Broadcast Concepts Inc
11700 NW 102
nd
Road Suite 4Medley FL 33178
Tel: 305.887.8400 FAX 305.887.8444
Broadcast Concepts Inc
Revised 8/20/2014
4
driving blind. You won’t be able to see what’s really going on and the end result could be
transistors that burn out.
Calculating Die Temperature:
How do you know if the amplifier is running to hot? There are symptoms to watch for. If there is a
significant drop in power of more than 5% over a short period of time the amplifier is likely overheating.
There is a scientific method that must be used to verify that the amplifier is installed and running within
thermal limits. We do not recommend the “dropping power with time test” described above.
Use the thermal resistance (Rth) of the device to calculate the junction temperature. The Rth from the
junction to the device flange for the BLF188XR is 0.11 °C /W. If the device is soldered down to the pallet
baseplate, this same value can be used to determine Tj. If the device is greased down to the pallet
baseplate, the Rth(j-h) value becomes 0.26K/W, as the thermal resistivity for the grease layer from the
flange to the baseplate is approximately 0.15 °C /W.
Note:
We always use thermal grease under transistors because soldering them directly would render the
pallets unserviceable.
Example:
Assuming that the device is running at 950W with the RF output power at 48V while
consuming 25.3 amps on a pallet baseplate (e.g. 70 °C). Tj can be determined based on this condition for
the given baseplate temperature:
•
Dissipated power (Pd) = (48V x 25.3A) – (950W) = 264.4W
•
Temperature rise (Tr) = Pd ´ Rth = 264.4W (0.26 °C/W) = 68.7 °C
•
Junction temperature (Tj) = Th + Tr = 70°C + 68.7 °C = 138.7 °C
The best way to make this calculation is to measure the temperature of the transistor flange directly. In this
case temperature rise is calculated using 0.11°C/W.
The maximum junction temperature for most LDMOS devices is 200C. We suggest staying under 175C.
In the example above a base plate temperature of 70C was used. When the pallet is mounted to an efficient
heatsink like one of our bonded fin models base plate temperatures in the 50C range are easily maintained
at full output power.