💡 IEC 61021: Standard Laminated Cores — YEE 2 Dimensions, Silicon Steel Selection, and Transformer/Inductor Design

IEC 61021: Standard Laminated Cores — YEE 2 Dimensions, Silicon Steel Selection, and Transformer/Inductor Design Engineering

Inside every piece of telecom equipment, every switch-mode power supply, and every audio amplifier lies a small laminated-core transformer quietly doing its job — stepping voltages up or down, providing galvanic isolation, and matching impedances. The physical dimensions, stacking precision, and material choices of these cores are precisely what IEC 61021 (Laminated Core Packages for Transformers and Inductors) standardizes. Prepared by IEC Technical Committee 51 (Magnetic Components and Ferrite Materials), Part 1 of this standard defines a comprehensive range of laminated core packages based on YEE 2 laminations — the workhorses of small transformer design in telecommunication and electronic equipment worldwide.

YEE 2-2 ~ 2-12

8 standard sizes (tongue width 8~40 mm)

0.10 ~ 0.35 mm

6 standard lamination thicknesses

2 Configs

Standard + Dual-E assemblies

C22/E1~E4/F1

5 silicon steel material grades

💡 1. Laminated Core Fundamentals: From EI Stamped Laminations to the YEE 2 System

1.1 Why Laminate a Transformer Core?

A transformer core sits in an alternating magnetic field. The changing flux induces eddy currents within the core material itself. If the core were a solid conductive block, these eddy currents would circulate freely across the entire cross-section, producing enormous I²R heating losses. Lamination is the classic solution: insulating layers between thin sheets force eddy currents into narrow, high-resistance paths within each individual lamination. This physics directly dictates two fundamental parameters of any laminated core design:

Lamination thickness (t): Thinner sheets = lower eddy current loss, but higher material and assembly cost. The eddy-current component of core loss scales with t².
Stacking factor (SF): The insulation coating and inevitable air gaps between laminations mean the actual steel cross-section is always less than the geometric cross-section, typically 0.88 to 0.97. This factor directly affects turns-per-volt calculations and winding window utilization.

1.2 The YEE 2 Lamination System: Standardizing the EI Shape

IEC 61021-1 builds upon the YEE 2 laminations defined in IEC 60740 to specify complete laminated core packages. YEE 2 belongs to the EI family of lamination shapes — a larger E-shaped piece combined with a closing piece (smaller E or I) to form a complete magnetic circuit. Unlike informal “EI-28” or “EI-35” vernacular designations, the YEE 2 system provides complete engineering dimensions with specified tolerance grades:

Core Designation	Tongue Width a (mm)	Stack Height b (mm)	Window Width c (mm)	Window Height e (mm)	Typical Power Range
YEE 2-2 d K	8	8	4.3	5.6	< 1 VA (signal/coupling)
YEE 2-3 d K	10	10	5.5	7	1~3 VA (small signal)
YEE 2-4 d K	12.6	12.6	6.7	8.8	3~8 VA (telecom interface)
YEE 2-5 d K	16	16	8.6	11.2	5~15 VA (general PSU)
YEE 2-6 d K	20	20	11	14	10~40 VA
YEE 2-8 d K	25	25	13.2	17.4	30~80 VA
YEE 2-10 d K	32	32	17.2	22.4	60~200 VA
YEE 2-12 d K	40	40	22	28	150~500 VA

In the designation, “d” indicates a square stack (b = a), and “K” identifies this as a laminated core package. A second variant using the suffix “K L” combines two larger E-pieces in a mating pair — this configuration offers a symmetric magnetic path and easier coil insertion, applicable to the larger YEE 2-6 through 2-12 sizes.

🎯 Engineering Insight: Precision Air-Gap Control
For inductors and flyback transformers, the centre-limb air gap is critical for controlling inductance and preventing saturation. IEC 61021 specifies two gap realization methods: grinding one or both centre limbs (highest precision, down to ±0.01 mm, but more expensive) or tooling the gap at the lamination stamping stage (lower cost, larger tolerance). For 50/60 Hz power transformers operating below saturation, no intentional gap is needed — the two core halves are simply ground flat and clamped together. But for any design carrying DC bias current (filter chokes, flyback transformers, single-ended audio output transformers), the air gap is your first line of defense against core saturation.

1.3 Butt-Stack vs. Interleaved Assembly

Two fundamental assembly strategies exist for EI laminated cores: butt-stack and interleaved. In butt-stack construction, all E laminations face one direction and the I pieces close the magnetic path in a separate layer, creating a distributed air gap at each E-I interface. This is ideal for inductors and transformers where controlled inductance is required. Interleaved stacking alternates E and I pieces so that the joint faces of one layer are bridged by solid material in the adjacent layer, dramatically reducing the effective air gap and maximizing permeability — the go-to choice for power transformers where minimum magnetizing current is desired. The trade-off is real: interleaved cores achieve 2~5x higher effective permeability but lose the air-gap tuning knob entirely.

⚙️ Key Parameter: Stacking Factor
Typical stacking factors: ~0.94~0.96 for 0.35 mm uncoated laminations; ~0.93~0.95 for 0.30 mm; as low as 0.88~0.92 for 0.10 mm thin-gauge material. When calculating turns-per-volt, always multiply the geometric core area by the stacking factor: A_e-eff = a × b × SF. Forgetting the stacking factor means underestimating turns by 5~12%, which can push flux density well past the saturation knee — especially in small transformers where the VA rating already forces conservative B_ac choices.

⚡ 2. Silicon Steel Selection: Grain-Oriented vs. Non-Oriented — The Critical Distinction

2.1 Two “Personalities” of Silicon Steel

Laminated cores are almost universally made from silicon steel (Fe-Si alloy, 2.5~4.5% Si). The silicon increases electrical resistivity, reducing eddy current losses. But the crucial design fork is: Grain-Oriented (GO/CRGO) or Non-Oriented (NGO/CRNGO)?

Property	Grain-Oriented (GO / CRGO)	Non-Oriented (NGO / CRNGO)
Grain Structure	Highly aligned along rolling direction (Goss texture)	Isotropic — uniform magnetic properties in all directions
B_sat (rolling dir.)	1.80~2.03 T	1.50~1.70 T
Permeability (rolling dir.)	Very high (30,000~100,000)	Moderate (1,500~4,000)
Permeability (transverse)	Drops sharply (500~2,000)	Uniform in all directions
50 Hz Core Loss at 1.5 T	0.8~1.3 W/kg	2.0~6.0 W/kg
Cost	Higher	Lower
Typical Use	50/60 Hz power transformers (cut cores, wound cores)	Motors, high-frequency transformers, EI stampings

2.2 Why EI Laminations Overwhelmingly Use Non-Oriented Steel

This is one of the most frequently misunderstood aspects of laminated core design. Grain-oriented silicon steel genuinely excels along the rolling direction, but the EI shape forces flux to travel in multiple directions. In an EI lamination, flux flows along the rolling direction in the centre limb (good), but must turn 90° at the corners where it crosses the rolling direction at the worst possible angle (bad). In the transverse direction, GO steel’s permeability collapses and core loss spikes dramatically. The net result: an EI core made from GO steel often performs no better than NGO steel, while costing 50~80% more. The EI shape and NGO steel are natural partners precisely because the isotropic magnetic properties match the multi-directional flux path. By contrast, C-cores (cut cores) and toroidal cores, where flux always follows the rolling direction, are where GO steel truly shines.

⚠️ Common Mistake: Applying C-Core Material Logic to EI Cores
C-cores are made by winding grain-oriented silicon steel strip, then cutting and grinding. The flux path is always along the rolling direction, so GO material’s advantages are fully realized. EI laminations, however, are stamped from flat sheets — the magnetic circuit contains unavoidable 90° corners where flux must cross the rolling direction. Using expensive GO steel in an EI lamination is an expensive design mistake. Unless you are designing for a highly specialized application where the centre-limb magnetizing inductance is the sole performance metric, stick with NGO grades (E3, E4 for general use; E1 for thin-gauge, low-loss applications).

2.3 How Frequency Dictates Lamination Thickness (Why High-Frequency Transformers Need Thinner Laminations)

Total core loss P_Fe in a laminated core has two components:

Hysteresis loss P_h: Proportional to frequency f. P_h = k_h × f × Bⁿ (n ≈ 1.6~2.0). This is the energy required to continuously reorient magnetic domains.
Eddy-current loss P_e: Proportional to f². P_e = k_e × (t × f × B)², where t is lamination thickness.

At 50/60 Hz, hysteresis typically dominates (60~70% of total loss), and 0.30 mm or 0.35 mm laminations are perfectly acceptable. But when frequency rises to 400 Hz (avionics) or 20 kHz (SMPS), the eddy-current component — scaling with f² — abruptly takes over! At these frequencies, laminations must be dramatically thinner: 0.10 mm is the minimum standardized by IEC 61021, and for switch-mode power supplies above ~20 kHz, even this is insufficient — ferrite cores (which are ceramic insulators with negligible eddy currents) replace silicon steel entirely. This is why IEC 61021 specifies six thicknesses from 0.10 to 0.35 mm: different frequencies demand different lamination gauges.

Application	Typical Frequency	Recommended Thickness	Recommended Material	Key Design Driver
Mains power transformer	50/60 Hz	0.30~0.35 mm	NGO (E3, E4)	Inductance, temperature rise, cost
Audio transformer	20 Hz~20 kHz	0.10~0.20 mm	NGO (E1, E3) or Permalloy	THD, bandwidth
Telecom interface transformer	300 Hz~3.4 kHz	0.10~0.15 mm	NGO (E1) or Permalloy	Return loss, longitudinal balance
SMPS power transformer	20~200 kHz	0.05~0.10 mm	Ferrite (not EI laminated)	Eddy-current dominant; laminations no longer viable
DC filter choke	DC + 100/120 Hz ripple	0.30~0.35 mm	NGO + air gap	DC bias, saturation margin

🔧 3. Engineering Design Practice: Building Reliable Transformers with Standard Laminated Cores

3.1 The Six-Step Core Selection Method

For small mains-frequency power transformers (below 500 VA), using IEC 61021 standard laminated cores dramatically simplifies the design workflow:

Determine VA requirement — Select the YEE 2 core designation from the power range table above.
Choose operating flux density B_ac — For 50 Hz with untreated NGO laminations, use 1.0~1.2 T (leaving saturation margin). For audio transformers, use 0.2~0.5 T (controlling THD).
Calculate turns per volt — N/V = 1 / (4.44 × f × B_ac × A_e × SF), where A_e = a × b and SF is the stacking factor.
Verify window utilization — Ensure copper loss balances core loss; window fill factor should not exceed 35~40%.
Select wire gauge and turns count — Based on current density (typically 2.5~3.5 A/mm² for naturally cooled small transformers).
Choose assembly type — Interleaved for power transformers (minimizing magnetizing current); butt-stack when inductance control is required.

🚫 Critical Design Error: Ignoring Magnetizing Current
Small transformers (below 50 VA) can exhibit magnetizing currents of 30~60% of full-load current, vastly higher than the 2~5% typical of large distribution transformers. This is because small cores have limited magnetizing inductance (often only tens of henries). In standby-power-sensitive applications, this no-load current must be explicitly evaluated. Mitigation options: increase primary turns, further reduce B_ac (may require upsizing the core), or switch to higher-permeability core material. A rule of thumb: if no-load primary current exceeds 10% of rated current, the design needs revisiting.

3.2 Audio Transformer Design Considerations

Audio transformers (input, output, and line-matching) represent a significant application domain for EI laminated cores. Unlike power transformers, the primary challenge is bandwidth, not power density:

Total Harmonic Distortion (THD): The non-linear B-H curve of silicon steel generates harmonic distortion. Reducing B_ac to 0.2~0.5 T dramatically improves linearity, at the expense of requiring a larger core for the same power handling.
Frequency Response: Thin-gauge, high-permeability material (0.10 mm E1 grade) extends high-frequency response. Low-frequency response depends on primary inductance — which depends on core cross-section, permeability, and turns count.
Shielding: Audio transformers commonly require both electrostatic shielding (copper foil between primary and secondary, grounded) and magnetic shielding (copper strap shorted turn around the core). The bobbin design must accommodate these additional layers.
Parasitic capacitance: Layer-wound rather than scramble-wound secondaries reduce leakage inductance for wideband applications, but increase winding capacitance. Sectionalized bobbins are preferred for high-performance designs.

3.3 Mechanical Assembly — Where Theory Meets Reality

After the electromagnetic design is complete, mechanical assembly details often determine whether the transformer performs to specification:

Clamping method: EI laminated cores are typically secured with shell-type clamp brackets or through-bolts. Excessive clamping pressure can abrade interlaminar insulation, creating inter-laminar short circuits that dramatically increase eddy current losses. Apply just enough pressure to eliminate audible vibration.
Vacuum impregnation: After winding and core assembly, vacuum impregnation with insulating varnish is standard practice. Beyond electrical insulation, this process effectively locks laminations in place, reducing magnetostriction-induced noise by 5~10 dBA and preventing moisture ingress.
Joint-face cleanliness: The mating faces between E and I (or E and E) pieces must be clean and burr-free. A single 0.01 mm particle on a 1 mm² contact area can create a significant unintended air gap, skewing inductance and increasing magnetizing current. Handle laminations with clean gloves during assembly.

❓ Frequently Asked Questions

Q1: EI laminated core vs. toroidal core — which is better?: A: There is no universal “better” — it depends entirely on the application. Toroidal cores have no air gap, resulting in very low magnetizing current and minimal stray field — ideal for small-signal transformers and common-mode chokes. But EI laminated cores offer distinct advantages: simple winding (the coil can be wound on a bobbin first, then the core assembled around it), precise inductance control via air gap (difficult to introduce in toroids), lower cost, and easier safety isolation for multi-winding transformers. For mains-frequency power transformers above ~10 VA, EI laminations are the more practical choice. For sensitive small-signal audio input transformers, toroids or mu-metal cans are preferred.
Q2: I have seen EI transformers with non-square stack heights. Is this valid?: A: Absolutely — this is what the “d” and other stack-height letters in the IEC 61021 core designation system represent. “d” specifies a square stack (b = a), but the standard allows for other stack heights. The purpose of varying stack height is to optimize the copper-to-iron loss balance. A taller stack (b > a) provides more core cross-section, reducing turns and therefore copper loss, at the expense of longer mean turn length. A shorter stack saves iron and window space but requires more turns. The optimum is typically when copper loss roughly equals core loss at the design operating point. Consult the full IEC 61021 tables for tolerance codes on non-square stacks.
Q3: Why do some laminated transformers hum, and how can I silence them?: A: Transformer hum originates from magnetostriction — the microscopic dimensional change of silicon steel laminations under alternating magnetization. Four practical countermeasures: (1) reduce operating flux density (1.0 T is dramatically quieter than 1.4 T); (2) use thinner laminations; (3) ensure uniform, moderate clamping pressure; (4) vacuum-impregnate with insulating varnish, which bonds laminations together and damps vibration. The combination of (1) and (4) is typically most effective. Note that higher silicon content (4~6.5% Si) materials exhibit lower magnetostriction, but are more brittle and harder to stamp — a manufacturing trade-off that ultimately affects cost.
Q4: Can I use IEC 61021 laminated cores for flyback transformer designs?: A: Yes, but with one mandatory modification: an air gap must be introduced in the centre limb. Flyback transformers store energy in the core during the switch-on period and release it during switch-off; the DC current component would rapidly saturate an ungapped silicon steel core. The gap stores the magnetic energy and prevents DC bias from driving the core into saturation. However, laminated silicon steel EI cores are only practical for line-frequency flyback designs (50/60 Hz). For high-frequency flyback converters (above ~20 kHz), eddy current losses in even the thinnest standard laminations become prohibitive — ferrite cores with their near-zero electrical conductivity are the standard choice. When designing a line-frequency flyback with an EI core, recalculate B_ac with adequate margin accounting for DC bias, and verify that the gap length does not cause excessive fringing flux that could overheat nearby windings.