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RE: [TCML] gate drive issues
> i am encountering this strange problem that i just cant wrap my head
> around. I suspect that i could have partially damaged igbt's however
i am
> hoping that is not the case. I am experiencing a very strange
problem
> with my gate drive waveforms. As i increase my bus voltage the gate
> drive waveforms seem to get worse and worse
<snip>
I suspect you are experiencing the Miller Effect. This causes an
increase in the effective input capacitance of the MOSFET and can play
havoc with the drive waveform.
See:
http://web.mit.edu/klund/www/papers/jmiller.pdf
for the original paper about it, and
http://wiki.answers.com/Q/What_is_the_miller_effect_in_mosfet
for a somewhat disjointed but simplified explanation of the problem.
Besides that problem, unless you are using a differential probe to
measure between the gate and the source of the MOSFET, you will have an
error due to the current flowing between the source terminal of the IGBT
or MOSFET and common ground. This can cause severe measurement errors if
not allowed for.
When I designed my PA1 3.1 MHz 500 watt RF amplifier I designed the gate
drive circuit to compensate for the Miller Effect so that the drive
waveform would be correct when the amplifier was running at full power.
http://rife-beam-ray.com/products.htm#PA1 and
http://rife-beam-ray.com/PA1.htm)
73,
Ralph W5JGV - WD2XSH/7
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