Control systems play a critical role in improving the economic viability of wave energy converters (WEC). This paper presents a framework for a causal power-maximising feedback controller inspired by Phi method. Quadratic damper control and PID control are combined to collectively achieve the optimal power absorption conditions for the peak wave periods, which results in a nonlinear controller being suboptimal in irregular waves. The proposed controller is tested on a fully submerged heaving spherical point absorber in regular and irregular wave conditions in time domain. The tuning of the PID gains are investigated in frequency-domain from a stability and control bandwidth perspective. The modelled WEC dynamics includes linear hydrodynamic forces (excitation and radiation) and the nonlinear viscous drag force. The main benefits of the proposed controller are (i) it does not require any time-consuming optimisation either online or offline; (ii) has a wider bandwidth and better power absorption performance as compared to the conventional spring-damper (PI) controller; and (iii) easy to implement in practice like standard PID control.