Converters with transformer decoupling

Converters with transformer decouplingTransformers are used in power supply units that work directly from AC mains. They are necessary for galvanic isolation of mains load. Mains power source is actually a DC supply which powers DC-to-DC converter into DC voltage with transformer decoupling.There are several types of converters with various power ranges and efficiency. Let's have a look at several converters with their operational principles.This is a model of a single-step reverse converter. Primary winding of the transformer (i.e. choke, to be precise) is used to accumulate electromagnetic energy, just like in an up-converter. Phasing of windings is opposite to that of a regular transformer. Energy accumulates in the choke core when the key is closed, and the current does not flow in the secondary winding. When the key is opened the current flows in the secondary winding and releases energy into load.A reverse converter is the only model that uses a choke. All other converters use a transformer. The described model has a certain advantage as it does not have an additional smoothing filter. The energy which is accumulated in the choke goes to capacitor and load. In this case ripple voltage is relatively high and thus requires a high-value capacitor.So here we have a single-step forward converter. Current flows in both primary and secondary windings when the key is closed. Current of the secondary winding charges the filter choke, just like in a down-converter. Upon opening the key the current flow still continues. This is facilitated by a commutating diode D2 in the secondary winding. It has the same function as in a down-converter.Actual transformers have stray inductance which can be presented in the form of a choke connected in series with primary winding of the transformer. Primary current which flows through the stray inductance coil should continue flowing when the key is opened. So when the key is opened currents of both primary and secondary windings cease. Restrictive winding stays in antiphase to primary and secondary windings of the transformer. Thus current starts flowing in the restrictive winding when the current flow ceases in primary and secondary windings and magnetic flow in the core drops. This current demagnetizes the core till a residual value of magnetic induction and ensures that everything is ready for processing the next pulse.

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