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!
and i am waiting for your answers and recommendations.

 

How about a schematic?

 

Here it is

 

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.

 

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.

i don't realy quite understand what you mean??

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.

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.

Assume: 12V input, 2 amp load, D = 0.416,​

Trise = Tfall = 55 ns, Fsw = 400 KHz​

Select P-Channel MOSFET for ease of driving gate.​

Select -30V, -9.3 amp MOSFET for low Rds (0.02 ohm)​

cost = $0.72​

Pconduction = (ID)2 · Rds(hot) · D = 22 · 0.02 · 0.416 = 0.033 watt​

Pswitching = (V · ID / 2) · (Ton + Toff) · Fsw + (Coss · V2 · Fsw)​

Pswitching = ((7 · 2/2) · 100 nsec · 400 kHz) + (890pf · 72 · 400 kHz)​

Pswitching = 0.28 watt + 0.017 watt = 0.297 watt
Ptotal = 0.3 watt​

To simplify the gate drive circuitry for the MOSFET, a P-channel device was​

selected. An N-channel device would require a gate drive circuit that​

incorporates a method to drive the gate voltage about the source. The cost​

of a level translator and charge pump will outweigh the savings of using an​

N-channel device versus a P-channel device.​

A 20 volt MOSFET was not selected because the available devices in the​

catalog had maximum gate to source voltage ratings of only 12 volts. With a​

12 volt input voltage, the applied gate volts might exceed the device​

specifications. If a 20 volt MOSFET was used, it would be good design​

practice to incorporate a voltage clamp in the gate driver circuit.​

A 30 volt device was selected on the basis of the 20 volt gate to source specification.​

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:​

 

Is your 22VRMS input 24VDC, or 24VAC?

Do you understand the formulas given in the PDF?