Hybrid Pulse Density Modulation of T-Type Resonant Inverter for Induction Heating

Pulse density modulation (PDM) is commonly used in induction heating (IH) to enhance inverter efficiency. However, PDM can cause significant fluctuations in the output current, especially under light-load conditions, leading to current intermittency and thereby causing the phase-locked loop to lose lock. Due to the limited output levels of the full-bridge inverters, the PDM improvement schemes applicable to LLC resonant tanks are constrained. To address this issue, an IH topology based on a T-type resonant inverter is proposed in this article. By coordinating the operation of each leg, this topology extends the output levels of the inverter while ensuring soft switching for all power devices. Aiming at the problems of PDM, a hybrid PDM (HPDM) is introduced, which can optimize the pulse sequence distribution and level configuration, effectively reduce current fluctuation amplitude and mitigate current intermittency. Moreover, a reconfigurable LLC-L resonant tank is proposed, utilizing dynamic impedance adjustment to promote more uniform pulse sequence distribution, and further alleviating current fluctuations. The experimental results validate the effectiveness of the proposed method. Compared with PDM, HPDM reduces current ripple by more than half and also improves system efficiency.