AC Power Supplies
An AC power supply is a type of power supply used to supply alternating current (AC) power to a load. The power input may be in an AC or DC form. The power supplied from wall outlets (mains supply) and...
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This article will take an in-depth look at AC DC Power Supply.
The article will look at topics such as:
This chapter will discuss what power supply is, what AC power supply is, what DC power supply is, and what AC DC Power Supply design and function is.
Power supplies are electrical circuits and devices that are designed to convert mains power or electricity from any electric source to specific values of voltage and current for the target device. The target device could be any of our everyday electric appliances or industrial grade equipment.
Any device that operates on electricity has voltage and currents limits that it requires to function optimally. These are usually specified by the suppliers and manufacturers of the device. If the minimum requirements are not met, the device will not switch on and exceeding these limits will burn components and damage the device. At times a single power supply may supply multiple devices at once. It is necessary that its output is constant despite any load variations or input power variations. Such power supplies are called regulated power supplies. Some power supplies, however, have an output that changes with changes in input because the output is a specified fraction of the input. These are referred to as unregulated power supplies. Power supplies exist in external portable forms such as chargers or they can be built into the device as is done in television sets and radios. Power supplies can convert AC or DC at the input to DC or AC at the output.
Direct current always flows in one direction. It alludes to the current flow from DC power supplies such as batteries and solar cells, etc.
Alternating current (AC) positive and negative cycles switch periodically and the direction of the flow of electrons changes accordingly.
This is the flow of current obtained from an outlet or generator. The electricity generated at power plants and transmitted to residential areas is also delivered as alternating current.
In direct current, there is always constant voltage, and the current flows in one direction. In disparity, in alternating current, voltage periodically switches from negative to positive and from positive to negative, and the direction of the current flow also accordingly changes periodically.
In alternating current, the direction of flow of the current changes constantly. Hence, when an inductor or capacitor is included in the circuit, for instance, there is an advance or delay in the current flow to the load relative to the voltage behavior. However, in a direct current, the direction of flow of current and voltage are always constant. Therefore the behavior of the coils and capacitors is also constant. Hence, in DC, there is no delay or advance in the circuit.
In alternating current, the direction of flow of current switches, and not all the current flows through the load, and some energy is produced just moving back and forth between the power source and the load. It is called reactive power. In direct current, all current flows through the load since the current flows in one direction. Therefore, reactive power is not produced, and power may be efficiently used.
It is hard to interrupt direct current. A constant voltage is always applied to direct current, mostly when there is a high voltage, problems like sparks might happen when interrupted, or there might be an electric shock risk in the close areas. In the instance of alternating current, the voltage drops momentarily to zero when the voltage changes from negative to positive or positive to negative. When aimed for an instance with low voltage, the current can be interrupted more safely than a direct current.
When changing DC voltage, it is important to change it to AC once and back to DC again. For this reason, direct voltage conversion machinery is bigger and more expensive than AC. It is direct current which is produced from stored products like batteries and capacitors. Hence, products relying on batteries are suitable for direct current.
The power source in an average residence is alternating current, but direct current is utilized in electronic devices like PCs and home appliances like televisions. To power such appliances, AC from the outlets is changed to DC utilizing capacitors and rectifiers. However, in data centers where direct current is primarily utilized, the usage of a DC power source is being endorsed in order to lower losses when changing AC to DC.
Each AC DC power supply has a unique design. Below are some of the construction considerations in which every design falls.
AC DC power supplies are classified in many ways by including functional features. For instance, a regulated power supply is one that maintains constant current output or voltage despite fluctuations in load current or voltage input. Equally, an unregulated power supply output may vary considerably when its load current or input voltage changes. The adjustable power supply allows the output current or voltage to be configured by mechanical control, such as knobs on power supplies front panels, by the usage of input control, or both. Adjustable regulated power supplies are ones that are both regulated and adjustable. Isolated power supplies have an output power which is electrically free of its input power, which is in disparity with other power supplies which share common connections between power output and input.
Power supplies are packed in differing ways and categorized accordingly. Bench power supplies are stand-alone desktop units utilized in applications like circuit testing and development. An open frame power supply has just a partial mechanical cover, sometimes consisting of just a mounting base. These are generally built into machines or other devices. A rack mount power supply is designed to be protected by regular electronic equipment racks. Integrated power supplies are ones that share common printed circuit boards with their load. External power supplies, AC adapters or power bricks, are power supplies found in the load's AC power cords which plug into wall outlets. Wall warts are external supplies incorporated with the outlet plugs itself. These are common in consumer electronics alluding to their safety. The hazardous 120V or 230V main voltage is stepped down to safer voltages before it flows to the appliance body.
AC DC power supplies may be largely divided into switching and linear types. Linear power supplies process the input voltage directly, with every active power conversion component operating in its linear operating region. In switching power supplies, the input voltage is converted to DC or to AC pulse before processing. This is by devices that function mainly in non-linear mode (like transistors which spend most of their time in saturation or cutoff). Power is "lost" (changed to heat) when a device operates in its linear region and, as a result, switching supplies are generally very efficient compared to linear supplies because their devices spend little time in linear operating areas.
AC voltage of 230V or a different amount depending on the region is applied to the input of an AC DC power supply. Inside the circuitry, the voltage is stepped down to the appropriate AC voltage for the supply rating. This voltage is usually 9V to 24V for most everyday power supplies. The AC voltage is then converted to DC voltage. Linear AC DC power supplies use diodes to convert and switching mode power supplies use transistors. When the voltage is converted to direct voltage it is then smoothed out to remove ripples and regulated. Some supplies use the voltage without regulation. Household appliances such as TVs and personal computers can now use this converted DC power to operate.
AC Input Voltage Range (VIN) – This is the range that a power supply requires, like single phase AC or 3 phase AC. A designer has to know the input AC voltage range of the intended use and the power supply to choose. The general AC input voltage range for many appliances are between 85 and 264 VAC, 47Hz to 60Hz. AC-DC power supplies also come with substitutable blade kits that may be switched easily to function in the region of choice.
Output Voltage Range (VOUT) – This is the range that would be needed for output voltage. For instance, an adjustable voltage output may be required in an LED or measurement and test usage. In that instance, picking an AC DC power supply that has a varying output range feature, saves money and time. In other instances, a fixed voltage output would be needed, like 12V, 48V, and so on.
Maximum Output Current (IOUT) – It is essential to know the total maximum power output needed from the AC DC power source since there are uses, where the current output fluctuates a lot. In instances like these, there are power supplies which may be paralleled for a lot more power.
Output Regulation – Some uses may cause the power source to have a high output ripple, and current output fluctuates a lot. This is primarily due to the load type like motor drives.
Space and Dimensions – Bearing in mind that power supplies are becoming very compact, one question to ask oneself is if there is enough space in usage to suit the power supply. If there’s extra space for parallel supplies for high power, if there’s space to put in a conduction plate or fan for cooling the power source, and so on.
Enclosure and Cooling With Fans – Does the power supply need an enclosed fan, or does it need conduction cooling, or an exterior fan to keep the unit cool? Many power sources have features referred to as over temperature protection. This helps to shut off the power supply to avoid more damage because of overheating.
Temperature Grade – Can the source withstand the severe temperature conditions like -40 degrees Celsius or up to +50 degrees Celsius.
Derating – The environment might affect the power source to an extent, causing some power loss, this is derating. Temperature and altitude are some factors that affect the derating.
Standards to Meet – Some applications need to meet particular specifications, like IP20, IP22, 60601 and so on. Some goods are tested to particular standards relying on general customer needs. Some products generally have TUV, EN, UL, and so more built into the power source.
These are important parameters that help choose the appropriate power supply for an application. Ultimately, it depends on the ultimate application and which parameter is essential when picking the power supply.
The different types of AC DC power supplies include:
Unregulated power supplies utilize the AC voltage as the input. The AC voltage is applied across the primary terminals of a step down transformer first. The voltage is stepped down and tapped across the secondary terminals. It is now rectified by a bridge rectifier and changed into a corresponding DC voltage. The common voltages are 9V, 12V, 15V, and 24V. A capacitor then smoothens the output voltage from the bridge rectifier. As the name implies, an unregulated power supply does not feature a regulator on the circuit.
Any changes in the AC input voltage will directly affect the output voltage. An unregulated power supply has a simple design, making it durable with a general efficiency of about 60%. The main usage of an unregulated power supply is electromechanical applications. These do not require fixed output voltage, for example supply of contactors. The types of unregulated power supplies are half wave, full wave center tapped, and full wave bridge rectifier.
Due to advancements in semiconductor technology, with fast switching MOSFETs, problems such as transformer size as well as voltage regulation in linear AC DC transformers can be eliminated. Efficiency is increased and less energy is dissipated in the form of heat. MOSFETS can switch off and switch on very fast even if there are high currents and voltages.
However, the design of switch mode AC DC power supplies is more complex than that of linear AC DC power supplies. To eliminate the use of large step-down transformers, the input voltage in switch mode AC DC power supplies undergoes rectification and filtration at the input. A dc chopper (DC to AC converter) is then used to convert this DC voltage into a pulse train of high frequency which then undergoes rectification and filtration as well to give a DC output. Now, the reason to first rectify and filter the input wave then convert it into a high frequency pulse train is that transformers are able to transfer ample power and not reach saturation. This implies that smaller transformers may be used.
This is the simplest AC DC Power Supply design. They have long served as a reliable method of converting the AC power supplied from the grid to the DC power that is utilized by most household appliances.
The use of a transformer is employed for the reduction of AC voltage into a smaller and more suitable value for home use. The reduced voltage is then rectified into a DC voltage that is then filtered to eliminate ripples.
The image above summarizes its structure and the changes in output voltage at each turn. A transformer is used to step down the main supply voltage from 230V.
The output voltage of the transformer depends on the ratio of turns of the primary coil to that of the secondary coil. The equation VsVp= NsNP is used to calculate the output voltage. Transformer operation utilizes the rules of electromagnetic induction. An alternating electric current in the primary coil produces a switching magnetic flux which is amplified by the core to induce an alternating electric current in the secondary coil. The shape of the wave is retained, but the amplitude is reduced.
The stepped down voltage is then passed through a rectifier to eliminate the negative halves of the waveform. There are two types of rectifiers: The half wave rectifier and full wave rectifier.
The half wave rectifier consists of two diodes and completely eliminates the negative half cycle by restricting the current flow in the negative direction. The diode only conducts current in the positive direction and does not conduct current when the polarity is reversed. The resultant output waveform is thus a simple pulsating dc waveform with zero value during negative half cycle periods.
The second is the full wave rectifier. As opposed to the half wave rectifier, this rectifier converts the negative half cycles into positive cycles. It does this by redirecting the current of the negative half cycle to pass through the load in the same direction as the positive current.
After rectification comes smoothing. This is the process of eliminating ripples from the rectified wave and approximating DC voltage as close as possible. After this waveform is rectified and smoothed, it is then passed through a regulator that oversees keeping the output at a constant level despite any varying in the input voltage.
Capacitive power supplies, also referred to as capacitive droppers, are a kind of AC DC power supplies that utilizes a capacitors’ capacitive reactance to lower the mains AC voltage. There are many important limitations. The high tolerating voltage needed of the capacitor, together with the capacitor’s high capacitance needed for a given current output, means that this kind of supply is practical only for low energy applications.
The capacitance required rises with the current being drawn; high capacitance AC capacitors mains voltage are bulky and expensive. A capacitive power supply generally has a low power factor. A capacitive power supply usually has a rectifier and filter, which generates DC current from the decreased AC voltage.
Such a supply includes a capacitor, C1 with reactance, limiting current flow through to the bridge rectifier D1. A resistor, R1, in series with the capacitor C1 prevents voltage spikes while switching. An electrolytic capacitor, C2, is utilized to smoothen the direct voltage and the max current (in the amps range) in switching functions. In the figure above a voltage regulator is seen, made by the resistor, R3 (current limiting), and the shunt regulator, Zener diode, IC1. If the voltage stabilization is not very important a Zener diode may be utilized as a regulator. The two terminal components will remove R5 and R4 utilized as a voltage divider in the circuit above.
This chapter will discuss the applications and benefits of AC DC power supplies.
Switched-mode power supplies in domestic appliances like PCs often feature universal inputs, implying that they can receive power from mains supplies across the world. However manual switches for voltage range may be needed. Switch mode power supply can handle a broad range of power voltages and frequencies.
Due to their high numbers cell phone chargers have always been generally cost sensitive. The first chargers used to be linear power supplies, but they promptly moved to the cheap ringing choke converter SMPS design, when new efficiency levels were required. Recently, the demands for more low no load power needs in the application has caused flyback topology to be used very widely. Primary side sensing controllers are also aiding in cutting the bill of materials by eliminating secondary side sensing parts like optocouplers.
Switched mode power supply is utilized for DC/DC conversion also. In vehicles where heavy trucks use nominal voltage DC 24V cranking supplies, 12V for fittings may be featured through DC/DC switch mode supplies. This has the benefit of using the batteries at the 12V positions (utilizing half cells) in which all the 12V loads are divided evenly over all 24 V battery cells. In industrial setups like telecommunications racks, a lot of power might be allocated at low DC voltages (from backup battery systems, for instance) and separate devices will feature DC/DC switched mode sources to supply whatever voltages are required.
A common utilization for a switched mode power supply is as very low voltage supplies for lighting, and for this use they are sometimes referred to as "electronic transformers."
In applications where cost is a huge factor and not so much for efficiency, linear power supplies are used because they have a relatively simple design and less components that switched mode power supply. Capacitive power supplies are applied in low power loads such as LEDs and small DIY projects because of their simplicity.
The benefits of AC DC supply include:
The benefits of unregulated supplies are simplicity and low cost. There are various applications that do not need precise voltage output. Additionally, it can be said that every linear power supply energized from the mains line features an unregulated power supply at the front followed by a voltage regulator.
The main benefit of the switched mode power supply is higher efficiency (gets to 96%) in comparison to linear regulators since the switching transistor dissipates low heat when acting as a switch.
Other benefits include compact size, low noise, and light weight from the exclusion of bulky line frequency transformers and relative heat generation. Standby power loss is at times much lower than transformers. The transformer in a switched mode power supply is also more compact than a typical line frequency (60 Hz or 50 Hz depending on location) transformer and needs a small amount of costly raw materials, such as copper.
The linear power supplies have numerous benefits, involving an overall comparatively low cost and simple design. A linear power supply is simplistic, reliable, generates low noise, and is affordable to manufacture. The need for fewer parts to make linear power supplies leads to more direct designs and reduced manufacturing costs. This also implies that engineers and designers have a tendency to choose them for the exact reasons.
Staying in line with the guidelines of electronics and mechanics, equipment (linear power supply) that uses fewer parts will, by nature, cause few issues. This increased dependability is another advantage of utilizing a linear power supply.
This design’s advantage is in its simplicity and low cost. Of all AC DC power supplies it is the simplest to build and the cheapest as well.
Some of the drawbacks of AC DC supply include:
If the mains AC voltage rises or falls, the voltage output will rise or fall. As the load current rises or falls, the output voltage will rise or fall accordingly because of the finite impedance of output of the power supply caused by stepdown transformer winding inductance, resistance, trace resistance, etc.
Drawbacks include greatly complicated, the creation of high frequency, high amplitude energy which a low pass filter should block to prevent electromagnetic interference (EMI), harmonic frequencies, and ripple voltages at the switched frequency thereof.
Very cheap SMPSs may pair electrical switching noises back to the mains power supply, causing interference with equipment connected on one phase, like A/V equipment. Non-PFC SMPSs also result in harmonic distortion.
Linear power supplies consist of excessive heat and reduced efficiency, which equals loss. The major problem alluded to the linear power supply’s shortcomings in high power applications is its weight and size. This comes from its need for big transformers and other ample parts in its construction. Besides size drawbacks are the problems of excessive heat loss that happen during high power load regulation. Because of its design, high current output passes through the transistor and thermal stresses need a heatsink to dissipate this heat energy.
Finally, the concern with efficiency is one of the noteworthy shortcomings of linear power supplies when evaluating a design. The lower efficiency implies a considerable difference between the output and input voltage that is an essential point in considering using a linear power supply in a design. There are other points to look at, such as dropout voltage and load voltage, when evaluating linear power supplies for a design. Generally, when assessing a power supply for a certain application, one should consider every factor, not only efficiency, cost, and size.
There is an absence of electrical isolation between output and input. Anything drawing power from the power supply should be dependably insulated to avoid the possibility of touching
The power supply comparison between linear power supply and SMPS denotes that:
A linear power supply is different from an SMPS in how it changes the mains AC voltage into output DC voltage. The SMPS uses a power transistor to make a high frequency voltage that passes through a miniature transformer, and then it is filtered to eliminate the AC noise. But, a linear power supply supplies DC voltage by allocating the primary AC voltage via a transformer before filtration to eliminate the AC noise.
SMPS include reduced weight, smaller size, higher efficiency, and increased durability, permitting a more extended input voltage range. The linear power supply is usually more affordable, larger in size, weighs more, is less capable, and is less efficient.
Linear power supplies operate at an efficiency around 60%, whereas SMPSs operate at efficiency around 80% or higher.
The linear power supplies possess a lengthier historical performance versus the SMPS. Linear power supplies are not without their flaws. Overall, the application needs will typically decide which AC DC Power Supply best suits individual needs.
AC and DC can be interchanged in order to suit the required input to a device. There are many aspects that have to be considered, from the device’s voltage, current and power limits to the areas in which they are operated to come up with the most appropriate power supply. Also, simple circuits may be able to supply the required power, but they have massive losses and dissipate a lot of heat. The better and more efficient solutions are more complex and consist of more expensive components. AC power supplies are usually preferred in large industries to power heavy load equipment. They are very efficient and minimize losses.
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