Electronic – How to switch a high-side transistor from logic referenced to ground


Problem: I have a logic signal (represented by SW1), say from a microcontroller, that is either 0V or some low voltage, maybe 5V or 3.3V. I have a higher voltage supply (V1) and a switching device (M1) on the high side of that supply, connected to a load.


simulate this circuit – Schematic created using CircuitLab

Sometimes, there is an H-bridge, or half an H-bridge, and I need to control the high-side transistor(s) (M2), but I think basically it's the same problem:


simulate this circuit

The difficulty is that my logic output is only 0V or 5V, but I need higher voltages, like maybe 50V and 45V, to switch the high-side transistor. I understand that the precise voltages required will depend on the transistors I select — I'm just wondering how to solve this problem, generally.

Is there a simple way to do this if my application is not very demanding? If I need to do the switching rapidly, say for a PWM application at 50kHz, is there a more complicated way to do it?

Best Answer

Aside from IC and isolated solutions (e.g. high side switch ICs, transformers, opto, etc - which is what you'd usually go for nowadays), one simple solution is a reasonably fast discrete PMOS driver circuit like this:

enter image description here

Obviously you can just use the one NPN version, but the push pull increases drive capability. This is just the rough idea, divider can be adjusted and protection added to ensure Vgs tolerance is met. Q1's emitter resistor is to prevent it saturating and lengthening the turn off time.

Simulation at 1MHz - I added the gate voltage (red trace), note it only swings ~7V, it follows the Q2/Q3 base voltage (a zener can be added as mentioned if needed):

enter image description here