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Im working on a project and i need 3v3,5v and 12V in this. I though smps circuits would be better than Linear Regulators. So i choose LM2676-adj IC. I designed schemetics for 3V3-3A SMPS and 12V-3A SMPS. Read all datasheet and get all recommended pieces from distributor. Printed the PCB and soldered on it. when i supply(16V) the Input on Pcb, i read 12.10V and 3.28V and its OK for me. But when load the circuit for get near of 3Ampere (3v3 - 1.5 ohm & 12v - 4.5 ohm) problems begin. I always read 1.2V from output of 3v3 smps and 11.90V from output of 12v smps.

Output of 3v3 smps is stabile at 1.2v but output of 12v smps is not stabile, its up to 16V and down to 6V by the time randomly.

Here is my all components list, Schemetics and Board images, What do you think about this problem, how i can handle it?

http://imgur.com/a/3DTFQ?gallery [1]

For 3v3 SMPS I use:

IC: 1x LM2676-ADJ, Inductor: 1x P0841, Output Cap: 2x 100uF (10V 1.1A(Irms)), Input Cap: 2x 4.7uF (50V 0.6A(Irms)), Boost Cap: 1x 10nF, Schottky: 1x 40V (3A), FeedBack Res: 1K & 1K74

For 12V SMPS:

IC: 1x LM2676-ADJ, Inductor: 1x PE54039S, Output Cap: 2x 47uF (20V 0.9A(Irms)), Input Cap: 2x 4.7uF (50V 0.6A(Irms)), Boost Cap: 1x 10nF, Schottky: 1x 40V (3A), FeedBack Res: 1K & 9K1

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  • \$\begingroup\$ What does the input voltage look like - is it in fact an unsuitable supply for these circuits? \$\endgroup\$ – Andy aka Nov 20 '14 at 21:53
  • \$\begingroup\$ I use adjustable DC power supply like this bit.ly/1xYlM8D and supply with 16V , 17V, i believe its stable power supply. \$\endgroup\$ – srt35 Nov 21 '14 at 7:18
  • \$\begingroup\$ Do you have a scope where you can monitor voltages, especially on the feedback path? Given all the gnd copper you might have some coupling, maybe read this a bit: electronics.stackexchange.com/questions/17822/… also do you have the possibility to simulate this circuit? \$\endgroup\$ – PlasmaHH Nov 21 '14 at 11:57
  • \$\begingroup\$ I work on this problem and get meaningful results with load, the output voltages not going down, i replace all output capacitors with 100uF. Now i have problem like that:***for 3v3 & 5V(this is not adj,its LM2676-5V IC) for example i want to load for 1A , they gives 320mA. i want to load for 600mA they gives 200mA VOUT stable. Seems like divide current with 3. ***for 12V smps i want to load 850mA, it gives 650mA. i want to load 2.4A it gives 2A VOUT stable. Reason not the inductor, i change with different options nothing change. When watch with scope, all smps give 260kHz pwm signals.Pls Help \$\endgroup\$ – srt35 Nov 26 '14 at 9:00
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For Lm2676 IC:

I changed my power supply and multimeter,assembly new pcbs today with used all components again. The pcbs is ok, no voltage going down or no current limits. So i changed my power supply and multimeter back and test again, nothing change there is no problem. I cant say anyhing exactly for problem source. I think the IC pad maybe didnt soldered or pcb quality is not enough for smps.

All smps work perfect. So we have 3v3-3A, 5v-3A and 12v-3A now. I cant test in high ampere mode more than 10 seconds. My resistors is not enough for that watt. When i test for a longer time, i write here the results.

I cant work on LM2696 IC yet.

Thank you for your replies.

for 5V smps:

IC: 1x LM2676-5V, Inductor: 1x P0841, Output Cap: 3x 100uF (10V 1.1A(Irms)), Input Cap: 3x 22uF (50V 0.6A(Irms)), Boost Cap: 1x 10nF, Schottky: 1x 40V (3A), FeedBack Res: none but connect fb pin to VOUT

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