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I'm trying to understand Buck Converters by designing & simulating a circuit in www.Falstad.com circuit simulation tool.

Here is the link to the circuit that I've Designed.

link to circuit designed in Falstad.com

(Its a long link, copy paste it if it the hyperlink won't work)

The objective is to design a circuit that can accept 230V 50Hz AC supply from mains and output a DC supply with a Voltage limit of 60V and a Current limit of 30A.

I'm still in the initial stages and was only able to implement the Voltage limiting feature.

Few points to note regarding the design.
1.I have used a 555 timer IC to generate the PWM signal to switch the N-MOSFET as that was the only method of generating a variable PWM I could find on this simulator.
2.The values of the capacitors and the Inductors are not yet fixed as Its a work in progress.
3.Oa is an Op-amp that is used to amplify the 5V output signal from the 555 timer IC to 350V required to drive the N-MOSFET gate.
4.Ob is an Op-amp that is used to send a feedback signal of 5V by comparing the voltage at the load w.r.t a constant reference voltage of 60V.
5.I have created a scope of the voltage and current across the load.

If you reset and run the simulation, you'll see that the load voltage(Vl) shoots higher than 60V initially but after a certain time settles down and is correctly regulated at around 60V.

1) Why does it shoot up initially?
2) I suspect that the inductor is releasing its stored energy after the MOSFET is switched off and causing this voltage spike. Is this correct?
3) But going through several circuits on the internet I have come across similar circuits used to design a buck converters. If this circuit has all the parts for a voltage controlled buck converter, how do commercially available converters address this voltage spike issue?

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  • \$\begingroup\$ Falstaff is giving you dodgy results. You have an N channel MOSFET that you are hoping to control the current into an inductor yet the internal diode will always be on and you will get hundreds of volts out. The fact that falstaff appears to be giving you the right voltage is purely coincidental. Try and show a proper circuit and try and use a more pro simulator like micro-cap (also free but excellent). \$\endgroup\$ – Andy aka May 8 at 13:16
  • \$\begingroup\$ Also, I don't see a proper feedback loop, there is nothing to limit the gain. Suggestion, if you are having trouble with closed loop, try to get open loop working. Set a fixed pulse width at the PWM, monitor the current at the inductor, get this working first. \$\endgroup\$ – Mattman944 May 8 at 13:30
  • \$\begingroup\$ @Mattman944 Correct me if I'm Wrong, The 555 IC is supposed to do the feedback here. Its configured as a PWM signal generator and the output of OpAmp Ob goes into the control pin of the 555 to control the PWM. If you right click the wire coming out of the outpin of the 555 IC and select 'view in scope', you can see its trace. \$\endgroup\$ – Techtroniks May 8 at 14:12
  • \$\begingroup\$ 1H is too big. 1mF draws huge current. Include ESR and change FET to beta =from 20m to ~ 5 ~ 1/5*RdsOn . FET backwards, Vgs 350V (not!) Try this , a bit better. Remember you have a 2nd order loop factors of Q, f0 , damping factor and watch or out FET power dissipation. tinyurl.com/yxvwomlf It works but must have all real ESR's ( also triangle wave + comparator = PWM) \$\endgroup\$ – Sunnyskyguy EE75 May 8 at 14:50
  • \$\begingroup\$ @SunnyskyguyEE75 Thanks for this. Will take a lot of time for me to understand all that you said and all the modifications you did. \$\endgroup\$ – Techtroniks May 8 at 15:21

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