In a low-voltage high-current SMPS, the secondary voltage is often rectified with the help of centre-tapped configuration. The main advantage is that at any instant of time just one diode is rectifying (as compared to the full-wave bridge rectifier). This improves energy efficiency, because the forward voltage drop on the diode (e.g. 0.7 V) can be a significant proportion of the output voltage (e.g. 3 V). The main disadvantage of such approach is that the transformer is oversized, because only half of the secondary winding conducts at any time.5)
For low voltage applications the secondary winding might have very few turns (usually just one or two), which complicates selection of an appropriate turn ratio of the transformer.
Current doubler alleviates such problems. Finer control over turns ratio is possible, because the secondary voltage of the transformer is higher and the current is doubled outside of the transformer.6)
A transformer Tx is driven in a bipolar way (push-pull, half-bridge or full-bridge). Two identical inductors L1 and L2 are connected across the secondary winding of Tx. These inductors are not magnetically coupled to each other or to the main transformer.
Because of bipolarity, an alternating voltage is driven across the two inductors connected in series. The load R is connected between the central point of the inductors (similar to a regular centre-tap configuration) and the shorted output of the two diodes D1 and D2. The low-pass filter can be implemented as the capacitor C, or as an additional output inductor (not shown).
The name “current doubler” comes from the fact that the average output current of the whole device is twice the average currents in each of the output inductors.
However, the RMS value which are responsible for temperature increase of the windings depend on the DC offset in each current. If the current ripple is very high, then the RMS values increase accordingly and become comparable with the RMS value of the total output current. Under such conditions the “doubling” property of the topology is not utilised properly.
For this reason, the current doubler is usually operated in continuous conduction mode, so that the currents in the two output inductors keep flowing at all times, with large DC offsets and relatively small ripple, which reduces the RMS values. This is achieved by using proportionally larger inductance of the output inductors. Under such conditions the average value of the output current remains the same, but its RMS is twice the inductor currents. Additionally, the ripple in the output current is also reduced by half. This occurs because most of the time one current increases and the other decreases, so the ripple in their sum appears accordingly reduced, as shown in the waveforms.