DC-to-DC Converters: What are They and How do They Work?

DC-DC converters can be found in almost any modern electronics. The applications of DC-DC converters range from smartphones and laptops to industrial and military systems. From this article, you can learn about their common types, their advantages and drawbacks, how they work, where and why they are used, and what issues one can face when designing electronics with DC-DC power converters.

What are DC-DC converters and their functions?

DC-to-DC converters are electromechanical devices or electronic circuits that convert one direct current voltage or current level to another. In most
cases, devices use only one power source. However, if different
sub-circuits require different voltages to work properly, one needs to
convert the input voltage to a lower or higher level. That can be done
with DC-DC converters. Additionally, they stabilize voltages, not
letting them drop or rise too much. For example, one of the purposes of
car DC-DC converters is to regulate the voltage fluctuations in
automotive alternators.

DC-to-DC converters are placed between the power source (an AC-to-DC converter in this case) and voltage consumers.

These circuits help distribute and manage power properly to provide each
power consumer with appropriate voltage or current level. It also
protects highly-sensitive sub-circuits. On top of that, in portable
devices, they can raise voltage when the batteries are partially
lowered, making power consumption more efficient. Such converters are
used in many electronic devices. According to EMR’s Global DC-DC Converter Market report, more than half of them are used in smartphones, but they have other applications as well: from consumer electronics and telecommunication equipment to industrial and medical equipment, factory automation, transportation, robotics, power electronics, etc. As an electronics developer, we also make full use of various DC-to-DC converters when designing power electronics, hardware, and robotics.

Common Types of DC-DC Converters

Linear & Switching DC-DC Converters

Linear converters reduce output voltage with a resistive load. In a
typical circuit of this type, the input and output are connected with a
transistor (RVT1 in the picture below). The input voltage is reduced by
the voltage across the transistor, which causes the output voltage to
drop.

Linear DC-DC converters are the simplest converter types, as the voltage here is reduced with a transistor placed between the input and output.

Such circuits are quite simple and cheap but have a number of serious
drawbacks. They can only be used to decrease voltage. Moreover, their
efficiency drops as the difference between the input and output voltage
rises. As long as the device isn’t powered by batteries, this drop is
irrelevant. On the other hand, this unused power dissipates as heat, and such models overheat easily if the input and output voltages differ
greatly.

Nevertheless, they are used in low-power devices and nodes that require high-quality output voltage and low output voltage ripple, or in devices that are sensitive to electromagnetic interference. They are simple in design, use few components, and can save a lot of space (unless there’s a need to use a heat sink). Linear converters are typically used in audio and video electronics, communication equipment, medical and measurement devices.

Switching converters use a switching element that charges a storage
capacitor with electrical pulses. This voltage is then smoothed with the
capacitor and transferred to the load. The output voltage level is
defined by the duty cycle of the switching element. 

Switching DC-DC converters consist of a power switch, conductor coil, diode, and a storage capacitor. The number of these components and their arrangement can differ, affecting the converter’s work. The input voltage is applied in pulses, but the capacitor smoothes it.

Their efficiency is much higher, compared to linear converters, and can
reach 85-90%. That is why engineers prefer using them in battery-powered devices. Since they’re more efficient, they do not generate as much
heat and can be used to decrease and increase the output voltage.
Therefore, they are used whenever linear types are not an option. On the
other hand, they generate more electromagnetic noise and require more
components, which makes them more expensive.

In one of our projects, we needed to power several sub-circuits in a
device with a number of radio transmitters. The sub-circuits required
5V, while the input voltage was 12V. The maximum electric current was
expected to reach 2A. In this case, using a linear converter would be
impractical because more than half of the energy would be dissipated as
heat (up to 14W at full power). Mounting a cooling radiator wasn’t an
option either, since the case was too small (10x10x1 cm). Instead, it
was decided to use a TPS54335 switching converter. 

In such cases, when the difference between the input voltage and the output voltage required by a sub-circuit is too big, switching converters let you avoid excessive heat generation.

As you can see, both these converter types have their own pros and cons,
so it can be a tough call. When deciding which converter to use, one
has to take into account a range of factors, including potential issues
related to electromagnetic compatibility, efficiency, overheating, etc.
That’s why you need a team of experienced professionals to do the job.
Don’t hesitate to contact us to discuss your project or ideas.

Non-Isolated & Isolated DC-DC Converters

The absence of direct connection between the input and output circuits is the key feature of isolated DC-DC converters, which makes them safer.

Non-isolated DC-DC converter design features a direct connection between
the input and output circuits (i.e. have a single circuit). They are
used in low-power devices to benefit from their relatively low cost,
smaller size, and higher efficiency compared to isolated models, as
there is no transformer where energy would be lost. Such types are used
in communications, computer, automotive, and other industries.

In isolated converters, the input and output are separated from each
other (typically with a transformer). It prevents direct current flow
between the two circuits. Often, the primary and secondary circuits are
separated for safety, which is why this design is widely used in
high-voltage DC-DC converters. Also, this design allows you to break up
ground loops to protect sensitive circuits from noise. 

They are used in programmable logic controllers, industrial
automation, as power supplies for IGBT drivers, etc. Particularly,
non-isolated DC-DC converters may not be allowed in devices due to
safety concerns. For example, one of the systems Integra Sources helped
develop was supposed to work in a humid environment. So, using an
isolated converter was a must. In this particular case, we used an
LM25017 fly-buck regulator:

The LM25017 fly-buck regulator is a good choice for systems that carry the risk of electric shock.

Step-down DC-DC Converters 

Also called buck converters and choppers, they produce lower output voltage, compared to input.

In a simple buck converter, the switching element (K) rapidly turns the
power on and off. The output voltage looks like a series of square
waves. While the switch is on, the coil (L) and the capacitor (C)
accumulate the energy. The capacitor also smoothes these waves into a DC
voltage. When the voltage reaches the required level, the switching
element is turned off and the diode (D) is turned on. The self-induced
emf makes the current flow through the diode. The energy accumulated in
the coil charges the load.

This is a simple schematic, but most step-down DC-DC converters are much more complicated.

Step-down DC-to-DC converters are used in many spheres, including
battery chargers, multimedia players, game consoles, monitors, and
television sets.

Step-up DC-DC Converters

Also called DC-DC boost converters, they can produce voltage higher than
the input voltage. In a typical boost converter, the induction coil
receives almost all the current, while the closed diode doesn’t let the
current charge the capacitor and the load. Due to a higher electric
current, the coil accumulates much more magnetic field energy compared
to a step-down schematic. When the voltage drops to a certain point, the
power key is turned off, while the diode is turned on. The input
voltage adds to the energy stored in the coil, which makes the output
voltage of boost DC-DC converters higher than the input voltage.

As you can see, a step-up DC-DC converter uses the same components as a step-down converter (a switching element, conductor coil, diode, and capacitor), but their arrangement is different.

Boost converters are used whenever you can’t provide a high enough input
voltage with batteries or there’s simply not enough room for more
batteries. They are typically used in hybrid vehicles, lighting systems
that use energy-saving lamps, portable lighting devices, etc.

Universal DC-to-DC Converters

They can increase and reduce the input voltage to produce a higher or
lower output level. They are also called buck-boost converters. This
type is often used when you deal with a wide input voltage range. For
example, it’s typical for car batteries.

In this case, the converter uses two conductors and two capacitors to raise and lower voltages, but the number and arrangement of components can be different.

Such circuits are often used in devices powered by Li-ion batteries. A
universal converter lowers the voltage to the required level. But as the
battery’s voltage drops with time, it starts raising it. They can also
be found in measuring equipment, photo and video cameras, MP3 players,
GPS systems, wireless devices (keyboards, mice, transmitters), LED
lighting, etc.

Inverting DC-to-DC Converters

Their primary function is to invert the polarity of the output voltage.
The output level can be higher or lower than the input level. Such
models are very useful when the device requires a dual supply (for
example, operational amplifiers).

The schematic of inverting converters is similar to that of buck converters, but the conductor, diode, and capacitor are arranged in a different way.

Essential Characteristics of DC-to-DC Converters

When choosing DC-to-DC converters for a device, engineers pay
attention to different characteristics and parameters, with the most
important ones being the following:

• Input voltage

This parameter is defined by the power source used. Different power
sources (for instance, AC-to-DC adapters or batteries) provide different
input voltages. When designing electronics, a hardware development company must make sure the DC-DC converter can withstand these voltages.

• Output voltage

DC-DC converters can produce either fixed or adjustable output voltage.
The latter can vary from a minimum to a maximum value. In both cases,
the choice of models is defined by the voltage range required by the
load.

• Output current

Output current (along with the output voltage) defines the electrical power that a converter can provide.

• Efficiency

Efficiency is the percentage of the input power delivered to the output. It can be calculated with the following formula:

As mentioned above, the efficiency of DC-DC converters can vary a lot. In some cases, this parameter is considered crucial. For example, if the device is powered by batteries, the efficiency defines how long the device can work before you have to replace them.

In some cases, efficiency itself is not as important. However, the energy lost during conversion dissipates as heat. This in turn can lead to undesirable consequences.

• Temperature

Since extra energy goes into heat, overheating can become a serious
issue. This problem can be partially solved with proper casing. But in
certain cases, one may have to use additional thermal protection.

• Size and mounting types

DC-DC converters are available in many package types. If it needs to be
mounted into a PCB, designers can choose from a variety of mounting
styles, including surface or through-hole mounting, single in-line or
dual in-line pin. The size also matters when engineers develop small
devices.

• Stable supply

And finally, when picking converters for a device, it’s also important
to make sure the manufacturer will not stop supporting the model in the
near future. That’s why our team always gives priority to the latest
models.

Common Issues Related to DC-DC Converters

Another common problem for any electronics design company is meeting
various regulatory and certification requirements. Using DC-DC
converters can affect the device’s characteristics and features from the
safety and electromagnetic compatibility perspectives. Furthermore,
different applications can impose additional requirements.

For example, replacing a switching converter with a cheaper linear one
may require a cooling system. But such a system may turn out to be even
more expensive. Solving such problems is Integra Source’s job, so if you have a product idea and need a consultation, feel free to contact us directly and ask any questions. 

EMC Issues

Electromagnetic compatibility is one of the most obvious issues one
may face when using DC-DC converters. Due to higher efficiency,
switching types are very popular. However, as mentioned above, they
generate electromagnetic noise.

Therefore, such devices must be tested for electromagnetic compatibility to make sure they don’t cause electromagnetic interference effects on other devices. You can read more about EMC testing in the article on common consumer electronics certifications.

These issues are generally solved with proper PCB layer stackup, additional
capacitors, and filter circuits. For example, designers must avoid mounting converters (and especially the conductor coil) near sensitive components and sub-circuits. On the schematic below, you can see a ferrite bead (L2), as well as four ceramic capacitors (C5-8) and one electrochemical capacitor (C4). They are all mounted there to protect an analog sub-circuit from EM interference. The ceramic capacitors suppress the high-frequency noise from the converter, while the electrochemical capacitor smoothes low-frequency fluctuations from different sources. This combination considerably improves the quality of the power supply.

You can see the four ceramic capacitors and one electrochemical capacitor in the upper-right corner of the schematic.

Safety Issues

In many devices, the difference between the input and output voltages
can reach hundreds of volts, which can be extremely dangerous.
Therefore, equipment using high-voltage (HV) DC-DC converters (for
instance, power electronics) must meet safety requirements.
They are regulated by different standards, with the most common being
IEC 60950-1 for Information technology equipment, IEC 60335-1 for
electrical appliances for household and similar purposes, and IEC
60601-1 for medical electrical equipment. Note that national standards
are based on International Electrotechnical Commission’s standards but
may contain deviations.

Whenever it is necessary to provide the required safety level, electronics designers use isolated converters, as they don’t have a direct connection between the input and output circuits. The IEC 60950-1 standard distinguishes five electrical insulation grades.

• Functional Insulation

The functional insulation between the input and output circuits is
only required for the proper work of the device. However, it does not
provide enough protection from electric shock if the input-to-output
insulation suffers a breakdown or fault. This level of protection must
meet at least one of the standard’s requirements sets:

a) Clearance and creepage distances;

b) Electrical strength tests;

c) Fault condition testing.

A device’s DC-DC converter is allowed to have functional insulation if:

- The AC-to-DC power supply uses reinforced or double insulation between the AC input and DC output.

- The AC-to-DC power supply uses basic or supplementary insulation, while
the secondary circuit of the DC-DC converter connects to protective
Earth.

- The AC-DC supply has basic or supplementary insulation,
while the primary circuit of the DC-DC converter connects to protective
Earth.

• Basic Insulation

This insulation grade provides basic protection against electric shock.
Devices must meet all three (a, b, and c) sets of requirements. This
level is required if the AC-to-DC power supply has functional insulation
between the AC input and DC output, while the secondary circuit of the
DC-DC converter connects to protective Earth.

• Supplementary Insulation

Apart from meeting the basic insulation requirements, this grade adds
one more level of protection, such as increasing the distance through
insulation by 0.4 mm for peak voltages above 71V. This level of protection is required if the AC-DC power supply uses basic insulation between the AC input and DC output.

• Double Insulation

This protection level combines basic and supplementary insulations.

• Reinforced Insulation

It’s a single insulation system capable of providing the same protection grade as double insulation. It can comprise several protection layers and is required if the AC-DC power supply has no insulation or functional insulation between the AC input and DC output.

Thermal Considerations

DC-DC converters are primarily used in portable devices. Mobile environments can be extremely harsh on electronics, so developers must take care of heat generation issues as early in the design stages as possible. 

Note that a DC-DC converter’s maximum full-power temperature can be very different from the temperature at which different standards (particularly, EN60950-1 and UL60950-1) begin to derate the device that
uses this converter. 

Most heat is generated by transformers. Therefore, thermal insulating systems are rated by standards according to what materials they are made from and how they interact at high temperatures. In most DC-DC converters, you will find planar transformers that are constructed within the main PCB. They are not regarded as safety hazards unless their temperatures exceed the maximum rating of the PWB (printed wiring board).

In rare cases, when transformers use unapproved (class A) thermal insulating systems, they are regarded as safety hazards when their internal temperatures exceed +105°C. But in most cases, such converters will require transformers with recognized thermal insulating systems – Class B that can withstand operating temperatures of up to + 130°C and class F that can withstand +155°C. 

Overheating issues can be solved with a combination of methods. For example, in the picture below, you can see four power converters that generate a lot of heat. To solve the issue, our team used MOSFET transistors with a lower static drain-to-source on-resistance. It means that less energy is converted into heat. Additionally, we made wide parallel traces (going through all four layers of the PCB) and added as many vias as possible.  It allows the heat to be dissipated from both sides of the board.

Pay attention to the width of the traces around the DC-DC converters on this printed circuit board. This, along with other measures, helps dissipate the heat.

Developing Firmware for DC-DC Converters

Time is always a critical factor for these circuits. Changing the voltage level too early or too late can result in the inefficient performance of the device or even malfunctions. In most converters, everything is controlled by hardware, and response time takes no longer than microseconds. Analog DC-DC converters modules are simple but perform well enough.

However, in some cases, electronics development companies may decide to use microcontrollers in the design of DC-to-DC converters. 

When there’s a need to control many parameters (in complex battery management systems, for instance), this function can be implemented with a microcontroller. It provides better flexibility in the realization of control algorithms. For example, in inverter welding machines, you need to
control the power keys and the output voltage and also measure the
output current. The device has different operating modes, including the
one designed for working under the conditions of increased humidity.
This mode requires reducing the output voltage to not exceed 30 V.

In such cases, the control functions can be handed over to a microcontroller. Microcontrollers fulfill the following functions:

1. House-keeping to ensure proper work of a DC-DC converter.

2. Parameter setting that implies dynamic control over the output voltage or current, etc.

3. Generating control signals for transistors. 

In such converters, almost all functions are managed through code. Therefore, it’s very important to properly organize the firmware structure and thus provide the highest speed possible. The microcontroller’s tasks can be divided into two types: time-critical, such as loop control, ADC measurements, system protection, state machine; and non-time-critical, such as fault logging, PMBus or universal asynchronous receiver/transmitter communication, etc. 

Thus, the firmware can also be divided into two parts: one that takes care of time-critical tasks (the interrupt loop) and one that controls non-time-critical tasks (the background loop).

This is how you can organize the firmware of the microcontroller for the best performance: the time-critical tasks are grouped in the interrupt loop, while the time-non-critical tasks are grouped in the background loop.

After the system is initialized, the central processing unit goes into
an infinite loop where non-time-critical tasks are carried out. The
timer here is responsible for generating fixed-frequency interrupts.
When an interrupt occurs, the CPU stops performing its current task to
store related data and then jumps to the interrupt routine. After
completing the interrupt routine, the CPU resumes the background loop.

The goal of the interrupt loop is to control the converter, measure the
ADC inputs and protect the system. It is crucial here to know the
current state of the converter, as well as its current and upcoming
tasks. For that purpose, the interrupt loop uses a state machine as its
key part.

State machine is a key element of the interrupt loop, as this algorithm controls the work of the converter.

The central processing unit monitors the input voltage until it exceeds a pre-defined threshold. This activates the converter that performs a
soft start to increase the output voltage up to a certain point. Then it switches to regulation mode. The converter stays in this mode until a
fault occurs (in which case, it will shut down and latch or restart if commanded to) or it is commanded to turn off. 

Conclusion

Most modern devices use distributed power architecture. Instead of using multiple power sources, developers now use a single power supply that is distributed between various points of need with DC-DC converters. They are especially useful for designing portable devices since their efficiency and run time are limited by the number of batteries and their voltages. 

When designing electronics, developers must choose from various DC-to-DC converter types that can meet technical specifications, efficiency requirements, as well as EMI and safety standards. A single device can contain multiple converters of different types that supply a number of sub-circuits. So, it’s a challenging task to find models suitable from technical and financial perspectives. 

This is one of the many issues electronics developers face when designing new products and systems. Thanks to our rich experience, the Integra Sources team can handle any task from designing printed circuit boards to developing electronic products and power electronics. Contact our team directly to discuss your project.

Originally published here.