Gapping a ferrite core for a Full Bridge SMPS transformer?

[Flyback]

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.

 

[crutschow]

It would appear that a gap would help alleviate your problem.
You would need to do some measurements of gap length versus the core characteristics to arrive at the desired trade-off between desired inductance and core size.
You will want to do this over temperature also, since there will still be some change with temperature, even with the gap.

 

[ronsimpson]

Mr. FlyBack,

This is a forward converter? I think you are not storing any energy on the core. Power in and power out at the same time.

What voltage will be across the primary and for what time? Voltage x Time
Turns=20. Area 5.7cm^2.

 

[Flyback]

The above example of 20 turns is just for example.

In our actual design it is a 36 turn primary , core = pm62/49 and with 0.2mm gap
Nominally it will be 390v across primary for 17.9us.
However, during overload or transients, it could be 20.5us of on time.
And of course, the 390vdc is a pfc output, and these can go up to 410v transiently.

I appreciate that in a bridge converter there is no need to store energy in the core...but the point here is that a tiny gap always ends up being needed, or else you need a too big core...if you work through it you will see why.

Another point is can you imagine going into output overload or output short circuit with an ungapped core?......you need the gap to give you gentler transition toward saturation, so that your sense resistor can catch it before you get smoke....that's just one (minor) reason for the gap.

 

[chemelec]

I would use a gap of about 3/1000 Inch on each leg of the ferrite.

 

[Tony Stewart]

Message is unavailable.

 

[Flyback]

Thanks Tony Stewart you confirm my beleif that those who don't gap have got a gap anyway...just they don't know it.