AN1506 datasheet(10/14 Pages) STMICROELECTRONICS

10 / 14 page

AN1506 - APPLICATION NOTE

10/14

Figure 10. Reverse conduction of the power MOSFET through the intrinsic body diode or through

the channel (ID=5A/div, VF=0.5V/div, VGS=5 V/div, t=5ms/div)

5.2. Power Losses Measurement from the Thermal Behavior.

The total power losses Ptot, in the steady-state conditions as before described, are related to the actual

heat sink temperature by relation:

where TH is the heat sink temperature, TA is the ambient temperature (25°C), and Rth, HA is the thermal

resistance between the heat sink and the ambient. Relation (10) can be used to indirectly measure the

total power losses, by measuring the ambient and heat sink temperatures and knowing the thermal

resistance.

First of all the thermal resistance has been experimentally established, by means of a specific test with

known power condition, at Rth, HA=15°C/W accounting for the actual layout. Hence, the total power

losses expressed by relation (9) have been evaluated by (10) measuring the temperatures in the

prototype in several loaded conditions. A separation in switching and conduction power losses has been

carried out by calculating Pcon through equation (8) and Psw as difference of the total and the conduction

power losses. The main results obtained are reported in the Table 2.

The same test procedure has been repeated with a different hysteresis band (about 18%), which implies

a different ripple on the phase currents of the motor, in order to determine the influence of the switching

frequency on the thermal behavior of the devices. In such a condition the switching frequency reduced to

1kHz. The traces of the motor currents for this new operating condition are reported in figure 11. The

tot

–

th HA

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AN1506 Datasheet(PDF) 10 Page - STMicroelectronics