Digital Control of Resonant Converters: Frequency Limit Cycles Conditions

The conditions for limit cycle oscillations in digitally controlled resonant converters were explored theoretically and tested by simulations and experiments. The analytical analysis reveals that, similar to the case of digital PWM control, limit cycles of such systems will occur when the LSB of the control is changing the output by a value that is larger than the resolution of the ADC. However, unlike the case of PWM, limit cycle oscillations is dependent on the operating point since both the power stage gain and the resolution of the digitally generated drive frequency, are not constant over the operational frequency range. Consequently, at high gains (close to resonant) the required frequency resolution may not be supported by the digital core. A time-domain simulation model, that was developed and verified experimentally, enables the steady state analysis of digitally controlled resonant converters including the limit cycles phenomenon as well as the closed loop response. The proposed static analysis and dynamic modeling were verified experimentally on a series-resonant parallel-loaded converter that was operated in closed current loop. The digital control algorithm was implemented on a TMS320F2808 DSP core. Very good agreement was found between the analytical derivations, simulations and the experimental results.