Phase Shifted Full Bridge - Transformer's magnetizing inductance

[tynnor]

Hello every one
I am still studying a PSFB 48V to 350V 1kW.
Despite all my researches, there is something I am still unable to understand : how to choose the transformer's magnetizing inductance ?
In the UCC28951 documentation, page 42, TI gives the minimum condition to stay in CCM operating. But what about the maximum value ?

I have unfortunately built a transformer with a much bigger magnetizing inductance than this minimum (~200µH vs 15µH). Will this trouble the current mode control ? I don't find any documentation about this

Thank you for your help !

 

[FvM]

There's no maximum value. Push-pull transformers are designed with a small air gap to avoid core saturation in case of duty cycle asymmetry. If the DC balancing is provided by the CM controller, you may omit the air gap.

 

[tynnor]

Thank you for your answer

How can I justify this ? Do you have documentation about this ?

 

[Easy peasy]

The magnetising inductance falls out from the peak design flux density (B) in the core ( which is usually a function of freq ) - if you have designed for a Bmax of 150mT each way at say 70kHz then the number of turns this turns out to be along with the AL of the ungapped core will give you the Lmag ( N^2 x Al )
you can have more pri turns than this ( to lower Imag, and raise Lmag ) or you can gap the core slightly to lower Lmag and raise Imag...

sometimes a lower Lmag ( higher Imag ) is useful to extend soft switching to lower values of load...

good luck ...!

 

[treez]

In the UCC28951 documentation, page 42, TI gives the minimum condition to stay in CCM operating. But what about the maximum value ?

....its nothing to do with "CCM".......with small Lmag, its saying you can be going towards voltage mode control because the magnetising current acts as a ramp which gets added to the current mode ramp.
In truth, going towards voltage mode control isnt the end of the world, but if you want current mode control, then make sure that a decent proportion of the control ramp is from the current sense resistor.

--- Updated Aug 1, 2020 ---

Attached is a PSFB sim in the free LTspice.
If you run it, i put an expression there, which if you put it in the waveform window, it will show you the magnetising current ramp.

 

[tynnor]

Hello,
Sorry for the very late answer. I had other tasks, holidays...
Thank you a lot both of you for for your answers and the simulation file, I will have a look
Bye