The simplest form of an inductor is winded copper wire with a thin layer of insulation so that one winding does not have contact with the next. When the copper wire conducts current, it is surrounded by an electric- and a magnetic field. When an alternating current run through the inductor, the magnetic field varies. The magnetic field oppose the change in current according to Lentz law. The copper wire is normally winded around an iron core or bobbin to strengthen the magnetic field. The more windings, the higher the inductance. Inductances is measured in Henry, which is a large unit. Most of the time inductance is given in mH or µH. The impedance of an inductor is measured in Ohm and is a measure of resistance to alternating current. The higher the frequency of the alternating current, the higher the impedance.
The simplest form of a transformer has a primary winding and a secondary winding that shares a common core. When an alternating current flow through the windings there is an alternating magnetic field around the core. In order to increase the voltage, the secondary winding has more turns than the primary winding. In order to reduce the voltage, the secondary winding should have less turns than the primary side. Many transformers have more than one secondary winding. Three commonly used cores are C-core, E-core and toroidal cores. The E-core is easy to use in automated production and the transformer can be manufactured at a low cost. The toroidal core has very low leakage and therefor high efficiency. Gate-Transformers and Pulse Transformers In a switch mode power supply, the input voltage is chopped up by a transistor in a series of pulses. By using capacitors and other components, the voltage is converted to direct current. The width of the pulses is controlled by one or several feedback loops depending on the load. Higher switching frequency makes it possible to use smaller inductive components.
Thyristors, MOSFET’s, IGBT’s or TRIAC-semiconductors performs the switching in a switch mode power supply. The terminals of a MOSFETs are named Gate, Drain and Source. The current channel between the Drain and the Source is controlled by the Gate. A gate drive transformer provides a voltage pulse or a series of pulses to the Gate terminal. Therefor, the gate drive transformer is also called a pulse transformer. A pulse transformer is optimized to transmit rectangular pulses with fast rise and fall times, and a constant altitude. Sometimes the pulse transformer is replaced with an optocoupled IC. The IC can be surface mount and it does not have the same requirement of a minimum frequency like the pulse transformer. Switching of high voltages and currents often requires galvanic isolation on the gate terminal of the transistor. Different standards define creepage distances and durability regarding partial discharge. A potting material is often used to improve isolation. If the potting material contains air bubbles, the voltage on one side of a bubble may be different to the voltage potential on the other side. This may lead to a spark that at some point in time destroys the isolating potting material. The risk of partial discharge increases with higher switching frequency. Air bubbles in the potting material are avoided by casting the transformers under vacuum. Switch Mode Transformers The four basic conversion technologies in switched mode power supplies are buck, boost, flyback and forward. The buck topology is used to lower the voltage. The boost topology creates a higher voltage than the input voltage. Buck and boost converters are based on coils and do not offer isolation. Flyback and forward topologies are based on transformers and are therefore used in isolated converters.
The forward topology is a development from the buck principal. Depending on the transformer, the voltage may be converted up or down. It is possible to design a forward converter with only one transistor delivering a few hundred Watt of power. The pulse ratio must be 0.5 or less and the transistor breakdown voltage must be equal to or greater than the double input voltage. The forward topology requires a core with air gap and very good magnetic coupling. Forward converters with two transistors can deliver a few kilo watts of power. The transformer should have a small core and no air gap, but otherwise the requirement on the pulse ratio and the transistor breakdown voltage is the same.
The flyback topology is a buck-boost version where the voltage can be regulated up or down. The flyback topology utilizes the magnetic storage capability of the transformer. Several output voltages can be regulated in the same circuit. Input voltage range is flexible and may be for instance 85 to 270V AC. Power up to a few hundred Watt is possible. The transistor breakdown voltage must be greater than or equal to the double input voltage. This topology requires a large core with air gap and good magnetic coupling.