Table of Contents
Thermal resistance of ferrite cores
Stan Zurek, Thermal resistance of ferrite cores, Encyclopedia Magnetica, https://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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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 cm^{3} 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 |
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 (cm^{2}) |
E (EE)
Core shape E | R_{th} (K/W)^{9)}^{10)} | R_{th} (K/W)^{11)} | R_{th} (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 | R_{th} (K/W)^{13)} | R_{th} (K/W)^{14)} | R_{th} (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 | R_{th} (K/W)^{16)} | R_{th} (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 | R_{th} (K/W)^{18)} | R_{th} (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
Core shape EFD | R_{th} (K/W)^{20)}^{21)} | R_{th} (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) | R_{th} (K/W)^{23)} | R_{th} (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 | R_{th} (K/W)^{25)} | R_{th} (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 | R_{th} (K/W)^{27)} | R_{th} (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 | R_{th} (K/W)^{29)} | R_{th} (K/W)^{30)} | R_{th} (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
Core shape ETD | R_{th} (K/W)^{32)}^{33)}^{34)} | R_{th} (K/W)^{35)} | R_{th} (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 | R_{th} (K/W)^{37)} | R_{th} (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 | R_{th} (K/W)^{39)} | R_{th} (K/W)^{40)} | R_{th} (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 | R_{th} (K/W)^{42)} ^{43)} | R_{th} (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 | R_{th} (K/W) | R_{th} (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 | R_{th} (K/W)^{47)} | R_{th} (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
Core shape RM | R_{th} (K/W)^{48)} ^{49)} | R_{th} (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 | R_{th} (K/W)^{51)} | R_{th} (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 | R_{th} (K/W)^{53)} | R_{th} (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 | R_{th} (K/W)^{55)} | R_{th} (K/W)^{56)} | R_{th} (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. |