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Topic: 555 buck converter (Read 650 times) previous topic - next topic

Wawa

but if Vin is larger than the max 555 voltage you'd need an isolating diode to keep the 555 healthy.
Like to see a diagram of that.
All I can think off is a transistor level shifter.
Leo..

Atomillo

How do I connect such diode? Thats something I dont understand. I have already use a Schotky one that protects the gate. Isnt that enough?

westfw

#17
Sep 13, 2017, 07:36 am Last Edit: Sep 13, 2017, 07:36 am by westfw
Wawa is right; it won't work; the pullup would forward bias the diode and allow harmful voltages to the 555; it'd work better if the 555 was running at a higher voltage than needed.

You should be able to use something like this, though...



Wawa

That circuit could harm the mosfet though. Many have a 20volt Gate/Source limit.
A resistor in the collector line could fix that (voltage divider).
That also would make gate charging as slow as gate discharging.
Not really suitable for high frequency switching.
Back to the drawing board.
Leo..

Atomillo

Maybe using a dedicated MOSFET driver would help?
If not should I return and use an NPN transistor?

MarkT

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.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Atomillo

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?

Wawa

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..

westfw

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...


Wawa

- 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..

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