Mosfet stack for higher breakdown voltage

[Jeg]

Hey guys, i really appreciate your help! I already started reading these!

Thanks again

[fritz]

Hi Jeg,

Some folks do it with ignition coils;-)))

http://www.instructables.com/id/Variable-voltage-ignition-coil-power-supply/
http://www.instructables.com/id/High-voltage-ignition-coil-supply/


rgds.

[Jeg]

Fritz
I think i 'll give a try with the ign. coil cause it's simple and quick! Thanks a lot.

Well, another obstacle occurred. As i told before, i have made a stack with 3 IRFP460 Mosfets and works like a charm. It switches 48V rail to rail and it has 1500V for avalanche for charging a coil and creating high voltage spikes (coil reaction after closing the switch). The problem is when i connect the coil in place of the Load resistance. It just doesn't work. It doesn't do anything. It reacts as a short circuit! The coil is an air coil and it is about 127μH! It seems that the switch doesn't like inductance. It's like that affects the input parasitic capacitance of the extra mosfets. I discard one of the three mosfets, and the problem still exists. It works only with one mosfet!
If i will not find the cause of this i will give a try with one IGBT at 1200V.
Any suggestions?

Thanks
Jeg     

[fritz]

Hi Jeg,

What concerns the ignition coil supply - I´d like to point out that you should operate that from batteries or isolation transformer. Be careful.

Mosfet Stack:
If you apply 48 Volts to a 127uH coil - you reach 0,4A within a microsecond, 4A within 10 microseconds.
If you triple the load capacity of the fet driver by stacking 3 fets - it takes probably triple the time to charge the gates.
So I would run for proper integrated mosfet driver for each fet.
For fast switching a driver capable of 1A peak.
I personally use FAN3100 from fairchild - thats a sot23-5 package - maybe to small for your flavour.
The "old school" way (for people using 555s) was to take 4049 or 4050 CMOS chips (inv or noninv.) and connect all gates in parallel,
or simply enhance current sourcing by using 2n2222 2n2907 complementary stage.

rgds.

[Jeg]

The old school way seems to fit to my needs! I will re design it today. Thanks Fritz!

About the ignition coil, i will follow your advice by feeding it through a mosfet!

Thanks again!