DC Power Supply: What Is It? Where Is It Used? AC vs. DC

25 Oct.,2022

 

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DC Power Supply

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Introduction

This page covers all the basic information you need to know about DC Power Supply.

As you go through the article, you will learn more about topics such as:

  • What is a DC power supply?
  • Different power inputs for DC power management subsystem
  • Regulated DC power supply block diagram
  • DC power supply designs
  • DC-DC converters
  • Basic outputs or modes of DC power supply
  • Different types of DC power supply and their applications
  • DC power supply specifications
  • And much more…

Chapter One – What is a DC Power Supply?

A DC power supply is a type of power supply that gives direct current (DC) voltage to power a device. Because DC power supply is commonly used on an engineer‘s or technician‘s bench for a ton of power tests, they are also often called a "bench power supply."

Chapter Two – Different Power Inputs for a DC Power Management Subsystem

A DC power supply has two major power inputs:

  • AC Input

    An AC input can be rectified and filtered to produce a DC voltage, which is then applied to a regulator circuit, generating a constant DC output voltage. The output can range from less than a volt to >1000 volts.

  • DC Input

    A DC voltage with typical values, 5V, 12V, 24V, or 48V, can also be accepted by a DC power supply as an input. The output voltage can also be generated; this ranges from less than a volt to >1000 volts DC. A battery or harvested energy (solar cells, fuel cells, etc.), which derive their electrical power from other energy sources, can also be used as power inputs for a DC power management subsystem.

  • Battery Input

    The DC power management subsystem is normally integrated with the electronic system of portable equipment. An AC adapter, a power unit that is plugged into the AC line outlet and gives a DC output voltage, is usually included in portable equipment that powers the unit. If there is a system battery, an AC adapter can also be used to recharge it.

  • Ultralow Voltage Input

    Small amounts of energy from solar power, thermal energy, wind energy, or kinetic energy can be harvested by a power converter that can operate with ultralow voltage inputs. Once harvested, this energy is accumulated and stored for future use as a power source.

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Chapter Three – Regulated DC Power Supply Block Diagram

As discussed in the previous chapter, a DC power supply can be generated from an AC line. Most electrical and electronic circuits require a DC voltage source that is constant regardless of the change in the input. Although DC batteries can be used as an input, this option is expensive and requires replacement from time to time. So, it is necessary to first convert an AC input into a DC voltage source and regulate it to serve this purpose. This conversion has four major steps and can be represented by a diagram called Regulated DC Power Supply Block Diagram.


Step Down Transformation

A step-down transformer is used in the first step of AC-DC voltage conversion. It is a device that converts a high primary voltage to a lower value (secondary voltage). A step-down transformer has two coil windings: primary and secondary, with the former having more coil winding turns than the latter.


The primary winding is connected to the main AC line. The secondary winding is isolated from the primary winding but electro-magnetically coupled with it.

There are three classifications of step-down transformers:

  • Single-Phase Step-down Transformer

    This type of step down transformer steps down both the current and input voltage ratings to produce a smaller current and voltage output.

  • Center-Tapped Step-down Transformer

    This type of step down transformer consists of a primary winding and a secondary winding with a center split. In effect, the voltage output will have a center split (eg. 12V to 0 to 12V).

  • Multi-Tapped Step-down Transformer

    This type of step down transformer is used to achieve the desired output through secondary coils (eg. 0-12V, 0-18V). This is possible because of the several tappings within the secondary winding.

The transformer output is then received by the rectifier circuit as an input.

Rectification

The second step of the AC-DC voltage conversion process is rectification. It is the step in which the AC voltage is converted into the corresponding DC quantity.

A rectifier is used to perform this process. It is an electronic circuit that consists of diodes.

Controlled and uncontrolled are the two major categories of rectifiers.

Rectifiers can also be classified as half-wave rectifiers and full-wave rectifiers.

A half-wave rectifier circuit utilizes a single diode. Half of the AC input signal is converted by a half-wave rectifier circuit into a pulsating DC output signal while the other half signal is lost. There are two types of half-wave rectifiers:

Positive Half-Wave Rectifier -
In this type of rectifier circuit, the positive half cycle of the AC is converted into DC while the negative half is lost.
Negative Half-Wave Rectifier -
In this type of rectifier circuit, the negative half cycle of the AC is converted into DC while the positive half is lost.

A full-wave rectifier circuit consists of more than one diode. Both-half cycles (positive and negative) of the AC are converted into DC. Therefore, a double output voltage is generated. A full-wave rectifier circuit is classified into two types:

Bridge Rectifier -
It has four diodes that allow conversion of both positive and negative half cycles of AC input into DC. The output voltage from this rectifier could be almost twice that of a full-wave center-tapped transformer rectifier.
Center-Tap Rectifier -
This full-wave rectifier circuit utilizes a center-tapped transformer to which two diodes are connected. Each of the diodes uses the half-cycle of the AC input. Since the AC input provides power to both the halves, this circuit is expected to have high output and efficiency.

DC Filtration

Since the output from the rectification process is a pulsating DC voltage with a high ripple content, a smoothening process called DC filtration is employed to remove these ripples from the waveform.

Capacitor filters, LC filters, choke input filters, and π type filters are the commonly used filters for this purpose.

In a capacitor filter, the capacitor charges as the instantaneous DC voltage increases until it peaks. When the voltage value reduces, the capacitor then discharges gradually through the regulator.

Regulation

The final step involves maintaining the output DC voltage to a constant value using a regulator.

With the help of a regulator, fluctuations in output DC voltage brought by changes in input from AC mainline, load current (at the output of the RPS), or other variable factors like temperature will be eliminated.

The following regulators can be used in this step:

  • Transistor series regulator
  • Fixed and variable IC regulator
  • Zener diode

Chapter Four – DC Power Supply Designs

Regulated power supplies and unregulated power supplies are the major classifications of DC power supplies. Regulated power supplies are subdivided into two types: linearly regulated and switch mode. Switch-mode power supplies can be primary or secondary switch mode.


The list below describes each DC power supply type and design.

Unregulated Power Supplies

An unregulated power supply uses the AC mainline as the input. The AC voltage passes through a step-down transformer first. The lower secondary voltage is then rectified and converted into the corresponding DC quantity. The output voltage of the rectifier is then smoothed by a capacitor. As the name suggests, unregulated power supplies do not have a regulator as part of the circuit. In effect, any changes in the mainline will directly affect the output.

Unregulated power supplies have a simple design, making them durable with a typical efficiency of around 80%.

The primary use of unregulated power supplies is electromechanical applications, which do not need definite output voltages, e.g. for the supply of contactors.

Main advantages include:

  • Highly efficient
  • Heavy-duty
  • Cost-efficient

Disadvantages include:

  • Bulk size
  • High residual ripple
  • Without DC supply (input)

Linearly Regulated Power Supplies

Linearly regulated power supplies employ the AC-DC conversion process discussed in Chapter 3.

The AC mains voltage is stepped down to a lower level using a transformer, and it is rectified and filtered. The final step involves regulation of the smoothed DC voltage, commonly using a power transistor, to maintain the output at a constant value.

For linearly regulated power supplies, the power transistor acts as a variable transistor.

As it passes through the power transistor, there will be high losses in energy; this energy is emitted as heat. The power supply is therefore required to be properly ventilated.

Due to these losses, linearly regulated power supplies normally have an efficiency of about 50%.

Highly precise medical devices require very exact output voltage. For this application, linearly regulated power supplies are often utilized.

Main advantages include:

  • Short regulation times
  • Small residual ripple
  • Simple circuitry

Disadvantages include:

  • Low efficiency
  • Large size
  • Without DC supply (input)

Primary Switch Mode Power Supplies

Primary switch mode power supplies employ rectification of the AC main line, filtering, and chopping/switching in the first few steps.

When a DC voltage is chopped, it only means that it is periodically switched at a frequency of 40-200 kHz with the use of a power transistor.

While power transistors act as variable transistors in linearly regulated power supplies, these are used as switches instead in primary switch mode power supplies.

A square-wave AC voltage is generated in the chopping/switching step, which is then used as input for the high-frequency transformer in the secondary circuit. The voltage is then rectified and smoothed again.

Depending on the load, the chopping rate can be varied to control the quantity of energy transformed to the secondary circuit.

Due to the use of high-frequency AC voltage, primary switch mode power supplies can use transformers which are typically smaller than required for low-frequency transformation.

Primary switch mode power supplies can use a wide range of input voltage. A DC voltage can also be used as an input. This is because the input voltage does not directly affect the output voltage.

A short-time buffer is also possible up to 200 ms, which is essential if the mains voltage breaks down.

It should be noted, however, that the power buffering failure time is restricted by the capacitor size.

A larger capacitor size can provide higher capacity and longer buffering time, but this is not desirable in small power supplies. Hence, the power supply should be optimized to have the "just right" buffering time and capacitor size.

Primary switch mode power supplies are widely used in electronics and electromechanical applications.

Main advantages include:

  • Small size
  • Light weight
  • Wide input voltage range
  • Easy to regulate
  • Highly efficient
  • DC supply
  • Buffering in case of mains voltage breakdown

Disadvantages include:

  • Complex circuitry
  • Mains pollution
  • High frequency requires interference suppression measures
  • Expensive

Secondary Switch Mode Power Supplies

Secondary switch mode power supplies are quite similar to primary switch mode power supplies, but the chopping is completed on the secondary side.

In effect, a larger transformer is required to transform the 50/60 Hz mains voltage.

Mains pollution, however, is reduced because the transformer can also act as a filter.

Main advantages include:

  • Highly efficient
  • Easy to regulate
  • Wide input voltage range
  • Low mains pollution

Disadvantages include:

  • Bulk size
  • No DC supply (input)
  • Expensive

In industrial applications, the primary switch mode power supply is the most widely used type because of its wide input voltage range, high efficiency, and small size.

Efficiency, regulation time, weight and size, residual ripple, costs, and fields of application are the most important factors to be considered for selecting the appropriate power supply in new engineering applications and the upgrade of existing installations.

Chapter Five – DC-DC Converters

A DC-DC converter is a type of DC power supply that utilizes DC voltage as an input. The main function of DC-DC converters is to generate regulated output voltage for electric and electronic applications. Unlike AC, DC cannot be changed from one voltage level to another (step up or step down) using a transformer. Instead, a DC-DC converter is used for this purpose. Hence, this type of DC power supply can be considered an equivalent of a transformer. Like transformers, DC-DC converters convert the input energy into a different impedance level. It should be noted that no energy is generated inside the converter since all the output power comes from the input power. In real applications, energy losses occur inside the converter; the energy is consumed by some components in the circuit. Due to advances in components and circuit techniques, DC-DC converters can have an efficiency as high as 90%. For the older models, the efficiency usually ranges from 80-85%.

DC-DC converters are categorized into two types:

Non-Isolating Converters

A type of DC-DC converter that is used for stepping up or down the voltage by a small ratio (< 4:1).

Non-isolating converters do not use dielectric isolation between the input and output.

Some of the examples include 24V/12V voltage reducers, 5V/3V reducers, and 1.5V/5V step-up converters.

Non-isolating converters have five types:
  • Buck Converter:

    For voltage step-down

  • Boost Converter:

    For voltage step-up

  • Buck-Boost:

    For either step-down or step-up

  • Cuk:

    For either step-down or step-up

  • Charge-Pump:

    Also used for either voltage step-up or inversion (low power applications)

Isolating Converters A type of DC-DC converter that completely isolates the output from the input using dielectric isolation. There are two types of isolating converters:
Flyback Converter
It has a similar operation to buck-boost converters, but instead of using a single conductor to store the energy, it utilizes a transformer.
Forward Converter
It utilizes the transformer in a more typical manner i.e., it directly transfers the energy between the input and output in one step.

Isolating Converters A type of DC-DC converter that completely isolates the output from the input using dielectric isolation. There are two types of isolating converters:

DC-DC converters are useful in the following applications:

Operation of car radio, CB transceiver, or mobile phone
Stepping down of 24V DC (truck battery) to 12V DC
Running a personal CD player
Stepping down of 12V DC (car battery) to 3V DC
Energizing one of the latest CPU chips
Stepping down of 5V DC (personal computer motherboard) to 3V, 2V or less
PC power supply application
Stepping down of 340V DC (from rectified 240V AC power) to 5V, 12V, and other DC voltages
Operation of electronic circuitry
Stepping up of 1.5V (single-cell) to 5V or more
Providing an insulation testing voltage
Stepping up of 6V or 9V DC to 500V DC or more
Running a car HiFi amplifier‘s circuitry
Stepping up of 12V DC to +/- 40V
DC-AC sine wave inverter application
Stepping up of 12V DC to 650V DC

Chapter Six – Basic Outputs or Modes of DC Power Supply

DC power supplies have four basic outputs or modes, including constant voltage, constant current, voltage limit, and current limit. Power supplies can be designed as various combinations of these outputs to fit various applications.

Constant Voltage Power Supply

Any changes in load, line, or temperature will not affect the output voltage. Hence, a constant output voltage is supplied.

Ideally, constant voltage power supplies would have zero output impedance at all frequencies.

Loads are connected in series.

There are four regulating techniques employed to provide a reasonable constant DC output voltage, and these include:

Series Regulation

The circuit consists of a control element in series with a rectifier and load device. Linear power supplies have this type of regulation. The main advantages of series regulation are:

  • Simple
  • High performance
  • Low power requirements
Series Regulation with Pre-Regulation

A pre-regulator added to a series regulator allows circuit techniques to be applied for medium and high-power design applications. The pre-regulation also increases the efficiency by 10-20% by minimizing power dissipation in the series regulating components. The main advantages of series regulation with pre-regulation are:

  • High efficiency
  • Excellent regulation
  • Low ripple and noise
  • Fewer series regulating transistors
Switching Regulation

Switching regulation in a basic switching supply is composed of a series of connected transistors that serve as opened and closed switches. The main advantages of switching regulation are:

  • Lower power dissipation
  • Higher efficiency than linear types
  • Low operational costs
  • Smaller regulator heat sinks
  • Smaller in size and weight (for high switching rate)
SCR (Silicon Controlled Rectifiers)

This type of regulating technique is used in high power applications. The main advantages of SCR are:

  • Low cost
  • Compact
  • Relatively high efficiency

Constant Current Power Supply

Any changes in load, line, or temperature will not affect the output current. Hence, a constant output current is supplied.

Ideally, constant current power supplies would have an infinite output impedance at all frequencies.

A change in load resistance is accommodated by the constant current power supply such that the output voltage is changed by just the right amount to remain the output current constant.

Common applications include semiconductor testing, circuit design, and fixed current supply to focus coils.

Loads are connected in series.

Voltage Limit

Like the constant voltage power supply, but it has less precise regulation characteristics.

Current Limit

Like the constant current power supply, but it has less precise regulation characteristics.

Chapter Seven – Different Types of DC Power Supply and their Applications

There are several types of DC power supply on the market. In this chapter, some of the most common types and their applications will be discussed briefly.

Battery Eliminator

Battery eliminators are the cheapest among DC power supplies and have a compact design. As the name suggests, they serve the functions of a battery whenever one is not available. Battery eliminators are typically used on battery-operated equipment.

Some battery eliminators can provide 18V DC power to devices normally powered by automobile batteries. These units can also be used in CB radios and automotive stereo systems.

Battery eliminators typically have an on-off switch and a rotary switch, which you can turn to select the target output DC voltage. For instance, there are units with outputs of 1.5-6V (with increments of 1.5V), 9V, and 12V. These are designed such that operations in a dead short can be done safely and continuously.

Constant Voltage Supply

A constant voltage supply provides a constant and adjustable voltage. Its design is much more complex than battery eliminators.

A typical unit has a voltage meter and current meter where you can monitor the voltage and current supply values respectively.

Regardless of the load‘s resistance, the voltage is maintained in this type of DC power supply.

The output voltage is adjusted using a knob. For some units, the output voltage may not be adjusted down to zero volts. Also, some models do not supply the rated current at any output voltage. In these instances, the maximum output current would be proportional to the output voltage.

Some models also provide tie points, with a current limit, to provide connections to an external digital meter (for accurate monitoring of output voltage) or other circuits.


Constant Voltage/Constant Current Supply

A constant voltage/constant current supply, the widely used lab power supply, allows a constant supply of both voltage and current.

Regardless of the load‘s resistance, the current is maintained in this type of DC power supply when in constant current mode.

Typical units include one adjustable voltage and fine and coarse controls for both voltage and current supply. In some models, 10-turn pots, thumbwheel switches, or pushbutton switches are used instead for adjustment. A meter is not necessary when using thumbwheel and pushbutton switches with accurate settings.

This type of DC power supply has the following features:
  • Remote Sensing

    The voltage of the load can be measured using a high-impedance input. The power supply performs corrections for the voltage drop in the leads, bridging the supply to the load.

  • Master/Slave Connections

    Power supplies from the same family can be connected in parallel or series using different methods to generate higher voltages or currents.

  • Remote Programming Terminal

    Input terminals for a voltage or resistance are present in some power supplies that are used to control output voltage/current.

Multiple Output Supply

As the name suggests, this type of DC power supply provides more than one DC output, usually two or three.

Multiple output supplies are cost-effective option systems requiring multiple voltages.

For example, a triple output supply is used in circuit development:
Digital Logic
where one output provides 0-6V
Bipolar Analog Circuitry
where the other two supply outputs provide 0-20V.

A knob or keypad is placed to set the three outputs independently i.e., turning on and off the outputs can be done separately or all at once. This feature allows a whole printed circuit board to be powered up.

A typical unit also has features like:

Output Operation Timer
This allows the user to set a time interval in which the output can turn off automatically after the time set has passed.
Voltage Limit for All Channels
The user can set a voltage limit to avoid accidental over-voltage settings in their prototype electrical design project.
Series or Parallel Connection
A higher voltage or current can be achieved by connecting two volt channels in series or parallel.
Storage Registers
This allows the user to save 50 instrument states for easy recollection of data in repetitive testing.
Last Power-On Settings
In case the AC mains line fails, this type of power supply will restart when the AC power restarts, then provide the same output as the last power-on settings.

Programmable Supply

Programmable supply, often known as system power supplies, are normally integrated into a computer-operated system during production or testing.


Multi-Range Supply

A multi-range DC power supply allows various combinations of voltage and current to operate and still provide maximum power. This is in contrast with most common power supplies that can only provide a maximum output power if operated at a certain fixed voltage and current ratings. Hence, the output power will be less than the maximum if other voltages/current combinations are used in conventional power supplies.

The main advantages of multi-range supply include:
  • Flexibility in output ratings
  • Savings in cost and bench space

Chapter Eight – DC Power Supply Specifications

The main advantages of multi-range supply include:

When selecting a DC power supply, one should consider the following specifications:

Constant Current and Constant Voltage Mode

DC power supplies with constant current and constant voltage modes are versatile and, thus, can be used in most applications.

Output

Another important specification to be considered is the power supply output. In general, the user should select a DC power supply with an output greater than the requirement since most projects require the addition of new functionalities at the later stage of the design cycle..

Regulation

Regulation can mean load regulation or line regulation. Load regulation (usually 0.1% to 0.01%) is the amount of change in the output voltage when the load changes. Line regulation (usually 0.1% to 0.01%) is the amount of change in the output voltage when the input AC voltage changes..

Temperature

Most parts of a DC power supply are temperature sensitive. Thus, one should consider checking the operating temperature range and temperature coefficient of the power supply. Ideally, a lab-quality power supply should have 0.05% /oC.

AC Input

Three-phase power is normally used by larger power supplies. These are more efficient than single-phase power supplies but with higher ripple frequency.

Other specifications include ripple and noise, tracking accuracy, and DC isolation.

Conclusion

  • A DC power supply, also known as a bench power supply, is a type of power supply that gives direct current (DC) voltage to power a device.
  • A DC power supply management subsystem can use AC, DC, battery, or ultralow voltage as inputs.
  • Step down transformation, rectification, DC filtration, and regulation are the four major steps in a regulated DC power supply block diagram.
  • DC power supply designs are categorized into two types: unregulated power supply and regulated power supply.
  • The regulated power supply can be linearly regulated or switched.
  • Primary switch mode and secondary switch mode are the two types of switch mode regulated power supplies.
  • A DC-DC converter is a type of DC power supply that utilizes DC voltage as an input.
  • Two types of DC-DC converters are non-isolating and isolating converters.
  • Operating car radio, CB transceiver, or mobile phone, running a personal CD player, energizing one of the latest CPU chips, engaging a PC power supply application, operating an electronic circuitry, providing an insulation testing voltage, running a car HiFi amplifier‘s circuitry, and engaging a DC-AC sine wave inverter application are some of the uses of DC-DC converters.
  • DC power supplies have four basic outputs: constant voltage, constant current, voltage limit, and current limit.
  • Battery eliminators, constant voltage power supply, constant voltage/constant current power supply, programmable supply, and multi-range power supply are the most common DC power supplies available on the market.
  • When selecting a DC power supply, one should consider the following specifications: constant current and constant voltage mode, output, regulation, temperature, AC input, ripple and noise, tracking accuracy, and DC isolation.

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