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Three different equipment selection schemes for low-frequency transformers
2026-07-07
EI type, ring-shaped, C-shaped low-frequency transformers: Comprehensive solution for different equipment selection
Low-frequency transformers are mainly divided into three structures: EI laminated type, ring-wound type, and C-shaped plug-in type. The magnetic circuit structure, leakage magnetic field level, temperature rise characteristics, cost, and shock resistance of these three types vary greatly. Many project selections have gone wrong, either resulting in excessive electromagnetic interference, noise exceeding standards, or being unable to fit the space, with costs seriously exceeding the limit, or even overheating and burning out due to long-term full-load operation.
Many engineers make selections solely based on experience, using EI transformers in precision instruments, which causes signal interference, or applying ring transformers to impact loads, resulting in frequent magnetic saturation. This article systematically disassembles the structures of three types of core transformers, analyzes their advantages and disadvantages, divides them into power ranges, load types, and usage environments, and provides clear selection schemes for different devices, taking into account performance, cost, structural installation, and long-term reliability. The entire article covers the entire process from research and development selection, batch procurement, to prototype modification.
- Comparison of Basic Structures and Core Performance of Three Types of Iron Core
1. EI Type Laminate Transformer (The Most Common Basic Model)
The EI iron core is formed by alternating insertion of E-shaped and I-shaped silicon steel sheets. There are gas gaps at the junctions, which determine the upper limits and advantages of all its performance.
The EI iron core is formed by alternating insertion of E-shaped and I-shaped silicon steel sheets. There are gas gaps at the junctions, which determine the upper limits and advantages of all its performance.
Core advantages:
First, the process is extremely mature, with high efficiency in automated winding and sheet stacking production, and the lowest procurement price for large quantities, making it the most cost-effective solution among the three;
Second, the iron core has gas gaps, which have strong resistance to DC bias magnetic fields, can withstand startup impact current and surge current, and is less likely to experience magnetic saturation;
Third, the windings are wound with a frame, making multi-winding and multi-output very easy to achieve, the primary and secondary isolation distance is easy to control, the safety standard creepage distance is easier to meet, and the withstand voltage and insulation rectification are simple;
Fourth, the structure is convenient for disassembly and assembly, making it easy to repair, disassemble and replace the coils in the later stage, and the gaps in the iron core form a natural ventilation channel, ensuring stable heat dissipation under continuous full-load conditions, and the temperature rise is controllable.
The shortcomings are also very obvious: the magnetic path has butt-joint gaps, the leakage magnetic flux is large, and it will generate electromagnetic stray magnetic fields externally, easily interfering with weak signals on the surrounding circuit boards; the no-load current is large, the overall conversion efficiency is low; the buzzing vibration noise after power-on is higher than that of the ring-shaped transformer; under the same power, the volume and weight are larger.
First, the process is extremely mature, with high efficiency in automated winding and sheet stacking production, and the lowest procurement price for large quantities, making it the most cost-effective solution among the three;
Second, the iron core has gas gaps, which have strong resistance to DC bias magnetic fields, can withstand startup impact current and surge current, and is less likely to experience magnetic saturation;
Third, the windings are wound with a frame, making multi-winding and multi-output very easy to achieve, the primary and secondary isolation distance is easy to control, the safety standard creepage distance is easier to meet, and the withstand voltage and insulation rectification are simple;
Fourth, the structure is convenient for disassembly and assembly, making it easy to repair, disassemble and replace the coils in the later stage, and the gaps in the iron core form a natural ventilation channel, ensuring stable heat dissipation under continuous full-load conditions, and the temperature rise is controllable.
The shortcomings are also very obvious: the magnetic path has butt-joint gaps, the leakage magnetic flux is large, and it will generate electromagnetic stray magnetic fields externally, easily interfering with weak signals on the surrounding circuit boards; the no-load current is large, the overall conversion efficiency is low; the buzzing vibration noise after power-on is higher than that of the ring-shaped transformer; under the same power, the volume and weight are larger.
2. Ring Transformer (High-performance, Low-interference Model)
The ring-shaped iron core is continuously wound from a continuous strip of silicon steel into a closed circular loop, without any breaks or butt-joint air gaps. The magnetic circuit is completely enclosed, and the magnetic flux is basically confined within the iron core. This is the structure with the best magnetic performance among the three. Core advantages: Closed magnetic circuit without air gaps, extremely low leakage flux, electromagnetic radiation is reduced by over 80% compared to EI transformers, no additional shielding cover is needed, and it will not interfere with sensors, audio circuits, or precise analog signals; Energy conversion efficiency can reach over 90%, with low iron loss and negligible no-load heating; The magnetic expansion is minimal, the operation is quiet, and the buzzing noise can be almost inaudible; Under the same power, it has a smaller volume, lighter weight, and is easier to fit into a compact housing.
The ring-shaped iron core is continuously wound from a continuous strip of silicon steel into a closed circular loop, without any breaks or butt-joint air gaps. The magnetic circuit is completely enclosed, and the magnetic flux is basically confined within the iron core. This is the structure with the best magnetic performance among the three. Core advantages: Closed magnetic circuit without air gaps, extremely low leakage flux, electromagnetic radiation is reduced by over 80% compared to EI transformers, no additional shielding cover is needed, and it will not interfere with sensors, audio circuits, or precise analog signals; Energy conversion efficiency can reach over 90%, with low iron loss and negligible no-load heating; The magnetic expansion is minimal, the operation is quiet, and the buzzing noise can be almost inaudible; Under the same power, it has a smaller volume, lighter weight, and is easier to fit into a compact housing.
Limitations: The structure without air gaps has weak resistance to direct current impact. Once a slight direct current component appears in the input, it is very likely to cause magnetic saturation, with the no-load current surging instantly and quickly overheating; The ring shape has no framework, and the difficulty of winding multiple groups and multiple circuits is high. The production cost of multi-output transformers will increase significantly; The coil is wound on the ring, and it cannot be disassembled and repaired later. Once the coil breaks down, it can only be scrapped as a whole; For high-power models, the internal heat dissipation channels are narrow, and the temperature rise in the sealed housing will significantly increase under full-load conditions.
3. C-Type Plug-in Transformer (C-type wound core, compromise preferred model)
The C-type core first rolls the silicon steel strips into a closed rectangular core, then cuts it in half to form two C-shaped components. During assembly, they are inserted together, which is what is commonly referred to as a plug-in transformer. It lies between EI and ring-type transformers, being a compromise product.
Advantages: There are only two magnetic junctions, far fewer than the multi-layer connection of EI, with much less leakage flux than the EI structure, approaching the level of ring-type transformers; the magnetic path is short, the magnetic flux density is high, the efficiency is high, and the noise is low; it retains a small air gap, has better resistance to direct current bias than pure ring-type transformers, and balances low interference and shock resistance; the core can be separated, the winding can be wound easily, and it is easier to design multi-winding and high-voltage insulation, balancing performance and process feasibility; the shape is flat and slender, suitable for horizontal installation on ultra-thin equipment.
Shortcomings: The cost for mass production of small power is higher than that of EI transformers, while the price of large power models is not as advantageous as ring-type transformers; the cutting surface of the core is prone to eddy current loss. If the annealing treatment is not done properly, the no-load loss will significantly increase; the overall structure is flat, and the cooling condition is poor when installed vertically, not suitable for long-term sealed full-load operation.
- Precise Selection by Scenario: Which type should be used for the specific equipment?
Scene 1: For cost-oriented civilian, ordinary industrial control, and security power supplies, the EI type transformer is the first choice.
Applicable equipment: Access control power supply, LED lighting transformer, ordinary monitoring power supply, relay control board, household appliance control power supply, power supply for industrial frequency isolation, ordinary industrial control cabinet power supply. These devices typically have multiple outputs, large quantities, and strict budgets. They often encounter voltage fluctuations in the power grid and large startup current during startup. The EI transformer can withstand surges and overloads, the multi-winding design is simple, the safety standards are easy to pass 3C and RoHS certifications, and the cost of large-scale procurement is the lowest. Supplementary selection suggestions: For ordinary indoor normal temperature conditions, use the ordinary EI core; for long-term 24-hour full-load operation models, use high-conductivity silicon steel sheets + vacuum impregnation technology to reduce noise and temperature rise; if there are weak signal circuits nearby, add an additional shielding winding to suppress magnetic interference.
Reasons not to recommend using ring transformers: The cost of multi-winding winding increases by a factor of two, the anti-shock capability is insufficient, it is prone to magnetic saturation and heating during power grid surges, and the maintenance cost is higher in the future.
Applicable equipment: Access control power supply, LED lighting transformer, ordinary monitoring power supply, relay control board, household appliance control power supply, power supply for industrial frequency isolation, ordinary industrial control cabinet power supply. These devices typically have multiple outputs, large quantities, and strict budgets. They often encounter voltage fluctuations in the power grid and large startup current during startup. The EI transformer can withstand surges and overloads, the multi-winding design is simple, the safety standards are easy to pass 3C and RoHS certifications, and the cost of large-scale procurement is the lowest. Supplementary selection suggestions: For ordinary indoor normal temperature conditions, use the ordinary EI core; for long-term 24-hour full-load operation models, use high-conductivity silicon steel sheets + vacuum impregnation technology to reduce noise and temperature rise; if there are weak signal circuits nearby, add an additional shielding winding to suppress magnetic interference.
Reasons not to recommend using ring transformers: The cost of multi-winding winding increases by a factor of two, the anti-shock capability is insufficient, it is prone to magnetic saturation and heating during power grid surges, and the maintenance cost is higher in the future.
Scene 2: Silent, low interference, high-precision low-voltage equipment, with priority given to ring-shaped transformers
Applicable equipment: Hi-Fi audio amplifiers, medical testing instruments, precision measuring instruments, laboratory voltage stabilizing power supplies, weak signal acquisition equipment, RF front-end power supply. The core requirements for these products are low EMI electromagnetic interference, low noise, and pure output voltage. The ring-shaped closed magnetic circuit has almost no magnetic leakage, which will not cause analog signal drift or increase background noise; the operation vibration is extremely small, and the entire machine has no buzzing noise, making it very suitable for enclosed silent equipment; high efficiency leads to lower no-load temperature rise, and the device will not heat up during long-term standby.
Usage precautions: Add common-mode inductors and voltage-sensitive resistors at the front end to suppress the DC component of the power grid and avoid magnetic saturation; do not design too many branch outputs; the sealed casing must have ventilation holes to prevent heat accumulation inside large-power models.
After many audio devices replaced EI transformers with ring-shaped ones, the power base noise significantly decreased, and the sound quality distortion was greatly reduced. This is the intuitive improvement brought by low magnetic leakage.
Scene 3: Ultra-thin body, medium power, capable of resisting interference and impact. Select C-shaped plug-in transformer.
Applicable devices: Ultra-thin wall-mounted power supply, desktop instruments, vehicle industrial control computer, medium-frequency isolation power supply, small UPS pre-stage transformer. Many device chassis heights are limited. The height of the EI square core exceeds the standard, the circular and circular structures occupy too much horizontal space, while the C-shaped flat structure fits perfectly in the narrow installation space. At the same time, it has low leakage magnetic field, low noise, and an air gap, which makes it less prone to magnetic saturation like a pure circular structure, perfectly balancing performance and stability. Power range suggestion: 100VA to 1000VA is the golden power range for C-shaped transformers. This range has the highest cost performance, with performance between EI and circular structures.
Scene 4: High-power industrial industrial-frequency transformers (above 1kVA)
For high-power models above 1000VA, there are two routes: ordinary industrial isolation transformers should preferentially use high-power EI laminated iron cores, with sufficient heat dissipation channels, capable of long-term continuous full-load operation, and with a sturdy and vibration-resistant structure; for high-power voltage stabilizing power supplies and inverter pre-stage power supplies with noise and leakage magnetic field requirements, a double C insert-type structure can be selected; ring-shaped high-power models have prominent heat dissipation defects. Unless the shell has sufficient holes, they are not recommended for long-term sealed full-load operation.
- Four key conditions to further determine the core structure
1. Consider the number of output windings
For 3 or more output channels: Prefer EI, followed by C-type inserts; Ring-type winding is difficult and will significantly increase costs.
For single or dual simple output: Ring-type has the highest cost-performance ratio and the lowest leakage flux.
For 3 or more output channels: Prefer EI, followed by C-type inserts; Ring-type winding is difficult and will significantly increase costs.
For single or dual simple output: Ring-type has the highest cost-performance ratio and the lowest leakage flux.
2. Consider the load characteristics
Resistive load, continuous stable load: Ring-type is the best choice, with high efficiency and low heat generation.
Rectifier load, motor impact load, frequent start-stop: EI is the first choice, the air gap can resist the DC component and prevent magnetic saturation; secondly, choose C-type iron core with a small air gap; be cautious of pure ring-type.
3. Consider the enclosure structure and cooling environment
Open-type chassis, good ventilation: All three structures can be freely selected.
Fully sealed encapsulation, sealed chassis: EI has the best cooling air duct, and temperature rise is easiest to control; Ring-type has severe internal heat accumulation, and full load is prone to overheating.
Ultra-thin flat chassis: C-type insert transformer is the only choice.
4. Consider cost and mass production scale
Mass production of civilian products at the 100,000 unit level: EI is an indispensable choice, with a huge price advantage.
Small batch precision instruments, medical equipment: Prioritize ring-type without considering cost.
Mid-range instrument prototypes, small batch mass production: C-type insert can be used as a compromise solution, balancing performance and price.
- Common Mistakes in Model Selection and Avoiding the Trap of Engineering Re-work
Mistake 1: Blindly Pursuing High Efficiency, Replacing EI Transformers with Ring Transformers Directly for Rectifier Power Supplies. The rectification circuit generates DC components, and transformers with no air gap are prone to magnetic saturation. The no-load current surges, and abnormal overheating occurs within just a few weeks. The EI transformer, which has an inherent air gap, can effectively counteract DC biasing, which is the main reason why industrial power supplies rarely use ring transformers.
Mistake 2: Believing that the performance of C-type transformers is definitely superior to EI. If the annealing process of the C-type iron core is not up to standard, the eddy current loss on the cutting surface will sharply increase, and the no-load heating can even be more severe than that of a high-quality EI transformer. One must not only consider the structure but also verify the iron core annealing and lamination processes.
Mistake 3: Focusing solely on power and ignoring the shape and structure. EI is square, ring is circular, and C-type is flat and elongated. The installation holes, length, width, and height of the three are completely different. If the iron core structure is selected incorrectly at the prototype stage, the PCB and the shell will need to be re-modified, directly delaying the delivery date.
Mistake 4: Pursuing low noise and choosing all ring transformers. Open EI transformers, after vacuum impregnation and iron core compression treatment, can have noise levels that meet safety standards. If it is only an ordinary device, to achieve silence, an additional 30% or more cost is added by choosing ring transformers, which will cause unnecessary cost waste.
Mistake 2: Believing that the performance of C-type transformers is definitely superior to EI. If the annealing process of the C-type iron core is not up to standard, the eddy current loss on the cutting surface will sharply increase, and the no-load heating can even be more severe than that of a high-quality EI transformer. One must not only consider the structure but also verify the iron core annealing and lamination processes.
Mistake 3: Focusing solely on power and ignoring the shape and structure. EI is square, ring is circular, and C-type is flat and elongated. The installation holes, length, width, and height of the three are completely different. If the iron core structure is selected incorrectly at the prototype stage, the PCB and the shell will need to be re-modified, directly delaying the delivery date.
Mistake 4: Pursuing low noise and choosing all ring transformers. Open EI transformers, after vacuum impregnation and iron core compression treatment, can have noise levels that meet safety standards. If it is only an ordinary device, to achieve silence, an additional 30% or more cost is added by choosing ring transformers, which will cause unnecessary cost waste.
- Summary: Standardized Selection Rules
1. Large-scale general industrial control, multiple output channels, impact loads, strict cost control → EI stack transformer;
2. Audio equipment, precise weak current, low electromagnetic interference, single or dual winding, pursuit of quiet and efficient performance → Ring transformer;
3. Ultra-thin chassis, medium power, need to have both low leakage magnetic field and anti-saturation, compromise solution → C-shaped plug-in transformer;
4. Priority for EI in closed full-load conditions, cautious use of pure ring transformer for impact rectification loads, recognize C-shaped core for flat structure.
2. Audio equipment, precise weak current, low electromagnetic interference, single or dual winding, pursuit of quiet and efficient performance → Ring transformer;
3. Ultra-thin chassis, medium power, need to have both low leakage magnetic field and anti-saturation, compromise solution → C-shaped plug-in transformer;
4. Priority for EI in closed full-load conditions, cautious use of pure ring transformer for impact rectification loads, recognize C-shaped core for flat structure.
When selecting low-frequency transformers, it is not about having the highest performance; instead, it is about ensuring that the core structure matches the load, installation space, and cost budget. There is no absolute superiority or inferiority among EI, ring, and C-type inserts; it all depends on whether they are suitable or not. By strictly selecting based on magnetic circuit characteristics, load conditions, and structural dimensions, there will be no electromagnetic interference, overheating, or noise faults, and no increase in costs. This ensures stable mass production of the project from the very beginning, avoiding repeated sample modifications and multiple mold changes.
Improve the batch consistency of the automated production line for small magnetic components.
Can 50Hz and 60Hz industrial frequency transformers be used interchangeably?
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