8.11 A 48V-to-5V Buck Converter with Triple EMI Suppression Circuit Meeting CISPR 25 Automotive Standards.

In automotive-used switching regulator applications, the characteristic of extreme conversion ratio (48V to 5V) and high switching frequency ($\mathrm{f}_{\mathrm{SW}}$) will give rise to considerable electromagnetic interference (EMI) issues, which is a tremendous obstacle to meeting CISPR 25 automotive standards. As shown in Fig. 8.11.1 (top), if $\mathrm{V}_{\text{Battery}}$ and $\mathrm{V}_{\mathrm{SW}}$ nodes are considered, the EMI problem can be divided into two areas. Region I is caused by $\mathrm{f}_{\mathrm{SW}}$ harmonic tones (<50MHz) and Region II is caused by $\mathrm{V}_{\mathrm{SW}}$ spurious ringing and high dv/dt of $\mathrm{V}_{\mathrm{SW}}(\gt 50 \mathrm{MHz})$. Prior techniques utilize spread-spectrum modulation (SSM) to reduce noise at ${\mathrm {V}}_{{\mathrm {SW}}}$ by increasing ${\mathrm {f}}_{{\mathrm {SW}}}$ spread spectrum $\left(\Delta {\mathrm {f}}_{{\mathrm {SW}}}\right)$ or decreasing modulation frequency $\left({\mathrm {f}}_{\mathrm {M}}\right)$ in Region I [1–3,6]. Moreover, EMI filters can be used to mitigate EMI problems in Region II [4, 5]. However, $\mathrm{f}_{\mathrm{SW}}$ harmonic overlap issues are more common in Region II, making EMI suppression less effective in conventional SSM techniques. Spurious Noise Compression (SNC) scheme in [1] modulates $f_{\mathrm{SW}}$ with noise $\left(\mathrm{V}_{\mathrm{n}}\right)$ from Zener diode to largely flatten EMI spikes. Besides, with the aid of Tri-Slope Gate Driving (TSGD), dv/dt and di/dt at $\mathrm{V}_{\mathrm{SW}}$ during rising edge can be finely tuned to further lower the noise level. Unfortunately, [1] cannot alleviate EMI at $\mathrm{V}_{\text{Battery}}$ and TSGD cannot control the falling edge of $\mathrm{V}_{\mathrm{SW}}$. The multi-rate SSM (MR-SSM) technique in [2] solves the EMI power aliasing spikes by modulating ${\mathrm {f}}_{{\mathrm {SW}}}$ with predefined multi-rate on-time pattern and adaptively synchronizing off-time; however, it fails to optimize the EMI performance as Random SSM does since the $\mathrm{f}_{\mathrm {M}}$ of MR-SSM will lie in a predefined value. The condition-adaptive $\Delta {\mathrm {f}}^{3}$-EMI control in [3] takes input voltage $\left({\mathrm {V}}_{\text{IN}}\right)$, load current $\left(\mathrm{I}_{\text{LOAD}}\right)$, and spectrum overlapping issues into consideration to adjust $\Delta \mathrm{f}_{\mathrm{SW}}$ and $\mathrm{f}_{\mathrm{M}}$ to optimize the EMI reduction. However, it lacks slew rate control when turning on and off the power switches, which induces unsatisfactory EMI reduction in Region II.