Newbie needing help in basic circuit help

Thread Starter

Mechasaiyan

Joined Jan 17, 2017
7
Hi,
I'm fairly new to electronics, only been studying it for about half a year now.
I was trying to design a low voltage solid state Marx Generator and I made the circuit in LTspice as attached.
This should be really basic, the voltage source charges the capacitor then when the MOSFET is turned on, the capacitor discharges to the load.
When I run my circuit, the capacitor charges to close to 5V from the power source, but when discharging the voltage is a lot smaller.
Can someone tell me where I'm going wrong, please?

Thank You
 

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ericgibbs

Joined Jan 29, 2010
15,734
hi Mech,
Consider what the NMOSFET Gate to Source voltage will be when the FET is conducting.
The VSource will pull up towards VGate.

Also what do you make the Time constant for a 100uF and a 100R.?
E

EDIT:

This modified LTS asc file shows what is happening, note the short discharge period.
 

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Last edited:

Thread Starter

Mechasaiyan

Joined Jan 17, 2017
7
hi Mech,
Consider what the NMOSFET Gate to Source voltage will be when the FET is conducting.
The VSource will pull up towards VGate.

Also what do you make the Time constant for a 100uF and a 100R.?
E

EDIT:

This modified LTS asc file shows what is happening, note the short discharge period.
Thank You!
Now I want to connect about 4 of this setup. The capacitors all charge in parallel and then when the MOSFETs activate they will discharge in series to the load. How would I go about doing this?

Also, could u explain in a little more detail what is happening with the MOSFETs gate/source, and about the rise time of capacitors. I taught electronics to myself so I'm not 100% sure on some of this stuff. It's my first time using Transistors.
 

ericgibbs

Joined Jan 29, 2010
15,734
Now I want to connect about 4 of this setup. The capacitors all charge in parallel and then when the MOSFETs activate they will discharge in series to the load. How would I go about doing this?
hi,
Will the four circuits be discharging sequentially into the same 100R load.?

EDIT:

If Yes, then consider the reverse internal diode of the MOSFET. it will become forward biassed when the next MOSFET connects to the load.
The 100uF of the previous switched ON MOSFET will be still close to 0V, so it will try to recharge via its own internal diode.

Do you follow that OK.?
E
 
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Thread Starter

Mechasaiyan

Joined Jan 17, 2017
7
hi,
Will the four circuits be discharging sequentially into the same 100R load.?
Yes, they will be. I recently read about Marx generators and thought it would be cool to make one myself.
The capacitors will charge in parallel, then discharge in series for a higher voltage pulse of around 30V.
 

Thread Starter

Mechasaiyan

Joined Jan 17, 2017
7
hi,
So they will discharge at the same time into a common load to give 30v pulse.?
E
yes, they will discharge in a common load.
Is this what you have in mind.

High Voltage Based On Marx Generator Principle Using MOSFETs
Scroll almost to the bottom article of this link.

http://efxkits.com/blog/marx-generator-design-principles/
Yes, that is the concept I had in mind, but instead of high voltage, I want to make it a low voltage one. I don't think I can build a high voltage one just yet! I want the first concept to be a little simple, then once I understand that, go to something more complicated.
How would my circuit design be for a simple 6 stage generator? I tried using the design you gave me and adding a couple more in, but then I get an error: Vo and vo are paralleled making an over defined circuit matrix. You will need to correct the circuit or add some series resistance.
 

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ericgibbs

Joined Jan 29, 2010
15,734
hi M,
You have two RED [F4] Labels with the same name, so LTS links then as fault.
You do not need the Diodes on the MOSFET, they already have internal diodes [ which could be a problem!}
 

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Thread Starter

Mechasaiyan

Joined Jan 17, 2017
7
hi M,
You have two RED [F4] Labels with the same name, so LTS links then as fault.
You do not need the Diodes on the MOSFET, they already have internal diodes [ which could be a problem!}
Thank You very much for all your help!
I finally have a working starting model and understand it a little more.
You have been incredibly helpful. Is there any way to upvote a user on this forum?
 
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