5.1 Introduction

Current-mode1 7 and current-fed9 20 topologies are grouped into one family despite their very significant differences. They are grouped together because they gain advantages by controlling both input current (although in different ways) and output voltage.

Current mode (Fig. 5.3) has two feedback loops: an outer one which senses DC output voltage and delivers a DC control voltage to an inner loop which senses peak power transistor currents and keeps them constant on a pulse-by-pulse basis. The end result is that it solves the magnetic flux imbalance problem in the current-mode version of the push-pull topology and restores push-pull as a viable approach in applications where the uncertainty of other solutions to flux imbalance is a drawback (Sec. 2.2.8). Further, the constant power transistor current pulses simplify the feedback-loop design.

Current-fed topology drives its power train (center tap of a push-pull or top end of a forward converter transformer) through an input inductor. Thus the power train is driven from high impedance of a current source (the input inductor) rather than the low impedance of a rectifier filter capacitor or the low source impedance of an input battery. This high output impedance of the prime power source very largely solves the flux imbalance problem when a push-pull converter is driven from the input inductor and offers other significant advantages.

In all topologies discussed thus far (voltage-mode circuits), output voltage alone is monitored and controlled directly. In those circuits, regulation against load current changes occur because current...