Segmented core cover technology for toroidal audio transformers

    A typical method for making a toroidal transformer involves creating a steel core, insulating the core, winding a magnetic wire around the core to create a primary winding, insulating the primary winding, winding a magnetic wire over the insulation to create a secondary winding, and insulating the secondary winding. To fasten the transformer, either a mounting washer and a bolt are used, or the center of the transformer is poured with epoxy with a bolt hole. Operations are almost always performed in this sequence.Медицинский трансформатор

Design and manufacture of segmented core covers

    Segmented core covers support the primary and secondary windings in alternating sectors to reduce leakage current. A plurality of modular electrical insulating segments are typically snapped or otherwise joined together to form annular or semi-annular core covers to cover or partially cover the transformer's annular toroidal core. Segments or modules are typically made from Zytel®, FR50, Rynite® FR530, or Zytel® E103HSL.

  The core cover modules isolate the copper windings from the core across the entire winding range and provide double layer insulation between adjacent windings, significantly reducing leakage current compared to conventional toroidal transformers. They also provide direct cooling of the transformer core with ambient or forced air without intermediate insulation. The core cap can also be assembled from component modules on a finished wound toroidal core.

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The segments of each module include a pair of spaced apart, usually electrically insulated walls, as well as a panel extension that separates the windings. The walls are positioned at a predetermined angle relative to each other, typically 30 degrees, 45 degrees, 60 degrees, etc., so that each modular segment spans an arc of approximately 30 degrees, 45 degrees, 60 degrees, etc. The walls include engaging, usually male and female, connecting portions, so that adjacent segments can repeatedly engage with each other, with a sufficient number of connected segments forming the cap of the annular core.

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    The number of segments required to complete the core cover is predetermined and is usually a function of the predetermined angle between the walls; for example, if the angle is 45 degrees, you would need to connect eight segments together to define the shape of the ring. If the angle is 60 degrees, it only takes six segments to get the ring shape. Although core covers are typically made from identical modules, they may alternatively include combinations of core cover modules spanning different arcs, for example, four core cover modules spanning 45 degrees each and six core cover units spanning 30 degrees each.

  While modules of the same size and shape are generally more convenient, there is little or no limitation on the combinations of sizes and shapes of core cover modules that can be combined to provide the desired core cover having the desired properties and characteristics.

Dividing walls

 When connected (interlocking), relatively flat and smooth sides (surfaces) are formed, and the barriers are located opposite each other. Barriers define the parameters that limit alternating wire windings, usually alternating primary and secondary windings.

  The segments include one or more baffles or walls positioned to extend partially or completely through the top of the panel to further define the parameters between which the wire windings are directed. One or more baffles are usually equidistantly spaced between the walls and / or each other, respectively. The spacers are typically oriented to extend radially outward from the center of the core and / or the annular space formed by the connected segments; in other words, each respective division usually lies within the radius of the annular space, although the division walls can have other convenient shapes and contours as desired.

  The segments further include a cover panel with an outer diameter D and / or a panel with an inner core diameter D, which extend downwardly so as to at least partially cover the outer D and inner D, respectively, of the core toroidal ring opposite the core cover panels of partially or fully formed annular space. These panels can be flat to cover a core ring having flat sides of the outer and inner diameters, or curved to follow the core ring having rounded or curved inner and outer diameters.

    The walls are truncated and do not fit through the panels. In some of them, the lower walls are located opposite the panel from the corresponding wall. The bottom wall can also include mating connectors for joint connection. Some of the segments contain ribs located on the top of the panels to create an air gap between the wire turns and the top of the ring. The air gap makes it easier to air-cool the windings by allowing air to circulate between the windings and the top of the cover.

  Segmented core lid winding tool

  The winding tool is used to facilitate winding of the single-bobbin lid core. The winding tool is usually a flat ring with a raised rim or flange protruding from the outside diameter. The ring usually has a slot, which gives it a C-shape. The ring is sized to fit the segment and the slot is sized to allow the wire to pass per segment. The winding tool also typically includes an elongated arcuate anchor wire having multiple partial slots and one or more anchoring holes for connecting the anchor wire to one or more segments during the wire winding process.инструмент для намотки медицинских трансформаторов

  During operation, several segments can be connected to each other to form a ring. The ring includes a top cap portion of the ring core formed by panels of individual segments. In most cases, the ring also includes (typically) equally spaced radial protrusions formed by mutually engaging connectors extending outwardly from the ring. Each radial protrusion is usually part of an elongated wall located on the upper side of the ring and extending radially inwardly partially or completely through the upper surface. Some of the walls terminate in radial ridges extending inward from the ring. These radial ridges are usually formed by joining the two bottom walls, although they can be formed separately.

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The ring may also include an annular core, an outer diameter cap and / or an inner diameter cap of the annular core, each cap being generally perpendicular to a portion of the upper core cap and extending downward.

Corresponding lids typically consist of adjacent lid panels when the segments are joined to form a ring.

Typically, a pair of cap rings are comprised of interconnected segments and are placed on opposite sides of a toroidal core with outwardly aligned protrusions. An even number of segments are connected to form each ring. The wire is wound continuously around alternating segments to define the primary windings, N turns per segment. Typically, all windings can be made from a single bobbin or shuttle in one continuous bobbin winding operation, with the wire guided from one segment to the next through a groove or gap between two opposing core covers. The wire is usually cut or cut to insulate the primary windings from the secondary windings, and then the wound core can be wrapped with insulation as in conventional winding of a toroidal transformer. Some windings may use a tool to facilitate winding of the core. Coils wound in this way retain the advantages of toroidal transformers, but are lighter, smaller, more efficient and quieter than dialed EI cores. Cores wound in this way exhibit lower interwinding leakage current compared to standard toroidal transformer cores.

Typically, the primary windings occupy the odd numbered segments starting from the winding of the first segment, and the secondary windings occupy the even numbered segments. Each ring can contain multiple segments such as six, nine, or twelve, and the core can be wound with primary, secondary and tertiary (not shown) windings as described above to form a three-phase transformer. Alternatively, the ring can contain segments of various configurations.

An insulating material, such as a strip of MYLAR, can be positioned to cover the portion of the core that is exposed by the gap, or the core can be partially or completely wrapped in insulating material prior to installing covers on it. In other designs, the walls are spaced and oriented relative to each other to form an annular space, but are not physically connected to each other. All leads are double insulated / braided and secured with cable ties.

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Summary: the benefits of a segmented closed core.

If it is necessary to design the segmented core cap to meet the safety requirements for leakage and clearances, the manufacturing time is reduced because:

-No need for grounding and interwinding insulation, as well as external winding.
-The primary and secondary windings can be wound on the same machine, which reduces maintenance time.
- Provided the cover has a mounting hole, there is no need to fill the center of the transformer with epoxy.
-Can be designed with a segmented core cover made up of repeating sections that "snap together" then the tooling and assembly costs of the covers will be even less. the tool cost for the smaller injection molding part is less than the tool cost for the larger part.

-Assembling "snap-fit" parts requires less skill than other methods of insulating conductors. 

    It is possible to design a segment transformer with a core cover that allows air flow around the core and windings, resulting in less temperature rise because:

-There is a direct path for heat to escape from the bare core to the environment.
-There is no interwinding insulation and an outer sheath that retains heat.
-All windings have a direct path to transfer heat from them to the environment.
                If it is possible to design a segment core cover with mounting holes, then the weight of the transformer will be less because:

  • No center epoxy required
  • No mounting washer required

Segment transformer with core cover and standard toroidal transformer - comparison.

сравнение обычного и медицинского трансформаторовSegment cap transformers provide significant reductions in leakage current and heat dissipation compared to standard toroidal transformers. Heat transfer in a segment transformer with a base is comparatively better, since the cover design provides all the necessary insulation. During the experiment, it was found that a simple isolation transformer with a simple circuit (1: 1 transformation ratio) covered with segment caps has heating-cooling advantages over a conventional 13 ~ 17 ° C transformer.


Test parameters Standard toroidal design (1500VA) Segmented coated core design (1500VA)
Open circuit voltage 240 V 239.64 V 239.60 V
Idle current 240 V 36 mA 48 mA
Core loss 240 V 7.9 W 8.8 W
Idle current 264 V 85 mA                        85 mA
Core loss 264 V 12.0 W 11.9 W
Maximum leakage current 264 V 81 µA 14 µA
Leakage (high potential) 5 kV, 50 Hz, 2 sec. 1030 µA 210 µA
DC resistance in primary winding 28 ° C 0.719 0.779
Secondary DC Resistance 28 ° C 0.784 0.781
Thermal equilibrium power output 1440 VA 1425 VA
Thermal equilibrium power input 1524 VA 1519 VA
Efficiency 94.49% 93.81%
Surface temperature 111.5 ° C 98.6 ° C
Ambient temperature 29.4 ° C 30 ° C
Temperature rise 82.1 ° C 68.6 ° C
The size Ø200 × 90mm Ø200 × 90 mm

Weight comparison

   The weight Standard toroidal design  Segmented core design
Core 7.80 KG 7.80 KG
Caps 400 gr.  360 gr.
Copper 1.90 kg 1.95 kg
Center fill 0.60 kg -
General 10.7 kg 10.1 kg

Temperature rise comparison

   Power Temperature rise (° C)
Standard toroidal design  Segmented core design  Difference 
1500VA (Rating) 82.1 68.6 13.5
1800VA 108.3 91.5 16.8

Comparison of working hours

Standard toroidal design 
 Segmented core design   Difference 
100% 66% 34%

The segmented core cover design of the transformer provides better heat dissipation, so they can be rated for increased power for the same volume, which is a major advantage. Thus, they are relatively smaller and lighter in weight compared to standard transformers for the same power levels. Other advantages are lower leakage current, lower manufacturing cost, and economical mounting design.

     For example, below is a comparison of a 1500 VA segment transformer (1800 VA extended power) versus our standard medical 1800 VA standard toroidal transformer.

Comparison of tests

Test parameters Standard toroidal design Segmented core design
Open circuit voltage 240 V 247.35 V 239.60 V
Idle current 240 V 55 mA 48 mA
Leakage current 264 V 86 µA 14 µA
Leakage at 5 kV, 50 Hz, 2 sec 1100 µA 210 µA
DC resistance in primary winding 28 ° C  0.414 0.779
DC resistance in the secondary winding 28 ° C 0,480 0.781
Efficiency 94..48050% 93.50%
Surface temperature 120 ° C 121.5 ° C
Ambient temperature 30 ° C 30 ° C
Temperature rise 90 ° C 91.5 ° C
The size Ø210 × 100 mm Ø200 × 90 mm

Comparison by weight

The weight  Standard toroidal design  Segmented core design
Core 11.3 kg 7.80 kg
Caps 500 gr. 360 gr.
Copper 2.70 kg 1.95 kg
Center fill 0.50 kg -
General 15.0 kg 10.1 kg

Comparison of working hours

Standard toroidal design Segmented core design  Difference
100% 66%     34%


While toroidal transformer designs are generally quite advanced, this technical analysis shows that there is still room for innovation and efficiency gains through the use of segmented core cover technology. We hope this work will be useful to medical device manufacturers, magnetic materials developers and anyone else who may be interested.

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