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[Amps] LDMOSFET questions for insiders

To: Amplifier Mailing List <>
Subject: [Amps] LDMOSFET questions for insiders
From: Manfred Mornhinweg <>
Date: Sat, 17 Nov 2018 18:48:09 +0000
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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.


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