15.7 An 8.3MHz GaN Power Converter Using Markov Continuous RSSM for 35dBμV Conducted EMI Attenuation and One-Cycle TON Rebalancing for 27.6dB VO Jittering Suppression
GaN power switches have gained fast-growing popularity in power electronics. With a similar R DS_ON resistance, they boast 2-to-3-order lower gate capacitance than silicon counterparts, making them highly desirable in high-frequency (fsw ), high-performance power converters. However, at high f sw , switching transitions have to be completed in much shorter times, creating much larger di/dt and dv/dt changes in power stage, which directly link to electromagnetic-interference (EMI) emissions . To suppress EMI, spread-spectrum-modulation (SSM) techniques [2-5] have been proposed. As depicted in Fig. 15.7.1, a periodic SSM (PSSM) is straightforward and easy to implement. However, its EMI suppression is not effective . A randomized SSM (RSSM) can outperform the PSSM, with lower peak EMI and near-uniform noise spreading, but its performance highly relies on the random clock design. In , an N-bit digital random clock was reported to achieve a discrete RSSM (D-RSSM). However, the bit number N has to be large in order to achieve satisfying EMI attenuation, significantly increasing circuit complexity, chip area, and power consumption. To overcome this, a thermal-noise-based random clock was proposed . Unfortunately, thermal noise is very sensitive to temperature and is hard to predict. To apply this approach to a practical implementation requires additional signal processing with periodic signals to confine its range of randomization, which, in turn, reduces the benefits of the RSSM. To achieve a near ideal RSSM, a continuous RSSM (C-RSSM) with a cost-effective implementation is highly preferable. Meanwhile, another challenge of applying SSM schemes lies in the fact that the schemes deteriorate V O voltage regulation. As shown in Fig. 15.7.1, as an SSM scheme continuously or periodically modulates f sw , a converter switching period fluctuates cycle by cycle, causing random errors on the duty ratio and thus jittering effect on V O. This is difficult to correct by a feedback control loop, as the duty-ratio error changes randomly between switching cycles. Due to a limited loop-gain bandwidth, the loop response usually lags far behind. Although a ramp compensation scheme was reported to resolve this , the improvement is very limited, and the scheme only works for voltage-mode converters. © 2019 IEEE.