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increasing output cap on SMPS

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Thunderchild

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I'm putting together a SMPS with the LT1074 (8-60 V in 5 A out 100 KHz with 100 uH inductor) the datasheet specifies a 500 uF output condenser but I don't have one. can i use a bigger one ? what about 1000 uF ? otherwise it will have to be 3 X 220 uF (660)
 

Thunderchild

New Member
ok but I'm also thinking am i better off with one 1000 uF or 3 X 220 uF for in/out current carachteristics ESR its called I beleive or am I being over fussy for 100 KHz (this is the second only SMPS I build last one was real simple - still proud of it)
 

Hero999

Banned
Remember to use a 100nF ceramic capacitor too.
 

Hero999

Banned
Both.






.
 

Thunderchild

New Member
bearing in mind I've already doubled the inductor size from 50 uH to 100 uH (on sound recomendations) so would larger capacity be even more so appropriate ?
 

Hero999

Banned
Output capacitors are more critical than input capacitors.

It depends on the frequency used as well but I'd say yes.
 

Hero999

Banned
A yes to both capacitance and lowest possible ESR.
 

sPuDd

New Member
I've used the LT1070 LT1170 and LT1268 quite a lot. Even though they are boost and yours is buck, they seem well built and very forgiving.

3x caps in parallel would be nicer.

Also I just find a chunk of ferrite big enough to choke on and wind it full of heavy gauge wire. The inductance value I've never bothered with. You might pay attention to it if you were designing for a mass produced item, as you would need a part you can replicate.

Oh, and always use a nice big schottky diode.

sPuDd..

Laptop12V-19V5ABoost.jpg
 

Thunderchild

New Member
I'm using a 10 A heat sinkable svotky but guess the heatsink is not needed for 5 A X 0.4 V = 2 W it will be more like 3 A
 

sPuDd

New Member
I'd still attach it to the heatsink - just in case (pun). It may stop thermal runaway in a serious fault.

sPuDd..
 

Hero999

Banned
Larger capacitors have lower ESR anyway, use what you have and then test it.
Unfortunately larger capacitors also have a higher ESL.

For example a 1000:mu:F capacitor with an ESL of 100nH will resonate at just under 16kHz
 

Thunderchild

New Member
so I've done best to put in 3 X 220 uF, so basically and capacitance in a "frequency circuit" is best made up of smaller capacitors if a large value
 

smanches

New Member
bearing in mind I've already doubled the inductor size from 50 uH to 100 uH (on sound recomendations) so would larger capacity be even more so appropriate ?
Why did you do this? Did you also halve the switching frequency to make up for it?

Inductance is inversely proportional to switching frequency and load current, so if you increase the inductance, you're decreasing the needed switching, as well as decreasing the total output current.
 

Thunderchild

New Member
I got the chip and inductor from picbits, he said the larger one would be fine and thats how he's using them, the chip will auto compensate and reduce frequency as neccesary
 
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