Absolute Maximum Values

[**Rafael**]

Hi there,

Could anybody tell me what would be the safe operating power/current/voltage/etc. selection criteria based on datasheet absolute maximum values?

Example; Transistor TIP31C, Absolute Maximum Junction Temperature 150 deg. C. BVceo = 100V, Pd = 40W.

What would be safe operating junction temperature/Voltage/Power for this transistor? One third? One half maximum value?

Thanks in Advance for your answers.

**Rafael**

 

[Harald Kapp]

Welcome to electronicspoint.

You cannot get the safe operating conditions simply from the data you mention. Typically a transistor datasheet states (or should state) a "safe operating area" (SOA). For the TIP31C it is to be found for example in this datasheeet, figure 3.

 

[ramussons]

There is another important parameter missing - The maximum collector current - 3 Amps.

All the 4 are inter related. It will not be possible to operate the device with all parameters together at their limit.

For eg; you cannot have Ic at 3 Amps AND Vceo = 100 Volts - the Pd would be 300 Watts! and the junction temperature would have surpassed 150 deg C.

You have, what is known as a safe operating area within the boundaries of the limits. And ....

<edit> Harald Kapp has beaten me to it

 

[Colin Mitchell]

You put the transistor on a large heatsink and turn the project ON.
If the transistor blows up, the wattage dissipation is too much. Feel the heatsink after 10 minutes. If you can hold the heatsink, the ratings are ok.

 

[Harald Kapp]

You put the transistor on a large heatsink and turn the project ON.
If the transistor blows up, the wattage dissipation is too much.

Coiln, that's not a very sensible approach and in my view bad advice:

• You will not know why the transistor has blown. Besides getting too hot from too much power dissipation, it could have burned from overvoltage or overcurrent without having seen too much power.
• You will not know whether the heatsink is just right in size, too big (and therefore too expensive) or just a tiny bit too small (so the transistor may work on the benchtop, but not within an enclosure)
• You will not know whether the circuit will operate reliably in the long term.
• You will not know whether the circuit will operate reliably with other specimens of the same transistor type due to tolerances.

Harald