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thermal_resistance_of_ferrite_cores

Thermal resistance of ferrite cores

Stan Zurek, Thermal resistance of ferrite cores, Encyclopedia Magnetica, E-Magnetica.pl
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.1)4)

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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 volume1)

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):

(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.5)

(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)1) Rth (K/W)3) Rth (K/W)
calculated from equation4)
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)2) Rth (K/W)3) Rth (K/W)
calculated from equation4)
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)1) Rth (K/W)
calculated from equation4)
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
Core shape EFD Rth (K/W)1)3) Rth (K/W)
calculated from equation4)
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)1) Rth (K/W)
calculated from equation4)
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)1) Rth (K/W)
calculated from equation4)
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)1) Rth (K/W)3) Rth (K/W)
calculated from equation4)
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
Core shape ETD Rth (K/W)1)3) Rth (K/W)2) Rth (K/W)
calculated from equation4)
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)1) Rth (K/W)
calculated from equation4)
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)1) Rth (K/W)3) Rth (K/W)
calculated from equation4)
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)1) 3) Rth (K/W)
calculated from equation4)
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 equation4)
PQ 16/11.6 49*
PQ 20/16 33*
PQ 20/20 30*
PQ 26/20 24 6) 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
Core shape RM Rth (K/W)1) 3) Rth (K/W)
calculated from equation4)
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)1) Rth (K/W)
calculated from equation4)
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)1) Rth (K/W)
calculated from equation4)
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)1) Rth (K/W)3) Rth (K/W)
calculated from equation4)
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: 2022/11/15 11:03 by stan_zurek

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