Surge current

At the instant a power supply is switched on, a sudden current surge occurs, even with no load at the output. This is because the filter capacitor(s) need an initial charge, and they draw a lot of current for a short time. The surge current is far greater than the operating current. This can destroy the rectifier diodes. The phenomenon is worst in high-voltage supplies and voltage-multiplier circuits. Diode failure can be prevented in at least four different ways.
The first method uses “brute force.” You can simply use diodes with a current rating of many times the operating level. The main disadvantage is cost. High-voltage, high-current diodes can get expensive.
A second method involves connecting several units in parallel wherever a diode is called for in the circuit. This is actually a variation on the first method. The overall cost might be less. Current-equalizing resistors are necessary.
A third scheme for surge protection is to apply the input voltage little by little. A variable transformer, called a Variac, is useful for this. You start at zero input and turn a knob to get up to the full voltage. This can completely get rid of the current surge. A fourth way to limit the current surge is to use an automatic switching circuit in the transformer primary. This applies a reduced ac voltage for a second or two, and then switches in the full input voltage.
Which of these methods is best? It depends on the overall cost, the operating convenience, and the whim of the design engineer.