Forward converter

Forward converter

 The forward converter is illustrated in Fig. 11. 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.

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>

When transistor Q1 turns off, the transformer magnetizing current forwardbiases 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.

 Voltsecond balance is maintained on the transformer windings provided that the magnetizing current reaches zero before the end of the switching period. It can be shown that this occurs when

D<1>

For the common choice n2 = n1, this expression reduces to

 D ≤ 1 /2 (13)

 Hence, the maximum duty cycle is limited. If this limit is violated, then the transistor off time is insufficient to reset the transformer. There will then be a net increase in the transformer magnetizing current over each switching period, and the transformer will eventually saturate. The converter output voltage can be found by application of the principle of inductor volt-second balance to the output filter inductor L. The result is

V = n3/n1DVg

This expression is subject to the constraint given in Eq. (12).

A two-transistor version of the forward converter is illustrated in Fig. 12. Transistors Q1 and Q2 are controlled by the same gate drive signal, such that they conduct simultaneously. After the transistors turn off, the transformer magnetizing current forward-biases diodes D1 and D2. This applies voltage – Vg across the primary winding, thereby resetting the transformer. The duty cycle is again limited to D < 0>