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thermal_resistance_of_ferrite_cores

Thermal resistance of ferrite cores

Stan Zurek, Thermal resistance of ferrite cores, Encyclopedia-Magnetica.com, {accessed 2020-11-26}
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)


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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 volume6)

by S. Zurek, Encyclopedia Magnetica, CC-BY-3.0

Calculator of thermal resistance of ferrite cores

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):


Vcore =

      Estimated Rth (K/W) =



E (EE)

Core shape E Rth (K/W)7) Rth (K/W)8) Rth (K/W)
calculated from equation9)
E 5.3/2.7/2 308 334
E 6.3/2.9/2 283 290
E 8.8/4.1/2 204 210
E 13/7/4 94 91
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
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)10) Rth (K/W)11) Rth (K/W)
calculated from equation12)
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)13) Rth (K/W)
calculated from equation14)
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

EFD

Transformer EFD25

by S. Zurek, Encyclopedia Magnetica, CC-BY-3.0

Core shape EFD Rth (K/W)15)16) Rth (K/W)
calculated from equation17)
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 LP

Core shape EI LP Rth (K/W)18) Rth (K/W)
calculated from equation19)
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)20) Rth (K/W)
calculated from equation21)
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)22) Rth (K/W)23) Rth (K/W)
calculated from equation24)
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

by S. Zurek, Encyclopedia Magnetica, CC-BY-3.0

Core shape ETD Rth (K/W)25)26) Rth (K/W)27) Rth (K/W)
calculated from equation28)
ETD 19/14/8 32.7
ETD 24/15/9 26.2
ETD 29/16/10 28 21.2
ETD 34/17/11 20 19 17.7
ETD 39/20/13 16 15 14.2
ETD 44/22/15 11 12 11.2
ETD 49/25/16 8 11 9.5
ETD 54/28/19 6 7.7
ETD 59/31/22 4 6.3

EV

Core shape EV Rth (K/W)29) Rth (K/W)
calculated from equation30)
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)31) Rth (K/W)32) Rth (K/W)
calculated from equation33)
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)34) 35) Rth (K/W)
calculated from equation36)
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 equation37)
PQ 16/11.6 49*
PQ 20/16 33*
PQ 20/20 30*
PQ 26/20 24 38) 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.

RM

Transformer RM8 rm8_core_3h1_a250_magnetica.jpg

by S. Zurek, Encyclopedia Magnetica, CC-BY-3.0

Core shape RM Rth (K/W)39) 40) Rth (K/W)
calculated from equation41)
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)42) Rth (K/W)
calculated from equation43)
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)44) Rth (K/W)
calculated from equation45)
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)46) Rth (K/W)47) Rth (K/W)
calculated from equation48)
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

thermal_resistance_of_ferrite_cores.txt · Last modified: 2020/07/19 01:53 by stan_zurek

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