Update: Dropping the capacitor back down to 0.001uF got streamers flying
again, as did unwinding the primary until just two turns were left. I also
realized that what I was seeing on the scope screen was the 555-driven 30khz
enabling/disabling of the MOSFET driver. The antenna was barely picking up
anything from the secondary! Extending the antennna to about double its
length fixed this.
However, class-E still appears out of reach. Here's how things look on the
scope right now:
- The MOSFET gate goes high (+3.5V*) in about 1uS. (*approximate as I have a
diode-protected opamp follower of unknown but probably low-ish input
impedance between probe and scope, and there's a slim chance I built a
voltage divider into it way back when I built it)
- Just before the MOSFET gate reaches the peak, the primary sees a .3uS dI/dt
spike (magnitude .6V, but this is horribly inaccurate due to the homemade
shielded loop being pretty crude), which reaches zero just as the MOSFET gate
reaches its peak.
- The MOSFET gate drops almost but not quite down to zero in approximately
0.6uS, then rises back up to 3.5V in about 1uS, and the process repeats.
On Tue, 4 Sep 2012, Andreas Hahn wrote:
I rebuilt the MicroSSTC's driver to Steve Ward's Class-E SSTC schematic.
Using a shielded magnetic loop (which gives output proportional to dI/dt)
scope probe on the primary lead suggests the primary is tuned way too
high... with the main spike lasting well under 1/10th of the MOSFET
turn-off / turn-on time. (the MOSFET gate goes HIGH for 10uS)
This seems odd, as I increased the capacitor value by a factor of 10 in the
rebuilding (to 0.01uF), so I figure it should be tuned too low [capacitor
taking longer to charge] if anything.
Tripling the number of turns on the primary and playing with coupling
didn't change much either.
Any thoughts?