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DC-DC Conversion Cookbook Slide 9

The power elements, the high side and low side switches, are the primary source of power dissipation in a DC/DC conversion. In an ideal converter, when the switches are on, they have zero voltage across drain to source, in actuality there is a voltage drop due to the RDSON and subsequently power dissipation. In first approximation, there are three components of the power lost in the conversion. The first is conduction losses, which are proportional to the square of the output current and to the resistance of the HS and LS during the respective on times. This is the power lost in the FETs when they are conducting, in the on state and can be minimized by reducing the RDSON of the switches, at the expense of the silicon area (die size). Next is the switching losses of the high side, proportional to the input voltage, Iout, switching frequency, and commutation times of the switches. These losses come from the fact that during the commutation on to off of the FETs, there is an intermediate state where current is flowing through the FET and a voltage is still present between drain and source. The faster the commutations on-off and off-on, the lower the losses, at the expense of higher EMI generated during the transitions. Finally, the power required by the internal circuitry, dependent on the Vin and quiescent current, is key when the device is in regulation at a zero or very light load. A particular case is a linear regulator, whose power dissipation is shown in the lower formula, where the voltage drop Vi-Vout is multiplied by the output current. The Vin Iq component is usually negligible, because the Iout is several orders of magnitude higher than the Iq. Therefore, this component can be ignored except when the load is comparable to the Iq.

PTM Published on: 2013-04-11