Buck Converter with NE555 in LTSpice

[Harun]

Hi, I am trying to make a Buck-Converter with a NE555 as a switch. But I do not understand why the output signal is not even a bit attenuated. I attached here my circuit and the measurements of the output and between the Gate and Source of the PMOS.

Thanks

[T3sl4co1l]

There's no load...

That's also way too much Vgs(on) for M1.  The simulation doesn't care, but don't do this with real parts.

The rise/fall times are also very poor.  Not that you're running much frequency, which is part of the problem as well, i.e. the L1-C3 time constant isn't much longer than the (R1, R2)-C2 time constant.

This is also not a great topology in general.  Suppose the load were shorted; the 555 should deliver almost zero duty cycle in that case, but how can it know?  Even if the load current is merely varying over a nominal range, or the supply voltage, the output voltage won't stay constant, it's completely open loop.

These problems are addressed with a control circuit like this,
https://www.seventransistorlabs.com/Images/555%20Boost.pdf
which uses the 555 as a flip-flop, and senses the switching current.  Operation is similar to the UC3843 boost/flyback current mode controller.

For buck architecture, if you don't need a common-ground output, you can simply use an NMOS switch, and buck towards +V; this allows the same (555 or other) control circuit to be used.  Otherwise, a level shifter (high side gate driver) is preferred.  Buck regulators simply integrate all this circuitry for you.

Tim

[Harun]

Thanks for your advice, it helped me a lot