1. Toroidal transformer is a large type of electronic transformer. It has been widely used in household electrical appliances and other electronic equipment with high technical requirements. Its main purpose is as a power transformer and an isolation transformer. The toroidal transformer has a complete series in foreign countries and is widely used in computers, medical equipment, telecommunications, instruments and lighting. In the past ten years, China's toroidal transformers have grown from scratch, and have formed a considerable scale of production so far, in addition to meeting domestic demand, but also a large number of exports. Domestically, it is mainly used in audio equipment and automatic control equipment for home appliances, as well as quartz lighting. Due to its excellent performance and price ratio, good output characteristics and anti-interference ability, the toroidal transformer is a competitive electronic transformer. This article introduces its characteristics.
2. Characteristics of toroidal transformer
The core of the toroidal transformer is made of high-quality cold-rolled silicon steel sheet (the sheet thickness is generally 0.35 mm or less), which is seamlessly rolled, which makes its core performance superior to that of the conventional laminated core. The coil of the toroidal transformer is evenly wound around the core, and the direction of the magnetic line generated by the coil is almost completely coincident with the magnetic circuit of the core. The excitation energy and the core loss are reduced by 25% compared with the laminated type, thereby bringing the following series: The advantages.
1) High electrical efficiency, no iron gap, stacking factor can be as high as 95% or more, core magnetic permeability can be 1.5~1.8T (the laminated core can only take 1.2~1.4T), the electrical efficiency is up to 95%, no load The current is only 10% of the laminated type.
2) The small size and light weight toroidal transformer can reduce the weight of the laminated transformer by half. As long as the cross-sectional area of ​​the core is equal, the toroidal transformer can easily change the length, width and height of the core, and can design the required size.
3) The magnetic interference is small. The toroidal core has no air gap, and the winding is evenly wound around the annular core. This structure leads to small leakage magnetic flux and small electromagnetic radiation. High-sensitivity electronic equipment can be used without additional shielding. On, for example, on low-level amplifiers and medical devices.
4) The vibration noise is small. The iron core has no air gap to reduce the noise of the core induction vibration. The winding evenly surrounds the toroidal core, effectively reducing the "click" caused by magnetostriction.
5) The operating temperature is low. Because the iron loss can be 1.1W/kg, the iron loss is small, the core temperature rises low, and the winding heats up well on the lower temperature core, so the transformer temperature rise is low.
6) Easy to install the toroidal transformer Only one mounting screw in the center is particularly easy to install and disassemble in electronic equipment.
3. Classification of toroidal transformers
Toroidal transformers can be divided into three types: standard type, economic type and isolated type.
1) Standard power transformer product series capacity 8 ~ 1500VA, with a small voltage adjustment rate, full load operation temperature rise of only 40 ° C, allowing short-time overload operation, suitable for demanding applications. Class B (130 ° C) polyester film insulation between the primary and secondary windings requires at least three layers of insulation tape to withstand 4000V, 1min withstand voltage test.
2) The economical power transformer product series capacity is 50-1500VA, which strives to reduce the cost based on the guaranteed performance. It is suitable for continuous operation without overloading. The operating temperature rise is 60 °C, and the insulation grade is A grade (105 °C). ), the output voltage error is less than 3% at full load.
3) The isolation transformer product series capacity is 50~1000VA, which can be divided into two series for industrial and medical equipment. The isolation transformer is mainly focused on its insulation performance. The polyester film with Class B insulation between the primary and secondary is at least 4 layers, the breakdown voltage is greater than 4000V, and all primary leads must be double insulated wires. The maximum temperature rise of the transformer is below 45 °C. In addition to meeting the above requirements, the medical isolation transformer must also comply with the UL544 standard, that is, the primary and secondary windings should have thermal protection, and the distance between the winding and the grounded copper shield should be greater than 13 mm.
In addition, the isolation transformer for medical use also requires a temperature protection switch on the primary winding. When the temperature of the core reaches 120 °C, the temperature protection switch is turned off. When the temperature returns to normal, the switch is automatically reset and closed. Since the toroidal transformer has no air gap and the coil is evenly wound around the core, theoretically, the magnetic leakage is small and there is no coil radiation. However, since the toroidal transformer has no air gap and has poor anti-saturation capability, it is prone to saturation when there is a DC component in the mains, and a strong magnetic leakage is generated. In many domestic areas, the waveform distortion of the mains is serious, so many users use the toroidal transformer and feel no better or even worse than the EI transformer.
The so-called toroidal transformer has no leakage. It is because the media is misleading, or because the manufacturer has made a request for commercial propaganda. The magnetic leakage of the toroidal transformer is extremely low. It is only established when the mains waveform is a strict sine wave. In addition, the toroidal transformer will also have strong electromagnetic leakage at the lead. Therefore, the leakage flux of the toroidal transformer also has a certain directionality. When the actual installation is performed, the toroidal transformer is rotated to obtain the highest signal-to-noise ratio at a certain angle. If conditions permit, consider installing a shield for the transformer and properly grounding it. The metal cover can only be made of iron. Generally, metals such as copper and aluminum have only electrical shielding and no magnetic shielding. They cannot be used as transformers. Shield.
The above analysis is based on the selection of materials and excellent production of transformers. Actually, most of the commercially available transformer products are not strictly designed according to industry norms due to cost pressure and competition, and sometimes even cut corners, and more unpredictable factors are analyzed. The first is the quality of the core material. Many companies use H50 iron cores, scraps and even noisy soft iron with low magnetic permeability to make transformers, resulting in high no-load current of the transformer, excessive iron loss and serious no-load heat generation. Reduce the cost, and at the same time cover the excessive voltage regulation caused by the high iron loss, greatly reduce the number of primary and secondary coil turns, reduce the copper loss to reduce the voltage adjustment rate, which further increases the air Current carrying, while the no-load current is too large will directly lead to increased magnetic leakage.
The toroidal transformer problem is more complicated. The regular toroidal transformer core is tightly wound from a strip of equal width silicon steel. Or for cost reasons, most low-cost toroidal transformers use several or even dozens of silicon steel strips for splicing, even using edge jagged edges to wrap, after winding, the machine is flat, due to the toroidal transformer coil package Around the iron core, it is difficult to find without destructive anatomy. Machining has severely damaged the lattice arrangement of silicon materials and the insulation between adjacent silicon steel strips. Such a toroidal transformer will be greatly reduced in performance and magnetic leakage characteristics, and even if it is annealed, it cannot compensate for serious defects in quality. .
Stray electromagnetic waves are mainly from the power output wires of active speakers, speakers and power dividers, wireless transmitters and computer mainframes. The reasons for this are not discussed in depth here. Stray electromagnetic waves are similar in transmission and induction to power transformers. The frequency range of stray magnetic fields is very wide. It is typical for spurious electromagnetic interference that useful speakers reflect that active speakers receive local radio broadcasts.
Another source of interference that needs attention is the rectifier circuit. After the filter capacitor enters the normal state, the charge is concentrated only on the peak of the AC current. The charge waveform is a strong pulse with a narrow width. The larger the capacitance, the greater the pulse strength. From the perspective of electromagnetic interference, the filter capacitor is not larger. Well, the trace between the rectifier and the filter capacitor should be as short as possible, and as far as possible away from the power amplifier circuit, PCB space is not allowed to use the ground envelope as much as possible.
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