This tutorial installment is: The Isolated Power Supply. This topic answers the following questions:
- What is power supply isolation?
- What are the main purposes of power supply isolation?
- What are the methods of power supply isolation?
- What are power supply isolation parasitics?
- How is power supply isolation measured?
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What is Power Supply Isolation?
In the context of power supply design, isolation is the electrical and sometimes magnetic separation between two circuits in close proximity to each other.
Main Purposes of Power Supply Isolation
There may be many purposes for isolation. Some of the most common are:
Electronic power supplies commonly process dangerous voltages with low impedances. Isolation is often required to prevent operator contact with dangerous voltages. Transformers used within power supplies function as a safety isolation transformer.
Voltage Level Shifting
Power supply designs must often accommodate loads at different potential voltages than the source. Isolation is often required to achieve the potential level shifting. Within the power supplies themselves, voltage level shifting devices may be required to couple energy or signals to circuits at two or more different voltages. Such an example is a gate drive transformer used for high side switch control.
Enable Step-Up Conversion Using Buck Derived Topologies
The buck converter has a conversion ratio that is a linear function of duty. This characteristic is very desirable. However, the buck converter is a step-down converter only. To achieve a conversion ratio greater than one without a transformer, one must use a converter topology such as a boost, buck-boost, non-inverting buck-boost, Cuk, SEPIC, etc. However, these topologies all have non-linear conversion ratios. To achieve conversion ratios greater than one with a linear conversion ratio, a transformer isolated derivative of the buck converter can be used. This type of converter can even be used to achieve a buck-boost type characteristic where the input voltage can range greater or less than the output voltage.
Non-buck derived topologies with non-linear conversion characteristics can also use transformers for isolation and assisting with step-up and step-down conversion. The most common example is the flyback topology (the transformer in the flyback more correctly functions as a multiple winding inductor).
Obtaining Multiple Output Converters
The isolating characteristics of a transformer also allow the design of power supplies with multiple outputs by adding windings to the transformer along with rectifier and filter components. A very common example of this type of power supply is the desktop computer supply with +12, +5, and +3.3 volt outputs.
Ground Loop Prevention
Isolation between circuits can also be used for the purpose of preventing ground loops. Ground loops occur when two or more circuits share a common return path. When ground loops occur, there is the high likelihood that the signal voltage developed by current from one of the circuits on the return path will disrupt the operation of the other circuit.
Providing Galvanic Isolation
Electrical isolation inherently provides galvanic isolation since galvanic corrosion depends on the conduction of electrical charge.
Power Supply Isolation Methods
There are at least three methods for achieving isolation in power supplies:
The most obvious form of power supply isolation is simple physical separation — used when there is no electrical, magnetic, or thermal interaction desired. The separation may employ different types of dielectric mediums between the conducting surfaces.
Transformers provide electrical isolation, but allow power to be magnetically coupled through from primary to secondary. They are used when it is desired to transfer power across the power supply isolation boundary and can be designed for safety isolation, voltage level translation, step-up, step-down functions, and for obtaining multiple outputs.
Opto-couplers are used to transfer signals across different voltage levels, and without introducing significant parasitics across the boundary. There are at least two basic types:
The optocoupler in an IC package consists of a light emitting device and a light receiving device inside the same IC package. Both are semiconductor devices. The efficiency of transfer is described by the Current Transfer Ratio (CTR). The CTR ages with input drive level and with operating temperature making careful design with these devices important. In power supplies, they are commonly used to transfer feedback from the secondary to the primary across the power supply isolation boundary. Typical isolation voltages for this type of device are on the order of 3kV.
High voltage power supplies can make use of optocouplers with a data transmission fiber between the transmitter and the reciever. This form of optocoupler can achieve tens and even hundreds of kilovolts of isolation while processing control and data signals.
Power Supply Isolation Parasitics
Isolation is never perfect. It is a poor practice to assume that if conductors do not touch, then they are isolated. The following parasitic parameters are responsible for undesirable isolation boundary performance:
All insulators have some degree of conductivity, resulting in leakage current.
Surface tracking is caused by conduction contaminants on the insulator surfaces between conductors. This type of parasitic is commonly responsible for circuits with extremely high lumped resistance values not working as planned. The greater the voltage potential between the conductors, the greater is the tracking current. If the combination of contaminant surface density is great enough or the voltage potential is great enough, an arc will occur leaving a carbon trail which is difficult to remove. Once the carbon trail is formed, the voltage holdoff capability at that location is compromised.
Dielectric breakdown occurs when the electric field strength across a dielectric medium is great enough to cause electrons to be stripped from their normal orbiting locations around the atoms. If the dielectric is not renewable, the reliability of the part is compromised. If the dielectric breakdown occurs along an insulation surface between two conductors, a carbon path is left which will result in surface tracking at that location.
Every insulator material used in isolation applications exhibits capacitance which is a function of the area of and distance between the effective conducting surfaces, as well as the dielectric constant of the material. At frequencies greater than DC, there will be some current flowing through the effective capacitor. This effect causes two circuits to be DC isolated but not AC isolated. If the circuits are physically close enough to each other, and the frequency components are high enough, one circuit will be able to disrupt the operation of the other circuit. This is known as “crosstalk”. Stray capacitance between primary and secondary windings is a common source of objectionable common mode currents and can cause power supplies to fail emissions testing.
Transformers will always have a parasitic called leakage inductance. It is caused by flux from one winding not linking with the other winding(s). Leakage is typically seen as an undesirable parasitic that should be minimized. In resonant supplies it is possible to use it advantageously.
Mutual inductance causes undesired linking between two different circuits. This is usually a problem when there are large magnitude currents with rapid switching transients in close proximity to sensitive nodes of other circuits.
Measurement of Power Supply Isolation
There are at least two methods of measuring isolation quality:
One method of measuring isolation quality is to measure the resistance between two isolated circuits.
Another method of measuring isolation quality is to perform a “Hi-Pot” test. This test applies either a DC or an AC voltage across two isolated circuits. The resulting leakage current is measured. If the leakage current fail exceeds a specified value, the device under test fails.
The next tutorial installment is Power Supply Topologies. This topic answers the following questions:
- What is a power supply topology?
- What are the most basic topologies?
- What are the derivative topologies of the most basic topologies?
- What are the other topologies in use?
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Next topic: Power Supply Topologies