Re: Gatedrive waveforms

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <dhmccauley-at-spacecatlighting-dot-com>


The ringing is actually caused by both the inductance of the transformer /
wire leading up to the FET along with the MOSFET gate impedance.

Typically, a small snubber network is used to suppress the ringing, however
since this ringing occurs well past the saturation point (turn-on) of the
FET, it really doesn't effect circuit operation.
However, if you're ringing extends well below say 5V, then you should be
concerned and probably need to design a simple snubber network (RC network)

But as Dan Strother stated below, you really need the FET installed (and
ideally under the nominal load conditions and voltage you are running at) to
see if the gate drive is acceptable.
Using gate capacitance alone is misleading.  You really want to use the gate
charge characteristic (which is dependent on operational frequency,
operating voltages, etc...) to model
the circuit.

Dan



 > You will not be able to accurately assess the gate drive waveforms until
 > the MOSFETs are installed. The MOSFETs' gate capacitance is the primary
 > cause of such undesirable effects.
 >
 >
 > Dan Strother
 >
 >
 >
 >
 >
 > >Original poster: "Finn Hammer by way of Terry Fritz
<teslalist-at-qwest-dot-net>"
 > ><f-h-at-c.dk>
 > >
 > >Dan, all:
 > >
 > >Thankyou, Dan, for this little tutorial, you are helping a lot in
 > >extending the experience of SSTC`s to the part of this community, that
 > >don`t have electronic experience, like myself.
 > >
 > >You write, that when testing the gate drive signal, overshoot and slight
 > >ringing is okay. I have recorded these waveforms from my board, this
 > >being without the FET`s mounted.
 > >http://home5.inet.tele.dk/f-hammer/gatedrive.jpeg
 > >
 > >The pink trace shows both turnons, and one turnoff, the green trace
 > >shows the turnoff extended in time.
 > >
 > >The turnons overshoot to 15.6V, then ring back to 9.4V
 > >
 > >The turnoff`s overshoot 5V, then ring 4.2V opposite.
 > >
 > >Having litle knowledge of this subject, I would like to ask, if these
 > >are acceptable values for overshoot and ringing, so that I may proceed,
 > >or is troubleshooting indicated.
 > >
 > >I should be getting 125 IRFP460A`s this week, and am looking forward to
 > >testing the rest of the features.
 > >
 > >Cheers, Finn Hammer
 >
 >
 >
 >
 >
 >
 >
 >