Single 48V to 1V, 200A Converter Powers Microprocessors (Part 2) (.PDF Download)

This converter utilizes a resonant capacitor in series with the primary winding. All power must be transferred from input to output by current passing through capacitor Cr. Capacitor Cr’s voltage rating is approximately 1V and yet it converts input voltage of 50V. Therefore, its voltage rating is 50 times lower than the input source voltage. In addition, the required capacitor value is in the order of 100µF, so a very small size capacitor is needed for this 100W, 1V @ 100A converter.

Furthermore, if a 5:1 step down is used to convert to 0.5V, the capacitor needs to handle 5 times reduced rms current of 20A instead of 100A. Likewise, the voltage scaling of the transformer leads to 5 times reduced voltage stresses of the output diode. This, in turn, significantly reduces diode conduction losses when a synchronous rectifier is used. 

Operation of this converter can be best understood when the models of the two circuit subintervals are analyzed with the inclusion of the leakage inductance Lr of the Hybrid transformer. Resonant inductance Lr resonates with the resonant capacitance Cr on the primary, resulting in resonant period Tr given by:

Tr = 2π√LrCr                             (11)

Note that the large magnetizing inductance of the transformer is effectively shorted during the OFF-time interval, leading to a purely sinusoidal AC current on the primary as illustrated in the waveforms in Figures 7 and 8, and the resulting sinusoidal current on secondary. This has crucial consequences for converter operation, as described below.