Hi:
After reading the chapter of Voltage Regulation of the very good book "Electronic Principles" of Malvino I decided to try and design my own buck converter. No any buck converter though, I wanted to make one with a 555. I came up with this design, which I think is decent. But before blowing something up, I wanted feedback from someone more experience than me. Sorry if the photo looks a bit bad, but its the best my camera can do. BTW, the resistor at pin 4 is a pull-down.
May be a nicer picture (schematic) with component values, together with a statement of what voltages, current capacity etc. the device is expected to handle may help you to get some interesting comments here.
The pass transistor is supposed to work as a switch (fully on/off).
Do you think the 555 can raise the base to or above the input voltage (40volt?).
Leo..
Once I come from school I will post it.
And I dont need a sufficient high voltage at the gate, only current. And the 20 mA of a 555 with a 1k resistir at the base should be enough, right?
But maybe a Darlington pair would be better. What do you think?
Knowing the value of L and the switching frequency would help, as would knowing how long the switching transistor will last, as it will be operating in linear mode and not saturated mode.
I suggest you go back to the drawing board and start again.
Yes thats what I though. Only problem is that I have no clue how to calc the best value of L. Is there some formula I should be aware of?
And I dont need a sufficient high voltage at the gate, only current. And the 20 mA of a 555 with a 1k resistir at the base should be enough, right?
Yes you do need higher voltage. As for 20ma, no one knows. One cannot calculate anything until specifications are established. You draw a BJT and then mention a gate and current in same sentence. Which is it? A BJT or a MOSFET? Do you understand the difference?
You still haven't posted the basic specs.
Input voltage
Output voltage
Output current
Frequency
You have drawn some lines on paper which will not function. People will help you if you answer their questions. So far, you've ignored everyone and you continue to ask what are pointless or unanswerable questions. You can either interact with the posters here or... continue with your folly while you talk to yourself, posting it here for unknown reasons.
Have a read of this.
This example uses a commutated FET design , which gives good efficiency .
Designing any kind of switching converter is not a 5 minute exercise, and does involve a fair bit of math, but its fairly straightforward once you understand the rationale of what all the components do.
The inductor is very important, and not only do you have to get the value right, you also have to get the maximum current rating right, or the inductor saturates, and the top FET goes bang.
Here are the link on how to calculate the best value of Inductor L.
http://www.dos4ever.com/flyback/flyback.html
Hi,
Did you google LM555 buck converter.
Suggested circuit from above link
There is even YouTube videos.
Tom... 
I didnt search anything in google because I wanted to try and design it by myself. I have been looking for youtube videos like you suggested and this one has helped me quiet a bit:
Trying to use all the advice received andhttps://www.youtube.com/watch?v=m8rK9gU30v4 the info. from the video I made this.
PS: the inductance is 500, not 40. Sorry.
But watching the video I realized that increasing the frequency is better and would allow smaller inductors.
Up to which freq. can I use the 555 for this? I know that the max. is 1 MHz but that is quiet distorted.
Same problem as in the first diagram. But now with a source follower
For the mosfet to work as a switch, the gate most swing between 0 and >=30volt.
The 555 can't do that. Most mosfets can't do that either.
Leo..
And a 555 has no hope of providing enough gate current to charge and discharge the Fets gate source capacitance quickly enough.
For the mosfet to work as a switch, the gate most swing between 0 and >=30volt.
If it's s P-channel moffet, pulling the gate to zero should turn it on, but it needs to get back up to the input voltage to turn off. Your pullup will do that, but if Vin is larger than the max 555 voltage you'd need an isolating diode to keep the 555 healthy.
And a 555 has no hope of providing enough gate current to charge and discharge the Fets gate source capacitance quickly enough.
A standard 555 sinks 200mA, which should be enough for all but the beefiest mostfets!
westfw:
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..
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?
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...
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..
Maybe using a dedicated MOSFET driver would help?
If not should I return and use an NPN transistor?