From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Transient problem !
References: <3E1F3BA5.7202C132@earthlink.net> <3E1FD1C3.640B7E8C@earthlink.net>
Date: Sun, 12 Jan 2003 09:30:34 GMT
NNTP-Posting-Date: Sun, 12 Jan 2003 01:30:34 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Peter Larsson wrote:
> > Well, for starters, your FET is switching a load on and off fairly
> > rapidly.
> > So that part is like a signal generator, and (maybe) you have looooong
> > wires to the battery (both the "ground" wire and the supply wire).
> > If not properly bypassed, that is more than enough to cause problems
> > like you describe.
> > Next is the generator (MAX 627 driver); i will assume that isproperly
> > bypassed, and so the length of the "ground" and supply wires to its own
> > PS then are not an issue.
> > But this signal has to travel many inches, feet, miles, furlongs, ??
> > to the load (FET gate and source).
> > Well, a twisted pair is better than 2 randumb wires, and a coax is
> > even better, because the gate should be switched in less than 100nSec.
> > BUT, a signal from a 50 ohm source, thru a 50 ohm coax cable 9 inches
> > long that is *not* terminated, will reflect back to the generator about
> > 2nSec later.
> > And if the generator is not 50 ohms, it will bounce back to the FET
> > gate, and ping-pong maybe a half a dozen times before it becomes
> > insignificant on a scope.
> > So, that is why i made mention of (1) careful layout and bypassing in
> > each circuit, and (2) proper termination.
> > BTW, that FET gate is a *large* capacitor.
> > A 10 ohm resistor (minimum) in series from drive signal to gate will
> > help slow down the risetime a little, and limit the current drive to
> > the gate (will not act like a transient short circuit to the drive
> > signal).
> > If the driver was very close to the gate, then connecting via a 10 ohm
> > resistor (as opposed to a very short wire) would be ideal.
> > But with a long coax, make it (say) 82 ohms and use a (say) 68 ohm
> > termination resistor.
> > Not perfect, but it is difficult to get good termination with a large
> > capacitive load.
> > If this was an RF CW circuit, an inductor in series would be better
> > (complex conjugate loading for you tube guys).
> > For gate safety, connect a resistor from the gate to source (ground);
> > anything from 10K to 20Megs will prevent static buildup if that driver
> > gets removed or worked upon.
> Well !!
> I do not see any problems on my scope with the signal to the gate (no
> ringing and no transient).
> The length to the gate from the driver is ~20mm with a 10 ohm resistor
> included. A resistor for gate safety is probably good to have !
> The coax is from my signal generator to the input of the MAX driver, and
> there is also the clips attached.
> I will try to put some caps over the supply line and see what happens. It
> seems to be some problem with a stray inductance somewhere in the system. A
> snubber solves the problem but should it really be necessary?
A snubber is used in "high" power work, to decrease current thru the
FET clamp diode when the LC load swings below ground.