, the iron loss is small. A variety of magnetic core physical properties and prices are listed in table 1. Ferrite material is a composite oxide sintered body, the resistivity is very high, suitable for high frequency use, but the Bs value is relatively small, often used in switching power supply. Ferrite material is used in this paper.
Table 1 Comparison of various magnetic core characteristics table
Magnetic core type amorphous alloy thin silicon steel sheet slope mullite ferrite
Low iron loss high low
High permeability and low permeability
Saturated magnetic density high
The effect of temperature on the small
Processing is difficult and easy
Low in price
The design of high frequency transformer usually adopts two methods: the first one is to first calculate the product of AP AW and core core window area effective area Ae (AP=AW * Ae, said core area product), according to the AP value, the look-up table to find the necessary number of magnetic materials; second is to first calculate the geometric parameters, check table to find core number, then design. This paper discusses in detail how to use the AP method to design the high frequency transformer.
The original side NP turn, side Ns turn of the transformer, the NP turn on the voltage V1 switch work, according to Faraday's law, there are
V1=KffsNPBWAe (1)
Kf for the waveform coefficient, that is, the effective value and the average value of the ratio of sine wave for 4.44, Fang Bo 4;
FS for working frequency;
BW for working magnetic flux density.
NP= (2)
The core area of a window window using AW multiplied by coefficient Ko (usually from 0.4) for the effective area, the area of the primary winding NP occupy the window area of NPAP 'and Ns occupy the side of the winding window area and NsAs', i.e.
NsAs' KoAW=NPAP '(3)
Type: AP 'and As' respectively for the primary winding and the secondary winding of each turn of the cross-sectional area.
The area occupied by each turn is related to the current value of I and the current density J, as shown in the formula (4).
'= AP
As' = (4)
The type (4) into the equation (3), then
KoAW=
That is, AWAe= (5)
The current density J directly affects the temperature rise, and it also affects the AWAe, which is expressed by the formula (6).
J=KJ (AWAe) X (6)
KJ is the current density coefficient; X is a constant, which is determined by the magnetic core.
If the apparent power transformer is PT=V1I1V2I2.
AWAe=
That is, AP= (7)
Type: AP units for the CM4, the rest of the units for the international unit system.
The apparent power varies with the line structure. As shown in figure 3. Transformer efficiency ETA, in Figure 3 (a).
PT=PoPi=Po=Po
In Figure 3 (b)
PT=Po
In Figure 3 (c)
This paper uses PT=Po in Figure 3 (b) structure, VDC=24V, Po=250W, =0.95 is set.
PT=Po = 617W
(a) two bridge rectifier
(b) two full wave rectifier
(c) a push-pull two half wave rectifier
Figure 3 the relationship between the power and the line structure
If the use of E type magnetic core, allowing the temperature rise of 25 degrees Celsius, there are KJ=323, X= - 0.14. A secret for 0.35T saturation at high temperature, considering saturation close down, at the same time, in order to prevent the saturation of the closing moments from the high frequency transformer, saturation magnetic induction 1/3 for magnetic transformer is dense, i.e. BW=0.117T. The working frequency is 20kHz, which can be got from the formula (7).
AP= = 6.65 (CM4)
Take 10% of the margin, namely AP=6.65 * (1 + 10%) = 7