Power Supply Circuits Primer & Tutorial Includes:
Power supply circuits overview Linear power supply Switch mode power supply Capacitor smoothing AC rectifier circuits Voltage regulator circuits Zener voltage regulator circuit Over-voltage protection PSU specs Digital Power Power management bus: PMbus Uninterruptible power supply
An uninterruptible power supply, UPS, sometimes also called an uninterruptible power source is a form of power supply that utilises the main power source such as the line supply, but is also able to maintain power to the equipment being powered when the main source fails or is interrupted.
The key feature of an uninterruptible power supply, UPS is that it provides instantaneous, or near instantaneous protection from power interruptions.
Uninterruptible power supplies are used in applications where maintaining power is of paramount importance. Typically UPS systems are used for data centres, computer systems and for some medical applications where continuity of power is crucial.
For many businesses, a power cut, even if very short can be catastrophic. Servers and files could become unreachable requiring to be rebooted. A worse scenario may be that data becomes corrupted by a sudden and disorderly shutdown.
The use of a UPS system can prevent disaster striking if the power is fails for a short time. The UPS seamlessly switches to some form of battery power to continue to power any devices until either the main power is restored, or possibly a backup generator can be activated or the devices can be short down properly.
Issues with line power systems
Before looking at what a UPS system is and how UPS backup can help, it is first necessary to understand the problems so that it is possible to see what can be done.
The power obtained from the mains or line power systems is not totally perfect. Issues arise that can cause problems with electronic systems. The various issues vary according to country, locations, etc.
The main power problems that can occur are:
Surge: A surge is brief spike in incoming voltage on the power line. It is commonly caused by lightning hitting a line on the network. It can also arise when large inductive loads are switched and a back EMF propagates along the line. Surges can damage and destroy electronics, as it can enter the electronic device and cause damage to the electronic components that are not designed to withstand the high voltage spikes.
Blackout: A blackout is a term used to describe a power outage. It can last anywhere from a few seconds upwards. They can be caused by faults on the power system, as a result of general usage, or as a result of severe weather: gales, snow, flooding, etc.
Brownout: A brownout is another form of reduction of service. It is a drop in the voltage being supplied. These can occur intentionally or unintentionally. Although power companies in many countries have strict limits for voltage they need to maintain, this is not so in all areas of the globe and power companies may reduce the voltage for a short or long period to reduce strained resources and prevent a complete blackout.
Under voltage: An under-voltage condition or voltage 'sag' is could be described as a short brownout. It is a short lived reduction in voltage.
Over Voltage: An over-voltage condition exists when the voltage provided by the power line is higher than it should be over an extended period, i.e. it is not a spike or a surge. Typically the voltage increase is not high enough for it to be defined as a surge or spike.
Power line noise: Power line noise is also known as frequency noise and it can disrupt or degrade the performance of a circuit by injecting unwanted signals into the system.
Frequency Variation: It is possible on some power systems for the frequency to vary. Normally national power systems are accurately timed so that the different power generation sources work in synchronism. However frequency variations could occur on a locally generated supply, e.g. a local diesel generator, etc.
Harmonic Distortion: Normally it is anticipated that the waveform from the power supply line will be a sine wave. On occasions it may have a high harmonic content, and this can disrupt the way that the internal equipment power supplies like switch mode power supplies, etc operate, causing issues with the equipment being used.
These different issues with power systems can give rise to problems in electronic equipment when powered from a mains line.
Computers, servers, and other IT related equipment can be particularly susceptible to issues with the power line and it is often necessary to protect them, otherwise costly interruptions to the service or damage can occur. This makes UPS backup systems worth considering.
Uninterruptible power supply basics
The aim of a UPS uninterruptible power supply is to provide AC power from the normal line or mains connection when it is available, but in the case of power failures, the uninterruptible power supply will use back-up alternatives, often in the form of batteries. Using techniques such as inverters, they will provide a simulated AC supply to maintain the power to the equipment.
There are several different types of uninterruptible power supply, and it is necessary to choose the correct type for the particular application in mind.
A UPS uninterruptible power supply differs from an auxiliary power supply in that the UPS provides instantaneous or virtually instantaneous protection from power interruptions. An auxiliary power supply may need some time to replace the power.
Often a UPS will provide the replacement power from batteries. These will only have a limited run time - often between 5 and 20 minutes, but this should be sufficient to allow for an orderly shutdown of the system to prevent data loss, or to allow an auxiliary power supply to be started up. It is possible to extend the power capacity in many instances to provide protection for longer periods of time. This is normally achieved by using larger batteries, or adopting other strategies that will be able to provide power over longer periods.
Uninterruptible power supplies, UPS range in their power ratings from small systems that may protect a single computer to much larger ones used to protect complete data centres, etc.
Uninterruptible power supply technologies
There are several different uninterruptible power supply technologies that are available and can be used.
Different types of these power supplies are used in different applications, some for higher power and other for lower power applications. The approach adopted depends upon the requirements.
- Stand-by or Off-line UPS technology: This form of uninterruptible power supply technology often referred to as an SPS - standby power supply - is used to provide a low cost solution to overcome the risk of data loss, etc from power failures. It is one of the most basics types. It provides surge protection along with battery back-up.
The equipment being protected is normally connected directly to incoming line or mains power. To provide the transient or surge protection, voltage transient clamping devices like those used in a common surge protected plug strip are employed - these are connected across the power line.
When the incoming utility voltage falls below a predetermined level the uninterruptible power supply detects the low voltage condition and activates its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The UPS will normally use mechanical switches or relays to change the equipment to be powered to the new supply. The switchover time can be as long as 25 milliseconds depending on the amount of time it takes the Standby UPS to detect the lost utility voltage and for the switches to change over.
It is also worth remembering that the power provided from this form of uninterruptible power supply technology is a square wave form factor rather than a sine wave as supplied by the normal line supply.
Line-interactive UPS technology: This form of UPS, uninterruptible power supply technology is aimed at the mid-range cost bracket. It builds on the technology of the Standby UPS, adding a multi-tap variable voltage transformer to provide protection against sustained low or over-voltage situations.
This form of uninterruptible power supply technology uses the autotransformer with selectable taps to compensate for any long term fluctuations in voltage. By altering the tap on the transformer the correct output voltage can be maintained. By doing this battery power in conserved as the UPS is able to operate from the mains or line power supply even when the input power does not conform fully to the normal specified voltages.
This form of UPS technology is particularly valuable in areas where the line or mains supply is not particularly reliable and may exhibit considerable fluctuations and occasional drop-outs. It provides the protection against all situations, although for extended failures, an auxiliary power supply may be needed to be incorporated as battery power is unlikely to be able to support long periods without mains power.
On-line or double conversion UPS technology: This form of uninterruptible power supply technology provide a greater level of protection. Initially this type of uninterruptible power supply technology was normally used for large installations, however costs and technology improvements have enabled them to be viable for smaller installations.
The On-Line or Double conversion UPS system uses the same building blocks as the other forms of uninterruptible power supply technology. However the big difference with an online UPS is that the batteries are in circuit all the time, and this means that no power switches are required, and the problems with drop-outs during switch-over are removed.
In essence the incoming line or mains power is transformed and rectified and applied to the batteries. The power for the load equipment is then taken from the batteries / rectified and through an inverter which brings the voltage back up to the required line input voltage. This form of UPS gains its name of a double conversion, because the power is converted from AC to DC to float charge the batteries and also power a DC to AC inverter.
In this way it can be seen that the batteries are always in circuit and this means that no switches are required. When power loss occurs, the input rectifier receives no supply, and as a result power is drawn from the batteries. Once line power is resumed, the rectifier will supply the equipment and also re-charge the batteries.
These different uninterruptible power supply technologies represent the major ones that are used. Other UPS technologies exist and are used, but not as widely as those described.
Buying an uninterruptible power supply, UPS
A good UPS, uninterruptible power supply is an essential purchase for many businesses that rely on PCs, servers and other electronics devices that are required to be powered on 24 hours a day, seven days a week.
However, the thought of buying an uninterruptible power supply may appear to be a very costly activity.
Surprisingly small UPS systems for home and small business use are very much less costly than many may consider.
When considering to buy a UPS system, it is worth stopping to consider what is needed and which UPS system meets the needs the best for the given cost
Type of UPS backup required: It is important to select the correct type of UPS system for the particular application, and the expected issues. The different UPS system types provide different levels of protection. I is worth analysing what is needed and hence being able to select the required type of US backup system..
Power line issue Standby UPS Line interactive UPS Double Conversion UPS Harmonic distortion ✘ ✘ ✔ Frequency variation ✘ ✘ ✔ Power line noise ✘ ✘ ✔ Over voltage ✘ ✔ ✔ Under voltage ✘ ✔ ✔ Brown-out ✔ ✔ ✔ Blackout ✔ ✔ ✔ Surge ✔ ✔ ✔
UPS power capability: It is necessary to select a UPS system that will provide the power required to supply the electronic devices that need to be protected and powered in the case of a line power failure. It is necessary to add up the power consumption of all the devices. As a rough guide a PC consumes about 120watts, a monitor around 50watts, an external drive around 20 watts, and a wireless router around 10 watts. For commercial systems, a server could consume about 1kW, although some will consume much less, a switch in a server room could consume up to 250 watts and storage devices up to 500 watts.
It is necessary to add up all of the power requirements and then add a margin on top. Sometimes there may be issues with VA ratings - these take account of the phase of the current and voltage. If this is the case, then add a margin for this.
Run time required: Apart from the power capability, it is necessary to consider the length of time the UPS system will need to hold the mains power for. Run time is a key issue. It may be that it is necessary to hold the mains up for a sufficient time to shut down the computer or other system in an orderly way. It may be that time is required to run a generator up. It is necessary to decide upon the time required. The longer the time, the larger the battery needed to provide the power, and the greater the cost.
Mechanical considerations: It is also wise to consider the mechanical considerations for the UPS backup system. Uninterruptible power supply systems can be large if they are to supply large amounts of power for a long time and they may need special accommodation.
Environmental aspects: It is worth thinking about the environmental aspects. If they are large and need fans, they are likely not to be wanted in an office environment as the noise could be annoying. They may also need to be kept within a certain temperature range for optimum operation. These factors need to be placed into the decision making process.
Uninterruptible power supplies are used in many areas where a continuous supply is essential. Uninterruptible power supply technology has advanced in recent years as the need for reliable power sources has grown. A wide range of applications require UPS systems, but some of the chief users are data and computer centres as well as cellular telecommunications or land line telecommunications centres where continuity of service is essential.
In many respects uninterruptible power supply systems pass unnoticed as they operate in the background, providing backup when needed and enabling the overall systems they power to operate reliably.
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