#### treez

##### Advanced Member level 5

Hello ,

We are designing a 1.7kW Full Bridge SMPS transformer using a PM62/49 core by Epcos TDK (N87 material).

We don’t want a gap in the core, but I believe We will have to have one because otherwise the change in the “saturation current” level {I(SAT)} varies too much as temperature changes….

As you know, the effective permeability (ue) of ungapped cores varies dramatically with temperature (see attached graph from Epcos)..

For example, an ungapped PM62/49 ferrite core using N87 material (by Epcos TDK) has ue = 1400 at 25degC..

However, at 120degC, ue = 3800

Now Consider that I have a 20 turn primary….

Now…..L(pri) = (N^2)/reluctance

[where N = Number of primary turns)]

Where reluctance = (core length)/ (uo.ue.A)

[where A = core area, uo = permeability of free space, ue = effective permeability)

At ue = 1400; L(PRI) = 3.03mH

….and I(SAT) = (Bsat.A.N.)/L = 0.77 Amps

At ue = 3800; L(PRI) = 8.24mH

…..and I(SAT) = (Bsat.A.N)/L = 0.34 Amps

[where Bsat = 300mT (saturation flux density); A=Core area; N = turns; L = inductance]

We need to know exactly what is our saturation current, and we simply cannot tolerate this level of variation in the saturation current level. If we work to the lowest level of saturation current, then we would in any case end up with a too big a core, as we would need to increase the core size.

Therefore, we will add a gap to the centre leg of the core…(just 0.2mm)….this means that the ue value will not vary over temperature, as is seen in Epcos graph attached.

Do you agree we need the gap?

(The gap will also help in making the saturation characteristic much less sharp, meaning that if we encounter sudden overload etc, then we are more likely to be able to ‘catch it’ with the current sense before the SMPS blows up. Also, during transients, our error amplifier can saturate high, meaning that our duty cycle goes to absolute maximum, and this can result in runaway saturation of the core if it is not gapped.)

So do you agree that a gap (just a tiny one) is needed?

PM62/49 Ferrite core datasheet:

https://www.datasheets.pl/ferrite_cores/B65684.pdf

The attached shows “ue vs Temperature” for gapped and ungapped PM62/49 cores.

We are designing a 1.7kW Full Bridge SMPS transformer using a PM62/49 core by Epcos TDK (N87 material).

We don’t want a gap in the core, but I believe We will have to have one because otherwise the change in the “saturation current” level {I(SAT)} varies too much as temperature changes….

As you know, the effective permeability (ue) of ungapped cores varies dramatically with temperature (see attached graph from Epcos)..

For example, an ungapped PM62/49 ferrite core using N87 material (by Epcos TDK) has ue = 1400 at 25degC..

However, at 120degC, ue = 3800

Now Consider that I have a 20 turn primary….

Now…..L(pri) = (N^2)/reluctance

[where N = Number of primary turns)]

Where reluctance = (core length)/ (uo.ue.A)

[where A = core area, uo = permeability of free space, ue = effective permeability)

At ue = 1400; L(PRI) = 3.03mH

….and I(SAT) = (Bsat.A.N.)/L = 0.77 Amps

At ue = 3800; L(PRI) = 8.24mH

…..and I(SAT) = (Bsat.A.N)/L = 0.34 Amps

[where Bsat = 300mT (saturation flux density); A=Core area; N = turns; L = inductance]

We need to know exactly what is our saturation current, and we simply cannot tolerate this level of variation in the saturation current level. If we work to the lowest level of saturation current, then we would in any case end up with a too big a core, as we would need to increase the core size.

Therefore, we will add a gap to the centre leg of the core…(just 0.2mm)….this means that the ue value will not vary over temperature, as is seen in Epcos graph attached.

Do you agree we need the gap?

(The gap will also help in making the saturation characteristic much less sharp, meaning that if we encounter sudden overload etc, then we are more likely to be able to ‘catch it’ with the current sense before the SMPS blows up. Also, during transients, our error amplifier can saturate high, meaning that our duty cycle goes to absolute maximum, and this can result in runaway saturation of the core if it is not gapped.)

So do you agree that a gap (just a tiny one) is needed?

PM62/49 Ferrite core datasheet:

https://www.datasheets.pl/ferrite_cores/B65684.pdf

The attached shows “ue vs Temperature” for gapped and ungapped PM62/49 cores.