The variable dc-dc transformer has boost modulator characteristics. It is the preferred output modulator for power converters requiring isolation and having a large step-down voltage ratio.

The primary excitation operates at 100 % duty-cycle, for maximum efficiency. The effective turns ratio is controlled on the secondary side, so no feed-back signals need to cross the isolation barrier.

The efficiency of the variable dc-dc transformer is highest when its effective turns ratio is maximum, just as in a boost converter operating at nearly 0 % duty-cycle. If the input voltage droops, the effective turns ratio is reduced to maintain the output voltage regulation. Because the effective turns ratio is lower, the input current increases and efficiency is reduced.

In the familiar boost and buck converters, the duty-cycle of the switch S1 is pulse width modulated to regulate the output voltage Vo. The entire current and voltage is switched on and off during each switch cycle. At high duty-cycles in the boost converter and low duty-cycles in the buck converter, the rms current and voltage are high and efficiency is poor.

In the variable dc-dc transformer, the effective turns ratio is modulated between two discrete steps of many to regulate the output voltage. Because the effective turns ratio varies by a small step, the voltage and current do not change nearly as much and the rms current and voltage are close to their dc values. At low effective turns ratios, the input current increases and the efficiency is reduced.

The graphs above are copied and pasted form SPICE simulations using the probe function.

In the graphs on the left, the input voltage Vi is varied from 100 to 140 volts. The effective turns ratio Tx is modulated between 8 : 1 and 9 : 1, then between 10 : 1 and 9 : 1 and so forth. The primary voltage is Vs and the output voltage is regulated at 12 V dc.

In the graph on the right, the input voltage has an extreme ac ripple, 100 to 140 volts. The effective turns ratio modulates in steps to regulate the output voltage to 12 V dc.

The variable dc-dc transformer is most efficient at its maximum effective turns ratio, but just as with a boost converter operating near 0 % duty-cycle, it saturates with over-voltage transients and no longer regulates. The output is determined, and follows the input voltage like a fixed ratio dc-dc transformer. The graphs above show the result.

The solution is to put a buck converter as an input stage, and have it operate at 100 % duty-cycle except when there is an over-voltage transient condition.

Vox is the curve that the output voltage Vo would follow with a fixed ratio dc-dc transformer alone. To the left of the vertical green line, the variable dc-dc transformer regulates the output voltage Vo. To the right, the buck regulator operates. Efficiency is maximized at the green line, and is very good over the normal operating range.

The "generic" variable dc-dc transformer can vary its effective turns ration over the full range, but the input currents get very large at low input voltages, unless there is load shedding, so its use at very low effective turns ratios should be restricted to transient conditions or "hold-up" time.

The coaxial transformer is very simple. The secondary windings are formed stampings inserted into a simple ferrite core.

The cores may be mounted on a printed circuit board. The transformer shown is for a 1 V dc output at 150 A dc.

A variant of the simple variable dc-dc transformer has three transformer elements. When operated at nominal input voltage, there is no ripple and the variable dc-dc transformer regulates for +/- 10 % variations in the input voltage. A buck converter input stage can be used for over-voltage transient protection.