March 2023

DC-DC CONVERTERS

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 Table of Contents

04 CHAPTER - 1 What is a DC-DC Converter?

04 CHAPTER - 2 Basic Concepts

07 CHAPTER - 3 DC-DC Converter (Switching Regulator) vs.

Voltage Regulator (LDO, Linear, Series)

08 CHAPTER - 4 Control Methods

09 CHAPTER - 5 DC-DC Converter Design Tips

12 CHAPTER - 6 Applications

14 CHAPTER - 7 Modeling and Simulation Software

15 CHAPTER - 8 DC-DC Converters

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 DC-DC CONVERTERS

element14 is a Community of over 800,000 makers, professional engineers, electronics enthusiasts, and
everyone in between. Since our beginnings in 2009, we have provided a place to discuss electronics,
get help with your designs and projects, show off your skills by building a new prototype, and much
more. We also offer online learning courses such as our Essentials series, video tutorials from element14
Presents, and electronics competitions with our Design Challenges.

Most electronic devices need power to function, typically a constant supply voltage. Because fluctuations
can happen at the load or the input power source, voltage regulators are used to supply constant
voltages. Increased circuit complexity has contributed to the demand for more reliable and efficient DC-
DC converter designs. This eBook covers the fundamentals of DC-DC converters and their industrial
and commercial applications.

element14 Community Team

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CHAPTER - 1 What is a DC-DC Converter?

DC-DC converters (also known as choppers) are circuits that adapt the voltage level of the power supply
to the load requirements. In other words, a DC-DC converter takes a DC input voltage and converts it to a
different DC output voltage. This output voltage can be higher or lower than the input DC voltage. A DC-DC
converter also regulates voltage level.

DC-DC converters are an important component in power electronics and energy drives, and can be found
in devices from almost every industry, including renewable energy, medical, communication, transportation,
smart lighting, and in various other small-scale electronic appliances. High quality DC-DC converters offer
smooth control, high efficiency, fast response, and regeneration.

CHAPTER - 2 Basic Concepts

2.1 Operation of a DC-DC Converter

Figures 1a and 1b depict a step-up DC-DC converter (boost converter) with an inductor L connected in
series with a voltage source V IN. A FET, such as a MOSFET, is switched on and off at a specific frequency.
When the MOSFET is turned ON for a given time period (T on), the current flows through the inductor and
stores the energy in the form of magnetic energy, as shown in Figure 1a. The dotted line depicts a slight
leakage current. When the MOSFET is turned OFF, the current stored in the inductor flows through the
diode and the load for the time T off, as shown in Figure 1b. The resulting voltage at the load is calculated
as V OUT = V IN + L (di/dt), where di/dt is a measure of how the current changes with time. If the MOSFET is in
the ON state for a longer duration, a much larger electric current is accumulated in the inductor, allowing
the retrieval of a larger voltage. However, if the MOSFET is in the ON state for too long, the time to supply
power to the load side becomes too short, resulting in increased losses and deterioration in conversion
efficiency. It is important to design a DC-DC converter so that the maximum duty cycle is not exceeded to
avoid damaging the components and ensure stable operation.

a. ON state a. OFF state

Figure 1: Operation of a step-up DC-DC converter. Image Source: Torex

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2.2 Types of DC-DC Converters

DC-DC converters are available in isolated and non-isolated configurations.

Isolated Converters

An isolated DC-DC converter has an electrical separation between the input and output. It uses a transformer
to eliminate the DC path between its input and output. Isolated DC-DC converters have high isolation voltage
properties, which enable them to block noise and interference, allowing them to produce a cleaner DC source.
They are categorized into two types: flyback and forward.

Flyback Converters

The operation of the flyback converter is similar to

the buck-boost converter. The only difference is that
the flyback uses the magnetizing inductance of a
transformer with an air gap to store energy instead of
an inductor in a buck-boost converter.

Figure 2: Flyback converter. Image Source: Robert W. Erickson

Forward Converters

The forward converter is a variation of a buck converter. The high-frequency transformer does not have an air gap,
so the forward converter relies on an additional choke to store energy.

Figure 3: Forward Converter. Image Source: Robert W. Erickson

Non–isolated Converters

Non-isolated DC-DC converters share a common ground between the input and output terminals in the circuit.
There is no isolation between the input and output. Non-isolated DC-DC converters are used when the voltage
needs to be stepped up or down by a relatively small ratio. For simplicity, the switching network in the following
figures is represented by an ordinary switch; however, in practice, they typically use MOSFETs, BJTs, or IGBTs with
an appropriate PWM signal to switch them on and off at the appropriate frequencies. More on this in Chapter 4.

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Non-isolated DC-DC converters include the following:

Buck Converter

The step-down DC–DC converter, commonly known as

a buck converter, steps down the DC supply voltage.
A buck converter operates in continuous conduction
mode (CCM), where the inductor will always have a
non-zero current. Referring to Figure 4, when switch
S is in the ON state, diode D is reverse biased. When
switch S is turned OFF, the diode conducts to support
an uninterrupted current in the inductor. A capacitor
Figure 4: Buck converter. Image Source: POWER
stabilizes the output voltage. The effective voltage can ELECTRONICS HANDBOOK 3rd Edition M Rashid
be lowered by controlling the duty cycle of the switch.

Boost Converter

A boost converter or step-up DC-DC converter provides

a higher output voltage than the input voltage. When
switch S is in the ON state, the current in the boost
inductor increases linearly. The diode D is in the OFF
state at this time. When switch S is turned OFF, the
energy stored in the inductor is released through the
diode to the input RC circuit.
Figure 5: Boost converter. Image Source: POWER
Buck-Boost Converter ELECTRONICS HANDBOOK 3rd Edition M Rashid

A buck-boost regulator provides an output voltage

that may be less than or greater than the input voltage
according to the duty cycle of the switch. When the
switch is in the ON state, the inductor current increases
while the diode is maintained OFF. When the switch is
turned OFF, the diode provides a path for the inductor
current. The output voltage polarity is opposite to that
of the input voltage. This type of regulator is also known Figure 6: Buck–Boost converter. Image Source: POWER
as an inverting regulator. ELECTRONICS HANDBOOK 3rd Edition M Rashid

Cúk Converter

A Cúk converter is a type of DC-DC converter that is used

to either step up or step down the DC output voltage.
It is essentially a buck and boost converter cascaded
with an intermediate series-connected capacitor. When
the switch is in the ON state, inductor L1 gets charged,
and capacitor C1 supplies the stored energy to the load.
Inductor L2 supplies energy to the capacitor C1, and C1 Figure 7: Cúk Converter. Image Source: POWER
acts as the voltage source for the load. ELECTRONICS HANDBOOK 3rd Edition M Rashid

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When the switch is turned OFF, inductor L1 discharges and supplies power to the capacitor C. Inductor L2 supplies
power to the load, which was energized when the switch was in the ON state. An important advantage of this
topology is a continuous current at the converter’s input and output.

SEPIC Converter

The single-ended primary inductance converter

(SEPIC), a non-inverting Cúk converter, can be formed
by interchanging the locations of diode D and inductor
L2 in Figure 8.
Figure 8: SEPIC converter. Image Source: POWER
ELECTRONICS HANDBOOK 3rd Edition M Rashid
2.3 Efficiency of a DC-DC Converter

Efficiency is expressed as the ratio of the converter’s output power Pout and the input power Pin. The difference
between the input and output power is the power loss: Ploss. The percentage efficiency is calculated as:

In an ideal case, there is no power loss and the efficiency reaches 100% under all circumstances. In the real world,
electronic components will introduce resistive losses, and electrical power is partially transferred into heat. No
matter how good the conditions are, some power will be lost. This loss can be calculated by subtracting the output
power from the input power.

Power loss (W) = Input power - Output power

DC-DC Converter (Switching Regulator)

CHAPTER - 3 vs. Voltage Regulator (LDO, Linear, Series) v

The key differences between a DC-DC converter (switching regulator) and the voltage regulator are:

Table 1: Differences between switching regulators and voltage regulators

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CHAPTER - 4 Control Methods

The input DC voltage is converted to the desired output DC voltage by switching or chopping the voltage ON/OFF.
The chopping cycle is controlled by using either PWM, PFM, or auto switching control. In the next sections, we
discuss these control methods.

PWM (Pulse Width Modulation)

The PWM method is commonly used to control the output of a DC-DC converter. The frequency remains constant
in this approach, but the duty cycle changes to adjust for variations in input voltage, output voltage, or output load.

PFM (Pulse Frequency Modulation)

PFM is classified into two types: fixed-on time and fixed-off time. The on-time of the fixed-on-time type does not
change; however, the off-time is variable (as illustrated in Figure 10). When there is an increase in load, the number
of on-times in a given time duration also increases to match the load. The fixed-off time type works in a similar
manner, but with the on-time being variable.

Figure 9: Comparison between PWM and Pulse Frequency Modulation (PFM) signals.
Image Source: MDPI

PWM/PFM Auto Switching Control

An auto-switching converter with PWM/PFM selection

can automatically switch between PFM and PWM
control based on the load. This feature helps to achieve
high efficiency and low ripple at light and heavy loads,
respectively. Figure 10 illustrates the power conversion
efficiency across a full range of loads.

Constant On-time (COT) Control

Constant On-time (COT) is a PFM control method that

operates at a constant ON time of ton = 1 / fosc x VOUT
/ VIN. COT operates in continuous-conduction mode
(CCM), where the duty ratio is VOUT / VIN when there is
no loss. The operation imitates a PWM control because Figure 10: Power conversion efficiency of an auto-switching
the frequency is almost constant; however, when a converter with PWM/PFM selection.
Image Source: Torex
transient where more current is required is detected in

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the feedback loop, the pulse generator outputs a higher pulse rate to maintain output voltage. COT Control is
capable of providing a transient response that is much faster than conventional PWM, changing the frequency
instantaneously to minimize changes in VOUT.

Figure 11: Load transient response comparison of a conventional PWM and COT control. Image Source: Torex

Comparison of PWM, PFM and PWM/PFM Auto Switching

The features of PWM, PFM and PWM/PFM auto switching controls are compared in the following table:

Table 2: Comparison of PWM, PFM and PWM/PFM Auto Switching

CHAPTER - 5 DC-DC Converter Design Tips

When specifying requirements for DC-DC converter circuits, the most important requirements are the following:

1. Stable operation

2. High efficiency

3. Small output ripple

4. Good load-transient response

The importance of these requirements varies with individual applications.

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5.1 Selection of Inductor and Capacitor

When designing a step-down DC-DC converter, the proper inductors and capacitor must be selected.
Inductor and capacitor selection influences performance and characteristics.

Inductor Selection

When choosing an inductor, one must consider three main points:

1. The inductor must be selected with an inductance value that is within the recommended L value range,
which can be found on the datasheet of the selected IC. In addition, an inductor should have a saturation
current sufficient for current limit protection and good DC superposition characteristics.

2. Shielding: a shielded type coil must be used to reduce electromagnetic interference (EMI).

3. Low DC resistance: a coil with low DC resistance must be selected to avoid losses due to the coil.
Selecting a coil with low AC resistance is also a good idea (if the specification is available).

Capacitor Selection

A DC-DC converter requires input and output capacitors to smooth and suppress voltage fluctuations. Ceramic
capacitors are favored due to their low ESR, small size, and low cost. Selecting capacitors with the incorrect
capacitance can lead to unstable operation, supply voltage fluctuation, and increased ripple and spike noises.
Another important specification is equivalent series resistance (ESR), which is the resistance component of
the capacitor. ESR can affect ripple and spike voltages. Many DC-DC converter ICs are designed to match
the characteristics of ceramic capacitors; if the ESR is not equivalent to that of a ceramic capacitor, abnormal
operation may occur. Conversely, DC-DC converter ICs designed for other types of capacitors may suffer unstable
operation when used with ceramic capacitors. Capacitors can also be required for frequency adjustment, phase
compensation, and various other purposes.

5.2 PCB Layout

Figure 12 depicts a basic circuit using an asynchronous rectification DC-DC controller with external switch
elements. Table 3 discusses the minimum requirements for the PCB layout of this circuit.

Figure 12: PCB layout of a basic circuit using an asynchronous rectification DC-DC controller. Image Source: Torex

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 Table 3: Requirements for the PCB layout for asynchronous rectification DC-DC controller

5.3 Noise Ripple and Spike Noise vs ESR

A DC-DC converter that repeatedly controls switching ON and OFF at the oscillation frequency
inevitably generates noise and harmonics.

Figure 13 illustrates the typical types of noise generated in DC-DC converters. This noise can be
broadly categorized into ripple noise and spike noise. Ripple noise occurs at the fundamental switching
frequency of the DC-DC converter. The magnitude of the ripple reaches its maximum at 50% of the duty
cycle. Input ripple can be reduced by increasing the capacitance or reducing the ESR of C IN . Ceramic
capacitors typically exhibit low ESR; they reduce the ripples and make the shape of a waveform similar
to a sine wave.

Spike noise is noise that can occur during the transitions where the Schottky Barrier Diode (SBD)
switches from ON to OFF. During the transition from ON to OFF, a reverse current can flow for a very
short duration. causing a spike noise. Spike noise can be reduced in several ways:

• For output capacitor C L , use a ceramic

capacitor with excellent high-frequency
characteristics.

• Insert a resistor between the gate of the

FET and the EXT terminal of the DC-DC
converter.

• Use an SBD with fast reverse recovery time.

• Insert a ferrite bead in series with the SBD.

• Insert a filter at the output (LC or RC low-

pass filter). Figure 13: Ripple and spike noise in a DC-DC converter.
Image Source: Torex

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5.4 Inductor Built-in “Micro DC-DC” Series

The Inductor Built-in micro DC-DC Series is an ultra-small DC-DC converter IC from Torex that can
operates in a similar fashion to a low-dropout regulator (LDO), by simply placing ceramic capacitors at
the input and output. This product also adheres to all the recommendations mentioned above regarding
coil selection and layout. Some of the advantages of this IC include:

• Built-in inductor with a small form factor

• The same usability as LDOs, but DC-DC means higher efficiency and less heat generation

• Fewer required parts, reducing the risk of component failure

• Reduced PCB and mounting costs due to reduced PCB area

• No requirements for the selection and purchase of coils

• Reduced development costs due to a significant reduction in development time

• Considerably reduced EMI over a wide frequency range, preventing interference with RF and
sensors

Figure 14: Standard DC-DC converter vs Inductor Built-in “Micro DC-DC” Series converter from TOREX
SEMICONDUCTOR LTD. Image Source: Torex

CHAPTER - 6 Applications

A cost-effective Point of Load (POL) DC-DC Converter and its typical application are discussed in the
following section.

6.1 Point of Load (POL) Converter

A point-of-load converter is a DC-DC converter that is placed as close to the load as possible. They
are used in applications with multiple power rails, such as newer generation microprocessors, DSPs, or
FPGAs. The complex core ICs of these devices demand low voltage power supply rails with high current.
Using multi-channel power management ICs (PMIC) for all the required power supply rails can result in
poor output regulation and complicated PCB layouts. PMICs localize heat dissipation, which can lead to

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difficult thermal management. Deploying several POL DC-DC converters in place of a single PMIC can
be an effective solution. POL DC-DC converter ICs are placed close to components requiring power.
PCB track lengths can be shortened, improving V OUT stability and reducing EMI. Integrating POL DC-DC
converters can also improve thermal management by spreading the system’s heat dissipation more evenly.

Figure 15: Multiple POL power rails for MPU / FPGA. Image Source: Torex

6.2 LED Backlight

Overview of the Project

The following project is a non-isolated LED backlight, designed using a Torex XC9133 fixed-frequency, constant
current, step-up DC-DC converter. The XC9133 is capable of driving four white LEDs in series, as well as two
parallel banks of three LEDs.

Circuit Operation

Figure 16 is a typical application

circuit for an LED backlight.

The constant current value for the

LEDs (I LED) is determined by R LED
(R LED = 0.2 / I LED)

Dimming of the LEDs is controlled

by adjusting the duty cycle of a
PWM signal, which is applied to
Figure 16: LED backlight system based on Torex XC9133.
the chip enable (CE) pin. Since the Image Source: Torex

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LEDs are in series, they are all turned on/off or dimmed at the same time. Voltage
is monitored via the feedback (FB) pin. When the voltage at FB becomes lower
than the reference voltage, the voltage at the error amplifier is increased. If the
LEDs are damaged, a detector at the Lx pin shuts down the oscillation in order to
prevent an excessive increase in output voltage.

Figure 17: Torex XC9133 driving four LEDs. Image Source: Torex

Bill of Materials

Item Quantity Value Description Part Number Manufacturer

IC 1 - LED Driver IC XC9133 TOREX

SBD 1 - Schottky Barrier Diode XBS053V15R-G TOREX

L 1 22 μH Power Inductor 74404054220 WÜRTH ELEKTRONIK
CIN 1 4.7 μF Ceramic Capacitor 22201C475KAT2A KYOCERA AVX
CL 1 0.22 μF Ceramic Capacitor CC1206KRX7R8BB224 YAGEO
LED 4 White LED CLM3C-WKW-CWBYA453 CREE-LED

CHAPTER - 7 Modeling and Simulation Software

Because characteristics such as junction temperature and switching frequency are difficult to simulate
using common tools, Torex offers an online DC-DC Simulation tool. The software can be used to accurately
simulate the operation of a DC-DC converter and provide design information for the selection of ICs or
peripheral components.

Torex also offers additional software tools, such as a DC-DC Measured Electrical Characteristics Tool and
SPICE models for its wide selection of efficient DC-DC converter ICs that enables engineers to design
voltage converters suitable for any kind of application.

Please check out more DC power related topics on the Community.

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CHAPTER - 8 DC-DC Converters

XCL211 DC-DC XCL212 DC-DC POL

POL Converter Converter

XCL205 DC-DC XCL206 DC-DC

POL Converter POL Converter

XC6119C Voltage XC9133 LED

Detector Backlight Driver

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