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P-Channel MOSFET behavior

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Rusttree

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I've modelled a simple charge-pump circuit in PSpice that uses some P-channel MOSFETs. I thought I had a handle on how P-channels worked, but PSpice is giving me results I don't understand. See the circuit attached. The two voltage sources at the bottom (V2 and V3) are essentially complimentary PWM. When one is high, the other is low, and vice versa.

When V2 is low, M1 turns on, and C1 is charged to 5V. During that time V3 is high, so M2 is off. Then, V2 goes high, which pushes C1 to 10V. V3 goes low, which turns M2 on, charging C3 up to 10V.

Next, V2 goes low and V3 goes high again. At this point, I thought M2 should go off again. But PSpice shows C3 rapidly losing charge. When I probed M2 with a current probe, there is a large negative current spike through it. So C3 is basically discharging itself through M2.

If V3 is high during that time, how is the transistor conducting current? Or is PSpice in error?

Thanks!
 
I believe you are losing charge through the intrinsic (parasitic) source-drain diode that every MOSFET has. For a PMOSFET the diode anode is connected to the drain so when the drain voltage is more positive than the source, the diode conducts and discharges the cap.
 
Ah, thank you Carl. You're exactly right. I put a diode on the drain of M2 and the problem went away. I also realized I had to put one on the drain of M1.
 
Oh wow. Just realized the two MOSFETs and the 2nd PWM were completely unnecessary. I learned a lot going through the design excercise, but my entire circuit can be reduced to this:
View attachment 61422
Note: the 1M Ohm resistor in my circuit is representative of a high-impedance input pin on an IC.
 
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Oh wow. Just realized the two MOSFETs and the 2nd PWM were completely unnecessary. I learned a lot going through the design excercise, but my entire circuit can be reduced to this:
View attachment 61422
Note: the 1M Ohm resistor in my circuit is representative of a high-impedance input pin on an IC.

oh a generic doubler...
why not just use a switched capacitor doubler chip?
 
Yep, that's exactly what it is. This was an academic exercise for me. I just wanted to see if I could derive the circuit on my own without just looking it up.

ok... would you like to expand on that? generate +15V and -10V from a 5V supply with a single FET :)
 
Hm, the +15V wouldn't need a FET. Add two more diodes and two more caps and you get a voltage tripler. Although, it's not quite 15V because of the diode drops.

The -10V I have to think more about. I'm assuming it's more or less the same circuit, just "upside-down". The PWM is attached to the top of the caps instead of the bottom. Still not sure where the FET would come in...
 
actually you need some sort of AC for a tripler to work... starting with DC... think in terms of a boost regulator....
 
This is what I was thinking for a tripler. PSpice's simulator seems to confirm it. The only thing I'd add in practice is a small resistor in series with the source for current limiting.
View attachment 61550

I do have an AC component (the VPulse block, labeled "V2") in the doubler and tripler. Is that what you're talking about?
 
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what does it show for diode drops under load?

What i was saying was that you need a source for the AC component.

have you ever used switching regulators?
 
The circuit I'm considering using the doubler/tripler in is actually very low current draw. It's the high-voltage programming pin (MCLR) on a PIC. The impedance of the input pin is a steady 1000000 ohm, and it'll be the only draw from the charge pump. I'll be using the PWM output of a uC for the AC source.

I have used switchers as boosters before, but that would be massive overkill for the job :).
 
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check the programming specs... the prg pin typically requires a fair amount of current. it is the power source for programming in chips that require it to be at a high voltage.
 
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