The test section was instrumented to measure the temperature. The passive components (capacitors, induct- ances and electric transformers) were instrumented with “T type” thermocouples. The surface temperature of the pipe was measured with four thermocouples (input and output, upstream and downstream of the coil). Six ther- mocouples were placed in different positions on the base of the throats of the finned heat sink. Two armoured “T type” thermocouples were placed upstream and downstream of the finned heat sink to measure the temperature of the internal air flow.

During the tests the SMPS was connected to a 5 kW dummy load. Both the electric power absorbed by the grid and that dissipated in the dummy load were measured.

The data acquisition of temperature and electrical parameters was carried out with a multimeter Agilent HP34970A. This multimeter was connected to a notebook by means of a GPIB-USB interface. The data acquisi- tion was carried out by means of a software developed in Labview (release 8.2) environment.

2.2. Hydraulic Loop

A schematic diagram of the hydraulic loop is shown in Figure 2. The water flow was maintained by a pump Laing S4-36/550 P. The water flow was cooled in an aluminium air-water heat exchanger Aavid Thermalloy. The flow rate was measured in an ultrasonic flow rate meter Danfoss Sonometer 1100.

The temperature of the air flow incoming and leaving the heat exchanger was measured with armoured “T type” thermocouples. The temperature of the water was measured by armoured “T type” thermocouples at the cold plate inlet and outlet. The flow-meter was connected to the notebook by means of a RS232-USB adapter.

2.3. Data Reduction and Measuring Uncertainties

In the discussion of the results (Par. 3) are shown measured values of temperature, heat released to the water, power absorbed from the grid and efficiency. The different runs are denoted by the power absorbed by the SMPS, its output voltage and the fan voltage. No particular procedures for data reduction was used. The water cooling flow rate was the same for the whole set of experiments.

The overall accuracy of temperature measurements was estimated to be better than 0.2˚C. The relative overall uncertainty of the power exchanged with the water reaches 15% at maximum, whereas for the power absorbed from the grid and for efficiency is lower than 3% and 4%, respectively. The high uncertainty of the heat released to the water, calculated through an energy balance based on water flow rate and temperature difference, is mainly due to the uncertainty of the temperature difference (≈14%) and less to the flow rate (≈3%).

3. Results and Discussion

Due to the presence of the air-cooling system, it was impossible for us an analysis of the temperature distribu- tion on the PCB with infrared thermography, as suggested in [6]. For the identification of the most critical ele- ments between the passive components placed on the PCB, we measured their temperature with thermocouples. For a typical experiment the temperature distribution is shown in Table 1. At high power (P ≥ 2.5 kW) the most critical point was for thermocouple 107, placed on the transformer. At lower power (P < 2.5 kW) the most stressed component was a winding (thermocouple 111). However, even if we are aware of this change, we used the values of the thermocouple 107 (transformer) as the reference for the discussion, because in this point the maximum temperature is attained.

Before discussing the thermal performance of the SMPS, it is opportune to discuss the distribution of temper- ature in the air inlet of the finned heat sink, because this parameter is particularly significant to characterize  the

 Table 1. Temperature on the different passive components.