Electronic – High Side P channel mosfet with totempole driver + floating / voltage follower pull down

Here's a better way to drive a high-side P-channel MOSFET:

^{simulate this circuit – Schematic created using CircuitLab}

Note that the BJT Q1 is a current sink; the current value is the control voltage divided by R2. If your control circuit puts out 12V, the current will be 12 mA.

This current will create a voltage drop across R1 as well, and if you happen to make the two resistors equal, this voltage drop will be essentially equal to the control voltage supply (minus the V_BE drop of Q1). Or you can make the resistors different in order to get a particular relationship.

You need to be aware of two things: The current level that you set will have some bearing on how fast the MOSFET M1 switches; higher current means faster switching. However, keep in mind that Q1 has a lot of voltage across it. When off, it has the full high voltage supply across it. And even when switched on, it has the high voltage minus 2× the control voltage across it. This means that it will be dissipating power equal to that voltage drop times the current when on.

If you are using a logic IO pin that can drive 3.3V, the current into the opto's diode will be about: -

\$\dfrac{3.3 volt - V_{F}}{330\Omega}\$ where Vf is forward diode voltage of about 1.2V (see opto's data sheet).

This is a current of about 6.4 mA. The current transfer ratio of the opto is a minimum of 50% therefore you can expect to deliver at least 3.2 mA to the emitter of the photo-transistor.

It could be more - according the the opto's data sheet, CTR can be as high as 600%. However, there are other problems to solve. If you look on page 7 of the data sheet you are pretty much limited to only being able to supply 5 mA from the device when the collector-emitter voltage is above 10V and because your supply is 36 volts you are likely to fry the chip if CTR is higher than 100%.

To avoid all of this I'd be tempted to use a voltage regulator from the 36 volts supply - bring it down to 12 volts then use the opto - no need for the zener protection.