Handling PWM Challenges in Resonant Converters
Abstract
Nowadays, topologies that requiring PWM phasing-shifting and frequency-changing have been widely used in digital power applications. Complicated topologies like multi-phase interleaved LLC have been introduced as the result of demand for higher efficiency and higher power level. To handle the challenge, Type-4 PWM is introduced on TI generation 3 C2000 devices. Especially, new features like global load and one shot reload would significantly simplify implementation phase-shifting and frequency changing topology. This application note discusses the benefit of these new features, and how to deal with corner cases in real practice.
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1 Introduction
With higher requirement of power rating and efficiency in power applications like server and telecom PSU, resonant converters like LLC have become the prioritized option of DC/DC stage in more and more new designs. Comparing with traditional full-bridge topology, LLC is easier to achieve soft switching due to resonant operation, leading to better EMI performance, higher efficiency and less stress on power stage.
Figure 1-1 Plot of Gain vs.
FrequencyNormally the LLC converter works under variable frequency but fixed duty cycle. The typical gain curve of a LLC converter is shown in the following picture. The converter works near frand adjust switching frequency through the control loop to stabilize the output voltage.
