I would like to make a SMPS which 'follows' the input voltage, but inverts it. For example, if I input 7 volts, I want approximately -7 volts out, and if I input 12 volts I want -12 volts out; preferably the output should go to around -3 volts to up to -18 volts. It needs to handle large currents so it looks like a charge pump is not a suitable solution.
You're free to design charge pump (switched capacitor) converters at any current level. The question is, what approximately means for you? A witchd capacitor converter als well as e.g. a push-pull forward converter can provide approximately 1:1 conversion, but with some internal resistance. To avoid any load dependant voltage drop, a converter with controllable output voltage and a control loop are needed.