Electronics > Beginners

Trying to drive a pmos

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albert22:
Hi, I am trying to make a buck preregulator based on a TL494. The unregulated input voltage is between 70 to 80v. Represented by V1 in the schematic. The TL494 ground will be at vun and it is represented by V3 its Vcc will be 16v referenced to vun. The PWM will drive Q3 (Vceo >100v). (Or perhaps a nmos). Q1 and Q2 are 2n2222 and 2n2907.
The only way that I found to clamp the voltage at the collector of Q3 is by the diode D. Is there a better way to do it?
I cannot use a dedicated gate driver IC
Simulation seems to work. Of course I will recalculate all the resistors (their values are just for for the sim)
I included a 15v zener between Gate and source of the pmos.
Thanks for any help

Giaime:
Hi, I've noticed some strange behavior in LTSPICE if you leave the default "NPN" or "PNP" devices. Run the simulations with an actual device instead.

Regarding your question, why you need to clamp Q3 collector? Why Q3 emitter is not at ground?
You're also missing the recirculating diode and the inductor...

Check this out:
http://www.jiggerjuice.info/electronics/projects/power/buck-diode-pmos.html

albert22:
Thanks for your answer Giaime.
The buck regulator is already working at 16v input. That is why I showed just the driver.  Now the TL494 collectors (pin 8 and 11) are driving Q1 and Q2.
All the circuits that I found are low voltage. My input voltage is 80v max so the idea is to use a floating supply (represented by V2) for Q1 and Q2.
I need the TL494 to be referenced to the negative of the input voltage (vun) so its output level should be translated to  drive Q1 and Q2. It cannot drive those transistors directly because the max voltage that the TL can handle is about 30v
I realized that placing a resistor in series with the collector of  Q3 forming a voltage divider with  R5, will work.
For Q3 I found a MJ13003 in my junk box. So I going to try it tonight.
Also I am going to find suitable values for the resistors to drive the gate of the mosfet as fast as possible without making them to low.
If the smoke is not too dense I am going to post the final circuit.
saluti affettuosi

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