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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