SMPS Design using lm78s40

[Lord Loh.]

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**

 

[Davood Amerion]

smps design

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This IC which you choose is obsolete.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

Regards
Davood Amerion

 

[Lord Loh.]

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.

 

[VVV]

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.

 

[Lord Loh.]

smps design step by step

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.

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.)

 

[omore]

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.

 

[Lord Loh.]

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.

 

[VVV]

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

 

[Lord Loh.]

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?

 

[Davood Amerion]

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.

 

[VVV]

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.

 

[Lord Loh.]

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.

 

[Davood Amerion]

unregulated power supply for smps

It is good for your purpose.