AN533 datasheet(3/22 Pages) STMICROELECTRONICS

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AN533

Through-hole packages

3/22

Through-hole packages

1.1

Thermal resistance

1.1.1

Review

The thermal resistance of semiconductor assembly is the parameter which characterizes its

resistance to the heatflow generated by the junction during operation. A temperature

exceeding the maximum junction temperature curtails the electrical performance and may

damage the device.

The maximum dissipated power capability is:

Where:

●

Tjmax is the maximum junction temperature of the semiconductor in degrees ( °C)

●

Ta is the ambient air temperature in degrees ( °C)

●

Rth(j-a) is the thermal resistance between junction and ambient air in °C/W

The Rth(j-a) takes into account all materials between the junction and ambient air.

An analogy between Ohm’s law and the thermal equivalent circuit can be made:

●

Electrical resistance corresponds to thermal resistance

●

Current corresponds to dissipated power

●

Voltage corresponds to temperature

Thus: V = R . I corresponds to

ΔT = Rth . P

1.1.2

Dissipated power for a thyristor or a TRIAC

The maximum power dissipation versus average on-state current (for SCRs) or RMS on-

state current (for TRIACs) is given in the datasheet for each product.

However, a more accurate result is obtained by using the Vto and Rd values with the

following calculation:

P = Vto . IT(AV) + Rd . I

T(RMS)

Where:

●

to is the threshold voltage specified in the datasheet

●

Rd is the dynamic on-state resistance specified as Rd in the datasheet

●

T(AV) is the average on-state current

●

T(RMS) is the RMS on-state current

Figure 1 shows the RMS and average values for different waveforms of current.

Pmax =

Tjmax -Ta

Rth(j-a)

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