LM5017: FLY-BUCK-QUICK-START-CALC

luck luck

Part Number: LM5017

couple of queries before attempting isolated multi-outputs with LM5017

* Given VINmin=16v & IVSEC1=15v, IVSEC2=2.5v, IVSEC3= minus 5v each @30mA, does it make sense to start design with non-isolated VPRI = 10v targeting isolated secondary outputs through proportionate tuns ratio of fly-buck transformer

* "Transformer" sheet not designed to pick input-output parameters from "Calc" sheet e.g. even Lpri is different between the two sheets 50uH and 40uH

over 5 years ago

0 Samuel Jaffe over 5 years ago

TI__Mastermind 25590 points

Luck Luck,

I would recommend selecting non-isolated VPRI so that the maximum duty cycle (at min VIN) is no more than 50%. This app note has lots of good information as to why. Then yes, you can use different turns ratios to get different VOUTs.

This is not an exact science due to the many different parameters for optimization. Depending on what type of optimization you're going for, 40uH or 50uH may be your choice but they should both work just fine. Remember that's the magnetizing inductance of the transformer. You want the leakage inductance to be as small as possible.

-Sam

0 luck luck over 5 years ago in reply to Samuel Jaffe

Intellectual 450 points

Great, gives good direction. Many thanks, will explore the app. - very best regards

0 luck luck over 5 years ago in reply to Samuel Jaffe

Intellectual 450 points

selecting non-isolated VPRI so that the maximum duty cycle (at min VIN) is no more than 50%

* Does above mean, keeping Dmax less than 0.5 i.e. "B15" with reference to "FLY-BUCK-QUICK-START-CALC"

* The app-note recommends keeping duty factor below 75% , with reference to figure 2, Vin/Vout less than 1 means Vout > Vin, which is invalid case for buck topology

0 Samuel Jaffe over 5 years ago in reply to luck luck

TI__Mastermind 25590 points

Yes, B15 is the max duty cycle. You should keep this under 50%. Anything higher may negatively affect your regulation on the secondaries (see the app note I linked before, figure 4.)

That mention of VIN/VOUT is a typo. You are correct. It should say VOUT/VIN as it does elsewhere in the document. Thanks for catching that.

-Sam

0 luck luck over 5 years ago in reply to Samuel Jaffe

Intellectual 450 points

Hello Sam,
It helps a lot, thank-you so much !!

Power management

Power management forum

LM5017: FLY-BUCK-QUICK-START-CALC