LM2596 Adj Output Current Problem - Page 1
Forum section: electronics / projects

author: sbdada
author_rank: Contributor
timestamp: January 15, 2016, 04:36:12 pm
content: Hello, I'm trying to make a DC-DC Buck Converter with LM2596 to get 5V 2A output from a 12V Battery. I used Adj version since only this version available here. also used the default diagram and pcb layout given in the datasheet.In my circuit:R1 = 4.7KR2 = 15KCin = 330uF 25VCout = 470uF 16VD1 = SB560L1 = I got it from an old PC power supply. It has a T80-26 core with 30 turns of 17/18 SWG wireThe problem is: In this way I got 5.2V with no load. But with a 1A load output decrease to 4.9/5V and with 1.5A load it decrease to 3.5V. I then re-winded the troide with 47turns of 20SWG, but the result is same.I'm a newbie in this area so have no idea about the problem. Can you please help me to solve it? Thanks in advance

author: Paul Price
author_rank: Super Contributor
timestamp: January 17, 2016, 03:06:06 am
content: The likely reason is that your inductor has too high an inductance, try to verify that you are winding your toroid core to create about 27uH.Try using one quarter to half as many turns on this core.Try to verify the inductance the easiest way would be using a LC meter, but it you don't have one:If you have an oscilloscope, you can parallel a known value capacitor (100pF to .001uF for example) across your inductor and determine the resonant frequency by ringing it with a single short square wave pulse(100nSec to 1mSec approx.) and observing the sine wave period. You can ring it by pulsing this resonant circuit by coupling a pulse to a couple turns of wire wound on the same core and you should see the core/cap sing its tone between applied pulses. Once the ringing period is known, then Freq=(1/Period) is known and you can solve the resonant frequency equation F= 1/(2pi*(sqr(L*C)) ) for L.https://en.wikipedia.org/wiki/LC_circuitYou can create the test pulse using a 555 timer if you don't have more sophisticated equipment for making test pulses.

author: sbdada
author_rank: Contributor
timestamp: January 17, 2016, 07:14:33 am
content: Thank you Mr. Paul for your kind attention. I've not any LC meter or oscilloscope. That inductor was at first 40-50uH according to datasheet. One of my fb friend who has a LC meter made an inductor with same core and same turn of same gauge wire for me and measured 57uH. It didn't worked which i mentioned before. Then I re-winded it to get arround 100uH followed by datasheet. ok, now i'll try with a lower value inductance and will inform here after the test

author: sbdada
author_rank: Contributor
timestamp: January 17, 2016, 08:13:34 am
content: Test 1:Inductor : 22 turns of 18 gauge wire.approximate inductance: 22uHresult:4.8V at 1.1A4.1V at 2.2Ainductor is little warm__Test 2:Inductor : 14 turns of 18 gauge wire.approximate inductance: 10uHresult:4.8V at 1.1A4.5V at 2.2Ainductor is hot within a few minutes

author: Paul Price
author_rank: Super Contributor
timestamp: January 17, 2016, 11:10:55 am
content: From the readings you show, the only other cause would be(in order of descending probability):(1) your 12V battery is not supplying enough current. I t must be able to deliver >16 watts and >3A peak.(2) You are running long and too slim wires to the battery and the input cap is open.(3) Your output cap is open.(4) Your chip is fried or the catch diode is open circuited.(5) The output filter cap is open.(6) Your measurements are in error...your original measurements posted are inconsistent at 4.9/5V output at >1A.Lowering the inductance should ensure the required current is delivered. If the saturation current of the core is too low, the nductance becomes too low or would saturate and overheat but the required current would be delivered. If the inductance is too large the ontime of the chip with a fixed sw. freq. will not be long enough to ramp up the current to the desired 3A output. So, if the output V drops with a smaller inductance, then the input voltage to the chip must also be dropping.

author: mij59
author_rank: Frequent Contributor
timestamp: January 17, 2016, 11:21:49 am
content: In test2 the losses in the iron powder core are too high, material 26 is not very well suited for frequencies greater than 100 kHz.I think it's a feedback or layout problem, please post a picture of the pcb, setup.