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Switch-Mode Power Supply Fundamentals (3)

Author: Apogeeweb Date: 21 Jun 2018  2341


Warm hint: This article is about 1000 words and reading time is about 3 minutes.



The switch-mode power supply fundamentals tutorials consist of five chapters: the type of topology, the relationship between the efficiency with input & output and duty cycle, the definitions of synchronization and non-synchronization, characteristics of isolation and non-isolation, pulse width modulation and frequency conversion and other various control modes. It explains the basic concept in simple language and tends to establish a platform to exchange ideas and info for power designers. This is the last part of the tutorial series on switch-mode power supply fundamentals.



Chapter 5 Pulse-Width Modulation & Pulse-Frequency Modulation


Ⅱ Examples of Typical Portable Power Supply Applications

Ⅲ Definitions of PWM and PFM

Ⅳ PWM Control Architecture

Ⅴ Hysteresis Mode Control

Ⅵ Pulse-Skipping Mode/Power-Save Mode

Ⅶ PWM Mode vs. Power-Save Mode

Ⅷ Efficiency and Loss of Converter

Ⅸ Dual-Mode Step-Down Converter

Ⅹ Comparison of Output Voltage Ripples

Ⅺ Comparison Between Power Save Mode and Forced PWM Mode

XII Multiple Power Save Modes

XIII Conclusions (Variable Frequency Converter)


Chapter 5 Pulse-Width Modulation & Pulse-Frequency Modulation


  • PWM and PFM are two types of DC-DC converter architectures;

  • The performance characteristics of each type are different;

  • Efficiencies under heavy and light loads conditions;

  • Load regulation;

  • Design complexity;

  • EMI / Noise considerations;

  • Integrated converter solutions should integrate these two modes of operation to take advantage of their respective advantages.


Ⅱ Examples of Typical Portable Power Supply Applications

  1. Significant changes in load levels are possible: 1~2mA at "sleep" mode and up to several hundred mA during "active" operation.

  2. Expect or need to achieve high efficiency throughout the entire load range.

  3. Good (sufficient) load regulation to handle transient conditions.

  4. Boost converter - LED backlight, audio bias power rails, or other "analog" loads.

  5. The sensitivity to noise/ripple depends to a large extent on the application.

  6. Different types of brightness control methods can be used for LED applications.


Ⅲ Definitions of PWM and PFM


FIG.1 Definitions of PWM and PFM


Ⅳ PWM Control Architecture


FIG.2 PWM Control Architecture


The figure is the architecture diagram of the classic design in the textbook. The two output voltage dividers are used for signal sampling, which then goes through a compensator, ready to be compared with a reference power supply. And the output error signal is compared with a Ramp (triangular wave) to obtain a pulse with a fixed cycle.


Merits and faults of PWM


  1. Good efficiency can be achieved under medium and heavy load conditions.

  2. The switching frequency is set by PWM ramp signal frequency.


  1. Switching loss caused by fixed switching frequency significantly reduces the efficiency under light loads.

  2. Fast transient response and high stability require a superior compensation network design.


Ⅴ Hysteresis Mode Control


FIG.3 Hysteresis Mode Control


The Bang-Bang control, also called hysteresis mode control, is actually a concept of a window comparator. The turn-on and turn-off of the MOSFET are based on the detection of the output voltage, so its response speed is the fastest and the output voltage is always just above or below the ideal set point. Comparator hysteresis is used to maintain predictable operation and to avoid "bounce" of the switch.


Ⅵ Pulse-Skipping Mode/Power-Save Mode

At light loads, the PWM converter automatically switches to a "low power" mode to minimize battery & current consumption. This mode is sometimes called "PFM", but it is actually a fixed frequency PWM converter that turns on and off intermittently.


Ⅶ PWM Mode vs. Power-Save Mode


FIG.4 Waveform difference between PWM Mode and Power-Save Mode


Figure 4 shows the waveform difference between the two modes, the right part shows the power-save mode and the PWM frequency of it is reduced, so is the switching loss. But the amplitude of ripple actually becomes larger because it is not adjusted every cycle in this mode, but only when the feedback signal reaches the upper or lower limit of the window comparator.


Ⅷ Efficiency and Loss of Converter

"Loss" =Any energy absorbed from the input but not transmitted to the output.


FIG.5 Efficiency and loss of the converter


Which components are your power wasters?

MOSFET: switch loss, gate driver loss, conduction loss.

Passive components: winding and core loss of inductor, resistive loss, capacitor losses ESR.

Converter IC: internal references, the power lost in the oscillator & gate drive circuits.


FIG.6 The loss of converter IC


Passive components and FET losses drop significantly at light loads. The internal current of the IC is controlled by the oscillator and therefore does not decrease with the load at a fixed frequency. The operating current of the IC will affect the efficiency of light loads. If the load current is about 1 mA and the internal current of the IC is about 4 mA, then even in the best-case scenario, it has an efficiency of only <20%. If the load current is approximately 200 mA and the internal current of the IC is around 4 mA, then the efficiency is >90% in the best-case scenario.


Ⅸ Dual-Mode Step-Down Converter

If the converter is operating in dual mode, the working current of the IC is 3.5mA in PWM mode, and the IC enters power-saving mode under light-load or no-load conditions. At this time, the working current of the IC is approximately 23μA, therefore the efficiency has increased.


FIG.7 Efficiency comparison of PWM Save Mode and Forced Power Mode


Ⅹ Comparison of Output Voltage Ripples

A trade-off in power saving mode is that the output voltage ripple is higher at a given load current, which is 15 mVPP in this example, while the PWM mode is only <5 mVPP, as shown below:


FIG.8 Comparison of output voltage ripples

The inter-pulse interval depends on the load. As the load increases, the frequency of the switching pulses increases (6.5μs at 40mA and 100μs at 1mA). If the load increases sufficiently, the converter will resume a constant frequency operation, as shown in the following waveform:


FIG.9 Converter resumes a constant frequency operation


Ⅺ Comparison Between Power Save Mode and Forced PWM Mode

The following figure shows the test results under 10 to 30 mA load transients.


FIG.10 Comparison Between Power Save Mode and Forced PWM Mode

In order to improve the efficiency of the full-range load, the chip enters the PFM control mode at light loads, and it is forced into the PWM mode at heavy loads.


XII Multiple Power Save Modes

The so-called fast PFM and light PFM is based on the output load current. When we set the threshold current, the circuit can naturally switch between the two modes. The efficiency of fast PFM is higher than that of PWM but lower than that of light PFM, which means that all should be carefully weighed. The following figure shows the relationship between output current and efficiency for the two modes.


FIG.11 Efficiency comparison of fast PFM/PWM and light PFM/PWM

Use light PFM mode if you need maximum efficiency at very light loads; use quick PFM mode for frequent switching between light and heavy loads with good transient response. The power-saving mode can be selected by sending a I2C command to the converter.


XIII Conclusions (Variable Frequency Converter)


  • Better efficiency at light loads;

  • No compensator required;

  • Easy to implement: multiple inductors can be used.


The variable-frequency control architecture may have some problems:

  • The EMI spectrum is spread across all frequencies and can be difficult to be filtered out;

  • Audible noise (f <20kHz)


Issues related to EMI can sometimes be alleviated:

  • The EMI is caused by high dv/dt and di/dt;

  • PWM uses a fixed frequency under high power conditions;

  • Using a variable frequency in power saving mode, the total output power is very low;

  • EMI may not be a problem;

  • The converter can be set to forced PWM mode when needed.

SMPS Tutorial (4): Boost Converters, Flyback Voltages, Switched Mode Power Supplies



Book Recommendations

Power Electronics Basics: Operating Principles, Design, Formulas, and ApplicationsApr 23, 2015

This book supplies graduate students, industry professionals, researchers, and academics with a solid understanding of the underlying theory while offering an overview of the latest achievements and development prospects in the power electronics industry.

by Yuriy Rozanov and Sergey E. Ryvkin


Pulse Width Modulation 1st Edition

In this book, the various space vector pulse width modulation-based algorithms for multilevel inverter-fed induction motors are proposed and implemented. The results have been analyzed and compared. The performance of these algorithms as evaluated in terms of inverter output regarding voltage, current waveforms, total harmonic distortion, speed of induction motor and torque ripples.

by Satish Kumar Peddapelli


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