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 $\mathrm {R}_{\mathrm {D}\mathrm {S}_{-}0\mathrm {N}}$ 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 fsw, 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 [1]. 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 [2]. 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 [3], 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 [4]. 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 Vo voltage regulation. As shown in Fig. 15.7.1, as an SSM scheme continuously or periodically modulates fsw, a converter switching period fluctuates cycle by cycle, causing random errors on the duty ratio and thus jittering effect on Vo. 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 [5], the improvement is very limited, and the scheme only works for voltage-mode converters.