Emulating diode behavior with transistors

Hi all,

I've come across a little problem here... I need to have a diode on the output of a high side switch, to prevent high voltage generated by a freewheeling motor from flowing back through the high side switch.

However, I don't want to use a diode as my load current may be well in excess of 5A, and even with a Schottky diode - the heat dissipation is unacceptable.

I'm looking for a low dissipation solution, so it looks like it will be some sort of active FET solution - but i'm stumped as to what sort of architecture. Using P FETs seem to present some issues, such as when the gate is pulled to 0V, the gate-source voltage could easily be exceeded and the gate destroyed depending on the voltage present at the source...

EDIT - I should clarify, the motor is a dual winding motor - hence when one of windings is powered, the other generates a high voltage, and one simply cannot shunt the high voltage to ground otherwise the motor is dynamically braked and stops spinning.

So another way I could word the question is - is I need to make it appear as so I have disconnected the slower winding completely from the drivers, when in reality I haven't.

"when the gate is pulled to 0V, the gate-source voltage could easily be exceeded and the gate destroyed depending on the voltage present at the source..."

There are many ways to accomplish your goal. The circuits are complicated. A PMOS with gate not at 0v. Use a gate voltage that tracks the source voltage by being -8v = Vgs. An NMOS with gate at a voltage higher than the usual voltage supply. A PMOS with a gate oxide that can withstand the voltage spike from the inductor.

Evaluate the magnitude of the voltage spike, tell me the voltage. Know the frequency components of the spike. If it is band limited to 2k hz, an active diode amplified circuit can act as a diode. This uses an amplifier that is slow but complicated, sophisticated, durable

A motor is not like a simple inductor. Are you sure that a diode is what you need in that circuit? Unlike an inductor, when you remove power from a PM DC motor, the freewheeling motor continues to generate voltage of the same polarity as the DC voltage that originally powered it.

And if you short the motor, it can cause -considerably- more current to flow than what it took to run the motor at speed. The motor will also quit turning very quickly, this is called dynamic braking.