Practical Topology of A Forward Converter Circuit
Practical Topology of a Forward Converter Circuit:
- The practical way to obtain the consequence of circuit losses is to over-design the power supply. The design is to achieve an output power of P0/η, where ‘P0’ is the required output power and ‘η’ is the efficiency of the converter.
- Another common non-ideality is the low frequency ripple and fluctuation in input dc supply voltage. In a practical circuit, the variation in input supply is taken care of by modulating the switch duty ratio in such a manner that it offsets the effect of supply voltage fluctuation and continues to give the required quality of output voltage.
- In fly-back transformer’s flux path some air-gap is deliberately introduced by creating a gap in the transformer core. Introduction of air gap in the mutual flux path increases the magnitude of leakage inductances.
- Transformer of a forward converter should have no air-gap in its flux path. The forward-converter transformer works like a normal power transformer where both primary and secondary windings conduct simultaneously with opposing magneto motive force (mmf) along the mutual flux path.
- The difference of the mmfs is responsible for maintaining the magnetizing flux in the core. When primary winding current is interrupted by switching off ‘S’, the dotted ends of the windings develop negative potential to oppose the interruption of current (in accordance with Lenz’s law). Negative potential of the dotted end of secondary winding makes diode ‘D1’ reverse biased and hence it also stops conducting.
- This results in simultaneous opening of both primary and secondary windings of the transformer. In case the basic circuit of Forward converter is used along with a practical transformer, turning off of switch ‘S’ will result in sudden demagnetization of the core from its previously magnetized state, a practical circuit cannot support sudden change in flux.
- Any attempt to change flux suddenly results in generation of infinitely large magnitude of voltage (in accordance with Lenz’s law). Such a large voltage in the circuit will have a destructive effect and that should be avoided. Thus, after switch ‘S’ is turned off, there must exist a convenient path for the trapped energy in the primary due to magnetizing current. Each time the switch ‘S’ is turned off the snubber circuit will dissipate the energy associated with the magnetizing flux.
- This reduces the power-supply efficiency considerably. A more preferred solution is to recover this energy. For this reason the practical forward converter uses an extra tertiary winding with a series diode.
- When both switch ‘S’ and ‘D1’ turn-off together, as discussed above, the magnetization energy will cause a current flow through the closely coupled tertiary winding and the diode ‘D3’.
- The dot markings on the windings are to be observed. Current entering the dot through any of the magnetically coupled windings will produce magnetic flux in the same sense. As soon as switch ‘S’ is turned off, the dotted end voltages of the windings will become negative in accordance with Lenz’s law.
- The sudden rise in magnitude of negative potential across the windings is checked only by the conduction of current through the tertiary winding.