How do I achieve well-formed pulses in a Dickson charge pump?

Question

A picture is worth a thousand words, and I tend to make a lot of mistakes in my words, so, first picture, the circuit in question:

I know the Dickson charge pump looks a little different but it is essentially the same.

EDIT: We can basically forget about the charge pump type whether it is MOSFET or diode or if the MOSFETs are from drain-source, etc. The problem remains the same.

Below is my pulse engine which does a fantastic job of providing me with two sets of pulses - until I hook them up as in the next two pictures.

Picture 1. The pulses are not hooked up to the negative side of the capacitors and

Picture 2. The pulses are then attached and the pulses are not at all the way I want them, that is, they do not alternate and one of the two no longer returns to ground.

Picture 1:

Picture 2:

I've used op amps to create the pulses and inverted pulses and bipolars - but that's not really the problem. It's that there needs to be an outlet or something for the capacitors.

I've tried to put PNP transistors that fire when the alternating pulse drops to zero. I've used resistors and also a lot of other embarrassing things I won't talk about.

I've tried to put high pass or band pass filters on each pulse line but that does not work at all.

The question, again, is: How to keep the pulses alternating and returning to ground after connecting them to the negative half of the capacitors in a topology like this?

Answer 1

I agree with jp314, that the MOSFETS are connected the wrong way and conduct in both directions. The difference in forward and reverse voltage across the FETs produces a small pump effect of 1-2V per FET.

The circuit you referred to with the researchgate link uses FETs as diodes because they can be produced straightforward inside a CMOS chip, there is no other advantage.

The other chargepump link works identical in the simulator if you just remove all MOSFETs, so this is not well understood either.

To drive a dual phase charge pump, you need two strong push pull driver stages, that can source and sink all the current for the charge transfers of all capacitors at once. It is a symmetrical requirement. Your driver stages may be capable to sink 1A or more, but can only source less than 100uA.

You can use H-bridge DC motor drivers or very powerful gate drivers (dual low side) between your pulse generator and the pump chain. R13, R16 and R17 should be <= 10kohm anyway.

Answer 2

Your MOSFETs are connected the wrong way. G-D means it conducts with a forward V of about Vthreshold. However the drain-bill body diode means it also conducts in the reverse direction.

Flip the FETs and connect gate-source. Not it will be off in reverse bias and in forward the FET and/or body diode will conduct. Note that if this is in an IC you probably don't want the drain-body diode to conduct...