Toroidal transformer and its application

Abstract: This paper introduces the characteristics and advantages of toroidal transformers, explains the matters that need to be paid attention to in applications, and introduces the design and calculation methods of toroidal transformers through examples. Keywords: transformer; toroidal transformer; design

1 Introduction

Toroidal transformer is a major type of electronic transformer, which has been widely used in household appliances and other electronic equipment with high technical requirements. Its main use is as a power transformer and isolation transformer. There are complete series of toroidal transformers abroad, which are widely used in computers, medical equipment, telecommunications, instruments and lighting.

In the past decade, my country has developed toroidal transformers from scratch, and has now formed a considerable production scale. In addition to meeting domestic demand, a large number of them are also exported. They are mainly used in home appliances such as audio equipment and automatic control equipment, as well as quartz lamp lighting.

The toroidal transformer is a competitive electronic transformer because of its excellent performance-price ratio, good output characteristics and anti-interference ability. This article intends to introduce its characteristics.

2 Characteristics of toroidal transformer

The core of the toroidal transformer is made of high-quality cold-rolled silicon steel sheets (thickness is generally less than 0.35mm), which is seamlessly rolled, making its core performance better than the traditional laminated core. The coil of the toroidal transformer is evenly wound on the core, and the direction of the magnetic lines of force generated by the coil is almost completely coincident with the magnetic circuit of the core. Compared with the laminated core, the excitation energy and core loss will be reduced by 25%, which brings the following series of advantages. 1) High electrical efficiency The core has no air gap, the stacking coefficient can be as high as 95% or more, the core magnetic permeability can be 1.5~1.8T (laminated core can only take 1.2~1.4T), the electrical efficiency is as high as 95% or more, and the no-load current is only 10% of the laminated core.

2) Small size and light weight. The weight of a toroidal transformer can be reduced by half compared to a laminated transformer. As long as the cross-sectional area of ​​the core is kept equal, the toroidal transformer can easily change the length, width and height ratio of the core, and can be designed to meet the required size.

3) Smaller magnetic interference: The toroidal transformer core has no air gap, and the winding is evenly wound on the toroidal core. This structure results in small magnetic leakage and electromagnetic radiation. It can be used in highly sensitive electronic equipment without additional shielding, such as low-level amplifiers and medical equipment.

4) The vibration noise is small. The core has no air gap, which can reduce the core

Table 1 Dimensions and weight of Canadian PLITRON toroidal transformer Output power P2/VA Transformer outer diameter Dw/mm Transformer height h1/mm Assembly height h2/mm Weight m/kg 85525300.25
156333370.35
307033380.45
508038450.9 809735391.00
1209543471.2 16011045501.8 22511050552.2 30011057622.6 50013563674.0 62514578835.0 75015080855.5 100016080856.3 1500200758011.7

Toroidal transformer and its application

Figure 1: Outline of toroidal transformer

The noise of induced vibration is reduced by the windings evenly and tightly wrapping the annular core, which effectively reduces the "buzzing" sound caused by magnetostriction. 5) Low operating temperature Since the iron loss can be as low as 1.1W/kg, the iron loss is very small, the core temperature rise is low, and the windings dissipate heat well on the core with lower temperature, so the transformer temperature rise is low.

6) Easy to install The toroidal transformer has only one mounting screw in the center, which makes it easy to install and disassemble quickly in electronic equipment.

3 Classification of toroidal transformers

According to foreign literature, toroidal transformers can be divided into three categories: standard type, economical type and isolated type. The characteristics of each type are

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 use in high-demand applications.

Class B (130°C) polyester film insulation is used between the primary and secondary windings and requires at least three layers of insulation tape to 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 comply with the UL544 standard, that is, the primary and secondary windings should have thermal protection, and the spacing 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.

The dimensions and weight of the standard toroidal transformer produced by Canadian PLITRON are listed in Table 1, and the appearance diagram is shown in Figure 1.

4 Issues that should be paid attention to in the application of toroidal transformers

4.1 Transformer power capacity

The power capacity of the transformer is the main basis for determining the core size. In many cases, the load of the transformer is intermittent, such as the power transformer in audio equipment. At this time, the volume and weight of the transformer are much smaller than when it is working continuously. As shown in Figure 2, the load section A is a smaller section than the entire section B. At this time, the working cycle of the transformer is much shorter than its thermal time constant. The rated power of the transformer can be calculated using formula (1). PN=PL(VA) (1)

Where: PN——transformer rated power (VA);

PL——Transformer load power (VA);

A——Load-on time;

B———Transformer working cycle.

4.2 Voltage Regulation

Voltage regulation is an important indicator to measure the load characteristics of transformers. Voltage regulation refers to the relative change in output voltage U2 when the input voltage remains unchanged and the load current rises from zero to the rated value, usually expressed in percentage.

Figure 2 Transformer intermittent load condition

Figure 3 Relationship between voltage regulation rate and output power of toroidal transformer

Figure 4 Relationship between toroidal transformer efficiency and load rate

Figure 5 Autotransformer circuit diagram

Figure 6 Relationship between temperature rise and load rate of toroidal transformer

Fractional expression, as shown in formula (2): ΔU = × 100% (2)

In the formula: ΔU——voltage regulation rate;

U20——no-load output voltage (V);

U2——Output voltage of transformer at rated load (V).

Table 2 lists the voltage regulation rate of the toroidal transformer of Canada PLITRON Company. Its characteristic curve is shown in Figure 3. The voltage regulation rate decreases as the transformer capacity increases.

Table 2 Toroidal transformer voltage regulation Transformer power/VA Voltage regulation ΔU/%
(Standard) (Economic) (Isolated)
822
1520
3018
501320.620.6
801216.116.1
1201113.713.7
160811.211.2
22579.39.3
30069.39.3
50047.47.4
62546.76.7
75046.56.5
100045.55.5
150044.9
4.3 Toroidal transformer efficiency

Since the transformer has iron loss and copper loss, the output power PO is always less than the input power Pi, and the efficiency η of the transformer is shown in formula (3). η= (3)

Figure 4 lists three sets of transformer efficiency curves of different powers. The efficiency increases significantly with the increase of capacity. For transformers with a capacity of more than 300VA, the efficiency can reach more than 95% under rated load.

4.4 Autotransformer

When only step-up or step-down is required, but isolation of the primary and secondary windings is not required, it is appropriate to use an autotransformer. The autotransformer has the advantages of small size, low cost, and high transmission power. The autotransformer wound with a toroidal core does not require insulation of the primary and secondary windings, so it is very convenient to process, smaller in size and weight, and lower in cost. It should be noted that the common end (COM) of the primary and secondary windings of the autotransformer must be connected to the neutral line for safety.

The autotransformer circuit is shown in FIG5 , and its rated power PAH is calculated according to formula (4).

PAH=PAO(UH－UL)/UH(VA)（4）

Where: PAO – autotransformer output power (VA);

UH——high voltage winding voltage (V);

UL - low voltage winding voltage (V).

4.5 Temperature rise problem

The temperature rise characteristic curve of the toroidal transformer is shown in Figure 6. It can be seen from Figure 6 that the temperature rise of the toroidal transformer is relatively low. For the standard series, even if it is overloaded by 120%, the temperature rise does not exceed 70°C.

The temperature rise of the transformer is determined by the iron loss and copper loss.

()

Toroidal transformer and its application

For laminated transformers, these two parts are basically equal, but because the toroidal transformer is wound with high-quality cold-rolled silicon steel sheets and has a good annealing process, its core loss is only (10-20)% of the total loss. Therefore, the temperature rise is mainly determined by the winding copper loss. A reasonable design is that the power consumption of the primary and secondary windings should be basically balanced.

Temperature rise is also closely related to the heat dissipation area. Since the temperature rise of the toroidal transformer core is low, the winding is evenly wound on the entire core, the heat dissipation area and heat dissipation conditions are relatively good, so a lower temperature rise can be obtained.

4.6 Closing current

Generally, transformers will generate a large closing impact current when they are closed, and toroidal transformers will cause a larger closing current due to the lack of air gaps and high magnetic permeability. Toroidal transformers below 300VA can be protected by general fuses, but in order to prevent the closing current from burning the fuse, the current of the selected fuse should be 8 to 10 times larger than the primary current of the transformer. Toroidal transformers above 300VA should consider using slow fuses or temperature fuses for protection. Sometimes, in order to reduce the impact current, the transformer flux density B value can be lowered.

4.7 Transformer and Rectifier Circuit

Most toroidal transformers used as power supplies are connected to rectifier circuits. The most commonly used rectifier circuits and the relationship between the transformer secondary voltage U2, secondary current I2 and DC voltage Ud and DC current Id are listed in Table 3 for reference during design.

Table 3 Rectification circuit and transformer parameters Circuit name Circuit diagram Transformer secondary voltage U2/V Transformer secondary current I2/A
Double rectifier circuit 0.8(Ud+2)1.8Id
Bridge rectifier circuit 0.8(Ud+2)1.8Id
Full wave center tap 1.7(Ud+1)1.2Id
5 Design calculation of toroidal transformer

The calculation method and steps are explained by designing a power transformer for a 50Hz quartz lamp, with a primary voltage U1=220V, a secondary voltage U2=11.8V, a secondary current I2=16.7A, and a voltage regulation rate ΔU≤7%.

1) Calculate the transformer secondary power P2

P2=I2U2=16.7×11.8=197VA(5)

2) Calculate the transformer input power P1 (assuming transformer efficiency η = 0.95) and input current I1 P1 === 207VA (6) I1 === 0.94A

3) Calculate the core cross-sectional area SS = K (cm2) (7)

Where: K - coefficient is related to transformer power, K = 0.6 ~ 0.8, take K = 0.75;

PO——average power of transformer, Po===202VA. Then S=0.75=10.66cm2, take S=11cm2.

According to the existing core specifications, the core size is: height H = 40mm, inner diameter Dno = 55mm, outer diameter Dwo = 110mm. Calculate the cross-sectional area of ​​the selected core S = H = × 40 × 10-2 = 11cm2

4) Calculate the number of turns per volt of the primary winding N10 and the number of turns N1N10 = (turns/V) (8)

Where: f——power frequency (Hz), f=50Hz;

B——Magnetic flux density (T), B=1.4T. Substituting N10==2.9 turns/V, taking N10=3 turns/V, then

N1=N10U1=3×220=660 turns.

5) Calculate the number of turns per volt of the secondary winding N20 and the number of turns N2N20 = (turns/V) (9) Substituting into N20, we get N20==3.23 turns/V, then

N2=N20·U2=3.23×11.8=38.1 turns, take N2=38 turns.

6) Select the wire diameter

Figure 7 Cross-section of toroidal transformer

The winding wire diameter d is calculated according to formula (10): d=1.13 (mm) (10)

Where: I – current passing through the wire (A);

j——current density, j=2.5～3A/mm2.

When j=2.5A/mm2 is used as input into formula (10), d=0.72 (mm) is obtained. The primary winding wire diameter is d1=0.72=0.69mm, and the outer diameter of the enameled wire is 0.72mm. The secondary winding wire diameter is d2=0.72=2.94mm, and two d=2.12mm (considering that the maximum outer diameter of the insulating varnish is 2.21mm) wires are selected and wound in parallel. Because the cross-sectional area of ​​the 2.94 wire is Sd2=6.78mm2, and the cross-sectional area of ​​the d=2.12mm wire is 3.53mm2, the cross-sectional area of ​​the two wires in parallel is: 2×3.53=7.06mm2, which fully meets the requirements and has a large margin.

6 Structural calculation of toroidal transformer

The winding of the toroidal transformer is wound by the winding ring of the winding machine rotating in the iron core, so the size of the inner diameter of the iron core is very important to the processing process. The purpose of the structural calculation is to check how much space is left in the inner diameter after all the windings are wound. If the calculated inner diameter space is too small and does not meet the winding requirements, the core size can be modified. As long as the cross-sectional area remains unchanged, the electrical performance will basically remain unchanged.

It is known that the inner diameter of the core Dno=55mm, the thickness of each insulation layer in Figure 7 is to=1.5mm, t1=t2=1mm.

1) Calculate the inner diameter Dn2 after winding the primary winding and wrapping the insulation

Calculate the number of turns of each layer of the primary winding n1n1= (turns) (11)

Where: Dn1——Inner diameter of the core after insulation, Dn1=Dno－2t0=55－(2×1.5)=52mm;

kp——lap winding coefficient, kp=1.15. Substituting into n1==197 turns

Then the number of layers of the primary winding Q1 is Q1===3.35, take the integer Q1=4 layers

The thickness of the primary winding δ1 is

δ1=Q1d1kp=4×0.72×1.15=3.3mm

Then the inner diameter Dn2 of the primary winding after insulation is

Dn2=Dn1－2(δ1＋t1)=52－2(3.3＋1)=43.4mm

2) Calculate the thickness of the secondary winding δ2

Calculate the number of turns n2 of each layer of the secondary winding. Considering that the secondary winding is wound with 2×d2=2×2.21mm wire, n2===27 turns

Then the number of layers of the secondary winding Q2 is Q2===1.41, and the integer Q2=2 layers.

The secondary winding thickness δ2 is

δ2=Q2d2kp=2×2.21×1.15=5.08mm

3) Calculate the inner diameter Dn4 after winding the primary and secondary windings and wrapping the insulation

Dn4=Dn2－2(δ2＋t2)=43.4－2(5.08＋1)=31.24mm

It can be seen that after winding, there is still margin in the inner diameter, and the selected core size is appropriate.

7Performance test of toroidal transformer samples

In order to verify the accuracy of the design method, the performance of the toroidal transformer samples made according to the design parameters was tested, and the results are as follows.

7.1 No-load characteristic test

The measurement circuit is shown in Figure 8. The measured data are listed in Table 4. According to the data in Table 4, the no-load characteristic curve shown in Figure 9 is drawn.

From the no-load characteristics of the transformer, it can be seen that the design meets the requirements. When the rated working voltage is 220V (working point is A), the no-load current of the transformer is only 13.8mA. Even if the power supply voltage rises to 240V, the core of the transformer is not saturated when working at point B, and there is a large margin.

7.2 Voltage Regulation Measurement

The secondary no-load voltage U20=12.6V measured when the transformer is unloaded. When the rated current I2=16.7A is passed, the secondary output voltage is U2=11.8V. The voltage regulation rate is calculated according to formula (2):

()

Table 4 Measurement data of no-load characteristics of toroidal transformer AC input voltage U1/V No-load current I0/mA
202.1
403.3
604.0
804.9
1005.6
1206.4
1407.3
1608.3
1809.6
20011.2
22013.8
24018
25022.7

Toroidal transformer and its application

Figure 9 No-load characteristic curve of toroidal transformer

Figure 8 No-load characteristic measurement circuit

ΔU=×100％==6.4％
The transformer voltage regulation rate reaches the indicator of ΔU<7％.

7.3 Temperature rise test

The transformer winding temperature rise test is carried out using the resistance method. The test is carried out after the transformer temperature rise stabilizes after energization for 4 hours, and the average winding temperature rise Δτm is calculated according to formula (12). Δτm=(k＋t1)－(t2－t1)（12）

The measured data and calculated results are listed in Table 5

Table 5 Temperature rise test data of 200VA toroidal transformer Winding type Ambient temperature when measuring cold resistance (r1) t1/℃ Ambient temperature when measuring hot resistance (r2) t2/℃ Winding resistance r1/Ω at t1 Winding resistance r2/Ω at t2 Constant k Average winding temperature rise Δτ/℃
Primary 34.8 35.5 5.2 755.9 58 234.5
34.2 Secondary 34.8 35.5 0.0 185 20.0 208 234.5 32.5
From the temperature rise test results, it can be seen that the designed transformer has reached the standard temperature rise standard, that is, Δτm<40℃, and the temperature rise of the primary and secondary windings is basically equal, that is, the power consumption of the two windings is relatively balanced.

7.4 Insulation performance test

1) Insulation resistance

The insulation resistance was tested with a 500V megger. The insulation resistance between the primary and secondary windings was greater than 100MΩ under normal conditions.

2) Dielectric strength

The transformer primary and secondary windings can withstand 50Hz, 4000V (effective value) voltage for 1min without breakdown and arcing. The limited leakage current is 1mA. This test proves that the transformer's dielectric strength meets IEC standards.

8 Conclusion

With its excellent performance and competitive performance-price ratio, it is expected that the toroidal transformer will replace the traditional laminated transformer in a larger field. As the technical performance of the toroidal transformer is further improved, it will have broader application prospects in the field of electronic transformers.