What is the purpose of the series diode?

[Detectorist#1]

Hi kt315,

First, thank you for support! But let to see "simple pulse pump capacitor scheme" from Moodz. It is too early to make comments.

[moodz]

Hi kt315,

First, thank you for support! But let to see "simple pulse pump capacitor scheme" from Moodz. It is too early to make comments.

Well it does involve setting up a cascode arrangment ... The top switch M3 is the voltage switch and the lower switch M6 is the current switch.

M3 cannot conduct unless M6 turns on ... when M6 turns on the Source of M3 falls to ground but the "pump" raises the Gate voltage of M3 to nearly 18 volts with respect to its source and M3 turns on.

M6 never sees the high voltage ... M6 should be a low R low voltage ( eg few milliohm ). M3 handles the voltage blocking. This arrangement can actually switch faster if you know how to set it up right.

[Detectorist#1]

Hi Moodz,

OK, this is an other idea. Where is the "simple pulse pump capacitor scheme" for SIC transistor with more than 15V Ugs from 9V battery voltage?

[Detectorist#1]

Hi Moodz,
Please escape from "virtual reality" in simulating programs and start to make publications for real working schematics (sorry).

[Detectorist#1]

Hi Moodz,

My last post is only the joke! LOL

[Detectorist#1]

Hi moodz,

Today, I tested diffusion capacitance of some schottky diodes (1N5819, SR504) with my tester (attached is the scheme of the tester) and the picture on the scope is very strange for me. After current pulse via diodes the voltage on the diodes is negative! With low power schottky diodes (BAT42 and BAT4 voltage on the diodes after pulse is positive. Please, use your powerful simulation to explain what happens. Thank you in advance!

[Detectorist#1]

BAT42 and BAT48 are small signal schottky under test

[Detectorist#1]

Hi moodz,

With this diffusion capacitance tester I noticed very strong dependence of this capacitance from the temperature of the diode. When the diode is still hot after soldering - the capacitance is two times bigger! This effect have to be mind when high pps in PI MD is used.

[Detectorist#1]

One more "discover" with this tester - low voltage schottky diodes for 1-5A current have negative TKC. Low power (<0.5A) and high voltage schottky diodes (>100V) have positive TKC. Is seems, have two different technologies for manufacturing of schottky diodes?

[moodz]

Hi moodz,

Today, I tested diffusion capacitance of some schottky diodes (1N5819, SR504) with my tester (attached is the scheme of the tester) and the picture on the scope is very strange for me. After current pulse via diodes the voltage on the diodes is negative! With low power schottky diodes (BAT42 and BAT4 voltage on the diodes after pulse is positive. Please, use your powerful simulation to explain what happens. Thank you in advance!

Even though the capacitance of the 1n4148 is very low ... its not zero.

One thing you can do in a simulation that you cant do for real when tracking down pesky capacitances is to start substituting in ideal diodes.

When we sub in the ideal diode for the 1n4148 the negative voltage goes away.. we add in two puff of capacitance and it comes back.

( most data sheets wont show it but alot of 1n4148s have about 4 pf at 0 volts reverse )

[moodz]

For the BAT54 the switching speed is closer to ideal so the negative voltage is either not there or minimal.

I am going to say the difference in switching speeds between your test diode and the 1n4148 causes the negative voltage or lack thereof.

[Detectorist#1]

Hi moodz,

Thank you for your explanation. But serial connection of 4pF capacitance of 1N4148 with 250-666pF of 1-5A schottky diodes not explane appr. 1V negative amplitude right after the end of pulse!

[Detectorist#1]

Hi moodz,

Attached is data for some fast diodes

[moodz]

Hi moodz,

Thank you for your explanation. But serial connection of 4pF capacitance of 1N4148 with 250-666pF of 1-5A schottky diodes not explane appr. 1V negative amplitude right after the end of pulse!

Well now that you have added further info regarding the voltage swing .. the small recovery current in the 1n1418 is driven by the charge on the capacitance of your shottky and current flows the other way briefly resulting in a negative swing across the shottky ...

When the 1n4148 is replaced with an ideal diode ( no recovery current ) the negative swing at the shottky does not happen ... just the discharge of the diffusion capacitance.
Ideal and 1n4148 diode swapped below.

[Detectorist#1]

Hi moodz,

Sound's good but 1N5819 (SS14 is the name in SMD variant) gives negative voltage after the end of pulse, and BAT42 ( BAT54 is the name in SMD variant) gives positive voltage after end of the pulse!
What points the simulation with real upper diode 1N4148?