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:
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."
A DC power supply has two major power inputs:
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.
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.
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.
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.
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.
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:
This type of step down transformer steps down both the current and input voltage ratings to produce a smaller current and voltage output.
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).
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.
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 where controlled rectifiers use SCRs or thyristors while an uncontrolled rectifier uses diodes.
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:
Three phase rectification is known as polyphase rectification circuits and are similar to single phase rectifiers in that it has three single phase rectifiers connected together. They are used when single phase rectifiers do not provide sufficient power.
Multi phase rectifiers are used to reduce harmonics in three phase rectifiers and consist of two 6 pulse rectifiers in series with 12 diodes to feed a DC bus. They stagger the current waveforms and improve the waveforms. They cancel the 5th and 7th harmonics making the first harmonic of significance the 11th one, which keeps voltage harmonic distortion below maximum levels.
The configuration of multi phase rectifiers continues into 18 pulse rectifier with three sets of 6 pulse rectifiers. As each additional rectifier is added, their cost and footprint increases.
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:
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.
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:
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.
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.
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.
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.
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.
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.
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%.
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.
For voltage step-down
For voltage step-up
For either step-down or step-up
For either step-down or step-up
Also used for either voltage step-up or inversion (low power applications)
DC-DC converters are useful in the following applications:
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.
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.
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:
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:
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:
This type of regulating technique is used in high power applications. The main advantages of SCR are:
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.
Like the constant voltage power supply, but it has less precise regulation characteristics.
Like the constant current power supply, but it has less precise regulation characteristics.
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.
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.
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.
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.
Power supplies from the same family can be connected in parallel or series using different methods to generate higher voltages or currents.
Input terminals for a voltage or resistance are present in some power supplies that are used to control output voltage/current.
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.
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:
Programmable supply, often known as system power supplies, are normally integrated into a computer-operated system during production or testing.
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:
When selecting a DC power supply, one should consider the following specifications:
DC power supplies with constant current and constant voltage modes are versatile and, thus, can be used in most applications.
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 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..
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.
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.
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