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 (fSW) 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 VBattery and VSW nodes are considered, the EMI problem can be divided into two areas. Region I is caused by fSW harmonic tones (<50MHz) and Region II is caused by VSW spurious ringing and high dv/dt of VSW(> 50 MHz). Prior techniques utilize spread-spectrum modulation (SSM) to reduce noise at V_SW by increasing f_SW spread spectrum (Δ f_SW) or decreasing modulation frequency (fM) in Region I [1–3,6]. Moreover, EMI filters can be used to mitigate EMI problems in Region II [4, 5]. However, fSW 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 fSW with noise (Vₙ) from Zener diode to largely flatten EMI spikes. Besides, with the aid of Tri-Slope Gate Driving (TSGD), dv/dt and di/dt at VSW during rising edge can be finely tuned to further lower the noise level. Unfortunately, [1] cannot alleviate EMI at VBattery and TSGD cannot control the falling edge of VSW. The multi-rate SSM (MR-SSM) technique in [2] solves the EMI power aliasing spikes by modulating f_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 fM of MR-SSM will lie in a predefined value. The condition-adaptive Δ f³-EMI control in [3] takes input voltage (VIN), load current (ILOAD), and spectrum overlapping issues into consideration to adjust Δ fSW and fM 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.