Duty cycle forward converter

As seen in Section 4.1, the major types of losses are the conduction and switching losses. Conduction losses are addressed by selecting a better power switch or rectifier with a lower conduction voltage. The synchronous rectifier can be used to reduce the conduction loss of a rectifier, but it can only be used for forward-mode topologies, and excludes the discontinuous boost-mode converters. The synchronous rectifier will improve the efficiency of a power supply about one to six percent depending upon the average operating duty cycle of the supply. For further improvements, other techniques must be pursued. 

Note The off-line forward converter transformer is probably the only known exception to the above logic. We will learn that if we for example double the duty cycle (i.e double toN), then almost coincidentally, Von halves, and therefore the voltseconds does not change (and nor does AI). In effect, AI is then independent of duty cycle. 

Note also that the duty cycle of such a forward converter can under no circumstances ever be allowed to exceed 50%. The reason for that is we have to unconditionally ensure that transformer reset will always occur, every cycle. Since we have no direct control on the transformer current waveforms, we have to just leave enough time for the current in the tertiary winding to ramp down to zero on its own. In other words, we have to allow voltseconds balance to occur naturally in the transformer. However, because the number of turns in the tertiary winding is equal to the primary turns, the voltage across the tertiary winding is equal to Vin when the switch is ON, and is also equal to Vin (opposite direction) when the switch is OFF. Reset will therefore occur when toFF becomes equal to toN- So, if the duty cycle exceeds 50%, toN would certainly always exceed toFF, and therefore transformer reset would never be able to occur. That would eventually destroy the switch. Therefore, just to allow toFF to be large enough, the duty cycle must always be kept to less than 50%. 

Note that the transformer of a forward converter is in discontinuous mode (DCM), but the duty cycle is determined by the choke, which is in CCM. Therefore, the duty cycle of the transformer also gets slaved at the CCM duty cycle of D = Vo/Vinr, despite the fact that it is in DCM. This rather coincidental CCM + DCM interplay leads to an interesting observation — the voltseconds across the forward converter transformer is a constant, irrespective of the input voltage. The following calculation makes that clear, by the fact that Yin cancels out completely ... 

The forward converter (Fig. 10.82) is a variation of the buck converter. It is usually operated in the CCM to reduce the peak currents and does not have the stabihty problem of the flyback converter. The HF transformer transfers energy directly to the output with very small stored energy. The output capacitor size and peak current rating are smaller than they are for the flyback. A reset windingis required to remove the stored energy in the transformer. The maximum duty cycle is about 0.45, which limits the control range. This topology is used for power levels up to about 1 kW. 

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