LNK304 design Support Reqd

Posted by: prabhdip_er on 03/11/2009

Hi,

I am designing one SMPS using LNK304PN from POWERINT.

AND der48.pdf application notes fits best to our design needs. Should i opt this FLYBACK design methodology and for this i have explored 80% of design formulas, but some of these are still little bit confusing. As in this application note, on page 12 under XMER spreadsheets...following parameters:

1). KP= 3.00 (How it has been chosen more than 1)

2). DMAX =0.32 Maximum Duty Cycle (how this has been calculated, as result do not fit best as per AN17.pdf)

3). LP =1795 uHenries Primary Inductance (how this has been derived?)

Could you please advice suitable formulas which have chosen to make calculation of these above three paramaters during design?

Looking forward to hear from you soon!

With Thanks n Regards,

Prabhdip Singh

M: +919899509909

Comments

It is still a little unclear to me as to which document you are referring to but here are some answers to the questions you posted
1) How can I choose KP to be greater than 1
You will recall that KP > 1 signifies a discontinuous design and thereafter KP is a measure of how discontinuous the design is. Larger the KP value, quicker the secondary diode current falls to zero
In on off control you CANNOT select KP of a design, KP is a function of the chosen device (ILIMIT) and value of inductance LP

2) DMAX - DMAX is calculated for discontinuous mode design. You should look at the general equations and methodology in AN 23 which is applicable for on off control and not AN17 which is applicable to PWM control.
http://www.powerint.com/sites/default/files/product-docs/an23.pdf

3) LP - Again look at An23. LP is chosen such that it is the minimum value that can deliver the power specified.

Hi,

Thanks for your valuable advice regarding design confusions of LNK.
FYIP: I was asking about following design example report:http://powerint.sphere.kharkov.com/PDFFiles/der48.pdf

Now query 1 and 2 are almost clear, as i have implemented formulas given AN23 and those have given exact values as published in design example report.

For query3: i have implemented Lp for discontinous mode operation. but didn't get exact value as published in design example report.

Could ypu please sapre your precious time for same.

With Thanks n regards,
Prabhdip Singh

The value should be very close to the value as specified by the expression for LP in AN 23.