Introduction to transformers in switched mode
Transformers in Switched Mode:
Fig: DC-DC buck converter (a) Isolated Type (b) Non-isolated type
- Transformers are required for galvanic isolation between input and output voltages and for voltage and current scaling. It also helps in optimizing the device voltage and current ratings.
- The switches, diodes and other circuit elements on the high voltage side of the transformer are subjected to higher voltages but only lower currents. Similarly the devices put on the low voltage side are subjected to less voltage stress but higher current stress. The circuit in a DC to DC buck converter uses a step down transformer with proper turn ratio and has the advantages of isolating the input form the output voltages.
- On the other hand the switch and diode and the filter inductor in a non-isolated DC-DC buck converter needs to withstand both input side voltage and output side current. Also, the switch in this case will be constrained to operate in a narrow range, which may cause lesser accuracy in output voltage control.
- Transformers used in switched mode power supply circuits are significantly different from the power transformers that are used in utility ac supply system. Following are the important differences:
- The input and output voltages and currents of a SMPS transformer are mostly non-sinusoidal, whereas the transformers connected to utility ac supply are almost always subjected to sinusoidal voltages and currents.
- The currents and voltages of SMPS transformer are of very high frequency whereas utility type transformers are subjected to low frequency supply voltages.
- SMPS transformers generally handle much smaller power than the utility transformer.
- SMPS transformer-core, because of high frequency operation, is generally made of hard magnetic material like ferrites whereas the low frequency power transformers mostly use soft magnetic material like silicon steel.
- Ferrites have very high ohmic resistance and the area enclosed under the hysteresis loop of their B-H magnetization curve is significantly lower than that of silicon steel.
- Even at very high frequency operation, the hysteresis and eddy current losses are low. [Low hysteresis loss is due to less B-H loop area and low eddy current loss is due to very high resistivity of the core material.] The ferrites have low magnetic permeability (typical value of relative permeability is around 100) and low saturating value of flux density (typical value is around 0.4 Tesla) that are considerably less than that of silicon steel. Ferrites are also brittle and fragile.
- The efforts are on to search for alternatives to ferrites that may have higher permeability, may handle higher flux density and may be more rugged.