step down switching power supply design

[kuldeep]

I want to design a step down switching power supply ckt with following specs:

I/P mains:
240V(~), 50Hz.

O/P:
15-20V, 2-3A

I'm new to power supply design, so please provide me information related to standard techniques, circuits and components.

 

[IanP]

If you dont have any experience with HV HF transformers you better start with a step-down transformer 240Vac/24Vac, use bridge rectifier and a ≈4700µ smoothing capacitor and then employ one of the switchmode step-down (buck) converters such as LM2576 or similar ( **broken link removed** ).

Once you gain experience in switchmode regulators you can switch to direct conversion from 300Vdc (240V after rectifying and smoothing capacitor will give you ≈300Vdc unregulated voltage) to 24/30Vdc.
Here is some info on the above:
http://www.onsemi.com/pub/Collateral/SMPSRM-D.PDF
and on SMPS transformer calculation
http://www.fairchildsemi.com/an/AN/AN-4102.pdf

 

[kuldeep]

Right now I'm using a step-down transformer (240Vac/12Vac) and LM2576T-3.3. The inductor (100uH, 3A) specified in datasheet is costly. Can I go for a approximate value inductor? The transformer cost makes the power supply even more costly.

I'm looking for a cheeper solution.

Thankyou, I'll go through the documents.

 

[homefree]

With little change on the output transformer, the following circuit may fit your need.

hXXp://focus.ti.com/lit/ml/slup058/slup058.pdf

 

[omore]

I suggest the following.

Visit www.powerint.com
Download PI Expert this will help you a lot in designing a SMPS like you wish, inclusive you will get detailes info in how to build the power inductor ( transformer )
Dont miss their application notes and tutorials.

Another site ( consider that most os power semis manufacturers give a lot of info ) is www.onsemi.com they bring a limited version of POWER 1-2-3 wich will allow to simulate your circuir ( of course using ON Semiconductor parts ) but anyway will be useful if you realize how ( askme if U don´t realize )

TI, FAIRCHILD, ST, PHILIPS, INFINEON, IR are also places where U willl be able to find lot of info and design guides.

Hope this help U