LM2576/2596-ADJ noise problem


I've designed a power supply unit on LM2576-adj. There are 27V DC on the input and on the output I've set 20V. There are 3 LM2576's connected parallel after greatz bridge together with 3 electrolitic capacitors 4700u. On the output there is coil 220uH, 3A shottky diode and 1000u electrolitic capacitor. On the PCB there is 3 IC mounted to the heat sink.

 The problem which I receive is that when I connect load with more than 1A current, one of the IC's which is in the middle gave me a noise on the otuput approx 60mVp-p and about 3kHz freq (similar to saw shape). I can hear this in speakers in my audio system. On the rest two IC's I doesn't hear anything. Could you advice what can cause this. I've already exchange all the parts and it doesn't help. 

The PCB board is already updated according to schematic diagram (the wire layout is old). Below is the diagram and pcb board. 

Best regards,


  • In reply to Prashanth Suvarna:

    Sorry... had to conclude as i want able to type much longer as the width of the post started dropping alarmingly . Well one portion was that there was the ac mains ripple that was coming through.I remember this was a very common problem in the olden days of radio reception esp on the Mw and Sw bands. This used to cause an annoying hum when the stn was tuned in. This was taken care of by adding a ceramic cap (usually .01 to.1uf depending on the inclination rather than type of the radio in question ) across the power diodes in the power supply. I hope this takes care of burgh the problems that you seem to be battling with.Also I suspect that the series choke that you are using isn't capable of supplying the desired power. Try to change it and you should have some Improvement.... Do let me know the final outcome..