Just FYI's
Most recent switchers operate in the 100Khz to 1Mhz range (some of the newer PWM converters operate up to 2Mhz). Higher frequencies are used for both better effeciencies and so that smaller inductors may be used.
Regarding generating negative voltages - a center tap is one way. More common is an inverting switcher design (which requires no transformers).
A good example of one is On Semi's NCV33063A controller which can be configured for step up, step down, or as an inverting switching supply.