So you are saying the signal from the uC PWM is converted to a much higher voltage close to the collector voltage and this drives the gate?

Yes, as you can see on the diagram T2 work as source follower.
But we use C1 bootstrap capacitor to provide a voltage needed to drive T2 (to full open T2).

If input voltage is equal to 5V the T1 is in saturation. And C1 capacitors star charging phase. And end the charging phase when voltage across C1 reach
V_C1 = Vcc - Vd = 20V - 0.7V = 19.3V

And now if we have a low input signal (0V). T1 is in cut-off. But T2 starts to conduct. So voltage at T2 source start to increase. This causes that the voltage at D1 cathode also increase thanks to C1 voltage. So this causes that the voltage at T2 source and the load increase even faster.
And voltage stop increase when T2 is full ON. (When T2 source voltage reach almost Vcc value).

 

CMOS is just NMOS and PMOS on the same chip. There is no reason that an analog CMOS cannot handle as much power as a bipolar one.

Can you please give a part number for a power rated CMOS? The only ones I'm familiar with are small signal type devices. Such as logic gates.

 

How about Googling: High power CMOS

From very first hit: Microchip TC429
From second page: http://www.eetimes.com/document.asp?doc_id=1261816

 

Can you please give a part number for a power rated CMOS? The only ones I'm familiar with are small signal type devices. Such as logic gates.

There are numerous devices in the MOS family.

At one extreme,LDMOS devices are used as high power linear RF amplifiers.
At the other,MOSFET devices are used in high sensitivity Receiver front ends.

Linear circuits,all!!

People hear MOSFET & only think of the High power switching devices,but these are only relative newcomers.

I've never had anything to do with IGBTs,but I cannot see why they can't be used in linear mode.
The OP's circuit just looks like a normal linear regulator.

 

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