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Re: [Amps] LDMOSFET questions for insiders

To: "'Amplifier Mailing List'" <>
Subject: Re: [Amps] LDMOSFET questions for insiders
From: "Doug Ronald" <>
Date: Sat, 17 Nov 2018 14:25:13 -0800
List-post: <>
This app note gives some information on repetitive avalanche ruggedness and
unclamped inductive switching which may help...


-----Original Message-----
From: Amps [] On Behalf Of Manfred
Sent: Saturday, November 17, 2018 10:48
To: Amplifier Mailing List <>
Subject: [Amps] LDMOSFET questions for insiders


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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