Determine the value of inductor in DC-DC (boost) converter

Hi all,

I am currently building a DC to DC boost converter and I have problem identifying the value of the inductor in the circuit. Can someone guide me for this? I am using a frequency of 5kHz and when the duty cycle is 0.75, it should have an output of 20V (5V as input).

I tried using different values of inductor along the way and found out the following:

Duty cycle of 0.75
4mH = 14V
0.220uH = 30V+
1mH = 15-16V

Check pls. such designer SWs as National`s WEBENCH® Designer:
**broken link removed**
or PI`s "PI Expert", free to use or downloadable:
**broken link removed**...
K.

Let your input dc voltage be Vd and output Vo, they are thus related if you desire your converter to be operating in the continous boundary current mode as:

Vo = 1/(1-D) * Vd where D is your duty ratio.

Now you must know the value of your load. ( in terms of Ohms) using your load information you cna canculate your load current.

You must assume that your load current will at least be equal or greater than the boundary output current so that your converter operates in the continous mode:

I0 = I0b

Now I0b is :

(a) for your input dc being constant I0b= D*T*Vd(1-D) / (2*L) where T = 1/f


(b) if your load(output) voltage is fixed, I0b= T*V0*D*((1-D)^2) / (2*L)


using either of these you cand find your f..... I used to find it that way when doing lab work.. hope this helps..