6.6 Efficiency Considerations

Efficiency is defined as the amount of useful output power, divided by the amount of power supplied.

$Efficiency\_\%=\left(\frac{V_{OUT}\times I_{OUT}}{V_{IN}\times I_{IN}}\right)\times 100$

Maintaining high efficiency serves two purposes. It reduces power dissipation in the power supply, reducing the need for heat sinks and thermal design considerations and it reduces consumption of current for battery-powered applications. Reduced current draw from a battery increases the devices operating time and is critical in handheld devices.

There are two types of losses in switching converters; DC losses and switching losses. DC losses are simply the power dissipation of I²R. Power is dissipated in the high side switch during the on cycle. Power loss is equal to the high-side MOSFET R_DSON multiplied by the switch current. During the off cycle, the low side N-channel MOSFET conducts, also dissipating power. Device operating current also reduces efficiency. The product of the quiescent (operating) current and the supply voltage is another DC loss.

Over 100 mA, efficiency loss is dominated by MOSFET R_DSON and inductor losses. Higher input supply voltages will increase the gate-to-source threshold on the internal MOSFETs, reducing the internal RD_DSON. This improves efficiency by reducing DC losses in the device. All but the inductor losses are inherent to the device. In which case, inductor selection becomes increasingly critical in efficiency calculations. As the inductors are reduced in size, the DC resistance (DCR) can become quite significant. The DCR losses can be calculated with the equation below:

${L\_P}_D={I_{OUT}}^2\times DCR$

From that, the loss in efficiency due to inductor resistance can be calculated in the equation below:

$Efficiency\_Loss=[1-(\frac{V_{OUT}\times I_{OUT}}{V_{OUT}\times I_{OUT}+{L\_P}_D}\left)\right]\times 100$

Efficiency loss due to DCR is minimal at light loads and gains significance as the load is increased. Inductor selection becomes a trade-off between efficiency and size in this case.