hello all, a problem with a smps anyone, pls, i am aware that smps's come in lots of guises, this is a fairly basic one with an active pfc circuit, and it supplies voltages of 24v, 12v and 5v. the 24 and 12 lines only come on when asked by the processor, but my dilemma is one of the 5v line, if the supply is started off load on the 5v line it starts ok, but as soon as the line is loaded from cold it trips and settles at about 2.4v, all obvious things have been checked, caps on the secondary and primary, however if the psu is then provided with a load(after starting) the supply continues to run, it is my own unit so im happy to mod it! what i will do is devise a method to only connect the load once the psu is started, a simple switching transistor with a short delay i need, although in the tv trade for many years design was never my field
many thanks to anyone who can help
tony walker

 

Starting with the obvious, is the 5 volt supply its own supply? As tv's have to hit that <1 watt standby draw this is frequently the case these days, in come cases the auxiliary 5/12 supply provides the power for the control logic for the rest of the board, saves using a resistor off the 120vac line.
Also, what is the minimum current needed to keep it running smoothly? 50 ma should be all that's necessary on a 5 volt line, and if this supply is intended to be run with the 5 volt connected to the rest of the TV then i'm not sure there is a real problem here?

 

thanks for your reply, the 5v line is rated at 4.5 amp, the reason it is this current is that the screen requires over 3 amps. however, i found the fault, on the power fet there is a .33 ohm resistor, it is there to provide current limiting in the event of a s\c on the secondary, if the voltage across it rises this is monitered and the psu goes into a protect mode that protects the fet. it had risen to just over 1ohm, bring back the mains dropper, it was easier, thanks again
tony walker