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2018.07.26 DC/DC

Introduction

Examination of Losses

An article on "Examination of Losses" is being added to “Characteristics and Evaluation Method of Switching Regulators” in the DC/DC Converter Evaluation edition.

Here "losses" means, of course, power losses relating to a power supply. In any event, power losses are directly converted into heat, and therefore are an extremely important subject for study. Heat generation is one major factor that detracts from the reliability of components and devices, and in the worst case, can even culminate in the occurrence of accidents involving smoke and flames. Hence the importance of understanding such losses should be obvious.

When erroneous design, defective components, and defective device assembly are excluded, nearly all problems with components and devices arising from heat generation are basically due to the failure to perform adequate thermal calculations and thermal design. Common examples are changes made to components, modifications of board layouts for greater device compactness, and other model-change scenarios, which are regarded as only slight changes, so that proper verification prior to executing the changes is neglected.

Regardless of the reason, if a heat generation problem is discovered, a countermeasure must be devised. Measures basically take the form of reducing the heat generation, or possibly increasing heat dissipation. In order to reduce heat generation, losses in the components of the power supply circuit are reduced. To increase heat dissipation, heat sinks may be added, or the heat dissipation effect may be enhanced, that is, thermal resistance reduced, by increasing the PCB copper area or the number of layers.

Reducing power losses of power supply circuit components

  • ・DC/DC converter IC losses (including package thermal resistance)
  • ・Losses of external FETs
  • ・Losses of external rectifying diodes (asynchronous rectification)
  • ・Coil losses

Increasing heat dissipation

  • ・Addition of heat sinks
  • ・Increasing PCB copper area
  • ・Increasing the number of layers of the PCB
20170509_graf_D51_01
Example of increasing allowable losses by increasing the PCB copper area and number of layers

In this new chapter on "Examination of Losses", we will explain what kinds of losses occur in which parts of a circuit, and the causes of such losses, in order to be able to implement countermeasures. The chapter is planned as follows.

  • Introduction
  • About losses
  • Losses in synchronous rectifying step-down converters
  • Conduction losses caused by MOSFET on-resistance
  • Switching losses
  • Dead time losses
  • Power consumption losses due to IC control circuits
  • Gate charge losses
  • Conduction losses due to coil DCR
  • Calculation example: IC power losses (case of synchronous rectification with internal FETs)
  • Package selection--1
  • Package selection--2
  • Regarding simple methods of loss calculation
  • Loss factors: output current, switching frequency, etc.
  • Points to be noted--1: When considering raising a frequency to reduce the application size
  • Points to be noted--2: When examining a high-voltage application
  • Points to be noted--3: When examining a large-current application (1)
  • Points to be noted--4: When examining a large-current application (2)

Key Points:

・Losses immediately produce heat, detracting from the reliability of components and devices.

・Thermal design is extremely important for improving the safety and reliability of devices.

・The parts of a power supply circuit that involve losses, the reasons for the losses, and countermeasures must be studied.

Characteristics of Switching Regulators and evaluation methods

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