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2019.06.06 DC/DC

Important Matters when Studying Large Output Currents Applications: Part 2

Examination of Losses

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In the previous article, the first of two points to note when studying applications with large output currents was explained. The first point was that in order to increase the output current, MOSFETs with low on-resistance values are used, fast switching is employed, and inductors with low DCR are selected. In this article, the second point is explained.

Important Points When Studying Applications with Large Output Currents: Part 2

As explained in the previous article, in order to increase the output current, MOSFETs with a low on-resistance are used. However, high-voltage MOSFETs with low on-resistances generally have a large gate capacitance, and there is a tendency for Qg to be large. Hence attention must be paid to gate charge losses.

We study losses for conditions used up to this point, with the gate change Qg varied from 1 nC to 50 nC.

・Gate charge losses
  

table

The following figure indicates the relationship between Qg and losses. As Qg increases, the gate charge loss increases monotonically.

Countermeasures

As measures to address this loss increase, MOSFETs with the on-resistance necessary to increase the output current and which have a low Qg are studied. In actuality, there are MOSFETs with sufficiently low Qg values and that have low on-resistances, so this problem can be avoided.

As the point to note, there are cases in which switching rising/falling edges are sharp for MOSFETs with low Qg values, and so there is the possibility of increased switching noise. By making switching faster, there is the advantage of reduced switching loss, but EMI issues must be carefully considered, and corresponding consideration must be paid during board design.

Summary

Our discussion of points to note when studying applications with large output currents has spanned two articles, and so we here present a summary.

When studying an application with a large output current, MOSFETs with a low on-resistance should be used, switching speeds should be raised, and inductors with low DCR should be selected.

Where MOSFETs are considered, devices with low on-resistances and low Qg values are used. Here, there is a tendency for switching to be fast, and so the possibility of increased switching noise must be studied.

Key Points:

・When increasing the output current, MOSFETs with low on-resistance are selected, switching speeds are raised, and low-DCR inductors are used.

・High-voltage MOSFETs with low on-resistances tend to have higher Qg values, and so in order to avoid the increase in gate charge loss that accompanies a higher Qg, MOSFETs with low on-resistance and low Qg are selected.

・Low-Qg MOSFETs tend to have faster switching speeds, and so attention must be paid to increases in switching noise.

Power Supply Design Technical Materials Free Download

Power Supply Design Technical Materials Free Download

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