Need for Variable Frequency Control in DC-DC Switching Converters– Challenges and Opportunities using Digital Implementation
DC-DC converters play a vital role in efficient power management in various portable devices. It offers fast dynamic performance and high energy efficiency over a wide operating range. In the context of fixed-frequency pulse-width modulation (PWM), multi-loop current-mode control (CMC) offers higher bandwidth and superior line regulation compared to single-loop voltage-mode control (VMC). However, CMC suffers from sub-harmonic oscillations with the duty ratio ‘d’>0.5, and a compensating ramp is needed for stabilization. This degrades the closed-loop bandwidth, which further degrades as the ramp slope increases with ‘d’. On the other hand, variable frequency control methods, such as hysteresis control and constant on/off-time control techniques offer improved stability, transient response, and light-load efficiency. However, undesirable steady-state frequency variations complicate the input filter design and eventually lead to EMI problems. While a digital platform is helpful for fast and precise frequency regulation using an all-digital-PLL, the choice of sampling remains an important concern for implementing variable frequency digital control. This paper highlights the above challenges and scopes. Then it presents event-driven digital control solutions and design insights for next generation power management solutions.
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