0-30 volt stailized power supply

[audioguru2]

Many power supplies use a fan blowing past the heatsink so that the heatsink can be smaller.

 

[Whizard]

Many power supplies use a fan blowing past the heatsink so that the heatsink can be smaller.

Should the fan blow the air pass through the heatsink or suck the air from inside the case where the heatsink located?