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I am designing a 3.3V to 24V non-inverting level shifter.

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

output : 24V , 24 mA (high low)to drive an SFH617A-3X007T opto-coupler, which has a series resistor to limit the current to the opto-coupler diode on the other side.

Which transistor sombination is best to use?

  1. NPN transistor and P-CH MOSFET
  2. NPN and PNP transistors

I am getting results in LTspice with both the methods.

I Know in terms of protection and power consumption that point 1 is the best sombination.

My output current is less than 24mA and for switching less current I have to use at least 5A (drain currrent) MOSFET, so I planned to go for BJTs.

With BJTs, my major concern is the Collector/base Reverse breakdown voltage.

When I switch 3.3 V to 24V using an NPN and PNP combination

  • When the base of the NPN is Low, then 24 V will be at the Collectro base of the NPN.

Since it is a positive voltage, is it okay to use the NPN and PNP combination, or is there any other risk in using that transistor combination with 24V (NPN and PNP) ?

Method 1 : NPN MOSFET combination enter image description here

Method 2: NPN and PNP combination enter image description here

Which transistor combination is best to use for a non-inverting 3.3V to 24V level shifter?

Is it okay to use the NPN and PNP combination?

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    \$\begingroup\$ This is a third question on the same subject. You can use either transistor combination, but you have not defined what "best" means, in respect to what metric? Price? Efficiency? Simplicity? Also your emitter-base voltages are nowhere near the rated value. You have drawn the circuit in a simulator, have you actually simulated the circuit how they work in respect to how you want them to work? \$\endgroup\$ Commented Sep 21 at 10:45
  • \$\begingroup\$ Why are you concerned about BE breakdown voltage in this circuit? Please explain. \$\endgroup\$ Commented Sep 21 at 11:12
  • \$\begingroup\$ Sorry Its Not Base Emitter breakdown , IEdited my question It about Collector Base reverse Voltage when NPN is OFF the collector is Pulled to 24V, Thats My exact Concern in the usage of NPN Transistors @Andyaka \$\endgroup\$ Commented Sep 21 at 11:53
  • \$\begingroup\$ Please explain "..for switching less current I have to use at least 5A (drain currrent) MOSFET.." in more detail. Why must it be such a large MOSFET? Smaller MOSFETs are available. This seems contradictory. Is there some limitation we don't know about -- availability? Cost? Please define. (This is a part of a "best" definition mandatory for a correct answer to your core question.) \$\endgroup\$ Commented Sep 21 at 18:50
  • \$\begingroup\$ Do you need to source current to the optocoupler, could you instead connect the optocoupler to +24V and use an NPN to ground the other end? \$\endgroup\$ Commented Sep 22 at 22:56

2 Answers 2

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With BJTs, my major concern is the Collector/base Reverse breakdown voltage.

The collector-base reverse voltage withstand is 50 volts as per this DigiKey datasheet: -

enter image description here

Given that your supply is 24 volts I don't see this as a problem at all.

is it okay to use the NPN and PNP combination, or is there any other risk in using that transistor combination with 24V (NPN and PNP)

Neither the BC547B nor the BC857A are in any form of jeopardy with a supply voltage of 24 volts. Both have the same reverse voltage rating of 50 volts.

Which transistor combination is best to use for a non-inverting 3.3V to 24V level shifter? Is it okay to use the NPN and PNP combination?

There is no problem with your choice. If you decide on different transistors read their relevant datasheets.

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    \$\begingroup\$ Thank you @Andyaka \$\endgroup\$ Commented Sep 21 at 13:22
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If you control the optocoupler input from the cathode, one simple inverter is enough:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Indeed if the diode is accessible, it can be direct driven from logic level through a 150Ω resistor. \$\endgroup\$ Commented Sep 21 at 18:47

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