Hey so i'm new to electronics and I've tried constructing a buck converter (using software only) circuit without the use of an external PWM source (microcontroller, 555 circuit) and i came up with this (steps down the voltage from 12 to 4.75 V using a p-MOSFET whose gate voltage is altered by the change of the output voltage).

Can you guys tell me what can be improved in the circuit and what impracticalities might present themselves when constructing it IRL?

Note: It would also be appreciated if someone were to show me how to calculate the efficiency of this circuit.


simulate this circuit – Schematic created using CircuitLab


1 Answer 1


What you've built (or tried to build, see below) is a linear regulator. Its efficiency will be the ratio of Vin to Vout. In this case, assuming your 4.75V output target, the efficiency will be about 21%.


Unfortunately, your design doesn't even work as a linear regulator. The reason why is that it has no reference to compare against to track the output voltage. Nevertheless I simulated it with a 1.5V threshold FET and got a 3.6V output with that load; the output varies under load. Try it yourself, here.

Below is a functioning linear regulator using a p-FET pass element, using feedback from the output and a 1.2V reference and an op-amp to drive the gate. Simulate it here.

enter image description here

A DCDC will have this same basic loop control: feedback, comparison to a reference, and some gain.

  • \$\begingroup\$ Thank you for your quick reply! from what I'm understanding, just because a P-Fet switches ON and OFF to keep a certain voltage from rising too high does not make your circuit a switching regulator nor does it give it its high efficiency advantage, plus as you've said the output varies under load. I'm going to look more into the Buck converter designs . cheers! \$\endgroup\$
    – A.H.Z
    Jul 20, 2021 at 21:54

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