DC-DC power converter; Fuel cell; Nonlinear control systems; Supercapacitor Backstepping; DC-DC converters; Electric inverters; Industrial electronics; Nonlinear control systems; State feedback; Supercapacitor; Voltage regulators; Backstepping technique; Constant output voltage; Control schemes; Lyapunov stability analysis; Modeling and control; Nonlinear current; Nonlinear state feedback control; Voltage source; Fuel cells