SPMD250STP

Thermal operating conditions

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Thermal operating conditions

In many cases the modules do not require any additional cooling because the dimensions

and the shape of the metal box are studied to offer the minimum possible thermal resistance

case-to-ambient for a given volume.

It should be remembered that these modules are a power device and, depending on

ambient temperature, an additional heath-sink or forced ventilation or both may be required

to keep the unit within safe temperature range. (Tcasemax < 85 °C during operation).

The concept of maximum operating ambient temperature is totally meaningless when

dealing with power components because the maximum operating ambient temperature

depends on how a power device is used.

What can be unambiguously defined is the case temperature of the module.

To calculate the maximum case temperature of the module in a particular applicative

environment the designer must know the following data:

●

Input voltage

●

Motor phase current

●

Motor phase resistance

●

Maximum ambient temperature

From these data it is easy to determine whether an additional heath-sink is required or not,

and the relevant size i.e. the thermal resistance.

The step by step calculation is shown for the following example.

Vin = 40 V, Iphase = 1 A, Rph Phase resistance = 10

Ω, max. T

●

Calculate the power dissipated from the indexer logic and the level shifter (see

electrical characteristics):

Plogic = (5 V x 60 mA) + (40 V x 20 mA) = 1.1 W

●

Calculate the average voltage across the winding resistance:

Vout = (Rph x Iout) = 10

Ω

●

Calculate the required ON duty cycle (D.C.) of the output stage to obtain the average

voltage (this D.C. is automatically adjusted by the SPMD250STP):

●

Calculate the power dissipation of the SPMD250STP output power stage. The power

dissipation depends on two main factors:

–

the selected operating mode (FAST or SLOW DECAY)

–

the selected drive sequence (WAVE, NORMAL, HALF STEP)

FAST DECAY. For this mode of operation, the internal voltage drop is Vsatsource + Vsatsink

during the ON period i.e. for 25 % of the time.

During the recirculation period (75 % of the time), the current recirculates on two internal

diodes that have a voltage drop Vd = 1 V, and the internal sense resistor (0.5

Ω). For this

example, by assuming maximum values for conservative calculations, the power dissipation

during one cycle is:

Ppw = 1.1 x [2 Vsat x Iph x D.C. + 2 Vd x Iph x (1 - D.C.) + 0.5 x Iph]

DC

V

OUT

V

IN

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

10

40

------

0.25

==

=