Can anyone please explain why increasing input voltage will decrease the efficiency of the buck convertor ?
As shown in the datasheet graph.
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The efficiency is:
Pout / Pin
Pout / (Pout + Pdiss)
Pdiss is primarily composed of losses in the IC and losses in the inductor. There are other losses as well, but they can generally be ignored. The IC losses are what we are concerned with here. At the first order they can be approximated by"
Pdiss (IC) = quiescent losses + conduction losses + switching losses + gate charge losses + dead time losses.
The switching losses are equal to (Vin * Iout * (trise + tfall) *Fsw) / 2
Vin is the input voltage
Iout is the output current
trise is the rise time of the switch node voltage
tfall is the fall time of the switch node voltage
Fsw is the switching frequency
The switching loss will increase as Vin increases. Since that loss term is in the denominator of the efficiency equation, as it increases, the efficiency will decrease.