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What should we pay attention to when designing high-frequency transformers?
Feb 22, 2018


What should we pay attention to when designing high-frequency transformers?


High frequency transformers are relative to audio and power frequency transformers. We refer to transformers that operate at frequencies above the audio frequency as high-frequency transformers. In general, we classify high-frequency transformers into two types, one called a single-frequency or narrow-band high-frequency transformer, whose operating frequency is a single frequency or a narrower frequency band, such as an inverter transformer and an oscillator transformer. The other is called a broadband transformer, which refers to a transformer operating in a relatively wide or wide frequency range, such as an impedance transformer transformer, a communication transformer, and the like. The main purpose of high-frequency transformers is to be used for impedance matching between two amplifiers, between the load and the feeder or between the feeder and the load. Compared with the power transformer, the design of the high-frequency transformer is more complicated; the materials used and core types are also better than Power transformers come much more. When designing high-frequency transformers, the following issues must be noted.


When the starting and ending windings are on the same side of BOBBIN. In principle, the outlet is laid out in one line and one groove. Entry and exit lines when using BOBBIN groove outlet. If multiple groups of the same PIN can use the same groove or adjacent groove outlets, care must be taken to prevent short circuiting during soldering and sleeve mounting. Except when there is a special prescription around the drawing surface. The principle of winding the BOBBIN winding area evenly and neatly is required during winding. The map shall prevail. The Teflon sleeves on the entry and exit lines must be flush with the BOBBIN notch (or at least 2/3 higher) and routed out of the BOBBIN groove to prevent the wire from tearing due to excessive tension on the sleeve. However, if LPIN is wrapped horizontally.

 

When high-frequency transformers need to add acetate cloth as a wall tape. Therefore, it is required that the overlap of acetic acid cloth above 2TS can not exceed 5mm. The acetic acid cloth wrapped in a circle should only be packaged with 0.9T to make a good penetration of the bottom layer. The width of acetic acid cloth is related to the requirements of high-frequency transformer safety regulations. VED winding Method ACT width 3.2mm package on both sides and to be added TUBE. Method: PIN end 6mm/4.8mm/4.4mm/4mm; TOP end 3mm/2.4mm/2.2mm/2mm without TUBE. Copper wire can not be on the winding If there is a casing wall, the casing must extend into the wall beyond 3mm.

 

Designing a high-frequency transformer must understand the working circuit in which the transformer is located. The high-frequency transformer operates in the amplifier circuit and is an integral part of the amplifier. In the analysis and design of high-frequency transformers, it must be combined with the amplifier circuit to determine the relevant parameters. For example, the secondary parameters of the transformer (voltage, current), in the power transformer, directly specify the secondary voltage and current. In a high-frequency transformer, the impedance matching principle is used, and the turns ratio is first determined according to the load impedance, and then the voltage and current are calculated: or the secondary voltage and current are calculated based on the output power and the load impedance. When planning a transformer, you should know in advance the circuit topology, operating frequency, input and output voltage, output power or output current, and environmental conditions. Together, you should also know how much loss your planned transformer will allow. Transformers are always planned to meet the worst-case conditions to ensure that the planned transformers will operate satisfactorily under any conditions. The primary purpose of a power transformer in a switching power supply is to transmit power. Instantly transmit the power of one power supply to the load.

 

At the beginning of planning switching power supply, based on output power, output voltage and output voltage conditioning scale, input voltage, environmental conditions and other factors, the planner gives a reachable power based on experience or with reference to the same type of prototype, thus obtaining the total loss value. . The total loss is then distributed to each loss component to obtain the promised loss of the transformer. The loss of the transformer makes the temperature of the coil and the core increase, and the temperature in the middle of the coil is close to the surface of the core. This maximum “hot” restricts the temperature rise of the transformer. According to equation (6-15), the temperature rise ΔT(°C) is equal to the transformer thermal resistance Rth(°C/W) multiplied by the power loss P(W):

 

In general industrial products, the civil ambient temperature is up to 40°C. The highest internal temperature of the high-frequency transformer is limited by the magnetic core and the insulating material. If the ferrite is used for insulation with Class A or Class E, the temperature rise of the transformer is usually set at 40-50°C. Its internal hot temperature is 100°C. If the temperature rises too high, larger cores should be used. If a smaller volume is required, alloy cores and high insulation grade insulation materials should be used to increase the temperature but lower the power. Transformer losses are divided into core losses and coil losses and are difficult to estimate accurately. Core losses include hysteresis and eddy current losses. Coil losses include DC and high frequency losses. The primary cause of transformer temperature rise is steady-state losses, not transient losses.


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