Smps load shut down

[davelectronic]

I'm trying to see if I can find a solution for a converted dell psu that shuts down with anything beyond a 50 watt halogen lamp load. This is the dell power brick that was originally powering the dell optiplex usff 745 / 760.
It's rated at 12 Volts 18 Amps, I've loaded it up with 50 watt lamp load and it's fine. But adding a second lamp, and it won't stay on, rather shutting down. I don't have a modest to large inductive load to try that, incase it was a resistive only issue. I think it might be seeing the higher resistive load as a short in the output.

What's thoughts on this , any ideas appreciated.  If it is seeing that 100 watt load as a short, what might be a likely solution ? Would some kind of soft start remedy this problem,  or maybe something in the power supplys feedback circuit. After looking at the under side of the pcb, reverse engineering this psu is probably beyond my ability. Just a way to add load and keep it running would be great. I've got remote and sense wires connected right at the load connection.  And have tried removing sense feedback wire, but it makes no difference.  I did think it might be adding the second lamp suddenly was the problem, but even connected from the start it won't power that 100 watt resistive load.
Thoughts appreciated and thanks for reading.

Ps. I have converted server power supplys over the years, and none shut down with 100 watt load. And I've even got up to 500 watts running fine on converted server power supplys. Just at a loss why this 12 Volt 18 Amp shuts down at 100 watts load, but happily runs a 50 watt load.

[MagicSmoker]

Halogen lamps - any incandescent lamp, actually - have a much lower resistance when cold. Something like a 16:1 change from 25C to the normal operating temp (2500C?).

[davelectronic]

So the cold incandescent halogen lamp is seen by the smps as a short circuit in the output stage ? Any ideas how to get round this, with this type of load ? Or maybe there isn't a way with this supply in particular.

[MagicSmoker]

The best solution is to modify the smps feedback loop so it operates in constant current mode when overloaded. Most cheap/commodity switchers go into hiccup mode when overloaded and these can have a tough time with incandescent lamps.

A next-best solution - more in the class of kludge - is to lengthen the soft-start time (usually only requires increasing a capacitor value) and make sure the lamp is connected to the smps before turning it on. You'll likely need a soft-start time in the range of 100-500ms.

Third-best - and really kludgey - is to put a fixed resistance in series with the lamp during startup then short it out with a relay or SCR or whatever after the filament heats to at least a dull red.