Switch-Mode Power Supplies for Beginners: An Efficiency Primer Part 2

May 27, 2008 3:28 PM
By Daniel Wagner, Associate Member of the Technical Staff, Applications, and Roger M. Kenyon, Director, Customer Applications Engineering, Maxim Integrated Products, Chandler, Ariz.

In part 1 of this article, the inefficiencies incurred by the MOSFET and the diode of the generic switch-mode power supply (SMPS) were examined. The conductive losses presented by the static characteristics of each of these devices, as well as the switching losses developed by their dynamics, proved to be main contributors to the power loss found in these types of circuits.

However, while high-quality switching devices can offer a much-improved efficiency score, they are not the only components that can be optimized to improve the efficiency of the SMPS circuit.

Fig. 1 details the basic components found in a typical IC-based stepdown converter. The control IC integrates two synchronous, low RDSon MOSFETs, and achieves a high efficiency — up to 97%.

In this circuit, the switching components are already chosen and optimized for the application. However, to maximize the overall efficiency potential, designers should turn their attention to the passive elements, namely the external inductor and capacitors since these passive components contribute a share of the power loss. Additionally, designers should be familiar with IC features and tradeoffs that can further enhance circuit efficiency.

In this article, the losses introduced by the capacitors and the inductor in the SMPS circuit will be discussed along with some popular SMPS IC features that are commonly seen in high-efficiency SMPS ICs.

Issues with Inductors

Power loss in an inductor is described by two basic phenomena: winding loss and core loss. Winding loss is due to the dc resistance (DCR) of the coil of wire used to make the inductor, while core loss depends on the inductor’s magnetic characteristics.

DCR is defined by the following well-known resistance equation:

DCR = ρ (ℓ/A)

where ρ is the resistivity of the wire’s material, ℓ is the length of the wire and A is the wire’s cross-sectional area.

Therefore, it is expected that DCR will increase for a longer wire and decrease for thicker wire. This principle can be applied to standard inductors to determine what to expect for different inductance values and case sizes. For a fixed inductance value, as inductor case size is reduced, DCR tends to increase since the cross-sectional area of the wire must decrease to fit the same number of turns. For a given inductor case size, DCR usually decreases for smaller inductances, since a shorter, larger gauge wire is used to accommodate a smaller number of turns in the case.

Knowing the DCR and the average inductor current (dependent on the SMPS topology), the inductor resistive power loss (P_Ldcr) can be estimated as:

P_Ldcr = I_Lavg² X DCR,

where I_Lavg is the average dc current flowing through the inductor. For the stepdown converter, the average inductor current is the dc output current. Although the magnitude of DCR directly affects the resistive power loss encountered in an inductor, power loss is related to the square of inductor current, as seen in the previous equation. Therefore, it is essential to minimize DCR to counter the effect of inductor current on power loss in higher-current SMPS systems.

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