Re: Software support Request for Full QSK BITX40

Jerry Gaffke


Simulating just the IRF510 and T7, minimal load, driving the gate hard with a 0-10v square wave,
I see spikes to several kV as current through the IRF510 shuts off, but only if I slow the square wave down enough.
The simulation gets kind of weird with a load of 10k ohms or more

At 7mhz the IRF510 stays on, even in the troughs, even though the gate pin goes to zero volts.
I haven't looked at the model for the IRF510, but all it would take is resistance going into the gate 
and miller capacitance from drain to gate to account for this.

So T7 does indeed behave like a flyback transformer at slow speeds, but at 7mhz the IRF510
stays on to where that coil current always has somewhere to go, T7 behaves more like a forward transformer.

Jerry, KE7ER

On Wed, Jan 31, 2018 at 01:26 pm, Jerry Gaffke wrote:
With no load at the antenna port, the simulation shows an IRF510 drain current that stays near the quiescent 100ma
with perhaps 20ma pk-pk of RF riding on top of it.  (An initial spike that I'm ignoring here, as we would be increasing 
drive at audio rates.)    The drain current never goes toward zero, even when Q13 is driven with 400mv pk-pk.
Gate voltage is quite distorted, but does get down to 1.2v which should be enough to shut down the IRF510.
I'd guess that as the IRF510 shuts down the drain voltage rises, the gate-drain capacitance causes the gate voltage to rise,
and so the IRF510 continues to conduct.     When operating more slowly the IRF510 does indeed shut down as Vgs rises.

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