The simplest and cheapest way to improve this cooling scheme – installation of a second fan at the back side of the PSU – is not very efficient and is only employed in inexpensive PSUs. The second fan is placed coaxially to the first one (or, better, with a slight shift to the center) and helps to blow at the power transformer and the both heatsinks as its airflow hits on these components directly. Below is an illustration of this cooling scheme – the Codegen 350X PSU:
More expensive PSUs – in newer models from Codegen as well as in the reviewed ASUS models – use different cooling schemes. First, we have the now-fashionable units with two 80mm fans, one of which is placed at the ordinary location and another is fastened to the cover of the PSU and is shifted to the center so that its airflow cooled the heatsinks as well as the group stabilization choke. The fact that the stream of cool air (well, not exactly very cool – it is taken from the computer’s case rather than from outside) is directed right at the heatsinks helps to improve their cooling considerably and, accordingly, allows using slower and quieter fans.
The more expensive model from ASUS, the A-30H, uses this scheme. Instead of the punched grill, there’s a wired one on the fans – it also helps to reduce the noise level.
The vent holes on the cover disappeared – the fan has replaced them – but they remained at the rear panel. The row of holes below of the passive PFC choke has been left intact, too.
Now, the last PSU cooling solution, also popular nowadays, although less so than the dual-fan scheme, puts a big 120mm fan on the PSU’s top, which blows at all the PSU components evenly due to its big size and creates a powerful airflow at a small rotation speed. Thus, the fan on the rear panel is not necessary anymore – there’s perforation instead of it. ASUS offers a model with one 120mm fan, too. It is the A-30G.
Of course, the back panel of the PSU is now without any holes. Otherwise, the hot air from the PSU would be exhausted back into the computer, which is definitely not what we need.