PWM problem in Buck-Boost

Thread Starter


Joined Oct 2, 2019
I am designing a buck-boost with 9V input and 35V output I am using MOSFET, so to generate the gate signal for the MOSFET I am using 555 timer (20khz) ,the problem is when I am trying to simulate the circuit on the pspice, I can't get the needed output value ,so can anyone help me?
The main circuitThe output


Joined Oct 7, 2019
1) The IRF640 is a N-mosfet. It turns on when the voltage from Gate to Source is more than 5 volts.
To turn on the mosfet the gate must be above the 9V supply by 5V so if the 555 can lift to 14V it is mostly on.
When turned "off" the mosfet needs to fall to -35V. (Source) But the 555 can not go below ground. With the gate on ground the Source can only go to -5 or -6V before the part turns on and keeps the output from going lower.

Try using a P-mosfet and putting the Source on +9V. Now there is no limit to how far negative the Drain can go negative.

Change the 1N4007 for a "fast" diode. 1A 100 volts.