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I apologise for the attached picture, I just made sure everything is readable so you can assist.

So I'm wondering if this is a logical idea, if 12 volts is supplied, to the circuit, as the capacitor reaches the zener 5.1v + 0.65v of the transistor Vbe, it will switch off the p channel MOSFET, and thus stop the current flow from supply, until the voltage in the capacitor drops below 5v1 + 0.65v again, which in turn will allow the p channel MOSFET to be switched on again.

Is this possible? Please feel free to correct me, thank you.my buck converter sketch

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    \$\begingroup\$ Without hysteresis, such circuits will often just settle in the linear zone, with the output a steady 5.7 ish volts. \$\endgroup\$
    – Unimportant
    Commented Mar 22, 2021 at 12:52
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    \$\begingroup\$ @BleedingEdgeLab No it's not, that will fry your transistor. What Unimportant is saying is that this is a linear regulator. \$\endgroup\$
    – Hearth
    Commented Mar 22, 2021 at 13:05
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    \$\begingroup\$ There will be half of power waist on Q2, no advantage of SMPS. \$\endgroup\$
    – user208862
    Commented Mar 22, 2021 at 13:06
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    \$\begingroup\$ The point is that this is NOT a buck regulator but rather a (poorly designed) linear regulator. Q2, however, will need to dissipate the entire load of dropping from 12 to 5V. \$\endgroup\$
    – jwh20
    Commented Mar 22, 2021 at 13:24
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    \$\begingroup\$ THIS CIRCUIT WILL FUNCTION AS A BUCK CONVERTER AS IT IS SHOWN. Adding formal hysteresis will improve efficiency BUT the energy stored in the inductor and Cout provides an energy transfer after turn off which allows operation. I have built many such hysteretic converters and used them in successful commercial products. Im out at present. I may provide more detail when back in my office. \$\endgroup\$
    – Russell McMahon ♦
    Commented Mar 23, 2021 at 4:27

2 Answers 2

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As above, what you have is a linear regulator.

BUT

If you add hysteresis to the circuit, you now have what is called a hysteretic converter, where the output bounces back and forth rapidly between slightly above and slightly below the desired output voltage. In this case, Q1 is now switching between off and fully enhanced (on) for much lower power dissipation. So, less heat, but more noise on the output. This can be done without an inductor, but it increases the stress on the switching transistor.

In its most simple form, a hysteretic converter is a voltage comparator driving the switch transistor.

Way back, the first switching power supply I had to (constantly) maintain was a 20 A hysteretic converter built onto the backplane of a Data General NOVA 1220. An undersized heatsink and poor airflow caused the transistor to burn up about every 6 months.

AND ...

Gold Star for using reference designators.

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    \$\begingroup\$ Ah yes the old hysterical regulator and with overshoot on startup \$\endgroup\$
    – Tony Stewart EE75
    Commented Mar 22, 2021 at 13:52
  • \$\begingroup\$ @tony works well, subject to obvious limitations. \$\endgroup\$
    – Russell McMahon ♦
    Commented Mar 23, 2021 at 4:29
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You could sense inductor current with an upper trip point and a lower trip point .This would self run .You could regulate the output volts using a simple error amp that changes the current trip point .This uses more parts and does work .

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