SMPS acting strange under heavy load

[SnipTheCat]

Hi,

  I have a 18V/30A switching power supply that is acting strange ("standard" chinese SMPS). It can be adjusted from 11.5V to 20.5V, and I'm using it to power an induction heating circuit. I usually start at minimum voltage and crank it up.

  I have made a new induction circuit which is power powerful, with larger coil and crucible. It starts as 11.5V/16A under load. I noticed that when I reached ~15V the voltmeter module connected to the supply showed decreasing voltage... I didn't pay much attention and thought it might be the voltemeter's wiring that has some issues, and as the cooling fan of the induction circuit kept having higher pitch. I ran like that for a while, wihtout any problem (ammeter reading 23-24A), melted some aluminium, no apparent problem.

  Today I took a closer look at highest setting (20.5V)... verified the voltmeter... same problem, I connected my multimeter to the supply, above 15V it started to have crazy readings... I switched to AC and it read a good stable 3V, brought the oscilloscope... see the picture attached... huge 3Vpp ripples at 100Hz (nothing visible < 15V/20A appears after that)

  I naturally suspect the power supply (100Hz is the frequency of rectified AC mains), and some kind of protection circuit kicking in and out.   

  I tried connecting 2 10000µF caps to the supply output... voltmeter then had a nice clean 15V reading and ammeter didn't go higher than 20A, also the cooling fan didn't seem to run faster as I tried to go above 15V.

  Any idea what to look for, is this fixable? Supply is supposed to be 540W not 300

David.

 

[bktemp]

Maybe some caps inside the power supply have failed (large caps after the mains rectifier) or the power supply is overrated and goes into some current limit mode when the input voltage drops.

[Mechatrommer]

Probably you have reached what the input capacitor or the main transformer can supply at that volt level...

[SnipTheCat]

Maybe some caps inside the power supply have failed (large caps after the mains rectifier) or the power supply is overrated and goes into some current limit mode when the input voltage drops.

Just tested the input caps... they don't look good... 4x 330µF / 200V : capacitance from 120 to 135µF ESR of ~0.4ohm for 3 of them 0.7 for the 4th.... I think that's a good lead...

[SeanB]

Do they look too small to be 330uF caps, and is the PCB layout where they are set up with space for larger ones? If so then they are relabelled 100uF caps, and should be replaced, along with the ones in the secondary side, as they likely are relabelled ones as well.