oscillating pwm
The output stage is fed with a higher voltage as the comparator, so some level shifters are needed. Also the gate- source voltage of the output fets must be limited to 20Volt!
With the Zener Diodes, the voltage at the bases of transistors 7430 and 7455 are kept between around +5.5 and -5.5V (zener voltage +1 forward drop). This sets also the level on the emittors of the transistors (because it is an emittor follower).
The pulse of +/-5V is transferred to the gates of the power mosfets through capacitors 2430 and 2455. They act as level-shifters. The value of the capacitors must be big with respect to the gate charge of the FETs. The zener diodes 6434 and 6435 limit the gate voltage.
This is a very cheap level-shifter, you need only the two transistors 7430/7455 for the two output fets.
An additional fact is that when there are no pulses at the base of 7430/7455, the power fets stop conducting, because the gates will discharge through the 4.7Meg resistors (3461/3436)
About the switching frequency, I did some calculations on the circuit you suggested, but with an ideal opamp (no switching delay, no input capacitance...). I also noticed that the ramp-threshold-setting resistor (R 3409) is 180 Ohm instead of 100 Ohm in your suggested schematic from before. This is very importand, because the ratio of R3408 and R3409 is in the equation of the switching frequency. (doubling R3409 (ok from 100 to 180
) will result in half the frequency.
Are you planning to build a thing like this? Well maybe I will...
Stefaan