[Amps] LDMOSFET questions for insiders

[Manfred Mornhinweg]

Hello,

is there anyone on this forum who knows LDMOSFETs really well and could 
answer a few questions?

My questions refer to a high power device like the BLF188XR or the 
MRF1K50N, operating in a broadband 1.8 to 30MHz amplifier, in saturated 
class AB, at about 6dB overdrive, into ham-typical loads, that is, a 
transmission-line transformer having a non-negligible line length, then 
a simple PI or T 5-pole lowpass filter, followed by a long transmission 
line and a real-world antenna, which could sometimes exhibit extreme SWR.

Now the questions:

1. If a load happens to be such that drain voltage peaks in excess of 
VDSmax occur, to what extent can the LDMOSFET safely clamp these? 
Datasheets claim "high avalanche energy absorption capability", but 
don't tell how much it actually is. Working it out from the high SWR 
test data given in the datasheets is hard, since they are in pulsed 
service and refer to a specific circuit at a specific frequency and with 
low or no overdrive. What I want to know is whether or not such an 
LDMOSFET will survive all avalanching as long as the total dissipation 
doesn't result in overheating the silicon, or if the tolerable amount of 
avalanche power dissipation is lower, and if so, how much.

2. If load conditions are such that the drain voltage becomes negative 
for a part of the RF cycle, what will happen? How does the body diode of 
LDMOSFETs behave? Is it fast enough to safely clamp such negative 
spikes, or is it so slow that it will remain in conduction for a long 
time and cause extreme power dissipation from cross-conduction with the 
other side?

3. Why isn't the IDmax given in the datasheets of most LDMOSFETs? How 
can I know how much current an LDMOSFET can take, and for how long, in 
the event of severe load mismatch (short circuit)? This is quite 
important, in order to know how much I can overdrive such an amplifier.


Manfred



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