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Thermal resistance of ferrite cores

Stan Zurek, Thermal resistance of ferrite cores, Encyclopedia Magnetica,
http://e-magnetica.pl/doku.php/thermal_resistance_of_ferrite_cores
See also the main article: Thermal resistance.

The values of thermal resistance of main core shapes for ferrite transformers, cooled by natural convection in air (no forced cooling) are given in the tables below.1)2)3)

Empirical data given by Epcos (see image) suggest that in the first approximation the thermal resistance is proportional to the reciprocal of square root of the ferrite core volume.4)5)

There are other publications which give thermal resistance in a different format, such as temperature rise vs. total dissipated power.6)

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Thermal resistance of ferrite cores, the dashed line shows the underlying trend proportional to the reciprocal of square root of core volume7)

S. Zurek, E-Magnetica.pl, CC-BY-4.0

Calculator of thermal resistance of ferrite cores

[open]

Some authors suggest that an empirical equation can be used as a first approximation of the thermal resistance of ferrite cores.

If the value of the core volume is used in cm3 then the thermal resistance can be calculated in (K/W):

(1) Maniktala fomula for thermal resistance
$$ R_{th} = 53 · (V_{core})^{-0.54} $$ (K/W)
after Sanjaya Maniktala, Switching power supplies A-Z, 2nd edition, Newnes, 2012, ISBN 978-0-12-386533-5, p. 155

Vcore =

      Estimated Rth (K/W) =

Amidon formula

There is also an alternative formula published by the Amidon company, which relates directly the temperature increase to the dissipated power and the surface area of the given core.8)

(2) Amidon formula for temperature rise (thermal resistance)
$$ T_{rise} \text{(°C)} = \left( \frac{P \text{ (mW)} }{A_{surf} \text{(cm}^2) } \right)^{0.833} $$
where: $T_{rise}$ - temperature increase above ambient, in free standing air, $P$ - power dissipated in the core, note that this value must be entered in (mW), $A_{surf}$ - area of the outer surface of the core, note that this value must be entered in (cm2)

E (EE)

Core shape E Rth (K/W)9)10) Rth (K/W)11) Rth (K/W)
calculated from (1)12)
E 5.3/2.7/2 308 334
E 6.3/2.9/2 283 290
E 8.8/4.1/2 204 210
EE 10/11-Z 63.8 98.9
E 13/7/4 94 91
EE 13-Z 62.5 75.7
E 14/8/4 79 75
E 16/6/5 76 73
E 16/8/5 65 62
E 19/8/5 60 56
E 20/10/6 46 50 43
E 21/9/5 59 55
E 25/13/7 40 40 29
E 25.4/10/7 41 37
EE 25/19-Z 32.5 37
E 30/15/7 23 23 25
E 32/16/9 22 22 20
E 32/16/11 21 22 18
E 34/14/9 23 20
E 36/18/11 18 16
E 40/16/12 20 20 14
E 42/21/15 19 19 11
E 42/21/20 15 15 10
E 47/20/16 13 13 10
E 55/28/21 11 11 7
E 55/28/25 8 8 6.9
E 56/24/19 9.5 7.7
E 65/32/27 6.5 6 5.0
E 70/33/32 (E71/33/32) 5.5 4.4
E 80/38/20 7 5.3

EC

Core shape EC Rth (K/W)13) Rth (K/W)14) Rth (K/W)
calculated from (1)15)
EC 35 18.5 18 19
EC 41 16.5 15 15
EC 52 11.0 11 11
EC 70 7.5 7 7.2

EE LP

Core shape EE LP Rth (K/W)16) Rth (K/W)
calculated from (1)17)
EE LP 14 105 102
EE LP 18 56 54.3
EE LP 22 35 32.0
EE LP 32 24 21.3
EE LP 38 18 15.1
EE LP 43 15 12.9
EE LP 58 11 9.3
EE LP 64 9 7.1

EER

Core shape EER Rth (K/W)18) Rth (K/W)
calculated from (1)19)
EER 25.5-Z 30 35.0
EER 28-Z 22.5 21.7 - 22.9
EER 28L-Z 22 19.9
EER 35-Z 17.5 15.5
EER 40-Z 15 12.5
EER 42-Z 14 10.8
EER 42/42/20-Z 12.8 9.59

EFD

Transformer EFD25

S. Zurek, E-Magnetica.pl, CC-BY-4.0

Core shape EFD Rth (K/W)20)21) Rth (K/W)
calculated from (1)22)
EFD 10/5/3 120 138
EFD 15/8/5 75 76
EFD 20/10/7 45 43
EFD 25/13/9 30 28
EFD 30/15/9 25 23

EI

Core shape EI (core) Rth (K/W)23) Rth (K/W)
calculated from (1)24)
EI 12.5 Z 62.5 100
EI 16 Z 45 65.1
EI 19 Z 38.5 54.5
EI 22 Z 45 54.5
EI 22/19/6 33.7 41.8
EI 25 Z 32 37.2
EI 28 Z 30 24.6
EI 30 Z 21.5 19.4
EI 33/29/13 18.6 17.2
EI 35 Z 16.7 18.9
EI 40 Z 14.3 14.2
EI 50 Z 11.4 10.1
EI 60 Z 9.1 8.96

EI LP

Core shape EI LP Rth (K/W)25) Rth (K/W)
calculated from (1)26)
EI LP 14 116 114
EI LP 18 61 59.7
EI LP 22 38 36.0
EI LP 32 26 23.4
EI LP 38 20 16.7
EI LP 43 16 14.2
EI LP 58 12 10.2
EI LP 64 9.5 7.6

EP

Core shape EP Rth (K/W)27) Rth (K/W)
calculated from (1)28)
EP 5 329 352*
EP 6 318 346*
EP 7 141 142*
EP 10 122 121*
EP 13 82 79*
EP 17 58 54*
EP 20 32 29*
*The calculated values are given here for comparison only, as the results can differ significantly for the non-E-like cores.

ER

Core shape ER Rth (K/W)29) Rth (K/W)30) Rth (K/W)
calculated from (1)31)
ER 9.5/2.5/5 164 166
ER 11/2.5/6 (ER 11/5) 134 136
ER 14.5/3/7 (ER 14.5/6) 99 96
ER 28/14/11 22
ER 28/17/11 22 20
ER 35/20/11 18 15
ER 42/22/15 14 12 12
ER 46/17/18 13 12
ER 49/27/17 9 9 8.7
ER 54/18/18 11 11 9.7

ETD

Transformer ETD44 transformer_etd44_160w_magnetica.jpg

S. Zurek, E-Magnetica.pl, CC-BY-4.0

Core shape ETD Rth (K/W)32)33)34) Rth (K/W)35) Rth (K/W)
calculated from (1)36)
ETD 19/14/8 32.7
ETD 19-Z 34 34.1
ETD 24/15/9 26.2
ETD 25-Z 30 27.0
ETD 29/16/10 28 21.2
ETD 29-Z 23.3 21.8
ETD 34/17/11 20 19 17.7
ETD 34-Z 19-20 17.7
ETD 39/20/13 16 15 14.2
ETD 39-Z 15.3 14.2
ETD 44/22/15 11 12 11.2
ETD 44-Z 12 11.1
ETD 49/25/16 8 11 9.5
ETD 49-Z 9.8 9.5
ETD 54/28/19 6 7.7
ETD 59/31/22 4 6.3

EV

Core shape EV Rth (K/W)37) Rth (K/W)
calculated from (1)38)
EV 15/9/7 55 51
EV 25/13/13 27 24
EV 30/16/13 21 18

P

Core shape P Rth (K/W)39) Rth (K/W)40) Rth (K/W)
calculated from (1)41)
P 3.2/2.6 773*
P 4.6/4.1 424 *
P 5.8/3.3 314*
P 7/4 223*
P 9/5 142 143*
P 11/7 106 104*
P 14/8 73 100 70*
P 18/11 51 60 47*
P 22/13 37 38 33*
P 26/16 27 30 24*
P 30/19 22 23 19*
P 36/22 17 19 14*
P 41/25 12*
P 42/29 13.5 11*
P 66/56 4.7*
*The calculated values are given here for comparison only, as the results can differ significantly for the non-E-like cores.

PM

Core shape PM Rth (K/W)42) 43) Rth (K/W)
calculated from (1)44)
PM 50/39 15 8.3*
PM 62/49 12 5.7*
PM 74/59 9.5 4.4*
PM 87/70 8 3.8*
PM 114/93 6 2.3*
*The calculated values are given here for comparison only, as the results can differ significantly for the non-E-like cores.

PQ

Core shape PQ Rth (K/W) Rth (K/W)
calculated from (1)45)
PQ 16/11.6 49*
PQ 20/16 33*
PQ 20/20 30*
PQ 26/20 24 46) 21*
PQ 26/25 19*
PQ 32/20 18*
PQ 32/30 15*
PQ 35/35 13*
PQ 40/40 10*
*The calculated values are given here for comparison only, as the results can differ significantly for the non-E-like cores.

PQ (large cores)

Core shape PQ Rth (K/W)47) Rth (K/W)
calculated from (1)
PQ60/42-Z 3.0 6.6*
PQ60/52-Z 3.5 6.0*
PQ65/44-Z 3.8 5.7*
PQ65/54-Z 4.4 5.2*
*The calculated values are given here for comparison only, as the results can differ significantly for the non-E-like cores.

RM

Transformer RM8 rm8_core_3h1_a250_magnetica.jpg

S. Zurek, E-Magnetica.pl, CC-BY-4.0

Core shape RM Rth (K/W)48) 49) Rth (K/W)
calculated from (1)50)
RM 4 120 98*
RM 5 100 72*
RM 6 80 52*
RM 7 68 46*
RM 8 57 33*
RM 10 40 24*
RM 12 25 17*
RM 14 18 13*
*The calculated values are given here for comparison only, as the results can differ significantly for the non-E-like cores.

RM LP

Core shape RM LP Rth (K/W)51) Rth (K/W)
calculated from (1)52)
RM 4 LP 135 112*
RM 5 LP 111 84*
RM 6 LP 90 59*
RM 7 LP 78 51*
RM 8 LP 65 38*
RM 10 LP 45 28*
RM 12 LP 29 20*
RM 14 LP 21 15*
*The calculated values are given here for comparison only, as the results can differ significantly for the non-E-like cores.

UI

Core shape UI Rth (K/W)53) Rth (K/W)
calculated from (1)54)
UI 93/104/16 5 4.1*
UI 93/104/20 4.5 3.6*
UI 93/104/30 4 2.9*
*The calculated values are given here for comparison only, as the results can differ significantly for the non-E-like cores.

UU and U

Core shape UU Rth (K/W)55) Rth (K/W)56) Rth (K/W)
calculated from (1)57)
U 11 46
U 15 35
U 17 30
U 20 24
U 21 22
U 25 15
U 26 13
U 30 4
UU 93/152/16 4.5
UU 93/152/20 4 1.7
UU 93/152/30 3 1.2
U 101/76/30 3.3 2.4*
U 126/91/20 2.6*
U 141/78/30 2.5 1.8*
*The calculated values are given here for comparison only, as the results can differ significantly for the non-E-like cores.

See also

1), 4), 7), 9), 16), 20), 25), 27), 29), 32), 37), 39), 42), 48), 51), 53), 55) Epcos, Ferrites and accessories, Application notes, September 2006, p. 23-29, {accessed 9 Nov 2012}
2), 13), 35) Robert W. Erickson, Dragan Maksimović, Fundamentals of power electronics, 2nd edition, Springer, Colorado, 2001, ISBN 0-7923-7270-0
3), 11), 14), 21), 30), 33), 40), 43), 49), 56) Alex Goldman, Handbook of modern ferromagnetic materials, Kluwer Academic Publishers, 2002, ISBN 0412146614, p. 500
5), 12), 15), 17), 19), 22), 24), 26), 28), 31), 36), 38), 41), 44), 45), 50), 52), 54), 57) Sanjaya Maniktala, Switching power supplies A-Z, 2nd edition, Newnes, 2012, ISBN 978-0-12-386533-5, p. 155
6), 10), 18), 23), 34) TDK, Ferrites, Mn-Zn, Ferrite Core for Switching Power Supplies, E-series, EI, EE, EF, EER, ETD, December 2021 {accessed 2023-03-31}
8) Iron powder core loss characteristics, Amidon Inc., {accessed 2022-11-15}
46) Magnetics in Switched-Mode Power Supplies, ON Semiconductor, p. 51, {accessed 10 Jul 2013}
47) Mn-Zn Ferrite Cores for Switching Power Supplies Large PQ series, TDK, Ferrites, May 2016 , {accessed 2023-02-218}
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thermal_resistance_of_ferrite_cores.txt · Last modified: 2023/09/05 14:44 by stan_zurek
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