Exciting a resonant tank with pulsed DC - help

[gyulasun]

Okay Dann, that is fine. The MOSFETs will work as long as their drain voltage and current ratings are violated.  In case you use a tank circuit in which the coil will have but 1 - 2 Ohm DC resistance or even less, then the drain current limits may easily be reached (depending on your supply votage amplitude of course).  Drain voltage ratings are added due to the series connection of the FETs  so it has higher safe margin.  Unfortunately the R_{DS ^'}on' resistances are also in series and added, doubling the loss of the switch.
One notice with respect to the bipolar power transistor you used so far: if a diode is connected in series with the collector in the forward direction for your 12V supply voltage, then this diode could block any current whenever the collector - emitter get the inverse voltage from the tank circuit swinging so that would also be a solution a few days ago.  Of course, the 0.7V forward voltage drop of the series diode in the on state of the transistor would add to the switching losses if that were important. But anyway the opto device could still have issues to drive the base-emitter correctly.

Gyula

[madddann]

Hi Gyula!

Before I started this thread I did play around a bit with the optos and mosfets and there was one configuration with the diode after the emitter that worked... well... for about 30 seconds  , after that the oscillations gradually decreased and stopped.  The mosfet became faulty and after that I did not want to sacrifice any more components.

Anyway, now I added the second channel on the protoboard (identical to the first) and when I tested it, it would not switch - the opto was not switching, then I replaced it with the opto from the first channel and everything worked as expected.
The optos are identical and I tested that opto before, I know it works, maybe just with a different resistor. So it might be that the optos are worn out to some extent, tomorrow I'll try some other ones in the circuit just to see their performance.
Will also post some scopeshots and a picture of the "rats nest" 


Dann

[madddann]

OK!
Here are some scope shots and a picture of the circuit (not the best picture) : https://postimg.cc/gallery/mpgxxk12/

I rechecked the opto's datasheet and figured out that the resistor for driving the opto's LED was not right (1k ohm - allowing only 5mA of current) and the opto was barely switching on the HV side. By measuring the current through the opto's LED I selected a 220 ohm resistor (for around 20mA of current, the forward voltage drop on the LED was about 1.24V).
The pull up resistor for the mosfet gates was also allowing too small current, so I replaced it with 470 ohm (about 18mA through the opto's transistor) and now the switching is more solid.

In the picture (of my bench) there are two channels - only the second one is used right now connected to only one LC tank (There is another LC tank on the right - not used).
The double coil in the middle (connected to a small lightbulb) was just used to check the output when all the coils are lined up and close to one another (Figuera setup)

That is all for now, I'm headed in the Figuera direction with this setup, so I'm not sure about posting here or in a new thread under the solid state devices section in the future. Are there any moderators there?

THX all!


Dann

[gyulasun]

Okay Dann,  thanks for reporting back.

Well, opto devices are different, some types are able to work fine with less than 10 mA input diode current, other types need higher currents.  (Maybe this was part of the problem when driving the bipolar transistor some days ago, maybe not.) 
Also, within the same types a single opto may work inferior or better to an another one of the same type.

I wish you further good luck.
I do not know about moderators.
Gyula