You know, it's over the GPU heatsink in S4 mini. I didn't test the PSU heat in open air, so maybe. I also didn't use the little fan, because my maximum power is only about 220W (GTX1066 with 65W i7). Strangely, when using the HDPLEX 400 PSU, my max power never jump over 190W. Probably my old PSU is very inefficient. But that doesn't explain the heat from HDPLEX. I don't have thermometer or the fancy temperature gun, so I don't know how hot it is. Maybe it's just something 40 or 50 Celsius degrees.