Some questions about buck converter circuitry

[long0900]

Hi!
I want to ask some question about it :
1. Is is ok to use the kind of Mosfet which attached with a diode in a basic buck converter?
2. I have read some basic calculations of buck converter, they recommend me a minimun value of inductance for a specific converter. So Is it alright to use a higher inductance value ?
3. Is this the formula to calculate switching frequency ( I'm using UC3843) ?
f=1/(2*pi*RC)
4. While searching the net, I have noticed in some circuits, there is a registor connected between the output pin of the controller and the gate of the mosfet, I want to ask what does it do, Is it necessary

That's all my question. Thanks for reply.

 

[barry]

1) ALL MOSFETs have an intrinsic diode, is that what you're talking about?
2) Your performance may not be optimal if the inductance is too much higher than the calculated value. Higher inductance means more turns of wire, more resistance and more losses. Also, your transient response may suffer.
3) READ THE DATA SHEET!!!!! It's right there on page 4.
4) This resistor limits the gate current from the driver (the gate looks like a capacitor). It can also prevent ringing.

 

[long0900]

Thanks barry,
I read the datasheet but I'm confused because if I use that formula, it give me a value ten times less than the one in tthe datasheet

 

[KlausST]

Hi,

I read the datasheet but I'm confused because if I use that formula, it give me a value ten times less than the one in tthe datasheet

Can you show us a reference document where your formula

f=1/(2*pi*RC)

is used to calculate a switching frequency?
I doubt this, because this formula commonly is used for filters, but not for frequency generation.

--> trust the datasheet.

Klaus