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Important Matters when Studying Large Output Currents Applications: Part 1

2019.05.23

Points of this article

・When the output current is increased, losses due to MOSFET on-resistances, switching, dead time, and inductor DCR increase.

・MOSFETs with low on-resistances are selected, switching is made faster, and inductors with low DCR are employed.

・In nearly all cases the controller IC dead time cannot be adjusted.

・MOSFET selection requires that matters other than on-resistance also be studied (as explained in Part 2).

table of contents

・Important Points When Studying Applications with Large Output Currents: Part 1
・Examination and Countermeasures

In the previous article, parts of a circuit where losses increase when the input voltage rises, as well as points to be noted and possible countermeasures, were explained. In this article, we explain the first of two points to note when studying applications with large output currents.

Important Points When Studying Applications with Large Output Currents: Part 1

The conditions used up to now have assumed specifications with a range of output currents from 1 A to 5 A.

Losses that increase with increasing output current include the on-resistance losses of low-side and high-side MOSFETs, switching losses, dead time losses, and inductor DCR losses.

Below are the equations for each type of loss described in the section on “Loss Factors”.

＜Loss Factors that Increase with Increasing Output Current ＞

・Conduction losses due to the on-resistance of the high-side MOSFET

・Conduction losses due to the on-resistance of the low-side MOSFET

・Switching losses

・Dead time losses

・Conduction losses due to the inductor (coil) DCR

As is seen from the equations, losses due to MOSFET on-resistance and inductor DCR are particularly large. Because the square of IO appears in these equations, the loss at 5 A is 25 times greater at 1 A, or five times greater than for the other losses. Below are shown losses of each type for IO varying between 1 A and 5 A.

Examination and Countermeasures

Conduction losses due to MOSFET on-resistances are large loss increase factors, and so in circuits based on controller ICs that use external switching MOSFETs, a MOSFET with a low on-resistance is chosen. If the IC uses internal MOSFETs, from the same standpoint, an IC with a low internal MOSFET on-resistance should be selected, but unlike when selecting MOSFETs separately, there is not a wide range of options, and so overall losses should be compared for selection.

Inductor DCR losses are also large, and so inductors with a small DCR are selected. In an IC-based power supply circuit, inductors are generally externally mounted, and so the approach is the same as when considering external- and internal-MOSFET type ICs.

Where switching losses are concerned, it is advantageous that tRISE and tFALL be short, that is, that MOSFET switching be fast. In essence, MOSFETs with a low Qg are chosen. A controller IC with good gate driving ability is also advantageous, but here this is not assumed as a condition. There are internal-MOSFET type ICs which feature fast switching.

In these conditional settings, it is assumed that the switching frequency is not changed, but there is also an approach in which the switching frequency is lowered to reduce losses. However, there is a tradeoff between this reduction in losses and inductor size. This is explained in “Matters to Consider When Studying Miniaturization by Raising the Switching Frequency”, which should be consulted.

Dead time losses are losses that occur due to the forward voltage VF in the body diode of a low-side MOSFET and IO during dead time, and so theory dictates that an IC with a short dead time and a MOSFET with low body diode VF should be used. However, in almost all cases the dead time is set to a value that is optimized for the controller IC and cannot be adjusted, and selecting the controller IC on the basis of the dead time is not very realistic. And where MOSFETs are concerned, searching for devices with a low body diode VF is also not very pragmatic. When dead time losses cannot be permitted, one possibility is to add a diode with a low VF such as a Schottky diode between the drain and source of the low-side MOSFET to lower the VF. And although deviating from the conditions adopted here, lowering the switching frequency is also an option.

Consequently, measures to be taken consist of using MOSFETs with low on-resistances, opting for fast switching, and selecting inductors with a low DCR. However, where MOSFET selection is concerned, there are a few other matters to be considered; these are explained in the next article, Part 2.

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Important Matters when Studying Large Output Currents Applications: Part 1

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