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Iam designing 3.3V to 24V Non Inverting Level Shifter by using NPN Transistor and P Channel Mosfet.

Input : STM32 Microcontroller GPIO- 3.3V (High ,Low)

output : 24V , 24 mA (High low)to Drive an SFH617A-3X007T Optocoupler (Which has series resistoir to limit the current to the Opto Coupler Diode on other side)

Note: In order to Keep the MOSFET Gate Source Voltage with in the Threshold i have used Two Methods .

Design 1 : I have used Resistor divider to the collector so when the NPN Transistor is ON The P-CH-MOSFET Gate Source voltage will be clamped to 12V (Within the Threshold) . enter image description here

Design 2 : I have used a 12V Zener Diode to Clamp the Gate Voltage with in the MOSFET Gate source threshold Voltage. enter image description here

Questions:

  • Which method is better to use it ? what are the pros and cons of both the designs In terms of Level Shifting and Protection?
  • is there any issue in the circuits ? If There Please Guide me ?
  • Is it okay to use NPN instead of N-CH to drive P-CH ? Do I need to consider Base collector reverse voltage for NPN?

Iam trying to understand the Transistor Switching , Please Guide me through.

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    \$\begingroup\$ Since this is just an LED you are powering from 24 V and a current limiting resistor, can't you just use a 3.3 V output to turn on an NPN to achieve this? Does it need to be complicated for some reason? \$\endgroup\$ Commented Sep 20 at 20:31
  • \$\begingroup\$ @periblepsis It's probably an industrial PLC with a certain input structure (for sensors called called "PNP"). \$\endgroup\$ Commented Sep 20 at 20:41

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Either will work okay. Unless you expect massive transients on the 24V bus there's not much difference.

I don't see much point in using an 8A MOSFET for the output if it is just driving an optocoupler.

You could also do this:

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here

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  • \$\begingroup\$ WWhat About Emitter Base Break down Voltage \$\endgroup\$ Commented Sep 21 at 9:22
  • \$\begingroup\$ There should be no reverse voltage on the base with any of these circuits as shown so it is not a concern. \$\endgroup\$ Commented Sep 21 at 12:04
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Also fond of this:

schematic

simulate this circuit – Schematic created using CircuitLab

R1 makes Q1 a switched current sink, and R2 (D1 optional) sets gate voltage.

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  • \$\begingroup\$ I like that better. Lower base current by a factor of 10, too. It has its pluses. \$\endgroup\$ Commented Sep 21 at 7:21

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