Buck converter calculations 1

[sbkenn]

Hi. Is there an easy calculation from voltage, & inductance to maximum power transfer ? I had expected to use a 500kHz converter with an 18V supply, but that won't allow the power that I need. I would need either to crank up the voltage, reduce the frequency, or use multiple phases. A lower inductance would allow an increase in peak current, but reduce the energy per cycle.

Thanks-in-advance
Shane

 

[zappedagain]

You don't state what power you need. That can have a big effect on which solution is right for you. Do you want one of these or a few million?

Try some of the website calculators from the buck converter manufacturers (TI, Analog Devices, National Semi, etc.). That may get you on the right path.

Z

 

[sbkenn]

I need about 100W from each of 4-phase converter.

 

[LiteYear]

The power transfer is basically the power you supply, minus that lost in the switching device, the inductor and the cap. To maximise power transfer you have to minimise those three. High quality components help because they give you low effective series resistance. Beyond that, very roughly speaking, the techniques are: lower switching freq to lower switching losses and AC resistance and lower ripple to lower power loss in output filter. Those two goals are mutually exclusive and need to be traded off to suit the design.

One guiding equation to determine minimum inductance is:

L(min) = (V(in) - V(hs) - V(out)) * D / (LIR * I(out) * f)

where:
* L(min) - minimum inductance
* V(in), V(hs), V(out) - input voltage, voltage drop across switching device in on state, output voltage
* D - cycle duty (approximately V(out) / V(in) )
* LIR - inductor current ripple ratio, ripple current divided by I(out)
* I(out) - maximum output current
* f - switching frequency

Often V(in), V(hs), V(out) and D are fixed, so all you have to play with is f, LIR and L to maximise your I(out).

See On Semiconductor Application Note AND9135/D for a handy worked example.