I am designing a PIN diode switch that must handle high power. How much power can the switch handle?
Answer: There is one primary determinant of power handling for a PIN switch: junction temperature.
The maximum junction temperature which a PIN diode switch can handle is specified in the Absolute Maximum Ratings table of its data sheet. For most Si, GaAs and AlGaAs PIN diodes such as these, the maximum rated junction temperature which produces a mean time between failures (MTBF) value of 1 million hours is 175 °C. IMPORTANT: CHECK THE DATA SHEET TO CONFIRM THIS VALUE!
Junction temperature (TJ) can be found by
where
| TJ | = | Junction temperature, in °C |
|---|---|---|
| TC | = | Case or contact temperature, in °C |
| PD | = | Dissipated power, in W |
| qJC | = | Thermal resistance from the diode junction to its outermost cathode terminal, in °C/W |
The value of TC is the temperature of the surface to which the cathode of the PIN diode is attached. This surface could be the top of a microstrip transmission line, the ground plane or substrate of a transmission line, a heat sink, etc. In any case, it is a surface whose temperature is known.
qJC is the thermal resistance from the junction of the PIN diode to its outermost cathode mounting surface. For an unpackaged die, qJC is the thermal resistance from the junction to the outermost layer of cathode metalization. For a packaged PIN diode, qJC is the thermal resistance from the junction to the cathode terminal of the package.
TJ may also be found by
where
| TA | = | Ambient temperature, in °C |
|---|---|---|
| qJA | = | Thermal resistance from the diode junction to the ambient surroundings of the system in which the PIN diode is contained, in °C/W |
qJA is the thermal resistance from the junction of the PIN diode to the ambient environment surrounding the system in which the PIN diode is contained. qJA is the sum of qJC and qCA. qCA is the thermal resistance of the system to which the diode is mounted, measured from the surface to which the diode is mounted to the ambient surroundings of the system. Since the diode manufacturer has no control over qCA, PIN diode manufacturers typically do not specify qJA.