PWM- what parts do I need?

The problem is you need a minimum of gate voltage above what the SOURCE has on it. for example, you want to drive a device in the high pole using N-Channels and you expect to put the full supply voltage on the output which will be the SOURCE in this case, the gate voltage must be well above the threshold point that turns it on.

So for example, lets say you have a Fet that needs 10 volts for a solid turn on and you want to pass 50v DC from the DRAIN to the SOURCE, SOURCE being the output side of the high side bridge. The voltage at the GATE must be 60 volts. That would be 10 volts higher than the SOURCE terminal.

Fets turn on by having their voltage on the GATE higher than what is at the SOURCE.

When ever you look at the specs on a FET, you'll see the Vbs(th) or Turn on voltage. That voltage is assumed that you have the SOURCE tied to common as you would being used in a low side or sink application.

So the voltage on the GATE is reference from the SOURCE as the common point. So what ever is going to be at the SOURCE, is your starting base line voltage you need ontop of the added voltage to turn the unit on at the GATE.

High side drivers will normally use your main supply to add a charge pumped voltage reference to it so that you'll get the main supply or at least what is at the SOURCE plus the added GATE drive voltage.

Have a look at the link I gave you. There are very simple basic high side gate drivers in IC's that work very well for this.

Jamie

They also handle the switching transition so that you don't get significant pass-through, that is, both devices being on for a short period, effectively shortcircuiting your rails. Get one and save yourself a heap of work, and possibly trouble.