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A power supply operating at 55 kHz refers to the switching frequency of the (switching) voltage supply regulator. There are two basic types of voltage regulators: linear and switching regulators. A basic switching regulator is some combination of diodes, capacitors, transistor switches and inductors. The speed and duty cycle of the transistor switches determine the DC output value. For your case, the transistors switch at 55 kHz. The power supply gives out a DC output. That means there is not any frequency except for the voltage ripple in the output.
See the book called "Switching Power Supply Design" by Abraham Pressman.
A higher switching frequency permits using smaller components. Particularly the coil. Thus the supply costs less to manufacture.
The designer constructs the prototype circuit. Then he adjusts it to yield maximum current output. One way is to use a high enough frequency to get maximum throughput of joules in and out of the inductor.