555 buck converter

Other Hardware General Electronics
21

You can't easily do high side switching with NPN or n-channel, use PNP or p-channel.

With a charge-pump such as in high-side MOSFET drivers you can drive a n-channel
MOSFET as a high-side switch, but you need to be careful it always keeps its charge pump
capacitor charged. Fortunately such drivers do the level shifting for you from the 12V
supply.

Caution is required with any SMPS using cored inductors, as core-saturation leads
to extremely rapid increase in current (you need a protection circuit to shut off the
switching device if this happens). Air cored inductors don't saturate, but are impractically
large.

22

So what I should do:
1: Use a dedicated N-Channel MOSFET driver.
2: Implement some kind of corrent limitation in order to prevent damage.
Luckily I already wanted to implement the 2 thing, so I will go back to the drawing board and post the resulting circuit when I am done.
PS: The list is correct right? But Leo said that the MOSFET would get damaged. So dedicated driver or transistor?

23

Atomillo:
But Leo said that the MOSFET would get damaged. So dedicated driver or transistor?

I think you lack fundamental understanding of transistors.
Try to "fix" that first, before you venture into the not so easy task of designing a buck converter.
Leo..

24

I'm a little confused that P-channel MOSFETs have Vgs(max) much lower than Vds(max) - aren't they usually used by driving their gates to ground? (A similar circuit should work with bipolar transistors, and is common in ie: H-bridges.)
OTOH, P-channel MOSFETs with Vgs > 30V don't seem to be THAT rare...

25

westfw:

  • aren't they usually used by driving their gates to ground?

Max gate voltage is referred to source.

In an n-channel fet circuit, source could be grounded. Then gate voltage is referred to ground.
In a p-channel fet circuit, source could be conected to VCC. Then gate is refered to VCC.

If you use a p-channel fet as pass element in OP's circuit (with 24volt input),
Then absolute max gate voltage for a 20volt Vgs fet could swing between 4volt and 44volt.
0volt from the 555 could in this case kill the fet.

A 30volt Vgs fet could fix this, but VCC (24volt) on the gate is needed to turn a p-channel fet off.
Not going to happen on the output of a 555 on a 12volt supply.
Leo..