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I have built a Voltage Controlled Current Source that has no ON/OFF switch. This VCCS simply receives a voltage and outputs current to a DUT. The DUT is basically a high-power variable resistor (10 to 20 ohm).

So for example, if I give 100mV to the VCCS's input, it will try to supply 100mA to the DUT (regardless of its current resistance). And if I give 2V to the VCCS, I get 2A at the DUT (again, regardless of its resistance). So the VCCS is all fine and dandy.

Now, I need to implement a high-side switching between the VCCS and the DUT, that can be controlled by a 3.3V logic from an MCU. I tried to come up with some solution, and here is where I am currently stuck at:

enter image description here

And here is the result:

enter image description here

Can you please guide me to wire this circuit correctly? Also, I will appreciate it if you can name a NMOS and PMOS that can be used in this configuration. I have chosen the mos-es in the LTSpice arbitrarily.

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    \$\begingroup\$ I think this is an XY problem... Your issue will be keeping the p-channel on when the current setting is low enough so the output is below the Vgs threshold. What does your current source circuit look like. \$\endgroup\$ – Trevor_G Nov 16 '17 at 17:26
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    \$\begingroup\$ I'm not sure what you are trying to accomplish with M1, for a simple switch it can be removed. You also need to watch the absolute maximum for Vgs on the P-FET with a 40V source you are probably right at the edge of damaging the device. A 10V zener in parallel with R2 can be used to limit Vgs regardless of the input voltage (watch the zener power rating when the switch is on, current governed by R3) \$\endgroup\$ – Dean Franks Nov 16 '17 at 17:26
  • \$\begingroup\$ @Trevor any ideas as to why I get 3.6A through the load? \$\endgroup\$ – Sean87 Nov 16 '17 at 22:30
  • \$\begingroup\$ Not sure how much source resistance? \$\endgroup\$ – Trevor_G Nov 17 '17 at 0:54
  • \$\begingroup\$ @trevor you are right i had 0 ohm on sources \$\endgroup\$ – Sean87 Nov 17 '17 at 8:11
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When your current setting is very low you will not be able to turn on the P-Channel unless you have a negative rail, which I see you do, so you would need something like this.

schematic

simulate this circuit – Schematic created using CircuitLab

A better solution would be to turn off the current source directly. Perhaps an N-MOSFET tied to the plus pin on your MP38CL.

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  • \$\begingroup\$ Thanks, do you mean putting a nmos on the (+Vs) of the MP38 or on the non-inverting input? How about a relay to switch between ground and Vin (e.g. if I want 0A from VCCS I short the Vin to ground) \$\endgroup\$ – Sean87 Nov 16 '17 at 22:28
  • \$\begingroup\$ @Sean87 just pulling the demand on the positive input to ground will effectively make the output zero. \$\endgroup\$ – Trevor_G Nov 17 '17 at 0:45

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