RF Power Amplifier Resilience to Loads with high VSWR

Question

Accepted Answer

Question: 
 How can I figure out the worst case VSWR that a RF Power Amplifier can withstand before it is damaged? 
 Answer: 
 This is a difficult question to answer. To establish a definitive answer requires a time consuming lab measurement where you connect the output of the power amplifier to a variable load. The impedance of the load should initially be set to 50 ohms, that is, a low VSWR such as 1:1.22 or -20dB. After the power amplifier has been turned on a the RF output power has been set, the VSWR is increased. If there is a target VSWR that we are testing for, we would set the load to that value and then vary the phase of the load over a 360 degree range to ensure that the device can survive that VSWR at all phases (we usually do this sweep in increments of 10 degrees). This step is repeated at -40 degC and +85 degC. After this measurement is complete, we measure the s-parameters and compare them to s-parameters that we would have collected before the load stress was applied. If there is no significant shift in the s-parameters, we deem that the device can survive that VSWR. In general, we will apply a higher stress level than out target so that there is some margin. For example, if we wan the amp to survive a VSWR of 5:1, we will set the VSWR during the test to maybe 6:1. Also because you don't want to destroy a lot of devices during repeated tests where you are trying to establish the breaking point, it is more practical to approach this experiment with a target VSWR that you want the device to survive (as described above). 
 When we are not in a position to perform this measurement in the lab (we generally need a strong business case to justify it), what guidance can we provide? There are a few points to note that may be helpful. 
 1. One failure mechanism to consider when a hostile load is present, is thermal. So, worst case, assume that all of the dc power entering the device (Vdd x Idd) along with the RF input power, is dissipating in the device as heat (because all of the RF output power is being reflected back into the device). Calculate this power level and then calculate the resulting junction temperature (Tcase + (Pdiss x THETAjc)). If that number exceeds the maximum junction temperature, you are in trouble. 
 2. The above calculation assumes that all of the power is coming back into the device, that is , VSWR is infinite. The calculation can be repeated for the case where the VSWR is lower. To do that, you add the RF input power (converted to watts) to the dc power (VDD x IDD) and subtract the RF power that is not being reflected back into the part. The recalculate the resulting junction temperature. 
 3. It follows form this that if the RF output power (in Watts) is relatively low compared to the internal dc power consumption (or low relative to the device's Psat), a load with a high VSWR is unlikely to be a problem. 
 These are some first order guidelines. But as stated above, to have a definitive answer, bench measurements must be taken.

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