How to choose the right flyback transformer - With ucc38c45

Dear P.A.M.

Try "Practical Magnetic Design: Inductors and Coupled Inductors" by Louis R. Diana. You can easily find it on Google. Also "Magnetic Design Handbook" by the master of magnetics design Lloyd Dixon. Just Google it. Coilcraft may have the transformer for you application off the shelf.

Chuck

Hello,

Thank you Chuck.
Wurth transformer 750311599 looks good. What do you think ?
( katalog.we-online.de/.../750311599.pdf )

P.A.M

P.A.M.

Yes, the Wurth transformer looks good. There are some things I would check:

Make sure the turns ratio works for the minimum input voltage and maximum DC Gain -or in the case of the flyback min duty cycle. Make sure you include power losses when you determine the minimum required duty cycle.

Connect the primary so that its dot is opposite the secondary. Remember that flybacks actually use coupled inductors -not transformers.

Check that the isolation requirement is sufficient. I think it is. 

-Chuck

P. A. M.

One more thing:

The leakage inductance is rated at 250 KHz. I assume this means you need to have a switching frequency of 250 KHz to maintain a 30 W output in continuous conduction mode (CCM) I am also assuming you want to operate the flyback in CCM. It's up to you as the designer. So really what this means is you need to slog through the equations or use a software tool and figure out the design before you make a final decision. But I think you are very close.

Good luck,
Chuck

Hello chuck

I was out of office last end of week. I'll give you a feedback ASAP

P.A.M

Chuck,
Please correct me I'm wrong, but with low input and heavy load isn't better to operate in DCM ?

I have worked hard through equations, with this Wurth transformer,
- Max Duty cycle is around 50%, because Dmax/(1-Dmax)= Vin/(n*Vout ) = n*Vout/Vin ( n=1.5 ).
- So switching frequency ( Fs ) shoud be higher than 850kHz 420kHz If I'd like to operate in CCM.
( Primary transformer inductance = ( Vin_min*Dmax)^2 / ( 2 * efficiency * Pout * Fs * ripple_factor ) (ripple factor<1 for CCM operation ))

( Primary transformer inductance = ( Vin_min*Dmax)^2 / ( 2 * (1/efficiency) * Pout * Fs * ripple_factor ) (ripple factor<1 for CCM operation ))

But as you said before, Fs has to be close to 250kHz to maintain full load.
So I need to find out a better transformer.

Am I right ?

P.A.M

Hello P.A.M

I am working through your equations to check. I didn't mean to imply that you shouldn't design a discontinuous mode flyback. In fact, with this light of a load -15 W- that's what I would choose. There are advantages and disadvantages to either mode. DCM has the advantage of a smaller flyback transformer and no right half-plane zero in the loop gain transfer function. The disadvantages are high peak currents in the diode and transistor and higher core losses in the transformer. CCM requires a larger transformer but has lower peak currents. Also the transfer function of the loop gain is more complicated.
As to the 250 KHz, I only mention that as the switching frequency since the data sheet specifies the measured leakage current at that frequency. I just assumed that was the frequency you wanted to switch at. I wouldn't go any higher than 250 KHz. You might be able to operate below 250 KHz if you wanted.

I will get back to you on those calculations,
-Chuck
I

Hello Chuck

Thank you for your feedback about DCM vs CCM.
I'm waiting for for your feedback on those calculations.

Thanks a lot.

P.A.M

There is an application note that uses the UCC28C44 in a 12V isolated flyback bias supply that has a lot of the design equations that you could refer to (www.ti.com/.../slua274a.pdf), the major difference between the UCC28C45 and UCC28C44 is the UVLO thresholds. But the overall design procedure may help guide you.