Caught out by the Coss - Effect of the output capacitance of a mosfet TX in PI.

[darov922]

High voltage probes are a standard accessory for oscilloscopes. x100, 100Megohm impedance, and 1-2.5kV for max applied voltage. I bought one for 12.00 GBP on eBay. Today I found I had one already by Hewlett Packard. Both give identical results.

It took me so long from when? 1982 when I started using mosfets, or 2019 at the beginning of November? I think this investigation has moved quite rapidly since my time on it has been little more that 1 hour per day. I have been looking for a while at high voltage mosfets that don't run into avalanche in the PI circuits used and also have a low Rds resistance. Low Rds resistance results in less losses in the device and much lower operating temperature. Important for what I am working on at the moment. Some of the later offerings I tried did not work due to serious ringing that required a low value of damping restance. Not good. That is where it started and it is still under investigation. Just tried a new device called a Superfet II. 650V, Rds ON is 0.133 milliohms typical at 12A. Works great with the diode and additional drain source capacitor.

Are they expensive? Fairchild FCP150N65F - 1.095 GBP. Vishay IRF740 - 2.00 GBP. Both are one off price.

Eric.

Very interesting analysis. I just finished my HH2 detector. I get a pulse of 3 uS and 650v amplitude... are these results correct? I can take my first sample at 2.5 uS to look for gold... what makes me wonder is: how do I define the position of the second sample and what duration? What criteria do I use? The components that I have used to achieve these features are:
-mosfet STF18N60M2 (Coss 40pf)
-Diode 1N4937 (12pF)
-1.3m long unshielded braided cable (same coil cable).
-coil: 300 uH 3dss coil made with category 5 AWG 26 UTP cable with 6.8 Ohm.

Do you think there is something that can be improved?

Dario