Why the minimum load?

[atomicbird]

I just replaced the power converter in a lighting fixture with 5 20W 12V halogen lights.

I had been thinking I'd replace the halogens with CFLs or maybe LEDs, but the power converter (PDF data sheet) indicates it has a minimum load of 50W. Lower power lights would likely take me below this limit. This converter takes standard 120VAC/60Hz and produces 11.8V at 30kHz.

I don't know how this kind of converter works, so I'm hoping someone could explain why there's a minimum load. If I go below it, will it just not turn on?

 

[Speakerguy]

Depends. It might just go out of regulation and give a different voltage, or have oscillations, or who knows what. My guess would be higher output voltage than spec, just looking at the load curve. Hook up a power resistor and see what it does (or just one halogen bulb @ 20W).

 

[RCinFLA]

Many switching power supplies require a minimum load.

For a fixed frequency, pulse width modulated, switching power supply, as the load get lighter the duty cycle gets shorter. Below about 5%, things get dicy for the stabilty of the feedback control voltage regulation.

Many new supplies allow pulse skipping to keep the duty cycle above a minimum level. This can give a bit more ripple in the supply voltage however.

 

[Sceadwian]

The different voltage could be as high as the supply voltage, and the oscillations could cause circuits to have complex and destructive AC interactions. Speakerguy pretty much said it all.