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problem with smps design

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sandhyak

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I am trying out an smps circuit using top switch ic. i need an output of 12v and 5v.. so i hav taken the circuit of 12v smps and i am generating 5v from it using 7805. but the problem i hav is that, when i connect smps to my circuit, the regulator ic 7805 is getting heated. the current requirement of my circuit is less than 1A. i would like to know why is this happening.. can anybody help??
 
sandhyak said:
I am trying out an smps circuit using top switch ic. i need an output of 12v and 5v.. so i hav taken the circuit of 12v smps and i am generating 5v from it using 7805. but the problem i hav is that, when i connect smps to my circuit, the regulator ic 7805 is getting heated. the current requirement of my circuit is less than 1A. i would like to know why is this happening.. can anybody help??

A 7805 is 1A maximum, if you're feeding it from 12V it will dissipate 7W while providing 1A of current - simple maths!.

This means it will require a substantial heatsink, I suggest you try looking up the datasheet for it, to check it's maximum dissipation, and suitable heatsinking.

You could also try inserting a resistor between the 12V supply and the input to the regulator (work it out from ohms law), this would need to be a wirewound resistor. This still requires the removal of 7W of heat, but shares it between the resistor and regulator.

You could also use a larger 5A regulator, but you're still going to need a heatsink - unless you change it for a switchmode 5V regulator.
 
hi Sandhyak,
Nigel Goodwin wrote a complete solution. if you want to use a series resistor between 12V supply and 7805, keep in mind that the input voltage of 7805 should not be less than 8 volts because of the biasing requirements of 7805 itself.
you can use LM7805CV or LM338 instead of normal 7805
 
If you use a resistor in series with the 7805 then the resistor will get very hot instead of the 7805.
 
If you don't want a heatshink then you'll need a separate SMPS for the 5V supply.
 
If you use a resistor in series with the 7805 then the resistor will get very hot instead of the 7805.
True but wirewound power resistors can be run smoking hot all day long while IC's get very upset if their junction temps exceed 130C.
 
If you haven't got the right resistor, string 10 off 1N4001 diodes in series with the 7805. It's messy but it worx. Add or subtract diodes as needed to get the 7805 working comfortably, not too hot and not losing regulation.
 
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you could wind a separate winding for the 5v section...
The trouble is it won't be regulated, it's better to have the 5V section regulated and 12V unregulated.
 
The trouble is it won't be regulated, it's better to have the 5V section regulated and 12V unregulated.


Ermmm.not really sure what the op is trying to achieve...Suffice to say, he already has one winding which is supplying him with juice for 12v yes? To that he added a5v reg which is dissipating a lot of heat...So my suggestion was to wind another winding with less turns to reduce the dissipation...

My orig response mebbe wasnt clear...well clear as mud I can see..:)

Perhaps now my suggestion might be a little clearer?

Sampling the 5v output would generally be the favoured option but I guess it depends on the loading of the 12v rail too?
 
The application was never mentioned, so the "degree" of regulation needed is unknown.

It is quite common for multiple output SMPS to have 2 or more outputs "contribute" to the feedback loop... it will depend on how much regulation is needed and in what proportion you want each output to dominate. If your familar with the SMPS books by Keith Billings or Marty Brown... it's explained in there in detail and examples are provided. If it's a forward converter, a coupled output inductor will help regulation... just make sure that the turns ratio of the inductor matches the transformer secondary ratios... Lloyd Dixon of (Unitrode) TI wrote some great app notes on SMPS magnetics design... they are available on TI's website.
 
hi Sandhyak,
If you are using a TOP SWITCH, you can use multiple secondary windings to create voltages you need. Fortunately, Power Integrations Co. supports its products with a plenty of documents. all you need is reading the application notes!
good luck!
 
Hey,
Does your SMPS has optical feedback ? If yes, Adjust the potential devider of the TL431 to adjust 5 volt insted of 12 Volt. Then no power loss wile occure like in series regulator.

for help, I am giving the lnk of the Application notes to adjust.
Code:
http://www.powerint.com/sites/default/files/product-docs/an43.pdf

--Thanks
 
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