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SMPS Design using lm78s40

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Lord Loh.

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lm78s40

I am planing to design an SMPS using μA 7840 IC.

I have a schamatic from a textbook (Op-Amp and Linear IC - Gaiakwad).

What I understand is that SMPS have an advantage of small size. But the datasheet shows that the SMPS IC requires a DC bias. Where do I get this DB bias from? A simple rectifier or a linear regulator.
Should I have to use a linear regulator, There shall remain no purpose of using an SMPS IC.

How exactly is the SMPS to be designed?

**broken link removed**
 

smps design

Hi Lord Loh.
This IC which you choose is obsolete.
you can use LM2575, LM2576, L4978, ...

Regards
Davood Amerion
 
78s40

Oh My!

I have ordered for one at Rs180/- (~US$3)


Any way, even in the data sheet of other ICs, I see that they all have a Vin.

What is Vin? An output from a linear regulator? How do I take advantage of small sized transformer in SMPS? I have to rectify the line frequency to feed my SMPS IC. :(
 

smps design using lm78s40

Vin is the unregulated input voltage. Typically, this comes from a transformer+rectifier+ filter cap. This voltage can be up to 40VDC. The 78S40 will then step down this voltage to the value you set by choosing the divider resistors.

The 78S40 is a DC/ DC converter, and not meant for AC/DC power supplies. So you cannot say that it will give you any transformer size reduction.
 

smps design step by step

VVV said:
The 78S40 is a DC/ DC converter, and not meant for AC/DC power supplies. So you cannot say that it will give you any transformer size reduction.
Click to expand...

So what can get me a transformer size reduction?

I am experimenting with my first SMPS (so far in thoughts only. Ihave not yet laid my hands on the hardware.)
 

smps design rules

Well although it is an experiment, think to gain knowledge in smps, U must give an idea of what you need. For example.

Input voltage. If U use a low voltage IC, you will need to step down the input voltage...then rectify it. In this case U will not see a reduction size improvement in the transformer as it will have to handle all the output power. If U wish an small transformer U will have to work on an OFF LINE SMPS, in this case, as the frequency will be higher than the line freq. U will get an smaller transformer or inductor.

Suggest U visit power integration site www.powerint.com they have devices that work from the line voltage.....

Of course onsemi TI, fairchild, ST and masny others have devices for this kind of apps.
 

my first smps

Okay...

But What about the laptops and other devices that use battery chargers? I believe that they use smps. And their size is small. Moreover, they do step down the voltage...

I am only making an SMPS to increase my knowledge of SMPS. However, as an end result, I would like to have a compact power supply that can supply about 3A(1A will do) at 5v to 12v.

My key fascination about SMPS is it's size and DC / DC conversion capability(especially step up) and high efficiency. These are all the features I would like to get hands on experience in designing.
 

step by step smps

Well, then you should start with the buck regulator and understand how it works. Especially that you have a chip to play with.
Once you've done that, you can try a forward, which is a buck-derived topology, but uses a transformer for isolation.

Then you can try the boost and later the flyback, which is boost-derived, but again uses a transformer.

Once you know these basic things, you can build almost anything you want. The only differences will voltages and currents
 

bias transformer in smps in detail

Can I design these on a bread board? Is 60KHz to 300Khz not too high for a bread board?
 

lm78s40

yes, but i dont recomend this metode for swithing power supply.
because if one special connection, say feedback path, disconnected, circuit components will dameged.
 

smps using ic

You can experiment with your 78S40 on a breadboard, if it is one mde by you, observing construction techniques. I do not recommend plug-in prototype boards, but pefboard work very well, again paying attention to how you lay it out, just like you would do on a normal PCB.
 

high power step down regulator with lm78s40

The board that I have is the ones with many holes 5 shorted in a line on both sides of a groove and a set of two wire buses on withe side of the board...Don't know what exactly it is called.

But a 10KHz diode rectification circuit showed distorted waveforms.
 

unregulated power supply for smps

It is good for your purpose.
 

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