transformer 101 35.9 37.6 40.1 44.2 48.9 54.8 61.4

transformer 104 34.8 36.4 38.3 41.3 45.0 49.4 55.0

transformer 107 33.8 36.6 39.8 36.1 50.9 58.5 66.7

winding 103 37.6 39.6 41.3 44.0 47.6 52.0 57.0

winding 111 38.4 41.2 43.2 46.4 50.5 55.7 61.6

coil 108 33.9 39.6 37.9 41.2 45.1 50.2 66.7

coil 109 31.6 35.6 34.7 37.0 40.0 42.9 56.0

performance of the air cooling system. Due to the high number of thermocouples and the limited space available inside the SMPS, we were able to measure the internal air temperature at the inlet of the finned heat sink just with a single thermocouple. In Figure 6, it is shown the cross distribution of this temperature, which is meas- ured in a plane placed 26.5 mm before the entrance of the finned heat sink. For reasons of symmetry, only half channel is shown. The vertical lines show the position of the openings of the fan. As evident in Figure 6, the temperature distribution in front of the fan is quite uniform along a cross section and this justifies the opportuni- ty to characterize the air temperature with a single measurement.

First of all, we need to discuss the efficiency of the cooling system. Figure 7 shows four distributions of temperature: ambient air, internal air after the finned heat sink, water at the inlet of the cold plate, surface of the transformer exposed to air flow. The whole set of data refers to a same DC power (3006.2 ± 0.4 W) dissipated in the dummy load for the same voltage (40.8 V). The temperature distributions are shown as a function of the fan voltage. This is proportional to the speed of rotation of the fan and consequently, through the fan similarity, to the air flow rate in the equipment. A higher fan voltage means a higher flow rate. The whole set of data was col- lected during the same day; it was difficult to obtain a constant ambient temperature in the room where the dummy load was dissipating 3 kW. We started with 28 V (high flow rate) and then lowered the fan tension up to 20 V (low flow rate). A drift of the ambient temperature is evident. The inlet temperature of the cooling water in the cold-plate follows strictly the ambient temperature. The temperature on the surface of the transformer dimi- nishes when increasing fan rotation. However, the dependency of the temperature on the surface of the trans- former on the fan speed is stronger than that of the ambient temperature. It is evident in Figure 7 that an in- creased flow rate means a lower transformer temperature and thus the efficiency of the cooling system is dem- onstrated.