help in buck converter design!

Discussion in 'The Projects Forum' started by idmond, Dec 22, 2010.

Oct 5, 2010
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1
i was asked to design a buck converter that takes a 22.4vrms and down them to 9vdc
for a load that draws 1amps.

the ripple current= 0.2 A
Fswitch= 400khz

i did all the math to choose the right values for the shottky diode, inductor
and the output capacitor, but i got stuck in choosing the MOSFET switch.

i don't know exactly whether i choose PMOS or NMOS.
if i chose NMOS, that will require a gate to source voltage higher that 22.4v to drive it
and i'm not sure i can do that.

and if i chose PMOS that will require at least 18v or more to turn the switch OFF
assuming the Vgs(th)= -5
(my info about MOSFETs are a little bit rusty, so i might got it all wrong)

so i am stuck now!

Mar 24, 2008
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Oct 5, 2010
18
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Here it is

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4. thatoneguy AAC Fanatic!

Feb 19, 2009
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720
If you don't have any voltage higher than 22.4V , that basically decides what you can use in your circuit, right? You can often get a lower voltage a lot easier than a higher one.

Do you have any schematics of examples you've been taught? Formulas? There are a few more things to consider than when creating a linear power supply.

Oct 5, 2010
18
1
i don't realy quite understand what you mean??

of course, my design was based on an example of designing a buck converter from Microship Webseminars.
i calculated every thing right with the output capacitor, inductor
and the Shottky diode, but in Microship Designing Example, he chose the P- channel MOSFET over the N- channel
because that will save him the higher gate voltage required to drive the N-channel MOSFET.

i am just confused. how i am to suppose to drive the MOSFET gate (N or P-channel)
to pass this input voltage (24.4V)??​

i already have stepped-down the 24.4V with a linear regulator to get +10V from 7810 and -10v from 7910,
but i want to design SMPS to compare their efficency with linear regulators.​

could i somehow just use the 10V output from 7805 to drive the gate,
or i will need another boost converter to step this 10v up to drive the MOSFET's gate of the Buck.​

i realy need help with this one.

Here is the PDF of the whole Designing Example:​

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6. thatoneguy AAC Fanatic!

Feb 19, 2009
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720
Is your 22VRMS input 24VDC, or 24VAC?

Do you understand the formulas given in the PDF?