Improving the peak to average power demand of a switching type capacitor/PFN charger

Commercially available switching type chargers for capacitor/PFN loads typically operate in a constant current mode. This results in a linear voltage ramp charging waveform. Since the current is constant, the power demand is also a linear ramp with a peak to average power ratio of two, over the charging period. At low repetition rates this two to one power fluctuation can be a troublesome interface with the prime power, especially a battery. Batteries have a specific power characteristic in kW/kg. If the peak to average power demand is two, twice the battery weight is required for the same application with a constant average power demand. Using a filter can smooth the power fluctuation but there is a significant weight and cost penalty for low repetition rate high-energy applications. The ideal solution for a constant power demand is a voltage waveform on the capacitor/PFN that is proportional to SQRT(time). This is difficult to implement. It is obvious that a variable or piece-wise turns ratio transformer could solve the problem technically; however, the cost, weight and complexity becomes a serious issue. A more productive solution is to modify the operation of the switching charger to reduce the two to one power demand ratio to a lower value. By studying the basic energy transfer mechanism in the switching charger design, modifications become apparent that are not complex or burdensome and can result in a reduction of the peak to average power demand to less than 1.25 to one. When applied to the total system including the prime power interface, a net advantage results.