Toroidal transformer classification and application

The main use of toroidal transformers is as power transformers and isolation transformers. There are complete series of toroidal transformers abroad, which are widely used in computers, medical equipment, telecommunications, instruments and lighting. Toroidal transformers have excellent performance-price ratio, good output characteristics and anti-interference ability.

Classification of toroidal transformers: standard type, economical type and isolated type

1) The standard power transformer product series has a capacity of 8 to 1500VA, a small voltage regulation rate, a full-load operating temperature rise of only 40°C, and allows short-term overload operation, which is suitable for high-demand applications. Class B (130°C) polyester film insulation is used between the primary and secondary windings, requiring at least three layers of insulation tape, and can withstand an AC 4000V, 1min withstand voltage test.

2) The economical power transformer product series has a capacity of 50 to 1500VA. It strives to reduce the cost while ensuring performance. It is suitable for continuous operation without overload. The operating temperature rise is 60°C, the insulation material grade is Class A (105°C), and the output voltage error is less than 3% when fully loaded.

3) The capacity of the isolation transformer product series is 50~1000VA, which can be divided into two series: industrial and medical equipment. The isolation transformer focuses on its insulation performance. The primary and secondary are wrapped with at least 4 layers of Class B insulation polyester film. The breakdown voltage is greater than 4000V, and all primary leads must use double-insulated wires. The maximum temperature rise of the transformer is less than 45℃. In addition to meeting the above requirements, medical isolation transformers must also meet 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 13mm.

In addition, medical isolation transformers are also required to have a temperature protection switch on the primary winding. When the core temperature reaches 120°C, the temperature protection switch is disconnected. When the temperature returns to normal, the switch automatically resets and closes. Since there is no air gap in the toroidal transformer and the coil is evenly wound around the core, the magnetic leakage is theoretically very small and there is no coil radiation. However, since there is no air gap, the toroidal transformer has poor anti-saturation ability. When there is a DC component in the mains, it is easy to saturate and produce strong magnetic leakage. In many areas of the country, the mains waveform is seriously distorted, so many users feel that the toroidal transformer is not better than the EI transformer, or even worse.

The so-called toroidal transformer has no leakage, which is misleading by the media or fabricated by the manufacturer for commercial promotion. The claim that the magnetic leakage of toroidal transformer is extremely low is only true when the mains wave is a strict sine wave. In addition, the toroidal transformer will also have strong electromagnetic leakage at the lead wire, so the leakage magnetic flux of the toroidal transformer is also directional. When actually installing the toroidal transformer, rotate the toroidal transformer to obtain the highest signal-to-noise ratio at a certain angle. If conditions permit, consider installing a shielding cover for the transformer and properly ground it. The metal cover can only be made of ferrous materials. General metals such as copper and aluminum only have electrical shielding but no magnetic shielding, and cannot be used as transformer shielding covers.

The above analysis is based on the selection of transformer materials and excellent production. In fact, most transformer products on the market are not strictly designed according to industry specifications due to cost pressure and competitive needs, and sometimes even cut corners. There are many unpredictable factors in the analysis. The first is the quality of the core material. Many companies use H50 cores with low magnetic permeability, scraps, or even mixed soft iron to make transformers, resulting in high no-load current, excessive iron loss, and serious no-load heating of the transformer. In order to reduce costs and cover up the problem of excessive voltage regulation caused by high iron loss, this type of transformer greatly reduces the number of turns of the primary and secondary coils to reduce the voltage regulation rate by reducing copper loss. This practice further increases the no-load current, and the excessive no-load current will directly lead to increased magnetic leakage.

The problem of toroidal transformers is more complicated. The core of a regular toroidal transformer is made of a tightly wound silicon steel strip of equal width. Still for cost reasons, most low-priced toroidal transformers use several or even dozens of silicon steel strips to splice, or even use scraps with jagged edges to wind, and then use machine tools to flatten them after winding. Since the coil of the toroidal transformer is wrapped around the core, it is difficult to find without destructive dissection. Mechanical processing has serious damage to the lattice arrangement of silicon materials and the insulation between adjacent silicon steel strips. Such toroidal transformers will greatly reduce both performance and leakage magnetic characteristics, and even annealing cannot make up for the serious quality defects.

Stray electromagnetic waves mainly come from the power output wires of active speakers, speakers and power dividers, wireless transmitters and computer hosts. The causes are not discussed in depth here. The transmission and induction forms of stray electromagnetic waves are similar to those of power transformers. The frequency range of stray magnetic fields is very wide. Some users have reported that their active speakers inexplicably receive local radio broadcasts, which is a typical example of stray electromagnetic wave interference.

Another interference source that needs attention is the rectifier circuit. After the filter capacitor is turned on and enters the normal state, the charging is concentrated only at the peak of the AC power. The charging waveform is a strong pulse with a narrow width. The larger the capacitance, the greater the pulse intensity. From the perspective of electromagnetic interference, the larger the filter capacitor, the better. The wiring between the rectifier tube and the filter capacitor should be shortened as much as possible and kept as far away from the power amplifier circuit as possible. If the PCB space does not allow, try to use the ground wire envelope.