Isolation Transformer

A transformer is a static electrical device which transfers power from one voltage level to another with the same frequency. Primary and secondary winding + coupled through a common iron core -The transformers are isolation transformers. The secondary winding is physically and electrically isolated from the primary winding. There is no literal electrical link between the two windings. They are magnetically coupled, however not electrically coupled. This “line isolation” is a very desirable characteristic for various application.
Because there is no electrical connection between the load and source, the transformer becomes a filter between the two. The isolation transformer will greatly reduce any voltage spikes, switching transients and noise that originate on the supply side before they are transferred to the load side. Some isolation transformers are built with a turn ratio of 1:1. A transformer of this type will have the same input and output voltage and is used for the purpose of isolation only. All transformers except auto transformers provide isolation.
When used with a switching power supply, an isolation transformer can prevent higher-order harmonic signals from degrading the performance of adjoining circuitry. This is especially important in computers or other equipment incorporating microprocessors. Improper isolation can cause interference to other function of the system and excess ripple at output voltage waveform.

Benefits of Isolation transformers are:
protect users from faulty equipment
enable safe and accurate measurements
avoid ground loops
physically separate one part of an electrical system from another
Drive Isolation Transformers also help to filter the electrical noise caused by SCR’s

The reason that the isolation transformer can greatly reduce any voltage spikes before they reach the secondary is because of the rise time of current through an inductor.
We know,

The current in an inductor rises at an exponential rate.
As the current increases –> the expanding magnetic field cuts through the conductors of the coil and induces a voltage that is opposed to the applied voltage(by virtue of Faraday’s law of electromagnetic induction and Lenz’s law)
The amount of induced voltage e α L * ( di/dt) is proportional to the rate of change of current.

The faster the current tends to rise, the greater will be the opposition to sudden change in current.
Spike voltages and currents are usually of very short period of time and they rise very rapidly. Such a surge causes the opposition to the change to increase just as rapidly. By the time the spike has been transferred to the secondary winding of the transformer, it has been eliminated or greatly reduced.
Another purpose of isolation transformers is to remove some piece of electrical equipment from ground. It is sometimes desirable that a piece of electrical equipment not be connected directly to ground. This is often done as a safety precaution to eliminate the hazard of an accidental contact between a person at ground potential and the ungrounded conductor. If the case of the equipment should come in contact with the ungrounded conductor, the isolation transformer would prevent a circuit being completed to ground through someone touching the case of the equipment. Many alternating current circuits have one side connected to ground. Neutral grounding transformer is generally used in turbo generator for the same reason.
Creepage is defined as the shortest distance between two conductors, measured along the surface of the insulators. Clearance is the shortest path through the air between two conductors that must be isolated. Each component subject to creepage and clearance must meet the standards. An isolation transformer can reduce the impact of meeting these requirements by reducing the line voltage from hazardous to nonhazardous levels.
Reduced Creepage and clearance consideration can simplify circuit lay out of electronic designs and reduce its cost. An isolation transformer is more effective than a full-wave bridge rectifier in screening electronic equipment from high input voltages. Unfortunately, an isolation transformer can add cost, weight, size, and increased cooling requirements to a design. But it represents a reliable solution for increased isolation, even for systems employing switching power supplies.
When an external isolation transformer is placed within an electronic design in series with a switching power supply, the output voltage from the transformer is reduced to a level that is no longer hazardous. Because the voltage following the transformer is low, smaller components can be used throughout the circuit with relaxed creepage and clearance requirement of low voltage circuitry.
Pulse Transformer – We need pulse type signals for firing circuits of thyristors which is a very low power circuit. Low power circuits cannot withstand the surges in high power circuits. Hence, they require this isolation. Pulse transformers can be used for galvanic isolation, low-power control circuits, as well as the main components in high-power switch mode power supply. Pulse transformers are gate drive transformer, trigger transformer, signal transformer. Low-power pulse transformers are used for controlling switching elements like power semi-conductors.
Austin Transformer
The purpose of this product is to provide a means of coupling the 50/60 Hz supply voltage for tower lighting to the tower while maintaining RF isolation between supply and load. Austin’s Ring Type Isolation Transformers are developed for use on AM radio towers, where the incoming AC power line must be isolated from the tower lighting system. These transformers utilize a high quality, low loss steel toroid core. This unique design allow the necessary 50/60 Hz AC power to be magnetically coupled through to the tower lights, while at the same time blocking the flow of RF energy from the tower to the AC power line by virtue of the extremely low primary to secondary capacitance (10-15 pF).

Benefits are-

Minimum RF loss
Effective tuning
Good regulation characteristic under normal loading
Efficiency is above 90 % under normal loading

Instrument transformer
Current Transformers, Inductive Voltage Transformers, Capacitive Voltage Transformers, Combined Current/Voltage Transformers and Station Service Voltage Transformers are designed to transform high current and high voltage levels down to low current and low voltage outputs in a known and accurate proportion specified by the product nameplate ratios.

Revenue metering for electric utilities, independent power producers, or industrial users.
Protective relaying for use with switchgear to monitor system current and voltage levels
High accuracy wide current range use for independent power facilities
Station service power needs within substations or for power needs at remote sites.

Isolation transformers operate at line frequencies of 50/60 Hz with a power range of 200 to 3500VA. Designed for use with UPS designs and linear power supplies, these transformers comply with safety specifications. A typical 1-kVA unit features 3-percent voltage regulation and 96-percent efficiency.
General recommendations for the selections are:

Isolation Transformer working voltages of 250 V or less.
High dielectric strength of 4 kV or more. This ensures that the level of isolation will meet general as well as specific medical standards.
Isolation Transformers should also meet minimum requirements for creepage and clearance, say 10 mm.
The Isolation Transformer’s minimum insulation
Temperature should be at least +130 ºC, and
Primary-to-secondary leakage current should be no more than 30 μA.

By meeting these minimum provisions, and evaluating related requirements, such as the type of operating environment (indoors, outdoors, surrounded by hazardous materials, etc.) and the number of fuses and circuit breakers in the remaining circuitry, product developers can ensure compliance with a large number of international standards for operating voltages of 250 V or less. Strategies can be applied similarly to higher or lower operating voltages. Meeting minimum requirements may add some expense to a design. But failure to achieve minimum requirements for a safety standard can be costly in terms of redesign time and lost time to market.

Isolation transformers provide an effective high isolation in distributed-power systems, such as computers and telecommunications systems. In a typical distributed power system, multiple DC-to-DC converters, rather than a single, centralized power source, provide voltage and current to the system’s subsystems and circuits. Small, efficient converters can typically generate 200 W or more at a specific location, helping to overcome voltage drops common when power is transmitted over a distance within a system. By locating converters on each of the system’s circuit boards, the system can be assembled in a modular fashion, speeding and simplifying manufacturing and testing processes.
Thermal design is simplified in the same way, since heat is distributed throughout the system, rather than concentrated in one location. Even though isolated DC-to-DC converters can be used to achieve high isolation in such a modular, distributed-power architecture, they are expensive compared to non-isolated converters. A better approach is the use of non-isolated DC-to-DC converters where necessary in a distributed-power system, with a single isolation transformer providing the necessary high-voltage isolation. In this way, each DC-to-DC converter need not meet the high-voltage isolation, creepage, and clearance safety requirements for a particular United States or international electronic safety standard. A single isolation transformer can provide the isolation and the low-voltage transformation to simplify the safety requirements of subsequent circuitry. Otherwise, each converter or separate power supply must be specified to applicable United States and international safety requirements, greatly increasing the overall cost of the equipment.
Isolation transformers are commonly used with linear power supplies to improve the amount of isolation in the overall circuit. But such transformers can also pay huge dividends when incorporated into high-frequency switching power supplies. They can improve the isolation of a design, as well as enable the overall power-supply circuitry to be made smaller, lighter, less complicated, and less expensive.
Isolation transformers are specified in terms of the amount of isolation that they provide, usually given as the root-mean-square (RMS) voltage, as well as the power rating, in terms of volts-amperes (VA). Additional specifications include efficiency (in percent) and the tolerance of the voltage regulation (in percent). The transformers, with heights as low as 0.69 in., have standard isolation of 4000 V RMS and can be supplied with ratings from 2 to 30 VA with dual primaries of 115/230-V, 50/60-Hz operation. These compact transformers are ideal for applications on densely packed PCBs.


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