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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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 |
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 (cm^{2}) |
Core shape E | R_{th} (K/W)^{1)} | R_{th} (K/W)^{3)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|---|
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 |
Core shape EC | R_{th} (K/W)^{2)} | R_{th} (K/W)^{3)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|---|
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 |
Core shape EE LP | R_{th} (K/W)^{1)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|
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 |
Core shape EFD | R_{th} (K/W)^{1)}^{3)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|
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 |
Core shape EI LP | R_{th} (K/W)^{1)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|
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 |
Core shape EP | R_{th} (K/W)^{1)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|
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. |
Core shape ER | R_{th} (K/W)^{1)} | R_{th} (K/W)^{3)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|---|
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 |
Core shape ETD | R_{th} (K/W)^{1)}^{3)} | R_{th} (K/W)^{2)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|---|
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 |
Core shape EV | R_{th} (K/W)^{1)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|
EV 15/9/7 | 55 | 51 |
EV 25/13/13 | 27 | 24 |
EV 30/16/13 | 21 | 18 |
Core shape P | R_{th} (K/W)^{1)} | R_{th} (K/W)^{3)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|---|
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. |
Core shape PM | R_{th} (K/W)^{1)} ^{3)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|
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. |
Core shape PQ | R_{th} (K/W) | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|
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. |
Core shape RM | R_{th} (K/W)^{1)} ^{3)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|
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. |
Core shape RM LP | R_{th} (K/W)^{1)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|
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. |
Core shape UI | R_{th} (K/W)^{1)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|
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. |
Core shape UU | R_{th} (K/W)^{1)} | R_{th} (K/W)^{3)} | R_{th} (K/W) calculated from equation^{4)} |
---|---|---|---|
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. |