Re: Bmax calculation for driver transformmer

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

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>



Tesla list wrote:
> 
> Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<jimlux-at-earthlink-dot-net>
> 
> But, the frequency you should be using is NOT the 250 kHz (unless you
> really want to drive the gate with a sine wave)... the real issue is the
> dV/dt, and the equation you used (popular for power transformers and CW
> sine applications) essentially calculates that from Vrms and frequency.
> If you want decent square edges, you probably need to figure your frequency
> at several tens of MHz, or possibly higher.
> 

And, I forgot to add, you also need to decide how much droop in the pulse
top you're willing to tolerate (which is related the amount of energy
stored in the core, and hence, the flux)...  Making nice square pulses is a
broadband problem, both high and low frequencies are important.


People building long pulse radar modulators (where you want, say, a 2
millisecond pulse with sub microsecond rise and fall time), for example,
use a transformer to just get the edges of the pulses from the Low voltage
to the (isolated) High Voltage deck, and then have some circuitry floating
at HV that regenerates the desired control waveform.  Then, the transformer
can be very small and low flux, since all you need is the + pulse at the
start and the - pulse at the end.  This separates the problem of moving
energy to the HV deck from the problem of getting timing to the HV deck.



> Tesla list wrote:
> >
> > Original poster: "rob by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <rob-at-pythonemproject-dot-com>
> >
> > I found this, so I thought I'd post it.  It shows that even with very
> > few turns Bmax in negligible on gate driver transformers:
> >
> > I am using material similar to Amidon #75 in
> > FT-82 size.  Thus Ae, the cross sectional area is about 0.25 cm
> > squared.  Gate drive is 9V rms, or 1/2 the square wave swing that I AC
> > couple to the transformers.  Using a derivative of the Biot-Savart law,
> > Bmax=
> > Vrms * 1e2 / (4.44 * N * Ae * F), where F is in Mhz.
> >
> > Thus for my 5 turn transformers at 250khz, Bmax= approx 650 gauss.  Bsat
> > is up in the thousands, so even with my small # turns, I am way below
> > the saturation point.
> >
> > Rob.
> >
> > --
> > -----------------------------
> > The Numeric Python EM Project
> >
> > www.pythonemproject-dot-com