Thermal operating conditions

SPMD250STP

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Ppw = 1.1 x [2x1.8x1x0.25+2x1x1x0.75 + 0.5 x1]

Ppw = 1.1 x [0.9 + 1.5 + 0.5] = 3.19 W

The factor 1.1 takes into account the power dissipation during the switching transient.

SLOW DECAY. The power dissipation during the ON period is the same. The

RECIRCULATION is made internally through a power transistor (Vsatsink) and a diode. The

power dissipation is, therefore:

Ppw = 1.1x [2 Vsat x Iph x D.C.+(Vsat+Vd) x Iph x (1-D.C.)]

Ppw = 1.1x [2 x 1.8 x 1 x 0.25 + (1.8 + 1) x 1 x 0.75]

Ppw = 1.1 x [0.9 + 2.1] = 3.3 W

WAVE MODE. When operating in this mode the power dissipation is given by values of

FAST and SLOW DECAY mode, because one phase is energized at any given time.

NORMAL MODE. At any given time, two windings are always energized. The power

dissipation of the power output stage is therefore multiplied by a factor 2.

HALF STEP. The power sequence, one-phase-on, two-phase-on forces the power

dissipation to be 1.5 times higher than in WAVE MODE when the motor is running. In stall

condition the worst case for power dissipation is with two-phase-on i.e. a power dissipation

as in NORMAL MODE.

The following table summarizes the power dissipations of the output power stage of the

SPMD250STP when running for this example:

●

Calculate the total power dissipation for the SPMD250STP :

Ptot = Plogic + Ppw

In this example, for slow decay and normal mode

Ptot = 1.1 + 6.6 = 7.7 W

●

The case temperature can now be calculated:

Tcase = Tamb + (Ptot x Rth) = 55 + (7.7 x 5) = 93.5 °C

●

If the calculated case temperature exceeds the maximum allowed case temperature, as

in this example, an external heat-sink is required and the thermal resistance can be

calculated according to:

Equation 5

Table 4.

Power dissipations

Wave

Normal

Half Step

Fast Decay

3.19 W

6.38 W

6.38 W

Slow Decay

3.30 W

6.60 W

6.60 W

Rth

tot

T

cmax

T

amb

–

P

tot

-----------------------------------

85

55

–

7.7

-------------------

3.9

°C

==

=