B. Full-Bridge Buck-Derived Converter
The full-bridge transformer-isolated buck converter is sketched in Figure 2.9. Typical waveforms are illustrated in Figure 2.8. The transformer primary winding is driven symmetrically such that the net volt-seconds applied over two switching periods is equal to zero. During the first switching period, transistors Q1 and Q4 conduct.
C. Forward Converter
The forward converter is illustrated in Figure 2.10. This transformer-isolated converter is also based on the buck converter. It requires a single transistor, and therefore finds application at power levels lower than those encountered in the full bridge circuit.
Figure 2.8 (a) Magnetic flux waveforms; (b) current waveform with a square excitation voltage
Figure 2.9 The full bridge transformer-isolated buck converter
Figure 2.10 A two-transistor version of the forward converter
The maximum transistor duty cycle is limited in value; for the common choice n1 = n2, the duty cycle is limited to the range D < 0.5. The transformer is reset while transistor Q1 is in the off state. While the transistor conducts, the input voltage Vg is applied across the transformer primary winding. This causes the transformer magnetizing current to increase. When transistor Q1 turns off, the transformer magnetizing current forward biases diode D1 and, hence, voltage - Vg is applied to the second winding. This negative voltage causes the magnetizing current to decrease. When the magnetizing current reaches zero, diode D1 turns off. Volt-second balance is maintained on the transformer windings, provided that the magnetizing current reaches zero before the end of the switching period.