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figure of merit for dc-dc converter

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DimaKilani

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Hello,

Does anyone know if there is a formula to calculate the figure of merit of a dc-dc converter? I need to compare between my design and other designs regardless of details.

Regards,
Dima
 

Hi,

you are talking about efficiency?
Then you should look for dissipated power.

* shutdown power (without switching)
* diode power dissipation (usually depends on average current)
* inductor power dissipation (RMS current, core loss)
* MOSFET power dissipation (gate drive power, condution loss in mosfet- accroding RMS current, body diode current according average current..)
* others

All these have their own power dissipation behavior as a function of load current.
You need to add them all and calculate the relation to the output power.

Klaus
 

Comparing things "regardless of details" is a pretty useless
exercise here, as in many places.

Would you call fixed frequency operation a "detail"? The
RF "client" won't.

Efficiency is the go-to differentiator. But efficiency how
and where? The marketing number, which is peak? Or a
minimum efficiency across the user's load-span (which
again is "details", so maybe you pick a converter that's
a percentage point better in the datasheet, but at your
real load-point is inferior because it's really peaky.

With a little effort you might be able to come up with
"housekeeping", conduction and switching losses from
datasheet entries and/or curves. Then Pdiss boils down to

Vin*(Ihousekeeping+fSw*Qsw)+((Vin-Vout)*duty%H+Vout*duty%L)*Iout

more or less, and

eff%=100%*Vout*Iout/(Vout*Iout+Pdiss)

neglecting things like ripple current and external losses
such as input filter ESR, output inductor R, etc.
 

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