Here's my goal: To create a capacitor bank to dump a load in to a load resistor/inductor, pulsed, without using switches and keeping it as isolated as I can from myself.
Here is what I have so far:
VG2 a square wave simply at around 10Hz, J1 being a net.
My thoughts (I tried with PNP and NPN with 10k Rs at base at first): Have the capacitors charge at full voltage in parallel, the control voltage (VG2) allowing this by opening a path for both to charge side by side.
When the control voltage goes to zero, the N channel fets will turn off (C2's path to ground) and also the diode's (ignore why it is there for now, but to prevent it from recharging C1 during discharge phase I believe) and connect the two in series to make the intended C1+C2=~240V on the load.
What I simulated:
C1 seems to be fine, but does not discharge.
C2 starts at negative (going down slightly)
Load seems to continuously have a voltage (no path to flow?)
I wonder if the fets or BJTs last time simply do not switch or work as I suspect, would they perform as I intend? Is there a more straightforward way of doing this parallel->series switching to isolate myself a little better?
Best Answer
You are using your transistors incorrectly. In fact, I think that the only transistor which behaves as a switch in your circuit is T2.
Note that a transistor, NMOS for example, will conduct while \$V_{GS}>V_T\$. You are biasing your NMOS transistors in such a way that the potentials at source terminals is much higher than at gate terminals.
I believe that the reason you see a constant voltage on the load is because T3 and T4 are conducting when \$VG_2=5V\$, and T1 and T5 are conducting when \$VG_2=-5V\$ (in all cases due to wrong biasing). You can easily check my guesses in simulation by measuring the currents.
Furthermore, take a look at transistors' datasheets - the voltages in your circuit exceed the max ratings for the transistors.
The component you are trying to implement is called "Charge Pump" - there are many standard implementations you can find on the internet. A short (but not complete) overview may be found here.