Luis didn't yet manage to say (but may when/if the list server accepts his mail) - 

> Hi Guys,
> I am looking at boost converter design at the moment and I came across a
> lot of useful and interesting information on the web, but with that I
> found a few discrepancies that I would like some advice with.
> On one of the sites I found it says that the forward diode should be
> dimensioned to be able to handle at least current equal to the peak
> current on the inductor, I can not see why, as the diode should never
> see that current. Am I missing something here? What value of max current
> would you use for this diode normally on your designs?
> Best Regards
>            Luis

Your question shows that you do not understand how inductors "work". When attempting to build boost or other converters it is useful (at least) to understand the characteristics of the main components, and the inductor is among the 'mainest' of them all.

"The law of natural cussedness" / how it happens / conservation of various things tells us the the current in an inductor can never vary as a step function - it's rate of change can change but, at a point in time the current an infinitely small period before and after is the same. (This is not true for a capacitor or a resistor. With a capacitor you can step change the current but not the voltage. With a resistor you can (and must) change both.)

SO if the inductor is carrying a current I just before the controlling "switch" (MOSFET or whatever) is turned off then it will be carrying the same current just after the switch is turned off. The current that was flowing through the switch now MUST find somewhere else to flow, and the diode is the path of choice. In a perfect design the peak inductor current WILL thus flow in the diode. 

Note that this is the PEAK diode current, not the mean current. Mean current will be smaller and MAY be much smaller depending on various aspects of the design. 

In a discontinuous converter Idiodemean is ~<= Idiode_peak/2 x Vin / Vo when the output inductor "stands on the Vin pedestal" which always happens with simple switching* and a single winding inductor, and ~<= Idiode_peak/2 x Idiode_peak/2 x Vin / (Vout + Vin) 
[[ OOMH, BOTE, YMMV, E&OE ... but you get the general idea.]]
I won't start in on the continuous case, but its worse :-). 

* I say "simple switching", as use of eg synchronous rectification allows you to relocate the inductor topologically between the input and output cycles so eg it is ground referenced during output. This is not relevant when a simple diode rectifier is used and a simgle inductor "stands on the Vin pedestal" during output. 

If you tell us the specifics of the application it may help put things in perspective. Small converters are usually far more forgiving than large ones. 

As a rule of thumb, the diode will on average carry Iout average. In most cases a diode will happily handle peak currents several time it average. Note that for higher power systems the "average" diode current does not tell the whole story due to higher Vdiode at higher currents than the average current which occur over only part of the cycle. So mean diode dissipation will be higher than if it carried the mean current as DC. 
In higher frequency systems there will also be losses due to reverse conduction while charge carriers are 'swept out' and other reasonably second order effects.


              Russell McMahon
              Applied Technology ltd
              New Zealand.
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