Buck converter

[siyom]

I want to design a buck converter capable stepping down a voltage of 17V to 4.2V.
The current should also be boosted by the same ratio.


This simulation does not work,I get an output voltage of 16.4V.Am not sure if its the switch,but i've used it in a boost converter and worked perfectly.I've calculated the values of the caps and the inductor from the best source available but still it does not work.This is only just a simulation but I dont think i should build a circuit when the simulation isn't even slightly working.

Specs are:
17-4.2V
Dutycycle of 25%
input ripple voltage of 1%
output ripple voltage of 1% as well.

 

[misterT]

You have the N-FET upside down. And I would actually use a P-FET.. easier to drive.

 

[siyom]

You have the N-FET upside down. And I would actually use a P-FET.. easier to drive.

Thanks a lot man,I don't think i would have detected it

 

[crutschow]

Notice the direction of the substrate diode on the MOSFET symbol. That always needs to be reversed biased to avoid conduction through the diode.

 

[MrAl]

I want to design a buck converter capable stepping down a voltage of 17V to 4.2V.
The current should also be boosted by the same ratio.
View attachment 73093

This simulation does not work,I get an output voltage of 16.4V.Am not sure if its the switch,but i've used it in a boost converter and worked perfectly.I've calculated the values of the caps and the inductor from the best source available but still it does not work.This is only just a simulation but I dont think i should build a circuit when the simulation isn't even slightly working.

Specs are:
17-4.2V
Dutycycle of 25%
input ripple voltage of 1%
output ripple voltage of 1% as well.

Hi,

Do you have any equations for buck circuits to start with?

 

[siyom]

Yes i did have the equations,now that i've sorted out the FET everything seems to be working correctly.I just need to built now.
But if you can furnish me with the buck equations please do incase I need to make any changes or additions

 

[MrAl]

Hi again,

Lets start with the basic approximations...

Vout=Vin*D

where D is the duty cycle expressed as a fraction, so for a 40 percent duty cycle D=0.40.

Since we've approximated the output voltage that means we can approximate the voltage differential input to output and then use the definition for the inductor:
V=L*di/dt

to approximate the peak ripple current (dt is the MOSFET 'on' time):

Vin-Vout=L*di/dt
or
di=(Vin-Vout)*dt/L=Iripple1

So the peak ripple current is di and that is added to the nominal current to get the peak current in the inductor:

Ipeak=Iripple1+Inominal

and Inominal is just:
Vout/Rload

where Rload is the load resistance.

We can then use the definition of the capacitor to approximate the ripple voltage at the output:
dv=I*dt/C

Vripple1=Iripple1*dt/C

and
Vripple2=Inominal*(T-dt)/C

where T is the total switch period, and so the total ripple is:
Vripple=Vripple1+Vripple2

and that's the peak to peak ripple voltage.

Does this make sense so far?

 

[siyom]

Yes it makes sense I think i had something similar except for the peak ripple current I used I just approximated that it should small percentage of the input current and then added it to the nominal current to get the peak current.But i can see yours gives an accurate approximation