UCC28950: Shim Inductor's value is negative calculated , what can be happen?

Hello Mikail

I assume you used the Excel calculator for the UCC28950 PSFB controller. If the calculator gives a negative value here is simply means that there is already enough energy available to force the ZVS transition without needing a shim inductor to supply extra energy. The shim inductance appears in series with the leakage inductance of the power transformer.  Here's a document that gives some more information about the energy needed to force the ZVS transitions in the PSFB.

/cfs-file/__key/communityserver-discussions-components-files/196/8726.ZVS_5F00_SubSet.docx

Further, I'd suggest you use the UCC28951 instead of the UCC28950. The UCC28951 is a newer device than the UCC28950 and it works better at wide duty cycles. The two devices are otherwise identical.

Please let me know if you need any further clarification.

Regards

Colin

Dear Colin

First of all , Thanks for the "very fast answer".

That's true i use the calculator. The reason that i ask this question is that the value's of the  "Tabset,Tcdset"  even "Cin" is going out of the rail
if a put "0uH's on the Ls. 
what should i do? Really i need you suggestions additionally  thanks for the suggestion about the new device's.

Best Regards

Mikail Unal

Hi Mikail

Probably the best thing is for you to send me your calculator and I'll review your numbers. You can send it to me at colingillmor@ti.com if you don't want to publish it here.

One thing you should try is to put the sum of the calculated shim inductor and the transformer primary leakage inductance into the cell at C61 ('Shim Inductance Used'). The calculator uses ONLY the shim inductor value when it calculates the delays whereas it should use the sum of the shim and leakage inductance. I'll try to get this fixed but for the moment you can use the workaround above.

Cin is a little strange - it's sized so that it has enough energy to allow the system to keep the output in regulation for 8.3ms after the input power source quits. If the input power source is a PFC stage then you will need a fairly large capacitor in any case but if the input power source is a DC supply or a battery then the calculator will overestimate the capacitance needed.  If you let me know how you want the PSFB to react to an input supply failure then I can review and make some suggestions. I'd need to know the input voltage range during normal operation and what holdup time you need, if any.

Regards

Colin