DC/DC BOOK OF KNOWLEDGE

Practical Tips for the User

RECOM's DC/DC Book of Knowledge is a detailed introduction to the various DC/DC converter topologies, feedback loops (analogue and digital), test and measurement, protection, filtering, safety, reliability, constant current drivers and DC/DC applications. The level is necessarily technical, but readable for engineers, designers and students.

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Years in the making

When we introduced our first DC/DC converter more than 25 years ago, there were little published technical material available and hardly any international standards to follow. There was a pressing need to communicate practical application information to our customers, which prompted us to add some simple Application Notes as an appendix to our first published product catalogue. The content of these guidelines grew over the years as we gained more and more expertise. Although they are still of a rudimentary nature, they are well received by our customer base and have today become a 70-page Application Notes package available for download from our website.

The advance of semiconductor technology and the shift towards highly integrated digital electronics has diminished the knowledge base of analogue techniques in many design labs, universities and technical colleges over the years. We often see a lack of practical know-how in analogue circuit design, particularly with regard to applied techniques, test and measurement and the understanding of filtering and noise suppression. Therefore, as experts in this arena, we saw the need for a much more comprehensive technical handbook that could be used as reference by hardware designers and students alike.

About the author

Steve Roberts, Technical Director for RECOM, has many years of experience answering customers’ questions, helping with design-ins, presenting at seminars, writing articles and even making YouTube videos.

The function of any AC/DC or DC/DC converter module is to meet one or more of the following requirements:
  • To match the secondary load to the primary power supply
  • To provide isolation between primary and secondary circuits
  • To provide protection against the effects of faults, short circuit or over heating
  • To simplify compliance with safety, performance or EMC legislation

DC/DC Book of Knowledge: Chapter 1

Introduction to Power Regulation

Introduction to Power Regulation

Fig. 1.1: 3-Pin Linear Regulator Block Diagram and Pinout

Modern AC/DC and DC/DC converters are designed to provide efficient power conversion to deliver a controlled, safe and well-regulated DC power supply for a variety of electronic instruments, devices and systems. It's not all too long ago that a transformer, rectifier and linear regulator was the main technology in power conversion, but just as the LED is slowly replacing the light bulb, the DC/DC converter is gradually edging out the linear regulator and the primary-side switching controller is replacing the simple 50 Hz mains transformer.

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Contents at a Glance
  • 1.1 Linear Regulators
  • 1.2 Switching Regulator

DC/DC Book of Knowledge: Chapter 2

Feedback Loops

Feedback Loops

Fig. 2.9: Subharmonic Instability Waveform

Some of the most important design criteria in DC/DC power conversion design are the calculations and methodologies involved in the feedback loop compensation. If the feedback loop parameters are not properly calculated, the converter can exhibit instability or regulation failure. The function of a feedback loop in a DC/DC converter is to maintain the output at a fixed value which is dependent on a reference value only – i.e. it is independent of load, input voltage or environmental variations.

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Contents at a Glance
  • 2.1 Introduction
  • 2.2 Open Loop Design
  • 2.3 Closed Loop Design
  • 2.4 Feedback Loop Compensation
  • 2.5 Slope Compensation
  • 2.6 Analyzing Loop Stability in Analogue and Digital Feedback Systems

DC/DC Book of Knowledge: Chapter 3

Understanding Datasheet Parameters

Understanding Datasheet Parameters

Fig. 3.1: Measurement Set Up

Every respectable manufacturer supplies a technical datasheet with their product that details at the very least the basic operating parameters, overall dimensions and pin connections, but to compare one DC/DC converter with another just relying on the datasheet information often requires interpretation rather than just a simple comparison of numbers.

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Contents at a Glance
  • 3.1 Measurement Methods – DC Characteristics
  • 3.2 Measurement Methods – AC Characteristics
  • 3.3 Understanding Thermal Parameters

DC/DC Book of Knowledge: Chapter 4

DC/DC Converter Protection

DC/DC Converter Protection

Fig. 4.2: Series Diode reverse Polarity Protection

As mentioned in the Preface, one of the functions of a DC/DC converter is to protect the application. At the most simple level, this protection consists of matching the load to the primary power supply and stabilizing the output voltage against input over-voltages and under-voltages, but a DC/DC converter is also a significant element ensuring system fault protection. For example, output overload limiting and short-circuit protection not only stops the converter.

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Contents at a Glance
  • 4.1 Introduction
  • 4.2 Reverse Polarity Protection
  • 4.3 Input Fuse
  • 4.4 Output Over-Voltage Protection
  • 4.5 Input Over-Voltage Protection
  • 4.6 Voltage Dips and Interruptions
  • 4.7 Inrush Current Limiting
  • 4.8 Load Limiting
  • 4.9 Under Voltage Lockout

DC/DC Book of Knowledge: Chapter 5

Input and Output Filtering

Input and Output Filtering

Fig. 5.1: Schematic of interferences generated by DC/DC Converters

All DC/DC converters have an output ripple voltage due to the charging and discharging of the output capacitor with each pulse of energy from the internal oscillator. This output ripple has a frequency of either the same or double the main oscillation frequency, depending on the topology, and is typically in the 100-200 kHz region. Superimposed on this ripple voltage are switching voltage spikes with a much higher frequency, typically in the MHz range.

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Contents at a Glance
  • 5.1 Introduction
  • 5.2 Back Ripple Current
  • 5.3 Output Filtering
  • 5.4 Full Filtering

DC/DC Book of Knowledge: Chapter 6

Safety

Safety

Fig. 6.4: Clearance/Creepage

The main aims of the various safety standards and regulations are to prevent injury, loss of life or damage to property by defining levels of protection against the following potential dangers: Electric Shock, Hazardous Energy, Fire and Smoke, Physical Injury, Radiation and Chemical Hazards. The terms “danger” and “hazard” are often used interchangeably.

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Contents at a Glance
  • 6.1 Electric Shock
  • 6.2 Hazardous Energy
  • 6.3 Inherent Safety
  • 6.4 Intrinsic Safety
  • 6.5 Injury Hazards
  • 6.6 Designing for Safety
  • 6.7 Medical Safety

DC/DC Book of Knowledge: Chapter 7

Reliability

Reliability

Equation 7.1: Calculation of Failure Rate

Almost since the advent of electronics, it has been vital for the user to know how long such devices will work properly. Since no one is able to know the future, statistical methods to predict the reliability of components, assemblies or devices have been developed.

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Contents at a Glance
  • 7.1 Reliability Prediction
  • 7.2 Environmental Stress Factor
  • 7.3 Using MTBF Figures
  • 7.4 Demonstrated MTBF
  • 7.5 MTBF and Temperature
  • 7.6 Designing for Reliability
  • 7.7 PCB Layout Reliability Consideration
  • 7.8 Capacitor Reliability
  • 7.9 Semiconductors Reliability
  • 7.10 ESD
  • 7.11 Inductors

DC/DC Book of Knowledge: Chapter 8

LED Characteristics

LED Characteristics

Fig. 8.4: LED String

The first rule of war is “know your enemy”. It is the same principle with Solid State Lighting (SSL) – if you don’t understand how an LED behaves, don’t be surprised when your application doesn’t succeed. LEDs are non-linear devices. If a low voltage is applied to an LED it does not conduct.

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Contents at a Glance
  • 8.1 Driving LEDs with Constant Currents
  • 8.2 Some DC Constant Currents Sources
  • 8.3 Connecting LEDs in String
  • 8.4 Connecting LED Strings in Parallel
  • 8.5 Balancing LED Current in Parallel Strings
  • 8.6 Parallel Strings or Grid Array – Which is better?
  • 8.7 LED Dimming
  • 8.8 Thermal Derating
  • 8.9 Temperature Derating
  • 8.10 Brightness Compensation
  • 8.11 Some Circuit Ideas using RCD driver

DC/DC Book of Knowledge: Chapter 9

DC/DC Application Ideas

DC/DC Application Ideas

Fig. 9.5: Cascaded Converters used in a Battery Health Monitor

Many applications require DC/DC conversion. So many, that it is estimated that the world market will exceed 35 billion dollars by 2020. But for many circuit designers, the DC/DC converter is a “black box”; a component to fulfil a function just like other components such as inductors or transistors.

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Contents at a Glance
  • 9.1 Introduction
  • 9.2 Polarity Inversion
  • 9.3 Power Doubler
  • 9.4 Combining Switching Regulators and DC/DC Converters
  • 9.5 Connecting Converters
  • 9.6 Increasing the Isolation
  • 9.7 5 V Rail Clean-up
  • 9.8 Using CTRL Pin
  • 9.9 Using Vadj. Pin

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