# Current Context and Research Trends in Linear DC-DC Converters
## 7 Current Context of Portable Power Supply Designs
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Item** & **Loss** & **Reason** & **Remarks** \\ \hline \multirow{4}{*}{1} & & \(\bullet\) & Nonzero collector-emitter voltages (in \\  & & bipolar transistors) & These show up a cumulative \\  & & \(\bullet\) & Nonzero RDS(ON) losses in MOSFETS & content known as “static losses” \\  & & such as transistors and diodes & \(\bullet\) & Similar losses in the output side of an IGBT \\  & & \(\bullet\) & Diode forward-drop-based losses & voltage and the load current. \\ \hline \multirow{4}{*}{2} & & \(\bullet\) & Transistors and diodes carry parasitic \\  & & \(\bullet\) & capacitances associated with junctions & switch-mode power supplies \\  & & and diodes increase with the & \(\bullet\) & These capacitors are charged and \\  & & switching frequency & discharged in each switching cycle & linearly with the frequency \\  & & & in general. \\ \hline \multirow{4}{*}{3} & & \(\bullet\) & Hysteresis loses & Except for ohmic losses, others \\  & & \(\bullet\) & Eddy current losses & are frequency-dependent and \\  & & \(\bullet\) & Ohmic losses & have a nonlinear behaviour \\  & & & with frequency. \\ \hline \multirow{4}{*}{4} & & \(\bullet\) & Losses associated with ESR of a capacitor & Smaller capacitors have high ESR \\  & & & while charging and discharging & and large capacitors have \\  & & & relatively lower ESR. \\ \hline \multirow{4}{*}{5} & & \(\bullet\) & Control circuit’s power consumption & \\  & & \(\bullet\) & Any associated secondary power supply & These losses depend on the \\  & & controller circuit associated & for the control IC or the control circuits & control circuit section of the \\  & & with the power stage & inside a power IC. & switch-mode power supply. \\ \hline \multirow{4}{*}{6} & & \(\bullet\) & PCB track & \\  & losses/interconnection losses & \(\bullet\) & Ohmic losses & The larger the current the higher \\  & & in an IC version & & these are. \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Secondary losses of DC–DC converters. A switch-mode power supply brings the additional major issue of RFI/EMI, which requires added circuitry of filters at the input and output side of a DC-DC converter in general. The non-ideal nature of capacitors and inductors used in the circuit for this filtering contributes additionally to Items 2 and 3 in general. When the secondary requirements of a converter are dictated by the load, sometimes a pure switch-mode converter does not satisfy the case. A classic example of this is in mobile phones and other portable devices where a large amount of mixed-signal circuitry is used. In these situations, the most useful secondary properties, such as Items 1 and 2 in an LDO or a linear converter, are used by cascading the output of the switch-mode converter with the input of an LDO for the overall achievement of the wider specifications of the power management system. In summary, a switch-mode converter is utilized when the primary specifications are critically important. However, when the load side cannot tolerate limitations such as Items 1 to 3 above, a low component count LDO is used or an LDO is placed in tandem with a switch-mode converter. A more detailed discussion on this is available in [3]. Therefore, inside each portable device, there is a requirement for multiple voltage levels, and DC-DC voltage converters are used to convert voltages to the various levels. Based on the application's requirements, the best technology is selected for portable devices. Efficiency, noise levels, EMI levels, and the compactness of the converter are some commonly considered factors when selecting a voltage converter for a portable device [61]. With the noise-sensitive nature of the portable devices due to communication requirements, LDO regulators are used as post-regulators in portable devices, which provides a sustainable market share for the linear regulator manufacturers [62].