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I need your advice: I am very new to electrical engineering, and I would like to design a PCB that allows me to rapidly charge/discharge capacitors. I would like to use a PIC to control MOSFETS that open/close voltage inputs from an external power supply to capacitors.

enter image description here

I thought about using this scheme for the switch:

enter image description here

... but I am not sure if this is a good approach since I would need a high OHM resistor to charge up my capacitor quickly (reduce drainage as much as possible) and to avoid excessive currents, but then on the downside, it would also slow down the discharge rate of the capacitor.

Hence, I was wondering, if someone knows a better approach of how to deal with such switching?

EDIT

Based on the comments and an answer, I tried to implement the two-MOSFET switch, as suggested:

enter image description here

Unfortunately, I must have done something wrong with my setup. Have a look at the transient analysis plot: The capacitor does not seem to be completely discharging.

enter image description here

The TINA simulation file can be downloaded here

EDIT 2

Another switching scheme that seems to work this time:

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Here is the result:

enter image description here

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – Voltage Spike
    Feb 25 at 23:08
  • \$\begingroup\$ You can't design anything without expectations "SPECS" \$\endgroup\$ Feb 26 at 15:52
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 Capacity: 14uF, Load/Unload to 100V-> as fast as possibel; What do you think about this: ti.com/lit/ds/symlink/… \$\endgroup\$
    – henry
    Feb 26 at 17:34
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That resistor will drain continuously your capacitor, so it will charge up and then slowly discharge. Also be wary of the gate requirement of your P-channel mosfet, unless you are charging to 5V you'll need to translate the level.

What do you need, as already commented, is two mosfet, a P-channel for charging and an N-channel for discharging. This is usually known as half-bridge and it's widely used for controlling, like, everything.

When using dedicaded ICs you can use an N-channel even for the high side (N-channels are cheaper and works better, but you need a floating Vgs to drive them).

They actually sell packages with one, two (brushed DC motor), three (brushless motor control) and maybe more half legs with all the drive circuitry and protection needed. Just supply a logic level. Major IC manufactures make them, I'd start with ON Semi, Infineon, Allegro and even TI (not an exhaustive list, of course)

Be careful with caps since the surge current can be quite big. Sometime for the discharging part only SCRs are used since they tend to perform better for the job.

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  • \$\begingroup\$ Thanks a lot for your answer! As you can see in my edited question, I tried to replicate your suggestion with the TINA simulation tool. However, I must have done something wrong, as the capacitor does not seem to be discharging completely. Do you see what I did wrong? \$\endgroup\$
    – henry
    Feb 25 at 9:11
  • \$\begingroup\$ when you give 10V to the gate the low mosfet sees Vgs=10V while the top one has Vgs=-20V so it's still in conduction. Typical shoot-through that would destroy the fets. You need to drive the gates up to 30V \$\endgroup\$ Feb 25 at 15:38
  • \$\begingroup\$ @Frog Thanks a lot for your input! What do you think about the new switching scheme that I posted in my second edit? \$\endgroup\$
    – henry
    Feb 25 at 20:55
  • \$\begingroup\$ @Henry since the capacitor is very small (230pF) it won't take long to charge even with a relatively large series resistance - in this case 450k, so the time constant is about 100us. Is that fast enough for you? The gate charge for the FET could be a few nF and the effective resistance of the IO pin might be 100R so the FET should switch on within 1us when driven directly from an IO pin. \$\endgroup\$
    – Frog
    Feb 25 at 22:30
  • \$\begingroup\$ @LorenzoMarcantonio What do you think about this one:ti.com/lit/ds/symlink/… \$\endgroup\$
    – henry
    Feb 26 at 11:03

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