I m supposed to design a dc to dc buck converter(SMPS)with following specifications .


input operating voltage range--18-30v
input voltage transient limit(converter survival)-44v for up to 1ms
output power range(resistive load)-75 -150W
allowed output voltage ripple(p-p,any load)-250mv
allowed input current ripple(p-p,ideal souce)-250ma
minimum efficiency(accross voltage,load)-80%
ambient temperature range= -20deg-40deg-

i m a begginner and need some guidance..this is the first designing project i m doing and hence HAVIN A LOT OF DOUBTS...which r as follows

Q:how do we calculate efficiency for a buck converter?
Qower ratings of resistive load is given..but what kind of load does it represent?dat means wt shud i use as a load,which can stand 12v and has low resistance??
Q:do we need an input filter in this project....?
Q:what shud be appropriate switching frequency for this buck converter without complicating the design and why?
Q:how do i select the mosfet type ...on what basis shud i choose?

plz guide me...smebdy..i need help...??

 

Q:how do we calculate efficiency for a buck converter?

http://en.wikipedia.org/wiki/Buck_converter : See "Effects of non-ideality on the efficiency"

Qower ratings of resistive load is given..but what kind of load does it represent?dat means wt shud i use as a load,which can stand 12v and has low resistance??

I don't know your output voltage specification, is it 12? If so, R = V^2 / W

Q:what shud be appropriate switching frequency for this buck converter without complicating the design and why?

50-100KHz I would think, there are some sites that talk about it. It has to do with the tradeoff between switching and conduction losses, and sizing of components.

Q:how do i select the mosfet type ...on what basis shud i choose?

low-gate charge = low switching losses, low RDSon = low conduction losses, these are inversely proportional though

Steve

 

thank u steve....

by "wt kind of load" i meant wt kind of practical load can i use here..eg.some bucks r used to charge battery etc..so wt kind of load can i use here...the output voltage is 12v,current is about 12A,resistance (max)=1ohms...

i hav few more questions....



as u can c in the specifications.. output power "RANGE" is given....it means the load is varying..and the input voltage is also varying(18-30)
what effect does the change in load have on the duty cycle ,ripple voltage and ripple current....shud i assume a fixed load n carry on with the calculations...???


n one more thing...
efficiency means=output/input=Vo/Vi=(D*Vi)/(Vi)=D(dutycycle)
so does dat mean dutycycle is equal to efficiency...????
how do i use different power losses to calculate efficiency?

 

You're welcome.

You need some more information about the load, since there is no specification on transient response. For now, and testing purposes, use resistors to test out the circuit. I would start out with a high value just to make sure you're getting the right voltage, then, go lower to ensure that you can supply the power. I have used lightbulbs in the past, since I don't have a lot of high power resistors on heatsinks lying around. Lightbulbs have dynamic resistance though, so be careful..

Qualitatively, as the output capacitor or switching frequency increase, the magnitude of the ripple decreases.

Practically, the ripple depends on many more factors. I cannot go into detail due to time and the fact someone else has already done it here: http://powerelectronics.com/power_s..._buckconverter_design_demystified/index2.html

For efficiency, use n = power out / power in , (your vout/vin gives you voltage gain, which isn't really important here)

You will get power out by the load power, then power in is this power plus the sum of all your losses. These losses are covered in detail in the wikipedia article I pointed you to.

Goodluck,

Steve

 

Although this sounds like a school project, nevertheless, in the real world there is Webench (because they want to sell you NS parts):

http://www.national.com/analog/power/webench